For a T cell to be active and proliferate we need signal two as well as signal one .. as its name implies it is co-stimulation. without signal 2 (co-sitimulation) the T cell will be anergied and that is what we want actually to succeed in immunosuppression to survive our engrafted organ.
the binding of CD3 to TCR (T cell receptor ) is the first signal. without it, T cells will not be activated
One of the characteristics of memory cells (both T and B lymphocytes) is their ability to produce an immune response with minimal need for co-stimulation as illustrated in the attached table (1).
What is CD3 and what is its role in T cell activation?
The CD3 complex serves as a T cell co-receptor that associates noncovalently with the T cell receptor (TCR) (Jennifer E Smith-2009).
The CD3 protein complex is a defining feature of the T cell lineage, therefore anti-CD3 antibodies can be used effectively as T cell markers .
CD3 protein complex is an important T cell marker for the classification of malignant lymphomas and leukemias (T cell neoplasms). CD3 can also be used for the identification of T cells in coeliac disease ,
Animal studies have shown that anti-CD3 antibodies induce tolerance to allografts (Nicolls et al. 1993).
OKT3, an anti-CD& antibody directed against CD3 ε, has been clinically approved for use in humans for the induction of immunosuppression in solid organ transplantation for the prevention and treatment of rejection (Norman 1995).
Interestingly, susceptibility to type I diabetes has been associated with the CD3 ε genetic locus and anti-CD3 antibodies have been shown to ameliorate the symptoms of this and other auto-immune disorders (Sprangers et al. 2011).
Jennifer E Smith-Garvin et al. Annu Rev Immunol. 2009.؟Tcell activation
Nicolls MR et al. (1993). Induction of long-term specific tolerance to allografts in rats by therapy with an anti-CD3-like monoclonal antibody. Transplantation 55, 459-468.
CD3 used as atool for monitoring of the following:-
*Polyclonal Lymphocyte-Depleting Antibodies (Antithymocyte globulins)
Thyroglobulin-mediated T-cell depletion can be monitored using daily flow cytometry to quantify circulating CD3 T cells
*Monoclonal Lymphocyte-Depleting Antibodies (OKT3)
• Efficacy can be monitored by daily flow cytometry to identify CD3 cells.
Peripheral CD3 cell count monitoring
its effective to determine the appropriate ATG doses needed its reliable useful and cost effective in one cohort study they use less ATG dosing by 45% and 20% less cost in the group underwent monitoring with CD3 cell count in comparison to the group use lymphocyte%.
Do. Yes it’s cost effective and reliable
References
1- monitoring of CD3 T cell count in patients receiving antithymocyte globulin induction after cadaveric Renal transplantation
P . ATa,M ,Kara, E, Ozdemir,M,Canbakan,A.M,Gokce,F,A.
What is CD3 complex …
it is present on all T cells and it’s importance in T cell activation is highlighted by the mechanism of action of OKT3 which is a monoclonal Ab which binds to one of the subunits of the CD3 complex, leading to destabilization and subsequent endocytosis of the TCR. Loss of the TCR from the cell surface inhibits activation and renders the T cell ineffective in generating an immune response.
co-stimulation is very important for T cell activation, if blocked ,no activation will happen, can be blocked with balatacept
CD3 is a part of complex with TCR, it comes in association with it on T cell surface and is very important during binding TCR with MHC on APC, without it no binding of TCR with MHC , can be blocked by OKT3
we need to know 3 signal model to block T cell activation at any stage to guard against its activation and proliferation to stop immune response to allogenic graft
-T lymphocytes need signal 2( co stimulation) to undergo full proliferation and differentiation to either cytotoxic or helper cells.
-Knowing the 3-signal models models helps in the invention and manufacturing of immunosuppressive drugs.
– the binding of antigens to TCR triggers a signal cascade that is not transduced through TCR proper but through an adjacent CD3 complex( CD3 which is a protein associated with TCR )that leads to T –lymphocytes activation.
CD28 is expressed in almost all murine T cells. It is expressed on 95% of the resting CD4+ T cells and 50% of the resting CD8+ T cells in human peripheral blood. A subpopulation of CD8 T cells with suppressor activity lack CD28 expression. Naive CD8+ T cells do not require co-stimulation for proliferation and differentiation into cytotoxic effector cells.
CD13 is transmembranous molecule in the surface of T lymphocytes required for binding to the HLA antigen. Muromunab is CD13 inhibitor, blocking the binding to the APC, and subsequently blocking the activation of T lymphocytes.
Knowing toe 3-signal model, we can be able to target the T lymphocytes activation, proliferation and differentiation at man levels and we would be able to know how cellular and humoral immune response are interconnected.
Thanks, Mohamed
Yes, “Knowing toe 3-signal model, we can be able to target the T lymphocytes activation, proliferation and differentiation at man levels and we would be able to know how cellular and humoral immune response are interconnected”.
Do you think that all T cells require co-stimulation?
Naïve T cell activation requires engagement of TCR by MHC complex , engagement of cell surface co-stimulatory molecules found on APC and T cell stimulation by cytokines
Sustainability of T cell activation requires engagement of surface co-stimulatory in addition to engagement of TCR by MHC complex
Co- stimulatory molecules such as Immunoglobulin superfamily ( B7), tumour necrosis factor family (CD 154), G protein coupled receptors (C3a and C5a)required during priming, expansion and death of T cells.
Without co-stimulatory signals, T cells might become anergic which make them unresponsive even if receives adequate second signal. Anergic cells can also inhibit activation of neighbouring T cells.
Why we need to know the 3-signal model?
Three signal model of T cell activation and subsequent cellular proliferation Is important tool for understanding the sites of action of the immunosuppression medications to prevent allograft rejection
What is CD3 and what is its role in T cell activation?
TCR by itself doesn’t have catalytic activity, requires to form complex with 6 CD3 subunits that contain cytoplasmic immunoreceptor tyrosine -based activation motifs (ITAMs)
References
Kidney Transplantation: Principles and Practice by Stuart J. Knechtle
Hand book of kidney transplantation by Gabriel M. Danovitch
1. We addressed the importance of co-stimulation of T cell activation. Do you think that all T cells require co-stimulation?
· T cells require the costimulatory signal second signal provided by CD28 and following its binding to B7-1 on APC to the naïve CD4 T – cells to undergo proliferation, differentiation and survival. So co stimulation rescues T cells from anergy (state of immune unresponsiveness).
· However, CD8+ T cells can be activated through the signal generated through TCR-alphabeta in the absence of any potential costimulatory molecule
2. Why we need to know the 3-signal model?
· We need to understand the 3 signal model to know how current immunosuppression work in inhibiting T cell activation.
· This will also help us to invent newer targeted immunosuppression.
3. What is CD3 and what is its role in T cell activation?
· CD3 is a member of the immunoglobulin superfamily
· CD3 is expressed by a high-percentage on circulating peripheral T cells (no other cell type) forming a complex with the T cell receptor (TCR).
· CD3 is present at all stages of T cell development so it is a highly effective T cell marker.
· T cell receptor/CD3 complex (TCR/CD3) plays a key role in antigen recognition, T cell activation and in consequence in triggering an antigen specific immune response.
· CD3 was the target of the first mAB (OKT3) that is not currently used because of side effects
· Monitoring of CD3 is a useful , reliable and cost effective in monitoring ATG therapy
Memory cells – they don’t require co-stimulation upon re-presentation of the antigen. This is nightmare in transplantation
The immuno-suppressive therapy is based on the understanding of the immunobiology of T cell signals
CD3 is TCR, it is important because the binding of antigen-complexed with HLA molecule is not complete until they bind with CD3or TCR inducing signal 1
Do you think that all T cells require co-stimulation?
In the absence of costimulatory signals, T cells become anergic so that they become unresponsive even they subsequently receive an adequate second signal, and these anergic cells can also inhibit the activation of neighboring T cells.
Why we need to know the 3-signal model?
To understand the different modes of action of immunosuppressive medications and helps in the discovery of new immunosuppressive medications.
What is CD3 and what is its role in T cell activation?
It is a protein complex and T cell coreceptor that involve in the activation of both cytotoxic T cells and T helper cells, it contains four chains and these chains together with TCR generate a signal that lead to T cells activation.
Do you think all t-cell required stimulation?
Memory cells can respond to antigen presenting cells independent of accessory co-stimulation.
Why we need to know the 3 signal module?
Knowing the 3 signal modules help us to understand T-cell activation ,proliferation and differentiation and how drugs can target these area .
What is CD3 ? what its role in T-cell activation?
Is a protein complex , composed of four distinct chains .these chains associate with the TCR and CD3-zeta to generate an activation signal in T lymphocytes . the TCR , CD3-zeta , and the other Cd3 molecule together constitute the TCR complex.
1-We need CD 80 ,86 to join CD28
CD40 join CD154
CD25
2- we need 3 signal model to know which drug is suitable for inhibiting each signal to prevent T cell proliferation
3-CD3 is complex transduce the TCR to be recognized by APC (this represents signal 1)
Not all T cells need co-stimulation , memory cells need minimal co-stimulation .
We need to know the 3-signal in order to know the mode of action of different IS so control the immune system and to help for future invention of new items.
CD3 is a T-cell co receptor associat with TCR necessary for signal 1
and it is used for ATG and OKT3 drug monitoring.
1- No. Memory T cells don’t need costimulation to be ativated.
2- To prevent and treat rejection.
3-CD3 complex is a group of protein chains that associate with T cell receptor and is important in signal 1.
Rehab Fahmy
3 years ago
The activation of naïve T cells is a tightly regulated event and requires three distinct signals for the generation of an optimal response,
including :
1)-T cell receptor (TCR) engagement (signal 1),
2)-Costimulation (signal 2), and
3)-cytokine stimulation (signal 3)
Signal 1: TCR engagement
The antigen binding component of the TCR complex is a heterodimer composed of an α and β chain that recognizes proteolytically processed short peptides (8–15 amino acids) presented in the context of self major histocompatibility complex (MHC) on antigen-presenting cells it was demonstrated that antigen-specific T cells recognize foreign peptides presented by self-MHC, by showing that cytotoxic T cells (CTL) lysed only virus-infected target cells that were matched at the MHC loci .Interestingly, a significant frequency of T cells also have the ability to recognize MHC molecules that are not present within the thymus during selection in a process referred to as allo-recognition, with both class I and II serving as targets . These alloreactive T cells are present at significantly higher frequencies (100–1000 fold higher) than T cells specific for individual foreign peptides presented by self-MHC, allowing for the generation of strong primary immune responses to transplanted non-self tissues and rapid rejection of allografts. Alloreactive T cells recognize alloantigens through two distinct pathways, the direct and the indirect pathways. In the direct pathway, T cells recognize an intact donor MHC antigen on the tissue allograft and in the indirect pathway, T cells recognize donor peptide antigens presented by self MHC .T cells recognizing alloantigens through either the direct or indirect pathways can mediate rejection of allografts.
Agents acts on signal 1:1)-Anti TCR agents: The murine anti-CD3 mAb Muromonab-CD3 (OKT3) was the first mAb. It targeted the CD3 subunit of the TCR complex and led to rapid elimination of functional T cells. It is now no longer in production because of waning utilization, primarily because of significant side effects related to the mitogenicity associated with its murine source.
2)-Calcineurin Inhibitors (Cyclosporin and Tacrolimus).
After initial TCR binding, a calcineurin-dependent signaling pathway is induced that leads to initial T cell gene transcription necessary for additional activation. Two commonly used calcineurin inhibitors (CNIs; cyclosporin and tacrolimus) and one investigational agent (voclosporin) inhibit the ability of calcineurin to dephosphorylate nuclear factor (NF) of activated T cells (NFAT), required for translocation from cytoplasm to nucleus, and prevent calcineurin-dependent gene transcription. One potential nonimmunosuppressive mechanism that could explain CNI efficacy in glomerular disease is the ability to inhibit synaptopodin degradation in the podocyte, thereby stabilizing the actin cytoskeleton and reducing proteinuria . alternative formulations of tacrolimus (extended release) as well as the novel CNI voclosporin have been developed and are approved or in late-phase clinical trials in transplantation.
Signal 2:Costimulation:
The second signal that is crucial for productive T cell activation is delivered by the engagement of one or more costimulatory molecules expressed by T cells with their ligands expressed on APC . One of the earliest costimulatory molecules to be identified and extensively studied is CD28, which is expressed on the cell surface of all naïve CD4 and CD8 T cells. TCR stimulation in the absence of CD28 engagement results in abortive activation of T cells and anergy . Binding of CD28 to its ligands (B7-1, CD80 and B7-2, CD86) on APC promotes optimal TCR signaling events that trigger IL-2 .production, clonal expansion and generation of effector and memory T cells .Following the discovery of CD28, cytotoxic T lymphocyte antigen-4 (CTLA-4, CD152) was identified based on structural homology to CD28, and a functional role for this molecule was suggested by the ability of a CD152-Ig fusion protein to inhibit T cell activation. Recently, additional costimulatory molecules related to the CD28 immunoglobulin superfamily have been discovered, including the inducible costimulatory molecule (ICOS)-B7h and the programmed death (PD)-PD-L1/PD-L2 pathway. The tumor necrosis factor family-tumor necrosis factor family receptor (TNF-TNFR) superfamily members also provide costimulatory signals important in the generation of antigen-specific T cell responses. Overall, there is a broad array of costimulatory pathways that either enhance or dampen T cell activation, and these pathways are all potential targets for blocking T cell activation and inducing T cell tolerance to alloantigens.
Agents acting on signal 2
1)- Blockade by CD80/86:CD28 Targeting (Abatacept and Belatacept).CTLA-4 Ig ,belatacept has high affinity to CD80/86.
2)-Costimulation Blockade by CD154:CD40 Targeting (Anti-CD40 mAb).: Targeting the induced surface molecule CD154 on activated T cells was a focus of drug development until it was recognized that CD154 was also present on platelets, and agents binding this cell surface molecule led to an increase in thrombotic events in both primate and early-phase human trials .Attention has, thus, turned to targeting CD40. A number of mAbs against CD40 are in development, with a fully human anti-CD40 (ASKP1240; Astellas) under study in phase 2 clinical trials in kidney transplantation.
Signal 3: CD4 help and inflammation:
naïve CD8 T cells also require a third signal for the optimal generation of effector and memory T cells .Two examples of third signals enhancing the induction of antigen-specific CD8 T cell responses are CD4 T cell help and the presence of inflammatory cytokines .The specific role of CD4 T cells in supporting the activation of CD8 T cell responses is dependent on the nature of the antigenic challenge. One mechanism for this help is the maturation or “licensing” of APC by engagement of CD40 on the APC by CD154 expressed by activated CD4 T cells. In contrast, infection with pathogens that induce significant levels of inflammation, such as influenza virus, lymphocytic choriomeningitis virus (LCMV) or vesicular stomatitis virus, does not require CD4 help to induce primary CD8 T cell responses .However, the generation of functional CD8 memory after infection is dependent on CD4 T cell help received during the initial activation of the CD8 T cells . A second mechanism by which CD4 T cells can provide help to CD8 T cell responses is by the production of IL-2
. IL2-signalling appears to have only a minimal role in the generation of primary CD8 T cell responses to viral infection or cellular antigens but is critical for the development of functional memory CD8 T cells.
Agents targeting signal 3:
No spescific cytokines inhibition:corticosteroids and Janus Kinase Inhibition (Tofacitinib).
Spescific cytokines inhibition:
A)-IL-2 Receptor Antagonist (Basiliximab),,
B)-TNF alpha inhibitors :Infliximab, adalimumab, golimumab, and certolizumab are mAbs to TNF-α that differ in the degree of chimerism and route of administration (intravenous versus subcutaneous) .Etanercept is a TNFR fusion protein bound to IgG. All carry the risk of increased susceptibility to intracellular pathogens, such as tuberculosis, coccidiomycosis, and Cryptococcus .
C)-IL-1 Inhibition (Anikinra, Rilonacept, and Canakinumab).
D)-IL-6 Inhibition (Tocilizumab).
E)-IL-17 Inhibition (Secukinumab).
There is also agents targeting B cells e.g Anti-CD20 Targeting: Rituximab, Ocrelizumab, Ofatumumab, and Veltuzuma ,,, Anti-CD22 Targeting: Epratuzumab ,,,, Targeting B Cell Differentiation: Belimumab and Atacicept,,,,, Plasma Cell Targeting: Bortezomib,,, Complement Inhibition (Eculizumab).
Agents targeting chemokines and cell adhesion: approved chemokine receptor antagonists include the CCR5 receptor antagonist maraviroc used in the treatment of HIV, the CXCR4 antagonist plerixafor approved for hematopoietic stem cell mobilization, and the CCR4 humanized mAb mogamulizumab approved for the treatment of T cell lymphoma. Relevant to nephrology practice, emapticap is an inhibitor to CCL2 (also known as monocyte chemotactic protein 1), which has been studied in phase 1 and 2 trials in diabetic nephropathy, with proof-of-concept studies showing a reduction in albuminuria presumably by inhibiting inflammatory cell migration into the kidney.
Pooled Polyclonal Antibodies as Immunosuppressive Agents:IVIG and ATG
Immunosuppressive Agents with Multiple Cellular Targets:
Panlymphocyte Depleting Agents: Anti-CD52 (Alemtuzumab)
Antiproliferative Agents: mTOR Inhibitors (Sirolimus and Everolimus)
Antimetabolites: Inhibition of DNA Synthesis:Azathioprine ,MMF and leflunamide.
Abdullah Raoof
3 years ago
.
Allorecognition and alloreaction is mediated by both naïve and memeory T cells .The memory cell reaction is more rapid and more sever than naïve one . rejection can take place by both of these group of cells .
Signal 1: TCR Signaling
Signal one is mediated by interaction between HLA-peptide complex ( antigen ) and T cell receptor .
T cell receptor binding with antigen on antigen presenting cells will activated t cell receptor . t cell receptor activation alone is not enough to induce t cell activation and proliferation , to complete this process the adjacent CD3 activation is needed . CD3 complex, a group of invariant (non polymorphic) protein chains that associate with the TCR. Engagement of the TCR by MHC, along with CD 3 the other required receptor engagements, initiates the signaling process from the CD3 complex, leading to cell proliferation and differentiation. CD4 and CD8 also participate in this reaction ,they has a co receptor role . CD4 combine with class II antigens while CD8 combine class I antigens .T cell receptor complex – antigen interaction induce cascade of signaling of T cell activation .
Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation. An example is OKT3, the first monoclonal antibody used in clinical medicine, and since withdrawn that was used to treat severe acute allograft rejection.
The activation signal transduced by the TCR-CD3
Cluster will lead to activation of enzymes tyrosine kinase and protein kinase which lead to generate second-messengers: Diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3).These messengers activate
calcineurin pathways that ultimately activate key transcriptionfactors (NFkB, NFAT, and AP-1) that induce the transcription of cytokine genes required for T-lymphocyte proliferation and differentiation.
The calcineurin pathway is the target of the CNIs . CNIs bind to specific proteins in the cell (immunophilins— cyclophilin for cyclosporin and FK-bindingprotein (FKBP) for tacrolimus. The drug immunophilincomplex then blocks the activation of calcineurin by the calcium-dependent enzyme calmodulin.
T Cell Costimulation: Signal 2
Binding of the TCR:CD3 complex to the peptide:MHC complex on APCs delivers a signal that can induce the clonal expansion of naïve T cells only when the appropriate co stimulatory signal is delivered (signal2). CD8 T cells require a stronger co stimulatory signal, and their clonal expansion is aided by CD4 T cells interacting with the same APC (i.e., T helper function). Co stimulation is likely a checkpoint developed by
the immune system to prevent the activation of self-reactive T cells that escaped negative selection in the thymus. Antigen binding to the TCR in the absence of co stimulation not only fails to activate the T cell but also leads to a state called anergy, in which T cells become refractory to subsequent activation, or even undergo apoptosis (programmed cell death). Thus co stimulation removes this inhibition and determines whether a T cell will proceed with clonal expansion and the development of effector functions. It is now clear that co stimulatory molecules
can provide positive or negative signals to T cells . It is the integration of both positive and negative co stimulatory signals during and after initial T cell activation, dictated by their temporal and spatial expression patterns, that ultimately determines the fate and functional status of the T cell response.
CTLA4- Ig (Belatacept), which binds with high affinity to B7 molecules and prevents them from engaging CD28, is in clinical use for the prevention of allograft rejection in kidney transplant recipients . Integrine :
Extended interaction between the DC presenting the alloantigen and the alloreactive T lymphocyte is necessary for sustained TCR signaling. Prolonged, stable interaction between the two is made possible by cell-adhesion molecules known as integrins. The integrins LFA-1 and CD2 (LFA-3) on T lymphocytes bind to their ligands ICAM-1/ICAM-2 and CD58, respectively,on dendritic cell.
The development and marketing of anti-LFA-1 (Efalizumab) and anti-CD2 antibodies (Alefacept) for clinical use in autoimmunity and transplantation have been halted owing to serious side effects such as progressive multifocal leukoencephalopathy (PML) caused by JC virus reactivation .
B7-CD28 ,CD40-CD154 and other co-Stimulatory Pathways like 41BBL-41BB (CD137), OX40L-OX40, CD70:CD27, and ICOSL-ICOS pathway are example .
Signal 3: Cytokines
They are produced by either APC or T cells .
They induces T cell proliferation and differentiation into different subset of effector T cell that have distinct
phenotypes and functions. cytokines can also regulate T lymphocytes or act on other immune and no nimmune cells to either enhance or suppress inflammation.
Interleukin-2 (IL-2) It is produced by T cells and it has autocrine and paracrine.
IL-2 binding to the high-affinity ( Active form ) IL-2R causes
the proliferation (clonal expansion) of antigen-activated T lymphocytes .Anti-CD25 monoclonal antibodies that block the α chain of the IL-2R are potent inhibitors of T-lymphocyte proliferation in vitro, but when used in
humans (for example, Basiliximab employed as induction therapy in kidney transplant recipients,are
relatively modest immunosuppressive agents. One explanation for this paradox is the presence of several other cytokines that also support the proliferation and survival of T lymphocytes. These include IL-4, IL-7, IL-9, IL-15,andIL-21.
A JAK3 inhibitor, Tofacitinib, is currently
available for the treatment of rheumatoid arthritis and psoriasis. Although it proved to be noninferior to cyclosporin in preventing rejection, it has not been approved for use in transplantation, possibly owing to increased incidence of infections when combined with other immunosuppressive agents.
Primary T cell activation involves the integration of three distinct signals delivered in sequence: (1) anti- gen recognition, (2) costimulation, and (3) cytokine- mediated differentiation and expansion. Strong im- munostimulatory events such as immunotherapy or infection induce profound cytokine release causing ‘‘bystander’’ T cell activation, thereby increasing the potential for autoreactivity and need for control. We show that during strong stimulation, a profound suppression of primary CD4+ T-cell-mediated im- mune responses ensued and was observed across preclinical models and patients undergoing high- dose interleukin-2 (IL-2) therapy. This suppression targeted naive CD4+ but not CD8+ T cells and was mediated through transient suppressor of cytokine signaling-3 (SOCS3) inhibition of the STAT5b tran- scription factor signaling pathway. These events re- sulted in complete paralysis of primary CD4+ T cell activation, affecting memory generation and induc- tion of autoimmunity as well as impaired viral clear- ance. These data highlight the critical regulation of naive CD4+ T cells during inflammatory conditions.
REFERENCES
Appay, V., and Rowland-Jones, S.L. (2002). Premature ageing of the immune system: the cause of AIDS? Trends Immunol. 23, 580–585.
Attia, P., Phan, G.Q., Maker, A.V., Robinson, M.R., Quezado, M.M., Yang, J.C., Sherry, R.M., Topalian, S.L., Kammula, U.S., Royal, R.E., et al. (2005). Autoimmunity correlates with tumor regression in patients with metastatic mel- anoma treated with anti-cytotoxic T-lymphocyte antigen-4. J. Clin. Oncol. 23, 6043–6053.
Berner, V., Liu, H., Zhou, Q., Alderson, K.L., Sun, K., Weiss, J.M., Back, T.C., Longo, D.L., Blazar, B.R., Wiltrout, R.H., et al. (2007). IFN-gamma mediates CD4+ T-cell loss and impairs secondary antitumor responses after successful initial immunotherapy. Nat. Med. 13, 354–360.
Boyman, O., Kovar, M., Rubinstein, M.P., Surh, C.D., and Sprent, J. (2006). Selective stimulation of T cell subsets with antibody-cytokine immune com- plexes. Science 311, 1924–1927.
Mohammed Sobair
3 years ago
interaction between the antigen-specific T cells and APCs provides the first signal (signal
1) for T-cell activation.
The most proximal event phosphorylation of the immunoreceptor tyrosine-based
activation motifs (ITAMs) of the TCR/CD3 complex,
followed by activation , Ras/ERK MAPK cascade,
the Ca/calcineurin/NFAT pathway, and the PKC/NFχB pathway.
immunosuppressive agents affecting signal 1 voclosporin,
Another alefacept, CD2-binding portion of the human lymphocyte function-associated
antigen-3 (LFA-3) linked to the Fc portion of human IgG1, is being tested in kidney
transplantation.
a co-stimulatory signal, signal 2, is required for T-cell; . Belatacept, a selective co-
stimulation blocker.
Signal 3: Signals provided by inflammatory cytokines, pre-dominantly IL-12 and type I
interferons (IFNα/β), and possibly IL-21, are now considered to be required for a
productive T-cell response and are termed signal 3.Basiliximab, Daclizumab is IL_2
inhibitors.
Ahmed Ziada
3 years ago
T cell activation signals are
signal 1
binding of antigen presenting cell to T lymphocytes which activate Series of reactions together with signal 2 cell proliferation occurs
examples of immunosuppression related its action with this signal are CNR inhibitors and anti CD3 like ATG
signal 2
co stimulatory signal together with signal 1 lead to cytokine release examples of immunosuppression its action related to this signal or anti-CD 28 like Blatacept
signal 3
activation signal lead to cytokine release and clonal proliferation
examples of immunosuppression depends on this signal is auntie CD 25 what is Basilliximap and daclizumab
Amit Sharma
3 years ago
What is the 3-signal model of T cell activation?
The activation and intended action of T cell depends on 3 signals:
Signal 1: Activation
Signal 2: Proliferation/ survival
Signal 3: Differentiation
Signal 1: It involves binding of MHC on APC to the T cell receptor (TCR) on the lymphocyte via CD3. It also involves CD4 and CD8 co-receptors on T lymphocytes. The TCR-CD3 cluster activation depends on tyrosine kinase which activate phospholipase C gamma catalyzing brakdown of PIP2 (phosphatidylinositol biphosphate) into DAG (Diacylglycerol) and IP3 (Inositol 1,4,5 triphosphate). IP3 increases intracellular calcium leading to activation of the calcineurin pathway. DAG activates protein kinase C and mitogen activated protein kinase pathways which, in association with the calcineurin pathway, activate transcription factors like NFkB, NFAT and AP-1 for the cytokine genes needed for signal 3.
Signal 2: It involves interaction between co-stimulatory molecules on T lymphocytes and their ligands on the APC. These co-stimulatory molecules are upregulated or induced after signal 1.
Examples of these molecules include:
1) Integrins: LFA-1, CD2 bind to ICAM-1/ICAM-2 and CD58 respectively.
2) B7-CD28: binds CD80 an CD86 on mature APCs
3) CD40: bind to CD40L (CD154)
4) CD27: bind to CD70
5) CD137: bind to 41BBl
6) OX40: bind to OX40L
Signal 3: It involves the secretion of cytokines from the APC as well as the lymphocytes, leading to T cell proliferation and differentiation into effector subsets. The cytokines involved are interleukines (IL), especially IL-2. Binding of IL-2 to IL-2 receptor (IL-2R) leads to T cell proliferation. Stimulation of IL-2R (CD25) causes mTOR activation, cytokine release and cell proliferation.
Give example of the mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model?
Signal 1: OKT3, (Anti CD3) Costicosteroids, Calcineurin inhibitors (Cyclosporine, Tacrolimus).
Signal 2: Efalizumab (anti LFA-1), Alefacept (Anti CD2), Belatacept (Anti B7), Corticosteroids
Signal 3: Anti IL-2 receptor antibodies (Basiliximab, Daclizumab), mTOR inhibitors (Sirolimus, Everolimus), MMF and azathioprine (cell-cycle inhibition), Corticosteroids
Mahmoud Hamada
3 years ago
CD3 is crucial for t Lymphocyte signaling.
it is associated with T cell receptor on cell membrane.
Ahmed Omran
3 years ago
Knowing of3- signal model is needed for development and manufacturing of immunosuppressive agents.
Signal 2;co stimulation is essential for T cell proliferation and differentiation into either cytotoxic or helper cells.
CD3 complex is a protein related to TCR. Antigen binding to TCR leads to signal cascade transduced through adjacent CD3 to TCR ,leading to T cell activation.
Mohamed Essmat
3 years ago
-signal 1 : signaling * In which TCR of lymphocytes binds to MHC peptide complex on the APCs . -signal 2 : co stimulation Which is binding co stimulatory molecules on T lymphocytes by their ligands on the APCs , -signal 3 : cytokine release Cytokines released from both APCs , and Lymphocytes, help in T cell activation and differentiation into effector T cells . *Drugs targeting Signal 1: Anti CD3 ( OKT3) *Drugs targeting Signal 2 Belatacept Calcineurin inhibitors (cyclosporine A, tacrolimus) *Drugs targeting Signal 3: Anti-IL-2 receptor (basiliximab, daclilumumab) mTOR inhibitors (rapamycin, everolimus)
Nasrin Esfandiar
3 years ago
Three single model describe T cells activation caused by reaction to allograft. Signal one is TCR (T cell receptor) binding to complex of HLA and donors specific antigen on antigen presenting cells.OkT3 is an example of monoclonal antibody that blocks T cell activation via inhibition of CD3 complex. This signal generates DAG and IP3. IP3 increases intracellular calcium and activates calcineurin pathways that finally activate transcription factors that are required for T cell activation. CNIs bind to immunophilins and their complex inhibit calcineurin pathway and so T cell activation. Signal 2 is co-stimulation .Full T cell activation needs engagement of these receptors on T cells by their ligands which are present on antigen presenting. Failure of this signal causes anergy on T cell death. Belatacept is an example of drug that inhibits this signal by binding to B7 molecules and prevents attachment to their ligand CD28. Signal 3 or cytokine refers to secretion of special proteins by T cells to proliferate and differentiate them in to effector subsets. Example for inhibition of this signal is provided by basiliximab which is an anti-CD25 monoclonal antibody and blocks α chain of IL -2 receptor.
Tahani Hadi
3 years ago
There are 3 signals responsible for T cells activation and initiation of the immune response direct indirect and semi direct signals
Direct or activation signal : responsible for CD8+T cells activation and acute cell mediated rejection and it’s occur by T cell activation through direct contact with donor APC this will lead to form complex with CD3 subunits causing subsequent activation and phosphorylation processes with different proteins leading to activation and proliferation of T cells ,but this signal alone is not sufficient to keep active T cells and it needs signal 2 activation for maintaining immune response.
Signal 2 indirect or co stimulatory pathway without signal 2 activated T cells through signal 1 will die ,by this signal T cells will become active by activation of recipient APC by donor cells inside the graft leading to tyrosin kinase activation which cause CD 4+ T cells activation and by this B cells and macrophages activity can be organized activation of this signal is responsible for chronic graft rejection.
Signal 3 or cytokines this occurs when both co stimulation and and Ag recognition happens lead to cytokines and cell gene activation causing signals activation and this cause molecules expression and clonal proliferation all will lead to T cells activation .
Signal 1 and 2 both are mandatory for naive T cells activation which will produce IL -2 and this help T cells activation and survival , interleukin- 2 production is 5he target of many immunosuppression medications.
Effects of immunosuppression on T cells activation signals :
Cyclosporin : binds to cyclophilin and inhibits calcineurin phosphatase and blocks IL-2 transcription and prevent cell activation.
Anti IL-2 (basiliximab ,daclizumab):is a monoclonal Ab bind to IL-2R which has similar affinity like IL-2 so it’s effect T cells activation and help to decrease the incidence of ACR within first 6 months post transplant, More specifically basiliximab directed against CD 25 which is a component of IL-2 receptor and by its binding it will cause cessation of clonal expansion and proliferation.
ATG : is polyclonal Ab that cause T cells depletion by it’s specificity against TCR CD3 ,CD4 and CD8 and also on cytokine receptors this will cause decrease T cell activation and decrease cytokines release .
Alemtuzumab: is monoclonal Ab that has specific effect in CD52 expressed on both T and B cells .
OKT 3 : it’s action by inhibiting CD3 – TCR.
Asmaa Khudhur
3 years ago
3-Model Signals ;
* Signal 1 (TCR binding signal) ; this signal occur when Antigen complexed with MHC molecule class I or II associated with APC bind T cell receptor. Drugs targeting signal 1 includes ; CNI, OKT3
* Signal 2 ( co-stimulation signal) ; interaction of co-stimulator molecules on APCs and T Cells e.g. CD80/86 and CD28. This activate pathways of protein kinases and together with signal 1 leads to IL-2 production. Therapies targeting signal 2 includes ; Belatacept, Afalizumab
* Signal 3 ( cytokine signal) ; signal1 and signal 2 lead s to induction of cytokine genes including ; IL-2 & IL2 receptors .This causes T cell proliferation and differentiation. Drugs targeting signal 3 includes ; mTORi, antiptroliferative, IL-2 blockade
Fatima AlTaher
3 years ago
For activation of naïve T cells to effector cells this requires 3 steps pathway
Step 1 : begins by binding of the Ag on surface of APCs to TCR /CD3 complex on T lymphocyte surface.
Step 2 (co stimulatory ) : via binding of B7 APC surface to CD28 on T cell. Both Step 1 and 2 will activate intracellular pathways leading to production of IL2 and other growth promoting cytokoines. Stimulation of the IL-2 receptor (CD25) leads to activation of mTOR and provides signal 3, that triggers cell proliferation
1- CNI : act by binding to their certain cytoplasmic receptor protiens to form a complex which then bind to calcinurin enzyme ( is a a phosphatase enzyme that dephosphorylates NFAT to facilitates its translocation
of NFAT to the nucleus with subsequent decrease production of IL2 required for T cell proliferation , So CNI inhibits T cell proliferation.
2-Drugs targeting signal 2:
a- Belatacept : binds to B7 with high affinity and inhibits the costimulatory pathway.
3- Drugs targeting signal 3:
a-Basiliximab : monoclonal antibody, anti–IL-2 receptor or
anti-CD25 antibody.
b- mTOR inhibitors eg sirolimus and everolimus
c- Corticosteroid: Blocks IL-1 and IL-2 production, suppressing early phase of immune response.
Nazik Mahmoud
3 years ago
T cell activation need 3 signals:
Signal 1:
Recognition of MHC complexes on antigen presenting cells by TCR lead to activation of multiple intercellular pathways. Drugs that inhibit this signal are CNI
Signal 2:
Is a co stimulation provided by interaction of T cell surface molecules with ligand on APC. Inhibited by Belatacept
Signal 3:
When TCR accompanied by co stimulation lead to activation of many genes required for T cell proliferation including IL-2,that why this signal blocked by basiliximab
Professor Ahmed Halawa
Admin
3 years ago
Dear All Remember the following:
CD3 is an essential component of T cell receptor. OKT3 (withdrawn from the market due to side effects) and ATG act on CD3 (bind to CD3 and cause inactivation of the Tcell receptor).
CD3 can be used in monitoring IS (ATG and OKT3). However ATG these days is monitored mainly by the lymphocyte count and platelet count.
Memory cells can be activated with minimal co-stimulation
Ben Lomatayo
3 years ago
3-Model Signals ;
Signal 1 (TCR binding signal) ; Antigen complexed with HLA molecule class I or II associated with APC bind T cell receptor. This lead to activation of multiple intracellular pathways including IL-2. Therapies targeting signal 1 includes ; CNI, OKT3
Signal 2 ( co-stimulation signal) ; interaction of complimentary co-stimulator molecules on APCs and T Cells e.g. CD80/86 and CD28. This activate pathways of protein kinases and together with signal 1 leads to IL-2 production. Therapies targeting signal 2 includes ; Belatacept, Afalizumab
Signal 3 ( cytokine signal) ; signal1 and signal 2 lead s to induction of cytokine genes including ; IL-2 & IL2 receptors .This causes T cell proliferation and differentiation. Thearpies targeting signal 3 includes ; mTORi, antiptroliferative, IL-2 blockade
Reference ; Oxford handbook of nephrology & hypertension, second edition
saja Mohammed
3 years ago
Why we need to know the 3-signal model?
with our understanding of the costimulatory signale and its role inTcell activation and proliferation to different subsets of effectors T cells and Tregs cells thier divers mechanisms of action on immune system, the long-term renal allograft survival has become an achievable goal with the innovation of the new selective costimulation blockade monoclonal ABs like antiCD40 AB which target CD40)/CD154 costimulatory pathway, has just completed a phase 2 trial with a CNI-free regimen. an anti-CD40, is also to be tested in a phase 2 trial in renal transplantation. another selective CD28 blocking AB Nonagonistic CD28 antibodies have re-emerged with two anti-CD28 candidates still in preclinical development.in addition to already used CTLA4Ig,belatocept
1-Current status of costimulatory blockade in renal transplantation:
Wojciechowski, David; Vincenti, Flavio
Current Opinion in Nephrology and Hypertension: November 2016 – Volume 25 – Issue 6 – p 583-590
2-An update on chemical pharmacotherapy options for the prevention of kidney transplant rejection with a focus on costimulation blockade
Florian Kälble 1, Matthias Schaier 1, Sebastian Schäfer 1, Caner Süsal 2, Martin Zeier 1, Claudia Sommerer 1, Christian Morath 1
Wessam Moustafa
3 years ago
The 3 signal model of T cell activation:-
*signal 1 : signaling *
In which TCR of lymphocytes binds to MHC peptide complex on the APCs .
* signal 2 : co stimulation
Which is binding co stimulatory molecules on T lymphocytes by their ligands on the APCs ,
Absence of this step could lead to deletion of T lymphocytes
* signal 3 : cytokine release
Cytokines released from both APCs , and Lymphocytes, help in T cell activation and differentiation into effector T cells , also help in regulation of T cells .
Handbook of kidney transplantation
Gabriel M.Danovitch
Theepa Mariamutu
3 years ago
Mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
Signal 1:
CNI
• immunosuppressive effect depends on the formation of a complex with cytoplasmic receptor proteins:
o cyclophilin for cyclosporine
o tacrolimus-binding protein (FKBP) for tacrolimus
• Cyclophilin or FKBP complex dephosphorylates NFAT cells and fascilitates passage thru membrane
• Inhibition of calcineurin will retard the expression of critical cytokine gene that promote T cell activation:IL-2,IL-4,IFN-G and TNF-A including transcription of genes such as H-ras, c-myc, CD40 ligand
• Enhances TGF-B which also inhibits IL-2 and Cytotoxic T cells
• When receiving CNI, CD4+ T cell have reduced IL-2 production to a degree that inversely correlate to drug levels
Corticosteroid
• Blocking T cell derived and APC derived cytokine and cytokine receptor expression.
• Inhibit the function of dendritic cells
• Inhibit translocation to the nuclear of nuclear factor KB
• Inhibit expression of IL-1,IL-2,IL-3,IL-6, TNF-a and IFN-g
Signal 2:
• Belatacept used as a substitute for CNIs at the time of transplant. They mimic the down regulatory T cell response (CTLA4 which competitively binds to CD80/86)
• Efalizumab(anti LFA-1)and Alefacept (anti CD2) where stopped usage because of serious side effects such as progressive multifocal leukoencephalopathy(PML)
• Co-stimulation Blockade by CD154:CD40 mABs were shifted towards CD40 as blocking CD154 with mABs was associated with increased thrombotic effects.
Signal 3:
• IL-2 receptor can be blocked by Basiliximab, targeting α chain or CD25
• The activated the mTOR signaling cascade can be inhibited by Sirolimus and Everolimus which are kinase inhibitors
• The cell cycle activation can be blocked by MMF, azathioprine
References
Bamoulid, J., Staeck, O., Crépin, T., Halleck, F., Saas, P., Brakemeier, S., Ducloux, D. and Budde, K., 2016. Anti-thymocyte globulins in kidney transplantation: focus on current indications and long-term immunological side effects. Nephrology Dialysis Transplantation, p.gfw368.
Handbook of Kidney Tramnsplantation by Gabriel M. Danovitch
Last edited 3 years ago by Theepa Mariamutu
Theepa Mariamutu
3 years ago
What is the 3-signal model of T cell activation?
Signal 1:TCR engagement
• Antigen binding component of TCR complex is heterodimer consists of an Alpha and Beta chain, that recognises proteolysed peptides presented as self major histocompatibility complex (MHC) on APC
• Allorecctive T cells recognise alloantigens via 2 pathways: direct and indirect pathways
o Direct pathway – T cell recognise an intact donor MHC antigens on tissue allograft
o Indirect pathway – T cell recognise donor peptide antigens presented by self MHC
Signal 2:co-stimulation
• Crucial for productive T cell activation
• CD 28 – expressed on the surface of all naïve CD4 and CD 8 T cells
• TCR stimulation without CD28 results in abortive activation of T cells and anergy
•. Binding CD28 to its ligands B7-1(CD80), B7-2(CD86) on APC promotes activation T cell and trigger IL-2 production, clonal expansion and generation of effector and memory T cell
• CTLA-4 ( cytotoxic T lymphocyte antigen -4) is a structural homologous to CD28
• Functional role of CTLA-4 was suggested by the ability of CD-152 Ig fusion protein to inhibit T cell activation
• CD152 builds to CD80,CD86 with higher affinity and provides inhibitory signal to T cells
• costimulatory molecules related to the CD28 immunoglobulin superfamily -inducible costimulatory molecule (ICOS)-B7h and the programmed death (PD)-PD-L1/PD-L2 pathwayhave been discovered
• ICOS-B7h engagement augments T cell activation is by inducing the expression of CD40L (CD154) on T cells which stimulates the activation of CD80 and CD86 molecules on APCs and provides a positive feedback in sustaining CD28 costimulation
• The (PD)-PD-L1/PD-L2 pathway is a negative costimulatory pathway that has an important role in the maintenance of self-tolerance. PD-1 is expressed on activated CD4 and CD8 T cells, B cells, NK cells and macrophages.
Signal 3: CD4 help and inflammation
• Naïve CD 8 T cell require 3rd signal for the optimal production of effector and memory T cell(CD4 T cell and presence of inflammatory cytokines)
REFERENCES
Bamoulid, J., Staeck, O., Crépin, T., Halleck, F., Saas, P., Brakemeier, S., Ducloux, D. and Budde, K., 2016. Anti-thymocyte globulins in kidney transplantation: focus on current indications and long-term immunological side effects. Nephrology Dialysis Transplantation, p.gfw368.
Alyaa Ali
3 years ago
T cell activation requires recognition of antigens on antigen presenting cells (APCs) , costimulators and cytokines produced by APCs and T cells themselves Signal 1 :
generated by interaction between the TCR of T cell and the MHC/peptide complex on the APCs and this is transduced through the CD3 complex. Signal 2: T cells require a number of secondary signals ( costimulatory ) through accessory molecules to become activated and respond to the threat.
The second signal comes from the recruitment of co-stimulators. common co-stimulators are the CD28 receptor and B7 molecules. Signal 3:
once T cell received first and second signals , it receives more instructions in the form of cytokines which lead to differentiation of T cells into two different types with distinct cytokines profile
type 1 helper cells Th1 secrete cytokines that include IL-2 , IFN and IL-12 , these cytokines stimulate delayed type hypersensitivity response , cytolytic activity and production of antibodies
type 2 helper cells Th2 secrete cytokines that include IL-4, IL-5, IL-10 and IL-13 , these cytokines activate eosinophils and stimulate production of IgE. Immunosuppression Drugs targeting Signal 1:
Monoclonal antibodies anti CD3 ( OKT3) Drugs targeting Signal 2
Belatacept : Inhibition of signal 2 in T-cells (competition with CD28 for CD80/CD86 binding) inhibiting T-cell co-stimulation
Daclizumab : Inhibition of signal 2 in T-cells (binds to CD25, the alpha subunit of the IL-2 receptor) preventing IL-2-induced T-cell activation.
Calcineurin inhibitors (cyclosporine A, tacrolimus) : Inhibition of signal 2 transduction in T-cells { inhibits calcineurin via cyclophilin (cyclosporine A) or via FKBP 12 (tacrolimus) blocking IL-2 transcription}. Drugs targeting Signal 3:
Anti-IL-2 receptor (basiliximab, daclilumumab) :Inhibition of T-cell proliferation and signal 3
mTOR inhibitors (rapamycin, everolimus) : Inhibition of signal 3 transduction in T-cells (inhibits mTOR), preventing IL-2-induced T-cell proliferation References
Baroja-Mazo A, Revilla-Nuin B, Ramírez P, Pons JA. Immunosuppressive potency of mechanistic target of rapamycin inhibitors in solid-organ transplantation. World J Transplant. 2016;6(1):183-192.
immunosuppressive agents targeting 3-signal model of T cell activation
Esmat MD
3 years ago
Complete activation of alloreactive T lymphocytes depends on three signals and every immunosuppressive agent can influence this process by acting on different signals.
Signal 1 is delivered by binding of TCRs on T lymphocytes to HLA-peptide complexes on APCs. This binding triggers a signaling cascade that leads to T lymphocyte activation. These signals are transduced not only through TCR but through the adjacent CD3 complex that associates with the TCR. CD4 and CD8 co-receptors on T lymphocytes also participate in the signal mediated by the TCR-CD3 cluster. Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation. A typical example is OKT3, the first monoclonal antibody that was used in transplantation and was withdrawn because of serious side effects.
Activation of PKC and MAP kinase, and calcineurin pathway by DAG and IP3 respectively, ultimately activates transcription factors such as NFkB that induce the transcription of cytokine genes required for T lymphocytes proliferation and differentiation. The calcineurin pathway is the target of CNIs. CNIs bind to immunophilins (cyclophilin in the case of cyclosporin and FKBP in the case of tacrolimus). Eventually, the drug-immunophilin complex blocks the activation of calcineurin by calmodulin, the calcium dependent enzyme.
Signal 2 is required for full proliferation and differentiation of T lymphocytes into effector lymphocytes (either cytotoxic or helper cells). Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs. Failure to provide the second signal results in failed T lymphocytes activation that leads to T lymphocyte depletion or anergy.
Main co-stimulatory pathways:
LFA-1 and CD2 on T lymphocytes and their ligand ICAM1/ICAM2 and CD58 on DC respectively. Antibodies against LFA-1 or CD2 delay kidney transplant rejection in animal models, but development of anti-LFA-1 (Efalizumab) and anti-CD2 antibodies (Alefacept) has been ceased because of serious adverse effects such as PML.
CD28 on all naïve T lymphocytes and B7.1 (CD80) and B7.2 (CD86) on mature APCs, namely DCs. This pathway potentiates the transcription of key genes that are required for T lymphocyte proliferation, for example IL-2 and IL-2 receptor genes.
In addition to these ligands that transduce a costimulatory or activating signal, cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), which also binds B7-1 and B7-2, provides an inhibitory signal. Although CD28 is expressed on resting T cells, CTLA4 is expressed on the cell surface only after initial T cell activation.
Engagement of CTLA4 by binding with high affinity to its B7 counter-receptors appears to downregulate immune responses, suggesting that CTLA4 plays a critical feedback role in terminating T cell responses.
CTLA-4-Ig (Belatacept), which binds with high affinity to B7 molecules and prevents them from engaging CD28, is utilized for the prevention of kidney allograft rejection.
However, the beneficial effect of CTLA4Ig is abrogated when coadministered with cyclosporine.
CD154 (CD40L) on activated CD4+ T lymphocytes and CD40 mainly on B lymphocytes, DC, and macrophages. Engagement of CD40 by CD154 is an important mechanism by which CD4+ lymphocyte provides help to B lymphocytes for isotope switching (shift from IgM producing isotypes to the more effective IgG antibody isotypes). It upregulates B7 expression and enhances cytokine production by DC and ultimately leads to further co-stimulation of T lymphocytes. Although antibodies that block the CD40-CD154 pathway could be very effective anti-rejection agents, in humans cause serious thromboembolic side effects.
Signal 3 is related to the cytokines that are involved in T lymphocyte activation, proliferation, and differentiation to the multiple effector subtypes. IL-2 is a cytokine with a strong capacity to induce clonal expansion of antigen activated lymphocytes. Upon activation, T lymphocytes express a third chain alpha or CD25, which increases the affinity of the IL2R to IL2 by approximately 1000 folds. Anti-CD25 monoclonal antibodies such as Basiliximab are relatively modest immunosuppressive agents for induction therapy in kidney transplantation. Daclizumab was removed from the market.
Stimulation of CD25 leads to activation of mTOR. The mTOR pathway is an important intracellular pathway involved in T lymphocyte proliferation, and immunosuppressive drugs such as Sirolimus and Everolimus inhibit this pathway.
MICHAEL Farag
3 years ago
Signal One T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen). Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of antigen presenting cells (APCs). The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex, on the surface of the APC. This triggers initial activation of the T cells. The CD4 and CD8 molecules then bind to the MHC molecule too, stabilising the whole structure. This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion. This normally takes place in the secondary lymphoid organs.
Signal Two In addition to TCR binding to antigen-loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat. In the case of helper T cells, the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) – and initiates T-cell proliferation. This process leads to the production of many millions of T cells that recognise the antigen. In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152). This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response. Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).
Signal Three Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines. These determine which type of responder the cell will become – in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine IL-12), Th2 (IL-4), or IL-17 (IL-6, IL-23). Each one of these cells performs a specific task in the tissue and in developing further immune responses.
**mechanisms of immunosuppressive on the 3 model signal activations are shown in the attached picture
Ibrahim Omar
3 years ago
Signal 1: TCR Signaling
Binding of antigen (HLA–peptide complex) to the TCR triggers a signaling cascade that leads to T- lymphocyte activation. These signals are not transduced through the TCR proper, but through the adjacentCD3 complex associated with the TCR. CD4 and CD8 co-receptors on T lymphocytes also participate in the activation signal mediated by the TCR– CD3 cluster by binding to the same MHC molecule that engages the TCR. The activation signal transduced by the TCR–CD3 cluster is dependent on tyrosine kinases that cause the recruitment and activation of the enzyme phospholipase C-g (PLC-g). PLC-g catalyzes the breakdown of the membrane lipid phosphatidylinositol biphosphate (PIP2) to generate two second-messengers: Diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). DAG activates the protein kinase C (PKC) and mitogen-activated protein (MAP) kinase pathways, whereas IP3 triggers the calcineurin pathway by increasing intracellular calcium concentration. Together, the PKC, MAP kinase, and calcineurin pathways ultimately activate key transcription factors (NFkB, NFAT, and AP-1) that induce the transcription of cytokine genes required for T-lymphocyte proliferation and differentiation. The calcineurin pathway is the target of the CNIs.
Signal 2: Co-Stimulation
T lymphocytes must receive a second signal to undergo full proliferation and differentiation into effector lymphocytes, which include either cytotoxic or helper cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs. Failure to provide the second signal results in aborted T-lymphocyte activation, which causes T- lymphocyte deletion (death) or anergy. Co-stimulatory molecules are either absent or are constitutively expressed on naïve T lymphocytes, but are induced or upregulated upon activation of T lymphocytes with antigen. Beltacept is an inhibitory drug for one of the co-stimulatory cell markers, namely CTLA-4. many other co-stimulatory cell markers are known and well-described.
signal 3: Cytokines
They are proteins secreted by mature APCs or the T lymphocytes themselves. They serve two main purposes: they stimulate T- lymphocyte proliferation and induce the differentiation of T lymphocytes into multiple effector subsets that have distinct phenotypes and functions. However, cytokines can also regulate T lymphocytes or act on other immune and non-immune cells to either enhance or suppress inflammation. Most cytokines are known by the term interleukin (IL) followed by a number that refers to the order in which they were discovered. Basiliximab is a monoclonal antibody and commonly used in induction of immunosuppression. it blocks the cell marker CD25, resulting in inhibition of release of these cytokines.
MOHAMMED GAFAR medi913911@gmail.com
3 years ago
1-Signal
bindinging of TCR/CD3 to antigenic peptides presented by the APC( donor or recipient).This trigger leads to dramatic increase in tyrosine phosphorylation of multiple cellular proteins, responsible for activation of calcineurin pathway, mitogen activated protein (MAP) and protein kinase C (PKC), which finally activate key transcription factors required for T cell activation, proliferation and differentiation.
DRUGS USED AT THIS SIGNAL LEVEL
Muromunab OKT3 targeting CD3 complex which blocks T cells activation. CNI block calcineurin pathway
2-Signal 2: Co-stimulation
engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen presenting cells. in the absence of this signal, T cells become anergic and these anergic cells can inhibit the activation of neighboring T cells.
many families involved in the co-stimulatory pathway including : CD7 ( Ig family), CD 154 ( tumor necrosis factor family), C3a & C5a receptors ( G protein coupled receptors) & lectin receptors,
DRUGS USED AT THIS SIGNAL,
Belatacept ( CTLA4Ig) acts on this signal.
3-Signal 3,cytokines:
after T cell activation , it will increase expression of many cytokines like IL2 . IL2 a receptor (CD25) on the surface of T cells once activated will lead to series of events through mTOR pathway leads to T cells proliferation and differentiation .
DRUGS USED AT THIS SIGNAL
a-basiliximab monoclonal antibody that blocks IL2a receptor used in induction of immunosuppression during transplantation of low risk patients.
b-mTOR inhibitors : everolimus and sirolimus
REFFERNCE JASN ePress. Published on April 15, 2015 as doi: 10.2215/CJN.06620714
Last edited 3 years ago by MOHAMMED GAFAR medi913911@gmail.com
Ahmed Fouad Omar
3 years ago
T cell activation:
It is the process in which naïve T cell become activated into effector T cell APC(dendritic cells play a prominent role) is capable to present antigen bound to MHC to naïve CD4 (T-helper) or CD8 (T –cytotoxic).
First signal (stimulation signal):
It is due to binding of the alloantigen on the graft (HLA –antigen complex) with the (TCR –CD3 complex) of the CD4 and CD8 T cell. The activation signal is dependent on tyrosine kinases that activate the enzyme phospholipase to activate 3 main pathways which are protein kinase C, Map kinase and calcineurin .this result in activation of transcription factors that induces transcription of the cytokines important for T lymphocyte proliferation and differentiation.
Agents targeting signal 1:
Anti-TCR Agents: OKT3, The murine anti-CD3 was the first mAb approved as a drug for human use in renal transplantation .It is now no longer in production because of significant side effects related to the mitogenicity associated with its murine source.
Calcineurin Inhibitors: Cyclosporine introduced in early 1980s and the more potent Tacrolimus introduced in late 1990s Besides, the newer Voclosporin . The introduction of these drugs resulted in dramatic reduction in rejection episodes
Second signal(Co-stimulation signal):
This co stimulatory signal allow T cells to undergo full proliferation and differentiation into effector lymphocytes( T helper and T cytotoxic). This occurs through engagement of the co-stimulatory receptors on T lymphocytes by their ligands on APCs resulting in calcineurin activation that dephosphorylates nuclear factors of activated T cell, enabling IL-2 transcription to start signal 3. This prevents T cell to go into anergy(death).
The most important currently is the interaction between CD28 on the T cell surface with its APC surface ligands, B7-1 or B7-2 (often known as CD80 or CD86) .
Other Co -stimulatory signals include interaction between integrins (LFA-1, and LFA-2(CD2)on T lymphocytes that bind to ICAM 1, ICAM2 and CD58 on APC.
Also, CD154 also called CD40L expressed on activated T helper cells that interacts with CD40 which is present on dendritic cells, macrophages and B lymphocytes . This interaction is important in switching B lymphocytes from producing IgM to IgG beside enhancing T cell activation.
Agents targeting signal 2:
Abatacept followed by Belatacept used as a substitute for CNIs at the time of transplant. They mimic the down regulatory T cell response (CTLA4 which competitively binds to CD80/86)
The use of Efalizumab(anti LFA-1) and Alefacept (anti CD2) where halted because of serious side effects like progressive multifocal leukoencephalopathy(PML) as a result of JC virus reactivation.
Additionally, Co-stimulation Blockade by CD154:CD40 mABs were shifted towards CD40 as blocking CD154 with mABs was associated with increased thrombotic effects.
Third signal( cytokine production):
It is a potent signal involving cytokine production mainly IL-2 that result in activation of the mTOR signaling pathway that causes the T cell to enter a cell cycle. Accordingly, T lymphocyte will undergo maturation and differentiation into multiple effector subsets with different phenotypes and functions. So, CD4 T lymphocytes differentiate to (TH1, TH2, TH17, Tfh) and a regulatory population called Treg, while CB8 T lymphocytes differentiate to T cytotoxic, together with the long lived memory T cells.
Agents targeting signal 3:
IL-2 receptor(has α , β, ϒ chains) can be blocked by mAB Basiliximab, targeting α chain or CD25( only modest immunosuppression) as the ϒ chain family can be still activated by other interleukins and targeting ϒ chain can cause severe lymphopenia.
The activated the mTOR signaling cascade can be inhibited by Sirolimus and Everolimus which are kinase inhibitors(used mainly in treatment of skin cancers like Kaposi sarcoma)
The cell cycle activation can be blocked by MMF, azathioprine
Primary T cell activation requires three signals in sequence:
signal 1, where T cell receptor (TCR) recognize antigen in the context of major histocompatibility complex (MHC) . This is blocked by anti CD3 monoclonal antibodies:OKT3 ,CNi
signal 2, involving binding of costimulatory molecules, B5,CD28, . This is blocked by Lulizumab , CTLA-4-IgG
signal 3, where cytokine,IL2 and IFN. direct and amplify T cell differentiation and proliferation .
This is blocked by antiCD25, mTOR, basiliximab , Daclizumab Halloran PF. Immunosuppressive drugs for kidney transplantation. N Engl J Med. 2004 Dec 23;351(26):2715-29. doi: 10.1056/NEJMra033540. Erratum in: N Engl J Med. 2005 Mar 10;352(10):1056. PMID: 15616206. Dirk R. J. Kuypers, Yves F. C. Vanrenterghem, Monoclonal antibodies in renal transplantation: old and new, Nephrology Dialysis Transplantation, Volume 19, Issue 2, February 2004, Pages 297–300, https://doi.org/10.1093/ndt/gfg555
Ramy Elshahat
3 years ago
Both cd4 and Cd8 tcell have TCR which recognize antigens by APC (adaptive immunity) which presents them on it’s MHC.
Signal 1:
TCR bind to antigen presented by APC then
Cd3 which is group of proteins on Tcell surface undergoes conformation changes through increase intracellular tyrosine phosphorylation,mitogen activated protein(MAP),calcinurin pathway and protein kinase c(PKC) leading to activation of transcription factor called nuclear factor of activated t cell(NFAT).
signal 2
Costimulatory pathway involving multiple families like cd28 on tcell with b7-1(Cd80)/b7-2(cd86) and integrins like LFA-1/LFA-2 on Tcell with ICAM-1and ICAM2 on APC which associated with expression of CTLD4 which usually not expressed on conventional tcell except after its activation and carry downregulation effect.
survival signals including ICOS, 4-1BB and OX40 on tcell which usually not expressed on tcell except after APC presentation of antigen (not like cd28 and TCR)
Signal 3
APC and Tcell secrets cytokines like IL2(th1),IL4(th2),IL17(th17)and IL 12.
IL2 bind to IL2 Rs on other naive tcells leading to its maturation and differentiate into effector Tcells(th1) and starting graft rejection.
Tcell proliferation is done by a cascade of intracellular events activates the mTOR signaling pathway which induces the T cell to enter the cell cycle resulting in T cell activation
mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
signal 1:
Steroids: the antiinflammatory effect of steroids affect almost all the 3signal of tcell activation..it inhibit Cd3complex and NFAT from signal 1
Okt3: monoclonal antibodies against Cd3complex
CNI:signal 1 inhibit the calcinurin pathway and include cyclosporine, tacrolimus and valcosporin
ATG polyclonal antibodies which cause depletion of both t cells and b cell
Alemutzumab anti CD52 on both tcell and b cell
Signal 2:
Belatacept which is 2aa modifications form abtacept which has high affinity to both b7-1/b7-2 on APC preventing its bind to cd28 on tcell leading to cell anergy and inhibition.
Signal 3:
Basilixmab: which bind to alpha part of IL2 Rs and prevent tcell proliferation (non depleting induction)
mTOR: sirolimus and evrolimus which block mTOR signaling pathway
Antiproliferative includes azathioprine and MMF by preventing tcell division.
After illustrative immunosuppressant medications site of action we can understand why steroids and CNI is the cornerstone of transplantation (signal 1)
Also how combination of immunosuppressant medications complete each other by multiple hits in tcell activation pathway.
Also explain why it’s not effective in treating ABMR.
Reference
-Glucocorticoids in T cell development, differentiation and function
Matthew D. Taves & Jonathan D. Ashwell
Nature Reviews Immunology volume 21, pages233–243
-Handbook of kidney transplantation 6th edition
-UpToDate
The activated antigen presenting cell (dendritic cells containing HLA antigens ) enter the recipient lymph nodes and activate the naïve T-cell . This activation is co-stimulated by (CD28, CTLA) molecules . The encounter between APC and TCR is the key step in activation of T-cell . The T cell receptor contains an α- and a β-chain as well as CD3 chains .The combination of dendritic cells and T cell receptors is called the “immunologic synapse,” which consists of donor HLA antigens, recipient T cell receptors and CD 4 or CD 8 molecules, co stimulatory molecules, and adhesion molecules. Maturation of these cells occurs through three intra cellular signaling pathways; (1) calcineurin pathway (2) mitogen-activated protein kinase pathway (3) JAK/STAT (Janus kinase/Signal Transducers and Activators of Transcription) pathway.
The first two pathways cause the production of IL-2, which binds to its receptor (IL-2R) on T cells . This binding leads to the activation of the mammalian target of rapamycin (mTOR), which allows the T cells to undergo cell division.
Mechanism and action of current immunosuppressive drugs
OKT3 targets CD3 chains . Belatacept blocks the costimulatory molecules . Cyclosporine and tacrolimus inhibit the calcineurin pathway . Daclizumab and basiliximab inhibit the interaction between IL-2 and its receptor . Sirolimus and everolimus inhibit mTOR . mycophenolate mofetil and azathioprine inhibit cell division. JAK3 inhibitors (CP-690550) inhibit JAK/STAT pathway.
Referance;
McKay DB, Park K, Perkins. What is transplant immunology and why are allografts rejected? In McKay DB, Steinberg SM (eds). Kidney Transplantation. A Guide to the Care of Kidney Transplant Recipients. New York, Springer, pp 25–39. Weltz A, Scalea J, Popescu M, et al. Mechanisms of immunosuppressive drugs. In Weir MR, Lerma EV (eds). Kidney Transplantation. Practical Guide to Management. New York, Springer, 2014, pp 127–141.
Thanks, Abdulrahman for your hard work Please see my question posted above
Shereen Yousef
3 years ago
Signal One; specific antigen signal
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen).
Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of APCs.
First The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held by the MHC complex, on the surface of the APC.
This triggers initial activation of the T cells. The CD4 and CD8 molecules act as coreceptors and bind to the MHC molecule , stabilising the whole structure. This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion. This normally takes place in the secondary lymphoid organs.
Activation of T cells generate second messengers DAG and IP3
DAG activate protein kinase C (PKC) and mitogen activated protein (MAP) while
IP3 trigger calcineurin pathway.
Drugs stopping signal 1 :
Calcineurin inhibitors target calcineurin pathway by calcium dependent enzyme Calmodulin
Alimtuzumab anti-CD52 monoclonal antibody also OKT3 is a monoclonal AB which binds to lymphocyte CD3 complex leading to rapid lysis.
ATG is polyclonal antibodies act against many hematopoietic antigens (CD2, 3, 4 ,8, 18)
Signal Two ;costimulation signals
In addition to TCR binding to antigen-loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat.
for helper T cells, the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) – and initiates T-cell proliferation.
This process leads to the production of many millions of T cells that recognise the antigen. In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152) which competes with CD28 for B7 and so reduces the activation signals to the T cell
to decrease the immune response.
Belatacept CTLA4-Ig bind B7 molecules preventing the engagement with CD28.to stop signal 2 activation. Belatacept is currently being tested for prevention of allograft rejection and preservation of kidney function in kidney transplant recipients as a potential replacement for calcineurin inhibitors (CNIs). It differs from abatacept by only two amino acids and has greater binding avidity to CD80 and CD86.
Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).
T cells must recognise foreign antigen strongly and specifically to mount an effective immune response and those that do are given survival signals by several molecules, including ICOS, 4-1BB and OX40. These molecules are found on the T-cell surface and are stimulated by their respective ligands which are found on APCs.
Unlike CD28 and the TCR, ICOS, OX40 and 4-1BB are not constitutively expressed on T cells. and their respective ligands are only expressed on APCs following pathogen recognition.
Signal Three cytokines
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines. These determine which type of responder the cell will become – in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine IL-12), Th2 (IL-4), or IL-17 (IL-6, IL-23). Each performs a specific task .
The resulting cell population moves out to the site of the infection or inflammation in order to deal with the pathogen. Other cells present at the tissue site of inflammation– such as neutrophils, mast cells, and epithelial cells – can also release cytokines, chemokines, short
peptides and other molecules which induce further activation and proliferation of the T cells.
T-cell activation Original author – Mary Cavanagh, Science Museum London, UK Updated by – Emily Gwyer Findlay, University of Edinburgh, UK
Thanks, Shereen Please see my question posted above
saja Mohammed
3 years ago
T cell activation. Antigens dependent T-cells receptor activation:
antigen-presentation singles when the TCRs engaged by APCs through the CD3 Complex, CD4, CD8 co-receptors also involved in this activation, monoclonal AB OKT3 blocking theTcell activation by binding to CD3 complex, and also CNI block the T cell activation at this level by binding to an intercellular protein cyclophilin and the cyclosporine /cyclophilin complex will inhibit the calcineurin pathway. T cells costimulation signal:
when second TCRS bind on the surface of the APCs expressing CD28 , B7 -1CD80) and B7-2 (CD86) this will regulate T cell clonal expansion and differentiation by B7-CD28 family of molecules the B7-1/B7-2-CD28/CTLA-4 pathway is crucial in regulating T-cell activation and induction of tolerance . New B7 and CD28 molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells (2).
Immunosuppressive drugs that act on costimulatory signals including CTLA-4Ig , belatocept.
CD40/CD154 Pathway and TNF/TNFR pathways are important for activation of APC and T cells, so blocking these pathways in animal model of transplantation was effective and hold promise for future therapeutic agents.
IFN-γ production also inhibits T-cell activation by inhibiting the expression of co-stimulators and MHC-II molecules on dendritic cells (DCs) and macrophages. cytokine production signal
which led to the activation of m TOR (mammalian target of the rapamycin) signale3 which lead to plenty of cytokines production for further activation and proliferation of the T cells effector subsets , Th1 t cells express IL2, CD25 IFN,IL12 and TNF while Th2 secrete IL4,IL5,IL10
mTOR inhibitor both sirolimus and everolimus act on signal 3 by inhibiting cytokine dependent cellular proliferation at G-S phase.
monoclonal AB anti CD25 Basiliximab and danosumab both effective as Induction therapy .
References
1-Hand book of kidney transplantation, GabrielM,Danovitch .
2-The B7-CD28 superfamily
Arlene H Sharpe 1, Gordon J Freeman, 2002 Feb;2(2):116-26. doi: 10.1038/nri727. Nat Rev Immunol.
3- T-cell activation
Original author – Mary Cavanagh, Science Museum London, UK
Updated by – Emily Gwyer Findlay, University of Edinburgh, UK
: Systems and Processe the differential activation of the two main classes of helper T cells
Binding of antigen (HLA–peptide complex) to the TCR triggers a signaling which-lead to Tlymphocyte activation through the adjacent CD3 complex.
CD4 and CD8 co-receptors on T lymphocytes also participate in the activation signal mediated by the TCR– CD3 cluster by binding to the same MHC molecule that engages the TCR. Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation.
Example for this signal is OKT3, the first monoclonal antibody used in clinical medicine, and since withdrawn that was used to treat severe acute allograft rejection.
Another example is CNI. CNIs bind to specialized proteins in the cell known as immunophilins—cyclophilin in the case of cyclosporin and FK-binding protein (FKBP) in the case of tacrolimus. The drug blocks the activation of calcineurin by the calcium-dependent enzyme calmodulin.
Signal 2—Costimulation
costimulatory molecules determine and mediate short and long-term function during priming, expansion, and death of T cells.
In addition to signal 1, T lymphocytes must receive a second signal to undergo full proliferation and differentiation into effector lymphocytes, which include either cytotoxic or helper cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs .
Activation of downstream signaling via CD28 results from ligation with B7 family proteins, CD80 or CD86. These proteins are expressed by APCs such as DCs and engage CD28 during antigen presentation.
Example for this signal is Belatacept ( CTLA4Ig)
Signal 3—Cytokines
The consequences of TCR engagement and costimulation are proliferation and differentiation of the T cells into effector cell and for production of cytokines which required to stimulate themselves, and other immune cell types, including CD8 T cells, macrophages, DCs, and B cells. IL-2 is a potent activator and proproliferative cytokine for T cells. Its effects are dependent on binding to its cell surface receptor, which has three subunits, α (CD25), β, and γ.
Blockade of the IL-2 receptor by targeting the α-chain profoundly inhibits T cell proliferation. Example : CD25 blockade with basiliximab or daclizumab have proven efficacy as induction agents in renal transplantation
1. Gabriel M. Handbook of kidney transplantation.Sixth Edition.
2. Stuart J.Kidney transplantation principles and practice . Eight Edition.
Reem Younis
3 years ago
T lymphocytes activation depend on 3 signals : signal 1: T- cell receptor for antigen (TCR) signaling:
-Here, the binding of antigens to TCR triggers a signal cascade that is not transduced through TCR proper, but through adjacent CD3 complex( which is a protein associated with TCR ).
-CD4 and CD8 co-receptors on T lymphocyte bind TCR-CD3 cluster on same MHC.
-So, antibodies target one or more proteins on CD3 complex block T lymphocytes activation. An example :
1-OKT 3:It bind to T cell receptor CD3 complex on the surface of circulating T cell, initially leading to activation but subsequently inducing the clearance of TCR complex from the cell surface and apoptosis of T cells.
2-CNI: TCR-CD3 cluster→activates phospholipase C-ɣ→generation of 2-second messangers diaacylglycerol(DAG ) and inositol 1,4,5 triphosphate(IP3) .
IP3→activates the calcineurin pathway which inhibits by CNI.
-CNI bind to specialized protein in the cell Known as immunophilins-cyclophilin in the case of cyclosporine, and FK- binding protein in the case of tacrolimus.
-The drug-immunophilin complex then blocks activation of calcineurin by calmodulin. Signal 2:Co Stimulation
-T lymphocytes need signal 2 to undergo full proliferation and differentiation to either cytotoxic or helper cells.
-It is delivered by the engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen-presenting cells (APCs)
-Failure to provide signal 2 causes T lymphocytes deletion or anergy.
-There are many co-stimulatory pathways:
1-Integrins: Integrins LFA-1 and CD2 on T lymphocytes bind to their ligands on dendritic cells. Efalizumab is anti-LFA-1 and Alefacept is anti CD2, they can lead to progressive multifocal leukoencephalopathy.
2-B7-CD28:CD28 is a co-stimulatory receptor present on all naive T lymphocytes. It is bound to the co-stimulatory molecules B7.1 and B7.2 on dendritic cells (DC). Belatacept has a high affinity to B7and prevents its bind toCD28.It is commonly used to prevent allograft rejection in kidney transplant recipients.
3-CD40-CD154:CD154 is expressed on activated CD4+ lymphocytes whereas CD40 is present on B lymphocytes, DC, and macrophages.
4- Other Co-Stimulatory pathways:41BBl-41BB, OX40L-OX40, CD70:CD70, and ICOSL-ICOS pathway. Signal 3:cytokines
-Cytokines are proteins secreted by mature APCs or T lymphocytes themselves,eg IL-2
-They stimulate T lymphocytes proliferation and induce the differentiation of T lymphocytes into multiple effector subsets.
-cytokines can also regulate T lymphocytes or act on other immune and nonimmune cells to either enhance or suppress inflammation.
-cytokines produced by APCs or by activated T lymphocytes direct the differentiation of proliferating T lymphocytes into multiple effector populations.CD4 lymphocytes differentiate into 4 major helper subpopulations and one regulatory subpopulation, while CD8 T lymphocytes differentiate into cytotoxic T cells.
-Th 1lymphosytes: their differentiation is driven by IL-12 and interferon-gamma. Ustekinumab is a monoclonal block of IL-12.
-Th17 lymphocytes: They produce IL-17 which can be blocked by monoclonal antibodies (Secukinumab ). References
1. Danovitch G.M handbook of kidney transplantation sixth edition
2.Bhorade, S. M.; Stern, E. (2009-01-15). “Immunosuppression for Lung Transplantation”. Proceedings of the American Thoracic Society. 6 (1): 47–53. doi:10.1513/pats.200808-096go. ISSN 1546-3222. PMID 19131530.
3.Benekli, M.; Hahn, T.; Williams, B. T.; Cooper, M.; Roy, H. N.; Wallace, P.; Stewart, C.; Bambach, B.; McCarthy, P. L. (September 2006). “Muromonab-CD3 (Orthoclone OKT3 ® ), methylprednisolone and cyclosporine for acute graft-versus-host disease prophylaxis in allogeneic bone marrow transplantation”. Bone Marrow Transplantation. 38 (5): 365–370. doi:10.1038/sj.bmt.1705450. ISSN 1476-5365. PMID 16862164. S2CID 31056997
Heba Wagdy
3 years ago
Alloimmune response involves 3 signals Signal 1: T cell receptor (TCR) signaling
Binding of HLA peptide complex on antigen presenting cells (APC) mostly dendritic cells (DC) to the TCR-CD3 complex on T lymphocyte
CD4 & CD8 coreceptors on T lymphocytes also help in activation of T lymphocytes
Activation depends on tyrosine kinase that activate phospholipase enzyme that catalyzes generation of second messengers DAG and IP3
DAG activate protein kinase C (PKC) and mitogen activated protein (MAP) while
IP3 trigger calcineurin pathway
PKC, MAP and calcineurin pathways activate transcription factors needed for T lymphocyte proliferation and differentiation.
Calcineurin inhibitors target calcineurin pathway
Cyclosporine bind immunophilins-cyclophilin while tacrolimus bind FK binding protein
Those complexes block activation of calcineurin by calcium dependent enzyme Calmodulin
Signal 2: Costimulation
Important for full proliferation and differentiation of T lymphocytes to effector cells
Integrins LFA-1 and CD2 on T cells bind their ligands on DC and act as costimulatory molecules for activation of T lymphocytes.
anti-LFA-1 (Efalizumab) and anti-CD2 antibodies (Alefacept) delay graft rejection in non human primates but not used clinically due to serious side effects.
T lymphocyte activation require costimulation which which involve engaging of CD80 (B7-1) and CD86 (B7-2) on mature APC with CD28 on T lymphocytes
Intracellular signalling by CD28 increase transcription of genes required for T cell proliferation as IL2 receptor genes.
CTLA4-Ig (Belatacept) bind B7 molecules preventing the engagement with CD28
used for prevention of allograft rejection
binding CD154 (on activated CD4+ T lymphocytes) with CD40 (on B lymphocytes, DCs and macrophages) induce switching of B cells from producing IgM to mor effective IgG antibodies isotypes
also, this binding enhances cytokine production by DCs leading to increase of costimulation of T cells especially CD8+
antibodies that block CD40-CD154 can prevent rejection but not used due to serious thromboembolic side effects
Other costimulatory pathways as OX40L-OX40, CD70:CD27 are induced with activation of T lymphocytes and leads to sustained activation Signal 3: Cytokines
secreted from T cells and APCs
stimulate proliferation and differentiation to effector cells and regulate T lymphocytes
IL-2:
Activation of T lymphocytes by antigen and costimulatory molecules increase expression of CD25 which increase binding of IL2 to receptors causing clonal expansion of T cells.
modest immunosuppression agents used as induction therapy
Other cytokines as IL-4,7,9,15 and 21 bind to receptor containing gamma chain (common gamma chain cytokine receptor), signalling is mediated by JAK3.
JAK3 inhibitor (Tofacitinib) available for treatment of rejection but not yet approved due to increased rate of infections and as it blocks IL-2 which is required for proliferation and maintenance of Treg
Cytokines direct the differentiation of proliferating T lymphocytes into effector cells:
CD4+ T cells differentiate into Th1,2,17, T follicular helper cells and T regulatory cells.
CD8+ T cells differentiate into cytotoxic T cells
cytokines also help transition of T cells to memory cells
Handbook of kidney transplantation by Gabriel M. Danovitch, Sixth Edition.
Thankyou Heba you all mentioned each drug and its site of action but somehow no mention of corticosteroids please mention the site of action and as a milestone in acute TCMR.
Corticosteroids inhibit cytokine receptor expression and dendritic cell function
they decrease the expression of IL-1, 2, 3, 6, TNF-a and IFN-g
so all stages of T cell activation are inhibited
Pulse methylprednisolone is the first line therapy for acute TCMR, it reverse 75% of first acute rejections Handbook of kidney transplantation by Gabriel M. Danovitch, Sixth Edition.
* Signal 1 : binding of the TCR with the MHC–peptide complex
* Signal 2 ( co-stimulation signal) ; engagement of “costimulatory” molecules found on the surface of T cells with their specific ligands on APCs ( CD80/86 and CD28).
* Signal 3 ( cytokine signal) ; signal1 and signal 2 lead s to induction of cytokine genes including ; IL-2 & IL2 receptors .This causes T cell proliferation and differentiation.
Drugs affecting signal 1:OKT3
Drugs affecting signal 2 ; Belatacept
Drugs targeting signal 3 ; mTORi, antiptroliferative, IL-2 blockade
TCR signal transduction.
Antigen recognition by the TCR leads to initiation of a phosphorylation cascade. The CD4/CD8 coreceptor phosphorylates CD3 and z (zeta) chains. ZAP-70 binds to two phosphotyrosines of the z (zeta) chain and, once activated, phosphorylates various adapter molecules including PLC g (gamma)1. These adapter proteins become docking sites for cellular enzymes that lead to activation of the mitogen-activated protein (MAP) kinase pathway, protein kinase C (PKC) pathway, and calcineurin pathway. These pathways converge to generate transcription factors AP-1 (activavtion protein-1), NF k (kappa)B (nuclear factor k (kappa)B), and NFAT (nuclear factor of activated T cells) which lead to gene transcription of various cytokines such as the T cell growth factor IL-2 (interleukin-2)
Sherif Yusuf
3 years ago
Three signals are required for T cell activation :
I- Signal 1 : antigen recognition
⦁ Occur when antigen peptide- MHC complex located on APC is attached to TCR
⦁ Immune response to a graft is either Alloantigen-dependent or alloantigen-independent (tissue injury such as ischemic injury leading to upregulation of expression of adhesion molecules, or shedding of intact HLA )
⦁ TCRs recognize antigen peptide complexed with MHC class I or II on the surfaces of APCs (TCR- AG- peptide-APC)
⦁ APC is either of the donor (interstitial denderitic cells, vascular endothelium) presenting intact HLA molecules and this is called the direct pathway (1) and is responsible for acute early graft rejection or of the recipient presenting processed HLA molecules and this is called indirect pathway and is responsible for chronic graft rejection (2)
Drugs acting on signal 1 :
1. OKT3 : is a monoclonal AB which binds to lymphocyte CD3 complex leading to rapid lysis.
2. rATG : polyclonal antibodies produced to many hematopoietic antigens (CD2, 3, 4 ,8, 18)
3. Alimtuzumab : humanized panlymphocytic ( B and T cells) anti-CD52 monoclonal antibody
II- Signal 2 : Costimulation
⦁ Occur when one or more TCR antigens (CD28, CTLA-4) interacts with its specific legend in APC (B7-1, B7-2). this is called co-stimulation
⦁ CD28 stimulate, while CTLA-4 supress T cells
Drugs acting on signal 2 :
1. Abatacept (CTLA4-Ig) is still under trial but it was found that the use of abatacept is associated with inhibition of T helper 1 and not T helper 2, leading to decrease in the incidence of acute rejection, but chronic rejection still occurs (3)
2. Belatacept (a high affinity variant CTLA4-Ig), has been developed with more immunosuppressive effect.
III- Signal 3 : Activation and differentiation of T cells
Once co-stimulation occur it activates signaling pathways that lead to calcineurin activation which in turn dephosphorylates NF-AT (Nuclear factor of activated T cell- a family of transcription factors) leading to IL-2 transcription and production of IL2 by T cell, IL-2 receptor stimulation activates the mTOR signaling pathway which induces the T cell to enter the cell cycle resulting in T cell activation and differentiation into :
A- CD4+ T helper which activate B cells (humoral immune response), macrophages (delayed type hypersensitivity), CD8+ T cells and produce inflammatory cytokines like TNF and IFN-𝛾 that cause direct injury to the graft.
B- CD8+ cytotoxic T cells which produce cell-mediated cytotoxicity by either killing of the cell or induction of apoptosis
Drugs acting on signal 3 :
1. IL-2 receptor antagonists Basiliximab and Daclizumab (4)
3. Calcineurin inhibitors : Cyclosporine and tacrolimus.
4. Cell cycle blocker : MMF, azathioprine.
REFERANCES
1. Matzinger P, Bevan MJ. Hypothesis: why do so many lymphocytes respond to major histocompatibility antigens? Cell Immunol 1977; 29:1.
2. Vella J, Knoflach A, Waaga A, Sayegh M. T cell mediated immune responses in chronic allograft rejection: Role of indirect allorecognition and costimulatory pathways. Graft 1998; 1:S11.
3. Vincenti F, Luggen M. T cell costimulation: a rational target in the therapeutic armamentarium for autoimmune diseases and transplantation. Annu. Rev. Med. 2007;58:347–358
4. Webster AC, Playford EG, Higgins G, Chapman JR, Craig JC. Interleukin 2 receptor antagonists for renal transplant recipients: a meta-analysis of randomized trials. Transplantation. 2004;77(2):166–176.
Thankyou Sherif
In signal 2 can you differentiate between the action of CTLA4(CD152) in slowing of the immune response as part of T reg action versus the block of costimulation when using Belatacept.
It would be helpful if you thought about keeping such a delicate balance between inhibiting T cell activation and autoimmunity. You need to consider the possible risks of increasing acute rejection while using costimulation blockade!
Primary T cell activation involves the integration of three distinct signals delivered in sequence:
(1) anti- gen recognition.
(2) costimulation.
(3) cytokine mediated differentiation and expansion.
●☆● SIGNAL ONE :
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen).
Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of antigen presenting cells (APCs).
The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex on the surface of the APC.
This triggers initial activation of the T cells.
The CD4 and CD8 molecules then bind to the MHC molecule too, stabilising the whole structure.
●☆● SIGNAL TWO
In addition to TCR binding to antigen loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat.
In the case of helper T cells, the first of these is provided by CD28.
This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) and initiates T-cell proliferation.
This process leads to the production of many millions of T cells that recognise the antigen.
In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152).
This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response
●☆● Signal Three
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines.
These determine which type of responder the cell will become in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine
IL-12), Th2 (IL-4), or IL-17, IL-6, IL-23).
Each one of these cells performs a specific task in the tissue and in developing further immune responses.
●The three main pathways activated through the TCR that control transcription are the MAPK, NF-κB, and calcium pathways.
● These pathways dramatically alter the expression and nuclear localization of various transcription factors that directly regulate genes involved in T cell activation.
____________________________
●●Example of immunosuppressive ttt that work in that way are:
CNI : Cyclosporin A
Anti cd 28 : BALATECEPT
Successful clinical trials established CD28 costimulation blockade mediated by the CTLA-4Ig derivative Belatacept as emerging treatment modality to prevent acute rejection and protect renal function in kidney transplant recipients .
The results of this study suggest synergistic effects during the combined use of CTLA-4Ig and CsA.
Reference
17. Larsen C.P., Grinyo J., Medina-Pestana J., Vanrenterghem Y., Vincenti F., Breshahan B. Belatacept-Based Regimens Versus a Cyclosporine A-Based Regimen in Kidney Transplant Recipients: 2-Year Results From the BENEFIT and BENEFIT-EXT Studies. Transplantation. 2010;90:1528–1535. [PubMed] [Google Scholar]
18. Rostaing L., Massari P., Garcia V.D., Mancilla-Urrea E., Nainan G., Rial M.D. Switching from Calcineurin Inhibitor-based Regimens to a Belatacept-based Regimen in Renal Transplant Recipients: A Randomized Phase II Study. Clin J Am Soc Nephrol. 2011;6:430–439. [PMC free article] [PubMed] [Google Scholar]
Signal 1 : Activation
activation of TCR and the nearby CD3 complex leads to a series of events that lead to activation of calcineurin which leads to dephosphorilation of NFAT ( nuclear factor of activated T cell) which then translocate to the nucleus and is responsible for transcription of genes required for T cells proliferation and differentiation. so signal 1 responsible for t cell activation.
Drugs that work by blocking signal 1 like : OKT3 : monoclonal antibody against CD3 , which leads to depletion of T cells.
another example is CNIs (cyclosporine and tacrolimus ) which inhibit calcineurin and prevent dephosphorilation of NFAT , and this leads to prevention of translocation of NFAT .
Signal 2 : Co-stimulation:
after TCR , CD3 complex activaion by the APC , there is increased expression of CD28 on the surface of T cells which will interact with CD80/86 on the surface of APC , this interaction is important for T cell activation . without this interaction T cell become anargic ( resistant to stimulation) and may lead to it’s apoptosis.
Drugs that inhibit co-stimulation : belatacept is CTLA 4 Ig that will bound to CD80/86 and prevent it’s interaction with CD 28. so the T cell can not be fully activated .
Signal 3 : cytokines:
after T cell activation , it will increase expression of many cytokines like IL2 . IL2 a receptor (CD25) on the surface of T cells once activated will lead to series of events through mTOR pathway leads to T cells proliferation and differentiation .
DRUGS :
basiliximab : monoclonal antibody that blocks IL2a receptor used in induction of immunosuppression during transplantation of low risk patients.
mTOR inhibitors : everolimus and sirolimus .
references:
(1) Gabriel M. Danovitch, MD Handbook of Kidney Transplantation sixth edition
(2) Alexander C. Wiseman Immunosuppressive Medications
Clin J Am Soc Nephrol 11: 332–343, 2016. doi: 10.2215/CJN.08570814
Signal-1 CNI block the activation of calcineurin to NFAT .
Signal -2 Blatacept bind to CD80and CD86 on APC blocking CD28 so preventing costimulation of T cell .
Signal -3 AntiCD28 mAb Basiliximab ,mTOR inhibitors {Sirolimus and Everolimus }.
Dalia Eltahir
3 years ago
Signal 1 recognition of foreign antigen within the MCH
Signal 2 Co stimulation between B7and CD28
Signal 3 cytokine-mediated differentiation and expansion
CTLA-4 or CTLA4, also known as CD152, is a protein receptor that functions as an immune checkpoint and downregulates immune responses. CTLA-4 is constitutively expressed in regulatory T cells but only upregulated in conventional T cells after activation – a phenomenon which is particularly notable in cancers.
Doaa Elwasly
3 years ago
3 Signal model of T cell activation
Signal one : is the recognition of the T-cell receptor (TCR) to antigenic peptides presented by major histocompatibility complex (MHC) class II molecules.
Signal two : co-stimulation by the triggering of CD28 on the T cell by CD80 and CD86 molecules on the Denderitic Cell.
Signal three : directs T-cell differentiation into phenotypes such as Th1 and Th2.
Both signals, two and three, result from the binding of microbial products to germline-encoded receptors such as toll-like receptors (TLR) on the Dendritic cells.(1)
Mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
Signal 1 starts due to MHC–antigen recognition by the T-cell receptor–CD3 complex, a process blocked by anti-CD3 monoclonalAbs and by rituximab.
Signal 2 can be blocked by belatacept. Costimulation resulting in calcineurin activation, a stage that can be inhibited by tacrolimus and cyclosporine A.
Activated calcineurin dephosphorylates Nuclear factor of activated T cell,enabling IL-2 transcription to start signal 3. IL-2 receptor stimulation, can be blocked by basiliximab,
IL 2 R stimulation activates the mTOR signaling cascade, which can be inhibited by sirolimus.
This pathway induces the T cell to enter the cell cycle which can be blocked by methotrexate, mycophenolate and azathioprine. AntiThymocyte Globulin exerts polyclonal effects while alemtuzumab binds to CD52, both leads to immunodepletion. (2)
1-Corthay A. A Three-cell Model for Activation of Naıve T Helper Cells .Scandinavian Journal of Immunology 2006 ,64, 93–96
Hi Dr Elwasly , Very nicely explained. Heller T cells and Cytotoxic cells are activated by different Cluster of differention proteins (CD). Can you highlight them in your reply.
agree with that but to clarify and help remove the confusion of mixing numbers. to help my colleagues realize what I mixed as well .. CD 80/86 are same b7.1, b.7.2 (alternatif names)
Last edited 3 years ago by Mahmud Islam
Weam Elnazer
3 years ago
T- Lymphocyte Activation:
T lymphocytes depend on three signals.
Signal 1 is delivered by binding of TCRs on T lymphocytes to HLA-peptide complexes on APCs. Signal 1 is necessary but not sufficient for T- lymphocyte proliferation and differentiation.
Signal 2 is delivered by binding of specialized accessory molecules on APCs to their receptors on T lymphocytes. Along with signal 1, signal 2 causes T-lymphocyte proliferation and differentiation.
cytokines produced by APCs deliver signal 3, which determines the differentiation pathway of T lymphocytes into specialized subsets.
Hi Dr Elnazer, point reply noted. Can you comment on ICOS, 4-1BB and OX40 molecule role in effector immune response
Assafi Mohammed
3 years ago
T-cell activation Signal One
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen).
Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of antigen presenting cells (APCs).
The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex, on the surface of the APC. This triggers initial activation of the T cells.
The CD4 and CD8 molecules then bind to the MHC molecule too, stabilising the whole structure.
This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion. This normally takes place in the secondary lymphoid organs.
Signal Two
In addition to TCR binding to antigen-loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat. In the case of helper T cells, the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) – and initiates T-cell proliferation. This process leads to:
Production of many millions of T cells that recognise the antigen.
In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152). This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response. Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).
T cells must recognise foreign antigen strongly and specifically to mount an effective immune response and those that do are given survival signals by several molecules, including ICOS, 4-1BB and OX40. These molecules are found on the T-cell surface and are stimulated by their respective ligands which are typically found on APCs. Unlike CD28 and the TCR, ICOS, OX40 and 4-1BB are not constitutively expressed on T cells. Likewise, their respective ligands are only expressed on APCs following pathogen recognition. This is important because it ensures T cells are only activated by APCs which have encountered a pathogen and responded. Interaction of the TCR with peptide-MHC in the absence of co-stimulation switches the T cells off, so they do not respond inappropriately.
Signal Three
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines. These determine which type of responder the cell will become – in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine IL-12), Th2 (IL-4), or IL-17 (IL-6, IL-23). Each one of these cells performs a specific task in the tissue and in developing further immune responses.
The resulting cell population moves out to the site of the infection or inflammation in order to deal with the pathogen.
Other cells present at the tissue site of inflammation– such as neutrophils, mast cells, and epithelial cells – can also release cytokines, chemokines, short peptides and other molecules which induce further activation and proliferation of the T cells.
T cell activation signals and drugs’ target: Signal 1: Antigen triggers T-cell receptors and synapse formation occurs.
Drug Example:
Several novel immunosuppressive agents affecting signal 1 have been in development: ISA247, also called Voclosporin, is a semisynthetic structural analog of cyclosporine A (CsA) that inhibits the calcineurin signal transduction pathway. Statistically longer allograft survival has been reported in animal studies for Voclosporin compared with CsA.
Another agent, alefacept, a dimeric fusion protein consisting of the CD2-binding portion of the human lymphocyte function-associated antigen-3 (LFA-3) linked to the Fc portion of human IgG1, is being tested in kidney transplantation. LFA-3-Ig is thought to neutralize the effect of CD2-expressing cells through several mechanisms including complement-mediated lysis and interruption of CD2’s interactions with LFA-3, limiting CD4+ T-cell adhesion to APCs and disrupting the engagement of effector TCR with antigen and MHC molecules.
Signal 2: Signal 1 allows co-stimulation of antigen-presenting cells to occur.
Drug Example: Belatacept A biological agent that blocks the co-stimulation necessary for activation of the T-cell (signal 2), Belatacept is associated with less metabolic side effects and nephrotoxicity compared to CNIs and mTOR inhibitors.
Signal 3: Signal 1 and signal 2 stimulate a cascade of intracellular events culminating in the
initiation of the T-cell cycle; stimulation of the T-cell cycle allows T cells to infiltrate the graft.
Drug Example: CNI :This class of drugs works by blocking calcineurin, an intricate protein in the signal transduction pathway that activates signal 3. ( eg: Tacrolimus and Cyclosporin)
Summary effect is to inhibit T-cell receptor activation, cytokine production, and subsequent lymphocyte proliferation to prevent rejection.
The activation of T cells primarily involves the integration of three distinct signals. first signal is antigen recognition followed by costimulation then cytokine mediated differentiation and expansion. As in picture attached anti cd25 (daclizumab) acts on signal 3 while anti CD3mAb acts on signal 1
DOI: 10.4172/2161-0991.1000169
Riham Marzouk
3 years ago
T cell activation happened through 3 signals:
1- signal 1 interaction between T cell receptor TCR on T cell (CD4 and CD8) and MHC on APCs , TCR is in association with CD3 on surface of T cell, this describe signal 1
can be blocked by block CD3 which is important to this step. OKT3 is the 1st monoclonal antibody block CD3. also cyclosporin act on this signal.
2- signal 2 it is costimulation , it is important for T cell activation , done by interaction between CD28 on T cell and CD80, 86 on APCs , balatacept block this costimulatory pathway.
3- signal 3 activation of signal 1 and 2 will end by release of cytokine IL-2 which interact with its receptor on T cell surface which is CD25. basiliximab or simulect block it as it is directed against CD25 anti-CD25 monoclonal antibody used in induction. mTor inhibitors act on this signal also by inhibit mTor which is previously stimulated by interaction between IL-2 and its receptor.
reference:
Gabriel M Danovitch. Handbook of kidney transplantation. 6th ed.
expression of CD80/86 on APCs (B7 protein), can bind to CD28 receptor on T cell either helper or cytotoxic this binding will leads to activation and proliferation of T cells
Ban Mezher
3 years ago
The full activation of & differentiation of T cells depends on 3 signals:
Signal 1: binding of Ag (on APC) to TCR leading to T cells activation. This signal is not selected through TCR properly but it occur through CD3 complex that in association of TCR & both CD4 & CD8. Blocking any protein present in Cd3 complex can prevent T cell activation. Immunosuppression acting by blocking signal 1 include OKT3 used for acute rejection( now withdraw ) & CNI.
Signal 2: needed to complete proliferation & differentiation of T cells into effector cells ( Cd4 & CD8). The signal is mediated by engagement of co stimulatory receptors on T cells to APC. There are numerous co stimulatory receptors as DC28 ( blocked by CTLA-4 Ig e.g. Belatacept), CD 154( CD40L, anti CD40 Ab undergoing clinical testing for renal transplant patients). Another co stimulatory receptors are CD137, CD70: Cd27.
Signal 3: Cytokines secreted by APC or T cells, these cytokines are important for stimulation & differentiation of T cells to specific effector subsets. Il-2 is a cytokines that produced by activated Ag. Naive T cells have low affinity form of IL-2R, but after activation of T cells, it will express CD25 which increase the affinity of Il-2R to 1000 fold. Immunosuppression that inhibit signal 3 include Basiliximab ( anti-monoclonal Abs block Il-2R), & mTOR inhibitors.
Reference: Danovitch G. Handbook of Transplantation.6th edition.
IL12 induce expansion & differentiation of CD8 T cells, IL4 had effects on both T cell ( induce differentiation of naive CD4 T cells into Th cells) & B cells ( generation go IgG1& IgE).Il23 has an important role in proliferation & maintenance of Th17.
Huda Al-Taee
3 years ago
Signal 1: T cell receptor activation
The binding of antigen to TCR trigger signals that transduced through CD3 and leads to T cell activation. CD4 & CD8 co-receptors on T cells also participate in the process of activation. OKT3 is the first monoclonal antibody used to block this signal. CNI also act on this signal.
Signal 2: Co-stimulation
The engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen presenting cells. in the absence of this signal, T cells become anergic and these anergic cells can inhibit the activation of neighboring T cells.
many families involved in the co-stimulatory pathway including : CD7 ( Ig family), CD 154 ( tumor necrosis factor family), C3a & C5a receptors ( G protein coupled receptors) & lectin receptors,
Belatacept ( CTLA4Ig) acts on this signal.
Signal 3: cytokines
Cytokines involved in T cell activation are proteins secreted by mature antigen presenting cells or the T lymphocytes themselves.
They have 2 main functions in the process of T cell activation:
stimulate T cell proliferation and differentiation into multiple effector cells which will cause graft rejection.
regulate T cells and act on other immune cells & non immune cells to enhance or suppress inflammation.
IL-2 is a potent activator and proliferative cytokine for T cells. It has a cell surface receptor which contain 3 subunits: alfa( CD25), beta, gamma. It’s effect depends on binding to cell surface receptor. During T cell activation, alfa subunit becomes associated with other subunits to form high affinity receptor. Blockade of the IL-2 receptor by targeting the alfa chain profoundly inhibits T cell proliferation.
CD25 blockade with Basiliximab or Daclizumab have a proven efficacy as induction therapy in kidney transplantation.
References:
Handbook of kidney transplantation by Gabriel M. Danovitch, Sixth Edition.
Kidney transplantation principles and practice by Stuart J. Knechtle, Loran P. Marson, Sir Peter J. Morris. Eight Edition.
T lymphocytes activation go through three signals:
–Signal 1 begins with the binding of TCR/CD3 to antigenic peptides presented by the APC( donor or recipient).This trigger leads to dramatic increase in tyrosine phosphorylation of multiple cellular proteins, responsible for activation of calcineurin pathway, mitogen activated protein (MAP) and protein kinase C (PKC), which finally activate key transcription factors required for T cell activation, proliferation and differentiation. Drugs used at this signal level include Muromunab OKT3 targeting CD3 complex which blocks T cells activation. CNI block calcineurin pathway.
–Signal 2 is an essential step for T lymphocytes to continue full activation and proliferation into full effector cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes to their ligands on APCs. Co-stimulatory receptors are absent or inactive in naive T lymphocytes, but they are upregulated in response to T cell activation by antigen. Signal 2 is essential for more sustained stimulation and amplification of T cell response. Co -stimulatory receptors include integrins (LFA-1, and LFA-2(CD2)) that bind to ICAM 1, ICAM2 and CD58 on APC.CD28 is a co-stimulatory receptor on T lymphocytes that bind to B7 on APC. The result of this complex is potentiation of tyrosine phosphorylation. Drug that act at this level is Belatacept which has high affinity to B7, preventing from binding to CD28. CD40L-CD40 is the complex formed between activated T lymphocyte and APC (B cells, DC, macrophages).
–Signal 3 includes the activation of T lymphocytes by cytokines released by APC or by the T lymphocytes themselves. Cytokines play major role in T cell proliferation and differentiation into multiple effector subsets. IL2 has been the target of therapies in induction immunosuppression for its role in T lymphocytes proliferation. IL2 is produced by activated T lymphocytes and acts on the same cell. IL2 receptors are inactive in naive T lymphocytes but upon activation by antigen and costimulatory pathways ), the T cell produce CD25, that with the pre-existed IL2 receptor, form more active IL2 receptor. IL-2 receptor monoclonal antibody to the alpha chain is used in the induction immunosuppressive therpies to prevent acute rejection.
Targeting T cell activation at many signal levels is still the site of researchers, as TCMR is still the major cause of rejection among kidney transplant recipients.
References
-Handbook of kidney transplantation 6th edition
-UpToDate
We addressed the importance of co-stimulation for T cell activation. Do you think that all T cells require co-stimulation? Naive CD8+ T cells do not require costimulation for proliferation and differentiation into cytotoxic effector cells.
Why we need to know the 3-signal model?
Different immunosuppressive work at different levels of the 3 signal model and knowing the 3 signa4l model will help in planning the immunosuppression.
What is CD3 and what is its role in T cell activation?
CD3 family of proteins is a complex of signaling molecules, which help to mediate T cell activation. Once the TCR is properly engaged with the peptide-MHC complex, conformational changes in the associated CD3 chains are induced, which leads to their phosphorylation and association with downstream proteins . Phosphorylated CD3 then recruit and activate the Syk family kinase zeta-activated protein.
signal 1
recognition of MHC-peptide complexes on antigen presenting cells by thr TCR leads to activation of multiple intracellular signalling pathways.
OKT3 is an antibody specific for CD3 and CNIs inhibit intracellular signalling.
signal 2
TCR signalling alone leads to T cell apoptosis and not activation.a second signal known as co-stimulation is required.Ex T cell CD28 and APC CD80 and 86.
Belatacept binds with CD80/86 and blocks their interaction with T cell receptor CD28,thus providing costimulation blockade.
signal 3
signalling through the TCR abd costimulation leads to the activation of many genes like IL2 and IL2R(CD25) which are required for T cell proliferation and effector functions.
Basiliximab and daclizumab are anti CD25 antibodies abd block binding of IL2 to the IL-2R .
sirolimus block the proliferative signal from the IL-2R and many other T cell growth factors mediated by mTOR.
Dear All
Great discussion
For a T cell to be active and proliferate we need signal two as well as signal one .. as its name implies it is co-stimulation. without signal 2 (co-sitimulation) the T cell will be anergied and that is what we want actually to succeed in immunosuppression to survive our engrafted organ.
the binding of CD3 to TCR (T cell receptor ) is the first signal. without it, T cells will not be activated
What about memory T cells?
Does it require costimulation?
Dear Dr Ahmed,
One of the characteristics of memory cells (both T and B lymphocytes) is their ability to produce an immune response with minimal need for co-stimulation as illustrated in the attached table (1).
References:
1) Jennifer Heimall. The adaptive cellular immune response: T cells and cytokines. © 2021 UpToDate. (Accessed on 3 December 2021).
memory cells are less dependent on accessory co-stimulation and can respond to many antigen presenting cells types including resting B cells.
M Croft, LM Bradley and S L Swain
J Immunol March 15 , 1994 , 152 (6) 2675-2685
What is CD3 and what is its role in T cell activation?
The CD3 complex serves as a T cell co-receptor that associates noncovalently with the T cell receptor (TCR) (Jennifer E Smith-2009).
The CD3 protein complex is a defining feature of the T cell lineage, therefore anti-CD3 antibodies can be used effectively as T cell markers .
CD3 protein complex is an important T cell marker for the classification of malignant lymphomas and leukemias (T cell neoplasms). CD3 can also be used for the identification of T cells in coeliac disease ,
Animal studies have shown that anti-CD3 antibodies induce tolerance to allografts (Nicolls et al. 1993).
OKT3, an anti-CD& antibody directed against CD3 ε, has been clinically approved for use in humans for the induction of immunosuppression in solid organ transplantation for the prevention and treatment of rejection (Norman 1995).
Interestingly, susceptibility to type I diabetes has been associated with the CD3 ε genetic locus and anti-CD3 antibodies have been shown to ameliorate the symptoms of this and other auto-immune disorders (Sprangers et al. 2011).
Jennifer E Smith-Garvin et al. Annu Rev Immunol. 2009.؟Tcell activation
Nicolls MR et al. (1993). Induction of long-term specific tolerance to allografts in rats by therapy with an anti-CD3-like monoclonal antibody. Transplantation 55, 459-468.
Is OKT3 still used if not what was its main side effect?
OKT3 is now no longer in production because of significant side effects related to the mitogenicity associated with its murine source.
Its use was stopped due its side effects especially sever cytokine release syndrome
Could you use CD3 as a tool for monitoring IS therapy? Any idea
CD3 used as atool for monitoring of the following:-
*Polyclonal Lymphocyte-Depleting Antibodies (Antithymocyte globulins)
Thyroglobulin-mediated T-cell depletion can be monitored using daily flow cytometry to quantify circulating CD3 T cells
*Monoclonal Lymphocyte-Depleting Antibodies (OKT3)
• Efficacy can be monitored by daily flow cytometry to identify CD3 cells.
Well done
monitor efficacy of OKT3
CD3 used to monitor OKT3 and ATG
Peripheral CD3 cell count monitoring
its effective to determine the appropriate ATG doses needed its reliable useful and cost effective in one cohort study they use less ATG dosing by 45% and 20% less cost in the group underwent monitoring with CD3 cell count in comparison to the group use lymphocyte%.
Do. Yes it’s cost effective and reliable
References
1- monitoring of CD3 T cell count in patients receiving antithymocyte globulin induction after cadaveric Renal transplantation
P . ATa,M ,Kara, E, Ozdemir,M,Canbakan,A.M,Gokce,F,A.
Excellent
Yes ;we can use CD 3 to monitor ATG dose and it was also used to monitor OKT3.
yes, for ATG & OKT3 monitoring
yes,it can be utilisein the monitoring of lypmpho-depleting agent e.g. rATG, OKT3
CD3 is used for monitoring both ATG and OKT3
for ATG is not clinically applied
Well done Shereen
Impressed with your comprehensive answer. You went far beyond transplantation and looked at other roles of CD3.
Dear All
We need to think always outside the box
What is CD3 complex …
it is present on all T cells and it’s importance in T cell activation is highlighted by the mechanism of action of OKT3 which is a monoclonal Ab which binds to one of the subunits of the CD3 complex, leading to destabilization and subsequent endocytosis of the TCR. Loss of the TCR from the cell surface inhibits activation and renders the T cell ineffective in generating an immune response.
co-stimulation is very important for T cell activation, if blocked ,no activation will happen, can be blocked with balatacept
CD3 is a part of complex with TCR, it comes in association with it on T cell surface and is very important during binding TCR with MHC on APC, without it no binding of TCR with MHC , can be blocked by OKT3
we need to know 3 signal model to block T cell activation at any stage to guard against its activation and proliferation to stop immune response to allogenic graft
-T lymphocytes need signal 2( co stimulation) to undergo full proliferation and differentiation to either cytotoxic or helper cells.
-Knowing the 3- signal models models helps in the invention and manufacturing of immunosuppressive drugs.
– the binding of antigens to TCR triggers a signal cascade that is not transduced through TCR proper but through an adjacent CD3 complex( CD3 which is a protein associated with TCR )that leads to T –lymphocytes activation.
Thanks, Reem
Yes, you are right. “Knowing the 3- signal model helps in the invention and manufacturing of immunosuppressive drugs”.
Well done
CD28 is expressed in almost all murine T cells. It is expressed on 95% of the resting CD4+ T cells and 50% of the resting CD8+ T cells in human peripheral blood. A subpopulation of CD8 T cells with suppressor activity lack CD28 expression. Naive CD8+ T cells do not require co-stimulation for proliferation and differentiation into cytotoxic effector cells.
CD13 is transmembranous molecule in the surface of T lymphocytes required for binding to the HLA antigen. Muromunab is CD13 inhibitor, blocking the binding to the APC, and subsequently blocking the activation of T lymphocytes.
Knowing toe 3-signal model, we can be able to target the T lymphocytes activation, proliferation and differentiation at man levels and we would be able to know how cellular and humoral immune response are interconnected.
Thanks, Mohamed
Yes, “Knowing toe 3-signal model, we can be able to target the T lymphocytes activation, proliferation and differentiation at man levels and we would be able to know how cellular and humoral immune response are interconnected”.
Do you think that all T cells require co-stimulation?
Naïve T cell activation requires engagement of TCR by MHC complex , engagement of cell surface co-stimulatory molecules found on APC and T cell stimulation by cytokines
Sustainability of T cell activation requires engagement of surface co-stimulatory in addition to engagement of TCR by MHC complex
Co- stimulatory molecules such as Immunoglobulin superfamily ( B7), tumour necrosis factor family (CD 154), G protein coupled receptors (C3a and C5a)required during priming, expansion and death of T cells.
Without co-stimulatory signals, T cells might become anergic which make them unresponsive even if receives adequate second signal. Anergic cells can also inhibit activation of neighbouring T cells.
Why we need to know the 3-signal model?
Three signal model of T cell activation and subsequent cellular proliferation Is important tool for understanding the sites of action of the immunosuppression medications to prevent allograft rejection
What is CD3 and what is its role in T cell activation?
TCR by itself doesn’t have catalytic activity, requires to form complex with 6 CD3 subunits that contain cytoplasmic immunoreceptor tyrosine -based activation motifs (ITAMs)
References
Kidney Transplantation: Principles and Practice by Stuart J. Knechtle
Hand book of kidney transplantation by Gabriel M. Danovitch
1. We addressed the importance of co-stimulation of T cell activation. Do you think that all T cells require co-stimulation?
· T cells require the costimulatory signal second signal provided by CD28 and following its binding to B7-1 on APC to the naïve CD4 T – cells to undergo proliferation, differentiation and survival. So co stimulation rescues T cells from anergy (state of immune unresponsiveness).
· However, CD8+ T cells can be activated through the signal generated through TCR-alphabeta in the absence of any potential costimulatory molecule
2. Why we need to know the 3-signal model?
· We need to understand the 3 signal model to know how current immunosuppression work in inhibiting T cell activation.
· This will also help us to invent newer targeted immunosuppression.
3. What is CD3 and what is its role in T cell activation?
· CD3 is a member of the immunoglobulin superfamily
· CD3 is expressed by a high-percentage on circulating peripheral T cells (no other cell type) forming a complex with the T cell receptor (TCR).
· CD3 is present at all stages of T cell development so it is a highly effective T cell marker.
· T cell receptor/CD3 complex (TCR/CD3) plays a key role in antigen recognition, T cell activation and in consequence in triggering an antigen specific immune response.
· CD3 was the target of the first mAB (OKT3) that is not currently used because of side effects
· Monitoring of CD3 is a useful , reliable and cost effective in monitoring ATG therapy
Do you think that all T cells require co-stimulation?
In the absence of costimulatory signals, T cells become anergic so that they become unresponsive even they subsequently receive an adequate second signal, and these anergic cells can also inhibit the activation of neighboring T cells.
Why we need to know the 3-signal model?
To understand the different modes of action of immunosuppressive medications and helps in the discovery of new immunosuppressive medications.
What is CD3 and what is its role in T cell activation?
It is a protein complex and T cell coreceptor that involve in the activation of both cytotoxic T cells and T helper cells, it contains four chains and these chains together with TCR generate a signal that lead to T cells activation.
Do you think all t-cell required stimulation?
Memory cells can respond to antigen presenting cells independent of accessory co-stimulation.
Why we need to know the 3 signal module?
Knowing the 3 signal modules help us to understand T-cell activation ,proliferation and differentiation and how drugs can target these area .
What is CD3 ? what its role in T-cell activation?
Is a protein complex , composed of four distinct chains .these chains associate with the TCR and CD3-zeta to generate an activation signal in T lymphocytes . the TCR , CD3-zeta , and the other Cd3 molecule together constitute the TCR complex.
1-We need CD 80 ,86 to join CD28
CD40 join CD154
CD25
2- we need 3 signal model to know which drug is suitable for inhibiting each signal to prevent T cell proliferation
3-CD3 is complex transduce the TCR to be recognized by APC (this represents signal 1)
Not all T cells need co-stimulation , memory cells need minimal co-stimulation .
We need to know the 3-signal in order to know the mode of action of different IS so control the immune system and to help for future invention of new items.
CD3 is a T-cell co receptor associat with TCR necessary for signal 1
and it is used for ATG and OKT3 drug monitoring.
1- No. Memory T cells don’t need costimulation to be ativated.
2- To prevent and treat rejection.
3-CD3 complex is a group of protein chains that associate with T cell receptor and is important in signal 1.
The activation of naïve T cells is a tightly regulated event and requires three distinct signals for the generation of an optimal response,
including :
1)-T cell receptor (TCR) engagement (signal 1),
2)-Costimulation (signal 2), and
3)-cytokine stimulation (signal 3)
Signal 1: TCR engagement
The antigen binding component of the TCR complex is a heterodimer composed of an α and β chain that recognizes proteolytically processed short peptides (8–15 amino acids) presented in the context of self major histocompatibility complex (MHC) on antigen-presenting cells it was demonstrated that antigen-specific T cells recognize foreign peptides presented by self-MHC, by showing that cytotoxic T cells (CTL) lysed only virus-infected target cells that were matched at the MHC loci .Interestingly, a significant frequency of T cells also have the ability to recognize MHC molecules that are not present within the thymus during selection in a process referred to as allo-recognition, with both class I and II serving as targets . These alloreactive T cells are present at significantly higher frequencies (100–1000 fold higher) than T cells specific for individual foreign peptides presented by self-MHC, allowing for the generation of strong primary immune responses to transplanted non-self tissues and rapid rejection of allografts. Alloreactive T cells recognize alloantigens through two distinct pathways, the direct and the indirect pathways. In the direct pathway, T cells recognize an intact donor MHC antigen on the tissue allograft and in the indirect pathway, T cells recognize donor peptide antigens presented by self MHC .T cells recognizing alloantigens through either the direct or indirect pathways can mediate rejection of allografts.
Agents acts on signal 1:1)-Anti TCR agents: The murine anti-CD3 mAb Muromonab-CD3 (OKT3) was the first mAb. It targeted the CD3 subunit of the TCR complex and led to rapid elimination of functional T cells. It is now no longer in production because of waning utilization, primarily because of significant side effects related to the mitogenicity associated with its murine source.
2)-Calcineurin Inhibitors (Cyclosporin and Tacrolimus).
After initial TCR binding, a calcineurin-dependent signaling pathway is induced that leads to initial T cell gene transcription necessary for additional activation. Two commonly used calcineurin inhibitors (CNIs; cyclosporin and tacrolimus) and one investigational agent (voclosporin) inhibit the ability of calcineurin to dephosphorylate nuclear factor (NF) of activated T cells (NFAT), required for translocation from cytoplasm to nucleus, and prevent calcineurin-dependent gene transcription. One potential nonimmunosuppressive mechanism that could explain CNI efficacy in glomerular disease is the ability to inhibit synaptopodin degradation in the podocyte, thereby stabilizing the actin cytoskeleton and reducing proteinuria . alternative formulations of tacrolimus (extended release) as well as the novel CNI voclosporin have been developed and are approved or in late-phase clinical trials in transplantation.
Signal 2:Costimulation:
The second signal that is crucial for productive T cell activation is delivered by the engagement of one or more costimulatory molecules expressed by T cells with their ligands expressed on APC . One of the earliest costimulatory molecules to be identified and extensively studied is CD28, which is expressed on the cell surface of all naïve CD4 and CD8 T cells. TCR stimulation in the absence of CD28 engagement results in abortive activation of T cells and anergy . Binding of CD28 to its ligands (B7-1, CD80 and B7-2, CD86) on APC promotes optimal TCR signaling events that trigger IL-2 .production, clonal expansion and generation of effector and memory T cells .Following the discovery of CD28, cytotoxic T lymphocyte antigen-4 (CTLA-4, CD152) was identified based on structural homology to CD28, and a functional role for this molecule was suggested by the ability of a CD152-Ig fusion protein to inhibit T cell activation. Recently, additional costimulatory molecules related to the CD28 immunoglobulin superfamily have been discovered, including the inducible costimulatory molecule (ICOS)-B7h and the programmed death (PD)-PD-L1/PD-L2 pathway. The tumor necrosis factor family-tumor necrosis factor family receptor (TNF-TNFR) superfamily members also provide costimulatory signals important in the generation of antigen-specific T cell responses. Overall, there is a broad array of costimulatory pathways that either enhance or dampen T cell activation, and these pathways are all potential targets for blocking T cell activation and inducing T cell tolerance to alloantigens.
Agents acting on signal 2
1)- Blockade by CD80/86:CD28 Targeting (Abatacept and Belatacept).CTLA-4 Ig ,belatacept has high affinity to CD80/86.
2)-Costimulation Blockade by CD154:CD40 Targeting (Anti-CD40 mAb).: Targeting the induced surface molecule CD154 on activated T cells was a focus of drug development until it was recognized that CD154 was also present on platelets, and agents binding this cell surface molecule led to an increase in thrombotic events in both primate and early-phase human trials .Attention has, thus, turned to targeting CD40. A number of mAbs against CD40 are in development, with a fully human anti-CD40 (ASKP1240; Astellas) under study in phase 2 clinical trials in kidney transplantation.
Signal 3: CD4 help and inflammation:
naïve CD8 T cells also require a third signal for the optimal generation of effector and memory T cells .Two examples of third signals enhancing the induction of antigen-specific CD8 T cell responses are CD4 T cell help and the presence of inflammatory cytokines .The specific role of CD4 T cells in supporting the activation of CD8 T cell responses is dependent on the nature of the antigenic challenge. One mechanism for this help is the maturation or “licensing” of APC by engagement of CD40 on the APC by CD154 expressed by activated CD4 T cells. In contrast, infection with pathogens that induce significant levels of inflammation, such as influenza virus, lymphocytic choriomeningitis virus (LCMV) or vesicular stomatitis virus, does not require CD4 help to induce primary CD8 T cell responses .However, the generation of functional CD8 memory after infection is dependent on CD4 T cell help received during the initial activation of the CD8 T cells . A second mechanism by which CD4 T cells can provide help to CD8 T cell responses is by the production of IL-2
. IL2-signalling appears to have only a minimal role in the generation of primary CD8 T cell responses to viral infection or cellular antigens but is critical for the development of functional memory CD8 T cells.
Agents targeting signal 3:
No spescific cytokines inhibition:corticosteroids and Janus Kinase Inhibition (Tofacitinib).
Spescific cytokines inhibition:
A)-IL-2 Receptor Antagonist (Basiliximab),,
B)-TNF alpha inhibitors :Infliximab, adalimumab, golimumab, and certolizumab are mAbs to TNF-α that differ in the degree of chimerism and route of administration (intravenous versus subcutaneous) .Etanercept is a TNFR fusion protein bound to IgG. All carry the risk of increased susceptibility to intracellular pathogens, such as tuberculosis, coccidiomycosis, and Cryptococcus .
C)-IL-1 Inhibition (Anikinra, Rilonacept, and Canakinumab).
D)-IL-6 Inhibition (Tocilizumab).
E)-IL-17 Inhibition (Secukinumab).
There is also agents targeting B cells e.g Anti-CD20 Targeting: Rituximab, Ocrelizumab, Ofatumumab, and Veltuzuma ,,, Anti-CD22 Targeting: Epratuzumab ,,,, Targeting B Cell Differentiation: Belimumab and Atacicept,,,,, Plasma Cell Targeting: Bortezomib,,, Complement Inhibition (Eculizumab).
Agents targeting chemokines and cell adhesion: approved chemokine receptor antagonists include the CCR5 receptor antagonist maraviroc used in the treatment of HIV, the CXCR4 antagonist plerixafor approved for hematopoietic stem cell mobilization, and the CCR4 humanized mAb mogamulizumab approved for the treatment of T cell lymphoma. Relevant to nephrology practice, emapticap is an inhibitor to CCL2 (also known as monocyte chemotactic protein 1), which has been studied in phase 1 and 2 trials in diabetic nephropathy, with proof-of-concept studies showing a reduction in albuminuria presumably by inhibiting inflammatory cell migration into the kidney.
Pooled Polyclonal Antibodies as Immunosuppressive Agents:IVIG and ATG
Immunosuppressive Agents with Multiple Cellular Targets:
Panlymphocyte Depleting Agents: Anti-CD52 (Alemtuzumab)
Antiproliferative Agents: mTOR Inhibitors (Sirolimus and Everolimus)
Antimetabolites: Inhibition of DNA Synthesis:Azathioprine ,MMF and leflunamide.
.
Allorecognition and alloreaction is mediated by both naïve and memeory T cells .The memory cell reaction is more rapid and more sever than naïve one . rejection can take place by both of these group of cells .
Signal 1: TCR Signaling
Signal one is mediated by interaction between HLA-peptide complex ( antigen ) and T cell receptor .
T cell receptor binding with antigen on antigen presenting cells will activated t cell receptor . t cell receptor activation alone is not enough to induce t cell activation and proliferation , to complete this process the adjacent CD3 activation is needed . CD3 complex, a group of invariant (non polymorphic) protein chains that associate with the TCR. Engagement of the TCR by MHC, along with CD 3 the other required receptor engagements, initiates the signaling process from the CD3 complex, leading to cell proliferation and differentiation. CD4 and CD8 also participate in this reaction ,they has a co receptor role . CD4 combine with class II antigens while CD8 combine class I antigens .T cell receptor complex – antigen interaction induce cascade of signaling of T cell activation .
Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation. An example is OKT3, the first monoclonal antibody used in clinical medicine, and since withdrawn that was used to treat severe acute allograft rejection.
The activation signal transduced by the TCR-CD3
Cluster will lead to activation of enzymes tyrosine kinase and protein kinase which lead to generate second-messengers: Diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3).These messengers activate
calcineurin pathways that ultimately activate key transcriptionfactors (NFkB, NFAT, and AP-1) that induce the transcription of cytokine genes required for T-lymphocyte proliferation and differentiation.
The calcineurin pathway is the target of the CNIs . CNIs bind to specific proteins in the cell (immunophilins— cyclophilin for cyclosporin and FK-bindingprotein (FKBP) for tacrolimus. The drug immunophilin complex then blocks the activation of calcineurin by the calcium-dependent enzyme calmodulin.
T Cell Costimulation: Signal 2
Binding of the TCR:CD3 complex to the peptide:MHC complex on APCs delivers a signal that can induce the clonal expansion of naïve T cells only when the appropriate co stimulatory signal is delivered (signal2). CD8 T cells require a stronger co stimulatory signal, and their clonal expansion is aided by CD4 T cells interacting with the same APC (i.e., T helper function). Co stimulation is likely a checkpoint developed by
the immune system to prevent the activation of self-reactive T cells that escaped negative selection in the thymus. Antigen binding to the TCR in the absence of co stimulation not only fails to activate the T cell but also leads to a state called anergy, in which T cells become refractory to subsequent activation, or even undergo apoptosis (programmed cell death). Thus co stimulation removes this inhibition and determines whether a T cell will proceed with clonal expansion and the development of effector functions. It is now clear that co stimulatory molecules
can provide positive or negative signals to T cells . It is the integration of both positive and negative co stimulatory signals during and after initial T cell activation, dictated by their temporal and spatial expression patterns, that ultimately determines the fate and functional status of the T cell response.
CTLA4- Ig (Belatacept), which binds with high affinity to B7 molecules and prevents them from engaging CD28, is in clinical use for the prevention of allograft rejection in kidney transplant recipients .
Integrine :
Extended interaction between the DC presenting the alloantigen and the alloreactive T lymphocyte is necessary for sustained TCR signaling. Prolonged, stable interaction between the two is made possible by cell-adhesion molecules known as integrins. The integrins LFA-1 and CD2 (LFA-3) on T lymphocytes bind to their ligands ICAM-1/ICAM-2 and CD58, respectively,on dendritic cell.
The development and marketing of anti-LFA-1 (Efalizumab) and anti-CD2 antibodies (Alefacept) for clinical use in autoimmunity and transplantation have been halted owing to serious side effects such as progressive multifocal leukoencephalopathy (PML) caused by JC virus reactivation .
B7-CD28 ,CD40-CD154 and other co-Stimulatory Pathways like 41BBL-41BB (CD137),
OX40L-OX40, CD70:CD27, and ICOSL-ICOS pathway are example .
Signal 3: Cytokines
They are produced by either APC or T cells .
They induces T cell proliferation and differentiation into different subset of effector T cell that have distinct
phenotypes and functions. cytokines can also regulate T lymphocytes or act on other immune and no nimmune cells to either enhance or suppress inflammation.
Interleukin-2 (IL-2)
It is produced by T cells and it has autocrine and paracrine.
IL-2 binding to the high-affinity ( Active form ) IL-2R causes
the proliferation (clonal expansion) of antigen-activated T lymphocytes .Anti-CD25 monoclonal antibodies that block the α chain of the IL-2R are potent inhibitors of T-lymphocyte proliferation in vitro, but when used in
humans (for example, Basiliximab employed as induction therapy in kidney transplant recipients,are
relatively modest immunosuppressive agents. One explanation for this paradox is the presence of several other cytokines that also support the proliferation and survival of T lymphocytes. These include IL-4, IL-7, IL-9, IL-15, andIL-21.
A JAK3 inhibitor, Tofacitinib, is currently
available for the treatment of rheumatoid arthritis and psoriasis. Although it proved to be noninferior to cyclosporin in preventing rejection, it has not been approved for use in transplantation, possibly owing to increased incidence of infections when combined with other immunosuppressive agents.
Danovitch GM. Handbook of Kidney Transplantation. Sixth Edition, Wolters Kluwer, eISBN 9781496388841, 2017.
Primary T cell activation involves the integration of three distinct signals delivered in sequence: (1) anti- gen recognition, (2) costimulation, and (3) cytokine- mediated differentiation and expansion. Strong im- munostimulatory events such as immunotherapy or infection induce profound cytokine release causing ‘‘bystander’’ T cell activation, thereby increasing the potential for autoreactivity and need for control. We show that during strong stimulation, a profound suppression of primary CD4+ T-cell-mediated im- mune responses ensued and was observed across preclinical models and patients undergoing high- dose interleukin-2 (IL-2) therapy. This suppression targeted naive CD4+ but not CD8+ T cells and was mediated through transient suppressor of cytokine signaling-3 (SOCS3) inhibition of the STAT5b tran- scription factor signaling pathway. These events re- sulted in complete paralysis of primary CD4+ T cell activation, affecting memory generation and induc- tion of autoimmunity as well as impaired viral clear- ance. These data highlight the critical regulation of naive CD4+ T cells during inflammatory conditions.
REFERENCES
Appay, V., and Rowland-Jones, S.L. (2002). Premature ageing of the immune system: the cause of AIDS? Trends Immunol. 23, 580–585.
Attia, P., Phan, G.Q., Maker, A.V., Robinson, M.R., Quezado, M.M., Yang, J.C., Sherry, R.M., Topalian, S.L., Kammula, U.S., Royal, R.E., et al. (2005). Autoimmunity correlates with tumor regression in patients with metastatic mel- anoma treated with anti-cytotoxic T-lymphocyte antigen-4. J. Clin. Oncol. 23, 6043–6053.
Berner, V., Liu, H., Zhou, Q., Alderson, K.L., Sun, K., Weiss, J.M., Back, T.C., Longo, D.L., Blazar, B.R., Wiltrout, R.H., et al. (2007). IFN-gamma mediates CD4+ T-cell loss and impairs secondary antitumor responses after successful initial immunotherapy. Nat. Med. 13, 354–360.
Boyman, O., Kovar, M., Rubinstein, M.P., Surh, C.D., and Sprent, J. (2006). Selective stimulation of T cell subsets with antibody-cytokine immune com- plexes. Science 311, 1924–1927.
interaction between the antigen-specific T cells and APCs provides the first signal (signal
1) for T-cell activation.
The most proximal event phosphorylation of the immunoreceptor tyrosine-based
activation motifs (ITAMs) of the TCR/CD3 complex,
followed by activation , Ras/ERK MAPK cascade,
the Ca/calcineurin/NFAT pathway, and the PKC/NFχB pathway.
immunosuppressive agents affecting signal 1 voclosporin,
Another alefacept, CD2-binding portion of the human lymphocyte function-associated
antigen-3 (LFA-3) linked to the Fc portion of human IgG1, is being tested in kidney
transplantation.
a co-stimulatory signal, signal 2, is required for T-cell; . Belatacept, a selective co-
stimulation blocker.
Signal 3: Signals provided by inflammatory cytokines, pre-dominantly IL-12 and type I
interferons (IFNα/β), and possibly IL-21, are now considered to be required for a
productive T-cell response and are termed signal 3. Basiliximab, Daclizumab is IL_2
inhibitors.
T cell activation signals are
signal 1
binding of antigen presenting cell to T lymphocytes which activate Series of reactions together with signal 2 cell proliferation occurs
examples of immunosuppression related its action with this signal are CNR inhibitors and anti CD3 like ATG
signal 2
co stimulatory signal together with signal 1 lead to cytokine release examples of immunosuppression its action related to this signal or anti-CD 28 like Blatacept
signal 3
activation signal lead to cytokine release and clonal proliferation
examples of immunosuppression depends on this signal is auntie CD 25 what is Basilliximap and daclizumab
The activation and intended action of T cell depends on 3 signals:
Signal 1: Activation
Signal 2: Proliferation/ survival
Signal 3: Differentiation
Signal 1: It involves binding of MHC on APC to the T cell receptor (TCR) on the lymphocyte via CD3. It also involves CD4 and CD8 co-receptors on T lymphocytes. The TCR-CD3 cluster activation depends on tyrosine kinase which activate phospholipase C gamma catalyzing brakdown of PIP2 (phosphatidylinositol biphosphate) into DAG (Diacylglycerol) and IP3 (Inositol 1,4,5 triphosphate). IP3 increases intracellular calcium leading to activation of the calcineurin pathway. DAG activates protein kinase C and mitogen activated protein kinase pathways which, in association with the calcineurin pathway, activate transcription factors like NFkB, NFAT and AP-1 for the cytokine genes needed for signal 3.
Signal 2: It involves interaction between co-stimulatory molecules on T lymphocytes and their ligands on the APC. These co-stimulatory molecules are upregulated or induced after signal 1.
Examples of these molecules include:
1) Integrins: LFA-1, CD2 bind to ICAM-1/ICAM-2 and CD58 respectively.
2) B7-CD28: binds CD80 an CD86 on mature APCs
3) CD40: bind to CD40L (CD154)
4) CD27: bind to CD70
5) CD137: bind to 41BBl
6) OX40: bind to OX40L
Signal 3: It involves the secretion of cytokines from the APC as well as the lymphocytes, leading to T cell proliferation and differentiation into effector subsets. The cytokines involved are interleukines (IL), especially IL-2. Binding of IL-2 to IL-2 receptor (IL-2R) leads to T cell proliferation. Stimulation of IL-2R (CD25) causes mTOR activation, cytokine release and cell proliferation.
Signal 1: OKT3, (Anti CD3) Costicosteroids, Calcineurin inhibitors (Cyclosporine, Tacrolimus).
Signal 2: Efalizumab (anti LFA-1), Alefacept (Anti CD2), Belatacept (Anti B7), Corticosteroids
Signal 3: Anti IL-2 receptor antibodies (Basiliximab, Daclizumab), mTOR inhibitors (Sirolimus, Everolimus), MMF and azathioprine (cell-cycle inhibition), Corticosteroids
CD3 is crucial for t Lymphocyte signaling.
Knowing of3- signal model is needed for development and manufacturing of immunosuppressive agents.
Signal 2;co stimulation is essential for T cell proliferation and differentiation into either cytotoxic or helper cells.
CD3 complex is a protein related to TCR. Antigen binding to TCR leads to signal cascade transduced through adjacent CD3 to TCR ,leading to T cell activation.
-signal 1 : signaling *
In which TCR of lymphocytes binds to MHC peptide complex on the APCs .
-signal 2 : co stimulation
Which is binding co stimulatory molecules on T lymphocytes by their ligands on the APCs ,
-signal 3 : cytokine release
Cytokines released from both APCs , and Lymphocytes, help in T cell activation and differentiation into effector T cells .
*Drugs targeting Signal 1:
Anti CD3 ( OKT3)
*Drugs targeting Signal 2
Belatacept
Calcineurin inhibitors (cyclosporine A, tacrolimus)
*Drugs targeting Signal 3:
Anti-IL-2 receptor (basiliximab, daclilumumab)
mTOR inhibitors (rapamycin, everolimus)
Three single model describe T cells activation caused by reaction to allograft. Signal one is TCR (T cell receptor) binding to complex of HLA and donors specific antigen on antigen presenting cells.OkT3 is an example of monoclonal antibody that blocks T cell activation via inhibition of CD3 complex. This signal generates DAG and IP3. IP3 increases intracellular calcium and activates calcineurin pathways that finally activate transcription factors that are required for T cell activation. CNIs bind to immunophilins and their complex inhibit calcineurin pathway and so T cell activation. Signal 2 is co-stimulation .Full T cell activation needs engagement of these receptors on T cells by their ligands which are present on antigen presenting. Failure of this signal causes anergy on T cell death. Belatacept is an example of drug that inhibits this signal by binding to B7 molecules and prevents attachment to their ligand CD28. Signal 3 or cytokine refers to secretion of special proteins by T cells to proliferate and differentiate them in to effector subsets. Example for inhibition of this signal is provided by basiliximab which is an anti-CD25 monoclonal antibody and blocks α chain of IL -2 receptor.
There are 3 signals responsible for T cells activation and initiation of the immune response direct indirect and semi direct signals
Direct or activation signal : responsible for CD8+T cells activation and acute cell mediated rejection and it’s occur by T cell activation through direct contact with donor APC this will lead to form complex with CD3 subunits causing subsequent activation and phosphorylation processes with different proteins leading to activation and proliferation of T cells ,but this signal alone is not sufficient to keep active T cells and it needs signal 2 activation for maintaining immune response.
Signal 2 indirect or co stimulatory pathway without signal 2 activated T cells through signal 1 will die ,by this signal T cells will become active by activation of recipient APC by donor cells inside the graft leading to tyrosin kinase activation which cause CD 4+ T cells activation and by this B cells and macrophages activity can be organized activation of this signal is responsible for chronic graft rejection.
Signal 3 or cytokines this occurs when both co stimulation and and Ag recognition happens lead to cytokines and cell gene activation causing signals activation and this cause molecules expression and clonal proliferation all will lead to T cells activation .
Signal 1 and 2 both are mandatory for naive T cells activation which will produce IL -2 and this help T cells activation and survival , interleukin- 2 production is 5he target of many immunosuppression medications.
Effects of immunosuppression on T cells activation signals :
Cyclosporin : binds to cyclophilin and inhibits calcineurin phosphatase and blocks IL-2 transcription and prevent cell activation.
Anti IL-2 (basiliximab ,daclizumab):is a monoclonal Ab bind to IL-2R which has similar affinity like IL-2 so it’s effect T cells activation and help to decrease the incidence of ACR within first 6 months post transplant, More specifically basiliximab directed against CD 25 which is a component of IL-2 receptor and by its binding it will cause cessation of clonal expansion and proliferation.
ATG : is polyclonal Ab that cause T cells depletion by it’s specificity against TCR CD3 ,CD4 and CD8 and also on cytokine receptors this will cause decrease T cell activation and decrease cytokines release .
Alemtuzumab: is monoclonal Ab that has specific effect in CD52 expressed on both T and B cells .
OKT 3 : it’s action by inhibiting CD3 – TCR.
3-Model Signals ;
* Signal 1 (TCR binding signal) ; this signal occur when Antigen complexed with MHC molecule class I or II associated with APC bind T cell receptor. Drugs targeting signal 1 includes ; CNI, OKT3
* Signal 2 ( co-stimulation signal) ; interaction of co-stimulator molecules on APCs and T Cells e.g. CD80/86 and CD28. This activate pathways of protein kinases and together with signal 1 leads to IL-2 production. Therapies targeting signal 2 includes ; Belatacept, Afalizumab
* Signal 3 ( cytokine signal) ; signal1 and signal 2 lead s to induction of cytokine genes including ; IL-2 & IL2 receptors .This causes T cell proliferation and differentiation. Drugs targeting signal 3 includes ; mTORi, antiptroliferative, IL-2 blockade
For activation of naïve T cells to effector cells this requires 3 steps pathway
Step 1 : begins by binding of the Ag on surface of APCs to TCR /CD3 complex on T lymphocyte surface.
Step 2 (co stimulatory ) : via binding of B7 APC surface to CD28 on T cell. Both Step 1 and 2 will activate intracellular pathways leading to production of IL2 and other growth promoting cytokoines. Stimulation of the IL-2 receptor (CD25) leads to activation of mTOR and provides signal 3, that triggers cell proliferation
1- CNI : act by binding to their certain cytoplasmic receptor protiens to form a complex which then bind to calcinurin enzyme ( is a a phosphatase enzyme that dephosphorylates NFAT to facilitates its translocation
of NFAT to the nucleus with subsequent decrease production of IL2 required for T cell proliferation , So CNI inhibits T cell proliferation.
2-Drugs targeting signal 2:
a- Belatacept : binds to B7 with high affinity and inhibits the costimulatory pathway.
3- Drugs targeting signal 3:
a-Basiliximab : monoclonal antibody, anti–IL-2 receptor or
anti-CD25 antibody.
b- mTOR inhibitors eg sirolimus and everolimus
c- Corticosteroid: Blocks IL-1 and IL-2 production, suppressing early phase of immune response.
T cell activation need 3 signals:
Signal 1:
Recognition of MHC complexes on antigen presenting cells by TCR lead to activation of multiple intercellular pathways. Drugs that inhibit this signal are CNI
Signal 2:
Is a co stimulation provided by interaction of T cell surface molecules with ligand on APC. Inhibited by Belatacept
Signal 3:
When TCR accompanied by co stimulation lead to activation of many genes required for T cell proliferation including IL-2,that why this signal blocked by basiliximab
Dear All
Remember the following:
3-Model Signals ;
Reference ; Oxford handbook of nephrology & hypertension, second edition
with our understanding of the costimulatory signale and its role inTcell activation and proliferation to different subsets of effectors T cells and Tregs cells thier divers mechanisms of action on immune system, the long-term renal allograft survival has become an achievable goal with the innovation of the new selective costimulation blockade monoclonal ABs like antiCD40 AB which target CD40)/CD154 costimulatory pathway, has just completed a phase 2 trial with a CNI-free regimen. an anti-CD40, is also to be tested in a phase 2 trial in renal transplantation. another selective CD28 blocking AB Nonagonistic CD28 antibodies have re-emerged with two anti-CD28 candidates still in preclinical development.in addition to already used CTLA4Ig,belatocept
1-Current status of costimulatory blockade in renal transplantation:
Wojciechowski, David; Vincenti, Flavio
Current Opinion in Nephrology and Hypertension: November 2016 – Volume 25 – Issue 6 – p 583-590
2-An update on chemical pharmacotherapy options for the prevention of kidney transplant rejection with a focus on costimulation blockade
Florian Kälble 1, Matthias Schaier 1, Sebastian Schäfer 1, Caner Süsal 2, Martin Zeier 1, Claudia Sommerer 1, Christian Morath 1
The 3 signal model of T cell activation:-
*signal 1 : signaling *
In which TCR of lymphocytes binds to MHC peptide complex on the APCs .
* signal 2 : co stimulation
Which is binding co stimulatory molecules on T lymphocytes by their ligands on the APCs ,
Absence of this step could lead to deletion of T lymphocytes
* signal 3 : cytokine release
Cytokines released from both APCs , and Lymphocytes, help in T cell activation and differentiation into effector T cells , also help in regulation of T cells .
Handbook of kidney transplantation
Gabriel M.Danovitch
Mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
Signal 1:
CNI
• immunosuppressive effect depends on the formation of a complex with cytoplasmic receptor proteins:
o cyclophilin for cyclosporine
o tacrolimus-binding protein (FKBP) for tacrolimus
• Cyclophilin or FKBP complex dephosphorylates NFAT cells and fascilitates passage thru membrane
• Inhibition of calcineurin will retard the expression of critical cytokine gene that promote T cell activation:IL-2,IL-4,IFN-G and TNF-A including transcription of genes such as H-ras, c-myc, CD40 ligand
• Enhances TGF-B which also inhibits IL-2 and Cytotoxic T cells
• When receiving CNI, CD4+ T cell have reduced IL-2 production to a degree that inversely correlate to drug levels
Corticosteroid
• Blocking T cell derived and APC derived cytokine and cytokine receptor expression.
• Inhibit the function of dendritic cells
• Inhibit translocation to the nuclear of nuclear factor KB
• Inhibit expression of IL-1,IL-2,IL-3,IL-6, TNF-a and IFN-g
Signal 2:
• Belatacept used as a substitute for CNIs at the time of transplant. They mimic the down regulatory T cell response (CTLA4 which competitively binds to CD80/86)
• Efalizumab(anti LFA-1)and Alefacept (anti CD2) where stopped usage because of serious side effects such as progressive multifocal leukoencephalopathy(PML)
• Co-stimulation Blockade by CD154:CD40 mABs were shifted towards CD40 as blocking CD154 with mABs was associated with increased thrombotic effects.
Signal 3:
• IL-2 receptor can be blocked by Basiliximab, targeting α chain or CD25
• The activated the mTOR signaling cascade can be inhibited by Sirolimus and Everolimus which are kinase inhibitors
• The cell cycle activation can be blocked by MMF, azathioprine
References
Bamoulid, J., Staeck, O., Crépin, T., Halleck, F., Saas, P., Brakemeier, S., Ducloux, D. and Budde, K., 2016. Anti-thymocyte globulins in kidney transplantation: focus on current indications and long-term immunological side effects. Nephrology Dialysis Transplantation, p.gfw368.
Handbook of Kidney Tramnsplantation by Gabriel M. Danovitch
What is the 3-signal model of T cell activation?
Signal 1:TCR engagement
• Antigen binding component of TCR complex is heterodimer consists of an Alpha and Beta chain, that recognises proteolysed peptides presented as self major histocompatibility complex (MHC) on APC
• Allorecctive T cells recognise alloantigens via 2 pathways: direct and indirect pathways
o Direct pathway – T cell recognise an intact donor MHC antigens on tissue allograft
o Indirect pathway – T cell recognise donor peptide antigens presented by self MHC
Signal 2:co-stimulation
• Crucial for productive T cell activation
• CD 28 – expressed on the surface of all naïve CD4 and CD 8 T cells
• TCR stimulation without CD28 results in abortive activation of T cells and anergy
•. Binding CD28 to its ligands B7-1(CD80), B7-2(CD86) on APC promotes activation T cell and trigger IL-2 production, clonal expansion and generation of effector and memory T cell
• CTLA-4 ( cytotoxic T lymphocyte antigen -4) is a structural homologous to CD28
• Functional role of CTLA-4 was suggested by the ability of CD-152 Ig fusion protein to inhibit T cell activation
• CD152 builds to CD80,CD86 with higher affinity and provides inhibitory signal to T cells
• costimulatory molecules related to the CD28 immunoglobulin superfamily -inducible costimulatory molecule (ICOS)-B7h and the programmed death (PD)-PD-L1/PD-L2 pathwayhave been discovered
• ICOS-B7h engagement augments T cell activation is by inducing the expression of CD40L (CD154) on T cells which stimulates the activation of CD80 and CD86 molecules on APCs and provides a positive feedback in sustaining CD28 costimulation
• The (PD)-PD-L1/PD-L2 pathway is a negative costimulatory pathway that has an important role in the maintenance of self-tolerance. PD-1 is expressed on activated CD4 and CD8 T cells, B cells, NK cells and macrophages.
Signal 3: CD4 help and inflammation
• Naïve CD 8 T cell require 3rd signal for the optimal production of effector and memory T cell(CD4 T cell and presence of inflammatory cytokines)
REFERENCES
Bamoulid, J., Staeck, O., Crépin, T., Halleck, F., Saas, P., Brakemeier, S., Ducloux, D. and Budde, K., 2016. Anti-thymocyte globulins in kidney transplantation: focus on current indications and long-term immunological side effects. Nephrology Dialysis Transplantation, p.gfw368.
T cell activation requires recognition of antigens on antigen presenting cells (APCs) , costimulators and cytokines produced by APCs and T cells themselves
Signal 1 :
generated by interaction between the TCR of T cell and the MHC/peptide complex on the APCs and this is transduced through the CD3 complex.
Signal 2:
T cells require a number of secondary signals ( costimulatory ) through accessory molecules to become activated and respond to the threat.
The second signal comes from the recruitment of co-stimulators. common co-stimulators are the CD28 receptor and B7 molecules.
Signal 3:
once T cell received first and second signals , it receives more instructions in the form of cytokines which lead to differentiation of T cells into two different types with distinct cytokines profile
type 1 helper cells Th1 secrete cytokines that include IL-2 , IFN and IL-12 , these cytokines stimulate delayed type hypersensitivity response , cytolytic activity and production of antibodies
type 2 helper cells Th2 secrete cytokines that include IL-4, IL-5, IL-10 and IL-13 , these cytokines activate eosinophils and stimulate production of IgE.
Immunosuppression
Drugs targeting Signal 1:
Monoclonal antibodies anti CD3 ( OKT3)
Drugs targeting Signal 2
Belatacept : Inhibition of signal 2 in T-cells (competition with CD28 for CD80/CD86 binding) inhibiting T-cell co-stimulation
Daclizumab : Inhibition of signal 2 in T-cells (binds to CD25, the alpha subunit of the IL-2 receptor) preventing IL-2-induced T-cell activation.
Calcineurin inhibitors (cyclosporine A, tacrolimus) : Inhibition of signal 2 transduction in T-cells { inhibits calcineurin via cyclophilin (cyclosporine A) or via FKBP 12 (tacrolimus) blocking IL-2 transcription}.
Drugs targeting Signal 3:
Anti-IL-2 receptor (basiliximab, daclilumumab) :Inhibition of T-cell proliferation and signal 3
mTOR inhibitors (rapamycin, everolimus) : Inhibition of signal 3 transduction in T-cells (inhibits mTOR), preventing IL-2-induced T-cell proliferation
References
Baroja-Mazo A, Revilla-Nuin B, Ramírez P, Pons JA. Immunosuppressive potency of mechanistic target of rapamycin inhibitors in solid-organ transplantation. World J Transplant. 2016;6(1):183-192.
Handbook of Kidney Transplantation :Fifth Edition
immunosuppressive agents targeting 3-signal model of T cell activation
Complete activation of alloreactive T lymphocytes depends on three signals and every immunosuppressive agent can influence this process by acting on different signals.
Signal 1 is delivered by binding of TCRs on T lymphocytes to HLA-peptide complexes on APCs. This binding triggers a signaling cascade that leads to T lymphocyte activation. These signals are transduced not only through TCR but through the adjacent CD3 complex that associates with the TCR. CD4 and CD8 co-receptors on T lymphocytes also participate in the signal mediated by the TCR-CD3 cluster. Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation. A typical example is OKT3, the first monoclonal antibody that was used in transplantation and was withdrawn because of serious side effects.
Activation of PKC and MAP kinase, and calcineurin pathway by DAG and IP3 respectively, ultimately activates transcription factors such as NFkB that induce the transcription of cytokine genes required for T lymphocytes proliferation and differentiation. The calcineurin pathway is the target of CNIs. CNIs bind to immunophilins (cyclophilin in the case of cyclosporin and FKBP in the case of tacrolimus). Eventually, the drug-immunophilin complex blocks the activation of calcineurin by calmodulin, the calcium dependent enzyme.
Signal 2 is required for full proliferation and differentiation of T lymphocytes into effector lymphocytes (either cytotoxic or helper cells). Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs. Failure to provide the second signal results in failed T lymphocytes activation that leads to T lymphocyte depletion or anergy.
Main co-stimulatory pathways:
LFA-1 and CD2 on T lymphocytes and their ligand ICAM1/ICAM2 and CD58 on DC respectively. Antibodies against LFA-1 or CD2 delay kidney transplant rejection in animal models, but development of anti-LFA-1 (Efalizumab) and anti-CD2 antibodies (Alefacept) has been ceased because of serious adverse effects such as PML.
CD28 on all naïve T lymphocytes and B7.1 (CD80) and B7.2 (CD86) on mature APCs, namely DCs. This pathway potentiates the transcription of key genes that are required for T lymphocyte proliferation, for example IL-2 and IL-2 receptor genes.
In addition to these ligands that transduce a costimulatory or activating signal, cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), which also binds B7-1 and B7-2, provides an inhibitory signal. Although CD28 is expressed on resting T cells, CTLA4 is expressed on the cell surface only after initial T cell activation.
Engagement of CTLA4 by binding with high affinity to its B7 counter-receptors appears to downregulate immune responses, suggesting that CTLA4 plays a critical feedback role in terminating T cell responses.
CTLA-4-Ig (Belatacept), which binds with high affinity to B7 molecules and prevents them from engaging CD28, is utilized for the prevention of kidney allograft rejection.
However, the beneficial effect of CTLA4Ig is abrogated when coadministered with cyclosporine.
CD154 (CD40L) on activated CD4+ T lymphocytes and CD40 mainly on B lymphocytes, DC, and macrophages. Engagement of CD40 by CD154 is an important mechanism by which CD4+ lymphocyte provides help to B lymphocytes for isotope switching (shift from IgM producing isotypes to the more effective IgG antibody isotypes). It upregulates B7 expression and enhances cytokine production by DC and ultimately leads to further co-stimulation of T lymphocytes. Although antibodies that block the CD40-CD154 pathway could be very effective anti-rejection agents, in humans cause serious thromboembolic side effects.
Signal 3 is related to the cytokines that are involved in T lymphocyte activation, proliferation, and differentiation to the multiple effector subtypes. IL-2 is a cytokine with a strong capacity to induce clonal expansion of antigen activated lymphocytes. Upon activation, T lymphocytes express a third chain alpha or CD25, which increases the affinity of the IL2R to IL2 by approximately 1000 folds. Anti-CD25 monoclonal antibodies such as Basiliximab are relatively modest immunosuppressive agents for induction therapy in kidney transplantation. Daclizumab was removed from the market.
Stimulation of CD25 leads to activation of mTOR. The mTOR pathway is an important intracellular pathway involved in T lymphocyte proliferation, and immunosuppressive drugs such as Sirolimus and Everolimus inhibit this pathway.
Signal One
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen). Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of antigen presenting cells (APCs). The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex, on the surface of the APC. This triggers initial activation of the T cells. The CD4 and CD8 molecules then bind to the MHC molecule too, stabilising the whole structure. This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion. This normally takes place in the secondary lymphoid organs.
Signal Two
In addition to TCR binding to antigen-loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat. In the case of helper T cells, the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) – and initiates T-cell proliferation.
This process leads to the production of many millions of T cells that recognise the antigen. In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152). This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response. Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).
Signal Three
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines. These determine which type of responder the cell will become – in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine IL-12), Th2 (IL-4), or IL-17 (IL-6, IL-23). Each one of these cells performs a specific task in the tissue and in developing further immune responses.
**mechanisms of immunosuppressive on the 3 model signal activations are shown in the attached picture
Binding of antigen (HLA–peptide complex) to the TCR triggers a signaling cascade that leads to T- lymphocyte activation. These signals are not transduced through the TCR proper, but through the adjacent CD3 complex associated with the TCR. CD4 and CD8 co-receptors on T lymphocytes also participate in the activation signal mediated by the TCR– CD3 cluster by binding to the same MHC molecule that engages the TCR. The activation signal transduced by the TCR–CD3 cluster is dependent on tyrosine kinases that cause the recruitment and activation of the enzyme phospholipase C-g (PLC-g). PLC-g catalyzes the breakdown of the membrane lipid phosphatidylinositol biphosphate (PIP2) to generate two second-messengers: Diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). DAG activates the protein kinase C (PKC) and mitogen-activated protein (MAP) kinase pathways, whereas IP3 triggers the calcineurin pathway by increasing intracellular calcium concentration. Together, the PKC, MAP kinase, and calcineurin pathways ultimately activate key transcription factors (NFkB, NFAT, and AP-1) that induce the transcription of cytokine genes required for T-lymphocyte proliferation and differentiation. The calcineurin pathway is the target of the CNIs.
T lymphocytes must receive a second signal to undergo full proliferation and differentiation into effector lymphocytes, which include either cytotoxic or helper cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs. Failure to provide the second signal results in aborted T-lymphocyte activation, which causes T- lymphocyte deletion (death) or anergy. Co-stimulatory molecules are either absent or are constitutively expressed on naïve T lymphocytes, but are induced or upregulated upon activation of T lymphocytes with antigen. Beltacept is an inhibitory drug for one of the co-stimulatory cell markers, namely CTLA-4. many other co-stimulatory cell markers are known and well-described.
They are proteins secreted by mature APCs or the T lymphocytes themselves. They serve two main purposes: they stimulate T- lymphocyte proliferation and induce the differentiation of T lymphocytes into multiple effector subsets that have distinct phenotypes and functions. However, cytokines can also regulate T lymphocytes or act on other immune and non-immune cells to either enhance or suppress inflammation. Most cytokines are known by the term interleukin (IL) followed by a number that refers to the order in which they were discovered. Basiliximab is a monoclonal antibody and commonly used in induction of immunosuppression. it blocks the cell marker CD25, resulting in inhibition of release of these cytokines.
1-Signal
bindinging of TCR/CD3 to antigenic peptides presented by the APC( donor or recipient).This trigger leads to dramatic increase in tyrosine phosphorylation of multiple cellular proteins, responsible for activation of calcineurin pathway, mitogen activated protein (MAP) and protein kinase C (PKC), which finally activate key transcription factors required for T cell activation, proliferation and differentiation.
DRUGS USED AT THIS SIGNAL LEVEL
Muromunab OKT3 targeting CD3 complex which blocks T cells activation. CNI block calcineurin pathway
2-Signal 2: Co-stimulation
engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen presenting cells. in the absence of this signal, T cells become anergic and these anergic cells can inhibit the activation of neighboring T cells.
many families involved in the co-stimulatory pathway including : CD7 ( Ig family), CD 154 ( tumor necrosis factor family), C3a & C5a receptors ( G protein coupled receptors) & lectin receptors,
DRUGS USED AT THIS SIGNAL,
Belatacept ( CTLA4Ig) acts on this signal.
3-Signal 3,cytokines:
after T cell activation , it will increase expression of many cytokines like IL2 . IL2 a receptor (CD25) on the surface of T cells once activated will lead to series of events through mTOR pathway leads to T cells proliferation and differentiation .
DRUGS USED AT THIS SIGNAL
a-basiliximab monoclonal antibody that blocks IL2a receptor used in induction of immunosuppression during transplantation of low risk patients.
b-mTOR inhibitors : everolimus and sirolimus
REFFERNCE
JASN ePress. Published on April 15, 2015 as doi: 10.2215/CJN.06620714
T cell activation:
It is the process in which naïve T cell become activated into effector T cell APC(dendritic cells play a prominent role) is capable to present antigen bound to MHC to naïve CD4 (T-helper) or CD8 (T –cytotoxic).
First signal (stimulation signal):
It is due to binding of the alloantigen on the graft (HLA –antigen complex) with the (TCR –CD3 complex) of the CD4 and CD8 T cell. The activation signal is dependent on tyrosine kinases that activate the enzyme phospholipase to activate 3 main pathways which are protein kinase C, Map kinase and calcineurin .this result in activation of transcription factors that induces transcription of the cytokines important for T lymphocyte proliferation and differentiation.
Agents targeting signal 1:
Anti-TCR Agents: OKT3, The murine anti-CD3 was the first mAb approved as a drug for human use in renal transplantation .It is now no longer in production because of significant side effects related to the mitogenicity associated with its murine source.
Calcineurin Inhibitors: Cyclosporine introduced in early 1980s and the more potent Tacrolimus introduced in late 1990s Besides, the newer Voclosporin . The introduction of these drugs resulted in dramatic reduction in rejection episodes
Second signal(Co-stimulation signal):
This co stimulatory signal allow T cells to undergo full proliferation and differentiation into effector lymphocytes( T helper and T cytotoxic). This occurs through engagement of the co-stimulatory receptors on T lymphocytes by their ligands on APCs resulting in calcineurin activation that dephosphorylates nuclear factors of activated T cell, enabling IL-2 transcription to start signal 3. This prevents T cell to go into anergy(death).
The most important currently is the interaction between CD28 on the T cell surface with its APC surface ligands, B7-1 or B7-2 (often known as CD80 or CD86) .
Other Co -stimulatory signals include interaction between integrins (LFA-1, and LFA-2(CD2)on T lymphocytes that bind to ICAM 1, ICAM2 and CD58 on APC.
Also, CD154 also called CD40L expressed on activated T helper cells that interacts with CD40 which is present on dendritic cells, macrophages and B lymphocytes . This interaction is important in switching B lymphocytes from producing IgM to IgG beside enhancing T cell activation.
Agents targeting signal 2:
Abatacept followed by Belatacept used as a substitute for CNIs at the time of transplant. They mimic the down regulatory T cell response (CTLA4 which competitively binds to CD80/86)
The use of Efalizumab(anti LFA-1) and Alefacept (anti CD2) where halted because of serious side effects like progressive multifocal leukoencephalopathy(PML) as a result of JC virus reactivation.
Additionally, Co-stimulation Blockade by CD154:CD40 mABs were shifted towards CD40 as blocking CD154 with mABs was associated with increased thrombotic effects.
Third signal( cytokine production):
It is a potent signal involving cytokine production mainly IL-2 that result in activation of the mTOR signaling pathway that causes the T cell to enter a cell cycle. Accordingly, T lymphocyte will undergo maturation and differentiation into multiple effector subsets with different phenotypes and functions. So, CD4 T lymphocytes differentiate to (TH1, TH2, TH17, Tfh) and a regulatory population called Treg, while CB8 T lymphocytes differentiate to T cytotoxic, together with the long lived memory T cells.
Agents targeting signal 3:
IL-2 receptor(has α , β, ϒ chains) can be blocked by mAB Basiliximab, targeting α chain or CD25( only modest immunosuppression) as the ϒ chain family can be still activated by other interleukins and targeting ϒ chain can cause severe lymphopenia.
The activated the mTOR signaling cascade can be inhibited by Sirolimus and Everolimus which are kinase inhibitors(used mainly in treatment of skin cancers like Kaposi sarcoma)
The cell cycle activation can be blocked by MMF, azathioprine
References:
Hand book of kidney transplantation, 6th edition
Immunosuppressive Medications. Alexander C. Wiseman. Clin J Am Soc Nephrol. 2016 Feb 5; 11(2): 332–343.
Immunosuppressive Drugs for Kidney Transplantation. Philip F. Halloran, M.D., Ph.D., NEJM, 2004;351:2715-29.
Thanks, Ahmed for your comprehensive reply
Primary T cell activation requires three signals in sequence:
signal 1, where T cell receptor (TCR) recognize antigen in the context of major histocompatibility complex (MHC) . This is blocked by anti CD3 monoclonal antibodies:OKT3 ,CNi
signal 2, involving binding of costimulatory molecules, B5,CD28, . This is blocked by Lulizumab , CTLA-4-IgG
signal 3, where cytokine,IL2 and IFN. direct and amplify T cell differentiation and proliferation .
This is blocked by antiCD25, mTOR, basiliximab , Daclizumab
Halloran PF. Immunosuppressive drugs for kidney transplantation. N Engl J Med. 2004 Dec 23;351(26):2715-29. doi: 10.1056/NEJMra033540. Erratum in: N Engl J Med. 2005 Mar 10;352(10):1056. PMID: 15616206.
Dirk R. J. Kuypers, Yves F. C. Vanrenterghem, Monoclonal antibodies in renal transplantation: old and new, Nephrology Dialysis Transplantation, Volume 19, Issue 2, February 2004, Pages 297–300, https://doi.org/10.1093/ndt/gfg555
Both cd4 and Cd8 tcell have TCR which recognize antigens by APC (adaptive immunity) which presents them on it’s MHC.
Signal 1:
TCR bind to antigen presented by APC then
Cd3 which is group of proteins on Tcell surface undergoes conformation changes through increase intracellular tyrosine phosphorylation,mitogen activated protein(MAP),calcinurin pathway and protein kinase c(PKC) leading to activation of transcription factor called nuclear factor of activated t cell(NFAT).
signal 2
Costimulatory pathway involving multiple families like cd28 on tcell with b7-1(Cd80)/b7-2(cd86) and integrins like LFA-1/LFA-2 on Tcell with ICAM-1and ICAM2 on APC which associated with expression of CTLD4 which usually not expressed on conventional tcell except after its activation and carry downregulation effect.
survival signals including ICOS, 4-1BB and OX40 on tcell which usually not expressed on tcell except after APC presentation of antigen (not like cd28 and TCR)
Signal 3
APC and Tcell secrets cytokines like IL2(th1),IL4(th2),IL17(th17)and IL 12.
IL2 bind to IL2 Rs on other naive tcells leading to its maturation and differentiate into effector Tcells(th1) and starting graft rejection.
Tcell proliferation is done by a cascade of intracellular events activates the mTOR signaling pathway which induces the T cell to enter the cell cycle resulting in T cell activation
mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
signal 1:
Steroids: the antiinflammatory effect of steroids affect almost all the 3signal of tcell activation..it inhibit Cd3complex and NFAT from signal 1
Okt3: monoclonal antibodies against Cd3complex
CNI:signal 1 inhibit the calcinurin pathway and include cyclosporine, tacrolimus and valcosporin
ATG polyclonal antibodies which cause depletion of both t cells and b cell
Alemutzumab anti CD52 on both tcell and b cell
Signal 2:
Belatacept which is 2aa modifications form abtacept which has high affinity to both b7-1/b7-2 on APC preventing its bind to cd28 on tcell leading to cell anergy and inhibition.
Signal 3:
Basilixmab: which bind to alpha part of IL2 Rs and prevent tcell proliferation (non depleting induction)
mTOR: sirolimus and evrolimus which block mTOR signaling pathway
Antiproliferative includes azathioprine and MMF by preventing tcell division.
After illustrative immunosuppressant medications site of action we can understand why steroids and CNI is the cornerstone of transplantation (signal 1)
Also how combination of immunosuppressant medications complete each other by multiple hits in tcell activation pathway.
Also explain why it’s not effective in treating ABMR.
Reference
-Glucocorticoids in T cell development, differentiation and function
Matthew D. Taves & Jonathan D. Ashwell
Nature Reviews Immunology volume 21, pages233–243
-Handbook of kidney transplantation 6th edition
-UpToDate
Thanks, Ramy
Please see my question posted above
Signal of T- cell activation
The activated antigen presenting cell (dendritic cells containing HLA antigens ) enter the recipient lymph nodes and activate the naïve T-cell . This activation is co-stimulated by (CD28, CTLA) molecules . The encounter between APC and TCR is the key step in activation of T-cell . The T cell receptor contains an α- and a β-chain as well as CD3 chains .The combination of dendritic cells and T cell receptors is called the “immunologic synapse,” which consists of donor HLA antigens, recipient T cell receptors and CD 4 or CD 8 molecules, co stimulatory molecules, and adhesion molecules. Maturation of these cells occurs through three intra cellular signaling pathways;
(1) calcineurin pathway
(2) mitogen-activated protein kinase pathway
(3) JAK/STAT (Janus kinase/Signal Transducers and Activators of Transcription) pathway.
The first two pathways cause the production of IL-2, which binds to its receptor (IL-2R) on T cells . This binding leads to the activation of the mammalian target of rapamycin (mTOR), which allows the T cells to undergo cell division.
Mechanism and action of current immunosuppressive drugs
OKT3 targets CD3 chains .
Belatacept blocks the costimulatory molecules .
Cyclosporine and tacrolimus inhibit the calcineurin pathway .
Daclizumab and basiliximab inhibit the interaction between IL-2 and its receptor .
Sirolimus and everolimus inhibit mTOR
.
mycophenolate mofetil and azathioprine inhibit cell division.
JAK3 inhibitors (CP-690550) inhibit JAK/STAT pathway.
Referance;
McKay DB, Park K, Perkins. What is transplant immunology and why are allografts rejected? In McKay DB, Steinberg
SM (eds). Kidney Transplantation. A Guide to the Care of Kidney Transplant Recipients. New York, Springer, pp
25–39.
Weltz A, Scalea J, Popescu M, et al. Mechanisms of immunosuppressive drugs. In Weir MR, Lerma EV (eds). Kidney
Transplantation. Practical Guide to Management. New York, Springer, 2014, pp 127–141.
Thanks, Abdulrahman for your hard work
Please see my question posted above
Signal One; specific antigen signal
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen).
Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of APCs.
First The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held by the MHC complex, on the surface of the APC.
This triggers initial activation of the T cells. The CD4 and CD8 molecules act as coreceptors and bind to the MHC molecule , stabilising the whole structure. This initial binding between a T cell specific for one antigen and the antigen-MHC it matches sets the whole response in motion. This normally takes place in the secondary lymphoid organs.
Activation of T cells generate second messengers DAG and IP3
DAG activate protein kinase C (PKC) and mitogen activated protein (MAP) while
IP3 trigger calcineurin pathway.
Drugs stopping signal 1 :
Calcineurin inhibitors target calcineurin pathway by calcium dependent enzyme Calmodulin
Alimtuzumab anti-CD52 monoclonal antibody also OKT3 is a monoclonal AB which binds to lymphocyte CD3 complex leading to rapid lysis.
ATG is polyclonal antibodies act against many hematopoietic antigens (CD2, 3, 4 ,8, 18)
Signal Two ;costimulation signals
In addition to TCR binding to antigen-loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat.
for helper T cells, the first of these is provided by CD28. This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) – and initiates T-cell proliferation.
This process leads to the production of many millions of T cells that recognise the antigen. In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152) which competes with CD28 for B7 and so reduces the activation signals to the T cell
to decrease the immune response.
Belatacept CTLA4-Ig bind B7 molecules preventing the engagement with CD28.to stop signal 2 activation. Belatacept is currently being tested for prevention of allograft rejection and preservation of kidney function in kidney transplant recipients as a potential replacement for calcineurin inhibitors (CNIs). It differs from abatacept by only two amino acids and has greater binding avidity to CD80 and CD86.
Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).
T cells must recognise foreign antigen strongly and specifically to mount an effective immune response and those that do are given survival signals by several molecules, including ICOS, 4-1BB and OX40. These molecules are found on the T-cell surface and are stimulated by their respective ligands which are found on APCs.
Unlike CD28 and the TCR, ICOS, OX40 and 4-1BB are not constitutively expressed on T cells. and their respective ligands are only expressed on APCs following pathogen recognition.
Signal Three cytokines
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines. These determine which type of responder the cell will become – in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine IL-12), Th2 (IL-4), or IL-17 (IL-6, IL-23). Each performs a specific task .
The resulting cell population moves out to the site of the infection or inflammation in order to deal with the pathogen. Other cells present at the tissue site of inflammation– such as neutrophils, mast cells, and epithelial cells – can also release cytokines, chemokines, short
peptides and other molecules which induce further activation and proliferation of the T cells.
T-cell activation Original author – Mary Cavanagh, Science Museum London, UK Updated by – Emily Gwyer Findlay, University of Edinburgh, UK
Thanks, Shereen
Please see my question posted above
T cell activation.
Antigens dependent T-cells receptor activation:
antigen-presentation singles when the TCRs engaged by APCs through the CD3 Complex, CD4, CD8 co-receptors also involved in this activation, monoclonal AB OKT3 blocking theTcell activation by binding to CD3 complex, and also CNI block the T cell activation at this level by binding to an intercellular protein cyclophilin and the cyclosporine /cyclophilin complex will inhibit the calcineurin pathway.
T cells costimulation signal:
when second TCRS bind on the surface of the APCs expressing CD28 , B7 -1CD80) and B7-2 (CD86) this will regulate T cell clonal expansion and differentiation by B7-CD28 family of molecules the B7-1/B7-2-CD28/CTLA-4 pathway is crucial in regulating T-cell activation and induction of tolerance . New B7 and CD28 molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells (2).
Immunosuppressive drugs that act on costimulatory signals including CTLA-4Ig , belatocept.
CD40/CD154 Pathway and TNF/TNFR pathways are important for activation of APC and T cells, so blocking these pathways in animal model of transplantation was effective and hold promise for future therapeutic agents.
IFN-γ production also inhibits T-cell activation by inhibiting the expression of co-stimulators and MHC-II molecules on dendritic cells (DCs) and macrophages.
cytokine production signal
which led to the activation of m TOR (mammalian target of the rapamycin) signale3 which lead to plenty of cytokines production for further activation and proliferation of the T cells effector subsets , Th1 t cells express IL2, CD25 IFN,IL12 and TNF while Th2 secrete IL4,IL5,IL10
mTOR inhibitor both sirolimus and everolimus act on signal 3 by inhibiting cytokine dependent cellular proliferation at G-S phase.
monoclonal AB anti CD25 Basiliximab and danosumab both effective as Induction therapy .
References
1-Hand book of kidney transplantation, GabrielM,Danovitch .
2-The B7-CD28 superfamily
Arlene H Sharpe 1, Gordon J Freeman, 2002 Feb;2(2):116-26. doi: 10.1038/nri727. Nat Rev Immunol.
3- T-cell activation
Original author – Mary Cavanagh, Science Museum London, UK
Updated by – Emily Gwyer Findlay, University of Edinburgh, UK
: Systems and Processe the differential activation of the two main classes of helper T cells
Excellent
Signal 1—TCR Activation
Binding of antigen (HLA–peptide complex) to the TCR triggers a signaling which-lead to Tlymphocyte activation through the adjacent CD3 complex.
CD4 and CD8 co-receptors on T lymphocytes also participate in the activation signal mediated by the TCR– CD3 cluster by binding to the same MHC molecule that engages the TCR. Antibodies that target one or more proteins in the CD3 complex block T-lymphocyte activation.
Example for this signal is OKT3, the first monoclonal antibody used in clinical medicine, and since withdrawn that was used to treat severe acute allograft rejection.
Another example is CNI. CNIs bind to specialized proteins in the cell known as immunophilins—cyclophilin in the case of cyclosporin and FK-binding protein (FKBP) in the case of tacrolimus. The drug blocks the activation of calcineurin by the calcium-dependent enzyme calmodulin.
Signal 2—Costimulation
costimulatory molecules determine and mediate short and long-term function during priming, expansion, and death of T cells.
In addition to signal 1, T lymphocytes must receive a second signal to undergo full proliferation and differentiation into effector lymphocytes, which include either cytotoxic or helper cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes by their ligands on APCs .
Activation of downstream signaling via CD28 results from ligation with B7 family proteins, CD80 or CD86. These proteins are expressed by APCs such as DCs and engage CD28 during antigen presentation.
Example for this signal is Belatacept ( CTLA4Ig)
Signal 3—Cytokines
The consequences of TCR engagement and costimulation are proliferation and differentiation of the T cells into effector cell and for production of cytokines which required to stimulate themselves, and other immune cell types, including CD8 T cells, macrophages, DCs, and B cells. IL-2 is a potent activator and proproliferative cytokine for T cells. Its effects are dependent on binding to its cell surface receptor, which has three subunits, α (CD25), β, and γ.
Blockade of the IL-2 receptor by targeting the α-chain profoundly inhibits T cell proliferation. Example : CD25 blockade with basiliximab or daclizumab have proven efficacy as induction agents in renal transplantation
1. Gabriel M. Handbook of kidney transplantation.Sixth Edition.
2. Stuart J.Kidney transplantation principles and practice . Eight Edition.
T lymphocytes activation depend on 3 signals :
signal 1: T- cell receptor for antigen (TCR) signaling:
-Here, the binding of antigens to TCR triggers a signal cascade that is not transduced through TCR proper, but through adjacent CD3 complex( which is a protein associated with TCR ).
-CD4 and CD8 co-receptors on T lymphocyte bind TCR-CD3 cluster on same MHC.
-So, antibodies target one or more proteins on CD3 complex block T lymphocytes activation. An example :
1-OKT 3:It bind to T cell receptor CD3 complex on the surface of circulating T cell, initially leading to activation but subsequently inducing the clearance of TCR complex from the cell surface and apoptosis of T cells.
2-CNI: TCR-CD3 cluster→activates phospholipase C-ɣ→generation of 2-second messangers diaacylglycerol(DAG ) and inositol 1,4,5 triphosphate(IP3) .
IP3→activates the calcineurin pathway which inhibits by CNI.
-CNI bind to specialized protein in the cell Known as immunophilins-cyclophilin in the case of cyclosporine, and FK- binding protein in the case of tacrolimus.
-The drug-immunophilin complex then blocks activation of calcineurin by calmodulin.
Signal 2:Co Stimulation
-T lymphocytes need signal 2 to undergo full proliferation and differentiation to either cytotoxic or helper cells.
-It is delivered by the engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen-presenting cells (APCs)
-Failure to provide signal 2 causes T lymphocytes deletion or anergy.
-There are many co-stimulatory pathways:
1-Integrins: Integrins LFA-1 and CD2 on T lymphocytes bind to their ligands on dendritic cells.
Efalizumab is anti-LFA-1 and Alefacept is anti CD2, they can lead to progressive multifocal leukoencephalopathy.
2-B7-CD28:CD28 is a co-stimulatory receptor present on all naive T lymphocytes. It is bound to the co-stimulatory molecules B7.1 and B7.2 on dendritic cells (DC).
Belatacept has a high affinity to B7and prevents its bind toCD28.It is commonly used to prevent allograft rejection in kidney transplant recipients.
3-CD40-CD154:CD154 is expressed on activated CD4+ lymphocytes whereas CD40 is present on B lymphocytes, DC, and macrophages.
4- Other Co-Stimulatory pathways:41BBl-41BB, OX40L-OX40, CD70:CD70, and ICOSL-ICOS pathway.
Signal 3:cytokines
-Cytokines are proteins secreted by mature APCs or T lymphocytes themselves,eg IL-2
-They stimulate T lymphocytes proliferation and induce the differentiation of T lymphocytes into multiple effector subsets.
-cytokines can also regulate T lymphocytes or act on other immune and nonimmune cells to either enhance or suppress inflammation.
-cytokines produced by APCs or by activated T lymphocytes direct the differentiation of proliferating T lymphocytes into multiple effector populations.CD4 lymphocytes differentiate into 4 major helper subpopulations and one regulatory subpopulation, while CD8 T lymphocytes differentiate into cytotoxic T cells.
-Th 1lymphosytes: their differentiation is driven by IL-12 and interferon-gamma.
Ustekinumab is a monoclonal block of IL-12.
-Th17 lymphocytes: They produce IL-17 which can be blocked by monoclonal antibodies (Secukinumab ).
References
1. Danovitch G.M handbook of kidney transplantation sixth edition
2.Bhorade, S. M.; Stern, E. (2009-01-15). “Immunosuppression for Lung Transplantation”. Proceedings of the American Thoracic Society. 6 (1): 47–53. doi:10.1513/pats.200808-096go. ISSN 1546-3222. PMID 19131530.
3.Benekli, M.; Hahn, T.; Williams, B. T.; Cooper, M.; Roy, H. N.; Wallace, P.; Stewart, C.; Bambach, B.; McCarthy, P. L. (September 2006). “Muromonab-CD3 (Orthoclone OKT3 ® ), methylprednisolone and cyclosporine for acute graft-versus-host disease prophylaxis in allogeneic bone marrow transplantation”. Bone Marrow Transplantation. 38 (5): 365–370. doi:10.1038/sj.bmt.1705450. ISSN 1476-5365. PMID 16862164. S2CID 31056997
Alloimmune response involves 3 signals
Signal 1: T cell receptor (TCR) signaling
Binding of HLA peptide complex on antigen presenting cells (APC) mostly dendritic cells (DC) to the TCR-CD3 complex on T lymphocyte
CD4 & CD8 coreceptors on T lymphocytes also help in activation of T lymphocytes
Activation depends on tyrosine kinase that activate phospholipase enzyme that catalyzes generation of second messengers DAG and IP3
DAG activate protein kinase C (PKC) and mitogen activated protein (MAP) while
IP3 trigger calcineurin pathway
PKC, MAP and calcineurin pathways activate transcription factors needed for T lymphocyte proliferation and differentiation.
Signal 2: Costimulation
Important for full proliferation and differentiation of T lymphocytes to effector cells
Integrins LFA-1 and CD2 on T cells bind their ligands on DC and act as costimulatory molecules for activation of T lymphocytes.
T lymphocyte activation require costimulation which which involve engaging of CD80 (B7-1) and CD86 (B7-2) on mature APC with CD28 on T lymphocytes
Intracellular signalling by CD28 increase transcription of genes required for T cell proliferation as IL2 receptor genes.
binding CD154 (on activated CD4+ T lymphocytes) with CD40 (on B lymphocytes, DCs and macrophages) induce switching of B cells from producing IgM to mor effective IgG antibodies isotypes
also, this binding enhances cytokine production by DCs leading to increase of costimulation of T cells especially CD8+
Other costimulatory pathways as OX40L-OX40, CD70:CD27 are induced with activation of T lymphocytes and leads to sustained activation
Signal 3: Cytokines
secreted from T cells and APCs
stimulate proliferation and differentiation to effector cells and regulate T lymphocytes
IL-2:
Activation of T lymphocytes by antigen and costimulatory molecules increase expression of CD25 which increase binding of IL2 to receptors causing clonal expansion of T cells.
Other cytokines as IL-4,7,9,15 and 21 bind to receptor containing gamma chain (common gamma chain cytokine receptor), signalling is mediated by JAK3.
Cytokines direct the differentiation of proliferating T lymphocytes into effector cells:
CD4+ T cells differentiate into Th1,2,17, T follicular helper cells and T regulatory cells.
CD8+ T cells differentiate into cytotoxic T cells
cytokines also help transition of T cells to memory cells
Handbook of kidney transplantation by Gabriel M. Danovitch, Sixth Edition.
Thankyou Heba you all mentioned each drug and its site of action but somehow no mention of corticosteroids please mention the site of action and as a milestone in acute TCMR.
Corticosteroids inhibit cytokine receptor expression and dendritic cell function
they decrease the expression of IL-1, 2, 3, 6, TNF-a and IFN-g
so all stages of T cell activation are inhibited
Pulse methylprednisolone is the first line therapy for acute TCMR, it reverse 75% of first acute rejections
Handbook of kidney transplantation by Gabriel M. Danovitch, Sixth Edition.
T cell activation need 3 signals:
* Signal 1 : binding of the TCR with the MHC–peptide complex
* Signal 2 ( co-stimulation signal) ; engagement of “costimulatory” molecules found on the surface of T cells with their specific ligands on APCs ( CD80/86 and CD28).
* Signal 3 ( cytokine signal) ; signal1 and signal 2 lead s to induction of cytokine genes including ; IL-2 & IL2 receptors .This causes T cell proliferation and differentiation.
Drugs affecting signal 1:OKT3
Drugs affecting signal 2 ; Belatacept
Drugs targeting signal 3 ; mTORi, antiptroliferative, IL-2 blockade
TCR signal transduction.
Antigen recognition by the TCR leads to initiation of a phosphorylation cascade. The CD4/CD8 coreceptor phosphorylates CD3 and z (zeta) chains. ZAP-70 binds to two phosphotyrosines of the z (zeta) chain and, once activated, phosphorylates various adapter molecules including PLC g (gamma)1. These adapter proteins become docking sites for cellular enzymes that lead to activation of the mitogen-activated protein (MAP) kinase pathway, protein kinase C (PKC) pathway, and calcineurin pathway. These pathways converge to generate transcription factors AP-1 (activavtion protein-1), NF k (kappa)B (nuclear factor k (kappa)B), and NFAT (nuclear factor of activated T cells) which lead to gene transcription of various cytokines such as the T cell growth factor IL-2 (interleukin-2)
Three signals are required for T cell activation :
I- Signal 1 : antigen recognition
⦁ Occur when antigen peptide- MHC complex located on APC is attached to TCR
⦁ Immune response to a graft is either Alloantigen-dependent or alloantigen-independent (tissue injury such as ischemic injury leading to upregulation of expression of adhesion molecules, or shedding of intact HLA )
⦁ TCRs recognize antigen peptide complexed with MHC class I or II on the surfaces of APCs (TCR- AG- peptide-APC)
⦁ APC is either of the donor (interstitial denderitic cells, vascular endothelium) presenting intact HLA molecules and this is called the direct pathway (1) and is responsible for acute early graft rejection or of the recipient presenting processed HLA molecules and this is called indirect pathway and is responsible for chronic graft rejection (2)
Drugs acting on signal 1 :
1. OKT3 : is a monoclonal AB which binds to lymphocyte CD3 complex leading to rapid lysis.
2. rATG : polyclonal antibodies produced to many hematopoietic antigens (CD2, 3, 4 ,8, 18)
3. Alimtuzumab : humanized panlymphocytic ( B and T cells) anti-CD52 monoclonal antibody
II- Signal 2 : Costimulation
⦁ Occur when one or more TCR antigens (CD28, CTLA-4) interacts with its specific legend in APC (B7-1, B7-2). this is called co-stimulation
⦁ CD28 stimulate, while CTLA-4 supress T cells
Drugs acting on signal 2 :
1. Abatacept (CTLA4-Ig) is still under trial but it was found that the use of abatacept is associated with inhibition of T helper 1 and not T helper 2, leading to decrease in the incidence of acute rejection, but chronic rejection still occurs (3)
2. Belatacept (a high affinity variant CTLA4-Ig), has been developed with more immunosuppressive effect.
III- Signal 3 : Activation and differentiation of T cells
Once co-stimulation occur it activates signaling pathways that lead to calcineurin activation which in turn dephosphorylates NF-AT (Nuclear factor of activated T cell- a family of transcription factors) leading to IL-2 transcription and production of IL2 by T cell, IL-2 receptor stimulation activates the mTOR signaling pathway which induces the T cell to enter the cell cycle resulting in T cell activation and differentiation into :
A- CD4+ T helper which activate B cells (humoral immune response), macrophages (delayed type hypersensitivity), CD8+ T cells and produce inflammatory cytokines like TNF and IFN-𝛾 that cause direct injury to the graft.
B- CD8+ cytotoxic T cells which produce cell-mediated cytotoxicity by either killing of the cell or induction of apoptosis
Drugs acting on signal 3 :
1. IL-2 receptor antagonists Basiliximab and Daclizumab (4)
2. IL2 signaling inhibitors : mTOR inhibitors : Sirolimus and everolimus.
3. Calcineurin inhibitors : Cyclosporine and tacrolimus.
4. Cell cycle blocker : MMF, azathioprine.
REFERANCES
1. Matzinger P, Bevan MJ. Hypothesis: why do so many lymphocytes respond to major histocompatibility antigens? Cell Immunol 1977; 29:1.
2. Vella J, Knoflach A, Waaga A, Sayegh M. T cell mediated immune responses in chronic allograft rejection: Role of indirect allorecognition and costimulatory pathways. Graft 1998; 1:S11.
3. Vincenti F, Luggen M. T cell costimulation: a rational target in the therapeutic armamentarium for autoimmune diseases and transplantation. Annu. Rev. Med. 2007;58:347–358
4. Webster AC, Playford EG, Higgins G, Chapman JR, Craig JC. Interleukin 2 receptor antagonists for renal transplant recipients: a meta-analysis of randomized trials. Transplantation. 2004;77(2):166–176.
Thankyou Sherif
In signal 2 can you differentiate between the action of CTLA4(CD152) in slowing of the immune response as part of T reg action versus the block of costimulation when using Belatacept.
Dear All
Dear Dr. Sherif
It would be helpful if you thought about keeping such a delicate balance between inhibiting T cell activation and autoimmunity.
You need to consider the possible risks of increasing acute rejection while using costimulation blockade!
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5590720/
Primary T cell activation involves the integration of three distinct signals delivered in sequence:
(1) anti- gen recognition.
(2) costimulation.
(3) cytokine mediated differentiation and expansion.
●☆● SIGNAL ONE :
T cells are generated in the Thymus and are programmed to be specific for one particular foreign particle (antigen).
Once they leave the thymus, they circulate throughout the body until they recognise their antigen on the surface of antigen presenting cells (APCs).
The T cell receptor (TCR) on both CD4+ helper T cells and CD8+ cytotoxic T cells binds to the antigen as it is held in a structure called the MHC complex on the surface of the APC.
This triggers initial activation of the T cells.
The CD4 and CD8 molecules then bind to the MHC molecule too, stabilising the whole structure.
●☆● SIGNAL TWO
In addition to TCR binding to antigen loaded MHC, both helper T cells and cytotoxic T cells require a number of secondary signals to become activated and respond to the threat.
In the case of helper T cells, the first of these is provided by CD28.
This molecule on the T cell binds to one of two molecules on the APC – B7.1 (CD80) or B7.2 (CD86) and initiates T-cell proliferation.
This process leads to the production of many millions of T cells that recognise the antigen.
In order to control the response, stimulation of CD28 by B7 induces the production of CTLA-4 (CD152).
This molecule competes with CD28 for B7 and so reduces activation signals to the T cell and winds down the immune response
●☆● Signal Three
Once the T cell has received a specific antigen signal and a general signal two, it receives more instructions in the form of cytokines.
These determine which type of responder the cell will become in the case of helper T cells, it will push them into Th1 type (cells exposed to the cytokine
IL-12), Th2 (IL-4), or IL-17, IL-6, IL-23).
Each one of these cells performs a specific task in the tissue and in developing further immune responses.
●The three main pathways activated through the TCR that control transcription are the MAPK, NF-κB, and calcium pathways.
● These pathways dramatically alter the expression and nuclear localization of various transcription factors that directly regulate genes involved in T cell activation.
Reference:
https://www.immunology.org/public-information/bitesized-immunology/systems-and-processes/t-cell-activation
____________________________
●●Example of immunosuppressive ttt that work in that way are:
CNI : Cyclosporin A
Anti cd 28 : BALATECEPT
Successful clinical trials established CD28 costimulation blockade mediated by the CTLA-4Ig derivative Belatacept as emerging treatment modality to prevent acute rejection and protect renal function in kidney transplant recipients .
The results of this study suggest synergistic effects during the combined use of CTLA-4Ig and CsA.
Reference
17. Larsen C.P., Grinyo J., Medina-Pestana J., Vanrenterghem Y., Vincenti F., Breshahan B. Belatacept-Based Regimens Versus a Cyclosporine A-Based Regimen in Kidney Transplant Recipients: 2-Year Results From the BENEFIT and BENEFIT-EXT Studies. Transplantation. 2010;90:1528–1535. [PubMed] [Google Scholar]
18. Rostaing L., Massari P., Garcia V.D., Mancilla-Urrea E., Nainan G., Rial M.D. Switching from Calcineurin Inhibitor-based Regimens to a Belatacept-based Regimen in Renal Transplant Recipients: A Randomized Phase II Study. Clin J Am Soc Nephrol. 2011;6:430–439. [PMC free article] [PubMed] [Google Scholar]
Well done
Signal 1 : Activation
activation of TCR and the nearby CD3 complex leads to a series of events that lead to activation of calcineurin which leads to dephosphorilation of NFAT ( nuclear factor of activated T cell) which then translocate to the nucleus and is responsible for transcription of genes required for T cells proliferation and differentiation. so signal 1 responsible for t cell activation.
Drugs that work by blocking signal 1 like : OKT3 : monoclonal antibody against CD3 , which leads to depletion of T cells.
another example is CNIs (cyclosporine and tacrolimus ) which inhibit calcineurin and prevent dephosphorilation of NFAT , and this leads to prevention of translocation of NFAT .
Signal 2 : Co-stimulation:
after TCR , CD3 complex activaion by the APC , there is increased expression of CD28 on the surface of T cells which will interact with CD80/86 on the surface of APC , this interaction is important for T cell activation . without this interaction T cell become anargic ( resistant to stimulation) and may lead to it’s apoptosis.
Drugs that inhibit co-stimulation : belatacept is CTLA 4 Ig that will bound to CD80/86 and prevent it’s interaction with CD 28. so the T cell can not be fully activated .
Signal 3 : cytokines:
after T cell activation , it will increase expression of many cytokines like IL2 . IL2 a receptor (CD25) on the surface of T cells once activated will lead to series of events through mTOR pathway leads to T cells proliferation and differentiation .
DRUGS :
basiliximab : monoclonal antibody that blocks IL2a receptor used in induction of immunosuppression during transplantation of low risk patients.
mTOR inhibitors : everolimus and sirolimus .
references:
(1) Gabriel M. Danovitch, MD Handbook of Kidney Transplantation sixth edition
(2) Alexander C. Wiseman Immunosuppressive Medications
Clin J Am Soc Nephrol 11: 332–343, 2016. doi: 10.2215/CJN.08570814
Nice elaborate answer, good use of evidence.
these arethe drugs which work in the different signal
Hi Dr Elthahir, Please use any drug example in my plain words to explain the 3 signal model.
Signal-1 CNI block the activation of calcineurin to NFAT .
Signal -2 Blatacept bind to CD80and CD86 on APC blocking CD28 so preventing costimulation of T cell .
Signal -3 AntiCD28 mAb Basiliximab ,mTOR inhibitors {Sirolimus and Everolimus }.
Signal 1 recognition of foreign antigen within the MCH
Signal 2 Co stimulation between B7and CD28
Signal 3 cytokine-mediated differentiation and expansion
Hi Dr Eltahir, Please elaborate your answer. What do you think is the role of CTLA-4 (CD152).
CTLA-4 or CTLA4, also known as CD152, is a protein receptor that functions as an immune checkpoint and downregulates immune responses. CTLA-4 is constitutively expressed in regulatory T cells but only upregulated in conventional T cells after activation – a phenomenon which is particularly notable in cancers.
3 Signal model of T cell activation
Signal one : is the recognition of the T-cell receptor (TCR) to antigenic peptides presented by major histocompatibility complex (MHC) class II molecules.
Signal two : co-stimulation by the triggering of CD28 on the T cell by CD80 and CD86 molecules on the Denderitic Cell.
Signal three : directs T-cell differentiation into phenotypes such as Th1 and Th2.
Both signals, two and three, result from the binding of microbial products to germline-encoded receptors such as toll-like receptors (TLR) on the Dendritic cells.(1)
Mechanism of action of the current immunosuppressive drugs in relation to the 3-signal model
Signal 1 starts due to MHC–antigen recognition by the T-cell receptor–CD3 complex, a process blocked by anti-CD3 monoclonalAbs and by rituximab.
Signal 2 can be blocked by belatacept. Costimulation resulting in calcineurin activation, a stage that can be inhibited by tacrolimus and cyclosporine A.
Activated calcineurin dephosphorylates Nuclear factor of activated T cell,enabling IL-2 transcription to start signal 3. IL-2 receptor stimulation, can be blocked by basiliximab,
IL 2 R stimulation activates the mTOR signaling cascade, which can be inhibited by sirolimus.
This pathway induces the T cell to enter the cell cycle which can be blocked by methotrexate, mycophenolate and azathioprine. AntiThymocyte Globulin exerts polyclonal effects while alemtuzumab binds to CD52, both leads to immunodepletion. (2)
1-Corthay A. A Three-cell Model for Activation of Naıve T Helper Cells .Scandinavian Journal of Immunology 2006 ,64, 93–96
2-Getts DR et al Current landscape for T-cell targeting in autoimmunity and transplantation Immunotherapy. 2011 Jul; 3(7): 853–870.
Hi Dr Elwasly , Very nicely explained. Heller T cells and Cytotoxic cells are activated by different Cluster of differention proteins (CD). Can you highlight them in your reply.
agree with that but to clarify and help remove the confusion of mixing numbers. to help my colleagues realize what I mixed as well .. CD 80/86 are same b7.1, b.7.2 (alternatif names)
T- Lymphocyte Activation:
T lymphocytes depend on three signals.
Signal 1 is delivered by binding of TCRs on T lymphocytes to HLA-peptide complexes on APCs. Signal 1 is necessary but not sufficient for T- lymphocyte proliferation and differentiation.
Signal 2 is delivered by binding of specialized accessory molecules on APCs to their receptors on T lymphocytes. Along with signal 1, signal 2 causes T-lymphocyte proliferation and differentiation.
cytokines produced by APCs deliver signal 3, which determines the differentiation pathway of T lymphocytes into specialized subsets.
Gabriel M. Danovitch :Kidney transplantation
Hi Dr Elnazer, point reply noted. Can you comment on ICOS, 4-1BB and OX40 molecule role in effector immune response
T-cell activation
Signal One
Signal Two
Signal Three
T cell activation signals and drugs’ target:
Signal 1: Antigen triggers T-cell receptors and synapse formation occurs.
Drug Example:
Signal 2: Signal 1 allows co-stimulation of antigen-presenting cells to occur.
Drug Example:
Belatacept A biological agent that blocks the co-stimulation necessary for activation of the T-cell (signal 2), Belatacept is associated with less metabolic side effects and nephrotoxicity compared to CNIs and mTOR inhibitors.
Signal 3: Signal 1 and signal 2 stimulate a cascade of intracellular events culminating in the
initiation of the T-cell cycle; stimulation of the T-cell cycle allows T cells to infiltrate the graft.
Drug Example:
CNI :This class of drugs works by blocking calcineurin, an intricate protein in the signal transduction pathway that activates signal 3. ( eg: Tacrolimus and Cyclosporin)
Summary effect is to inhibit T-cell receptor activation, cytokine production, and subsequent lymphocyte proliferation to prevent rejection.
Very nicely elaborated and explained. Well done.
The activation of T cells primarily involves the integration of three distinct signals. first signal is antigen recognition followed by costimulation then cytokine mediated differentiation and expansion. As in picture attached anti cd25 (daclizumab) acts on signal 3 while anti CD3mAb acts on signal 1
DOI: 10.4172/2161-0991.1000169
T cell activation happened through 3 signals:
1- signal 1 interaction between T cell receptor TCR on T cell (CD4 and CD8) and MHC on APCs , TCR is in association with CD3 on surface of T cell, this describe signal 1
can be blocked by block CD3 which is important to this step. OKT3 is the 1st monoclonal antibody block CD3. also cyclosporin act on this signal.
2- signal 2 it is costimulation , it is important for T cell activation , done by interaction between CD28 on T cell and CD80, 86 on APCs , balatacept block this costimulatory pathway.
3- signal 3 activation of signal 1 and 2 will end by release of cytokine IL-2 which interact with its receptor on T cell surface which is CD25. basiliximab or simulect block it as it is directed against CD25 anti-CD25 monoclonal antibody used in induction. mTor inhibitors act on this signal also by inhibit mTor which is previously stimulated by interaction between IL-2 and its receptor.
reference:
Gabriel M Danovitch. Handbook of kidney transplantation. 6th ed.
Brief but point reply. Can you comment on activation of Helper T cells and Cytotoxic T cells by co-stimulatory pathway
expression of CD80/86 on APCs (B7 protein), can bind to CD28 receptor on T cell either helper or cytotoxic this binding will leads to activation and proliferation of T cells
The full activation of & differentiation of T cells depends on 3 signals:
Reference: Danovitch G. Handbook of Transplantation.6th edition.
Hi Dr Mezher, Nice and succint reply. What do you think is the role of IL12, IL4, IL6 and IL23 in regards to signal 3 pathway.
IL12 induce expansion & differentiation of CD8 T cells, IL4 had effects on both T cell ( induce differentiation of naive CD4 T cells into Th cells) & B cells ( generation go IgG1& IgE).Il23 has an important role in proliferation & maintenance of Th17.
Signal 1: T cell receptor activation
The binding of antigen to TCR trigger signals that transduced through CD3 and leads to T cell activation. CD4 & CD8 co-receptors on T cells also participate in the process of activation. OKT3 is the first monoclonal antibody used to block this signal. CNI also act on this signal.
Signal 2: Co-stimulation
The engagement of co-stimulatory receptors on T lymphocytes by their ligands on antigen presenting cells. in the absence of this signal, T cells become anergic and these anergic cells can inhibit the activation of neighboring T cells.
many families involved in the co-stimulatory pathway including : CD7 ( Ig family), CD 154 ( tumor necrosis factor family), C3a & C5a receptors ( G protein coupled receptors) & lectin receptors,
Belatacept ( CTLA4Ig) acts on this signal.
Signal 3: cytokines
Cytokines involved in T cell activation are proteins secreted by mature antigen presenting cells or the T lymphocytes themselves.
They have 2 main functions in the process of T cell activation:
IL-2 is a potent activator and proliferative cytokine for T cells. It has a cell surface receptor which contain 3 subunits: alfa( CD25), beta, gamma. It’s effect depends on binding to cell surface receptor. During T cell activation, alfa subunit becomes associated with other subunits to form high affinity receptor. Blockade of the IL-2 receptor by targeting the alfa chain profoundly inhibits T cell proliferation.
CD25 blockade with Basiliximab or Daclizumab have a proven efficacy as induction therapy in kidney transplantation.
References:
Well explained.
thanks
T lymphocytes activation go through three signals:
–Signal 1 begins with the binding of TCR/CD3 to antigenic peptides presented by the APC( donor or recipient).This trigger leads to dramatic increase in tyrosine phosphorylation of multiple cellular proteins, responsible for activation of calcineurin pathway, mitogen activated protein (MAP) and protein kinase C (PKC), which finally activate key transcription factors required for T cell activation, proliferation and differentiation. Drugs used at this signal level include Muromunab OKT3 targeting CD3 complex which blocks T cells activation. CNI block calcineurin pathway.
–Signal 2 is an essential step for T lymphocytes to continue full activation and proliferation into full effector cells. Signal 2 is delivered by engagement of co-stimulatory receptors on T lymphocytes to their ligands on APCs. Co-stimulatory receptors are absent or inactive in naive T lymphocytes, but they are upregulated in response to T cell activation by antigen. Signal 2 is essential for more sustained stimulation and amplification of T cell response. Co -stimulatory receptors include integrins (LFA-1, and LFA-2(CD2)) that bind to ICAM 1, ICAM2 and CD58 on APC.CD28 is a co-stimulatory receptor on T lymphocytes that bind to B7 on APC. The result of this complex is potentiation of tyrosine phosphorylation. Drug that act at this level is Belatacept which has high affinity to B7, preventing from binding to CD28. CD40L-CD40 is the complex formed between activated T lymphocyte and APC (B cells, DC, macrophages).
–Signal 3 includes the activation of T lymphocytes by cytokines released by APC or by the T lymphocytes themselves. Cytokines play major role in T cell proliferation and differentiation into multiple effector subsets. IL2 has been the target of therapies in induction immunosuppression for its role in T lymphocytes proliferation. IL2 is produced by activated T lymphocytes and acts on the same cell. IL2 receptors are inactive in naive T lymphocytes but upon activation by antigen and costimulatory pathways ), the T cell produce CD25, that with the pre-existed IL2 receptor, form more active IL2 receptor.
IL-2 receptor monoclonal antibody to the alpha chain is used in the induction immunosuppressive therpies to prevent acute rejection.
Targeting T cell activation at many signal levels is still the site of researchers, as TCMR is still the major cause of rejection among kidney transplant recipients.
References
-Handbook of kidney transplantation 6th edition
-UpToDate
Explained in detail. Good use of references
We addressed the importance of co-stimulation for T cell activation. Do you think that all T cells require co-stimulation?
Naive CD8+ T cells do not require costimulation for proliferation and differentiation into cytotoxic effector cells.
Why we need to know the 3-signal model?
Different immunosuppressive work at different levels of the 3 signal model and knowing the 3 signa4l model will help in planning the immunosuppression.
What is CD3 and what is its role in T cell activation?
CD3 family of proteins is a complex of signaling molecules, which help to mediate T cell activation. Once the TCR is properly engaged with the peptide-MHC complex, conformational changes in the associated CD3 chains are induced, which leads to their phosphorylation and association with downstream proteins . Phosphorylated CD3 then recruit and activate the Syk family kinase zeta-activated protein.
signal 1
recognition of MHC-peptide complexes on antigen presenting cells by thr TCR leads to activation of multiple intracellular signalling pathways.
OKT3 is an antibody specific for CD3 and CNIs inhibit intracellular signalling.
signal 2
TCR signalling alone leads to T cell apoptosis and not activation.a second signal known as co-stimulation is required.Ex T cell CD28 and APC CD80 and 86.
Belatacept binds with CD80/86 and blocks their interaction with T cell receptor CD28,thus providing costimulation blockade.
signal 3
signalling through the TCR abd costimulation leads to the activation of many genes like IL2 and IL2R(CD25) which are required for T cell proliferation and effector functions.
Basiliximab and daclizumab are anti CD25 antibodies abd block binding of IL2 to the IL-2R .
sirolimus block the proliferative signal from the IL-2R and many other T cell growth factors mediated by mTOR.
Oxford handbook of transplant