Anti-HLA antibodies especially DSA either preformed or donovo are associated with poor graft outcomes. Understanding the pathogenesis of anti-body formation considers the future hope for finding selective therapies targeting humoral responses. the human immune system consists of 2 main types of cells. Granulated innate immunity and adaptive nongranulated immune cells. Adaptive immunity (including T lymphocyte and B lymphocyte) needs antigens recognition by antigen-presenting cells (Follicular Dendritic Cells (FDC), B cells, or macrophages) either by direct, indirect, or semi-direct pathway to be activated. activation of B cells leading to the formation of plasma cells and antibodies formation including DSA, memory B cells, and mature B cells via high endothelial venules (HEV) under the guidance of chemotactic factors like TNF alpha. Immature B Cells enter lymph nodes in the cortex near the subcapsular sinus. Antigen-presenting cells capture antigens and deliver them to lymph nodes. Lymphoid tissues including lymph nodes and spleen consist of peripheral white pulp in which resident cells capture antigens and introduce them to APC and act on T cells in the T cell zone in the periarteriolar lymphoid area leading to T cell activation. Antigen-presenting cells and T-cell crosstalk usually take 5 days then rapid T cell transformation under the effect of IL to follicular helper cell (TFH) which can be either extrafollicular TFH or intrafollicular inside GC (Germinal Centre) TFH cells. T/B cells crosstalk and B cell priming usually occur in the spleen leading to the formation of early plasma cells, memory B cells, and B cells that migrate back to the follicle inside the lymph nodes. The GC has two zones a light peripheral zone contains FDCs and TFH cells while the dark central zone of GC contains no FDCs. B cells undergo apoptosis in the light zone, while dark zone B cells undergo activation and proliferation to plasma cells with intact B cell receptors (BCR) which re-enter the light zone again to test BCR for binding to antigen. After testing the BCR binding affinity to antigen inside the light zone, 2 types of B cells are diagnosed: 1-Early pre-GC memory B cells, which mainly secrets IgM with lower affinity 2-post-GC memory B cells have higher affinity and are able to secret both IgM and IgG. Memory B cells on re-exposure to the same antigen either undergo direct transformation into plasma cells secreting high titer antibody response, or they differentiate into GC B cells which transform to new higher affinity plasma cells.
Wael Jebur
2 years ago
The most influencing factor for chronic allograft dysfunction and failure is chronic ABMR ,especially the De novo DSAs., which is further featuring vascular obliterative lesions.
How the B lymphocyte stimulated by the donor HLA antigens?
Usually the B lymphocytes interact with HLA protein displayed on follicular dendritic cells FDC, B lymphocytes and macrophages. Consequently it would differentiate to immunoglobules secreting plasma cells and memory cells.
Several mechanisms pertain to excel the function of B lymphocytes:
1-the B lymphocytes enter the lymph nodes once every day driven by the chemokine gradient established by the FDC and fibroblast reticular cells FRCs.The it congregate in the sub capsular zone in the lymph nodes where the antigen accumulate by means:
a) complement system deposited on surface of Antigen.
b) antibody directed against the antigen.
c) circulating dendritic cells that acquire the antigen at the tissue site.
It can be stimulated in the Spleen as well.
d) Another mechanism of allo-recognition is accomplished by the means of delivering intact HLA antigen extracellular vesicle or exosome which circulate reaching to the lymph nodes.Where its captured and presented by specialized macrophages.
e) Donor Dendritic cells might circulate and reach to lymph nodes to dump the HLA antigen.
When DCs capture the donor HLA it will be cross-dressed which stimulate the CD8 lymphocytes directly,
Of interest:
1-Interestingly DCs cross dressed with MHCII are not effeciontly priming CD4 lymphocytes.
2-When exosomes taken by macrophages they will deliver it to FDCs within the lymph nodes.
3- When BCR binds to antigen on Macrophages ,dendritic cells or non antigen specific B lymphocytes it trigger B cell activation.
4- BCR bound antigen internalized and processed by B-lymphocytes is presented in combination with MHC II on cell surface to be interacting with T cell receptor on CD 4 lymphocytes.
5-T cells can recognize intact MHC or part of it in combination of MHC .
6- Only CD 4 Lymphocyte can trigger the conversion of B lymphocytes to Plasma cells producing DSAs.
ahmed saleeh
3 years ago
*In the past the center of focus of rejection was directed twards T cell mediated rejection which was challenged over the past decad where AMR esp. De Novo DSA is the main claim for the cause of chronic kidney transplant dysfunction and failure
*B cells plus specefi antigen start the process of differentiating into PCs producing antibody and memory B cells.
*Antigen presented to the conjugate BCR in the lymph nodes where it reaches LNDs with the help of FDCs (dendrites and chemokines) where antibodies are formed activating the complement pathway .
*humoral response arise from the spleen, two distinct compartments: the red pulp (venous sinuses) that encounter old or damaged erythrocytes by macrophages, and the white pulp immune responses . The white pulp contains T cell zone (PALS), B cell follicles and marginal zone .
The MZ contains resident cells capturing antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells.
*soluble antigens (≤14 kD), gain direct access to the B cell via passive flow through FRC conduit in the sinus floor of the lymph node or in the marginal sinus in the spleen .
*FRC conduits intersect with FDCs and provide direct connection for soluble antigen to be taken and presented on FDC surface.
*Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins
*Early B cell activation and the extrafollicular T/B cell interaction :
Antigen presented to BCR through dendritic cells reaches T/B border where MHCII antigens interact with CD4 T Helper cells causing B cell activation and production of T dependant DSA
*Memory B cells are produced in two phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG
*successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation
*only patients without DSA and without HLA sensitizing events such as pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory.
*IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies,
* the fate of memory B cells on reencounter with antigens is dependent, on how and type of memory B cell was generated and the conditions of the antigen-reencounter.
Nasrin Esfandiar
3 years ago
· In the past, only T cell were thought that have an important role in rejection but in the last decade, important role for donor specific antibodies (DSA)was considered.
Now, AMR is known as a major cause of chronic renal dysfunction. This article is a review about B cell activation and DSA generation post Tx. B cell can differentiate in to plasma cell and memory B cells. B cells expose to a variety of antigens in lymph nodes or spleen, especially with pulp. Antigens of transplanted kidney less than 14 KD are presented on the FDS also they activate complement after binding to DSA.
Non antibody-binding antigens are linked to DCS via innate recognition
DCS in the lymph nodes or spleen can activate complement.
In addition, intact MHC complexes of donor can accumulated in these sites and can activate recipient CD8+ T cell (direct path way).
· B cell activation and T/B cell interaction:
Ags which are displayed on FDS, SSM, or non-Ag –specific B cells, binds to BCR and can activate B cells causing movement to the T/B border. Ag peptides are presented on MHC class ΙΙ and have interaction with TCR on CD4+ T cells.
Interaction between T and B cells causes differentiation of B cells to early PCs and memory B cells and pre- GC response.
T cell exposure to Ags presented on DCS with signals of Co-stimulation and inflammation, causes T folicular helper (Tfh) responses. T fh express CXCR5 and Bcl-6 localize them toward B cell follicle and T zone (T/B) interfaces activate DCs expressing ICOS ligand and CD25 and Tfh cell differentiate.
· Germinal center response:
The GC is divided into the light zone and dark zone.
Light zone B cells receive signal from GC-Tfh cells and migrate in to dark zone where they are selected to emerge as PCS or memory B cells. CD40: CD154 interaction between T and B cells are important to maintain GC response.
Both anti CD154 and CTLA-4 Ig at 7 days can decrease GC B cells responses and DSA production. So, belatacept can control humoral responses by inhibition of B cells-Tfh interaction and B cells differentiation in to PCS, in spite of higher rate of AR. Germinal center responses to multiple complex antigens: In TX, B cells expose to multiple complex Ags and 15% of them develop de novo DSA during 46. Years of TX which is correlated which higher degree of HLA epitope mismatch.
· Regulation the germinal center response:
Recently a subset of CD4+ T cells named follicular regulatory (Tfr) cells are recognized that express FOXP3.The cells regulate Ab response by preventing long-lived interaction at the T/B border and prevent Tfh cells entry in to GCS and B cells differentiation in to GC B cells, memory B cells or Pcs.
Expression of CD69, IL-10, CT-10, CTLA-4 and GARP were associated with B cell suppression by Tfr. So may be TGF-β production is the mechanism of B cell inhibition by Tfr.
· Generation of memory B cells and PCs:
B cell differentiation in to PCs at the T/B border and within GCs is promoted by high BCR affinity BCR binding Ag on FDS light zone causes PC differentiation and Tfh cells drive their migration at of the GC.
Memory B cells are generated is an early pre-GC and post –GC memory B cells. Reduced IRF-4 and higher Bacl2 which is seen with lower-BCR signaling and T cell help can drive memory B cells which is opposite to PCs.
B cells lack CXCR4 which failed entering DZ, will turn to memory B cells.
So they have different mechanisms which means that lack of DSAs is not equate to absence of memory B cells.
So history of HLA sensitization events are important for considering patient as immunologic low risk.
· Recall B cell responses:
With exposing to specific Ags memory B cell will differentiate in to PCs or GC B cells.
Recall DSA response was associated with rapid differentiate in to PCs and low GC responses and was dependent on T cell help. So co-stimulation blockage by CTLA-4 Ig completely prevented this.
Long-lived PCs (LLPcs) repress CD28 and inhibition of CD25-CD80/CD86 interaction by CTLA-4Ig could describe belatacept ability in controlling DSA response in sensitized patients.
Better understanding of these mechanism will result in better control of DSA production and graft outcome.
Manal Malik
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation
ABMR is a major cause of Graft dysfunction and Graft loss,so B cell plays major role in initiation the immunological process .
Routes of antigen presentation to B cells
Intact soulble Ag activate B cell through B cell receptor (BCR) enags intact Ag displayed on FDCs,B cells or macrophages
B cells entering LN through multiple routes depend on Ag size,presence of AgAb and complement and migrating DCs,
B cell direct activate by small soluble protein through FRC conducts in LN or spleen .
DSA and polyreactve antibodies bind to soluble Antigen and activate complement (respecter CR3,CD11D/CD18) so opsonized Ags by B cells in the follicle through complement receptor CD21(CR2) and CD 35(CR1) or FCRY11B receptors.
Extracellular vesicle exosomes release by allograft one captured by SIGN-R1M2 macrophages,Fa180 then drain into LN or spleen where resident DCs donor MHC delivered by exosomes to recipient DCs and stimulate CD8 T cell via direct way.
In the Bcell zone ,glycogenprotein with sialic acid capture by sialodhesion(CD169) the macrophages deliver exosomes donor MNC to FDCs.
B cells activation by non antigens specific B cells or specific Ag(BCR-bound Ag)
B cell activate by Ag specific T cell receptor CD4+ T cells and recognize MHC of recipient,so activate or differentiate B cells recipient B cell into PCS producing DSA
Activation of T cells requires DCs that acquired Ag from allograft and migrate to LN when DCs arrived with tissue.
DCs and present capture Ag to T cells response more rapidly than independents migrate.
Germinal center response.
TCR signaling strength are unclear and their differentiation into Tfh also requre specific signals derived from TCR co-stimulation that facilitate different into other CD4T effect cell lineage ,understanding of these differentiation lead to identification way to prevent and treat ABMR and TCMR
The pre GC response generate PCs that lower affinity than post GC PC so important of preGC response to DSA production following organ transplant and contribution of this response to AMR has not been delineated
Germinal center response.
consist of dark and light zone which the later one contain FDCs and Tf
light zone Bcells express CD86 and CD38
B cells with high affinity BCR for antigens has an important role for persistent circulating antibodies or function as memory B cell for antibody production upon antigen exposure in future.
CTLA-4 on T FR limit the expansion of Ag specific T FR and reduce antigen specific antibody.
In the absence of T cell survival signals ,B cells undergo apoptosis in the light zone
Beltaacept inhibit B cell and Tfh interaction so can control humeral response but still under study.
Germinal Center responses to multiple complex antigens
When initial antibodies is produced these antibodies bind to the same Ag epitopes driving their production,either selection of B cells with higher affinity than circulating antibodies or in the selection of B cell with specified for new epiotpes
The affinity of maturation within GC1 based on the analysis following immunization with single hatpton or small proteins.
presence of many antigenic epitopes will reduce the relative of Abs repertoire.
Recently has been described that regulating the germinal center response subset of CD4 T cells that express the regulatory TC(treg)cell master regulator FOXP3 showing many phenotypic characteristics of Tth cells .
Generate memory B cells and plasma cells.
Plasma cell function is controlling of infection produce memory cells and protection against reinfection.
BCR receptor affinity and Tfh is crucial for differentiation and controlling of B cells.
Patients without DSA and HLA sensitizing consider as low risk for transplant rejection.
Memory B cells has 2 phases:
Early preG(memory B cell) with lower affinity for IgM
Later post-GC memory B cells with higher affinity and expressing IgM or IgG
Memory B cells generated in with pre-GC and early GC so pc later long lived
Affinity and kinetics of memory B cell versus pc differently suggest that treat of acute ABMR and reduction of circulating DSA may not prevent memory donor specific B cell generation
Recall B cell response
Memory B cells differentiated into PCs by two ways:
a) Faster and higher titre of antibody response
b) Higher affinity and class switch PCs generated from Gc B cells
IgG memory B cells predominant over IgM memory B cells in the presence of circulating antibodies
Factors affect fating of memory B cells up the antigen is depend on:in part and how and type of memory B cells
Long life PC express CD28 that provide survival signal by engaging CD80-CD86 express by B.M stromal cells as result CD28-CD80/CD86 interaction with CTLA/Ig resolution on long life PC deposition and explain recently ability of belatacept to control reverse DSA response in sensitized recipient
Conclusion:
De novo DSA is major cause of allograft loss
This review summarize the cellular process that generate an antibody response but not non antibody producing B cells
Insights into mechanism lead to antibody production and humeral memory from study of model antigens and how this process are alter by feature of organ transplant require further investigation
Failure of successful transplant influence by factors that impact on:
The cellular response that generate B cells
Quality of producing DSA
The quality of memory donor specific B cells
A new way to control DSA production
Ahmed Omran
3 years ago
T cells are considered major player in graft rejection. It was suggested that there is association of DSA with increased rejection risk and worse graft outcomes have addressed the role of B cells in organ transplant. Cognate antigen on follicular dendritic cells , B cells and macrophages act on B cell receptors leading to activation of B cells, resulting plasma cell formation which give rise to donor specific antibodies and memory B cells. B cells ;mature ones through high endothelial venules enter lymph nodes and assemble in cortex close to subcapsular sinus. Migrating dendritic cells catch antigens and deliver them to lymph nodes. White pulp of spleen starts immune response. Marginal zone of white pulp has resident cells which catch antigens and shuttle them to FDCs. Migrating dendritic cells act on T cells in the T cell zone ;PALS-periarteriolar lymphoid sheath resulting in T cell activation, but it taking up to 5 days to act. Rapid response develops with delivery of antigen to T cell zone through follicular reticular cell conduits. Activation of T cells will result in acquiring follicular helper cell properties which can be either early extrafollicular mantle Tfh and germinal centre Tfh cells. B cell priming at the T/B interface in spleen result in formation of early plasma cells, memory B cells and B cells which migrate back to follicle. GC has a light zone with FDCs and Tfh cells while the dark zone of GC have no FDCs. B cells undergo apoptosis in light zone, and dark zone B cells with intact B cell receptor reenter light zone where BCR are tested for binding to antigen & T cell help. Differentiation into plasma cells relies on higher BCR affinity. Early pre-GC memory B cells, IgM mainly , have lower affinity and then post-GC memory B cells have higher affinity expressing IgM and IgG. Memory B cells, on re-encountering with antigen will either directly form plasma cells leading to a high titer antibody response, or differentiate into GC B cells to generate new high affinity plasma cells.
Wee Leng Gan
3 years ago
Development of B cell responses. The new insights and its implications for solid organ transplantation.
Antibody mediated rejection ( AMR ) due to donor specific antibodies ( DSA ) is associated with acute graft rejection and may serve as one of the reason for chronic kidney transplant dysfunction and failure. Knowledge on B cell activation and antibody production particularly the DSA is crucial for solid organ transplant. It provide wider prospects in managing kidney transplant rejection.
Mechanism of B cells activation.
1） Mature B cell entering lymphatic system through high endothelial venules ( HEV ), migrating along processes extending from follicular dendritic cell ( FDC ) and chemokine CXCL13. Mature B cell congregate within the cortical region near the subcapsular sinus and bind to the soluble or particulate antigens. Ultimately this lead to formation of antigen specific antibodies and complement activation via classical or alternative pathways.
2） Spleen has a role in B cell activation. Antigen antibody binding process occur at whitle pulp area of the spleen especially the B cell follicles and marginal zone ( MZ) which contain large reservoir of resident cells.
3） Small soluble protein ( less than 14 kD) from transplanted organs activated B cells through the lymphatic system and margical sinus in the spleen.
4） Subcapsular sinus macrophages ( SSM ) opsonized antigens and relayed to B cells via complement receptors. B cells then transport opsonized antigens via FDC to the lymphatic system .
5） Antigen not bound by antibodies can bind to macrophages and dendritic cells at lymph nodes via innate recognition proteins which activate complement system at the spleen.
6） Exosomes released by allograft, alloantigen and donor derived intact MHC complexes accumulate in lymphatic system and B cell zone of the spleen which trigger B cell activation .
Early B cell activation and extrafollicular T/B cell interaction.
1） B cells receptors bind to antigen on MHC class II molecules through cascade of immunological reaction lead to activation of antigen specific T cell receptors on CD4+ T cells.
2） Only CD4+ T cells that have indirect specificity for donor derived antigens expressed by recipient B cells are capable to stimulate differentiation of recepient B cells into plasma cells producing DSA. This cognate interaction between T and B cells initiates the T dependent DSA response.
3） B cell priming at the T and B cells interface lead to early plasma cells generation, memory B cells and transformation into B cells that are destined to move back to the follicle to stimulate germinal center ( GC ) responses.
4） Pre GC responses generate plasma cells with lower affinity.
5） bcl-6 expression at T cells stimulate extrafollicular production of Ig G2 which is crucial to constrain infection.
6） However, pre GC response to DSA production after solid organ transplantation and trigger AMR has not been delineated.
Germinal center response.
1） GC consist of light zone which contain stroma derived FDC and T fh cells. On the other hand , the dark zone of GC is devoid of FDC.
2） Light zone B cells stimulated by the activation markers CD86 and CD83. The activated B cells bind to antigen presented to FDC and receive survival signals from GC T fh cells.
3） Selected B cells up regulate CXCR4 and migrate into dark zone to undergo cell proliferation and BCR diversification mediated by activation induced cytidine deaminase (AID).
4） Without T cell survival signals, B cells undergo apoptosis at the light zone. B cells with BCR gene defect due to AID undergo apoptosis at the dark zone.
5） Dark zone B cells with intact BCR down regulate CXCR4 and re enter light zone to bind to antigen surface and access to T cell help.
6） B cells with high affinity BCR for antigens has an important role for persistent circulating antibodies or function as memory B cell for antibody production upon antigen exposure in future.
7） CD 40 : CD 154 are important for initial activation of T cells and maintaining GC response. Unfortunately, no FDA approved drug available at the moment to target CD40:CD154 interaction.
8） CTLA-4 on T fr limit the expension of antigen specific T fr and reduce antigen specific antibody. CTLA-4 Ig is an FDA approved immunosuppressive to prevent post transplant kidney rejection.
9） Both anti CD 154 and CTLA-4Ig reverse GC response and prevent further DSA increase.
10) Belatacept able to control humoral responses by inhibiting B cell and T fh interaction. However the indication of Belatacept in kidney transplant recipients still under investigation.
Germinal Center responses to multiple complex antigens.
1) Majority of organ transplantation involves multiple MHC and non MHC incompatibilities.
2) B cells clone with different antigenic specificity and affinity compete for stimulation during episodes of somatic hyoermutation within GC. Hence, presence of multiple antigenic epitopes tends to reduce the relative breadth of antibody repertoire.
3) Most kidney transplant recipients likely encoungter multiple complex antigen. Approximately 15% of kidney transplant recipients develop de novo DSA within 4.6 years of translantation.
Regulating the germinal center response.
1) Subset of CD4+ T cells express regulatory T cells ( Treg ). The T follicular regulatory cell ( TFr ) function by controlling GC responses.
2) The TFr cells migrate to B cell follicles and suppressed self reactivate B cell generated in GC.
3) Without TFr , self reactivated B cells differentiate into plasma cells that produce anti histone and ANA.
4) TGF β production control by ability of TFr inhibit B cell response.
Generate memory B cells and plasma cells.
1) Rapid plasma cells production following antigen exposure is crucial for protection against reinfection
2) Both B cell receptors affinity and TFh is important for plasma cells production.
3) Memory B cells consist of early pre GC memory cells with lower affinity and enriched for Ig M. On the other hand, post GC memory B cells has higher affinity and expressing Ig M or Ig G.
4) Memory B cells are derived from low affinity B cell receptorssignaling and reduced T cell help.
5) Humoral sensitization without detectable DSA in kidney transplant are due to absorption of DSA by allograft , the loss of shorter lived plasma cells and generation of repertoire of memory B cells.
6) Only patients without DSA and HLA sensitizing consider as immunological low risk for alloimmune memory.
Recall B cell response.
1) Memory B cells upon binding to antigen will either differentiate into plasma cells or differentiate into GC B cells that produce GC B cells that generate new , high affinity and class switched plasma cells.
2) The B cell recall response remain dependant on T cell and co stimulation blockade with CTLA-4Ig.
Key massages
1) De novo DSA remains strong predictor of allograft loss.
2) Following kidney transplant, recipient is exposed to enormous antigenic epitopes .
3) Cellular responses that produce plasma cells .
4) Proper understanding the mechanistic difference between the immune responses to antigens, pathogens and transplant antigens may lead to new ways to control DSA production which maintaining protective immunity.
MOHAMED Elnafadi
3 years ago
Terminally differentiated B cell, the plasma cell, is the sole cell type capable of producing antibodies in our body. Over the past 30 years, the identification of many key molecules controlling B cell activation and differentiation has elucidated the molecular pathways for generating antibody-producing plasma cells. Several types of regulation modulating the functions of the important key molecules in B cell activation and differentiation add other layers of complexity in shaping B cell responses following antigen exposure in the absence or presence of T cell help. Further understanding of the mechanisms contributing to the proper activation and differentiation of B cells into antibody-secreting plasma cells may enable us to develop new strategies for managing antibody humoral responses during health and disease. Herein, we reviewed the effect of different types of regulation, including transcriptional regulation, post-transcriptional regulation and epigenetic regulation, on B cell activation, and on mounting memory B cell and antibody responses. We also discussed the link between the dysregulation of the abovementioned regulatory mechanisms and B cell-related disorders.
Despite an emerging appreciation of the limits of DSA in predicting ABMR andsensitization, the appearance of de novo DSA remains a strong predictor of allograft loss . Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression. These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation. Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity .
Mohamed Essmat
3 years ago
DSA’s is associated with high risk allograft rejection. The article shows the role of B cell activation in formation of AMR . They showed experimental models on mice by the use of MHC class I & class II immunized into the donor of spleen cell which showed activation of B cell in presence of T cell inside germinal center and formation of donor specific antibody after 7 days with activation of CD40 & CD154. Belatacept shows best control of humoral response . In renal transplant recipients , B cell exposure to recurrent different antigens and under usual current immunosuppressive agents there is about 15% of transplant kidney develops de novo DSA after 4-6 years post-transplant. Memory B cell are generated into 2 phase also known as long living plasma cell. Absence of DSA’s doesn’t indicate absence of donor specific memory cell; so good history is important to roll out the causes that may lead to development of DSA’s ; blood transfusion , previous transplants , pregnancy and HDx . the term “Recall B cell response” means memory B cell with antigen differentiated into GC B cell with higher affinity and class switched plasma cell; memory B cell differentiate into plasma cell and the recall DSA response.
Mohammed Sobair
3 years ago
Introduction:
New insights into processes that lead to antibody production upon primary and
secondary antigen encounter are discussed, and the potential implications to DSA
production and future areas of investigation.
Routes of antigen presentation to B cells:
B cell engaged with antigen in order to start the process of differentiating into PCs
producing high affinity antibody and memory B cells.
increasing evidence suggest that optimal B cell activation occurs when the B cell
receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages .
Several strategies exist to increase the opportunities for B cells to encounter soluble and
membrane-bound antigen in the draining lymph nodes:
Mature B cells circulate though the lymph nodes approximately every 24 hours, by
leaving the vascular system and entering the lymph nodes through specialized high
endothelial venules (HEV), migrating along processes extending from follicular dendritic
cell (FDCs) and following the chemokine CXCL13 gradient established by FDCs and
fibroblastic reticular cells (FRCs). Eventually these B cells congregate within the cortical
region near the sub capsular sinus where they may encounter soluble or particulate
antigens that enter the draining lymph node via multiple routes depending on antigen
size, the presence of circulating antigen-specific antibodies, and the deposition of
complement on the antigen by the classical or alternative pathways.
Humoral response can also arise from the spleen:
MZ contains a large reservoir of resident cells that participate in capturing and shuttling
the antigen to FDCs within the B cell follicles and cells that are capable of processing
and presenting the antigen to T CELL.
Early B cell activation and the extrafollicular T/B cell interaction:
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B
cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B
border .
Concurrently, B cells internalize the BCR-bound antigen, process and present the
peptides derived from the antigen on MHC Class II molecules, in preparation for
interaction with antigen-specific T cell receptors on CD4+ T cells.
This cognate interaction between T and B cells initiates the Dependents DSA response.
The activation of T cells requires an initial encounter with migratory DCs that had
acquired antigen from the allograft and migrated via afferent lymph and along the same
FRC network in the lymph node or spleen to reach the T cell zone.
Initial encounter of T cells with cognate antigen presented on DCs result in a subset
acquiring T follicular helper (Tfh) cell properties. How CD4+ T cell fates are determined
has been the subject of a number of investigations and remains incompletely resolved.
Concluded that increasing T cell receptor (TCR) .strength results in more Tfh at the
expense of Th1 cells, consistent with our findings that optimum Tfh responses were
elicited by higher doses of T cell epitopes compared to Th1/Th2 responses.
Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh
cells.
Germinal center response:
extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they
downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase
expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich
lymph in the subcapsular sinus toward the follicle center.
Germinal Center responses to multiple complex antigens:
Initial antigen encounter, B cell clones with different antigenic specificity and affinity
compete for stimulation during rounds of somatic hypermutation within GCs. As a result,
the presence.
When the initial repertoire of circulating antibodies is generated, these antibodies will
bind to the same antigenic epitopes driving their production, resulting either in the
selection of B cells with higher affinity than circulating antibodies, or in the selection of
B cells with specificity for new epitopes.
Regulating the germinal center response:
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator
Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been
described [65; 66; 67]. These T follicular regulatory (TFr) cells express CXCR5, PD-1,
Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by
controlling GC responses.
Generation of memory B cells and PCs:
Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR
affinity [73; 74; 75]. Sciammas and reported that graded expression of interferon
regulatory factor-4 (IRF4) is an early measure of BCR signaling intensity: high levels o
f IRF4 induced Blimp-1 expression, promoted the PC program and shut down the
expression of Bach2 controlling GC B cell fate [78; 79], while modest levels of IRF4
promote differentiation into GC B cells.
Recall B cell responses:
Memory B cells upon antigen reencounter will either differentiate directly into PCs,
generating a faster, high-titer and class-switched recall antibody response compared to
a primary response, or they will differentiate into GC B cells that generate new, higher
‐affinity and class‐switched PCs.
Dogan et al. and Pape et al. Reported that IgM + memory B cells preferentially
differentiated into GC cells, whereas the IgG1+ memory B cells gave rise primarily to
PCs.
As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in
long lived PC depletion and explain the recently described ability of belatacept to control
and reverse DSA responses in sensitized recipient.
Conclusion:
Despite an emerging appreciation of the limits of DSA in predicting ABMR and
sensitization, the appearance de novo DSA remains a strong predictor of allograft loss.
In this review, we have provided a summary of the cellular processes that generate an
antibody response.
nawaf yehia
3 years ago
B cells play an important role in antibody Mediated rejection AMR , which is a major cause of dysfuntion and organ transplant failure in both acute and chronic setting . Routes of Antigen presentation to B cells
B cells have to encounter cognate antigen in order to start the process of differentiating into PCs nd memory B cells. Optmal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages
Mature B cells circulate though the lymph nodes approximately every 24 hours, where they may encounter soluble or particulate antigens that enter the draining lymph node via multiple routes. In addition, there may be additional contribution by migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
The humoral response can also arise from the spleen white pulp . The Marginal zone contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells. The routes for antigen delivery to B cells in the spleen and lymph nodes, identified through the study of model antigens or pathogens, are applicable to antigens derived from solid organ transplantation. Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow In addition, natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement. These opsonized antigens are captured by complement receptor 3 (CR3) expressed by subcapsular sinus macrophages (SSM) in draining nodes. Opsonized antigens captured and displayed by the SSM are relayed to non-antigen-specific naïve B cells in the underlying follicles via the complement receptors, (CR2) and (CR1). These B cells then transport opsonized antigens to FDCs. that capture the immune complexes by CR1 binding C3b or C4d . Likewise, opsonins draining into the spleen are captured in a C3d/CR2-dependent manner by MZ macrophages or B cells located just underneath the marginal sinus, which are then delivered to FDC . Early B cell activation and the extrafollicular T/B cell interaction When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation .B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells. It is notable that while alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA . This cognate interaction between T and B cells initiates the Tdependent DSA response.
Generation of memory B cells and PCs The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
Factors that determine the PC fate during primary antigen encounter have been extensively investigated, with critical roles for BCR affinity and Tfh . Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity .
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG . Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. features of memory B cell versus plasma cell differentiation are consistent with memory B cells being preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later . The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. Indeed, in a recent meeting report by the Sensitization in Transplantation: Assessment of Risk 2017 working group [87], a major finding was that the absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC , the generation of a repertoire of memory B cells that is not identical to the long-lived PC . In the absence of a clinical test of donor specific memory B cells, the recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events such as pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory. Recall B cell responses Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cellsthat generate new, higher‐affinity and class‐switched PCs. However, the rules that predict memory B cell fate in the recall response remain to be fully clarified. Dogan et al. and Pape et al reported that IgM + memory B cells preferentially differentiated into GC cells, whereas the IgG1+ memory B cells gave rise primarily to PCs. Furthermore, Pape et al reported that the IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies, because these antibodies outcompeted the low-affinity IgM BCR for access to limiting amounts of antigens. Data suggest that the fate of memory B cells upon reencounter with antigens is dependent, in part, on how and type of memory B cell was generated and the conditions of the antigen-reencounter. When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses. Nevertheless, the recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells . As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients . Conclusion Despite an emerging appreciation of the limits of DSA in predicting ABMR and sensitization, the appearance of de novo DSA remains a strong predictor of allograft loss . Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression. These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation. Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity .
saja Mohammed
3 years ago
This review summarize thesteps that underlie the primary and recall phases of B cell activation and antibody production. The presence of preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and that de novo DSA generated after transplantation is associated with poor outcomes and vascular obliterative lesions. Quantification of DSA by MFI might not be enough as predictor of sensitization and AMR, so efforts to improve DSA quantification. Absence of donor-specific antibody does not rule out an absence of sensitization or of memory B cells. In the absence of a clinical test of donor specific memory B cells, the STAR recommendation is that an accurate patient history of previous sensitization event and only patients with negative DSAs, , negative sensitizing history like previous blood transfusion , pregnancies , re-transplant should be considered as low immunological risk for allo-immune memory
B cell activation and differention:
Optimal B cell activation dependent on B cell receptor (BCR) affinity to engages intact antigen Displayed on APC (DCs, B cells or macrophages) with the Tfh in addition to the modest levels of IRF4 which further facilitate the differentiation into GC B cells [76; 78]. Within GCs, PC differentiation was induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC. Recall B cell response:
Memory B cells are generated in two distinct phases:
Early pre-GC memory B cells with lower affinity and enriched for IgM,
Late post-GC memory B cells with higher affinity and expressing IgM or IgG [54; 81].
The IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies, because these antibodies outcompeted the low-affinity IgM BCR for access to limiting amounts of antigens.
Memory B cells that express CD80, PD‐L2, and CD73 were mostly IgM+ and were mostly likely to produce PC. The recall response remains dependent on T cell help, and co-stimulation
Blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and
The recall DSA response.
Inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in long-lived PC depletion and this high light the role of the belatocept to control and reverse DSA responses in sensitized recipient.
Murad Hemadneh
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation.
Introduction:
Antibody mediated rejection AMR is considered a major cause of chronic renal transplant dysfunction and failure after it was clear that presence of preformed DSA is associated with increased risk for acute rejection and that de novo DSA is associated with poor long term outcomes. B cell responses may play an important role in chronic and sustained DSA production after solid organ transplantation.
Routes of antigen presentation to B cells.
There are many routes that B cell can encounter the antigen which will lead to B cell differentiation to Plasma cells which is able to produce antibodies and memory B cells. The optimal way of B cell activation occurs when B cell receptor engages intact antigen displays on Follicular dendritic cells FDCs, B cells or macrophages. This can occur in many ways which includes:
1. In the draining lymph nodes: Mature B cells circulate through the lymph nodes every 24 hours, congregate within cortical region to increase the chances to encounter antigens.
2. In the Spleen. Spleen divided into red and white pulp. White pulp is comprised of T cell zone, B cell follicles and Marginal zone. Marginal zone is between white and red pulp and contains many cells which facilitate in capturing the antigens and present them to B cells follicles which are then process and presents the antigens to T cells.
3. Small antigens (Small soluble proteins ≤14 kD) can be encountered to B cells through passive through gabs in the fibroreticular cells FRCs in the lymph nodes or spleen.
4. Natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement which will lead eventually that opsonized antigens presented to B cells.
5. Antigens secreted and shed from the allograft (alloantigen delivery) can be carried by extracellular vesicles or exosomes released by the allograft and accumulate in the draining lymph node and spleen. Eventually they are able to stimulate recipient CD8 T cells through direct pathway.
Early B cell activation and the extrafollicular T/B cell interaction.
B cell activation occurs after BCR binds to antigen presented by the previously described ways. B cell then internalize the CBR-antigen complex, then it will processed and will presents the MHC class II peptides derived from the antigen. T cells can recognize the antigen in direct way as mentioned or indirectly through B cells. Here CD4 T cells are the only with the ability to be activated directly with antigen-MCHII-peptides presented by B cell, As a result of this interaction T cells will drive the differentiation of recipient B cells into Plasma Cells which are able to produce antibodies (DSA).
T cells can be activated directly by dendritic cells as they are able to process detected antigens and can present them to T cells and induce T cell response directly. This direct encounter of T cells and antigen results in a subset of T follicular helper T Cells (Tfh). Tfh cells are divided into early extra follicular mantel Tfh and Germinal center GC Tfh cells. Finally, B cell priming at the T/B interface results in the generation of early PCs, memory B cells as well as B cells that are destined to migrate back to the follicle to initiate GC responses.
Germinal center response
The GC is divided into two zones. First, the light zone comprising stromal-derived FDCs and Tfh cells. Second, the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 which are able to engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells. Positively selected B cells migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase (AID). In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone. Post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
Belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection. Belatacept was able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
Germinal Center responses to multiple complex antigens
Most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities. How the immune response responds to multiple antigens to culminate in these diametrically opposite outcomes remains unresolved. In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation. The development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells. T follicular regulatory (TFr) cells function by controlling GC responses. They suppress self-reactive GC B cells that may have inadvertently arose during the GC response. Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border. Inhibition at this early stage of B cell response, would prevent further activation of Tfh cells and entry into GCs, as well as B cell differentiation into GC B cells, and post-GC memory B cells or PCs.
Generation of memory B cells and PCs
PC fate during primary antigen encounter have been extensively investigated, with critical roles for BCR affinity and Tfh. Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity. Within GCs, PC differentiation was induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG. Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. Memory B cells being preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later. Successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs. The recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response
Ben Lomatayo
3 years ago
Introduction; The existence of DSA is a risk factor for poor transplant outcomes including acute rejection & graft failure(3). B cells responses that causes Chronic-antibody mediated rejection is not well understood.
Routes of antigen presentation to B cells ; B cell activation is enhanced after BCR engages intact antigen displayed on Follicular Dendritic Cells(FDC), B cells , or macrophages(5) Mature B cells passes through lymph node every day and they usually accumulate in the cortical region where they interact with antigens. The spleen is another organ that produces humeral responses(7) Small soluble proteins may reach B cells directly by passive flow through Fibroblastic Reticular Cells(FB) conduit or gaps in the sinuses of lyphm nodes and spleen(5)(6)(8) B cells carries opsonised antigen to follicular dendretic cells taht capture the immune-complexes by CR1 binding C3 or C4d(9) Free antigen can bind macrophages and PCs of the spleen through innate recognition proteins e.g MBL, Ficolin, CRP. Donor extracellular vesicles or exosomes is considered new allo-antigen and accumulates in the draining lymph nodes and spleen(13)(14)
Early B cells activation and the extra-follicular T/B cell interface ; CD4 cells cells provide help for B cells differentiation into PCs producing DSA(20). This is the bases on T cells-dependent DSA response. DC also help to T cells as they process and present the captured antigens to T cells, therefore resulting fast T cell response independent of migratory DCs and the T cells became T follicular helpher(Tfh)(24,25,26). T follicular cells up-regulate CXCR5 & transcription factors(34,35). Priming B cells at T/B interface generates antibody-producing plasma PCs and memory B cells(38).
Germinal Center Response ; B cells interact with antigens on FDC and receive survival signals from GC-Tfh cells(38)(45) Positively selected B cells up-regulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase(AID)(44,46). Within GC,following selection, post GC B cells became PCs which generates antibodies and/or a quiescent memory B cells that produces rapid and more vigorous response upon re-exposure the same previous antigen(38). Anti-CD154 and CTLA-Ig inhibit GC- B cells and prevent increasing DSA in animal models(49). Belatacept was able to control humeral responses in human by preventing B cells-Tfh interactions and preventing B cells differentiation into PCs(50).
Germinal center responses to multiple complex antigens ; In transplant setting, recipient B cells usually interact with multiple complex antigens. DSA against class II HLA antigens was seen commonly in pairs with high degree of HLA epitope mismatch(64). Regulating the germinal center response ;
The key regulatory cells are subsets of CD4 known as T follicular regulatory cells TFr{FOXP3}(66,67). They control GC- response. They are located in the GC and suppress self-reactive B cells taht may have accidentally came up during GC- response (66). TFr was found not to have effects in the inhibition of influenza- specific B cells in some studies. Other studies reported the opposite(65,66,67). Suppression of B cells by Tfr was independent of PD-1 expression and correlated with elevated expression of CD69,IL-10, CTLA-4, GARP which is a protein critical for surface expression of latent TGF-B(71).
Generation of memor cells and plasma cells ; Differentialtion to plasma cells depends on the grade of BCR affinity which is expressed as interferon-regulatory factor 4 {IFRF4} (73,74,75) Early pre-GC memory B cells has low affinity(56,81) Late post-GC memory B cells has high affinity(54,81) Memmory cells are drived from cells receiving lower-affinity BCR signaling and reduced T cell help which in turn induces reduced IRF-4 and higher Bach2 respectively(82)
Recall B cell responses ; Memory B cells may differentiate into PCs or GC- B cells when they encounter the antigen. This depend on the conditions of antigen re-encounter. Recall response also depend on the T cell help. Co-stimulation blockade completely prevents memory B cell differentiation into PCs and the recall DSA response
Conclusion ; The presence of DSA is important risk factors for graft loss. Understanding the process of anti-body production and B cells response are the basics for future selective therapies targeting humoral responses. Identifying the mechanistic differences between the immune responses to model antigens, pathogens, and transplant antigens may lead to different methods to handle DSA production while preserving protective immunity.
Dalia Ali
3 years ago
the role of B cells following solid organ
transplantation is antibody mediated rejection
(AMR)
Quantification of donor-specific antibodies (DSA) are an imperfect predictor of AMR, and efforts to improve DSA quantification anticipate that this will result in improved predictive power
antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure
Routes of antigen presentation to B cells
optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages
Mature B cells circulate though the lymph nodes approximately every 24 hours, by leaving the vascular system and entering the lymph nodes through specialized high endothelial venules (HEV), migrating along processes extending from follicular dendritic cell (FDCs) and following the chemokine CXCL13 gradient established by FDCs and fibroblastic reticular cells (FRCs)
The humoral response can also arise from the spleen, which is divided into two distinct compartments: the red pulp comprising a network of venous sinuses that traps old or damaged erythrocytes by red pulp macrophages, and the white pulp involved in the initiation of immune responses (reviewed in
The white pulp is comprised of the T cell zone (also referred to as the periarteriolar lymphoid sheath; PALS), B cell follicles and marginal zone (MZ), which is strategically situated at the interface of the red and white pulp.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins such as natural IgM; C-type lectins such as MBL (mannose binding lectin) and ficolins; and pentraxins, including C-reactive protein.
In addition to antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated reviewed
Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen.
exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages
These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border .
alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA
The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the allograft and migrated via afferent lymph and along the same FRC network in the lymph node or spleen to reach the T cell zone. These DCs arrive between 12–18 hours to 5 days after antigen encounter in the tissues, and it has been argued that such delivery would be asynchronous to the more rapid delivery of antigen to FDCs and B cell activation, and that there has to be another more rapid route of antigen delivery to DCs in the T cell zone.
The initial encounter of T cells with cognate antigen presented on DCs result in a subset acquiring T follicular helper (Tfh) cell properties
How CD4+ T cell fates are determined has been the subject of a number of investigations and remains incompletely resolved independently concluded that increasing T cell receptor (TCR) strength results in more Tfh at the expense of Th1 cells, consistent with our findings that optimum Tfh responses were elicited by higher doses of T cell epitopes compared to Th1/Th2 responses
Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh cells Both Tfh subsets are characterized by the upregulated expression of CXCR5 and the transcription factor, Bcl-6. CXCR5 together with the G-protein-coupled receptor EBI2 promotes the initial localization of mantel Tfh cells towards the B cell follicle and T zone (T/B) interface where they interact with specialized activated DCs expressing inducible co-stimulator (ICOS) ligand and CD25 . Membrane and soluble CD25 quench Tcell-derived IL-2, together with ICOS engagement and IL-21 produced by Tfh cells, promote Bcl-6-dependent Tfh cell differentiation.
B cell priming at the T/B interface results in the generation of early PCs, memory B cells as well as B cells that are destined to migrate back to the follicle to initiate GC responses
The relative importance of the pre-GC response to DSA production following solid organ transplantation, and the contribution of this response to AMR, has not been delineated.
Germinal center response
When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 [43; 44] and gene signatures associated with signaling from the BCR, CD40 and Myc.
Within each GC, and following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter
MHC Class I and Class II tetramers to track the fate of alloreactive B cells in mice immunized with donor spleen cells or following heart allograft transplantation
Within 7 days, donor-specific B cells acquire an activated phenotype (downregulated IgD and upregulated Class II and CD86), and differentiate into GC B cells or PC. The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well documented
In contrast, the necessity of CD28:B7 interactions in sustaining the GC response was less clear. Because there is currently no FDA approved drug that targets the CD40:CD154 interaction, while CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses
delayed treatment with CTLA-4Ig significantly diminished the frequency of memory alloreactive B cells generated, and the recall DSA response upon re-immunization in the absence CTLA-4Ig
The efficacy of CTLA-4Ig in controlling ongoing GC B cell responses and memory B cells in experimental models are congruent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection
Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which is currently more extensively used for kidney transplant recipients, requires further investigation.
Germinal Center responses to multiple complex antigens
For most part, the events shaping affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins, however most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities. Therefore, it is critical that we understand how the B cell and antibody repertoire is shaped following the simultaneous introduction of multiple complex antigens.
the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly
Using a computation method to evaluate HLA epitope mismatch,reported that the development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been described [65; 66; 67]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC responses.
Using multiplexed quantitative imaging of human mesenteric lymph node, and functional assays, Sayin et al. [70] recently reported that the majority of CD3+FOXP3+ Tfr cells expressing CD25 but low levels of CD5, resided at the T/B or GC-mantle borders, with very few located in the GC. The median Tfh/Tfr ratio was 1.3:1; 2:1 and 24:1 at the T-B border, B cell follicle and GC, respectively, leading the authors to suggest an alternative model wherein Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. Indeed, in a recent meeting report by the Sensitization in Transplantation: Assessment of Risk 2017 working group a major finding was that the absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs. However, the rules that predict memory B cell fate in the recall response remain to be fully clarified.
When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses
Nevertheless, the recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells
As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients
Amit Sharma
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation
Conventionally, T cells have been implicated as major player in graft rejection. But the data suggesting association of DSA with higher rejection risk and poorer graft outcomes have emphasized on the role of B cells in organ transplant. Cognate antigen on Follicular dendritic cells (FDC), B cells and macrophages act on B cell receptors causing activation of B cells, leading to plasma cell formation which give rise to donor specific antibodies and memory B cells. Mature B cells, via high endothelial venules (HEV), enter lymph nodes and assemble in cortex near subcapsular sinus. Migratory dendritic cells capture antigens and deliver them to lymph nodes. The white pulp of spleen initiates immune response. The marginal zone of white pulp has resident cells which capture antigens and shuttle them to FDCs. Migratory dendritic cells act on T cells in the T cell zone (PALS-periarteriolar lymphoid sheath) leading to T cell activation, but it takes upto 5 days to act. A rapid response is seen with delivery of antigen to T cell zone via Follicular reticular cell (FRC) conduits. The activation of T cells will lead to acquiring follicular helper cell (Tfh) properties which can be either early extrafollicular mantle Tfh and GC (Germinal Centre) Tfh cells. The B cell priming at the T/B interface in spleen leads to formation of early plasma cells, memory B cells and B cells that migrate back to the follicle. The GC has a light zone with FDCs and Tfh cells while the dark zone of GC does not have FDCs. B cells undergo apoptosis in light zone, while dark zone B cells with intact B cell receptor (BCR) reenter light zone where BCR are tested for binding to antigen and T cell help. Differentiation into plasma cells depends on high BCR affinity. Early pre-GC memory B cells, mainly IgM, have lower affinity and later post-GC memory B cells have higher affinity and express both IgM and IgG. Memory B cells, on re-encounter with antigen will either directly form plasma cells leading to a high titre antibody response, or will differentiate into GC B cells generating new higher affinity plasma cells. The review paper provided the summary of the cellular processes generating antibody response.
Weam Elnazer
3 years ago
There is a greater risk of allograft rejection related to the presence of donor-specific antibodies (DSA).
An important role in chronic renal transplant dysfunction and failure is played by the development of Antibody-Mediated Rejection (AMR).
This article focuses on the evidence for and the function of B cell activation in the creation of AMR, which is responsible for the loss of grafts after transplantation.
The following is the mechanism of antigen presentation to the B cell:
The B cell Receptor binds to the intact soluble antigen on the surface of the B cell (BCR).
The B cell is a critical component of the adaptive immune system response.
B cells are produced in the bone marrow and subsequently circulate through the bloodstream and into the spleen.
Inflammatory responses are elicited by the spleen, which is comprised of red pulp and white pulp; the red pulp is responsible for the destruction of undesired RBC by macrophages, and the white pulp is responsible for the destruction of white pulp.
T cells, B cells, and the marginal zone are found in the white pulp, which is responsible for starting the immunological response.
The Marginal Zone has a big cell that is responsible for collecting and transporting antigen to the follicular dendritic cell.
Antigens are obtained through the transplantation of solid organs.
The presence of circulating antigen-specific antibodies as well as the deposition of complement on antigen via a traditional or alternative route are required.
B cells deliver opsonized antigen to follicular dendritic cells, which collect immune complexes by attaching to C3b and C4d on the surface of the CR1.
An antigen that is not coupled to an antibody may bind to macrophages and dendritic cells in the lymph node via the action of innate recognition proteins such as IgM and CRP.
These dendritic cells and macrophages in lymph nodes, together with CD8, M2 metallophilic macrophages, and M2 macrophages in the spleen, capture antigen, and then the antigen displayed on the surface of the cell activates complement, which then delivers the antigen to the CR on follicular dendritic cells via the fallow route of opsonisation.
Memory B cells and PCs are being produced in large quantities.
There are significant disparities between memory development by B cells and memory generation by plasma cells. Because of the affinity and kinetics for BCRs and T follicular helper cells, as well as the expression of IgG and IgM, this type of memory is perpetuated, and it is possible that even successful treatment of acute AMR and reduction of circulating DSA did not prevent the generation of memory donor-specific B cells. Recall the reactions of B cells
Recall the contents of your recollection B cells are projected to develop straight into plasma cells, therefore producing antibodies more quickly than the main response or forming germinal centre B cells, however, there are no regulations governing this process at this time.
Various studies have demonstrated the presence of IgM+ antibodies, while others have demonstrated the presence of IgG+ antibodies, as well as the interaction with certain signal expressions. They have also revealed the need for T cell assistance, as well as the use of CTLA-4Ig antibodies, which are involved with CD28/CD80/CD86. Belatacept, on the other hand, has the ability to regulate and deplete this connection.
Theepa Mariamutu
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation
The pathogenesis of chronic AMR is complex and poorly understood. There is paradigm shift from role of T cells to B cell in AMR.
B cells interact with APC and differentiate into plasma cells and memory cells. Antigens presented by B cells, macrophages and follicular dendritic cells will interact with B cell receptors. B cells migrate from blood stream to lymph nodes through small venules
B cells response will depend on antigen size presented to them . They accumulate in cortex near subcapsular sinus and then they get access to antigens. Antigens can also be transported from blood to lymph nodes by dendritic cells. Splenic white pulp B cells have a role in immune response .
B Cell Activation
Antigens are presented to BCR by APC and leads to B cell activation. Antigens are internalized and interaction happens with CD 4 T cells. These B cells then differentiate into plasma cells which produce DSA. T cell will interact with migrating dendritic cells which bear antigens. Antigens are presented to T cells by macrophages in spleen . Activated T cells gain follicular Helper characters and it will form follicular mantle and germinal centre cell.
Germinal Centre Response
It consist of light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Regulation of Germinal centre response is done by CD4 cells which suppress self reactive germinal B cells.
Generation of memory B cells and Plasma Cells
Memory generation is done through B cells and plasma cells. Affinity and kinetics of T follicular helper as well as BCR and antibody expression leads to this type of memory. Variable affinity of BCR can explain the reason of sensitization in patients without detectable DSAs. Upon re exposure to same antigen B cells develop higher affinity for BCR and convert to plasma cells.
Professor Ahmed Halawa
Admin
3 years ago
Thank you All
Tahani Ashmaig
3 years ago
☆ In this review, new insights into processes that lead to antibody production upon primary and secondary antigen encounter, the potential implications to DSA production and future areas of investigation to control AMR are discussed.
INTRODUCTION:
▪︎Antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause
of chronic kidney transplant dysfunction and failure.
▪︎Less is understood of the B cell responses that result in chronically sustained antibody production mediating chronic AMR and transplant failure.
■Routes of antigen presentation
to B cells
▪︎B cells have to encounter cognate antigen in order to start the process of differentiating into Plasma cells (PCs) producing high affinity antibody and memory B cells.
▪︎ B cell activation occurs when the B cell receptor (BCR) engages intact antigen
displayed on FDCs, B cells or macrophages.
▪︎Mature B cells leave the vascular system and enter the lymph nodes through specialized high endothelial venules (HEV), migrate along processes extending from follicular dendritic cell (FDCs) and follow the chemokine CXCL13 gradient established by FDCs and fibroblastic reticular cells (FRCs) every 24 hours and congregate within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens.
▪︎ The humoral response can also arise in the white pulp of the spleen which is comprised of the T cell zone, B cell follicles and marginal zone (MZ) which contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and antigen presenting cells.
▪︎ The routes for antigen delivery to B cells in the spleen and lymph nodes, are applicable to antigens derived from solid organ transplantation.
▪︎Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen. These FRC conduits intersect with FDCs and thus provide direct connection for soluble antigen to be captured and presented on the FDC surface.
▪︎Natural polyreactive antibodies and DSA can bind to soluble antigens and activate complement. These antigens are captured by subcapsular sinus macrophages (SSM) in draining lymph nodes which relayed the opsonized antigens to non-antigen-specific naïve B cells in the underlying follicles which transport opsonized antigens to FDCs. In the same manner, opsonins draining into the spleen are captured by MZ macrophages or B cells located just underneath the marginal sinus, which are delivered to FDCs.
▪︎ Free antigens can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins. These, together with CD8- DCs, MZ metallophilic macrophages and MZ macrophages in the spleen capture antigens. Antigens displayed on the surface of these cells activate complement, and then follow the route of opsonized antigen delivery to CR1 on FDCs.
▪︎ A new type of alloantigen delivery is that donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by the macrophages in the red pulp of the spleen, SCS macrophages and DCs. Alternatively, a relatively limited number of donor DCs migrate from the graft into draining lymph nodes or spleen, where they deliver exosome to resident DCs.
▪︎Donor MHC delivered by exosomes to recipient DCs, results in cross-dressed DCs that have the ability to stimulate recipient CD8+ T cells via the direct pathway. But, recipient APCs cross-dressed with donor MHC II cannot prime efficiently CD4+ T cells.
▪︎The exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages.
▪︎These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone
■Germinal center response:
▪︎ When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they:
1. down regulte T cells release from the
outer follicle
2. repels cells toward the follicle center
3. Tfh cells retention in the GC is regulated.
4. Tfh cell exhaustion is prevented.
5. The ability of the GC-Tfh cells to secrete
IL-12 is preserved.
▪︎The GC-Tfh cells express higher levels of CXCR5, Pdcd1 and IL-21 compared to extrafollicular Tfh.
▪︎ The GC is divided into:
1) The light zone comprising stroma-derived FDCs and Tfh cells.
2) The dark zone, which is devoid of FDCs.
▪︎ The Light zone B cells express the activation markers engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells
▪︎ Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification.
▪︎ In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone.
▪︎ Dark zone B cells with intact BCR downregulate CXCR4 and reenter the light zone where their newly generated BCR are tested for binding to antigen and access to T cell help.
▪︎Following the selection for B cells with high affinity BCR for antigens presented in the GC by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
■The fate of alloreactive B cells:
▪︎Within 7 days, donor-specific B cells acquire an activated phenotype and differentiate into GC B cells or PC. There is interactions between B cells and T cells in the initial activation of T cells with maintenance of the GC response .
■Germinal Center responses to
multiple complex antigens
▪︎ In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens
▪︎The affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins. But, most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities.
▪︎Upon initial antigen encounter, B cell clones with different antigenic specificity and affinity compete for stimulation within GCs. As a result, the presence of many antigenic epitopes tends to reduce the antibody repertoire and the initial affinity of the generated antibodies to multiple antigens would be reduced,.
▪︎ When the initial repertoire of circulating antibodies is generated, they bind to the same antigenic epitopes driving their production, resulting either in the selection of B cells with higher affinity than circulating antibodies.
▪︎The B cell response will terminate with a relatively restricted repertoire if there are no alternative immunogenic epitopes.
▪︎ The development of DSA to Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
■Regulating the germinal center response
▪︎ T follicular regulatory (TFr) cells are located in GCs and control their responses. They suppress self-reactive GC B cells generated during infection.
▪︎ In the absence of Tfr, these selfreactive B cells differentiated into PC that produced anti-histone and anti-nuclear antibodies.
▪︎Some Tfr cells inhibit antigen-specific GC B cells and antibody production.
▪︎Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border which prevent Tfh cells activation and entry into GCs, as well as B cell differentiation into GC B cells, and post GC memory B cells or PCs.
▪︎TGF-ß production may be the mechanism by which Tfr inhibit B cell responses.
■Generation of memory B cells and PCs
▪︎ B cell differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity.
▪︎Within the GC, PC differentiation is induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.
▪︎Memory B cells are generated in two distinct phases:
1. Early pre-GC memory B cells with lower affinity and enriched for IgM
2. Later post-GC memory B cells with higher affinity and expressing IgM or IgG.
▪︎ Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help.
▪︎Classswitching to membrane IgG1, which signals more efficiently compared to IgM, biased the fate choice to PC over memory fate.
▪︎GC B cells which fail to enter the DZ ( lack CXCR4) are more likely to enter the memory compartment.
▪︎Memory B cells are preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later.
▪︎The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
☆Concerning Sensitization in Transplantation:
– The absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells
– Humoral sensitization without detectable DSA in transplant recipients, include the absorption of DSA by the allograft, the loss of shorter-lived PC and the generation of a repertoire of memory
B cells that is not identical to the long-lived PC.
▪︎In the absence of a clinical test of donor specific memory B cells, the STAR recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events should be considered to be immunologically low risk for alloimmune memory.
■Recall B cell responses:
The fate of memory B cells upon reencounter with antigens is dependent, in part, on how and type of memory B cell was generated and the conditions of the antigen-reencounter.
▪︎Memory B cells upon antigen reencounter will either differentiate directly into PCs, or they will differentiate into GC B cells.
▪︎IgM+ memory B cells preferentially differentiated into GC cells, whereas the IgG1+ memory B
cells gave rise primarily to PCs. But, memory B cells that express CD80, PD‐L2, and CD73 which are IgM+ produce PC.
▪︎ IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies
▪︎ High‐affinity IgM+ memory B cells rapidly differentiated (≤3 days) into IgM+ and IgG+ PC.
▪︎The recall response remains dependent on T cell help.
▪︎Co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response.
▪︎Inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in long lived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients.
manal jamid
3 years ago
The initiation of B-cell responses need many steps. Initially, recognition of antigens by the B-cell receptor (BCR) and costimulation to triggers intracellular signalling and antigen internalization ,Secondly, processing and presentation of internalized antigens to helper T cells which lead to B-cell activation and of plasma cells generation and antibody secretion
Which consederd a major cause of antibody mediated dysfunction and failure.
The current immunotherapies are failing to control the development of alloantibodies responses, or failing to reverse the production or the effects of the alloantibodies.
B cells is one of the APC – antigen presenting cells , it can bind intact soluble antigen . B cells travel from blood stream to lymph nodes through small venules ,following the chemokine CXCL13 gradient at the end it accumulate in cortex near subcapsular sinus where they encounter the antigen.
Antigen can also be transported from blood to lymph nodes by dendritic cells.
Humoral response can also arise from the spleen specifically in the area of white pulp which is comprised of 1.TCell zone 2.B cell follicles
3.marginal zone
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules,
Alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC.
Germinal Centre Response
It has light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Germinal Center responses to multiple complex antigens :
-In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, & sharing many phenotypic characteristics of Tfh cells, has recently been described. [3; 4; 5]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 & IL-10, but not CD25, & function by controlling GC responses.
Generation of memory B cells and PCs.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG .
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
Recall B cell responses
Upon re-exposure to antigen, memory B cells differentiate into either PCs or GC B cells. Compared to primary response, the PCs produce a faster, high titer & class-switched recall antibody response, while the GC B cell generates new PCs that have higher‐affinity & are class‐switched. Data suggest that the fate of memory B cells upon re-exposure to antigens is dependent, in part, on how & type of memory B cell was generated & the conditions of the antigen-reencounter.
amiri elaf
3 years ago
* Role of B cells following solid organ transplantation is being driven by clinical data suggesting that antibody mediated rejection (AMR) is a major cause of dysfunction and organ transplant failure.
* The current immunotherapies are failing to control the development of allo anti body responses, or failing to reverse the production or the effects of the alloantibodies.
* Routes of antigen presentation to B cells:
– B cells have to encounter antigen to start the process of differentiating into :
# PCs producing high affinity antibody #Memory B cells.
* B cells can bind intact soluble antigen, also optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
*B cell encounter solubl and membrane bound antigen in the draining lymph nodes.
* Mature B cells circulate though the
lymph nodes every 24 hours, by leaving the vascular system and entering the
lymph nodes through specialized high endothelial venules (HEV) in response to chemokine CXCL13 established by FDCs and fibroblastic reticular cells (FRCs) in the cortical region near the subcapsular sinus where they may encounterantigen. The presence of circulating antigen specific antibodies lead to complement activation (classical and alternative) pathways.
* The humoral response can also arise from the spleen, which is divided into two distinct compartments:
– Red pulp traps old or damaged erythrocytes by red pulp macrophages .
-White pulp involved in the initiation
of immune responses is comprised of the (T cell zone, B cell follicles and marginal zone MZ).
* Early B cell activation and the extrafollicular T/B cell interaction:
– When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, internalize the BCR-bound antigen,process and present the peptides derived from the antigen on MHC Class II molecules for interaction with antigen-specific T cell receptors on CD4+ T cells.
– Alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells intoPCs producing DSA . This cognate interaction between T and B cells initiates the T dependent DSA response.
* Germinal center response:
– The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells.
– Dark zone, which is devoid of FDCs. Light zone B cells express the activation markers,
– CD86 and CD83 a signatures associated with signaling from the BCR, CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive
survival signals from GC-Tfh cells
provided by the interaction between costimulatory molecules, Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase (AID).
– In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes undergo apoptosis in the dark zone.
– Dark zone B cells with intact BCR downregulate CXCR4 and reenter the light zone where their newly
generated BCR are tested for binding to antigen and access to T cell help.
– Following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response.
– The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well documented, but the rule of CD28:B7 interactions in sustaining the GC response was less clear, because there is currently no FDA approved drug that targets the CD40:CD154 interaction, while CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection.
– It is important to test the effect
of CTLA-4Ig on established B cell responses, since it may promote humoral responses .
– Delayed treatment with CTLA-4 significantly diminished the frequency of memory alloreactive B cells generated, and the recall DSA.
– Controlling ongoing GC B cell responses and memory B cells in experimental models are congruent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection.
– Belatacept was able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
– Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which iscurrently more extensively used for kidney transplant recipients, requires further investigation.
* Germinal Center responses to multiple complex antigens:
– For most part, the events shaping affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins.
– Study suggested that upon initial antigen encounter, B cell clones with
different antigenic specificity and affinity compete for stimulation during roundd ofsomatic hypermutation within GCs.
– As a result, the presence of many antigenic epitopes tends to reduce the relative breadth of the antibody repertoire.
– Initial affinity of the generated antibodies to multiple antigens would be reduced, due to a reduction in the number of GCs and B cells available for each epitope, which compromises affinity selection.
– When the initial repertoire of circulating antibodies is generated, these antibodies will bind to the same antigenic epitopes driving their production, resulting either :
in the selection of B cells with higher affinity than circulating antibodies
or in the selection of B cells with specificity for new epitopes.
– In the majority of kidney transplant recipients, B cells will most likely encounter multiple complex antigens.
– Development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
– Regulating the germinal center response subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been described.
These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1,
FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC
responses.
* Generation of memory B cells and PCs:
– The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
– Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity
– Graded expression of interferon
regulatory factor-4 (IRF4) is an early measure of BCR signaling intensity
High levels of IRF4 induced Blimp-1 expression, promoted the PC program and shut down the expression
of Bach2 controlling GC B cell fat, while modest levels of IRF4 promote
differentiation into GC B cells .
Memory B cells are also generated in two distinct phases:
early pre-GC memory B cells with
lower affinity and enriched for IgM
and later post-GC memory B cells with higher affinity and expressing IgM or IgG .
-Successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
– The absence of donor-specific antibody does not equate to an absence of
sensitization or of memory B cells.
There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC.
– In the absence of a clinical test of donorspecific memory B cells, the STAR recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events such as
pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory.
* Recall B cell responses:
– Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a
primary response
or they will differentiate into GC B cells that generate new, higher‐affinity
and class‐switched PCs.
* Conclusion
Despite an emerging appreciation of the limits of DSA in predicting ABMR and
sensitization, the appearance de novo DSA remains a strong predictor of allograft loss.
– summary of the cellular processes that generate an antibody response.
– Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression.
– These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation.
– Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity.
–
Huda Al-Taee
3 years ago
Routes of antigen presentation to B cells:
B cells have to encounter cognate antigen to start the process of differentiation into plasma cells and memory cells. Optimal B cells activation occurs when the B cell receptor engages intact antigen displayed on FDCs, B cells, or macrophages. Several strategies exist to increase the opportunities for B cells to encounter soluble and membrane-bound antigens in the draining lymph nodes. Mature B cells circulate through the lymph nodes every 24 hours, eventually, they gathered within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens that enter the lymph node via multiple routes, in addition, DCs may encounter antigens at the tissue site and transport them to the lymph node.
The humoral response can also arise from the spleen, particularly the white pulp which is comprised of the T cell zone, B cell follicles, and the marginal zone( MZ). The MZ contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cells follicles and cells that are capable of processing and presenting the antigen to T cells.
Early B cell activation and the extrafollicular T/B cell interaction:
When BCR binds to an antigen, B cells activation and migration to the B/T cell border will happen. B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells( indirect allorecognition). the interaction between B & T cells initiates the T cell-dependent DSA response.
Germinal center response:
Consist of light and dark zones. The light zone contains FDCs and Tfh while the dark zone is devoid of FDCs.
Light zone B cells express CD86 and CD83.
Germinal center responses to multiple complex antigens:
How the immune system responds to multiple antigens remains unresolved, and could be due to the accessibility of binding sites on the antigen, stochastic founding events, genetic predisposition affecting the T helper response, and chance mutations during affinity maturation.
Regulating the germinal center response:
A subset of CD4+ T cells that express the regulatory T cell master regulatory foxp3 and sharing characteristics of Tfh cells has been described. These T follicular regulatory cells function by controlling the germinal center.
Generation of memory B cells and PCs:
Differentiation into PCs at the T/B border and within the germinal center is facilitated by high BCR affinity.
Within the GC, PC differentiation was induced in a discrete subset of high affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.(according to Sciammas et al.).
Memory B cells are also generated in to 2 distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG.
Relative to PCs, memory B cells are derived from cells receiving lower-affinity and reduced T cells help.
The differences in affinity and kinetics of memory B cell versus PC suggest that successful treatment of acute AMR and reduction of DSA level may not prevent memory B cell generation.
Recall B cell response:
After encountering antigen, memory B cells will either differentiate into PCs generating a faster and high titer antibody response or they will differentiate into GC B cells that generate newer, high affinity, and class PCs.
The recall response remains dependent on T cell help and co-stimulation blockade with CTLA-4Ig completely prevented memory B cells differentiation into PCs and the recall DSA response.
mai shawky
3 years ago
· B cells play a crucial role in both active AMR and chronic AMR and eventually graft loss.
· B cell differentiate into antibody producing plasma cells and memory B cell which produce stronger and more rapid immune response on re-exposure to same antigen with higher titers and binding affinity antibodies formation.
· Antigens are presented by antigen presenting cells either (macrophages, B cells or dendritic cells) on their MHC class II molecules, then interaction with T cell receptors present on T helper (CD4+ T cells)
· The interaction between B and T cells occur through costimulatory pathway leading to B cell activation and formation of plasma cells that produce antibody (DSA) and T helper cell activation. This is mediated by interaction between:
o CD 40: CD 154 on B cells & T cells respectively in the initial activation of T cells. No available FDA approved drug to inhibit this costimulation signal.
o CD80: CD 28 on B cell and T cell respectively. However, interaction depending on CTLA 4 was implicated in clinical practice by use of belatacept (anti CTLA4 Ig) as costimulation inhibition to prevent further production of DSA and decrease generation of memory B cells.
· Circulating antibodies (denovo DSA) can be produced against mismatched epitopes. New high-resolution techniques to detect this submicroscopic mismatch may improve long term graft outcome.
· The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not prevent memory B cell generation.
AMAL Anan
3 years ago
*Routes of antigen presentation to B cells :
Activation of B cell occur when B cell receptors engaged antigen displayed on FDCS , B cell or macrophages.
*Mature B cells circulate in lymph nodes every 24 hours through specialised high endothelial venules.
*The presence of circulating B cells antigen specific antibodies and complement deposition on antigen affect B cells aggregation with cortical region near the subcapsular sinus which meet soluble or particular antigen that enter draining lymph nodes via multiple routes .
*Spleen generate humoral response which differentiate into two compartment :
1- Red pulp is full of venous sinus trap damaged erythrocytes by red pulp macrophages.
2- White pulp : consists of >>>
* T cell zone.
* B cell follicles.
* Marginal zone .
Marginal zone has large number of residents cells which capture and shuttle of antigen to FDCs within B cell follicles.
Natural poly reactive antibodies and DSA in naiive and sensitised recipients bind to soluble antigens and activate complement.
Antigen which not binds to antibody , binds to macrophages and DCs located in the lymph nodes by recognising by innate protein as natural IgM , C-type lectins as MBL , ficolins and pentraxins including CRP.
*Early B cell activation and te exrafollicular T/B cell interaction :
-B cells internalize the BCR- bound antigen, process and present peptides drived from the antigen on MHC class II molecules to interact with antigen specific T cell receptors on CD4+ T cell.
-The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the allograft and migrated via afferent lymph and along the same FRC network in the lymph node or spleen to reach the T cell zone.
-Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh cells . Both Tfh subsets are characterized by the upregulated expression of CXCR5 and the transcription factor, Bcl-6. CXCR5 together with the G-protein-coupled receptor EBI2 promotes the initial localization of mantel Tfh cells towards the B cell follicle and T zone (T/B) interface.
*Germinal Center response :
-When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
**Germinal Center responses to multiple complex antigens :
-In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal
transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly.
**Regulating the germinal Center response:
Early studies utilizing adoptive transfer or mixed bone marrow chimeras showed that Tfr are located in the GC, and that they suppress self-reactive GC B cells that may have inadvertently arose during the GC response .
*Using multiplexed quantitative imaging of human mesenteric lymph node, and functional
assays, Sayin et al. [70] recently reported that the majority of CD3+FOXP3+ Tfr cells expressing CD25 but low levels of CD5, resided at the T/B or GC-mantle borders, with very few located in the GC.
**Generation of memory B cells and PCs :
The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
*Within GCs, PC differentiation was induced in a
discrete subset of high-affinity B cells residing within the light zone upon BCR engagement
with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for
completing the PC differentiation and driving their migration out of the GC.
*The differences in affinity and kinetics of memory B cell versus PC generation suggest that
successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
**Recall B cell response:
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating
a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class-
switched PCs.
*When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen
following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses.
References:
[1]. Billingham RE, Krohn PL, and Medawar PB, Effect of cortisone on survival of skin homografts in rabbits. Br Med J 1 (1951) 1157–63. [PubMed: 14830863] .
[2]. Benichou G, Yamada Y, Yun SH, Lin C, Fray M, and Tocco G, Immune recognition and rejection
of allogeneic skin grafts. Immunotherapy 3 (2011) 757–70. [PubMed: 21668313] .
[3]. Pouliquen E, Koenig A, Chen CC, Sicard A, Rabeyrin M, Morelon E, Dubois V, and Thaunat O,
Recent advances in renal transplantation: antibody-mediated rejection takes center stage.
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[4]. Sellares J, de Freitas DG, Mengel M, Reeve J, Einecke G, Sis B, Hidalgo LG, Famulski K, Matas
A, and Halloran PF, Understanding the causes of kidney transplant failure: the dominant role of
antibody-mediated rejection and nonadherence. Am J Transplant 12 (2012) 388–99. [PubMed:
22081892].
[5]. Heesters BA, van der Poel CE, Das A, and Carroll MC, Antigen Presentation to B Cells. Trends Immunol 37 (2016) 844–854. [PubMed: 27793570] .
[6]. Gonzalez SF, Degn SE, Pitcher LA, Woodruff M, Heesters BA, and Carroll MC, Trafficking of B
cell antigen in lymph nodes. Annu Rev Immunol 29 (2011) 215–33. [PubMed: 21219172] .
[7]. Hey YY, and O’Neill HC, Murine spleen contains a diversity of myeloid and dendritic cells
distinct in antigen presenting function. J Cell Mol Med 16 (2012) 2611–9. [PubMed: 22862733] .
[8]. Pape KA, Catron DM, Itano AA, and Jenkins MK, The humoral immune response is initiated in
lymph nodes by B cells that acquire soluble antigen directly in the follicles. Immunity 26 (2007)
491–502. [PubMed: 17379546] .
[9]. Heesters BA, Chatterjee P, Kim YA, Gonzalez SF, Kuligowski MP, Kirchhausen T, and Carroll
MC, Endocytosis and recycling of immune complexes by follicular dendritic cells enhances B
cell antigen binding and activation. Immunity 38 (2013) 1164–75. [PubMed: 23770227].
Abdul Rahim Khan
3 years ago
The pathogenesis of chronic AMR is complex and poorly understood. There is paradigm shift from role of T cells to B cell in AMR. This article highlights and gives new insight in role of B cell in AMR. This paper highlights the activation of B cells and subsequent development of Plasma cells which produce DSA and memory cells which will lead to severe response to re exposure
Routes of Antigen Presentation to B cells
B cells interact with APC – antigen presenting cells and differentiate into plasma cells and memory cells. Antigens presented by B cells, Macrophages and Follicular dendritic cells will interact with B cell receptors. B cells travel from blood stream to lymph nodes through small venules ,
B cells response will depend on antigen size delivered to them . They accumulate in cortex near subcapsular sinus and then they get access to antigens. antigens can also be transported from blood to lymph nodes by dendritic cells. Splenic white pulp B cells have a role in immune response .
B Cell Activation
Antigens are presented to BCR by APC and leads to B cell activation. Antigens are internalized and interaction happens with CD 4 T cells. These B cells then differentiate into plasma cells which produce DSA. T cell will interact with migrating dendritic cells which bear antigens. Antigens are presented to T cells by macrophages in spleen . Activated T cells gain follicular Helper characters and it will form follicular mantle and germinal centre cell.
Germinal Centre Response
It has light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Regulation of Germinal centre response is done by CD4 cells which suppress self reactive germinal B cells.
Generation of memory B cells and Plasma Cells
Memory generation is done through B cells and plasma cells. Affinity and kinetics of T follicular helper as well as BCR and antibody expression leads to this type of memory. Variable affinity of BCR can explain the reason of sensitization in patients without detectable DSAs. Upon re exposure to same antigen B cells develop higher affinity for BCR and convert to plasma cells.
Innocent lule segamwenge
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation
This paper is a review article on B cell responses and how these relate to solid organ transplantation.
There is now new data which is re-emphasizing the major the role of antibody mediated rejection (AMR) in chronic graft dysfunction and is shifting current thinking from the previously help ideas of a more superior or central role of T cells.
The mechanisms underlying chronic antibody mediated rejection are poorly understood, but more information being gained from research is helping to provide more insight into this phenomenon.
Routes of antigen presentation to B cells.
The interaction of B cells with antigen presenting cells is important for their differentiation into plasma cells and memory cells.
The B cell receptor (BCR) interacts with antigens presented by profession antigen presenting cells (APC) like Follicular dendritic cells (FDC), B cells and Macrophages.
Mature B cells make their way from the blood stream to lymph nodes through endothelial venules and follow a CXCL13 chemokine gradient established by follicular dendritic cells and fibroblast reticular cells (FRC).
B cells encounter antigen delivered to them depending on antigen size;
By concentrating in the cortical area near the subscapular sinus they are able to access soluble antigens.
Migrating dendritic cells also participate in transporting antigens from the blood to lymph nodes.
The white pulp of the spleen also participates in the immune. In this white pulp B cells reside in the B cell follicles and marginal zone, which is strategically located at the interface between red and white pulp to access large antigens.
B cell activation
B cell activation follow interaction of the BCR with antigens displayed on antigen presenting cells. Antigen internalization into B cells is followed by interaction with CD4+ T cells. B cells which have interacted with CD4+ T cells differentiate into plasma cells capable of producing donor specific antibodies (DSA).
T cells are located in the periarteriolar sheath (PALS), interact with migratory dendritic cells which bring in allograft antigens from the blood stream. In the spleen resident macrophages process antigen, which they present to T cells and this leads to a more rapid response than that produced by migratory macrophages.
The activated T cells acquire T follicular helper (Tfh) properties which form early follicular mantle and germinal centre Tfh cells.
Germinal center response
The germinal centre comprises of the light and dark zone.
The light zone has follicular dendritic cells and T follicular helper cells. For apoptosis of B cells
The dark zone has CD86 and CD83 B cells. B cells selected proliferate and develop BCR specificity.
Generation of memory B cells and PCs
Memory cells and plasma cells play an important role in infection control and in generating memory for protection against future infection.
The high affinity of BCR favours formation of plasma cells promoted by regulatory factor-4 (IRF4) while T follicular helper cells provide the final signal for differentiation.
Memory cells are produced form B cells which receive less T cell help and have lower BCR affinity.
This difference in affinity of the BCR between plasma cells and memory cells explains why patients without detectable DSA may still be sensitized and may have memory cells to that exposure.
When memory cells are re-exposed to the same antigen, they may either differentiate into plasma cells or differentiate into germinal center B cells to develop higher affinity of the BCR and become plasma cells.
To sum it all up, this paper highlights are complex and yet important process of B cell activation and subsequent development of plasma cells that produce DSA as well as memory cells that lead to a more rapid and aggressive immunological response on re-exposure to the same antigen.
Zahid Nabi
3 years ago
New insights into the development of B cell responses: implications for solid organ transplantation
Antibody mediated rejection is major cause of graft dysfunction and ultimately graft failure.Attempts to control ABMR have focused on non specific elimination of B cells , plasma cells or circulating antibodies.
In this review processes that lead to antibody production upon primary and secondary antigen encounter along with future areas of investigation to control AMR are discussed.
B cell activation occurs when the B cell receptors (BCR) engages intact antigens displayed on FDCs, B cells or macrophages. Mature B cells circulate through lymph node approximately every 24 hrs these B cells congregate within the cortical region where they encounter the soluble antigen entering the draining lymph nodes through different routes.
The humoral response can also arise from spleen . The routes for antigen delivery to B cells in spleen and lymph nodes are applicable to antigens derived from solid organ transplantation.
Small soluble proteins such as those secreted by transplanted organs may gain direct access to the B cells via lymph nodes or spleen.
When the BCR binds to antigen it triggers B cell activation concurrently B cells internalize the BCR bound antigen process and interacts with antigen specific T cell receptors on CD4 cells.This interaction between T and B cell initiates the T cell dependent DSA response. B cell priming at the T/B interface results in generation of early plasma cells , memory B cells as well as B cells which migrate back to follicle to initiate Germinal center response. The relative importance of the pre GC response to DSA production following solid organ transplant and the contribution of this response to AMR has not been delineated.
Germinal center response
Within each GC, & following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as either PCs or quiescent memory cells. The former are responsible for persistent circulating antibodies, while the latter are responsible for the recall antibody response upon re-exposure to antigen.
CTLA-4Ig is an FDA agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses.
reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection.
Belatacept was found to able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
Germinal center response to multiple complex antigens
In majority of kidney transplant recipients , recipients B cell s will encounter multiple complex antigens and approximately 15% of renal transplant recipients develop de novo DSA within 4.6 yrs. TGF beta production may be the mechanism by which Tfr inhibit B cell response.
Memory B cells are generated in 2 distinct phases: early pre-GC memory B cells with lower affinity BCR & enriched for IgM, & later post-GC memory B cells with higher affinity & expressing IgM or IgG. The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
Possible explanations for humoral sensitization without detectable DSA in transplant recipients include: absorption of DSA by the allograft, the loss of shorter-lived PC, the generation of memory B cells that are not identical to the long-lived PC.
Recall B cell response
Memory B cell upon antigen re exposure will either differentiate into PC or GC B cell that generate new higher affinity and class switches PC. The fate of memory cells upon recall still needs to be clarified.
Mohamed Mohamed
3 years ago
II. New insights into the development of B cell responses: Implications for solid organ transplantation Anita S. Chong Department of Surgery, The University of Chicago, Chicago, IL
Introduction
B cells play a major role in both AMR & chronic kidney transplant dysfunction & failure. The current immunosuppressive medications fail, in subset of transplant patients, to reverse the production or the effect of antibodies. Improving our understanding of the role of B cells & its differentiation into memory B cells & antibody-secreting plasma cells (PCs) will give new ways to more specifically target the DSA response & thus improve long-term allograft survival. This review discusses the processes that lead to antibody production & its implications to DSA production & future areas of investigation to control AMR.
Routes of Antigen presentation to B cells
B cells should bind to an antigen before starting the differentiation into PCs, that produce high affinity antibody, & memory B cells.
B cells encounter soluble & membrane-bound particulate antigen in the draining lymph nodes &also in the spleen. Mature B cells circulate though the lymph nodes every 24 hours. Antigens, enter the draining lymph node via multiple routes depending on their size, the presence of circulating antigen-specific antibodies, & the deposition of complement. The routes for antigen delivery to B cells in the spleen & lymph nodes, were studied in model antigens, but are applicable to antigens derived from solid organ transplantation.
Early B cell activation & the extrafollicular T/B cell interaction B cell activation starts when its BCR binds to antigen presented on FDCs. The BCR-bound antigen is then internalized & processed. The peptides thus derived from the antigen on MHC Class II molecules interact with antigen-specific TCRs on CD4+ T cells.
Alloreactive T cells can directly or indirectly recognize intact donor MHC or processed donor MHC presented on recipient MHC, respectively. But only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA.This interaction between T & B cells initiates the T-dependent DSA response. On binding antigen presented on DCs, a sub-set of T cells differentiate into T follicular helper (Tfh) cells. It is not well known how CD4+ T cell fates are determined, but it is clear that the differentiation into Tfh cells require specific signals derived from the TCR, co-stimulation & inflammatory milieu that are different from those that drive differentiation into the other CD4+ T effector cell lineages. Understanding these differences may lead to identify ways to more precisely prevent & treat AMR versus T cell-mediated rejection.
B cell priming at the T/B interface results in the generation of early PCs, memory B cells & B cells that migrate back to the follicle to initiate germinal centre (GC) responses. These pre-GC responses generate PCs that are of lower affinity than post-GC PCs. The relative importance of the pre-GC response to DSA production following solid organ transplantation, & the contribution of this response to AMR needs to be resolved. Germinal center responses: Within each GC, & following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as either PCs or quiescent memory cells. The former are responsible for persistent circulating antibodies, while the latter are responsible for the recall antibody response upon re-exposure to antigen. The importance of CD 40:CD 154 interactions between B cells & T cells in the initial activation of T cells & in the maintenance of the GC response is well documented. Since there is no currently FDA approved drug targeting the CD40:CD154 interaction, while CTLA-4Ig is an FDA agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses. Chen et al. [1]compared the ability of anti-CD154 & of CTLA-4Ig to reverse established GC response in mice. Both anti-CD154 & CTLA-4Ig reduced GC B cell responses & prevented further DSA increase. Delayed treatment with CTLA-4Ig also significantly decreased the frequency of memory alloreactive B cells generated, & the recall DSA response upon re-immunization in the absence CTLA-4Ig. These experimental model reports are consistent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection. Belatacept was found to able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC. [2] Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which is currently more extensively used for kidney transplant recipients, requires further investigation. Germinal Center responses to multiple complex antigens It remains unresolved how the immune system responds to multiple antigens leading to these variable outcomes. Childs et al. , using computational modeling methods , suggested that upon initial exposure to antigen, B cell clones with different antigenic specificity & affinity compete for stimulation during rounds of somatic hypermutation within GCs. The resulting presence of many antigenic epitopes leads to narrowing of the breadth of the antibody repository. Their model also predicted ‘affinity selection’ ; meaning that the initial affinity of the produced antibodies to multiple antigens would be reduced, due to a reduction in the number of GCs & B cells available for each epitope. Initially produced circulating antibodies will bind to the same antigenic epitopes, resulting either in the selection of B cells with higher affinity than circulating antibodies, or in the selection of B cells with specificity for new epitopes. Thus circulating antibodies may increase the avidity, but either increase or decrease the breadth of the antibody reserve. In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Nearly 15% of renal transplant recipients develop de novo DSA within 4.6 years, some of them responding with a limited DSA repertoire while others more broadly. Wiebe et al. reported that the development of antibodies directed at donor Class II was most frequent in recipients with a higher degree of HLA epitope mismatch. Understanding the process of selection of alloantibody in response to multiple antigen-mismatched allografts, their evolve over time, & the impact of immunosuppression on these process, will result in a better understanding of when pathogenic versus non-pathogenic DSA develops. Regulating the germinal center response A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, & sharing many phenotypic characteristics of Tfh cells, has recently been described.[3; 4; 5]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 & IL-10, but not CD25, & function by controlling GC responses. These, & many other observations led the authors to suggest that TGF-ß production may be the mechanism by which Tfr inhibit B cell responses. Generation of memory B cells and PCs Appropriate & rapid generation of PCs is required for the successful control of infection, & of memory cells & long-lived PCs for the protection against re-infection. BCR affinity & Tfh have critical roles in deciding the fate of PC during primary exposure to antigen. High BCR affinity facilitate differentiation into PCs at the T:B border & within GCs. Memory B cells are generated in 2 distinct phases: early pre-GC memory B cells with lower affinity BCR & enriched for IgM, & later post-GC memory B cells with higher affinity & expressing IgM or IgG. These features of memory B cell versus PCs differentiation are consistent with memory B cells being preferentially generated in the pre-GC & early GC period, & long-lived PC emerging significantly later. The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not have prevented memory donor-specific B cell generation. In a recent meeting report by the Sensitization in Transplantation: Assessment of Risk (STAR) 2017 working group, a major finding was that the absence of DSA does not mean an absence of sensitization or of memory B cells. Possible explanations for humoral sensitization without detectable DSA in transplant recipients include: absorption of DSA by the allograft, the loss of shorter-lived PC, the generation of memory B cells that are not identical to the long-lived PC. The STAR recommendation, in the absence of a clinical test of donor-specific memory B cells, is that an accurate history be obtained, & only patients without DSA & also without HLA sensitizing events such as pregnancies, transfusions, previous transplantation & implants should be considered to be immunologically low risk for alloimmune memory. Recall B cell responses
Upon re-exposure to antigen, memory B cells differentiate into either PCs or GC B cells. Compared to primary response, the PCs produce a faster, high titer & class-switched recall antibody response, while the GC B cell generates new PCs that have higher‐affinity & are class‐switched. Data suggest that the fate of memory B cells upon re-exposure to antigens is dependent, in part, on how & type of memory B cell was generated & the conditions of the antigen-reencounter.[6,7,8,9] Conclusion In spite of the emerging limits of DSA role in predicting ABMR & sensitization, de novo DSA remains a strong predictor of allograft loss [10; 11; 12]. This review summarized the cellular processes that generate an antibody response & humoral memory. The study of these processes was in model antigens in reductionist mouse models. Further investigation is needed to know how this would apply to human solid organ transplantation. The recipient is exposed to a huge number of antigenic epitopes, antigens that are persistent, as well as immunosuppressive medications. These factors impact the cellular responses that generate PCs, & the quality of the DSA they produce, as well as quality of memory donor-specific B cells; all these factor form barriers to successful transplantation. More understanding of the mechanistic differences between the immune responses to model antigens & transplantation antigens may pave the road to new means to help control DSA production without compromising protective immunity of the host. References [1]. Chen J, Yin H, Xu J, Wang Q, Edelblum KL, Sciammas R, and Chong AS, Reversing endogenous alloreactive B cell GC responses with anti-CD154 or CTLA-4Ig. Am J Transplant 13 (2013) 2280–92. [PubMed: 23855587] [2]. Leibler C, Thiolat A, Henique C, Samson C, Pilon C, Tamagne M, Pirenne F, Vingert B, Cohen JL, and Grimbert P, Control of Humoral Response in Renal Transplantation by Belatacept Depends on a Direct Effect on B Cells and Impaired T Follicular Helper-B Cell Crosstalk. J Am Soc Nephrol 29 (2018) 1049–1062. [PubMed: 29321143] [3]. Chung Y, Tanaka S, Chu F, Nurieva RI, Martinez GJ, Rawal S, Wang YH, Lim H, Reynolds JM, Zhou XH, Fan HM, Liu ZM, Neelapu SS, and Dong C, Follicular regulatory T cells expressing Foxp3 and Bcl-6 suppress germinal center reactions. Nat Med 17 (2011) 983–8. [PubMed: 21785430] [4]. Linterman MA, Pierson W, Lee SK, Kallies A, Kawamoto S, Rayner TF, Srivastava M, Divekar DP, Beaton L, Hogan JJ, Fagarasan S, Liston A, Smith KG, and Vinuesa CG, Foxp3+ follicular regulatory T cells control the germinal center response. Nat Med 17 (2011) 975–82. [PubMed: 21785433] [5]. Wollenberg I, Agua-Doce A, Hernandez A, Almeida C, Oliveira VG, Faro J, and Graca L, Regulation of the germinal center reaction by Foxp3+ follicular regulatory T cells. Journal of immunology 187 (2011) 4553–60. [6]. Pape KA, Taylor JJ, Maul RW, Gearhart PJ, and Jenkins MK, Different B cell populations mediate early and late memory during an endogenous immune response. Science 331 (2011) 1203–7. [PubMed: 21310965] [7]. Dogan I, Bertocci B, Vilmont V, Delbos F, Megret J, Storck S, Reynaud CA, and Weill JC, Multiple layers of B cell memory with different effector functions. Nature immunology 10 (2009) 1292–9. [PubMed: 19855380] [8]. Zuccarino-Catania GV, Sadanand S, Weisel FJ, Tomayko MM, Meng H, Kleinstein SH, Good-Jacobson KL, and Shlomchik MJ, CD80 and PD-L2 define functionally distinct memory B cell subsets that are independent of antibody isotype. Nature immunology 15 (2014) 631–7. [PubMed: 24880458] [9]. Krishnamurty AT, Thouvenel CD, Portugal S, Keitany GJ, Kim KS, Holder A, Crompton PD, Rawlings DJ, and Pepper M, Somatically Hypermutated Plasmodium-Specific IgM(+) Memory B Cells Are Rapid, Plastic, Early Responders upon Malaria Rechallenge. Immunity 45 (2016) 402–14. [PubMed: 27473412] [10]. Wiebe C, Gibson IW, Blydt-Hansen TD, Pochinco D, Birk PE, Ho J, Karpinski M, Goldberg A, Storsley L, Rush DN, and Nickerson PW, Rates and Determinants of Progression to Graft Failure in Kidney Allograft Recipients With De Novo Donor-Specific Antibody. Am J Transplant 15 (2015) 2921–30. [PubMed: 26096305] [11]. Cooper JE, Gralla J, Cagle L, Goldberg R, Chan L, and Wiseman AC, Inferior kidney allograft outcomes in patients with de novo donor-specific antibodies are due to acute rejection episodes. Transplantation 91 (2011) 1103–9. [PubMed: 21403588] [12]. Ho EK, Vlad G, Vasilescu ER, de la Torre L, Colovai AI, Burke E, Deng M, Schwartz J, Marboe C, Mancini D, Opelz G, and Suciu-Foca N, Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival. Hum Immunol 72 (2011) 5–10. [PubMed: 20971146]
Thank you Dr Mohamed
You do not need to write references when you answer questions in the Journal Club. We know that we are referring the the given article.
This article serves as a bridge between the past and present understanding of the pathology behind transplant outcomes. It also aims to shed light on future areas of investigations that would prove to be fruitful in the development of improved and better transplant outcomes.
Our knowledge on transplants and related issues have been growing over the past few decades. While T cells were mainly blamed previously for poor transplant outcomes, now there are other concepts and principles that challenge this point of view. Pre-formed DSAs and de novo DSA development have been recognized to have a major role in graft rejection and poor outcome. AMR is a major challenge since it potentially leads to failure of graft with a significant impact on patient mortality. Although we have moved from focussing on T cells as the major culprits, immunosuppressive regimens that are currently employed are far from perfect. This is because ABMR control is now attempted to be achieved by eliminating B cells and plasma cells, circulating antibodies. This article suggests that better understanding of B cell differentiation might help significantly in developing better sustainable long term allograft outcomes while at the same time preserving the immunity of the recipient in order to protect them from ancillary disease and infection. With this background, the article aims to focus on how antibodies are produced and how they affect the graft, patient and outcome, along with future possibilities of further ABMR control.
In order to understand the grand picture, it’s important to start from the beginning. How exactly this whole process comes about. B cells when met with an antigen are matured into plasma cells, which then create antibody and memory B cell. How these antigens come in contact with the B cell is what drives the start of AMR. There are two routes of delivery – from lymph nodes and spleen. This is similar to antigens derived from SOT.
Since mature B cells circulate around lymph nodes every 24 hours, they accumulate within the cortex of the lymph node where they encounter the antigen. The enter the lymph node via the HEV or high endothelial venules and move along using FDCs or follicular dendritic cells.
In the spleen, the action is mainly in the marginal zone, which is located between the red and white pulp. The resident cells in the marginal zone capture the antigen and take it to the FDR in the B cells.
The proteins secreted by the transplanted kidney reach the B cell through gaps in the lymph node or similar gaps in the spleen. DSA in both naive and sensitized recipients bind to these antigens and activate complement pathway leading to cell destruction or ultimately transplant failure.
Once the BCR binds to the antigen, the B cell response is activated. They then present it to the CD4+ T cells. This combined interaction leads to the T cell response to DSA. The differentiation of the B cells of the recipient into plasma cells is key to this interaction and ultimate DSA production.
T cells then exit the lymph node and enter into the PALS region of the spleen using FRC networks. DCs with antigen from the graft moving through afferent lymph and using same FRC pathways present the antigen to the T cells inducing more rapid T cell response. Antigen affinity determines Th1 accumulation and response.
B cell priming in this interaction of B and T cells leads to early production of PCs before GC response. This is of lower affinity than GC response. GC has light zone and dark zone. Light zone has FDCs while dark zone is devoid of them. B cells in the light zones express activation markers and receive signals from GC Tfh cells. Positively selected B cells move into the dark zone and proliferate. Ultimately both light and dark zone B cells undergo apoptosis. Post GC B cells emerge as PCs, leading to persistent circulating antibodies.
Tregs regiulate what happens in the GC, and inhibit early stage B cell response.
All of these processes culminate in the formation of DSAs and ultimately possible graft rejection and failure of transplant. De novo DSA still remains a significant factor resulting in loss of graft.
This summarizes the various pathogenic processes outlined in the given article with focus on B cell antigen encounter, and the various methods relative to it, along with how T cells are involved in this fight against the transplanted graft. Better understanding of B and T cells is needed to create more effective immunosuppressive regimens that will provide smoother post transplant phases for transplant recipients as well as long term good outcome of graft. Preservation of natural immunity of the recipient needs to be a major goal to focus on while researching deeper into the pathogenic processes leading to ABMR.
This article points to a paradigm shift in pre-transplant evaluation as it recognizes the role of B lymphocytes in the production of antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure. So, it summarize the processes that underlie the primary and recall phases of B cell activation and antibody production and draws attention to the singular characteristics of this process.
1 – Routes of antigen presentation to B cells
There is increasing evidence suggest that optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on follicular dendritic cell (FDCs), B cells or macrophages. This encounter ocorre in the draining lymph nodes via multiple routes depending on antigen size, the presence of circulating antigen-specific antibodies, and the deposition of complement on the antigen by the classical or alternative pathways. This antignes are opsonized and complement is actived.
2 – Early B cell activation and the extrafollicular T/B cell interaction
There is a group of B cells responsible for the presentation of antigens from donor MHC cells to cells T CD4+. Following, cells T CD4 will driving the differentiation of recipient B cells into plasma cells producing donor-specific antibody (DAS).
The activation of T cells requires an initial encounter with migratory antigen presented on dendritic cells (that had acquired antigen from the allograft and migrated via afferent lymph) result in a subset acquiring T follicular helper, which will differentiate into CD4+ T cells.
3- Germinal center response
In the germinal center is divided in light zone and dark zone. There is interaction between B cells, dendritic cells, CD4+ T cells and T follicular helper in these zones, providing downregulation and upregulation, through cytokines and costimulatory molecules, responsible for persistente circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
4 – Germinal Center responses to multiple complex antigens
The chain of interactions mentioned above is capable of responding to the simultaneous introduction of multiple complex antigens, mimicking the stimulus of multiple MHC and non-MHC antigens found in the transplant. So, instead of a relative decrease in the antibody repertoire, we have increase in the breadth of the antibody repertoire.
5 – Regulating the germinal center response
The Germinal Center itself has a subset of CD4+ T cells that express the regulatory of this system. This T follicular regulatory are responsible to suppress self-reactive Germinal Center B cells that may have inadvertently arose during the Germinal center response.
6 – Generation of memory B cells and PCs
There are differences between memory generation through B cells and plasma cells. The affinitys and kinetics for BCRs and T follicular helper as well as by IgG and IgM expressions, lead to a perpetuation of this type of memory so that even with successful treatment of acute AMR and reduction of circulating DSA may not have prevented the generation of memory donor-specific B cells.
7 – Recall B cell responses
Recall memory B cells are expected to differentiate directly into plasma cells, generating antibodies faster than the primary response or generating germinal center B cells, however there are no rules in this way.
Different studies demonstrate prevalence of IgM+, others of IgG + and interaction with some signal expressions and revealed dependence on T cell help, and co-stimulation blockade with CTLA-4Ig, which are involved with CD28/CD80/CD86. And, this interaction can be controlled and depleted by belatacept.
REFER TO THE ATTACHED PICTURE FOR BETTER UNDERSTANDING OF B- T CELL INTERACTIONS
Mohamad Habli
3 years ago
Preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and that de novo DSA generated after transplantation is associated with poor outcomes and vascular obliterative lesions.
Antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure.
B cells have to encounter cognate antigen in order to start the process of differentiating into PCs producing high affinity antibody and memory B cells.
optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
Mature B cells circulate though the lymph nodes approximately every 24 hours, by leaving the vascular system and entering the lymph nodes through specialized high endothelial venules. Eventually these B cells encounter soluble or particulate antigens that enter the draining lymph node.
The humoral response can also arise from the spleen, which is divided into two distinct compartments: the red pulp, and the white pulp involved in the initiation of immune response.
The white pulp is comprised of the T cell zone (also referred to as the periarteriolar lymphoid sheath; PALS), B cell follicles and marginal zone (MZ). MZ contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells.
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
Only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA.
Germinal center response
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 and gene signatures associated with signaling from the BCR, CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells.
Survival signals include those provided by the interaction between costimulatory molecules, CD28-B7, CD40-CD154 and ICOS-ICOSL, by cytokines such as IL-21 and BAFF as shown in the attached picture. Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation. In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone.
Generation of memory B cells and PCs
Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity. Memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. t BCR-signaling and T cell help is sufficient to drive memory differentiation, without the need for further proliferation or somatic hypermutation. The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
The recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells [92; 93; 94]. As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients
-Antibody-mediated rejection (AMR), is a significant cause of renal allograft failure.
-When B cells stimulate by antigens differentiate into plasma cells that produce high-affinity antibodies and memory B cells
-When the B cell receptor binds to antigen displayed on follicular dendritic cell, triggers B cell activation, the upregulation of CCR7, and migration to the T/B border.
-B cells process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
-The interaction between T and B cells initiates the T-dependent DSA response.
-Extrafollicular Tfh cells can stimulate extrafollicular B cells.
– CTLA-4Ig ( belatacept ) is an FDA-approved immunosuppressive agent for preventing kidney rejection. CTLA-4 on Tfr limited the expansion of antigen-specific Tfh cells and reduce antigen-specific antibody responses that may promote humoral responses.
-The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the donor and migrated via afferent lymph and along the same fibroblastic reticular cells network in the lymph node or spleen to reach the T cell zone.
– For larger antigens, the localized CD11b+DCs within the lymphatic sinus can process and present antigens to T cells, which induce T cell responses rapidly.
-Memory B cells upon antigen reencounter will either differentiate into Plasma cells ( high-titer and class-switched recall antibody response), or GC B cells that (generate new class‐switched PCs).
-Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation.
– Renal transplantation recipients with a higher degree of HLA mismatch develop DSA against class II.
Donor specific antibody (DSA)is associated with high risk of allograft rejection.
Formation of Antibody Mediated Rejection (AMR) has significant role in chronic renal transplant dysfunction & failure.
This article focus on the evidence and role of B cell activation in formation of AMR which are responsible for lost of graft post transplant.
Mechanism of antigen presentation to B cell:
B cell is bind to intact soluble antigen by B cell Receptor (BCR).
B cell is major part of adaptive immune system response.
B cell arise from bone marrow and then circulate to blood into spleen.
Humoral response are arise from spleen which is composed of red pulp and white pulp; red pulp is responsible for damaging unwanted RBC by Macrophage.
white pulp responsible for initiating immune response which contain T cell/ B cell & marginal zone.
Marginal Zone contain large cell which capturing & shuttling antigen to follicular dentric cell.
Antigen are derived from solid organ transplant.
Presence of circulating Antigen specific antibody & deposition of complement on antigen by classical or alternative pathway.
B cell transport opsonised antigen to follicular dentric cell that capture immune complex by CR1 binding C3b , C4d.
antigen not bound to antibody can bind to macrophage & dentric cell located in lymph node by innate recognition protein like IgM & CRP.
these dentric cell and macrophage in lymph nodes together with CD8, M2 metallophilic macrophage & M2 macrophage in spleen capture antigen & then Antigen displayed on surface of cell activated complement after that fallow route of opsonisation antigen delivery to CR on follicular dentric cell.
all new alloantigen captured by MHC on the surface of dentric cell form exosome which migrate around lymph nodes and spleen. exosome surface contain glycoproteins decorated with sialic acid that attach to macrophage. Donor MHC delivered by exosome to recipient dentric cell result in cross dressed DC that stimulate CD 8 T cell direct pathway.
Early B cell activation and extra follicular T and B cell interaction.
it’s a reaction between T & B cell lead intiate depended DSA response.
T cell & B cell directly and indirectly recognise donor MHC on MHC of recipient B cell. T cell migrate from blood to lymph nodes and activate by donor presenting antigen.
Germinal Ceńter response:
it’s present in side B cell; it’s divided to light zone & dark zone.
light zone consist from stroma derived follicular dentric cell & there’s multiple processing and activation of markers regulated by T follicular regulatory cell. Dark zone contain network produce reticular cell for B cell proliferation. Germinal center response are responsible to produce of long lived antibody ( secreting plasma cell & memory B cell.
plasma cell is activated B cell with high affinity B cell receptor for antigen responsible for persist circulating antibody. Memory B cell is responsible for recall antibody.
In this article, they are do experimental models on mice by use of MHC class I & class II tetramer immunized into donor of spleen cell; they show activation of B cell in presence of T cell inside germinal center and formation of donor specific antibody after 7 days from immunisation. they noticed activation of CD40 & CD154. this markers currently still there is no drug target against them. while CTLA-41g approved as immunosuppressive agent to prevent kidney rejection by reduce donor specific antibody number and reduce humoral response by inhibiting B cell T helper cell interaction and prevent B cell differentiation to plasma cell.
So belatacept more control humoral response in comparison to cyclosporine & Tacrolims .
Germinal center response to multiple complex antigens with MHC & Non MHC antigens incompatibility.
circulating repertoire antibody bind to epitope of antigen and they selected B cell with high affinity epitope antigen to production & differentiation.
In kidney transplant, recipient B cell exposure to multiple complex antigen and under current immunosuppressive agents around 15% of transplant kidney develops de novo DSA repertoire within 4-6 years post transplant.
by using method to evaluate HLA epitope mismatch shows that development of antibody directed at donor class II have higher affinity to HLA epitope mismatch.
regulating the germinal center response:
it’s function of Tfr cell control and suppression inadvertent B cell. in absence of Tfr cell will result the self reactive B cell differentiated to plasma cell which produce anti histone and ANA.
Other studies the Tfr cell inhibit Ag specific GC B cell and antibody production.
Generation of memory B cell and plasma cell:
it’s very important for control of reinfection. Memory B cell generated into 2 phase (early GC memory B cell with low affinity & enriched of IgM / late post GC memory B cell with higher affinity and expressing IgM & IgG) also long lived plasma cell. Absence of DSA doesn’t indicate absence of donor specific memory cell; so good history important to role out exposure to transfusion & transplant or pregnancy to identify low risk for alloimmune memory.
Recall B cell response: memory B cell with antigen differentiated into GC B cell with higher affinity and class switched plasma cell.
memory B cell differentiated into plasma cell and the recall DSA response.
This paper describes the role of the humoral response in hyperacute or chronic antibody-mediated rejection, re-discussing the concept of antibody-mediated rejection.
B lymphocytes have chemotaxis via the reticuloendothelial system, increasing their exposure by dendritic cells and macrophages in lymph nodes and the spleen. This exposure can occur in the bloodstream by binding by soluble antigens (causing immune complexes) or by antigen-induced complement activation. Upon activation, the B lymphocyte becomes an immunoglobulin-producing plasma cell specific for that antigen.
MHC exposure to B lymphocytes leads to interaction with CD4 T lymphocytes, linking these pathways. It is believed that increased exposure to these antigens can increase the CD4/CD8 response and, consequently, increase DSA values chronically and progressively.
Germinal Center controls the erratic production of plasma cells exposed to alloantibodies, inducing them to apoptosis or inactivating their activity when leaving the light zone. In this way, it prevents the production of DSA by blocking the CTLA-4 pathway, pharmacologically evidenced when using belatacept, cyclosporine or tacrolimus.
HLA mismatch DR is most closely related to de novo DSA after 4 to 6 years of kidney transplantation.
Plasma cells fight recent infections (prioritizing the production of IgM) and B lymphocytes prevent reinfections when that antigen has already triggered a previous humoral response (IgG subclasses – IgG1, IgG2, IgG3, IgG4). This previous response occurred in the Germinal Center and is apparently the reason why donor B lymphocytes do not produce alloantibodies. In this way, CTLA-4 blocks the transformation of B lymphocytes into plasma cells, minimizing their production of IgM and at the same time increasing the risk of infection from viral and bacterial agents.
In conclusion, patients who were more exposed to epitopes and antigens during the pre-transplantation period (transfusions, pregnancy, previous transplantation) as well as those who did not perform immunosuppression adequately, are more prone to de novo ASD and, consequently, to antibody-mediated rejection.
RULE OF B CELLS IN TRANSPLANTATION
Immune response is either innate or adaptive
Adaptive immunity involves specific reaction to a certain agent and memorization of these agent
Adaptive immunity is either T cell mediated (when antigen peptide binds to HLA antigens) or B cell medicated (when antigen peptide bind to immunoglobulin receptors on B cell or through activation by CD4- T cells) (2)
The 3 systems (natural immunity, T cell immune system, B cell immune system ) are usually integrated.
B cells have an important rule in T cell medicated rejection (through acting as APC that present antigen to T cell and through activation of T cells) or in ABMR (through transformation into plasma cells and production of immunoglobulins which act as DSA)
Full activation of B cell occurs when BCR binds to an intact antigen on the follicular dendritic cell or macrophages
Upon activation of B cell the following occur :
1- B cell secretes many cytokines such as IL-6 and TNF-a , that lead to activation and Differentiation of T lymphocytes which leads to graft damage. Some of the cytokines which is secreted by B cells like INF-gama, TNF-a are directly injurious to the graft.
2- B cells can differentiate into plasma cells which secrete antibodies DSA and lead to antibody mediated rejection. for complete plasma cell differentiation and migration out of the germinal center Tfh cells are needed.
3- B cells can differentiate into memory B cells which can stay in the body for years and can induce memory response once reactivated. thery are either grenerated early pre-germinal center (secrete IgM, has low affinity) or late post germinal center (secrete IgG or IgM , has high affinity)
Good ,concise explanation for B cell activation
Please refer to fig 2 for clinical application and use of Belatacept.
Mohamed Saad
3 years ago
New insights into the development of B cell responses: Implications for solid organ transplantation. Introduction:
Antibody mediated rejection (AMR), is recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure.
Role of B cell activation and antibody production, may be applied to understanding the generation of DSA following solid organ transplantation. Routes of antigen presentation to B cells
– B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages (reviewed in [5]).
Mature B cells circulate though the lymph nodes approximately every 24 hours, these B cells congregate within the cortical region encounter soluble or particulate antigens that enter the draining lymph node via multiple routes depending on antigen size, the presence of circulating antigen-specific antibodies, and the deposition of complement on the antigen by the classical or alternative pathways. Also migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
Antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated, Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by macrophages in the red pulp of the spleen, SCS macrophages and DCs,somtimes limited number of donor DCs migrate from the graft into draining lymph nodes or spleen, where they deliver exosome to resident DCs.
Donor MHC delivered by exosomes to recipient DCs, results in cross-dressed DCs that have the ability to stimulate recipient CD8+ T cells via the direct pathway.
The exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages .These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone. Early B cell activation and the extrafollicular T/B cell interaction.
B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
T cells can directly or indirectly recognize intact donor MHC but only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA. Germinal center response.
Within each GC, and following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as memory B cells that are responsible for the recall antibody response after another antigen stimulation.
The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well known. In contrast, the necessity of CD28:B7 interactions in sustaining the GC response was less clear. CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses. Germinal Center responses to multiple complex antigens:
B cell clones with multiple different antigenic specificity and affinity compete for stimulation during rounds of somatic hypermutation within GCs. As a result, the presence of many antigenic epitopes tends to educe the relative breadth of the antibody repertoire and generated antibodies to multiple antigens would be reduced. Generation of memory B cells and PCs:
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation so absence of DSA dose not mean absence of sensitization.
so we still need a clinical test of donor specific memory B cells. Recall B cell responses:
Sensitization of Memory B cell by same antigen again lead to direct differentiation to PCs stronger than primary response or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
So, recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response.
So, belatacept has the ability to control and reverse DSA responses in sensitized recipients.
Thankyou Mohamad for your hard work in reading and extensive summary Iam sure this is very rewarding for you however you can conclude the useful clinical information in final points as
figure 2 clearly gives detailed diff. of what happens in a NAIVE and a SENSITISED recipient.
It demonstrate the T ,B cell cognitive crosstalk.
Detailed sites of costmulation block byCTLA-4Ig.
Effect of CTLA-4Ig on Tregs which still needs an answer.
This goes on during reading any paper to extract useful clinical conclusions.
A resurgent interest in the role of B cells following solid organ transplantation is being driven by
clinical data suggesting that antibody mediated rejection (AMR) is a major cause of dysfunction
and organ transplant failure. In this review, new insights into processes that lead to antibody production upon primary and secondary antigen encounter are discussed, and the potential implications to DSA production and future areas of investigation to control AMR are discussed.
Routes of antigen presentation to B cells
B-cell + Antigen >>>>>plasma cells >>>>antibody & B memory cells
While it has long been established that B cells can bind intact soluble antigen, there is increasing evidence suggest that optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
How can B cell encounter soluble or membrane bound antigen
– Mature B cells circulate though the lymph nodes approximately every 24 hours.
– Eventually these B
– cells congregate within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens that enter the draining lymph node.
– In addition, there may be additional contribution by migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
The humoral response can also arise from the spleen.
Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain
direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen.
In addition, natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph
nodes, via innate recognition proteins such as natural, Antigens displayed on the surface of these cells activate complement.
In addition to antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated
Alternatively, a relatively limited number of donor DCs migrate from the graft into draining lymph nodes or spleen.
Early B cell activation and the extrafollicular T/B cell interaction
– When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border.
– Concurrently, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
– only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA. This cognate interaction between T and B cells initiates the T-dependent DSA response.
Germinal center response
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the
dark zone, which is devoid of FDCs. Light zone B cells express the activation markers,
CD86 and CD83 [43; 44] and gene signatures associated with signaling from the BCR,
CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive
survival signals from GC-Tfh cells
Germinal Center responses to multiple complex antigens
In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal
transplant recipients develop de novo DSA within 4.6 years of transplantation.
Understanding how the alloantibody repertoire is selected in response to multiple antigen-mismatched allografts, how this repertoire evolves over time, and the impact of immunosuppression on these process, will result in a better understanding of when pathogenic versus nonpathogenic DSA develops.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3,
and sharing many phenotypic characteristics of Tfh cells, has recently been described
In the absence of Tfr, these selfreactive B cells differentiated into PC that produced anti-histone and anti-nuclear antibodies.
Interestingly, Tfr cells did not inhibit the expansion of influenza-specific B cells or their
differentiation into PC, and had no significant effect on influenza-specific antibody responses. In contrast, other studies have reported that Tfr cells inhibit antigen-specific GC B cells and antibody production
Generation of memory B cells and PCs
The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
Within GCs, PC differentiation was induced in a
discrete subset of high-affinity B cells residing within the light zone upon BCR engagement
with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for
completing the PC differentiation and driving their migration out of the GC
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with
lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity
and expressing IgM or IgG
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
Professor Ahmed Halawa
Admin
3 years ago
Thank you All I’m still waiting for all colleagues to complete their logging
Doaa Elwasly
3 years ago
The preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and de novo DSA detected after transplantation is associated with poor outcomes and vascular obliterative lesions Ag presentation routes to Bcells
Donor derived MHC complex are captured by SIGN-R1+ MZ macrophages, F4/80+ macrophages in the red pulp of the spleen, SCS macrophages and DCs, stimulating recipient CD8+ T cells directly . These macrophages transfer intact donor MHC to FDCs within the B cell zone. B cell activation and T/B cell interaction
BCR binding to antigen triggers B cell activation.
CD4+ T cells can drive the differentiation of recipient B cells into PCs producing DSA thereby starting DSA effect.
The activation of T cells encountering with migratory DCs activated T cell .
The strategically localized CD11b+DCs within the lymphatic endothelium present captured antigens to T cells, stimulating rapid T cell response .
Belatacept, a mutant of CTLA-4Ig, acts by inhibiting B cell-Tfh interactions,blocking humoral response therefore preventing DSA in kidney transplant recipients .
Renal transplantation recipients having higher degree of HLA epitope mismatch developed antibodies directed towards donor Class II.
It was supposed that Tfr inhibit B cell responses through producing TGF-ß .
BCR signaling intensity is assessed early by expression of interferon regulatory factor-4 (IRF4).
Acute AMR treatment and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
IgM + memory B cells preferentially differentiated into GC cells, and the IgG1+ memory B cells gave rise primarily to PCs.
clever conclusion about the effect of treating ABMR by removal of DSAs effect on memory donor-specific B cell recognition.
Fig 2 well explains diff. between naive and sensitised recipient.
New insights into the development of B cell responses: Implications for solid organ transplantation
Routes of antigen presentation to B cells
B cells can bind intact soluble antigen or B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
The humoral response can also arise from the spleen.
Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins such as natural IgM; C-type lectins such as MBL (mannose binding lectin) and ficolins; and pentraxins, including C-reactive protein.
Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by SIGN-R1+ MZ macrophages, F4/80+ macrophages in the red pulp of the spleen, SCS macrophages and DCs.
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border (reviewed in [5; 6]). Concurrently, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
Germinal center response
When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they down- regulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
Germinal Center responses to multiple complex antigens
In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly .
Regulating the germinal center response
T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC responses. Early studies utilizing adoptive transfer or mixed bone marrow chimeras showed that Tfr are located in the GC, and that they suppress self-reactive GC B cells that may have inadvertently arose during the GC response.
Generation of memory B cells and PCs
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG. Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help, which in turn, induces reduced IRF-4 and higher Bach2, respectively.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
Anti-HLA antibodies especially DSA either preformed or donovo are associated with poor graft outcomes.
Understanding the pathogenesis of anti-body formation considers the future hope for finding selective therapies targeting humoral responses.
the human immune system consists of 2 main types of cells. Granulated innate immunity and adaptive nongranulated immune cells. Adaptive immunity (including T lymphocyte and B lymphocyte) needs antigens recognition by antigen-presenting cells (Follicular Dendritic Cells (FDC), B cells, or macrophages) either by direct, indirect, or semi-direct pathway to be activated. activation of B cells leading to the formation of plasma cells and antibodies formation including DSA, memory B cells, and mature B cells via high endothelial venules (HEV) under the guidance of chemotactic factors like TNF alpha. Immature B Cells enter lymph nodes in the cortex near the subcapsular sinus. Antigen-presenting cells capture antigens and deliver them to lymph nodes.
Lymphoid tissues including lymph nodes and spleen consist of peripheral white pulp in which resident cells capture antigens and introduce them to APC and act on T cells in the T cell zone in the periarteriolar lymphoid area leading to T cell activation. Antigen-presenting cells and T-cell crosstalk usually take 5 days then rapid T cell transformation under the effect of IL to follicular helper cell (TFH) which can be either extrafollicular TFH or intrafollicular inside GC (Germinal Centre) TFH cells. T/B cells crosstalk and B cell priming usually occur in the spleen leading to the formation of early plasma cells, memory B cells, and B cells that migrate back to the follicle inside the lymph nodes. The GC has two zones a light peripheral zone contains FDCs and TFH cells while the dark central zone of GC contains no FDCs. B cells undergo apoptosis in the light zone, while dark zone B cells undergo activation and proliferation to plasma cells with intact B cell receptors (BCR) which re-enter the light zone again to test BCR for binding to antigen.
After testing the BCR binding affinity to antigen inside the light zone, 2 types of B cells are diagnosed:
1-Early pre-GC memory B cells, which mainly secrets IgM with lower affinity
2-post-GC memory B cells have higher affinity and are able to secret both IgM and IgG. Memory B cells on re-exposure to the same antigen either undergo direct transformation into plasma cells secreting high titer antibody response, or they differentiate into GC B cells which transform to new higher affinity plasma cells.
The most influencing factor for chronic allograft dysfunction and failure is chronic ABMR ,especially the De novo DSAs., which is further featuring vascular obliterative lesions.
How the B lymphocyte stimulated by the donor HLA antigens?
Usually the B lymphocytes interact with HLA protein displayed on follicular dendritic cells FDC, B lymphocytes and macrophages. Consequently it would differentiate to immunoglobules secreting plasma cells and memory cells.
Several mechanisms pertain to excel the function of B lymphocytes:
1-the B lymphocytes enter the lymph nodes once every day driven by the chemokine gradient established by the FDC and fibroblast reticular cells FRCs.The it congregate in the sub capsular zone in the lymph nodes where the antigen accumulate by means:
a) complement system deposited on surface of Antigen.
b) antibody directed against the antigen.
c) circulating dendritic cells that acquire the antigen at the tissue site.
It can be stimulated in the Spleen as well.
d) Another mechanism of allo-recognition is accomplished by the means of delivering intact HLA antigen extracellular vesicle or exosome which circulate reaching to the lymph nodes.Where its captured and presented by specialized macrophages.
e) Donor Dendritic cells might circulate and reach to lymph nodes to dump the HLA antigen.
When DCs capture the donor HLA it will be cross-dressed which stimulate the CD8 lymphocytes directly,
Of interest:
1-Interestingly DCs cross dressed with MHCII are not effeciontly priming CD4 lymphocytes.
2-When exosomes taken by macrophages they will deliver it to FDCs within the lymph nodes.
3- When BCR binds to antigen on Macrophages ,dendritic cells or non antigen specific B lymphocytes it trigger B cell activation.
4- BCR bound antigen internalized and processed by B-lymphocytes is presented in combination with MHC II on cell surface to be interacting with T cell receptor on CD 4 lymphocytes.
5-T cells can recognize intact MHC or part of it in combination of MHC .
6- Only CD 4 Lymphocyte can trigger the conversion of B lymphocytes to Plasma cells producing DSAs.
*In the past the center of focus of rejection was directed twards T cell mediated rejection which was challenged over the past decad where AMR esp. De Novo DSA is the main claim for the cause of chronic kidney transplant dysfunction and failure
*B cells plus specefi antigen start the process of differentiating into PCs producing antibody and memory B cells.
*Antigen presented to the conjugate BCR in the lymph nodes where it reaches LNDs with the help of FDCs (dendrites and chemokines) where antibodies are formed activating the complement pathway .
*humoral response arise from the spleen, two distinct compartments: the red pulp (venous sinuses) that encounter old or damaged erythrocytes by macrophages, and the white pulp immune responses . The white pulp contains T cell zone (PALS), B cell follicles and marginal zone .
The MZ contains resident cells capturing antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells.
*soluble antigens (≤14 kD), gain direct access to the B cell via passive flow through FRC conduit in the sinus floor of the lymph node or in the marginal sinus in the spleen .
*FRC conduits intersect with FDCs and provide direct connection for soluble antigen to be taken and presented on FDC surface.
*Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins
*Early B cell activation and the extrafollicular T/B cell interaction :
Antigen presented to BCR through dendritic cells reaches T/B border where MHCII antigens interact with CD4 T Helper cells causing B cell activation and production of T dependant DSA
*Memory B cells are produced in two phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG
*successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation
*only patients without DSA and without HLA sensitizing events such as pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory.
*IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies,
* the fate of memory B cells on reencounter with antigens is dependent, on how and type of memory B cell was generated and the conditions of the antigen-reencounter.
· In the past, only T cell were thought that have an important role in rejection but in the last decade, important role for donor specific antibodies (DSA)was considered.
Now, AMR is known as a major cause of chronic renal dysfunction. This article is a review about B cell activation and DSA generation post Tx. B cell can differentiate in to plasma cell and memory B cells. B cells expose to a variety of antigens in lymph nodes or spleen, especially with pulp. Antigens of transplanted kidney less than 14 KD are presented on the FDS also they activate complement after binding to DSA.
Non antibody-binding antigens are linked to DCS via innate recognition
DCS in the lymph nodes or spleen can activate complement.
In addition, intact MHC complexes of donor can accumulated in these sites and can activate recipient CD8+ T cell (direct path way).
· B cell activation and T/B cell interaction:
Ags which are displayed on FDS, SSM, or non-Ag –specific B cells, binds to BCR and can activate B cells causing movement to the T/B border. Ag peptides are presented on MHC class ΙΙ and have interaction with TCR on CD4+ T cells.
Interaction between T and B cells causes differentiation of B cells to early PCs and memory B cells and pre- GC response.
T cell exposure to Ags presented on DCS with signals of Co-stimulation and inflammation, causes T folicular helper (Tfh) responses. T fh express CXCR5 and Bcl-6 localize them toward B cell follicle and T zone (T/B) interfaces activate DCs expressing ICOS ligand and CD25 and Tfh cell differentiate.
· Germinal center response:
The GC is divided into the light zone and dark zone.
Light zone B cells receive signal from GC-Tfh cells and migrate in to dark zone where they are selected to emerge as PCS or memory B cells. CD40: CD154 interaction between T and B cells are important to maintain GC response.
Both anti CD154 and CTLA-4 Ig at 7 days can decrease GC B cells responses and DSA production. So, belatacept can control humoral responses by inhibition of B cells-Tfh interaction and B cells differentiation in to PCS, in spite of higher rate of AR. Germinal center responses to multiple complex antigens: In TX, B cells expose to multiple complex Ags and 15% of them develop de novo DSA during 46. Years of TX which is correlated which higher degree of HLA epitope mismatch.
· Regulation the germinal center response:
Recently a subset of CD4+ T cells named follicular regulatory (Tfr) cells are recognized that express FOXP3.The cells regulate Ab response by preventing long-lived interaction at the T/B border and prevent Tfh cells entry in to GCS and B cells differentiation in to GC B cells, memory B cells or Pcs.
Expression of CD69, IL-10, CT-10, CTLA-4 and GARP were associated with B cell suppression by Tfr. So may be TGF-β production is the mechanism of B cell inhibition by Tfr.
· Generation of memory B cells and PCs:
B cell differentiation in to PCs at the T/B border and within GCs is promoted by high BCR affinity BCR binding Ag on FDS light zone causes PC differentiation and Tfh cells drive their migration at of the GC.
Memory B cells are generated is an early pre-GC and post –GC memory B cells. Reduced IRF-4 and higher Bacl2 which is seen with lower-BCR signaling and T cell help can drive memory B cells which is opposite to PCs.
B cells lack CXCR4 which failed entering DZ, will turn to memory B cells.
So they have different mechanisms which means that lack of DSAs is not equate to absence of memory B cells.
So history of HLA sensitization events are important for considering patient as immunologic low risk.
· Recall B cell responses:
With exposing to specific Ags memory B cell will differentiate in to PCs or GC B cells.
Recall DSA response was associated with rapid differentiate in to PCs and low GC responses and was dependent on T cell help. So co-stimulation blockage by CTLA-4 Ig completely prevented this.
Long-lived PCs (LLPcs) repress CD28 and inhibition of CD25-CD80/CD86 interaction by CTLA-4Ig could describe belatacept ability in controlling DSA response in sensitized patients.
Better understanding of these mechanism will result in better control of DSA production and graft outcome.
New insights into the development of B cell responses: Implications for solid organ transplantation
Routes of antigen presentation to B cells
Germinal center response.
Germinal center response.
Germinal Center responses to multiple complex antigens
Generate memory B cells and plasma cells.
Recall B cell response
a) Faster and higher titre of antibody response
b) Higher affinity and class switch PCs generated from Gc B cells
Conclusion:
T cells are considered major player in graft rejection. It was suggested that there is association of DSA with increased rejection risk and worse graft outcomes have addressed the role of B cells in organ transplant. Cognate antigen on follicular dendritic cells , B cells and macrophages act on B cell receptors leading to activation of B cells, resulting plasma cell formation which give rise to donor specific antibodies and memory B cells. B cells ;mature ones through high endothelial venules enter lymph nodes and assemble in cortex close to subcapsular sinus. Migrating dendritic cells catch antigens and deliver them to lymph nodes. White pulp of spleen starts immune response. Marginal zone of white pulp has resident cells which catch antigens and shuttle them to FDCs. Migrating dendritic cells act on T cells in the T cell zone ;PALS-periarteriolar lymphoid sheath resulting in T cell activation, but it taking up to 5 days to act. Rapid response develops with delivery of antigen to T cell zone through follicular reticular cell conduits. Activation of T cells will result in acquiring follicular helper cell properties which can be either early extrafollicular mantle Tfh and germinal centre Tfh cells. B cell priming at the T/B interface in spleen result in formation of early plasma cells, memory B cells and B cells which migrate back to follicle. GC has a light zone with FDCs and Tfh cells while the dark zone of GC have no FDCs. B cells undergo apoptosis in light zone, and dark zone B cells with intact B cell receptor reenter light zone where BCR are tested for binding to antigen & T cell help. Differentiation into plasma cells relies on higher BCR affinity. Early pre-GC memory B cells, IgM mainly , have lower affinity and then post-GC memory B cells have higher affinity expressing IgM and IgG. Memory B cells, on re-encountering with antigen will either directly form plasma cells leading to a high titer antibody response, or differentiate into GC B cells to generate new high affinity plasma cells.
Development of B cell responses. The new insights and its implications for solid organ transplantation.
Antibody mediated rejection ( AMR ) due to donor specific antibodies ( DSA ) is associated with acute graft rejection and may serve as one of the reason for chronic kidney transplant dysfunction and failure. Knowledge on B cell activation and antibody production particularly the DSA is crucial for solid organ transplant. It provide wider prospects in managing kidney transplant rejection.
Mechanism of B cells activation.
1） Mature B cell entering lymphatic system through high endothelial venules ( HEV ), migrating along processes extending from follicular dendritic cell ( FDC ) and chemokine CXCL13. Mature B cell congregate within the cortical region near the subcapsular sinus and bind to the soluble or particulate antigens. Ultimately this lead to formation of antigen specific antibodies and complement activation via classical or alternative pathways.
2） Spleen has a role in B cell activation. Antigen antibody binding process occur at whitle pulp area of the spleen especially the B cell follicles and marginal zone ( MZ) which contain large reservoir of resident cells.
3） Small soluble protein ( less than 14 kD) from transplanted organs activated B cells through the lymphatic system and margical sinus in the spleen.
4） Subcapsular sinus macrophages ( SSM ) opsonized antigens and relayed to B cells via complement receptors. B cells then transport opsonized antigens via FDC to the lymphatic system .
5） Antigen not bound by antibodies can bind to macrophages and dendritic cells at lymph nodes via innate recognition proteins which activate complement system at the spleen.
6） Exosomes released by allograft, alloantigen and donor derived intact MHC complexes accumulate in lymphatic system and B cell zone of the spleen which trigger B cell activation .
Early B cell activation and extrafollicular T/B cell interaction.
1） B cells receptors bind to antigen on MHC class II molecules through cascade of immunological reaction lead to activation of antigen specific T cell receptors on CD4+ T cells.
2） Only CD4+ T cells that have indirect specificity for donor derived antigens expressed by recipient B cells are capable to stimulate differentiation of recepient B cells into plasma cells producing DSA. This cognate interaction between T and B cells initiates the T dependent DSA response.
3） B cell priming at the T and B cells interface lead to early plasma cells generation, memory B cells and transformation into B cells that are destined to move back to the follicle to stimulate germinal center ( GC ) responses.
4） Pre GC responses generate plasma cells with lower affinity.
5） bcl-6 expression at T cells stimulate extrafollicular production of Ig G2 which is crucial to constrain infection.
6） However, pre GC response to DSA production after solid organ transplantation and trigger AMR has not been delineated.
Germinal center response.
1） GC consist of light zone which contain stroma derived FDC and T fh cells. On the other hand , the dark zone of GC is devoid of FDC.
2） Light zone B cells stimulated by the activation markers CD86 and CD83. The activated B cells bind to antigen presented to FDC and receive survival signals from GC T fh cells.
3） Selected B cells up regulate CXCR4 and migrate into dark zone to undergo cell proliferation and BCR diversification mediated by activation induced cytidine deaminase (AID).
4） Without T cell survival signals, B cells undergo apoptosis at the light zone. B cells with BCR gene defect due to AID undergo apoptosis at the dark zone.
5） Dark zone B cells with intact BCR down regulate CXCR4 and re enter light zone to bind to antigen surface and access to T cell help.
6） B cells with high affinity BCR for antigens has an important role for persistent circulating antibodies or function as memory B cell for antibody production upon antigen exposure in future.
7） CD 40 : CD 154 are important for initial activation of T cells and maintaining GC response. Unfortunately, no FDA approved drug available at the moment to target CD40:CD154 interaction.
8） CTLA-4 on T fr limit the expension of antigen specific T fr and reduce antigen specific antibody. CTLA-4 Ig is an FDA approved immunosuppressive to prevent post transplant kidney rejection.
9） Both anti CD 154 and CTLA-4Ig reverse GC response and prevent further DSA increase.
10) Belatacept able to control humoral responses by inhibiting B cell and T fh interaction. However the indication of Belatacept in kidney transplant recipients still under investigation.
Germinal Center responses to multiple complex antigens.
1) Majority of organ transplantation involves multiple MHC and non MHC incompatibilities.
2) B cells clone with different antigenic specificity and affinity compete for stimulation during episodes of somatic hyoermutation within GC. Hence, presence of multiple antigenic epitopes tends to reduce the relative breadth of antibody repertoire.
3) Most kidney transplant recipients likely encoungter multiple complex antigen. Approximately 15% of kidney transplant recipients develop de novo DSA within 4.6 years of translantation.
Regulating the germinal center response.
1) Subset of CD4+ T cells express regulatory T cells ( Treg ). The T follicular regulatory cell ( TFr ) function by controlling GC responses.
2) The TFr cells migrate to B cell follicles and suppressed self reactivate B cell generated in GC.
3) Without TFr , self reactivated B cells differentiate into plasma cells that produce anti histone and ANA.
4) TGF β production control by ability of TFr inhibit B cell response.
Generate memory B cells and plasma cells.
1) Rapid plasma cells production following antigen exposure is crucial for protection against reinfection
2) Both B cell receptors affinity and TFh is important for plasma cells production.
3) Memory B cells consist of early pre GC memory cells with lower affinity and enriched for Ig M. On the other hand, post GC memory B cells has higher affinity and expressing Ig M or Ig G.
4) Memory B cells are derived from low affinity B cell receptorssignaling and reduced T cell help.
5) Humoral sensitization without detectable DSA in kidney transplant are due to absorption of DSA by allograft , the loss of shorter lived plasma cells and generation of repertoire of memory B cells.
6) Only patients without DSA and HLA sensitizing consider as immunological low risk for alloimmune memory.
Recall B cell response.
1) Memory B cells upon binding to antigen will either differentiate into plasma cells or differentiate into GC B cells that produce GC B cells that generate new , high affinity and class switched plasma cells.
2) The B cell recall response remain dependant on T cell and co stimulation blockade with CTLA-4Ig.
Key massages
1) De novo DSA remains strong predictor of allograft loss.
2) Following kidney transplant, recipient is exposed to enormous antigenic epitopes .
3) Cellular responses that produce plasma cells .
4) Proper understanding the mechanistic difference between the immune responses to antigens, pathogens and transplant antigens may lead to new ways to control DSA production which maintaining protective immunity.
Terminally differentiated B cell, the plasma cell, is the sole cell type capable of producing antibodies in our body. Over the past 30 years, the identification of many key molecules controlling B cell activation and differentiation has elucidated the molecular pathways for generating antibody-producing plasma cells. Several types of regulation modulating the functions of the important key molecules in B cell activation and differentiation add other layers of complexity in shaping B cell responses following antigen exposure in the absence or presence of T cell help. Further understanding of the mechanisms contributing to the proper activation and differentiation of B cells into antibody-secreting plasma cells may enable us to develop new strategies for managing antibody humoral responses during health and disease. Herein, we reviewed the effect of different types of regulation, including transcriptional regulation, post-transcriptional regulation and epigenetic regulation, on B cell activation, and on mounting memory B cell and antibody responses. We also discussed the link between the dysregulation of the abovementioned regulatory mechanisms and B cell-related disorders.
Despite an emerging appreciation of the limits of DSA in predicting ABMR andsensitization, the appearance of de novo DSA remains a strong predictor of allograft loss . Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression. These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation. Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity .
DSA’s is associated with high risk allograft rejection. The article shows the role of B cell activation in formation of AMR . They showed experimental models on mice by the use of MHC class I & class II immunized into the donor of spleen cell which showed activation of B cell in presence of T cell inside germinal center and formation of donor specific antibody after 7 days with activation of CD40 & CD154. Belatacept shows best control of humoral response .
In renal transplant recipients , B cell exposure to recurrent different antigens and under usual current immunosuppressive agents there is about 15% of transplant kidney develops de novo DSA after 4-6 years post-transplant.
Memory B cell are generated into 2 phase also known as long living plasma cell. Absence of DSA’s doesn’t indicate absence of donor specific memory cell; so good history is important to roll out the causes that may lead to development of DSA’s ; blood transfusion , previous transplants , pregnancy and HDx .
the term “Recall B cell response” means memory B cell with antigen differentiated into GC B cell with higher affinity and class switched plasma cell; memory B cell differentiate into plasma cell and the recall DSA response.
Introduction:
New insights into processes that lead to antibody production upon primary and
secondary antigen encounter are discussed, and the potential implications to DSA
production and future areas of investigation.
Routes of antigen presentation to B cells:
B cell engaged with antigen in order to start the process of differentiating into PCs
producing high affinity antibody and memory B cells.
increasing evidence suggest that optimal B cell activation occurs when the B cell
receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages .
Several strategies exist to increase the opportunities for B cells to encounter soluble and
membrane-bound antigen in the draining lymph nodes:
Mature B cells circulate though the lymph nodes approximately every 24 hours, by
leaving the vascular system and entering the lymph nodes through specialized high
endothelial venules (HEV), migrating along processes extending from follicular dendritic
cell (FDCs) and following the chemokine CXCL13 gradient established by FDCs and
fibroblastic reticular cells (FRCs). Eventually these B cells congregate within the cortical
region near the sub capsular sinus where they may encounter soluble or particulate
antigens that enter the draining lymph node via multiple routes depending on antigen
size, the presence of circulating antigen-specific antibodies, and the deposition of
complement on the antigen by the classical or alternative pathways.
Humoral response can also arise from the spleen:
MZ contains a large reservoir of resident cells that participate in capturing and shuttling
the antigen to FDCs within the B cell follicles and cells that are capable of processing
and presenting the antigen to T CELL.
Early B cell activation and the extrafollicular T/B cell interaction:
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B
cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B
border .
Concurrently, B cells internalize the BCR-bound antigen, process and present the
peptides derived from the antigen on MHC Class II molecules, in preparation for
interaction with antigen-specific T cell receptors on CD4+ T cells.
This cognate interaction between T and B cells initiates the Dependents DSA response.
The activation of T cells requires an initial encounter with migratory DCs that had
acquired antigen from the allograft and migrated via afferent lymph and along the same
FRC network in the lymph node or spleen to reach the T cell zone.
Initial encounter of T cells with cognate antigen presented on DCs result in a subset
acquiring T follicular helper (Tfh) cell properties. How CD4+ T cell fates are determined
has been the subject of a number of investigations and remains incompletely resolved.
Concluded that increasing T cell receptor (TCR) .strength results in more Tfh at the
expense of Th1 cells, consistent with our findings that optimum Tfh responses were
elicited by higher doses of T cell epitopes compared to Th1/Th2 responses.
Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh
cells.
Germinal center response:
extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they
downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase
expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich
lymph in the subcapsular sinus toward the follicle center.
Germinal Center responses to multiple complex antigens:
Initial antigen encounter, B cell clones with different antigenic specificity and affinity
compete for stimulation during rounds of somatic hypermutation within GCs. As a result,
the presence.
When the initial repertoire of circulating antibodies is generated, these antibodies will
bind to the same antigenic epitopes driving their production, resulting either in the
selection of B cells with higher affinity than circulating antibodies, or in the selection of
B cells with specificity for new epitopes.
Regulating the germinal center response:
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator
Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been
described [65; 66; 67]. These T follicular regulatory (TFr) cells express CXCR5, PD-1,
Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by
controlling GC responses.
Generation of memory B cells and PCs:
Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR
affinity [73; 74; 75]. Sciammas and reported that graded expression of interferon
regulatory factor-4 (IRF4) is an early measure of BCR signaling intensity: high levels o
f IRF4 induced Blimp-1 expression, promoted the PC program and shut down the
expression of Bach2 controlling GC B cell fate [78; 79], while modest levels of IRF4
promote differentiation into GC B cells.
Recall B cell responses:
Memory B cells upon antigen reencounter will either differentiate directly into PCs,
generating a faster, high-titer and class-switched recall antibody response compared to
a primary response, or they will differentiate into GC B cells that generate new, higher
‐affinity and class‐switched PCs.
Dogan et al. and Pape et al. Reported that IgM + memory B cells preferentially
differentiated into GC cells, whereas the IgG1+ memory B cells gave rise primarily to
PCs.
As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in
long lived PC depletion and explain the recently described ability of belatacept to control
and reverse DSA responses in sensitized recipient.
Conclusion:
Despite an emerging appreciation of the limits of DSA in predicting ABMR and
sensitization, the appearance de novo DSA remains a strong predictor of allograft loss.
In this review, we have provided a summary of the cellular processes that generate an
antibody response.
B cells play an important role in antibody Mediated rejection AMR , which is a major cause of dysfuntion and organ transplant failure in both acute and chronic setting .
Routes of Antigen presentation to B cells
B cells have to encounter cognate antigen in order to start the process of differentiating into PCs nd memory B cells. Optmal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages
Mature B cells circulate though the lymph nodes approximately every 24 hours, where they may encounter soluble or particulate antigens that enter the draining lymph node via multiple routes. In addition, there may be additional contribution by migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
The humoral response can also arise from the spleen white pulp . The Marginal zone contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells. The routes for antigen delivery to B cells in the spleen and lymph nodes, identified through the study of model antigens or pathogens, are applicable to antigens derived from solid organ transplantation. Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow In addition, natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement. These opsonized antigens are captured by complement receptor 3 (CR3) expressed by subcapsular sinus macrophages (SSM) in draining nodes. Opsonized antigens captured and displayed by the SSM are relayed to non-antigen-specific naïve B cells in the underlying follicles via the complement receptors, (CR2) and (CR1). These B cells then transport opsonized antigens to FDCs. that capture the immune complexes by CR1 binding C3b or C4d . Likewise, opsonins draining into the spleen are captured in a C3d/CR2-dependent manner by MZ macrophages or B cells located just underneath the marginal sinus, which are then delivered to FDC .
Early B cell activation and the extrafollicular T/B cell interaction When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation .B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells. It is notable that while alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA . This cognate interaction between T and B cells initiates the Tdependent DSA response.
Generation of memory B cells and PCs The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
Factors that determine the PC fate during primary antigen encounter have been extensively investigated, with critical roles for BCR affinity and Tfh . Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity .
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG . Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. features of memory B cell versus plasma cell differentiation are consistent with memory B cells being preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later . The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. Indeed, in a recent meeting report by the Sensitization in Transplantation: Assessment of Risk 2017 working group [87], a major finding was that the absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC , the generation of a repertoire of memory B cells that is not identical to the long-lived PC . In the absence of a clinical test of donor specific memory B cells, the recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events such as pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory.
Recall B cell responses Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs. However, the rules that predict memory B cell fate in the recall response remain to be fully clarified. Dogan et al. and Pape et al reported that IgM + memory B cells preferentially differentiated into GC cells, whereas the IgG1+ memory B cells gave rise primarily to PCs. Furthermore, Pape et al reported that the IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies, because these antibodies outcompeted the low-affinity IgM BCR for access to limiting amounts of antigens. Data suggest that the fate of memory B cells upon reencounter with antigens is dependent, in part, on how and type of memory B cell was generated and the conditions of the antigen-reencounter. When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses. Nevertheless, the recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells . As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients .
Conclusion Despite an emerging appreciation of the limits of DSA in predicting ABMR and sensitization, the appearance of de novo DSA remains a strong predictor of allograft loss . Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression. These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation. Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity .
This review summarize thesteps that underlie the primary and recall phases of B cell activation and antibody production. The presence of preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and that de novo DSA generated after transplantation is associated with poor outcomes and vascular obliterative lesions. Quantification of DSA by MFI might not be enough as predictor of sensitization and AMR, so efforts to improve DSA quantification. Absence of donor-specific antibody does not rule out an absence of sensitization or of memory B cells. In the absence of a clinical test of donor specific memory B cells, the STAR recommendation is that an accurate patient history of previous sensitization event and only patients with negative DSAs, , negative sensitizing history like previous blood transfusion , pregnancies , re-transplant should be considered as low immunological risk for allo-immune memory
B cell activation and differention:
Optimal B cell activation dependent on B cell receptor (BCR) affinity to engages intact antigen Displayed on APC (DCs, B cells or macrophages) with the Tfh in addition to the modest levels of IRF4 which further facilitate the differentiation into GC B cells [76; 78]. Within GCs, PC differentiation was induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.
Recall B cell response:
Memory B cells are generated in two distinct phases:
Early pre-GC memory B cells with lower affinity and enriched for IgM,
Late post-GC memory B cells with higher affinity and expressing IgM or IgG [54; 81].
The IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies, because these antibodies outcompeted the low-affinity IgM BCR for access to limiting amounts of antigens.
Memory B cells that express CD80, PD‐L2, and CD73 were mostly IgM+ and were mostly likely to produce PC. The recall response remains dependent on T cell help, and co-stimulation
Blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and
The recall DSA response.
Inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in long-lived PC depletion and this high light the role of the belatocept to control and reverse DSA responses in sensitized recipient.
New insights into the development of B cell responses: Implications for solid organ transplantation.
Introduction:
Antibody mediated rejection AMR is considered a major cause of chronic renal transplant dysfunction and failure after it was clear that presence of preformed DSA is associated with increased risk for acute rejection and that de novo DSA is associated with poor long term outcomes. B cell responses may play an important role in chronic and sustained DSA production after solid organ transplantation.
Routes of antigen presentation to B cells.
There are many routes that B cell can encounter the antigen which will lead to B cell differentiation to Plasma cells which is able to produce antibodies and memory B cells. The optimal way of B cell activation occurs when B cell receptor engages intact antigen displays on Follicular dendritic cells FDCs, B cells or macrophages. This can occur in many ways which includes:
1. In the draining lymph nodes: Mature B cells circulate through the lymph nodes every 24 hours, congregate within cortical region to increase the chances to encounter antigens.
2. In the Spleen. Spleen divided into red and white pulp. White pulp is comprised of T cell zone, B cell follicles and Marginal zone. Marginal zone is between white and red pulp and contains many cells which facilitate in capturing the antigens and present them to B cells follicles which are then process and presents the antigens to T cells.
3. Small antigens (Small soluble proteins ≤14 kD) can be encountered to B cells through passive through gabs in the fibroreticular cells FRCs in the lymph nodes or spleen.
4. Natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement which will lead eventually that opsonized antigens presented to B cells.
5. Antigens secreted and shed from the allograft (alloantigen delivery) can be carried by extracellular vesicles or exosomes released by the allograft and accumulate in the draining lymph node and spleen. Eventually they are able to stimulate recipient CD8 T cells through direct pathway.
Early B cell activation and the extrafollicular T/B cell interaction.
B cell activation occurs after BCR binds to antigen presented by the previously described ways. B cell then internalize the CBR-antigen complex, then it will processed and will presents the MHC class II peptides derived from the antigen. T cells can recognize the antigen in direct way as mentioned or indirectly through B cells. Here CD4 T cells are the only with the ability to be activated directly with antigen-MCHII-peptides presented by B cell, As a result of this interaction T cells will drive the differentiation of recipient B cells into Plasma Cells which are able to produce antibodies (DSA).
T cells can be activated directly by dendritic cells as they are able to process detected antigens and can present them to T cells and induce T cell response directly. This direct encounter of T cells and antigen results in a subset of T follicular helper T Cells (Tfh). Tfh cells are divided into early extra follicular mantel Tfh and Germinal center GC Tfh cells. Finally, B cell priming at the T/B interface results in the generation of early PCs, memory B cells as well as B cells that are destined to migrate back to the follicle to initiate GC responses.
Germinal center response
The GC is divided into two zones. First, the light zone comprising stromal-derived FDCs and Tfh cells. Second, the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 which are able to engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells. Positively selected B cells migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase (AID). In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone. Post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
Belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection. Belatacept was able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
Germinal Center responses to multiple complex antigens
Most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities. How the immune response responds to multiple antigens to culminate in these diametrically opposite outcomes remains unresolved. In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation. The development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells. T follicular regulatory (TFr) cells function by controlling GC responses. They suppress self-reactive GC B cells that may have inadvertently arose during the GC response. Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border. Inhibition at this early stage of B cell response, would prevent further activation of Tfh cells and entry into GCs, as well as B cell differentiation into GC B cells, and post-GC memory B cells or PCs.
Generation of memory B cells and PCs
PC fate during primary antigen encounter have been extensively investigated, with critical roles for BCR affinity and Tfh. Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity. Within GCs, PC differentiation was induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG. Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. Memory B cells being preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later. Successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs. The recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response
Introduction;
The existence of DSA is a risk factor for poor transplant outcomes including acute rejection & graft failure(3). B cells responses that causes Chronic-antibody mediated rejection is not well understood.
Routes of antigen presentation to B cells ;
B cell activation is enhanced after BCR engages intact antigen displayed on Follicular Dendritic Cells(FDC), B cells , or macrophages(5)
Mature B cells passes through lymph node every day and they usually accumulate in the cortical region where they interact with antigens.
The spleen is another organ that produces humeral responses(7)
Small soluble proteins may reach B cells directly by passive flow through Fibroblastic Reticular Cells(FB) conduit or gaps in the sinuses of lyphm nodes and spleen(5)(6)(8)
B cells carries opsonised antigen to follicular dendretic cells taht capture the immune-complexes by CR1 binding C3 or C4d(9)
Free antigen can bind macrophages and PCs of the spleen through innate recognition proteins e.g MBL, Ficolin, CRP.
Donor extracellular vesicles or exosomes is considered new allo-antigen and accumulates in the draining lymph nodes and spleen(13)(14)
Early B cells activation and the extra-follicular T/B cell interface ;
CD4 cells cells provide help for B cells differentiation into PCs producing DSA(20). This is the bases on T cells-dependent DSA response. DC also help to T cells as they process and present the captured antigens to T cells, therefore resulting fast T cell response independent of migratory DCs and the T cells became T follicular helpher(Tfh)(24,25,26). T follicular cells up-regulate CXCR5 & transcription factors(34,35). Priming B cells at T/B interface generates antibody-producing plasma PCs and memory B cells(38).
Germinal Center Response ;
B cells interact with antigens on FDC and receive survival signals from GC-Tfh cells(38)(45)
Positively selected B cells up-regulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase(AID)(44,46). Within GC,following selection, post GC B cells became PCs which generates antibodies and/or a quiescent memory B cells that produces rapid and more vigorous response upon re-exposure the same previous antigen(38).
Anti-CD154 and CTLA-Ig inhibit GC- B cells and prevent increasing DSA in animal models(49).
Belatacept was able to control humeral responses in human by preventing B cells-Tfh interactions and preventing B cells differentiation into PCs(50).
Germinal center responses to multiple complex antigens ;
In transplant setting, recipient B cells usually interact with multiple complex antigens. DSA against class II HLA antigens was seen commonly in pairs with high degree of HLA epitope mismatch(64).
Regulating the germinal center response ;
The key regulatory cells are subsets of CD4 known as T follicular regulatory cells TFr{FOXP3}(66,67). They control GC- response.
They are located in the GC and suppress self-reactive B cells taht may have accidentally came up during GC- response (66).
TFr was found not to have effects in the inhibition of influenza- specific B cells in some studies. Other studies reported the opposite(65,66,67).
Suppression of B cells by Tfr was independent of PD-1 expression and correlated with elevated expression of CD69,IL-10, CTLA-4, GARP which is a protein critical for surface expression of latent TGF-B(71).
Generation of memor cells and plasma cells ;
Differentialtion to plasma cells depends on the grade of BCR affinity which is expressed as interferon-regulatory factor 4 {IFRF4} (73,74,75)
Early pre-GC memory B cells has low affinity(56,81)
Late post-GC memory B cells has high affinity(54,81)
Memmory cells are drived from cells receiving lower-affinity BCR signaling and reduced T cell help which in turn induces reduced IRF-4 and higher Bach2 respectively(82)
Recall B cell responses ;
Memory B cells may differentiate into PCs or GC- B cells when they encounter the antigen. This depend on the conditions of antigen re-encounter.
Recall response also depend on the T cell help.
Co-stimulation blockade completely prevents memory B cell differentiation into PCs and the recall DSA response
Conclusion ;
The presence of DSA is important risk factors for graft loss.
Understanding the process of anti-body production and B cells response are the basics for future selective therapies targeting humoral responses.
Identifying the mechanistic differences between the immune responses to model antigens, pathogens, and transplant antigens may lead to different methods to handle DSA production while preserving protective immunity.
the role of B cells following solid organ
transplantation is antibody mediated rejection
(AMR)
Quantification of donor-specific antibodies (DSA) are an imperfect predictor of AMR, and efforts to improve DSA quantification anticipate that this will result in improved predictive power
antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure
Routes of antigen presentation to B cells
optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages
Mature B cells circulate though the lymph nodes approximately every 24 hours, by leaving the vascular system and entering the lymph nodes through specialized high endothelial venules (HEV), migrating along processes extending from follicular dendritic cell (FDCs) and following the chemokine CXCL13 gradient established by FDCs and fibroblastic reticular cells (FRCs)
The humoral response can also arise from the spleen, which is divided into two distinct compartments: the red pulp comprising a network of venous sinuses that traps old or damaged erythrocytes by red pulp macrophages, and the white pulp involved in the initiation of immune responses (reviewed in
The white pulp is comprised of the T cell zone (also referred to as the periarteriolar lymphoid sheath; PALS), B cell follicles and marginal zone (MZ), which is strategically situated at the interface of the red and white pulp.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins such as natural IgM; C-type lectins such as MBL (mannose binding lectin) and ficolins; and pentraxins, including C-reactive protein.
In addition to antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated reviewed
Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen.
exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages
These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border .
alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA
The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the allograft and migrated via afferent lymph and along the same FRC network in the lymph node or spleen to reach the T cell zone. These DCs arrive between 12–18 hours to 5 days after antigen encounter in the tissues, and it has been argued that such delivery would be asynchronous to the more rapid delivery of antigen to FDCs and B cell activation, and that there has to be another more rapid route of antigen delivery to DCs in the T cell zone.
The initial encounter of T cells with cognate antigen presented on DCs result in a subset acquiring T follicular helper (Tfh) cell properties
How CD4+ T cell fates are determined has been the subject of a number of investigations and remains incompletely resolved independently concluded that increasing T cell receptor (TCR) strength results in more Tfh at the expense of Th1 cells, consistent with our findings that optimum Tfh responses were elicited by higher doses of T cell epitopes compared to Th1/Th2 responses
Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh cells Both Tfh subsets are characterized by the upregulated expression of CXCR5 and the transcription factor, Bcl-6. CXCR5 together with the G-protein-coupled receptor EBI2 promotes the initial localization of mantel Tfh cells towards the B cell follicle and T zone (T/B) interface where they interact with specialized activated DCs expressing inducible co-stimulator (ICOS) ligand and CD25 . Membrane and soluble CD25 quench Tcell-derived IL-2, together with ICOS engagement and IL-21 produced by Tfh cells, promote Bcl-6-dependent Tfh cell differentiation.
B cell priming at the T/B interface results in the generation of early PCs, memory B cells as well as B cells that are destined to migrate back to the follicle to initiate GC responses
The relative importance of the pre-GC response to DSA production following solid organ transplantation, and the contribution of this response to AMR, has not been delineated.
Germinal center response
When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 [43; 44] and gene signatures associated with signaling from the BCR, CD40 and Myc.
Within each GC, and following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter
MHC Class I and Class II tetramers to track the fate of alloreactive B cells in mice immunized with donor spleen cells or following heart allograft transplantation
Within 7 days, donor-specific B cells acquire an activated phenotype (downregulated IgD and upregulated Class II and CD86), and differentiate into GC B cells or PC. The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well documented
In contrast, the necessity of CD28:B7 interactions in sustaining the GC response was less clear. Because there is currently no FDA approved drug that targets the CD40:CD154 interaction, while CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses
delayed treatment with CTLA-4Ig significantly diminished the frequency of memory alloreactive B cells generated, and the recall DSA response upon re-immunization in the absence CTLA-4Ig
The efficacy of CTLA-4Ig in controlling ongoing GC B cell responses and memory B cells in experimental models are congruent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection
Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which is currently more extensively used for kidney transplant recipients, requires further investigation.
Germinal Center responses to multiple complex antigens
For most part, the events shaping affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins, however most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities. Therefore, it is critical that we understand how the B cell and antibody repertoire is shaped following the simultaneous introduction of multiple complex antigens.
the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly
Using a computation method to evaluate HLA epitope mismatch,reported that the development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been described [65; 66; 67]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC responses.
Using multiplexed quantitative imaging of human mesenteric lymph node, and functional assays, Sayin et al. [70] recently reported that the majority of CD3+FOXP3+ Tfr cells expressing CD25 but low levels of CD5, resided at the T/B or GC-mantle borders, with very few located in the GC. The median Tfh/Tfr ratio was 1.3:1; 2:1 and 24:1 at the T-B border, B cell follicle and GC, respectively, leading the authors to suggest an alternative model wherein Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation. Indeed, in a recent meeting report by the Sensitization in Transplantation: Assessment of Risk 2017 working group a major finding was that the absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells. There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs. However, the rules that predict memory B cell fate in the recall response remain to be fully clarified.
When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses
Nevertheless, the recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells
As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients
New insights into the development of B cell responses: Implications for solid organ transplantation
Conventionally, T cells have been implicated as major player in graft rejection. But the data suggesting association of DSA with higher rejection risk and poorer graft outcomes have emphasized on the role of B cells in organ transplant. Cognate antigen on Follicular dendritic cells (FDC), B cells and macrophages act on B cell receptors causing activation of B cells, leading to plasma cell formation which give rise to donor specific antibodies and memory B cells. Mature B cells, via high endothelial venules (HEV), enter lymph nodes and assemble in cortex near subcapsular sinus. Migratory dendritic cells capture antigens and deliver them to lymph nodes. The white pulp of spleen initiates immune response. The marginal zone of white pulp has resident cells which capture antigens and shuttle them to FDCs. Migratory dendritic cells act on T cells in the T cell zone (PALS-periarteriolar lymphoid sheath) leading to T cell activation, but it takes upto 5 days to act. A rapid response is seen with delivery of antigen to T cell zone via Follicular reticular cell (FRC) conduits. The activation of T cells will lead to acquiring follicular helper cell (Tfh) properties which can be either early extrafollicular mantle Tfh and GC (Germinal Centre) Tfh cells. The B cell priming at the T/B interface in spleen leads to formation of early plasma cells, memory B cells and B cells that migrate back to the follicle. The GC has a light zone with FDCs and Tfh cells while the dark zone of GC does not have FDCs. B cells undergo apoptosis in light zone, while dark zone B cells with intact B cell receptor (BCR) reenter light zone where BCR are tested for binding to antigen and T cell help. Differentiation into plasma cells depends on high BCR affinity. Early pre-GC memory B cells, mainly IgM, have lower affinity and later post-GC memory B cells have higher affinity and express both IgM and IgG. Memory B cells, on re-encounter with antigen will either directly form plasma cells leading to a high titre antibody response, or will differentiate into GC B cells generating new higher affinity plasma cells. The review paper provided the summary of the cellular processes generating antibody response.
There is a greater risk of allograft rejection related to the presence of donor-specific antibodies (DSA).
An important role in chronic renal transplant dysfunction and failure is played by the development of Antibody-Mediated Rejection (AMR).
This article focuses on the evidence for and the function of B cell activation in the creation of AMR, which is responsible for the loss of grafts after transplantation.
The following is the mechanism of antigen presentation to the B cell:
The B cell Receptor binds to the intact soluble antigen on the surface of the B cell (BCR).
The B cell is a critical component of the adaptive immune system response.
B cells are produced in the bone marrow and subsequently circulate through the bloodstream and into the spleen.
Inflammatory responses are elicited by the spleen, which is comprised of red pulp and white pulp; the red pulp is responsible for the destruction of undesired RBC by macrophages, and the white pulp is responsible for the destruction of white pulp.
T cells, B cells, and the marginal zone are found in the white pulp, which is responsible for starting the immunological response.
The Marginal Zone has a big cell that is responsible for collecting and transporting antigen to the follicular dendritic cell.
Antigens are obtained through the transplantation of solid organs.
The presence of circulating antigen-specific antibodies as well as the deposition of complement on antigen via a traditional or alternative route are required.
B cells deliver opsonized antigen to follicular dendritic cells, which collect immune complexes by attaching to C3b and C4d on the surface of the CR1.
An antigen that is not coupled to an antibody may bind to macrophages and dendritic cells in the lymph node via the action of innate recognition proteins such as IgM and CRP.
These dendritic cells and macrophages in lymph nodes, together with CD8, M2 metallophilic macrophages, and M2 macrophages in the spleen, capture antigen, and then the antigen displayed on the surface of the cell activates complement, which then delivers the antigen to the CR on follicular dendritic cells via the fallow route of opsonisation.
Memory B cells and PCs are being produced in large quantities.
There are significant disparities between memory development by B cells and memory generation by plasma cells. Because of the affinity and kinetics for BCRs and T follicular helper cells, as well as the expression of IgG and IgM, this type of memory is perpetuated, and it is possible that even successful treatment of acute AMR and reduction of circulating DSA did not prevent the generation of memory donor-specific B cells. Recall the reactions of B cells
Recall the contents of your recollection B cells are projected to develop straight into plasma cells, therefore producing antibodies more quickly than the main response or forming germinal centre B cells, however, there are no regulations governing this process at this time.
Various studies have demonstrated the presence of IgM+ antibodies, while others have demonstrated the presence of IgG+ antibodies, as well as the interaction with certain signal expressions. They have also revealed the need for T cell assistance, as well as the use of CTLA-4Ig antibodies, which are involved with CD28/CD80/CD86. Belatacept, on the other hand, has the ability to regulate and deplete this connection.
New insights into the development of B cell responses: Implications for solid organ transplantation
The pathogenesis of chronic AMR is complex and poorly understood. There is paradigm shift from role of T cells to B cell in AMR.
B cells interact with APC and differentiate into plasma cells and memory cells. Antigens presented by B cells, macrophages and follicular dendritic cells will interact with B cell receptors. B cells migrate from blood stream to lymph nodes through small venules
B cells response will depend on antigen size presented to them . They accumulate in cortex near subcapsular sinus and then they get access to antigens. Antigens can also be transported from blood to lymph nodes by dendritic cells. Splenic white pulp B cells have a role in immune response .
B Cell Activation
Antigens are presented to BCR by APC and leads to B cell activation. Antigens are internalized and interaction happens with CD 4 T cells. These B cells then differentiate into plasma cells which produce DSA. T cell will interact with migrating dendritic cells which bear antigens. Antigens are presented to T cells by macrophages in spleen . Activated T cells gain follicular Helper characters and it will form follicular mantle and germinal centre cell.
Germinal Centre Response
It consist of light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Regulation of Germinal centre response is done by CD4 cells which suppress self reactive germinal B cells.
Generation of memory B cells and Plasma Cells
Memory generation is done through B cells and plasma cells. Affinity and kinetics of T follicular helper as well as BCR and antibody expression leads to this type of memory. Variable affinity of BCR can explain the reason of sensitization in patients without detectable DSAs. Upon re exposure to same antigen B cells develop higher affinity for BCR and convert to plasma cells.
Thank you All
☆ In this review, new insights into processes that lead to antibody production upon primary and secondary antigen encounter, the potential implications to DSA production and future areas of investigation to control AMR are discussed.
INTRODUCTION:
▪︎Antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause
of chronic kidney transplant dysfunction and failure.
▪︎Less is understood of the B cell responses that result in chronically sustained antibody production mediating chronic AMR and transplant failure.
■Routes of antigen presentation
to B cells
▪︎B cells have to encounter cognate antigen in order to start the process of differentiating into Plasma cells (PCs) producing high affinity antibody and memory B cells.
▪︎ B cell activation occurs when the B cell receptor (BCR) engages intact antigen
displayed on FDCs, B cells or macrophages.
▪︎Mature B cells leave the vascular system and enter the lymph nodes through specialized high endothelial venules (HEV), migrate along processes extending from follicular dendritic cell (FDCs) and follow the chemokine CXCL13 gradient established by FDCs and fibroblastic reticular cells (FRCs) every 24 hours and congregate within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens.
▪︎ The humoral response can also arise in the white pulp of the spleen which is comprised of the T cell zone, B cell follicles and marginal zone (MZ) which contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and antigen presenting cells.
▪︎ The routes for antigen delivery to B cells in the spleen and lymph nodes, are applicable to antigens derived from solid organ transplantation.
▪︎Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen. These FRC conduits intersect with FDCs and thus provide direct connection for soluble antigen to be captured and presented on the FDC surface.
▪︎Natural polyreactive antibodies and DSA can bind to soluble antigens and activate complement. These antigens are captured by subcapsular sinus macrophages (SSM) in draining lymph nodes which relayed the opsonized antigens to non-antigen-specific naïve B cells in the underlying follicles which transport opsonized antigens to FDCs. In the same manner, opsonins draining into the spleen are captured by MZ macrophages or B cells located just underneath the marginal sinus, which are delivered to FDCs.
▪︎ Free antigens can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins. These, together with CD8- DCs, MZ metallophilic macrophages and MZ macrophages in the spleen capture antigens. Antigens displayed on the surface of these cells activate complement, and then follow the route of opsonized antigen delivery to CR1 on FDCs.
▪︎ A new type of alloantigen delivery is that donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by the macrophages in the red pulp of the spleen, SCS macrophages and DCs. Alternatively, a relatively limited number of donor DCs migrate from the graft into draining lymph nodes or spleen, where they deliver exosome to resident DCs.
▪︎Donor MHC delivered by exosomes to recipient DCs, results in cross-dressed DCs that have the ability to stimulate recipient CD8+ T cells via the direct pathway. But, recipient APCs cross-dressed with donor MHC II cannot prime efficiently CD4+ T cells.
▪︎The exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages.
▪︎These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone
■Germinal center response:
▪︎ When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they:
1. down regulte T cells release from the
outer follicle
2. repels cells toward the follicle center
3. Tfh cells retention in the GC is regulated.
4. Tfh cell exhaustion is prevented.
5. The ability of the GC-Tfh cells to secrete
IL-12 is preserved.
▪︎The GC-Tfh cells express higher levels of CXCR5, Pdcd1 and IL-21 compared to extrafollicular Tfh.
▪︎ The GC is divided into:
1) The light zone comprising stroma-derived FDCs and Tfh cells.
2) The dark zone, which is devoid of FDCs.
▪︎ The Light zone B cells express the activation markers engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells
▪︎ Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification.
▪︎ In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone.
▪︎ Dark zone B cells with intact BCR downregulate CXCR4 and reenter the light zone where their newly generated BCR are tested for binding to antigen and access to T cell help.
▪︎Following the selection for B cells with high affinity BCR for antigens presented in the GC by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
■The fate of alloreactive B cells:
▪︎Within 7 days, donor-specific B cells acquire an activated phenotype and differentiate into GC B cells or PC. There is interactions between B cells and T cells in the initial activation of T cells with maintenance of the GC response .
■Germinal Center responses to
multiple complex antigens
▪︎ In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens
▪︎The affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins. But, most organ transplantation involves multiple MHC and non-MHC antigen incompatibilities.
▪︎Upon initial antigen encounter, B cell clones with different antigenic specificity and affinity compete for stimulation within GCs. As a result, the presence of many antigenic epitopes tends to reduce the antibody repertoire and the initial affinity of the generated antibodies to multiple antigens would be reduced,.
▪︎ When the initial repertoire of circulating antibodies is generated, they bind to the same antigenic epitopes driving their production, resulting either in the selection of B cells with higher affinity than circulating antibodies.
▪︎The B cell response will terminate with a relatively restricted repertoire if there are no alternative immunogenic epitopes.
▪︎ The development of DSA to Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
■Regulating the germinal center response
▪︎ T follicular regulatory (TFr) cells are located in GCs and control their responses. They suppress self-reactive GC B cells generated during infection.
▪︎ In the absence of Tfr, these selfreactive B cells differentiated into PC that produced anti-histone and anti-nuclear antibodies.
▪︎Some Tfr cells inhibit antigen-specific GC B cells and antibody production.
▪︎Tfr cells regulate antibody responses by preventing long-lived interactions at the T/B border which prevent Tfh cells activation and entry into GCs, as well as B cell differentiation into GC B cells, and post GC memory B cells or PCs.
▪︎TGF-ß production may be the mechanism by which Tfr inhibit B cell responses.
■Generation of memory B cells and PCs
▪︎ B cell differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity.
▪︎Within the GC, PC differentiation is induced in a discrete subset of high-affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.
▪︎Memory B cells are generated in two distinct phases:
1. Early pre-GC memory B cells with lower affinity and enriched for IgM
2. Later post-GC memory B cells with higher affinity and expressing IgM or IgG.
▪︎ Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help.
▪︎Classswitching to membrane IgG1, which signals more efficiently compared to IgM, biased the fate choice to PC over memory fate.
▪︎GC B cells which fail to enter the DZ ( lack CXCR4) are more likely to enter the memory compartment.
▪︎Memory B cells are preferentially generated in the pre-GC and early GC period, and long-lived PC emerging significantly later.
▪︎The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
☆Concerning Sensitization in Transplantation:
– The absence of donor-specific antibody does not equate to an absence of sensitization or of memory B cells
– Humoral sensitization without detectable DSA in transplant recipients, include the absorption of DSA by the allograft, the loss of shorter-lived PC and the generation of a repertoire of memory
B cells that is not identical to the long-lived PC.
▪︎In the absence of a clinical test of donor specific memory B cells, the STAR recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events should be considered to be immunologically low risk for alloimmune memory.
■Recall B cell responses:
The fate of memory B cells upon reencounter with antigens is dependent, in part, on how and type of memory B cell was generated and the conditions of the antigen-reencounter.
▪︎Memory B cells upon antigen reencounter will either differentiate directly into PCs, or they will differentiate into GC B cells.
▪︎IgM+ memory B cells preferentially differentiated into GC cells, whereas the IgG1+ memory B
cells gave rise primarily to PCs. But, memory B cells that express CD80, PD‐L2, and CD73 which are IgM+ produce PC.
▪︎ IgG+ memory B cells predominated over IgM+ memory B cells in the presence of circulating antibodies
▪︎ High‐affinity IgM+ memory B cells rapidly differentiated (≤3 days) into IgM+ and IgG+ PC.
▪︎The recall response remains dependent on T cell help.
▪︎Co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response.
▪︎Inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in long lived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients.
The initiation of B-cell responses need many steps. Initially, recognition of antigens by the B-cell receptor (BCR) and costimulation to triggers intracellular signalling and antigen internalization ,Secondly, processing and presentation of internalized antigens to helper T cells which lead to B-cell activation and of plasma cells generation and antibody secretion
Which consederd a major cause of antibody mediated dysfunction and failure.
The current immunotherapies are failing to control the development of alloantibodies responses, or failing to reverse the production or the effects of the alloantibodies.
B cells is one of the APC – antigen presenting cells , it can bind intact soluble antigen . B cells travel from blood stream to lymph nodes through small venules ,following the chemokine CXCL13 gradient at the end it accumulate in cortex near subcapsular sinus where they encounter the antigen.
Antigen can also be transported from blood to lymph nodes by dendritic cells.
Humoral response can also arise from the spleen specifically in the area of white pulp which is comprised of 1.TCell zone 2.B cell follicles
3.marginal zone
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules,
Alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC.
Germinal Centre Response
It has light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Germinal Center responses to multiple complex antigens :
-In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, & sharing many phenotypic characteristics of Tfh cells, has recently been described. [3; 4; 5]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 & IL-10, but not CD25, & function by controlling GC responses.
Generation of memory B cells and PCs.
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG .
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
Recall B cell responses
Upon re-exposure to antigen, memory B cells differentiate into either PCs or GC B cells. Compared to primary response, the PCs produce a faster, high titer & class-switched recall antibody response, while the GC B cell generates new PCs that have higher‐affinity & are class‐switched. Data suggest that the fate of memory B cells upon re-exposure to antigens is dependent, in part, on how & type of memory B cell was generated & the conditions of the antigen-reencounter.
* Role of B cells following solid organ transplantation is being driven by clinical data suggesting that antibody mediated rejection (AMR) is a major cause of dysfunction and organ transplant failure.
* The current immunotherapies are failing to control the development of allo anti body responses, or failing to reverse the production or the effects of the alloantibodies.
* Routes of antigen presentation to B cells:
– B cells have to encounter antigen to start the process of differentiating into :
# PCs producing high affinity antibody #Memory B cells.
* B cells can bind intact soluble antigen, also optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
*B cell encounter solubl and membrane bound antigen in the draining lymph nodes.
* Mature B cells circulate though the
lymph nodes every 24 hours, by leaving the vascular system and entering the
lymph nodes through specialized high endothelial venules (HEV) in response to chemokine CXCL13 established by FDCs and fibroblastic reticular cells (FRCs) in the cortical region near the subcapsular sinus where they may encounterantigen. The presence of circulating antigen specific antibodies lead to complement activation (classical and alternative) pathways.
* The humoral response can also arise from the spleen, which is divided into two distinct compartments:
– Red pulp traps old or damaged erythrocytes by red pulp macrophages .
-White pulp involved in the initiation
of immune responses is comprised of the (T cell zone, B cell follicles and marginal zone MZ).
* Early B cell activation and the extrafollicular T/B cell interaction:
– When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, internalize the BCR-bound antigen,process and present the peptides derived from the antigen on MHC Class II molecules for interaction with antigen-specific T cell receptors on CD4+ T cells.
– Alloreactive T cells can directly recognize intact donor MHC or indirectly recognize processed donor MHC presented on recipient MHC, only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells intoPCs producing DSA . This cognate interaction between T and B cells initiates the T dependent DSA response.
* Germinal center response:
– The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells.
– Dark zone, which is devoid of FDCs. Light zone B cells express the activation markers,
– CD86 and CD83 a signatures associated with signaling from the BCR, CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive
survival signals from GC-Tfh cells
provided by the interaction between costimulatory molecules, Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation and BCR diversification mediated by activation-induced cytidine deaminase (AID).
– In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes undergo apoptosis in the dark zone.
– Dark zone B cells with intact BCR downregulate CXCR4 and reenter the light zone where their newly
generated BCR are tested for binding to antigen and access to T cell help.
– Following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that responsible for persistent circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response.
– The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well documented, but the rule of CD28:B7 interactions in sustaining the GC response was less clear, because there is currently no FDA approved drug that targets the CD40:CD154 interaction, while CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection.
– It is important to test the effect
of CTLA-4Ig on established B cell responses, since it may promote humoral responses .
– Delayed treatment with CTLA-4 significantly diminished the frequency of memory alloreactive B cells generated, and the recall DSA.
– Controlling ongoing GC B cell responses and memory B cells in experimental models are congruent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection.
– Belatacept was able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
– Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which iscurrently more extensively used for kidney transplant recipients, requires further investigation.
* Germinal Center responses to multiple complex antigens:
– For most part, the events shaping affinity maturation within the GC is based on the analysis following immunization with single haptens or small proteins.
– Study suggested that upon initial antigen encounter, B cell clones with
different antigenic specificity and affinity compete for stimulation during roundd ofsomatic hypermutation within GCs.
– As a result, the presence of many antigenic epitopes tends to reduce the relative breadth of the antibody repertoire.
– Initial affinity of the generated antibodies to multiple antigens would be reduced, due to a reduction in the number of GCs and B cells available for each epitope, which compromises affinity selection.
– When the initial repertoire of circulating antibodies is generated, these antibodies will bind to the same antigenic epitopes driving their production, resulting either :
in the selection of B cells with higher affinity than circulating antibodies
or in the selection of B cells with specificity for new epitopes.
– In the majority of kidney transplant recipients, B cells will most likely encounter multiple complex antigens.
– Development of antibodies directed at donor Class II was most frequent in renal transplantation recipients with a higher degree of HLA epitope mismatch.
– Regulating the germinal center response subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, and sharing many phenotypic characteristics of Tfh cells, has recently been described.
These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1,
FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC
responses.
* Generation of memory B cells and PCs:
– The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
– Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity
– Graded expression of interferon
regulatory factor-4 (IRF4) is an early measure of BCR signaling intensity
High levels of IRF4 induced Blimp-1 expression, promoted the PC program and shut down the expression
of Bach2 controlling GC B cell fat, while modest levels of IRF4 promote
differentiation into GC B cells .
Memory B cells are also generated in two distinct phases:
early pre-GC memory B cells with
lower affinity and enriched for IgM
and later post-GC memory B cells with higher affinity and expressing IgM or IgG .
-Successful treatment of acute AMR and reduction of circulating DSA may not have
prevented memory donor-specific B cell generation.
– The absence of donor-specific antibody does not equate to an absence of
sensitization or of memory B cells.
There may be other explanations for humoral sensitization without detectable DSA in transplant recipients, including the absorption of DSA by the allograft, the loss of shorter-lived PC the generation of a repertoire of memory B cells that is not identical to the long-lived PC.
– In the absence of a clinical test of donorspecific memory B cells, the STAR recommendation is that an accurate patient history be obtained, and only patients without DSA and also without HLA sensitizing events such as
pregnancies, transfusions, previous transplantation and implants should be considered to be immunologically low risk for alloimmune memory.
* Recall B cell responses:
– Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a
primary response
or they will differentiate into GC B cells that generate new, higher‐affinity
and class‐switched PCs.
* Conclusion
Despite an emerging appreciation of the limits of DSA in predicting ABMR and
sensitization, the appearance de novo DSA remains a strong predictor of allograft loss.
– summary of the cellular processes that generate an antibody response.
– Following solid organ transplantation, the recipient is exposed to an enormous diversity of antigenic epitopes, large amounts of antigens that are persistent, as well as pharmacological immunosuppression.
– These factors will impact on the cellular responses that generate PCs, and the quality of the DSA they produce, as well as quality of memory donor-specific B cells, all of which remain formidable barriers to successful transplantation.
– Finally, defining the mechanistic differences between the immune responses to model antigens, pathogens and transplantation antigens may lead to new ways to control DSA production while preserving protective immunity.
–
Routes of antigen presentation to B cells:
B cells have to encounter cognate antigen to start the process of differentiation into plasma cells and memory cells. Optimal B cells activation occurs when the B cell receptor engages intact antigen displayed on FDCs, B cells, or macrophages. Several strategies exist to increase the opportunities for B cells to encounter soluble and membrane-bound antigens in the draining lymph nodes. Mature B cells circulate through the lymph nodes every 24 hours, eventually, they gathered within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens that enter the lymph node via multiple routes, in addition, DCs may encounter antigens at the tissue site and transport them to the lymph node.
The humoral response can also arise from the spleen, particularly the white pulp which is comprised of the T cell zone, B cell follicles, and the marginal zone( MZ). The MZ contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cells follicles and cells that are capable of processing and presenting the antigen to T cells.
Early B cell activation and the extrafollicular T/B cell interaction:
When BCR binds to an antigen, B cells activation and migration to the B/T cell border will happen. B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells( indirect allorecognition). the interaction between B & T cells initiates the T cell-dependent DSA response.
Germinal center response:
Consist of light and dark zones. The light zone contains FDCs and Tfh while the dark zone is devoid of FDCs.
Light zone B cells express CD86 and CD83.
Germinal center responses to multiple complex antigens:
How the immune system responds to multiple antigens remains unresolved, and could be due to the accessibility of binding sites on the antigen, stochastic founding events, genetic predisposition affecting the T helper response, and chance mutations during affinity maturation.
Regulating the germinal center response:
A subset of CD4+ T cells that express the regulatory T cell master regulatory foxp3 and sharing characteristics of Tfh cells has been described. These T follicular regulatory cells function by controlling the germinal center.
Generation of memory B cells and PCs:
Differentiation into PCs at the T/B border and within the germinal center is facilitated by high BCR affinity.
Within the GC, PC differentiation was induced in a discrete subset of high affinity B cells residing within the light zone upon BCR engagement with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for completing the PC differentiation and driving their migration out of the GC.(according to Sciammas et al.).
Memory B cells are also generated in to 2 distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG.
Relative to PCs, memory B cells are derived from cells receiving lower-affinity and reduced T cells help.
The differences in affinity and kinetics of memory B cell versus PC suggest that successful treatment of acute AMR and reduction of DSA level may not prevent memory B cell generation.
Recall B cell response:
After encountering antigen, memory B cells will either differentiate into PCs generating a faster and high titer antibody response or they will differentiate into GC B cells that generate newer, high affinity, and class PCs.
The recall response remains dependent on T cell help and co-stimulation blockade with CTLA-4Ig completely prevented memory B cells differentiation into PCs and the recall DSA response.
· B cells play a crucial role in both active AMR and chronic AMR and eventually graft loss.
· B cell differentiate into antibody producing plasma cells and memory B cell which produce stronger and more rapid immune response on re-exposure to same antigen with higher titers and binding affinity antibodies formation.
· Antigens are presented by antigen presenting cells either (macrophages, B cells or dendritic cells) on their MHC class II molecules, then interaction with T cell receptors present on T helper (CD4+ T cells)
· The interaction between B and T cells occur through costimulatory pathway leading to B cell activation and formation of plasma cells that produce antibody (DSA) and T helper cell activation. This is mediated by interaction between:
o CD 40: CD 154 on B cells & T cells respectively in the initial activation of T cells. No available FDA approved drug to inhibit this costimulation signal.
o CD80: CD 28 on B cell and T cell respectively. However, interaction depending on CTLA 4 was implicated in clinical practice by use of belatacept (anti CTLA4 Ig) as costimulation inhibition to prevent further production of DSA and decrease generation of memory B cells.
· Circulating antibodies (denovo DSA) can be produced against mismatched epitopes. New high-resolution techniques to detect this submicroscopic mismatch may improve long term graft outcome.
· The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not prevent memory B cell generation.
*Routes of antigen presentation to B cells :
Activation of B cell occur when B cell receptors engaged antigen displayed on FDCS , B cell or macrophages.
*Mature B cells circulate in lymph nodes every 24 hours through specialised high endothelial venules.
*The presence of circulating B cells antigen specific antibodies and complement deposition on antigen affect B cells aggregation with cortical region near the subcapsular sinus which meet soluble or particular antigen that enter draining lymph nodes via multiple routes .
*Spleen generate humoral response which differentiate into two compartment :
1- Red pulp is full of venous sinus trap damaged erythrocytes by red pulp macrophages.
2- White pulp : consists of >>>
* T cell zone.
* B cell follicles.
* Marginal zone .
Marginal zone has large number of residents cells which capture and shuttle of antigen to FDCs within B cell follicles.
Natural poly reactive antibodies and DSA in naiive and sensitised recipients bind to soluble antigens and activate complement.
Antigen which not binds to antibody , binds to macrophages and DCs located in the lymph nodes by recognising by innate protein as natural IgM , C-type lectins as MBL , ficolins and pentraxins including CRP.
*Early B cell activation and te exrafollicular T/B cell interaction :
-B cells internalize the BCR- bound antigen, process and present peptides drived from the antigen on MHC class II molecules to interact with antigen specific T cell receptors on CD4+ T cell.
-The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the allograft and migrated via afferent lymph and along the same FRC network in the lymph node or spleen to reach the T cell zone.
-Tfh cells can be divided into two subsets: early extrafollicular mantel Tfh and GC Tfh cells . Both Tfh subsets are characterized by the upregulated expression of CXCR5 and the transcription factor, Bcl-6. CXCR5 together with the G-protein-coupled receptor EBI2 promotes the initial localization of mantel Tfh cells towards the B cell follicle and T zone (T/B) interface.
*Germinal Center response :
-When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they downregulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
**Germinal Center responses to multiple complex antigens :
-In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal
transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly.
**Regulating the germinal Center response:
Early studies utilizing adoptive transfer or mixed bone marrow chimeras showed that Tfr are located in the GC, and that they suppress self-reactive GC B cells that may have inadvertently arose during the GC response .
*Using multiplexed quantitative imaging of human mesenteric lymph node, and functional
assays, Sayin et al. [70] recently reported that the majority of CD3+FOXP3+ Tfr cells expressing CD25 but low levels of CD5, resided at the T/B or GC-mantle borders, with very few located in the GC.
**Generation of memory B cells and PCs :
The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
*Within GCs, PC differentiation was induced in a
discrete subset of high-affinity B cells residing within the light zone upon BCR engagement
with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for
completing the PC differentiation and driving their migration out of the GC.
*The differences in affinity and kinetics of memory B cell versus PC generation suggest that
successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
**Recall B cell response:
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating
a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class-
switched PCs.
*When memory alloreactive B cells in sensitized murine recipients reencounter alloantigen
following heart transplantation, they generate a recall DSA response that is largely dependent on the rapid differentiation into PC, with minimal GC responses.
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The pathogenesis of chronic AMR is complex and poorly understood. There is paradigm shift from role of T cells to B cell in AMR. This article highlights and gives new insight in role of B cell in AMR. This paper highlights the activation of B cells and subsequent development of Plasma cells which produce DSA and memory cells which will lead to severe response to re exposure
Routes of Antigen Presentation to B cells
B cells interact with APC – antigen presenting cells and differentiate into plasma cells and memory cells. Antigens presented by B cells, Macrophages and Follicular dendritic cells will interact with B cell receptors. B cells travel from blood stream to lymph nodes through small venules ,
B cells response will depend on antigen size delivered to them . They accumulate in cortex near subcapsular sinus and then they get access to antigens. antigens can also be transported from blood to lymph nodes by dendritic cells. Splenic white pulp B cells have a role in immune response .
B Cell Activation
Antigens are presented to BCR by APC and leads to B cell activation. Antigens are internalized and interaction happens with CD 4 T cells. These B cells then differentiate into plasma cells which produce DSA. T cell will interact with migrating dendritic cells which bear antigens. Antigens are presented to T cells by macrophages in spleen . Activated T cells gain follicular Helper characters and it will form follicular mantle and germinal centre cell.
Germinal Centre Response
It has light and dark zones for dendritic cells and T follicular helper cells respectively. Interaction between dendritic cells B cells and CD4 T Cells and T follicular helper cells will down regulate or upregulate the immune response.
Regulation of Germinal centre response is done by CD4 cells which suppress self reactive germinal B cells.
Generation of memory B cells and Plasma Cells
Memory generation is done through B cells and plasma cells. Affinity and kinetics of T follicular helper as well as BCR and antibody expression leads to this type of memory. Variable affinity of BCR can explain the reason of sensitization in patients without detectable DSAs. Upon re exposure to same antigen B cells develop higher affinity for BCR and convert to plasma cells.
New insights into the development of B cell responses: Implications for solid organ transplantation
This paper is a review article on B cell responses and how these relate to solid organ transplantation.
There is now new data which is re-emphasizing the major the role of antibody mediated rejection (AMR) in chronic graft dysfunction and is shifting current thinking from the previously help ideas of a more superior or central role of T cells.
The mechanisms underlying chronic antibody mediated rejection are poorly understood, but more information being gained from research is helping to provide more insight into this phenomenon.
Routes of antigen presentation to B cells.
The interaction of B cells with antigen presenting cells is important for their differentiation into plasma cells and memory cells.
The B cell receptor (BCR) interacts with antigens presented by profession antigen presenting cells (APC) like Follicular dendritic cells (FDC), B cells and Macrophages.
Mature B cells make their way from the blood stream to lymph nodes through endothelial venules and follow a CXCL13 chemokine gradient established by follicular dendritic cells and fibroblast reticular cells (FRC).
B cells encounter antigen delivered to them depending on antigen size;
By concentrating in the cortical area near the subscapular sinus they are able to access soluble antigens.
Migrating dendritic cells also participate in transporting antigens from the blood to lymph nodes.
The white pulp of the spleen also participates in the immune. In this white pulp B cells reside in the B cell follicles and marginal zone, which is strategically located at the interface between red and white pulp to access large antigens.
B cell activation
B cell activation follow interaction of the BCR with antigens displayed on antigen presenting cells. Antigen internalization into B cells is followed by interaction with CD4+ T cells. B cells which have interacted with CD4+ T cells differentiate into plasma cells capable of producing donor specific antibodies (DSA).
T cells are located in the periarteriolar sheath (PALS), interact with migratory dendritic cells which bring in allograft antigens from the blood stream. In the spleen resident macrophages process antigen, which they present to T cells and this leads to a more rapid response than that produced by migratory macrophages.
The activated T cells acquire T follicular helper (Tfh) properties which form early follicular mantle and germinal centre Tfh cells.
Germinal center response
The germinal centre comprises of the light and dark zone.
The light zone has follicular dendritic cells and T follicular helper cells. For apoptosis of B cells
The dark zone has CD86 and CD83 B cells. B cells selected proliferate and develop BCR specificity.
Generation of memory B cells and PCs
Memory cells and plasma cells play an important role in infection control and in generating memory for protection against future infection.
The high affinity of BCR favours formation of plasma cells promoted by regulatory factor-4 (IRF4) while T follicular helper cells provide the final signal for differentiation.
Memory cells are produced form B cells which receive less T cell help and have lower BCR affinity.
This difference in affinity of the BCR between plasma cells and memory cells explains why patients without detectable DSA may still be sensitized and may have memory cells to that exposure.
When memory cells are re-exposed to the same antigen, they may either differentiate into plasma cells or differentiate into germinal center B cells to develop higher affinity of the BCR and become plasma cells.
To sum it all up, this paper highlights are complex and yet important process of B cell activation and subsequent development of plasma cells that produce DSA as well as memory cells that lead to a more rapid and aggressive immunological response on re-exposure to the same antigen.
New insights into the development of B cell responses: implications for solid organ transplantation
Antibody mediated rejection is major cause of graft dysfunction and ultimately graft failure.Attempts to control ABMR have focused on non specific elimination of B cells , plasma cells or circulating antibodies.
In this review processes that lead to antibody production upon primary and secondary antigen encounter along with future areas of investigation to control AMR are discussed.
B cell activation occurs when the B cell receptors (BCR) engages intact antigens displayed on FDCs, B cells or macrophages. Mature B cells circulate through lymph node approximately every 24 hrs these B cells congregate within the cortical region where they encounter the soluble antigen entering the draining lymph nodes through different routes.
The humoral response can also arise from spleen . The routes for antigen delivery to B cells in spleen and lymph nodes are applicable to antigens derived from solid organ transplantation.
Small soluble proteins such as those secreted by transplanted organs may gain direct access to the B cells via lymph nodes or spleen.
When the BCR binds to antigen it triggers B cell activation concurrently B cells internalize the BCR bound antigen process and interacts with antigen specific T cell receptors on CD4 cells.This interaction between T and B cell initiates the T cell dependent DSA response. B cell priming at the T/B interface results in generation of early plasma cells , memory B cells as well as B cells which migrate back to follicle to initiate Germinal center response. The relative importance of the pre GC response to DSA production following solid organ transplant and the contribution of this response to AMR has not been delineated.
Germinal center response
Within each GC, & following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as either PCs or quiescent memory cells. The former are responsible for persistent circulating antibodies, while the latter are responsible for the recall antibody response upon re-exposure to antigen.
CTLA-4Ig is an FDA agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses.
reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection.
Belatacept was found to able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC.
Germinal center response to multiple complex antigens
In majority of kidney transplant recipients , recipients B cell s will encounter multiple complex antigens and approximately 15% of renal transplant recipients develop de novo DSA within 4.6 yrs. TGF beta production may be the mechanism by which Tfr inhibit B cell response.
Memory B cells are generated in 2 distinct phases: early pre-GC memory B cells with lower affinity BCR & enriched for IgM, & later post-GC memory B cells with higher affinity & expressing IgM or IgG. The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
Possible explanations for humoral sensitization without detectable DSA in transplant recipients include: absorption of DSA by the allograft, the loss of shorter-lived PC, the generation of memory B cells that are not identical to the long-lived PC.
Recall B cell response
Memory B cell upon antigen re exposure will either differentiate into PC or GC B cell that generate new higher affinity and class switches PC. The fate of memory cells upon recall still needs to be clarified.
II. New insights into the development of B cell responses: Implications for solid organ transplantation
Anita S. Chong Department of Surgery, The University of Chicago, Chicago, IL
Introduction
B cells play a major role in both AMR & chronic kidney transplant dysfunction & failure. The current immunosuppressive medications fail, in subset of transplant patients, to reverse the production or the effect of antibodies.
Improving our understanding of the role of B cells & its differentiation into memory B cells & antibody-secreting plasma cells (PCs) will give new ways to more specifically target the DSA response & thus improve long-term allograft survival.
This review discusses the processes that lead to antibody production & its implications to DSA production & future areas of investigation to control AMR.
Routes of Antigen presentation to B cells
B cells should bind to an antigen before starting the differentiation into PCs, that produce high affinity antibody, & memory B cells.
B cells encounter soluble & membrane-bound particulate antigen in the draining lymph nodes &also in the spleen. Mature B cells circulate though the lymph nodes every 24 hours. Antigens, enter the draining lymph node via multiple routes depending on their size, the presence of circulating antigen-specific antibodies, & the deposition of complement.
The routes for antigen delivery to B cells in the spleen & lymph nodes, were studied in model antigens, but are applicable to antigens derived from solid organ transplantation.
Early B cell activation & the extrafollicular T/B cell interaction
B cell activation starts when its BCR binds to antigen presented on FDCs. The BCR-bound antigen is then internalized & processed. The peptides thus derived from the antigen on MHC Class II molecules interact with antigen-specific TCRs on CD4+ T cells.
Alloreactive T cells can directly or indirectly recognize intact donor MHC or processed donor MHC presented on recipient MHC, respectively. But only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA. This interaction between T & B cells initiates the T-dependent DSA response.
On binding antigen presented on DCs, a sub-set of T cells differentiate into T follicular helper (Tfh) cells. It is not well known how CD4+ T cell fates are determined, but it is clear that the differentiation into Tfh cells require specific signals derived from the TCR, co-stimulation & inflammatory milieu that are different from those that drive differentiation into the other CD4+ T effector cell lineages.
Understanding these differences may lead to identify ways to more precisely prevent & treat AMR versus T cell-mediated rejection.
B cell priming at the T/B interface results in the generation of early PCs, memory B cells & B cells that migrate back to the follicle to initiate germinal centre (GC) responses. These pre-GC responses generate PCs that are of lower affinity than post-GC PCs.
The relative importance of the pre-GC response to DSA production following solid organ transplantation, & the contribution of this response to AMR needs to be resolved.
Germinal center responses:
Within each GC, & following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as either PCs or quiescent memory cells. The former are responsible for persistent circulating antibodies, while the latter are responsible for the recall antibody response upon re-exposure to antigen.
The importance of CD 40:CD 154 interactions between B cells & T cells in the initial activation of T cells & in the maintenance of the GC response is well documented.
Since there is no currently FDA approved drug targeting the CD40:CD154 interaction, while CTLA-4Ig is an FDA agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses.
Chen et al. [1] compared the ability of anti-CD154 & of CTLA-4Ig to reverse established GC response in mice. Both anti-CD154 & CTLA-4Ig reduced GC B cell responses & prevented further DSA increase. Delayed treatment with CTLA-4Ig also significantly decreased the frequency of memory alloreactive B cells generated, & the recall DSA response upon re-immunization in the absence CTLA-4Ig.
These experimental model reports are consistent with reports of belatacept, a high affinity mutant of CTLA-4Ig, preventing DSA development in kidney transplant recipients despite high rates of acute rejection.
Belatacept was found to able to control humoral responses in humans by inhibiting B cell-Tfh interactions, thereby preventing B cell differentiation into PC. [2]
Whether this superiority of belatacept in controlling humoral responses compared to cyclosporine, will be maintained when compared to tacrolimus, which is currently more extensively used for kidney transplant recipients, requires further investigation.
Germinal Center responses to multiple complex antigens
It remains unresolved how the immune system responds to multiple antigens leading to these variable outcomes. Childs et al. , using computational modeling methods , suggested that upon initial exposure to antigen, B cell clones with different antigenic specificity & affinity compete for stimulation during rounds of somatic hypermutation within GCs. The resulting presence of many antigenic epitopes leads to narrowing of the breadth of the antibody repository.
Their model also predicted ‘affinity selection’ ; meaning that the initial affinity of the produced antibodies to multiple antigens would be reduced, due to a reduction in the number of GCs & B cells available for each epitope.
Initially produced circulating antibodies will bind to the same antigenic epitopes, resulting either in the selection of B cells with higher affinity than circulating antibodies, or in the selection of B cells with specificity for new epitopes. Thus circulating antibodies may increase the avidity, but either increase or decrease the breadth of the antibody reserve.
In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Nearly 15% of renal transplant recipients develop de novo DSA within 4.6 years, some of them responding with a limited DSA repertoire while others more broadly.
Wiebe et al. reported that the development of antibodies directed at donor Class II was most frequent in recipients with a higher degree of HLA epitope mismatch.
Understanding the process of selection of alloantibody in response to multiple antigen-mismatched allografts, their evolve over time, & the impact of immunosuppression on these process, will result in a better understanding of when pathogenic versus non-pathogenic DSA develops.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3, & sharing many phenotypic characteristics of Tfh cells, has recently been described. [3; 4; 5]. These T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 & IL-10, but not CD25, & function by controlling GC responses.
These, & many other observations led the authors to suggest that TGF-ß production may be the mechanism by which Tfr inhibit B cell responses.
Generation of memory B cells and PCs
Appropriate & rapid generation of PCs is required for the successful control of infection, & of memory cells & long-lived PCs for the protection against re-infection. BCR affinity & Tfh have critical roles in deciding the fate of PC during primary exposure to antigen. High BCR affinity facilitate differentiation into PCs at the T:B border & within GCs.
Memory B cells are generated in 2 distinct phases: early pre-GC memory B cells with lower affinity BCR & enriched for IgM, & later post-GC memory B cells with higher affinity & expressing IgM or IgG.
These features of memory B cell versus PCs differentiation are consistent with memory B cells being preferentially generated in the pre-GC & early GC period, & long-lived PC emerging significantly later.
The differences in affinity & kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR & reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
In a recent meeting report by the Sensitization in Transplantation: Assessment of Risk (STAR) 2017 working group, a major finding was that the absence of DSA does not mean an absence of sensitization or of memory B cells.
Possible explanations for humoral sensitization without detectable DSA in transplant recipients include: absorption of DSA by the allograft, the loss of shorter-lived PC, the generation of memory B cells that are not identical to the long-lived PC.
The STAR recommendation, in the absence of a clinical test of donor-specific memory B cells, is that an accurate history be obtained, & only patients without DSA & also without HLA sensitizing events such as pregnancies, transfusions, previous transplantation & implants should be considered to be immunologically low risk for alloimmune memory.
Recall B cell responses
Upon re-exposure to antigen, memory B cells differentiate into either PCs or GC B cells. Compared to primary response, the PCs produce a faster, high titer & class-switched recall antibody response, while the GC B cell generates new PCs that have higher‐affinity & are class‐switched. Data suggest that the fate of memory B cells upon re-exposure to antigens is dependent, in part, on how & type of memory B cell was generated & the conditions of the antigen-reencounter.[6,7,8,9]
Conclusion
In spite of the emerging limits of DSA role in predicting ABMR & sensitization, de novo DSA remains a strong predictor of allograft loss [10; 11; 12].
This review summarized the cellular processes that generate an antibody response & humoral memory.
The study of these processes was in model antigens in reductionist mouse models. Further investigation is needed to know how this would apply to human solid organ transplantation.
The recipient is exposed to a huge number of antigenic epitopes, antigens that are persistent, as well as immunosuppressive medications. These factors impact the cellular responses that generate PCs, & the quality of the DSA they produce, as well as quality of memory donor-specific B cells; all these factor form barriers to successful transplantation.
More understanding of the mechanistic differences between the immune responses to model antigens & transplantation antigens may pave the road to new means to help control DSA production without compromising protective immunity of the host.
References
[1]. Chen J, Yin H, Xu J, Wang Q, Edelblum KL, Sciammas R, and Chong AS, Reversing endogenous alloreactive B cell GC responses with anti-CD154 or CTLA-4Ig. Am J Transplant 13 (2013) 2280–92. [PubMed: 23855587]
[2]. Leibler C, Thiolat A, Henique C, Samson C, Pilon C, Tamagne M, Pirenne F, Vingert B, Cohen JL, and Grimbert P, Control of Humoral Response in Renal Transplantation by Belatacept Depends on a Direct Effect on B Cells and Impaired T Follicular Helper-B Cell Crosstalk. J Am Soc Nephrol 29 (2018) 1049–1062. [PubMed: 29321143]
[3]. Chung Y, Tanaka S, Chu F, Nurieva RI, Martinez GJ, Rawal S, Wang YH, Lim H, Reynolds JM, Zhou XH, Fan HM, Liu ZM, Neelapu SS, and Dong C, Follicular regulatory T cells expressing Foxp3 and Bcl-6 suppress germinal center reactions. Nat Med 17 (2011) 983–8. [PubMed: 21785430]
[4]. Linterman MA, Pierson W, Lee SK, Kallies A, Kawamoto S, Rayner TF, Srivastava M, Divekar DP, Beaton L, Hogan JJ, Fagarasan S, Liston A, Smith KG, and Vinuesa CG, Foxp3+ follicular regulatory T cells control the germinal center response. Nat Med 17 (2011) 975–82. [PubMed: 21785433]
[5]. Wollenberg I, Agua-Doce A, Hernandez A, Almeida C, Oliveira VG, Faro J, and Graca L, Regulation of the germinal center reaction by Foxp3+ follicular regulatory T cells. Journal of immunology 187 (2011) 4553–60.
[6]. Pape KA, Taylor JJ, Maul RW, Gearhart PJ, and Jenkins MK, Different B cell populations mediate early and late memory during an endogenous immune response. Science 331 (2011) 1203–7. [PubMed: 21310965]
[7]. Dogan I, Bertocci B, Vilmont V, Delbos F, Megret J, Storck S, Reynaud CA, and Weill JC, Multiple layers of B cell memory with different effector functions. Nature immunology 10 (2009) 1292–9. [PubMed: 19855380]
[8]. Zuccarino-Catania GV, Sadanand S, Weisel FJ, Tomayko MM, Meng H, Kleinstein SH, Good-Jacobson KL, and Shlomchik MJ, CD80 and PD-L2 define functionally distinct memory B cell subsets that are independent of antibody isotype. Nature immunology 15 (2014) 631–7. [PubMed: 24880458]
[9]. Krishnamurty AT, Thouvenel CD, Portugal S, Keitany GJ, Kim KS, Holder A, Crompton PD, Rawlings DJ, and Pepper M, Somatically Hypermutated Plasmodium-Specific IgM(+) Memory B Cells Are Rapid, Plastic, Early Responders upon Malaria Rechallenge. Immunity 45 (2016) 402–14. [PubMed: 27473412]
[10]. Wiebe C, Gibson IW, Blydt-Hansen TD, Pochinco D, Birk PE, Ho J, Karpinski M, Goldberg A, Storsley L, Rush DN, and Nickerson PW, Rates and Determinants of Progression to Graft Failure in Kidney Allograft Recipients With De Novo Donor-Specific Antibody. Am J Transplant 15 (2015) 2921–30. [PubMed: 26096305]
[11]. Cooper JE, Gralla J, Cagle L, Goldberg R, Chan L, and Wiseman AC, Inferior kidney allograft outcomes in patients with de novo donor-specific antibodies are due to acute rejection episodes. Transplantation 91 (2011) 1103–9. [PubMed: 21403588]
[12]. Ho EK, Vlad G, Vasilescu ER, de la Torre L, Colovai AI, Burke E, Deng M, Schwartz J, Marboe C, Mancini D, Opelz G, and Suciu-Foca N, Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival. Hum Immunol 72 (2011) 5–10. [PubMed: 20971146]
Thank you Dr Mohamed
You do not need to write references when you answer questions in the Journal Club. We know that we are referring the the given article.
OK
Thanks a lot dear Prof Halawa
ARTICLE SUMMARY
This article serves as a bridge between the past and present understanding of the pathology behind transplant outcomes. It also aims to shed light on future areas of investigations that would prove to be fruitful in the development of improved and better transplant outcomes.
Our knowledge on transplants and related issues have been growing over the past few decades. While T cells were mainly blamed previously for poor transplant outcomes, now there are other concepts and principles that challenge this point of view. Pre-formed DSAs and de novo DSA development have been recognized to have a major role in graft rejection and poor outcome. AMR is a major challenge since it potentially leads to failure of graft with a significant impact on patient mortality. Although we have moved from focussing on T cells as the major culprits, immunosuppressive regimens that are currently employed are far from perfect. This is because ABMR control is now attempted to be achieved by eliminating B cells and plasma cells, circulating antibodies. This article suggests that better understanding of B cell differentiation might help significantly in developing better sustainable long term allograft outcomes while at the same time preserving the immunity of the recipient in order to protect them from ancillary disease and infection. With this background, the article aims to focus on how antibodies are produced and how they affect the graft, patient and outcome, along with future possibilities of further ABMR control.
In order to understand the grand picture, it’s important to start from the beginning. How exactly this whole process comes about. B cells when met with an antigen are matured into plasma cells, which then create antibody and memory B cell. How these antigens come in contact with the B cell is what drives the start of AMR. There are two routes of delivery – from lymph nodes and spleen. This is similar to antigens derived from SOT.
Since mature B cells circulate around lymph nodes every 24 hours, they accumulate within the cortex of the lymph node where they encounter the antigen. The enter the lymph node via the HEV or high endothelial venules and move along using FDCs or follicular dendritic cells.
In the spleen, the action is mainly in the marginal zone, which is located between the red and white pulp. The resident cells in the marginal zone capture the antigen and take it to the FDR in the B cells.
The proteins secreted by the transplanted kidney reach the B cell through gaps in the lymph node or similar gaps in the spleen. DSA in both naive and sensitized recipients bind to these antigens and activate complement pathway leading to cell destruction or ultimately transplant failure.
Once the BCR binds to the antigen, the B cell response is activated. They then present it to the CD4+ T cells. This combined interaction leads to the T cell response to DSA. The differentiation of the B cells of the recipient into plasma cells is key to this interaction and ultimate DSA production.
T cells then exit the lymph node and enter into the PALS region of the spleen using FRC networks. DCs with antigen from the graft moving through afferent lymph and using same FRC pathways present the antigen to the T cells inducing more rapid T cell response. Antigen affinity determines Th1 accumulation and response.
B cell priming in this interaction of B and T cells leads to early production of PCs before GC response. This is of lower affinity than GC response. GC has light zone and dark zone. Light zone has FDCs while dark zone is devoid of them. B cells in the light zones express activation markers and receive signals from GC Tfh cells. Positively selected B cells move into the dark zone and proliferate. Ultimately both light and dark zone B cells undergo apoptosis. Post GC B cells emerge as PCs, leading to persistent circulating antibodies.
Tregs regiulate what happens in the GC, and inhibit early stage B cell response.
All of these processes culminate in the formation of DSAs and ultimately possible graft rejection and failure of transplant. De novo DSA still remains a significant factor resulting in loss of graft.
This summarizes the various pathogenic processes outlined in the given article with focus on B cell antigen encounter, and the various methods relative to it, along with how T cells are involved in this fight against the transplanted graft. Better understanding of B and T cells is needed to create more effective immunosuppressive regimens that will provide smoother post transplant phases for transplant recipients as well as long term good outcome of graft. Preservation of natural immunity of the recipient needs to be a major goal to focus on while researching deeper into the pathogenic processes leading to ABMR.
Reference :
Given article
Thanks
This article points to a paradigm shift in pre-transplant evaluation as it recognizes the role of B lymphocytes in the production of antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure. So, it summarize the processes that underlie the primary and recall phases of B cell activation and antibody production and draws attention to the singular characteristics of this process.
1 – Routes of antigen presentation to B cells
There is increasing evidence suggest that optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on follicular dendritic cell (FDCs), B cells or macrophages. This encounter ocorre in the draining lymph nodes via multiple routes depending on antigen size, the presence of circulating antigen-specific antibodies, and the deposition of complement on the antigen by the classical or alternative pathways. This antignes are opsonized and complement is actived.
2 – Early B cell activation and the extrafollicular T/B cell interaction
There is a group of B cells responsible for the presentation of antigens from donor MHC cells to cells T CD4+. Following, cells T CD4 will driving the differentiation of recipient B cells into plasma cells producing donor-specific antibody (DAS).
The activation of T cells requires an initial encounter with migratory antigen presented on dendritic cells (that had acquired antigen from the allograft and migrated via afferent lymph) result in a subset acquiring T follicular helper, which will differentiate into CD4+ T cells.
3- Germinal center response
In the germinal center is divided in light zone and dark zone. There is interaction between B cells, dendritic cells, CD4+ T cells and T follicular helper in these zones, providing downregulation and upregulation, through cytokines and costimulatory molecules, responsible for persistente circulating antibodies, or as quiescent memory B cells that are responsible for the recall antibody response upon antigen reencounter.
4 – Germinal Center responses to multiple complex antigens
The chain of interactions mentioned above is capable of responding to the simultaneous introduction of multiple complex antigens, mimicking the stimulus of multiple MHC and non-MHC antigens found in the transplant. So, instead of a relative decrease in the antibody repertoire, we have increase in the breadth of the antibody repertoire.
5 – Regulating the germinal center response
The Germinal Center itself has a subset of CD4+ T cells that express the regulatory of this system. This T follicular regulatory are responsible to suppress self-reactive Germinal Center B cells that may have inadvertently arose during the Germinal center response.
6 – Generation of memory B cells and PCs
There are differences between memory generation through B cells and plasma cells. The affinitys and kinetics for BCRs and T follicular helper as well as by IgG and IgM expressions, lead to a perpetuation of this type of memory so that even with successful treatment of acute AMR and reduction of circulating DSA may not have prevented the generation of memory donor-specific B cells.
7 – Recall B cell responses
Recall memory B cells are expected to differentiate directly into plasma cells, generating antibodies faster than the primary response or generating germinal center B cells, however there are no rules in this way.
Different studies demonstrate prevalence of IgM+, others of IgG + and interaction with some signal expressions and revealed dependence on T cell help, and co-stimulation blockade with CTLA-4Ig, which are involved with CD28/CD80/CD86. And, this interaction can be controlled and depleted by belatacept.
Thanks
REFER TO THE ATTACHED PICTURE FOR BETTER UNDERSTANDING OF B- T CELL INTERACTIONS
Preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and that de novo DSA generated after transplantation is associated with poor outcomes and vascular obliterative lesions.
Antibody mediated rejection (AMR), is now recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure.
B cells have to encounter cognate antigen in order to start the process of differentiating into PCs producing high affinity antibody and memory B cells.
optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
Mature B cells circulate though the lymph nodes approximately every 24 hours, by leaving the vascular system and entering the lymph nodes through specialized high endothelial venules. Eventually these B cells encounter soluble or particulate antigens that enter the draining lymph node.
The humoral response can also arise from the spleen, which is divided into two distinct compartments: the red pulp, and the white pulp involved in the initiation of immune response.
The white pulp is comprised of the T cell zone (also referred to as the periarteriolar lymphoid sheath; PALS), B cell follicles and marginal zone (MZ). MZ contains a large reservoir of resident cells that participate in capturing and shuttling the antigen to FDCs within the B cell follicles and cells that are capable of processing and presenting the antigen to T cells.
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
Only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA.
Germinal center response
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the dark zone, which is devoid of FDCs. Light zone B cells express the activation markers, CD86 and CD83 and gene signatures associated with signaling from the BCR, CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive survival signals from GC-Tfh cells.
Survival signals include those provided by the interaction between costimulatory molecules, CD28-B7, CD40-CD154 and ICOS-ICOSL, by cytokines such as IL-21 and BAFF as shown in the attached picture. Positively selected B cells upregulate CXCR4 and migrate into the dark zone to undergo cell proliferation. In the absence of T cell survival signals, B cells undergo apoptosis in the light zone, while B cells with damaged BCR genes as a result of AID activity also undergo apoptosis in the dark zone.
Generation of memory B cells and PCs
Differentiation into PCs at the T:B border and within GCs is facilitated by high BCR affinity. Memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help. t BCR-signaling and T cell help is sufficient to drive memory differentiation, without the need for further proliferation or somatic hypermutation. The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
The recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response. In addition, long-lived PC express CD28 that provides survival signals by engaging CD80/CD86 expressed by bone marrow stromal cells [92; 93; 94]. As a result, inhibition of the CD28-CD80/CD86 interaction with CTLA-4Ig may result in longlived PC depletion and explain the recently described ability of belatacept to control and reverse DSA responses in sensitized recipients
Thanks
-Antibody-mediated rejection (AMR), is a significant cause of renal allograft failure.
-When B cells stimulate by antigens differentiate into plasma cells that produce high-affinity antibodies and memory B cells
-When the B cell receptor binds to antigen displayed on follicular dendritic cell, triggers B cell activation, the upregulation of CCR7, and migration to the T/B border.
-B cells process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
-The interaction between T and B cells initiates the T-dependent DSA response.
-Extrafollicular Tfh cells can stimulate extrafollicular B cells.
– CTLA-4Ig ( belatacept ) is an FDA-approved immunosuppressive agent for preventing kidney rejection. CTLA-4 on Tfr limited the expansion of antigen-specific Tfh cells and reduce antigen-specific antibody responses that may promote humoral responses.
-The activation of T cells requires an initial encounter with migratory DCs that had acquired antigen from the donor and migrated via afferent lymph and along the same fibroblastic reticular cells network in the lymph node or spleen to reach the T cell zone.
– For larger antigens, the localized CD11b+DCs within the lymphatic sinus can process and present antigens to T cells, which induce T cell responses rapidly.
-Memory B cells upon antigen reencounter will either differentiate into Plasma cells ( high-titer and class-switched recall antibody response), or GC B cells that (generate new class‐switched PCs).
-Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation.
– Renal transplantation recipients with a higher degree of HLA mismatch develop DSA against class II.
This isa very short summary Reem
Donor specific antibody (DSA)is associated with high risk of allograft rejection.
Formation of Antibody Mediated Rejection (AMR) has significant role in chronic renal transplant dysfunction & failure.
This article focus on the evidence and role of B cell activation in formation of AMR which are responsible for lost of graft post transplant.
Mechanism of antigen presentation to B cell:
B cell is bind to intact soluble antigen by B cell Receptor (BCR).
B cell is major part of adaptive immune system response.
B cell arise from bone marrow and then circulate to blood into spleen.
Humoral response are arise from spleen which is composed of red pulp and white pulp; red pulp is responsible for damaging unwanted RBC by Macrophage.
white pulp responsible for initiating immune response which contain T cell/ B cell & marginal zone.
Marginal Zone contain large cell which capturing & shuttling antigen to follicular dentric cell.
Antigen are derived from solid organ transplant.
Presence of circulating Antigen specific antibody & deposition of complement on antigen by classical or alternative pathway.
B cell transport opsonised antigen to follicular dentric cell that capture immune complex by CR1 binding C3b , C4d.
antigen not bound to antibody can bind to macrophage & dentric cell located in lymph node by innate recognition protein like IgM & CRP.
these dentric cell and macrophage in lymph nodes together with CD8, M2 metallophilic macrophage & M2 macrophage in spleen capture antigen & then Antigen displayed on surface of cell activated complement after that fallow route of opsonisation antigen delivery to CR on follicular dentric cell.
all new alloantigen captured by MHC on the surface of dentric cell form exosome which migrate around lymph nodes and spleen. exosome surface contain glycoproteins decorated with sialic acid that attach to macrophage. Donor MHC delivered by exosome to recipient dentric cell result in cross dressed DC that stimulate CD 8 T cell direct pathway.
Early B cell activation and extra follicular T and B cell interaction.
it’s a reaction between T & B cell lead intiate depended DSA response.
T cell & B cell directly and indirectly recognise donor MHC on MHC of recipient B cell. T cell migrate from blood to lymph nodes and activate by donor presenting antigen.
Germinal Ceńter response:
it’s present in side B cell; it’s divided to light zone & dark zone.
light zone consist from stroma derived follicular dentric cell & there’s multiple processing and activation of markers regulated by T follicular regulatory cell. Dark zone contain network produce reticular cell for B cell proliferation. Germinal center response are responsible to produce of long lived antibody ( secreting plasma cell & memory B cell.
plasma cell is activated B cell with high affinity B cell receptor for antigen responsible for persist circulating antibody. Memory B cell is responsible for recall antibody.
In this article, they are do experimental models on mice by use of MHC class I & class II tetramer immunized into donor of spleen cell; they show activation of B cell in presence of T cell inside germinal center and formation of donor specific antibody after 7 days from immunisation. they noticed activation of CD40 & CD154. this markers currently still there is no drug target against them. while CTLA-41g approved as immunosuppressive agent to prevent kidney rejection by reduce donor specific antibody number and reduce humoral response by inhibiting B cell T helper cell interaction and prevent B cell differentiation to plasma cell.
So belatacept more control humoral response in comparison to cyclosporine & Tacrolims .
Germinal center response to multiple complex antigens with MHC & Non MHC antigens incompatibility.
circulating repertoire antibody bind to epitope of antigen and they selected B cell with high affinity epitope antigen to production & differentiation.
In kidney transplant, recipient B cell exposure to multiple complex antigen and under current immunosuppressive agents around 15% of transplant kidney develops de novo DSA repertoire within 4-6 years post transplant.
by using method to evaluate HLA epitope mismatch shows that development of antibody directed at donor class II have higher affinity to HLA epitope mismatch.
regulating the germinal center response:
it’s function of Tfr cell control and suppression inadvertent B cell. in absence of Tfr cell will result the self reactive B cell differentiated to plasma cell which produce anti histone and ANA.
Other studies the Tfr cell inhibit Ag specific GC B cell and antibody production.
Generation of memory B cell and plasma cell:
it’s very important for control of reinfection. Memory B cell generated into 2 phase (early GC memory B cell with low affinity & enriched of IgM / late post GC memory B cell with higher affinity and expressing IgM & IgG) also long lived plasma cell. Absence of DSA doesn’t indicate absence of donor specific memory cell; so good history important to role out exposure to transfusion & transplant or pregnancy to identify low risk for alloimmune memory.
Recall B cell response: memory B cell with antigen differentiated into GC B cell with higher affinity and class switched plasma cell.
memory B cell differentiated into plasma cell and the recall DSA response.
Thanks
This paper describes the role of the humoral response in hyperacute or chronic antibody-mediated rejection, re-discussing the concept of antibody-mediated rejection.
B lymphocytes have chemotaxis via the reticuloendothelial system, increasing their exposure by dendritic cells and macrophages in lymph nodes and the spleen. This exposure can occur in the bloodstream by binding by soluble antigens (causing immune complexes) or by antigen-induced complement activation. Upon activation, the B lymphocyte becomes an immunoglobulin-producing plasma cell specific for that antigen.
MHC exposure to B lymphocytes leads to interaction with CD4 T lymphocytes, linking these pathways. It is believed that increased exposure to these antigens can increase the CD4/CD8 response and, consequently, increase DSA values chronically and progressively.
Germinal Center controls the erratic production of plasma cells exposed to alloantibodies, inducing them to apoptosis or inactivating their activity when leaving the light zone. In this way, it prevents the production of DSA by blocking the CTLA-4 pathway, pharmacologically evidenced when using belatacept, cyclosporine or tacrolimus.
HLA mismatch DR is most closely related to de novo DSA after 4 to 6 years of kidney transplantation.
Plasma cells fight recent infections (prioritizing the production of IgM) and B lymphocytes prevent reinfections when that antigen has already triggered a previous humoral response (IgG subclasses – IgG1, IgG2, IgG3, IgG4). This previous response occurred in the Germinal Center and is apparently the reason why donor B lymphocytes do not produce alloantibodies. In this way, CTLA-4 blocks the transformation of B lymphocytes into plasma cells, minimizing their production of IgM and at the same time increasing the risk of infection from viral and bacterial agents.
In conclusion, patients who were more exposed to epitopes and antigens during the pre-transplantation period (transfusions, pregnancy, previous transplantation) as well as those who did not perform immunosuppression adequately, are more prone to de novo ASD and, consequently, to antibody-mediated rejection.
Thanks Filipe
Thank you, Professor Halawa. I will get better
RULE OF B CELLS IN TRANSPLANTATION
Immune response is either innate or adaptive
Adaptive immunity involves specific reaction to a certain agent and memorization of these agent
Adaptive immunity is either T cell mediated (when antigen peptide binds to HLA antigens) or B cell medicated (when antigen peptide bind to immunoglobulin receptors on B cell or through activation by CD4- T cells) (2)
The 3 systems (natural immunity, T cell immune system, B cell immune system ) are usually integrated.
B cells have an important rule in T cell medicated rejection (through acting as APC that present antigen to T cell and through activation of T cells) or in ABMR (through transformation into plasma cells and production of immunoglobulins which act as DSA)
Full activation of B cell occurs when BCR binds to an intact antigen on the follicular dendritic cell or macrophages
Upon activation of B cell the following occur :
1- B cell secretes many cytokines such as IL-6 and TNF-a , that lead to activation and Differentiation of T lymphocytes which leads to graft damage. Some of the cytokines which is secreted by B cells like INF-gama, TNF-a are directly injurious to the graft.
2- B cells can differentiate into plasma cells which secrete antibodies DSA and lead to antibody mediated rejection. for complete plasma cell differentiation and migration out of the germinal center Tfh cells are needed.
3- B cells can differentiate into memory B cells which can stay in the body for years and can induce memory response once reactivated. thery are either grenerated early pre-germinal center (secrete IgM, has low affinity) or late post germinal center (secrete IgG or IgM , has high affinity)
Good ,concise explanation for B cell activation
Please refer to fig 2 for clinical application and use of Belatacept.
New insights into the development of B cell responses: Implications for solid organ transplantation.
Introduction:
Antibody mediated rejection (AMR), is recognized as a significant, and possibly the major cause of chronic kidney transplant dysfunction and failure.
Role of B cell activation and antibody production, may be applied to understanding the generation of DSA following solid organ transplantation.
Routes of antigen presentation to B cells
– B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages (reviewed in [5]).
Mature B cells circulate though the lymph nodes approximately every 24 hours, these B cells congregate within the cortical region encounter soluble or particulate antigens that enter the draining lymph node via multiple routes depending on antigen size, the presence of circulating antigen-specific antibodies, and the deposition of complement on the antigen by the classical or alternative pathways. Also migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
Antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated, Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by macrophages in the red pulp of the spleen, SCS macrophages and DCs,somtimes limited number of donor DCs migrate from the graft into draining lymph nodes or spleen, where they deliver exosome to resident DCs.
Donor MHC delivered by exosomes to recipient DCs, results in cross-dressed DCs that have the ability to stimulate recipient CD8+ T cells via the direct pathway.
The exosome surface also bears numerous glycoproteins decorated with sialic acid that allow their capture by sialoadhesin (CD169) expressed on SSM and MZ macrophages .These macrophages then deliver exosomes bearing intact donor MHC to FDCs within the B cell zone.
Early B cell activation and the extrafollicular T/B cell interaction.
B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
T cells can directly or indirectly recognize intact donor MHC but only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA.
Germinal center response.
Within each GC, and following the selection for B cells with high affinity BCR for antigens presented by the FDCs, post-GC B cells emerge as PCs (PC) that are ultimately responsible for persistent circulating antibodies, or as memory B cells that are responsible for the recall antibody response after another antigen stimulation.
The importance of CD40:CD154 interactions between B cells and T cells in the initial activation of T cells and in the maintenance of the GC response is well known. In contrast, the necessity of CD28:B7 interactions in sustaining the GC response was less clear. CTLA-4Ig is an FDA approved immunosuppressive agent for preventing kidney rejection, it was important to test the effect of CTLA-4Ig on established B cell responses.
Germinal Center responses to multiple complex antigens:
B cell clones with multiple different antigenic specificity and affinity compete for stimulation during rounds of somatic hypermutation within GCs. As a result, the presence of many antigenic epitopes tends to educe the relative breadth of the antibody repertoire and generated antibodies to multiple antigens would be reduced.
Generation of memory B cells and PCs:
The differences in affinity and kinetics of memory B cell versus PC generation suggest that successful treatment of acute AMR and reduction of circulating DSA may not have prevented memory donor-specific B cell generation so absence of DSA dose not mean absence of sensitization.
so we still need a clinical test of donor specific memory B cells.
Recall B cell responses:
Sensitization of Memory B cell by same antigen again lead to direct differentiation to PCs stronger than primary response or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
So, recall response remains dependent on T cell help, and co-stimulation blockade with CTLA-4Ig completely prevented memory B cell differentiation into PC and the recall DSA response.
So, belatacept has the ability to control and reverse DSA responses in sensitized recipients.
Thankyou Mohamad for your hard work in reading and extensive summary Iam sure this is very rewarding for you however you can conclude the useful clinical information in final points as
Thanks .
professor Dawlat.
I will.
A resurgent interest in the role of B cells following solid organ transplantation is being driven by
clinical data suggesting that antibody mediated rejection (AMR) is a major cause of dysfunction
and organ transplant failure. In this review, new insights into processes that lead to antibody production upon primary and secondary antigen encounter are discussed, and the potential implications to DSA production and future areas of investigation to control AMR are discussed.
Routes of antigen presentation to B cells
B-cell + Antigen >>>>>plasma cells >>>>antibody & B memory cells
While it has long been established that B cells can bind intact soluble antigen, there is increasing evidence suggest that optimal B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
How can B cell encounter soluble or membrane bound antigen
– Mature B cells circulate though the lymph nodes approximately every 24 hours.
– Eventually these B
– cells congregate within the cortical region near the subcapsular sinus where they may encounter soluble or particulate antigens that enter the draining lymph node.
– In addition, there may be additional contribution by migratory DCs that acquire antigen at the tissue site and transport them into the lymph node.
The humoral response can also arise from the spleen.
Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain
direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen.
In addition, natural polyreactive antibodies and DSA in naïve and sensitized recipients, respectively, can bind to soluble antigens and activate complement.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph
nodes, via innate recognition proteins such as natural, Antigens displayed on the surface of these cells activate complement.
In addition to antigens secreted and shed from the allograft, a new type of alloantigen delivery has become increasingly appreciated
Alternatively, a relatively limited number of donor DCs migrate from the graft into draining lymph nodes or spleen.
Early B cell activation and the extrafollicular T/B cell interaction
– When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border.
– Concurrently, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
– only CD4+ T cells that have indirect specificity for donor-derived antigens presented by the MHC molecules expressed by recipient B cells are capable of driving the differentiation of recipient B cells into PCs producing DSA. This cognate interaction between T and B cells initiates the T-dependent DSA response.
Germinal center response
The GC is divided into the light zone comprising stroma-derived FDCs and Tfh cells and the
dark zone, which is devoid of FDCs. Light zone B cells express the activation markers,
CD86 and CD83 [43; 44] and gene signatures associated with signaling from the BCR,
CD40 and Myc. These B cells engage immobilized antigen presented on FDCs, and receive
survival signals from GC-Tfh cells
Germinal Center responses to multiple complex antigens
In the majority of kidney transplant recipients, recipient B cells will most likely encounter
multiple complex antigens. Under current immunosuppression, approximately 15% of renal
transplant recipients develop de novo DSA within 4.6 years of transplantation.
Understanding how the alloantibody repertoire is selected in response to multiple antigen-mismatched allografts, how this repertoire evolves over time, and the impact of immunosuppression on these process, will result in a better understanding of when pathogenic versus nonpathogenic DSA develops.
Regulating the germinal center response
A subset of CD4+ T cells that express the regulatory T (Treg) cell master regulator Foxp3,
and sharing many phenotypic characteristics of Tfh cells, has recently been described
In the absence of Tfr, these selfreactive B cells differentiated into PC that produced anti-histone and anti-nuclear antibodies.
Interestingly, Tfr cells did not inhibit the expansion of influenza-specific B cells or their
differentiation into PC, and had no significant effect on influenza-specific antibody responses. In contrast, other studies have reported that Tfr cells inhibit antigen-specific GC B cells and antibody production
Generation of memory B cells and PCs
The appropriate and rapid generation of PCs is essential for the successful control of infection, and of memory cells and long-lived PCs for the protection against reinfection.
Within GCs, PC differentiation was induced in a
discrete subset of high-affinity B cells residing within the light zone upon BCR engagement
with antigen presented on FDC, while Tfh cells provided the subsequent signals essential for
completing the PC differentiation and driving their migration out of the GC
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with
lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity
and expressing IgM or IgG
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.
Thank you All
I’m still waiting for all colleagues to complete their logging
The preformed circulating donor-specific antibodies (DSA) is associated with high risk for acute rejection, and de novo DSA detected after transplantation is associated with poor outcomes and vascular obliterative lesions
Ag presentation routes to Bcells
Donor derived MHC complex are captured by SIGN-R1+ MZ macrophages, F4/80+ macrophages in the red pulp of the spleen, SCS macrophages and DCs, stimulating recipient CD8+ T cells directly . These macrophages transfer intact donor MHC to FDCs within the B cell zone.
B cell activation and T/B cell interaction
BCR binding to antigen triggers B cell activation.
CD4+ T cells can drive the differentiation of recipient B cells into PCs producing DSA thereby starting DSA effect.
The activation of T cells encountering with migratory DCs activated T cell .
The strategically localized CD11b+DCs within the lymphatic endothelium present captured antigens to T cells, stimulating rapid T cell response .
Belatacept, a mutant of CTLA-4Ig, acts by inhibiting B cell-Tfh interactions,blocking humoral response therefore preventing DSA in kidney transplant recipients .
Renal transplantation recipients having higher degree of HLA epitope mismatch developed antibodies directed towards donor Class II.
It was supposed that Tfr inhibit B cell responses through producing TGF-ß .
BCR signaling intensity is assessed early by expression of interferon regulatory factor-4 (IRF4).
Acute AMR treatment and reduction of circulating DSA may not have prevented memory donor-specific B cell generation.
IgM + memory B cells preferentially differentiated into GC cells, and the IgG1+ memory B cells gave rise primarily to PCs.
clever conclusion about the effect of treating ABMR by removal of DSAs effect on memory donor-specific B cell recognition.
Fig 2 well explains diff. between naive and sensitised recipient.
New insights into the development of B cell responses: Implications for solid organ transplantation
Routes of antigen presentation to B cells
B cells can bind intact soluble antigen or B cell activation occurs when the B cell receptor (BCR) engages intact antigen displayed on FDCs, B cells or macrophages.
The humoral response can also arise from the spleen.
Small soluble proteins (≤14 kD), such as those secreted by the transplanted organs, may gain direct access to the B cell via passive flow through FRC conduits or gaps in the sinus floor of the lymph node or in the marginal sinus in the spleen.
Antigens not bound by antibodies can bind to macrophages and DCs located in the lymph nodes, via innate recognition proteins such as natural IgM; C-type lectins such as MBL (mannose binding lectin) and ficolins; and pentraxins, including C-reactive protein.
Donor-derived intact MHC complexes carried by extracellular vesicles or exosomes released by the allograft can accumulate in the draining lymph node and spleen. These exosomes are then captured by SIGN-R1+ MZ macrophages, F4/80+ macrophages in the red pulp of the spleen, SCS macrophages and DCs.
Early B cell activation and the extrafollicular T/B cell interaction
When the BCR binds to antigen displayed on FDCs, SSMs or non-antigen-specific B cells, it triggers B cell activation, the upregulation of CCR7 and migration to the T/B border (reviewed in [5; 6]). Concurrently, B cells internalize the BCR-bound antigen, process and present the peptides derived from the antigen on MHC Class II molecules, in preparation for interaction with antigen-specific T cell receptors on CD4+ T cells.
Germinal center response
When extrafollicular B cells receive costimulation from extrafollicular Tfh cells, they down- regulate EBI2 EBI2, which releases T cells from the outer follicle, and increase expression of the chemorepulsive receptor S1PR2, which repels cells from the S1P-rich lymph in the subcapsular sinus toward the follicle center.
Germinal Center responses to multiple complex antigens
In the majority of kidney transplant recipients, recipient B cells will most likely encounter multiple complex antigens. Under current immunosuppression, approximately 15% of renal transplant recipients develop de novo DSA within 4.6 years of transplantation, with some patients responding with a limited DSA repertoire while others more broadly .
Regulating the germinal center response
T follicular regulatory (TFr) cells express CXCR5, PD-1, Bcl-6. Blimp-1, FoxP3, GITR, ICOS, CTLA-4 and IL-10, but not CD25, and function by controlling GC responses. Early studies utilizing adoptive transfer or mixed bone marrow chimeras showed that Tfr are located in the GC, and that they suppress self-reactive GC B cells that may have inadvertently arose during the GC response.
Generation of memory B cells and PCs
Memory B cells are also generated in two distinct phases: early pre-GC memory B cells with lower affinity and enriched for IgM, and later post-GC memory B cells with higher affinity and expressing IgM or IgG. Relative to PCs, memory B cells are derived from cells receiving lower-affinity BCR signaling and reduced T cell help, which in turn, induces reduced IRF-4 and higher Bach2, respectively.
Recall B cell responses
Memory B cells upon antigen reencounter will either differentiate directly into PCs, generating a faster, high-titer and class-switched recall antibody response compared to a primary response, or they will differentiate into GC B cells that generate new, higher‐affinity and class‐switched PCs.