Antigen Presentation and T Lymphocyte Biology PDF

Slide 26 T Cell Activation & Survival (Signals 1 & 2) Slide 27 Activation: upstream events (Signal 1) TMHC # & associated CD3 up w) made of mostly > - 11 zet chains - zeth chains docking tite forzAP crea 10 Mansegna serves dockingnaling sig of station lot LAT or pathways as a...

Slide 26 T Cell Activation & Survival (Signals 1 & 2) Slide 27 Activation: upstream events (Signal 1) TMHC # & associated CD3 up w) made of mostly > - 11 zet chains - zeth chains docking tite forzAP crea 10 Mansegna serves dockingnaling sig of station lot LAT or pathways as a Ac va on – Signal 1 The :-TCR is in fact a complex of polypeptide chains: the TCR -chain, the TCR -chain, a chain, a -chain, two -chains, as well as two -chains. The :-TCR heterodimer forms the antigen recognition component of the TCR. Two heterodimers, one containing the - and chains and one the - and -chains form the CD3 signaling complex. The two -chains form a homodimer associated with the transmembrane tail of the :-TCR heterodimer. The CD3 and  homodimer cytoplasmic tails are loosely associated with a Src family tyrosine kinase, Fyn. Associated with the TCR complex is the (1) T cell co-receptor, either CD4 or CD8, both of which are tightly associated with another Src family tyrosine kinase, Lck, on their cytoplasmic domain, and (2) CD45 which is a phosphatase required for the activation of Fyn (Fyn needs to be dephosphorylated to be activated). It is estimated that 10 to 50 peptide:MHC-II complexes displayed on the surface of pAPCs is sufficient to activate CD4 T lymphocytes; CD8 T lymphocytes are estimated to require as little as 1 to 3 peptide:MHC-I complexes. When T cells are presented with sufficient amounts of peptide:MHC complexes, TCR complex clustering takes place to initiate intracellular signaling. The cytoplasmic domains of CD3 and the  homodimer contain Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) that increase the efficiency of receptor signaling by creating docking sites for the kinases involved in signaling. Clustering of two or more :-TCR heterodimers with CD4 (or CD8) co-receptors and CD45 triggers the activation of Fyn when CD45 dephosphorylate it. Fyn then phosphorylates tyrosine residues in the ITAMs of CD3 and the  homodimer. As a result, ZAP-70 (-chain-associated protein kinase 70), another tyrosine kinase, binds (docks to) the phosphorylated ITAMs of the  homodimer and is in turn phosphorylated (activated) by Lck. ZAP-70 then phosphorylates the scaffold proteins LAT (Linker of Activated T cells) and SLP-76, which are linked by an adaptor protein (GADS); the LAT:SLP-76:GADS complex then recruits phospholipase C- (PLC-) which then cleaves phosphatidylinositol biphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). Further downstream signaling include (1) the activation and nuclear translocation of the transcription factor nuclear factor kappa B (NF-B) as a result of PKC- activation by DAG, (2) the activation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NF-AT) resulting from Ca2+/calcineurin signaling by IP3, and (3) the activation and nuclear translocation of the transcription factor AP-1 (Activator Protein 1) as a result of MAPK (Mitogen-Activated Protein Kinase) activation by RasGRP and DAG (refer to the Cellular Signaling I & II lecture for a review on cell signaling). Slide 28 Activation: Ca2+ signaling (Signal 1) X Ac va on – Signal 1 The :-TCR is in fact a complex of polypeptide chains: the TCR -chain, the TCR -chain, a chain, a -chain, two -chains, as well as two -chains. The :-TCR heterodimer forms the antigen recognition component of the TCR. Two heterodimers, one containing the - and chains and one the - and -chains form the CD3 signaling complex. The two -chains form a homodimer associated with the transmembrane tail of the :-TCR heterodimer. The CD3 and  homodimer cytoplasmic tails are loosely associated with a Src family tyrosine kinase, Fyn. Associated with the TCR complex is the (1) T cell co-receptor, either CD4 or CD8, both of which are tightly associated with another Src family tyrosine kinase, Lck, on their cytoplasmic domain, and (2) CD45 which is a phosphatase required for the activation of Fyn (Fyn needs to be dephosphorylated to be activated). It is estimated that 10 to 50 peptide:MHC-II complexes displayed on the surface of pAPCs is sufficient to activate CD4 T lymphocytes; CD8 T lymphocytes are estimated to require as little as 1 to 3 peptide:MHC-I complexes. When T cells are presented with sufficient amounts of peptide:MHC complexes, TCR complex clustering takes place to initiate intracellular signaling. The cytoplasmic domains of CD3 and the  homodimer contain Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) that increase the efficiency of receptor signaling by creating docking sites for the kinases involved in signaling. Clustering of two or more :-TCR heterodimers with CD4 (or CD8) co-receptors and CD45 triggers the activation of Fyn when CD45 dephosphorylate it. Fyn then phosphorylates tyrosine residues in the ITAMs of CD3 and the  homodimer. As a result, ZAP-70 (-chain-associated protein kinase 70), another tyrosine kinase, binds (docks to) the phosphorylated ITAMs of the  homodimer and is in turn phosphorylated (activated) by Lck. ZAP-70 then phosphorylates the scaffold proteins LAT (Linker of Activated T cells) and SLP-76, which are linked by an adaptor protein (GADS); the LAT:SLP-76:GADS complex then recruits phospholipase C- (PLC-) which then cleaves phosphatidylinositol biphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). Further downstream signaling include (1) the activation and nuclear translocation of the transcription factor nuclear factor kappa B (NF-B) as a result of PKC- activation by DAG, (2) the activation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NF-AT) resulting from Ca2+/calcineurin signaling by IP3, and (3) the activation and nuclear translocation of the transcription factor AP-1 (Activator Protein 1) as a result of MAPK (Mitogen-Activated Protein Kinase) activation by RasGRP and DAG (refer to the Cellular Signaling I & II lecture for a review on cell signaling). Slide 29 Activation: MAPK signaling (Signal 1) Ac va on – Signal 1 The :-TCR is in fact a complex of polypeptide chains: the TCR -chain, the TCR -chain, a chain, a -chain, two -chains, as well as two -chains. The :-TCR heterodimer forms the antigen recognition component of the TCR. Two heterodimers, one containing the - and chains and one the - and -chains form the CD3 signaling complex. The two -chains form a homodimer associated with the transmembrane tail of the :-TCR heterodimer. The CD3 and  homodimer cytoplasmic tails are loosely associated with a Src family tyrosine kinase, Fyn. Associated with the TCR complex is the (1) T cell co-receptor, either CD4 or CD8, both of which are tightly associated with another Src family tyrosine kinase, Lck, on their cytoplasmic domain, and (2) CD45 which is a phosphatase required for the activation of Fyn (Fyn needs to be dephosphorylated to be activated). It is estimated that 10 to 50 peptide:MHC-II complexes displayed on the surface of pAPCs is sufficient to activate CD4 T lymphocytes; CD8 T lymphocytes are estimated to require as little as 1 to 3 peptide:MHC-I complexes. When T cells are presented with sufficient amounts of peptide:MHC complexes, TCR complex clustering takes place to initiate intracellular signaling. The cytoplasmic domains of CD3 and the  homodimer contain Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) that increase the efficiency of receptor signaling by creating docking sites for the kinases involved in signaling. Clustering of two or more :-TCR heterodimers with CD4 (or CD8) co-receptors and CD45 triggers the activation of Fyn when CD45 dephosphorylate it. Fyn then phosphorylates tyrosine residues in the ITAMs of CD3 and the  homodimer. As a result, ZAP-70 (-chain-associated protein kinase 70), another tyrosine kinase, binds (docks to) the phosphorylated ITAMs of the  homodimer and is in turn phosphorylated (activated) by Lck. ZAP-70 then phosphorylates the scaffold proteins LAT (Linker of Activated T cells) and SLP-76, which are linked by an adaptor protein (GADS); the LAT:SLP-76:GADS complex then recruits phospholipase C- (PLC-) which then cleaves phosphatidylinositol biphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). Further downstream signaling include (1) the activation and nuclear translocation of the transcription factor nuclear factor kappa B (NF-B) as a result of PKC- activation by DAG, (2) the activation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NF-AT) resulting from Ca2+/calcineurin signaling by IP3, and (3) the activation and nuclear translocation of the transcription factor AP-1 (Activator Protein 1) as a result of MAPK (Mitogen-Activated Protein Kinase) activation by RasGRP and DAG (refer to the Cellular Signaling I & II lecture for a review on cell signaling). Slide 30 Activation: transcription factors (Signal 1) Ac va on – Signal 1 The :-TCR is in fact a complex of polypeptide chains: the TCR -chain, the TCR -chain, a chain, a -chain, two -chains, as well as two -chains. The :-TCR heterodimer forms the antigen recognition component of the TCR. Two heterodimers, one containing the - and chains and one the - and -chains form the CD3 signaling complex. The two -chains form a homodimer associated with the transmembrane tail of the :-TCR heterodimer. The CD3 and  homodimer cytoplasmic tails are loosely associated with a Src family tyrosine kinase, Fyn. Associated with the TCR complex is the (1) T cell co-receptor, either CD4 or CD8, both of which are tightly associated with another Src family tyrosine kinase, Lck, on their cytoplasmic domain, and (2) CD45 which is a phosphatase required for the activation of Fyn (Fyn needs to be dephosphorylated to be activated). It is estimated that 10 to 50 peptide:MHC-II complexes displayed on the surface of pAPCs is sufficient to activate CD4 T lymphocytes; CD8 T lymphocytes are estimated to require as little as 1 to 3 peptide:MHC-I complexes. When T cells are presented with sufficient amounts of peptide:MHC complexes, TCR complex clustering takes place to initiate intracellular signaling. The cytoplasmic domains of CD3 and the  homodimer contain Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) that increase the efficiency of receptor signaling by creating docking sites for the kinases involved in signaling. Clustering of two or more :-TCR heterodimers with CD4 (or CD8) co-receptors and CD45 triggers the activation of Fyn when CD45 dephosphorylate it. Fyn then phosphorylates tyrosine residues in the ITAMs of CD3 and the  homodimer. As a result, ZAP-70 (-chain-associated protein kinase 70), another tyrosine kinase, binds (docks to) the phosphorylated ITAMs of the  homodimer and is in turn phosphorylated (activated) by Lck. ZAP-70 then phosphorylates the scaffold proteins LAT (Linker of Activated T cells) and SLP-76, which are linked by an adaptor protein (GADS); the LAT:SLP-76:GADS complex then recruits phospholipase C- (PLC-) which then cleaves phosphatidylinositol biphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). Further downstream signaling include (1) the activation and nuclear translocation of the transcription factor nuclear factor kappa B (NF-B) as a result of PKC- activation by DAG, (2) the activation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NF-AT) resulting from Ca2+/calcineurin signaling by IP3, and (3) the activation and nuclear translocation of the transcription factor AP-1 (Activator Protein 1) as a result of MAPK (Mitogen-Activated Protein Kinase) activation by RasGRP and DAG (refer to the Cellular Signaling I & II lecture for a review on cell signaling). Slide 31 Costimulation: (Signal 2) Cos mula on – Signal 2 While the ligation of the T cell receptor with specific peptide:MHC is necessary for activation, it is not sufficient. T lymphocytes also require a second signal, termed costimulatory signal or survival signal. The pAPC provides that signal; CD80 and/or CD86 on the surface of the pAPC bind CD28 on the surface of the T cell. Professional APCs usually do not express costimulatory molecules in the absence of infection; signaling through toll-like receptors or other receptors of innate immunity (inflammation) triggers the expression of costimulatory molecules by pAPCs. The combined peptide:MHC and CD28:CD80/86 interaction triggers complete activation of the T cell. When a naïve T cell binds to peptide:MHC on a cell that does not express CD80/86, the cell becomes non-responsive to antigen (anergic) and cannot subsequently become activated or becomes a Treg cell; many anergic T cells eventually die from apoptosis. CD28-dependent costimulation reinforces the expression of IL-2 and high affinity IL-2 receptors (IL-2R – :: heterotrimers; naïve, resting T lymphocytes express low affinity IL-2 receptors – : heterodimers), which in turn ensure T cell proliferation and allows for differentiation to take place. CD28-mediated signaling is achieved by a variety of second messengers, including the phosphatidylinositide 3-kinase/Akt pathway, the Ras-MAPK pathway, as well as Lck. Slide 32 Activation & costimulation: outcome (Signals 1 & 2) CD28-dependent co-stimulation drives the expression of IL-2 and high affinity IL-2 receptors (IL2R – :: heterotrimers; naïve, resting T lymphocytes express low affinity IL-2 receptors – : heterodimers), which in turn triggers T cell proliferation and allows for differentiation to take place. CD28-mediated signaling is achieved by a variety of second messengers, including the phosphatidylinositide 3-kinase/Akt pathway, the Ras-MAPK pathway, as well as Lck. Slide 33 Antigen Presentation: Differentiation (Signal 3) Slide 34 Differentiation: Signal 3 cytokines Depending on the cytokines present during antigen presentation, CD4 T lymphocytes will differentiate into either TH1, TH2, TH17, or Treg cells (following slides). Most differentiation signals (signal 3) would use type I or type II cytokine receptors which signal through JAK/STAT: As previously seen, JAK/STAT signaling occurs following these steps: • • Cytokine engagement of cytokine receptor chains; • • Activation of JAKs by cross-phosphorylation (sometimes called trans-phosphorylation); • Recruitment and docking of STATs; Dimerization (or trimerization in the case of cytokine receptors with three receptor subunits) of receptor chains bringing receptor-associated JAK molecules in close proximity; JAK-mediated phosphorylation of receptor chain ITAMs (Inducible Tyrosine-based Activation Motif); • • • JAK-mediated phosphorylation and dimerization of STATs; • Gene transcription. STAT dimer nuclear translocation; STAT dimer binding to promoter palindromic GAS-binding sites (Gamma interferon Activation Site); Slide 35 Differentiation: TH1 Cells For intracellular microorganisms & tumor cells: – TH1 cells require IL-12 & IFN- for their differentiation; the IL-12 comes from the pAPC (DC or macrophage), and the IFN- comes from NK cells – TH1 cells in turn secrete IFN-, IL-12 & TNF-α, and express CD40L, a membrane-bound cytokine involved in the activation of macrophages, B lymphocytes, and in helping pAPCs augment their CD80/CD86 expression for CD8 T lymphocyte activation TH1 cell differentiation. Generally, antigen presentation achieved in the presence of IL-12 and IFN- (often with IL-18 & type I IFNs) yields TH1 cells which in turn secrete IFN-, IL-12, TNFα and other proinflammatory cytokines; TH1 cells also express CD40L (CD154) on their surface which is another crucial tool required for the activation of other cells, e.g. B lymphocytes, and macrophages. TH1 lymphocytes are important in the control and/or elimination of intracellular microbes such as viruses and intracellular bacteria, fungi and protozoans. Slide 36 Differentiation: Cytotoxic T Cells (CTLs) For the killing of infected & tumor cells: – Generally requires TH1 help in the form of IL-2, IL-12, & IFN-, as well as CD40L for the upregulation of pAPC CD80/CD86 expression Cytotoxic T lymphocyte differentiation. Major histocompatibility complex class I antigen presentation to CD8 T lymphocytes differentiates these into cytotoxic T lymphocytes (CTLs) which in turn express high levels of CD95L (CD178 or FasL), perforin, granzyme, granulolysin, as well as IFN- and other type I immunity cytokines. Cytotoxic T cells are very important in the control and/or elimination of cytosolic intracellular parasites, especially viruses. CD8+ T lymphocytes as a general rule require a higher costimulatory threshold as compared to CD4+ T lymphocytes. Consequently, CD8+ cell activation usually requires the help of TH1 cells. This help comes in the form of CD40L stimulation of the pAPC and IL-2 stimulation of the CD8+ T lymphocyte; CD40L (provided by TH1 cells) engagement of pAPC CD40 leads to increased CD80/CD86 pAPC expression to fully costimulate CD8+ cells and IL-2 helps in the proliferation and differentiation of the lymphocytes. Slide 37 Differentiation: TH2 Cells For extracellular parasites: – TH2 cells require TSLP (thymic stromal lymphopoietin), IL-25 & IL-33 for their differentiation – TH2 cells in turn secrete IL-4, IL-5, IL-10, & IL-13 TH2 cell differentiation. Antigen presentation achieved in the presence of thymic stromal lymphopoietin (TSLP), IL-25, and IL-33 generates TH2 cells which in turn express IL-4, IL-5, and IL-13, as well as CD40L. Interleukin 4 promotes B lymphocyte isotype-switching to IgE and, with IL-13, promotes gastrointestinal tract peristalsis, eosinophil recruitment by inducing chemokine and endothelial adhesion molecule expression at the site of injury, and the alternative activation of macrophages (M2 macrophages). Interleukin 13 stimulates peristalsis and mucus production, whereas IL-5 stimulates bone marrow production of eosinophils (leading to the characteristic eosinophilia seen in worm infections & allergic reactions) and eosinophil activation. TH2 cells are important in the control and/or elimination of large extracellular parasites such as worms, but they are also involved in hypersensitivity reactions (e.g. asthma and type 1 hypersensitivity i.e. allergic reactions). The role of IL-4 in TH2 differentiation is not as clear-cut as is expressed in many reference and review books; for NBME/USMLE exams use IL4, but the reality is edging toward the cytokines mentioned here. Slide 38 Differentiation: TH17 Cells For extracellular microorganisms: IL-6 & TGF-β – TH17 cells require IL-6 & TGF-β for their differentiation – TH17cells in turn secrete IL-17 & Il-22 TH17 cell (or Treg17) differentiation. Antigen presentation achieved in the presence of IL-6 and TGF- yields TH17 cells which in turn express IL-17, IL-21, and IL-22. Antigen presentation achieved in the presence of IL-1β and IL-23 generates TH17 as well, but these are involved in autoimmune and inflammatory diseases and will not be further addressed here. Interleukin 17 is important in the production of neutrophils by the bone marrow where it stimulates the expression of G-CSF, the most important cytokine in the differentiation of neutrophils; IL-17 also promotes the expression of CXCL8 (IL-8) at the site of injury for the purpose of neutrophil recruitment as well as antimicrobial mediators such as defensins. Interleukin 22 also is involved in the stimulation of antimicrobial mediators at the site of infection, as well as increased epithelial barrier functions. Slide 39 Differentiation: TFH Cells For B lymphocyte activation, isotype-switching, affinity maturation, & germinal centre formation: – TFH cells require IL-12 & activin A for their differentiation – TFH cells in turn express CXCR5, ICOS (inducible T cell co-stimulator), PD-1 (Programmed Death 1), BCL-6 (B Cell Lymphoma 6), CD40L and secrete IL-21 as well as other cytokines: • IFN- for IgG isotype-switching • IL-4 for IgE isotype-switching • TGF-β for IgA sotype-switching TFH differentiation. Of note is the manner helper T lymphocytes help activate B lymphocytes. Resting naïve CD4+ T lymphocytes (TH0) are engaged by a pAPC in the T cell zone of secondary lymphoid tissue. Once activated, a fraction of the helper T lymphocytes (TH1, TH2, or TH17, depending on the infection) down-regulate CCR7 and up-regulate CXCR5; consequently, some activated T cells leave the T cell zone and migrate to the follicle, in response to CXCL13 (the ligand for CXCR5) secreted by follicular DCs and stromal cells, where helper T lymphocytes can help with the activation of B lymphocytes. Meanwhile, B lymphocytes in the follicle bind antigen drained by the secondary lymphoid tissue (the Ag is bound in its native conformation by the BCR), endocytose it, and process it through the exogenous pathway of antigen presentation for presentation to helper T cells. The likelihood of B and T cell interaction is increased by the convergence of helper T lymphocytes and B lymphocytes to the edge of the primary follicle. Just as helper T cells modify their chemokine receptor profile to migrate towards the follicle, B lymphocytes that have processed antigen modify their chemokine receptor expression so as to migrate towards the T cell zone; consequently, B cells down-regulate CXCR5 expression and increase CCR7 expression. As a result of this convergence, T and B lymphocytes interact at the edge of the follicle where B cell activation occurs. Restimulation of helper T lymphocytes by B lymphocytes drives B cell activation through CD40L and cytokines provided by helper T cells. This extrafollicular B lymphocyte activation yields an early antibody response with limited isotype-switching and somatic hypermutation; these lowaffinity antibodies then circulate and serve to limit the spread of the infection. Isotype-switching and generation of high-affinity antibodies is done in the germinal centre. Extrafollicular T and B cells then migrate back to the follicle where the T cells become follicular helper T lymphocytes (TFH). These TFH cells will generate germinal centres. Expression of CD40L, ICOS (inducible T cell costimulator; another membrane-bound molecule on the surface of TFH cells), as well as secretion of IL-21 by TFH cells drives germinal centre formation and the generation of long-lived plasma cells with higher rates of somatic hypermutation. Depending on the initial activation of TH0 cells by pAPCs, TFH cells also secrete either IFN-, IL4, IL-21 or other cytokines, to drive isotype-switching that is appropriate for the pathogen and its localization; therefore these TFH cells are further differentiated into TFH1, TFH2 and TFH17 cells. TH17 profile cytokine IL-21 drives isotype-switching towards IgG1, IgG2a, IgG2b, and IgG3. The TH1 profile cytokine IFN- drives IgG1 and IgG3 class-switching. The TH2 profile cytokine IL-4 drives IgE and IgG4 class-switching. Finally, in mucosa, TGF- and other signals induce IgA class-switching. The resulting high-affinity antibodies are what most likely help in the clearance of the infection, especially when dealing with extracellular bacteria, fungi, protozoans, and helminths; high-affinity antibodies are also useful in fighting viruses by the generation neutralizing antibodies, and antibodies involved in antibody-dependent cell cytotoxicity (ADCC). Slide 40 Differentiation: Treg Cells For the regulation of immune responses: – Treg cells require TGF-β for their differentiation – Treg cells in turn express high levels of CD25 (IL-2R α-chain), immune checkpoint receptors and ligands such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 and its ligand (PD-1 & PD-1L), as well as secrete IL-10 and TGF-β Treg differentiation. Antigen presentation achieved in the presence of TGF- generates Treg cells which in turn express TGF- and IL-10, as well as high levels of CD25 (IL-2R α-chain) and immune checkpoint receptors and ligands such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 and its ligand (PD-1 & PD-1L). Treg cells are important in peripheral tolerance and the termination of immune responses. Slide 41 TASK • What is a TH1 response better suited for? • What is a TH17 response better suited for? • What is a TH2 response better suited for? • What is the purpose of TFH cells? • What is the purpose of TREG cells? Slide 42 Regulation of T Lymphocyte Response Slide 43 Control of the Immune System Important concepts to remember: Normally, to avoid tissue damage and chronic inflammation, immune reactions are inherently short-lived: Examples: Once activated, the life-span of an effector T lymphocyte (e. g. TH cell) is dramatically shortened unless it is continuously being re-stimulated by antigen (i. e. activated effector T cells are reprogrammed to carry out their functions and then undergo apoptosis after only a few days); Cytokines have a rapid turnover; furthermore, cytokine and cytokine receptor mRNAs are unstable and have a very short half-life – this enables the immune system to react rapidly to changes in the response. So how are immune reactions sustained when needed? By continual antigen stimulation: as long as the antigen persists, T cells are given survival signals and continue performing their functions, such as cytokine production (a great clinical correlation is strepthroat!) This reasoning, as well as that developed in the next slides, are paramount in understanding how immune responses are sustained and turned off… Slide 44 Control of the Immune System  Antigen if  Inflammation more  Inflammation foreign proinflammatory  CD80/CD86 (B7) expression costim molecules cytokines As long as there is enough Ag, there is inflammation and, as long as there is inflammation, CD80/86 expression keeps being expressed… Slide 45 Control of the Immune System AFFINITY versus AVIDITY As far as we are concerned: Affinity: how tight is the interaction… Avidity: how much binding there is... (thought of in terms of valence) Slide 46 Termination/Regulation of the Immune Response CD28 (activation signal) and CTLA-4 (inhibition signal) both expressed on activated T lymphocytes The affinity of CTLA-4 for CD80/CD86 is much greater than that of CD28 Under inflammatory conditions, CD80/CD86 avidity is increased to offset CTLA-4 engagement, i. e. the higher expression of CD80/CD86 under inflammatory conditions increases the likelyhood of CD28 engagement to maintain co-stimulation Important for Pharmacology!!! Control of T cell responses: Cell intrinsic inhibitory signaling: Activated T cells express both CD28 (whose engagement yields activation) and CTLA-4 (whose engagement leads to termination of T cell activation). Under inflammatory conditions (so as long as there is enough Ag present), enough CD80/86 expression is present to counteract the effect of CTLA-4 (i. e. the likelihood of CD28 engagement is greater – concept of avidity compensating for the greater affinity of CTLA-4 for CD80/86 – and T cell activation is maintained). As Ag dwindles, the greater affinity of CTLA-4 for CD80/86 translates into inhibition of T cell activation and immune responses are terminated. Blockage or removal of CD80/86 access to T lymphocytes by Treg cells: Regulatory T lymphocytes (Treg), which express high levels of CTLA-4, can "mop up", so to speak, pAPC CD80/86 to leave little CD80/86 to interact with T cell CD28 (render most pAPC CD80/86 unavailable for T cell CD28 engagement – i. e. reducing the CD80/86 valence hence the avidity of CD80/86…). Slide 47 Termination/Regulation of the Immune Response Similar effect for PD1/PD1-L and PD2/PD2-L (important for Pharmacology!) Activated T lymphocytes also express other types of inhibitory receptors, PD-1 and PD-2, whose ligands are PD-L1 and PD-L2 respectively; PD-1/PD-L1 is particularly important in therapeutics. Engagement of PD-1 by PD-L1 leads to the termination of T cell activation in a manner that is similar as that seen for CTLA-4. Slide 48 When T cell activation is incomplete… Slide 49 Activation in absence of co-stimulation… Peripheral tolerance When a naïve T cell binds to peptide:MHC on a cell that does not express CD80/86, the cell becomes non-responsive to antigen (anergic) and cannot subsequently become activated; these usually eventually die from apoptosis or become Treg cells (depending on context).