3. 1 Antigen Presentation And T Lymphocyte Biology.pdf

Slide 50 T Cell Biology Slide 51 The life and functions of T lymphocytes Intro videos: https://www.youtube.com/watch?v=9E_UxnC_L2o https://ca.video.search.yahoo.com/search/video?fr=mcafee&p=youtube+immunology+to ronto#id=5&vid=3aaa27ffdff7df4cddfd237a3b2e367f&action=click Slide 52 Antigen Pre...

Slide 50 T Cell Biology Slide 51 The life and functions of T lymphocytes Intro videos: https://www.youtube.com/watch?v=9E_UxnC_L2o https://ca.video.search.yahoo.com/search/video?fr=mcafee&p=youtube+immunology+to ronto#id=5&vid=3aaa27ffdff7df4cddfd237a3b2e367f&action=click Slide 52 Antigen Presentation & Timeline: IMPORTANT to understand where and when Ag presentation occurs, and why! Process Site Antigen presentation for the purpose of T lymphocyte development (Positive and negative selection) Antigen presentation to double-positive thymocytes by thymic cortical epithelial cells (positive selection) and bone marrow derived macrophages and dnedritic cells (negative selection – i. e. central tolerance) Thymus - Development (Primary lymphoid organ) Requires co-stimulatory signals (which you need not know…) Antigen presentation for the purpose of T lymphocyte activation (Response to infection) Antigen presentation to naïve T lymphocytes by professional antigen presenting cells (mostly dendritic cells & macrophages) Lymph nodes, spleen - Activation (Secondary lymphoid organs) Requires co-stimulatory signals (CD80/CD86) Antigen presentation for the purpose of effector responses (B lymphocyte help, macrophage activation, etc.) Antigen presentation to T helper (TH) lymphocytes (activated T cells) at site of infection so that TH cells can deliver further signals to highten the effector responses of the cells presenting antigen (killing of microbes, antibody production, etc.) Antigen presentation for the purpose of reactivation of memory T lymphocytes Antigen presentation to memory T lymphocytes upon re-exposure to infection to activate the anamnestic response Site of infection & secondary follicules - Help (Affected tissue) We will not go into further detail… No requirement for co-stimulation It is important to note that Ag presentation is a part of several stages of the T cell’s life cycle, and that at each stage, Ag presentation serves a different function (and these are important when considering certain diseases...): • Ag presentation to select developing thymocytes in the thymus; • Ag presentation for activation of mature, naïve T lymphocytes in secondary lymphoid tissue for the purpose of driving effector T cell responses (TH, TFH, Treg, & CTLs); • Ag presentation by B lymphocytes in secondary lymphoid tissue for the purpose of seeking T cell help for further development into plasma cells capable of isotype-switching and affinity maturation, as well as memory B cell development; • Ag presentation by macrophages in tissue (e. g. sites of infection) for the purpose of seeking T cell help for further activation and enhanced microbicidal activity; • Ag presentation for the purpose of eliciting T cell memory responses upon re-exposure to the same Ag. Slide 53 Effector T cells and their functions Slide 54 Effector T cells • Change gene expression profiles and thereby gain the capacity to express proteins they will need to perform their effector functions – CD4 T cells (T helper cells, T regulatory cells) • Secrete cytokines (e. g. IL-4 & IFN-) • Express surface molecules (e. g. CD40L & FasL) – CD8 T cells (Cytotoxic T Lymphocytes) • Secrete cytokines (e. g. IFN-) • Express surface molecules (FasL) • Release cytotoxins – proteins that are used to kill infected cells During differentiation, activated T cells acquire the capacity to express the proteins they will need to perform their effector functions. For CD4 T cells, these proteins are generally cytokines (e.g. IFN-, IL-4, IL-17, IL-21, IL-22 etc.) and surface molecules (e.g. CD40 ligand, or CD40L, & Fas ligand, or FasL). For CD8 T lymphocytes, these usually take the form of cytokines (e.g. IFN-), cytotoxins (e.g. perforin, granulysin, granzyme etc.), and surface molecules (e.g. CD95L or FasL). Slide 55 Effector T cells During differentiation, activated T cells acquire the capacity to express the proteins they will need to perform their effector functions. For CD4 T cells, these proteins are generally cytokines (e.g. IFN-, IL-4, IL-17, IL-21, IL-22 etc.) and surface molecules (e.g. CD40 ligand, or CD40L, & Fas ligand, or FasL). For CD8 T lymphocytes, these usually take the form of cytokines (e.g. IFN-), cytotoxins (e.g. perforin, granulysin, granzyme etc.), and surface molecules (e.g. CD95L or FasL). Slide 56 TH1 responses Slide 57 1. Killing of cells infected with microorganisms residing in the cytosol, e.g. viruses, and tumor cells Slide 58 Cytotoxic T cell killing mechanisms J Leuk Biol (2001) 70:18-29 Target cells include: • Infected cells • Tumor cells Cytosolic pathogens. These pathogens are better handled by a TH1 response that elicits predominantly a CTL response. For this case, let us use the example of viruses since all viral infections are primarily controlled by CTLs. But some bacteria, fungi and protozoans can also primarily reside in the cytosol and thus would be handled the same way as viruses. Because of their cellular localization, these pathogens are generally protected from complement and antibody while inside cells. Control of these pathogens mainly requires the death of the pathogen-harboring cell. Intracellular pathogens produce antigen that is readily processed by the MHC-I-restricted antigen presentation pathway. Because CD8+ T lymphocytes require a higher threshold of CD86 costimulation than CD4+ T lymphocytes, CD4+ T lymphocyte help (TH1) is more often than not required for CD8+ T lymphocyte activation in response to such pathogens: TH1 cell interaction with an pAPC give it the nudge it needs to increase the CD86 costimulatory molecule expression required for effective CD8+ T lymphocyte activation. CD4+ T lymphocyte help comes in the form of CD40L and IL-2. Interleukins 12 and 18, as well as IFN-, also increase the efficiency of CTLs. Once activated, most clonally-expanded CTLs leave the secondary lymphoid tissue (e.g. lymph node) to search for and destroy infected cells in affected tissues, but some also become memory CD8+ T cells and leave the secondary lymphoid tissue to set up residence in other tissues until they are called upon in case of an infection with the same virus. The clonal expansion of TH1 cells and CTLs are what leads to the lymphocytosis usually observed in viral infections. Upon recognition of an infected cell in the affected tissue, the CTL will kill the cell and move on to kill another infected cell. Since CTLs are already activated, the infected cells need only to display MHC-I/viral peptide complexes (& adhesion molecules) to CTLs for their destruction. The death of the infected cell is by apoptosis, and this death is mediated by CD95L binding to CD95 on the infected cell or by release of perforin to create pores in infected cellular membranes, and granzymes (serine proteases) and granulolysins which enter the infected cell’s cytosol and induce apoptotic pathways. Because apoptosis is a ‘clean’ death (i.e. there is little release of DAMPs as opposed to necrotic death), there is little pus formation in these infections which are rather characterized by clear discharges, clear vesicular fluids etc. In some cases, noncytotoxic CD8 T lymphocytes also arise from MHC class I-restricted activation, and these are important in the control of some viral diseases (e.g. viral hepatitides). These noncytotoxic CD8 T lymphocytes exert their action mainly through IFN- secretion at the CTL/infected cell interface and function to clear viruses from infected cells without killing them (e.g. IFN--mediated APOBEC expression by the infected cell generates deleterious mutations in viral genomes that lead to viral genome degradation; this phenomenon is referred to as viral restriction). We further develop viral restriction in the Viral Pathogenesis & Immunity activity. Finally, a substantial number of TH1 cells are set aside for immunological memory. Slide 59 2. Killing of cells infected with microorganisms residing in vacuoles Slide 60 TH1 cells and macrophage activation Vacuolar pathogens. Some intracellular pathogens other than viruses (e.g. intracellular bacteria) primarily reside in host cell vacuolar compartments such as endosomes. Just like cytosolic pathogens, vacuolar pathogens are generally shielded from the effects of complement and antibody. Vacuolar pathogens are also controlled by TH1 cells, but without the intervention of CTLs (as previously mentioned, their vacuolar localization essentially precludes MHC-I-restricted antigen presentation). Therefore, the TH1 response to vacuolar pathogens is geared toward TH1 help for the activation of the infected cell; TH1 help mainly comes in the form of IFN- and CD40L. Incidentally, many intracellular organisms actually infect macrophages. Infected macrophages process pathogen protein via the MHC-II-restricted antigen presentation pathway and present pathogen antigen to TH1 cells that have infiltrated the affected tissue. Upon antigen presentation, TH1 cells provide CD40L and IFN- to the macrophage which in turn activates the macrophage (M1) and increases its microbicidal functions (enhanced lysosomal enzyme production, induction of NADPH oxidase (respiratory burst), sequestration of iron & tryptophan, & production of nitric oxide etc.). Finally, a substantial number of TH1 cells are set aside for immunological memory. Slide 61 First question: Is the pathogen mainly found in the cytosol, or is it mainly found in vacuoles? If mainly cytosolic, then CD8 T cell adaptive response is the main response (same as for viruses); If mainly vacuolar, then IFN- secreted by TH1 cells is the main response. Second question: Is the infected cell capable of microbicidal activity if stimulated to do so, or does the pathogen need to be killed by another cell? If capable, then IFN- released by TH1 cells can activate the infected cell’s microbicidal mechanisms; If not capable, then IFN- released by TH1 cells can activate macrophages to produce nitric oxide (NO) which is released in the vicinity of the infected cell; NO, which can diffuse across membranes, reacts with superoxide (O2-) – which all cells can produce as a result of mitochondrial physiology – to yield peroxynitrite (ONOO-) which is toxic to most microbes and metazoan parasites. Slide 62 Cytosolic Examples: Rickettsia rickettsii Listeria monocytogenes Innate Nitric oxide (iNOS) production by macrophages that are activated by NK cell-derived IFN-. Adaptive Cytotoxic CD8 lymphocytes: 1) MHC-I recognition followed by perforin/granzyme or FasL-mediated killing (requirement for CD8 cell activation by TH1 cells); 2) iNOS (nitric oxide production) and other intracellular killing mechanisms in cells capable of microbicidal activity (such as TH1activated (IFN-) macrophages & endothelial cells); 3) Nitric oxide release by TH1-activated (IFN-) macrophages; nitric oxide can diffuse into neighboring infected cells to kill the microorganisms inside. Slide 63 Vacuolar Examples: Mycobacterium tuberculosis* Legionella pneumophila *M. tuberculosis is very resistant to killing and induce granuloma formation. Innate Nitric oxide (iNOS) production by macrophages that are activated by NK cell-derived IFN-. Adaptive TH1 cells: IFN- released by these cells induce: 1) iNOS (nitric oxide production) and other intracellular killing mechanisms in cells capable of microbicidal activity (such as macrophages & endothelial cells); 2) Nitric oxide release by IFN--activated macrophages (iNOS); nitric oxide can diffuse into neighboring infected cells to kill the microorganisms inside. Slide 64 3. When all else fails… wall off the focus of infection… Slide 65 TH1 cells granuloma formation Finally, as you have learned in Pathology, when TH1 cells and macrophages fail to clear the infection with particularly resistant vacuolar microorganisms, then these cells switch gears and work to isolate the foci of infection by creating granulomas. Slide 66 B lymphocyte activation (all pathogens) Slide 67 Help for B cell activation (link with B Cell Biology lecture…) Intro video in your book: https://studentconsult.inkling.com/read/abbas-basic-immunology-5th/videos/sequence-of-events-in-helper-t 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) 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. Re-stimulation 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 low-affinity antibodies then circulate and serve to limit the spread of the infection. Generation of high-affinity antibodies is done in the germinal center. 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 centers. Expression of CD40L, ICOS (inducible costimulatory, another membrane-bound molecule on the surface of TFH cells), as well as secretion of IL-21 by TFH cells drives germinal center 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-, IL-4, IL-17, 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. The resulting highaffinity 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 68 Isotype-switching (link with B Cell Biology lecture…) Ligand IL-21 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-, IL-4, 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 69 TH17 response: Killing of extracellular microorganisms Slide 70 TH17 cells: for extracellular bacteria and fungi TH17 response. Extracellular microorganisms are better controlled by neutrophils and antibodies. When activated in an IL-6 and TGF- rich environment, TH0 cells differentiate into TH17 cells, many of which migrate to the periphery; TH17 cells secrete IL-2, IL-17, IL-21, and IL-22, among others, to help orchestrate the immune response to extracellular microorganisms. T helper 17 lymphocytes express immune checkpoint molecules (CTLA-4 & PD-1) involved in the control of the immune response as well. In the periphery, infiltrating TH17 cells secrete IL-17 and IL-22 to stimulate fibroblasts and epithelial cells to in turn secrete CXCL8 and CXCL2 to recruit newly formed neutrophils to the site of infection. Interleukin 22 also stimulates the release of defensins and increase barrier protection at the site of infection. In the bone marrow, TH17 cells stimulate stromal cells to secrete G-CSF and GM-CSF to stimulate neutrophil production (hence the sustained neutrophilia observed in bacterial & fungal infections) as well as monocyte production. As seen for the TH1 response, the TH17 response also stimulates antibody production by B lymphocytes (mediated by TFH cells & secondary follicle formation as seen for the TH1 response). Immunoglobulin serve to (1) opsonize pathogens for phagocytosis by neutrophils mainly, and (2) activate the classical pathway of complement thereby destroying these by lysis through the membrane attack complex. TFH cells mainly promote IgG isotype-switching, as this is the better isotype for promoting both phagocytosis and the classical pathway of complement activation. Again, as for any adaptive immune response, a substantial number of TH17 cells are set aside for immunological memory. Slide 71 TASK • What cytokines are involved in TH17 differentiation? • What cytokines are secreted by TH17 cells? Slide 72 Killing of helminths (worms) and other large extracellular parasites Slide 73 TH2 cells: for worms TH2 response. Worms and other large extracellular parasites are controlled by TH2 responses; TH2 responses are also the primary mechanisms involved in the pathophysiology of asthma and type I hypersensitivity (allergic reactions). T helper 2 cells secrete IL-4, IL-5, IL-10, and IL-13; they also express high levels of CD40L as well as immune checkpoint molecules (CTLA-4 & PD-1). T helper 2 cells are primarily responsible for (1) eosinophil production by the bone marrow, their (2) activation in the periphery, and (3) stimulating peristalsis and mucus production; TH2-derived TFH cells are also responsible for IgE isotype-switching. Interleukin 4 and IL-13 (1) promote B lymphocyte isotype-switching to IgE (mediated by TFH cells derived from TH2 cells & germinal centre formation as seen for TH1 and TH17 responses) and (2) gastrointestinal tract peristalsis (mediated by TH2 cells infiltrating the affected tissue), (3) eosinophil recruitment by inducing chemokine and endothelial adhesion molecule expression at the site of injury (TH2 tissue infiltration), and (4) the alternative activation of macrophages (M2 macrophages; again by tissue infiltrating TH2 cells). Interleukin 5 stimulates bone marrow production of eosinophils (leading to the characteristic eosinophilia seen in worm infections & allergic reactions) and eosinophil activation at the site of injury. Finally, as for the TH1 and TH17 responses, a substantial number of TH2 cells are set aside for immunological memory. Slide 74 Treg cell functions Slide 75 Treg cells: Functions Tools: TGF- IL-10 CTLA-4 PD-1 CD25 Treg cells serve to: • promote self-tolerance; • tolerance to innocuous Ag; • prevent T cell responses; • terminate T cell responses; • inhibit other leukocyte functions. Slide 76 TREG cells: Mechanism – CTLA-4 Cell-intrinsic suppression: Activated T cells express CTLA-4 which has greater affinity for B7 than CD28; when B7 expression diminishes as inflammation decreases with disappearance of Ag, the likelihood of CTLA-4 engagement is greater and the response is ended by killing or inactivating the effector T cell. Cell extrinsic suppression: Treg cells can also ‘soak up’ B7 on the surface of pAPCs leaving little pAPC co-stimulation potential. 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 77 Fig. 1. Mechanisms of action of the PD-L1 and PD-1 pathway. Such as tumor cells & Treg cells… Modified from Weiping Zou et al., Sci Transl Med 2016;8:328rv4 Published by AAAS Mechanisms of action of the PD-L1 and PD-1 pathway. Cells that express high levels of PD-L1 may include tumor cells, APCs (DCs, macrophages, MDSCs, and B cells), T lymphocytes, epithelial cells, fibroblasts, and others. Engagement of PD-L1 by PD-L1+ cells induces T cell apoptosis, anergy, functional exhaustion, or IL-10 production. Slide 78 Quick assessment of what I know so far… • What are the : T cell subsets, their functions, the cytokines required for their differentiation during activation, as well as the effector cytokines they secrete and the ligands they express? In other words… • What cytokines are required for TH1 differentiation? What cytokines do TH1 cells secrete? What ligands do TH1 cells express? What are TH1 cells useful for? How do TH1 cytokines exert their functions? • How do TH1 cells provide help for… – CD8 activation? – Macrophage activation? • How do CTLs kill infected/transformed cells? • What cytokines are required for TH2 differentiation? What cytokines do TH2 cells secrete? What ligands do TH2 cells express? What are TH2 cells useful for? How do TH2cytokines exert their functions? • What cytokines are required for TH17 differentiation? What cytokines do TH17 cells secrete? What are TH17cells useful for? How do TH17 cytokines exert their functions? • What cytokines are required for TFH differentiation? What cytokines do TFH cells secrete? What ligands do TFH cells express? What are TFH cells useful for? How do TFH cytokines exert their functions? • What cytokines are required for TREG differentiation? What cytokines do TREG cells secrete? What ligands do TREG cells express? What are TREG cells useful for? How do TREG cytokines exert their functions? • Can you explain T cell suppression by CTLA-4 and PD-L1?