4. 1 Antigen Presentation And T Lymphocyte Biology.pdf

Slide 79 Memory T lymphocytes Learning objectives: 17. Describe the importance of memory T cells for long-term immunity; describe the phenotype of memory T lymphocytes. Slide 80 Memory T lymphocytes See the next slide for memory T lymphocyte explanation… Slide 81 Memory T lymphocytes The mem...

Slide 79 Memory T lymphocytes Learning objectives: 17. Describe the importance of memory T cells for long-term immunity; describe the phenotype of memory T lymphocytes. Slide 80 Memory T lymphocytes See the next slide for memory T lymphocyte explanation… Slide 81 Memory T lymphocytes The memory response is not only produced by circulating antibodies, but also by long-lived memory B cells and memory T cells. These memory cells are generated during the primary immune response against the pathogen from a subset of effector cells. While most effector cells are short-lived, these memory cells survive throughout the course of the primary immune response; once the infection is resolved, memory cells proliferate and go on to survive for long periods of time – some throughout the life of the individual in the case of memory T lymphocytes. After the primary adaptive immune response, pathogen-specific memory cells outnumber their naïve counterparts by several orders of magnitude. Memory cells have the capacity to respond to specific antigen faster and better than naïve cells. While the molecular and cellular interactions necessary for activation of memory cells are similar to naïve cells, memory cells are more sensitive to infection, more easily activated, and more abundant (10 – 1,000 fold) than primary naïve lymphocytes specific for the same antigen. For example, memory B cells, in addition to being more numerous than their naïve counterparts, have already undergone isotype switching and somatic hypermutation and, as a consequence, produce antibodies with (1) higher affinity (affinity maturation) and (2) the appropriate isotype for the pathogen, as compared to antibody produced during the primary response. Also, upon subsequent infection, memory cells activation inhibits the activation of their naïve counterparts so that responses are skewed towards the activation of the most efficient cells. As previously mentioned, memory T cells specific for a particular pathogen are present in much higher numbers than their naïve counterparts and therefore a greater expansion of specific T cells can occur during the secondary immune response. Additionally, memory T cells express a different profile of cell-surface molecules that allow them to perform their function and enter the peripheral tissues faster. Unfortunately, the processes involved in memory T lymphocyte development are poorly understood. The best characterized T cell memory development is that of CD8 T lymphocytes. Memory CD8 T lymphocyte development relies on CD4 T lymphocyte help and IL-2 stimulation since CD8 T lymphocytes that do not express CD40 cannot become memory T cells whereas those that express CD40 can. Likewise, CD8 T lymphocytes that do not express the -subunit of the IL-2 receptor cannot become memory T cells whereas those that express it can. A marker of memory T cells is CD45RO (as opposed to CD45RA on naïve T cells). Slide 82 Immunosenescence Slide 83 Immunosenescence Thymus development (embryonic), fully developed before birth, begins degenerating one year after birth (replaced by fat) – progresses steadily through life (thymic involution) – reduced production of new T lymphocytes does not impair T cell immunity (neither does thymectomy in adults) because T lymphocytes are longed-lived, self-renewing, and because of memory T cell expansion upon antigen exposure – by adulthood, individuals possess memory T lymphocytes to the most common potentially life-threatening pathogens (childhood infections). Notes are on the slide itself… Slide 84 Sites of adaptive immune activation Slide 85 Sites of adaptive immune activation Adaptive immune reactions for most tissues occur in lymph nodes. For blood infections, the primary site of adaptive immune response is the spleen. And for mucosal infections, mucosaassociated lymphoid tissues (MALTs) are where adaptive immune reactions are primarily held. Slide 86 Through Skin  Lymph nodes Adaptive immune reactions for most tissues occur in lymph nodes. Slide 87 Through BloodSpleen For blood infections, the primary site of adaptive immune response is the spleen. Slide 88 Through Mucosa  MucosaAssociated Lymphoid Tissue and Lymph Nodes And for mucosal infections, mucosa-associated lymphoid tissues (MALTs) are where adaptive immune reactions are primarily held. Slide 89 Other T lymphocyte subsets Slide 90 Innate Lymphoid Cells (ILCs) 3 types – innate counterparts of TH1, TH2, & TH17 cells • ILC1 (intracellular infections): secrete IFN- • ILC2 (helminths): secrete IL-5 & IL-13 • ILC3 (extracellular microorganisms): secrete IL-17 & IL-22 Innate lymphoid cells. There exist 3 types of ILCs: ILC1, ILC2, and ILC3. Innate lymphoid cells are the innate counterparts of the TH1, TH2, and TH17 cells, they reside in epithelial barrier tissue, they mainly secrete cytokines, and they mainly provide cytokine signaling help during innate immune responses. Innate lymphoid cells 1 mainly secrete IFN-, ILC2s IL-5 and IL-13, whereas ILC3s mainly produce IL-17 and IL-22. Innate lymphoid cells are activated by cytokines (alarmins). ILC1s are involved in innate immune responses to intracellular pathogens like viruses. Because ILC1s, TH1 lymphocytes and CD8+ T lymphocytes respond to the same types of pathogens, they are often grouped under the heading of type I immunity. ILC2s are involved in innate immune responses to large extracellular pathogens like worms. Similarly, because ILC2s and TH2 lymphocytes respond to the same types of pathogens, they are often grouped under the heading of type II immunity. ILC3s are involved in innate immune responses to microscopic extracellular pathogens like bacteria and fungi, and they strongly contribute to the maintenance of epithelial barrier integrity. And again, because ILC3s and TH17 lymphocytes respond to the same types of pathogens, they are often grouped under the heading of type III immunity. Slide 91 Natural Killer cells (NK cells) (Seen in Innate Immunity lecture…) Natural Killer cells (NK cells): T lymphocytes that do not express a TCR 1- Pathogen-derived ligands: • • Viral proteins Heparan sulfate proteoglycans 2- MHC-I-like molecules (or altered-self) ligands: • • • MICA MICB (HLA-E) NK cells. These cells respond to MHC-I-like proteins (refer to the MHC & Antigen Presentation handout). Such proteins include HLA-E (human leukocyte antigen E), MICA (MHC-I chainrelated gene A) and MICB (MHC-I chain-related gene B), which are induced by cellular stress to signal cytotoxic cells that stressed cells need to be eliminated (MICA & MICB: modified-self binding & activating the killer activating receptor receptor or KAR) or not (HLA-E: modified-self binding the KAR receptor but inhibiting the killing signal). For example, binding of NKG2D homodimers, a KAR (killer activating receptor) of NK cells, to MICA or MICB on the surface of a stressed or infected cell, leads to the destruction of the cell by NK cells, provided that the stressed cell expresses little or no MHC-I. This aspect of NK cell biology has been dealt with in the Innate Immunity lecture and will no longer be developed here. Another important function of NK cells is the secretion of IFN-; along with ILC1s, NK cells are the primary IFN- producers during innate immune responses. We have seen that, during acute inflammation, macrophages release IL-12 which then activates NK cells; IL-12 stimulation of NK cells results in the secretion of IFN- which in turn activate macrophages to become more microbicidal. NKG2-mediated recognition of infected or transformed cells A special case of antigen presentation is that of the NKG2 family of receptors found on the surface of NK cells and IELs. NKG2-mediated cell killing often relies on binding to non-classical MHCI molecules that are usually only expressed on stressed, infected or transformed cells; such nonclassical MHC-I molecules include MICA and MICB. For example, NKG2D homodimers, one of the most common KARs present on the surface of NK cells, bind MICA or MICB; binding of MICA or MICB to NKG2D homodimers signals the NK cell to kill the cell it has docked to. This works because, usually, cells that express non-classical MHC-I proteins down-regulate their expression of MHC-I to minimize KIR signaling. Other KARs on the surface of NK cells include NKG2A/NKG2C heterodimers as well as CD94/NKG2A, CD94/NKG2B, CD94/NKG2C heterodimers, and these all bind the non-classical MHC-I HLA-E. So these are all cases where NK cell KARs bind what are referred to as modified-self proteins (i.e. MICA, MICB & HLA-E are considered modified-self proteins). Lastly, another example of KAR, but which only recognizes pathogen determinants (PAMPs, not modified-self proteins), is NKp46; this KAR binds viral protein and heparan sulfate proteoglycans embedded in cells. Finally, as part of the adaptive immune response, NK cells participate in antibody-dependent cellcytotoxicity (ADCC; syn. antibody-mediated cell-cytotoxicity AMCC), as you have seen in the Innate Immunity lecture. Slide 92 Natural Killer cells (NK cells) (Seen in Innate Immunity lecture…) Natural Killer cells (NK cells): innate source of IFN- Another important function of NK cells is the secretion of IFN-; along with ILC1s, NK cells are the primary IFN- producers during innate immune responses. We have seen that, during acute inflammation, macrophages release IL-12 which then activates NK cells; IL-12 stimulation of NK cells results in the secretion of IFN- which in turn activate macrophages to become more microbicidal. Slide 93 Natural Killer cells (NK cells) (Seen in Innate Immunity lecture…) Natural Killer cells (NK cells): antibody-mediated cell cytotoxicity (AMCC) or antibody-dependent cell cytotoxicity (ADCC) Finally, as part of the adaptive immune response, NK cells participate in antibody-dependent cellcytotoxicity (ADCC; syn. antibody-mediated cell-cytotoxicity AMCC), as you have seen in the Innate Immunity lecture. Slide 94 Natural Killer T lymphocytes (NKT cells) Natural Killer T lymphocytes (NKT cells): • React to glycolipid Ag presented by MHC-I-like molecules (CD1a, CD1b, CD1c, CD1d & CD1e), which are structurally similar to MHC-I molecules, associate with 2m, but have deeper binding grooves to accommodate hydrocarbon chains of glycolipids (hydrophobic groove binds alkyl chains to allow exposure of variable carbohydrate head and other hydrophilic groups to TCR) • Secrete cytokines (e. g. IL-4 or IFN-) in a context-dependent manner; some subsets can be cytotoxic • Are important in the innate control of some infections and help direct adaptive immune responses; e. g. recognition of mycobacteria Another group of MHC-I-like molecules (CD1a, CD1b, CD1c, CD1d & CD1e) present glycolipid antigen to NKT cells. Natural killer T cells share properties common to both T lymphocytes (CD3+ & : TCR, but that can either be CD4+CD8-, CD4-CD8- or CD4-CD8+) and NK cells (NK1.1). Glycolipid antigen presentation in this manner mainly leads NKT cells to secrete cytokines (e. g. IL-4 or IFN-) in a context-dependent manner, i. e. depending on the pathogen; some NKT cell subsets can also be cytotoxic and destroy infected cells. PROPERTIES OF NKT CELLS: Are different from NK cells and T lymphocytes; Are generally considered innate-like lymphocytes; Are a diverse set of T lymphocytes that express both T lymphocyte markers (CD3+, CD4+CD8-, CD4-CD8-, CD4-CD8+, : TCR) and NK cell markers (NK1.1+); Two groups of CD1: Group 1: CD1a, CD1b and CD1c: bind glycolipids, phospholipids and lipopeptides derived from microbes, such as mycolic acid, lipoarabinomannan, glucose monomycolate, phosphoinositol mannosides and isoprenoid glycolipids of mycobacteria; Group 2: CD1d: thought to mainly bind self-lipid Ag such as sphingolipids and diacylglycerols CD1e is considered an intermediate between groups 1 and 2; React to glycolipid Ag presented by MHC-I-like molecules (CD1a, CD1b, CD1c, CD1d & CD1e), which are structurally similar to MHC-I molecules, associate with 2m, but have deeper binding grooves to accommodate hydrocarbon chains of glycolipids (hydrophobic groove binds alkyl chains to allow exposure of variable carbohydrate head and other hydrophilic groups to TCR); Secrete cytokines (e. g. IL-4, IFN-) in a context-dependent manner; some subsets can be cytotoxic; Are important in the innate control of some infections and help direct adaptive immune responses; e. g. recognition of mycobacteria. Slide 95 Intraepithelial T lymphocytes Intraepithelial T lymphocytes (IEL): – Mostly CD8 : T cells Intraepithelial cells Intraepithelial cells fall into two classes: Type a IELs and Type b IELs. Type a IELs are IELs composed of T lymphocytes that follow the conventional development of T lymphocytes in the thymus (i. e. they go through both positive and negative selection), and the vast majority of them are CD8+ T lymphocytes. These possess a conventional :TCR, as well as a conventional : heterodimer CD8 co-receptor. They are activated in MALTs and the skin. Since these cells possess a conventional :TCR and a conventional : heterodimer CD8 co-receptor, they kill cells in an MHC-I-restricted manner the way CTLs do. Also, since they undergo negative selection in the thymus, few self-reactive IELs leave the thymus. Type a IELs differ from CTLs in that they express high levels of NKG2D homodimers (also found on the surface of NK cells), a receptor that activates killing by binding to non-classical MHC-I molecules (e. g. MICA & MICB). Type b IELs, on the other hand, are unconventional CD8+ T lymphocytes in that they possess an :CD8 homodimer co-receptor instead of the conventional : heterodimer CD8 co-receptor. This :CD8 homodimer co-receptor is paired with either a conventional :TCR or a :TCR. However, Type b IEL :TCRs or :TCRs do not bind conventional peptide:MHC-I ligands; instead, they bind non-classical MHC-I molecules and other ligands (e. g. PAMPs). Since these cells do not interact with conventional peptide:MHC-I ligands and they possess an unconventional :CD8 homodimer co-receptor, there is little potential for autoimmunity; unconventional :CD8 homodimer co-receptors have very low affinity for conventional peptide:MHC-I ligands. Type b IELs, like Type a IELs, possess high levels of NKG2D homodimers which can also activate killing. The development of Type b IELs is poorly understood. Unconventional CD8+ T lymphocytes may arise from late DN/early DP thymocytes that partially undergo positive selection in the thymus, but escape negative selection due to the low-affinity :CD8 homodimer co-receptor. They then exit the thymus to finalize their maturation in mucosal epithelia or the skin, a process that seems to also require IL-15. Slide 96 Quick assessment of what I know so far… • How do NK cells identify the cells they need to kill? • How do NK cells kill their target? • What are intraepithelial T lymphocytes, and how do they identify the cells they need to kill? • What are NKT lymphocytes, and to which antigen/antigenbearing molecule do they respond to? Slide 97 Superantigens Slide 98 Superantigen Activation Non-specific activation of CD4 T lymphocytes leads to an excess secretion of proinflammatory cytokines that affect vascular permeability Depletion of CD4 T lymphocyte population leading to immunosuppression * Most often involves CD28 also Superan gen ac va on Superantigens (superAg) are molecules that bind both the class II MHC molecule and the TCR (cross-link the class II MHC and TCR) in such a way that CD4+ T lymphocytes are activated independently from the presence or absence of a peptide in the peptide-binding groove: the superAg is therefore not a result of Ag processing by the pAPC, but cross-links MHC-II/TCR from the outside of the cells if you like. The cross-linking occurs outside of the peptide-binding cleft and requires CD28 dimerization and cross-linking with the superAg/MHC-II/TCR complex. It is therefore a non-specific, polyclonal, activation of CD4+ lymphocytes (up to 20% of the total T cells!). Superantigen activation can therefore lead to a cytokine storm which can be associated with very serious, even life-threatening, consequences. Depletion of large numbers of activated T cells following superAg activation also leads to a state of immunosuppression (since activated T cells are short lived and die rapidly by apoptosis, a built-in mechanism designed to avoid chronic inflammation, another problem in and of itself…). Typical examples of superAg are the staphylococcal and streptococcal toxic shock syndrome toxins and staphylococcal enterotoxins (SEs), but other organisms such as viruses and fungi can produce superAg. Some superAg can also cross-link class I MHC and the TCR of CD8 T lymphocytes, with similar consequences. Slide 99 Quick assessment of what I know so far… • What is the structure of the TCR? • What are the three signals required for T lymphocyte activation? • What are the individual functions of these three signals? • What are these signals made up of? • What are their respective receptors? • Which cells are providing these activation signals? • Which cells are the intended target of these signals? • Through what intracellular communication systems (signaling pathways) are these signals transmitted? Do I remember the ones I saw in the Cell Signaling lectures? Do I need to know the details of the relevant signaling pathways? Yes, they are important for immunodeficiencies and pharmacology! – For now, we focus on the upstream events of TCR signaling (CD3 & subunits, Lck, ZAP-70, PLC); the downstream events you’ve already seen in Biochemistry and that you are expected to know here as well are: PI3K, GTP/GDP-Ras MAPK & SAPK/JNK, PLC (Ca2+, calcineurin, DAG, PKC), and Jak/STAT. • Why is a high-affinity IL-2R (CD25) required? What is the difference between a low- and high-affinity IL-2R? • Which genes (name 3 to 5) are expressed following T lymphocyte activation? And why are these genes being expressed? • What happens if one signal is missing? • What happens when a T cell is presented antigen without co-stimulation? • Why do CD8 T lymphocytes require CD4 T cell help for their activation? • What is peripheral tolerance? • Be able to answer the questions of the previous task! • Explain the manner in which superantigens activate T lymphocytes in a non-specific manner… Slide 100 Ag Presentation – Summary MHC-I-restricted MHC-II-restricted iDC iDC Help for… Ag Secondary lymphoid tissue BCR mDC mDC 1. Activation of CD8 T lymphocytes Secondary lymphoid tissue Periphery MHC I CD80/CD86 CD8 CD8 CD28 TCR MHC I CD80/CD86 CD8 CD28 TCR 1. cell + activation CD8 CD4 TCR Naïve CD4 MHC II B cell TCR CD4+ CD40 CD40L CD4+ Thelper CD4 TCR 2. B lymphocyte activation CD40L CD8+ CTL CTL CD8 TCR CD80/CD86 MHC II CD28 2. Killing of infected or tumor cells MHC I MHC II MΦ CD40 3. Macrophage activation Periphery Y IgG      Plasma cell Memory B cell IgM Overview of an gen presenta on: MHC-I-restricted ac va on of naïve CD8 T lymphocytes by professional an gen presen ng cells (pAPCs) occurs in secondary lymphoid tissue (1): activated CD8 T lymphocytes become armed with CD95L (FasL) on their surface, and synthesize perforin, granzyme, and granulolysin, among others, which accumulate in granules and are delivered to target cells (infected cells or tumor cells) to kill them by apoptosis (activated CD8 cells are referred to as cytotoxic T lymphocytes or CTLs) (2). CD8 T cells are activated in secondary lymphoid tissue but exert most of their function in the periphery where infected cells or tumor cells are present; CTLs can exert their function in secondary lyphoid tissue as well when the infected cells are found in these tissues (e. g. HIV infections or lymphomas). MHC-II-restricted ac va on of naïve CD4 T lymphocytes by professional an gen presen ng cells (pAPCs) occurs in secondary lymphoid tissue as well: activated CD4 T lymphocytes can then provide help in the form of CD40L and secretion of cytokines to other leukocytes either in the secondary lymphoid tissues (e. g. (1) help pAPCs activate CD8 lymphocytes and (2) help activate B lymphocytes to induce germinal centres and antibody production), or in the periphery (e. g. (3) macrophage activation for the killing of intracellular microorganisms, either phagocytosed or through macrophage infection). Slide 101 Genetics of the MHC Slide 102 Genetics of the MHC There are three MHC-I genes (-chain): HLA-A, HLA-B, & HLA-C (HLA stands for Human Leukocyte Antigen) There are three MHC-II genes as well (but one for the -chain, & one for the chain): HLA-DP, HLA-DQ, & HLA-DR Nomenclature of MHC: examples – HLA-DQA*0025 stands for allele number 25 for the -chain of HLA-DQ – Similarly, HLA-DQB*0003 stands for allele number 3 for the -chain of HLA-DQ Gene cs of MHC The human MHC contains over 200 genes and is located on chromosome 6. These include the a chain of class I MHC as well as the a and b chains of class II MHC, components of the immunoproteasomes and transporters associated with Ag processing 1 and 2 (TAP1 & TAP2), to name but a few of the important ones. Other very important genes associated with Ag processing and presentation are found on chromosomes 5 (e. g. the invariant chain) and 15 (e. g. b2-microglobulin). Human MHC genes are called human leukocyte Ag (HLA). Although present on nearly every nucleated cell, these were named HLA because they were first discovered on leukocytes. There are 3 sets of class I MHC genes called HLA-A, HLA-B and HLA-C, present on all somatic, nucleated cells (hence erythrocytes and gametes do not normally express HLAs); similarly, there are 3 pairs of class II MHC gene clusters referred to as HLA-DR, HLA-DP and HLA-DQ, and these are usually restricted to pAPCs. But because many individuals possess an extra b chain in their HLA-DR cluster, there actually exist 4 types of class II MHC molecules. HLA-A, HLA-B and HLA-C all encode the a chain of class I MHC molecules, whereas HLA-DR, HLA-DP and HLA-DQ each encode both the a and b chains of class II MHC molecules (except for HLA-DR which encodes an extra b chain). An important characteristic of the MHC is that it is polygenic; this means that there are many different MHC genes (both class I & class II). A functional consequence of this is that individuals possess a discreet, but different, set of MHC molecules (both class I & class II). Because single MHC molecules can bind a range of peptides and individuals carry sets of MHC molecules on their cell surface (but always the same set), this translates in an increase of Ag peptides that can be displayed by an individual, as compared to a single MHC molecule. Slide 103 Genetics of the MHC Another very important characteristic of the MHC is the fact that it is highly polymorphic, i. e. there are many different versions or alleles (sometimes numbered in the hundreds) of the same gene within the population. In fact, the MHC possesses the most polymorphic loci known, thereby dramatically increasing the range of peptides that can be displayed on the cell surface. Slide 104 Genetics of the MHC MHC expression is co-dominant! As Dr. Larson would put it, it’s like having an AB blood type! MHCs are expressed in a co-dominant fashion. Slide 105 Number of expressed MHC types on cells • MHC-I: minimum of 3 and maximum of 6, average of 6 • MHC-II: minimum of 3 and maximum of 12 (in theory…), average of 8 (this average of 8 is lower than the expected average because, (1) even though people are heterozygous at most loci, some people may be homozygous, and (2) some : pairings are unstable and never get expressed on the cell surface) • Combined MHC-I & MHC-II: minimum of 6 and maximum of 18 (again, in theory…), average of 14 Slide 106 Quick assessment of what I know so far… • How many MHC-I genes do we have? Name them… • How many MHC-II genes do we have? Name them… • How polymorphic are MHC genes? • What exactly is co-dominant gene expression? • How do I calculate the number of MHC-I types a person possesses from her/his genetic make up? Refer to the Handout & Study Guide on MHC & Ag Presentation