Antigens and Antigen Presentation - BIOM 611G (PDF)

BIOM 611G, Medical Microbiology PCOM Georgia ANTIGENS, ANTIGEN PROCESSING & PRESENTATION Valerie E. Cadet, PhD Assistant Dean of Health Equity Integration Professor of Microbiology and Immunology...

BIOM 611G, Medical Microbiology PCOM Georgia ANTIGENS, ANTIGEN PROCESSING & PRESENTATION Valerie E. Cadet, PhD Assistant Dean of Health Equity Integration Professor of Microbiology and Immunology BMS1 & BMS2 Department of Biomedical Sciences January 9, 2025  Cellular and Molecular Immunology, 10th Ed., Abbas Chapter 6: Antigen Presentation to T Lymphocytes and the Function of MHC Molecules LECTURE OBJECTIVES Through the study of this content and recommended reading, the successful student will be able to……. 1. Differentiate between antigens, immunogens, epitopes and haptens. 2. Recognize the different types of antigens and their respective immunogenicity. 3. Define T-dependent and T-independent antigens. 4. Describe the inheritance pattern of the MHC. 5. Contrast the cellular expression of MHC Class I vs MHC Class II molecules. 6. Identify the most polymorphic MHC Class I and Class II genes and the importance of this polymorphism for antigen processing and transplantation. 7. Diagram and label a Class I and a Class II MHC molecule as it appears on the surface of the cell. Label the polypeptide chains of the molecules, beta 2 microglobulin, the peptide binding groove, bound peptide, anchor residues and pockets, binding sites for the TCR, and the binding sites for CD4 or CD8. 8. Identify the key steps in exogenous and endogenous antigen processing, including the role of TAP, proteasomes, chaperones, invariant chain/CLIP, and HLA-DM. 9. Describe the cellular/molecular mechanism and significance of cross-presentation in activation of CD8+ T cells. 2  WHAT IS IT Differentiate THAT OUR between antigens, immunogens, epitopes and IMMUNE haptens SYSTEM Recognize the different types of RECOGNIZES antigens and their respective immunogenicity ? 3  ANTIGENS Neoplastic or normal host cells Microbial proteins and carbohydrates Streptococcus pneumoniae with polysaccharide antigens Viral proteins and carrier protein Environmental factors Animal dander 4 Pollen TERMS TO KNOW: ANTIGENS & IMMUNOGENS ▪ Antigen (Ag) ▪ Substance that reacts with the products of a specific immune response ▪ Molecule recognized by specific receptors on T or B cells 🡺 ANTIbody GENerating Agent ▪ May be microbial or otherwise foreign to the body, neoplastic or normal host cell in origin ▪ Immunogen ▪ Substance that induces a specific immune response (binding to TCR or BCR/antibody) ▪ Antibody (Ab) ▪ A specific protein which is produced in response to an immunogen, and which reacts with an antigen NOTE: ▪ All immunogens are antigens but NOT all antigens are immunogens. 5 AN ANTIGEN BY ANY OTHER NAME…. ▪ Allergen ▪ Substance that causes detrimental allergic reaction ▪ May result after exposure via ingestion, inhalation, injection or contact with skin ▪ Tolerogen ▪ Antigen that invokes a specific immune non-responsiveness due to its molecular form ▪ If its molecular form is changed, a tolerogen can become an immunogen ▪ Tumor-specific antigens (TSAs) ▪ Typically result from tumor specific mutation ▪ Are targeted for non-self attack when displayed on class I MHC ▪ Tumor-associated antigens (TAAs) ▪ Antigens presented both by tumor cells and normal cells ▪ More common than TSAs 6 TERMS TO KNOW: EPITOPE ▪ Epitope aka antigenic determinant ▪ Portion of an antigen that combines with the products of a specific immune http://highered.mheducation.com/sites/0072507470/st response udent_view0/chapter22/ani mation__antigenic_determi B cell epitopes nants__epitopes_.html Bc epitope ▪ Specific portion of a macromolecular Ag to which an Ab binds Tc epitope ▪ Peptide portion that is recognized by a TCR when it is displayed bound to an MHC molecule 7 CHEMICAL NATURE OF DIFFERENT TYPES OF ANTIGENS ▪ Proteins: Usually very good immunogens ▪ Hapten: Low molecular-weight small ▪ Pure proteins, glycoproteins or lipoproteins compound that induces an immune response only when coupled (attached) ▪ Most immunogens are proteins to larger carrier molecule (usually protein). ▪ Polysaccharides: Good immunogens ▪ The carrier protein has a different and ▪ Pure polysaccharides and unique antigenic structure than the lipopolysaccharides hapten. ▪ The immune response is directed at both ▪ Nucleic Acids: Poorly immunogenic the hapten and the carrier protein. ▪ May become immunogenic when single ▪ Free, uncoupled haptens CAN react with stranded or when complexed with proteins products of a specific immune response after such products have been elicited 🡺 ▪ Lipids: Non-immunogenic antigenic but not immunogenic ▪ May be haptens 8 RELATIVE IMMUNOGENICITY OF VARIOUS ANTIGENS 9 FEATURES OF BIOLOGICAL ANTIGENS RECOGNIZED B cell Epitopes T cell Epitopes ▪ Forms of antigens recognized: ▪ Forms of antigens recognized: ▪ Macromolecules ▪ Protein/polypeptide ▪ Protein/polypeptide (5-15 aa) ▪ CD8+: 8-10 aa ▪ Polysaccharide (5-7 monosaccharides) ▪ CD4+: 12-17 aa ▪ Nucleic acid (5-7 nucleotides) ▪ Linear ▪ Lipids ▪ Small chemicals ▪ Bound by cell-surface TCR only after being processed and displayed via MHC ▪ Conformational and linear ▪ Due to being processed, can be an intracellular or ▪ Recognized and bound by cell-surface (BCR) extracellular antigen originally or soluble antibody ▪ Extracellular or on cell-surface 10 ANTIGENS STIMULATE ADAPTIVE IMMUNE RESPONSES 11  HOW DOES THE INNATE IMMUNE SYSTEM STIMULATE ADAPTIVE RESPONSES? ▪ Warns adaptive immune system that an effective response is needed Two-signals are required for lymphocyte activation ▪ Signal 1: Antigen recognition by lymphocytes results in lymphocyte activation ▪ Signal 2: Cytokines, microbial peptides and microbial degradation products during innate immune responses 12 Big Abbas RECOGNITION BY ▪ Lymphocytes LYMPHOCYTES ▪ Humoral: B cell 🡺 antibody secreting plasma cell ▪ Cell-mediated: T cell ▪ T Helper Cells (CD4+)🡺 cytokines ▪ Cytotoxic T Cells (CD8+)🡺kill infected/altered cells ▪ Receptors ▪ Membrane-bound: BCR/TCR ▪ Soluble: Antibodies ▪ Accessory Molecules ▪ Major histocompatibility complex (MHC) types I and II ▪ Also referred to as human leukocyte antigen (HLA) ▪ Class I found on all nucleated cells ▪ Class II found on professional APCs: dendritic cells, macrophage, B lymphocytes and some thymocytes 13 Baby Abbas Fig. 4.1 Properties of Abs as BCRs and TCRs  ANTIGENS CAN ELICIT A B CELL RESPONSE IN TWO PRIMARY WAYS 🡺 B CELL ACTIVATION 14 https://youtu.be/5YiMhRetj B CELLS CAN BE ACTIVATED Define T-dependent and WITH OR T-independent antigens WITHOUT T CELL HELP 15  ANTIGENS CAN ELICIT A B CELL RESPONSE IN TWO PRIMARY 1. WAYS T-dependent antigen (T-D) ▪ T cell help is required for immune response (humoral-IgG, A or E) ▪ Proteins are processed by B cell as pAPC🡺presented via MHC-II on APC🡺bound by CD4+ TCR ▪ The requirement for MHC-associated antigen presentation for T cell activation accounts for the MHC restriction of T cell–B cell interactions ▪ Endogenous Ags: The body’s own cellular components or intracellular pathogens. These are further classified into: ▪ Autoantigens: self-antigen ▪ Alloantigens: tissue specific antigen, which is present in one individual of a species but not in others. ▪ Ex: ABO, HLA etc. ▪ Intracellular Pathogens: viruses, intracellular bacteria and parasites https://courses.lumenlearning.com/microbiology/chapter/b-lymphocyt 16 es-and-humoral-immunity/ https://youtu.be/5YiMhRetj  ANTIGENS CAN ELICIT A B CELL RESPONSE IN TWO 2. PRIMARY WAYS T-independent antigen (T-I) ▪ Does not require T cell help at all (TI-1) or partially requires (TI-2) to induce immune response (humoral-IgM) ▪ TI-1: no T cell help required to produce appropriate response ▪ TLR activation by LPS, CpG, etc the ▪ TI-2: no direct T cell help required but need cytokines produced by T cells for additional signal 🡺 appropriate response ▪ Polysaccharides and glycoproteins are bound by BCR ▪ Exogenous Ags: These antigens enter the body or system and freely circulate in the body fluids and are trapped by the APCs. The uptake of these exogenous antigens by APCs are mainly mediated by the phagocytosis (extracellular). 17 https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/1094/2016/11/03172714/OSC_Microbio_18_04_indact.jpg 18  Antigen ANTIGEN Adaptive immune response PROCESSING AND Cell-Mediated response PRESENTATI T-cell response ON TO T CELLS MHC-restricted Ag processing ▪ Ability of antigen presenting cell (APC) to converting naturally occurring proteins into peptides able to bind to MHC molecules 19  HOW ARE Describe inheritance pattern of MHC MHC Identify MOLECULE most polymorphic MHC Class I and Class II genes S and the importance of this polymorphism for antigen INHERITED? processing and transplantation 20 DISCOVERY OF THE HUMAN MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) Discovered by searching for cell surface molecules in one individual recognized as foreign by another individual ▪ Studied people who received multiple blood transfusions & kidney transplants ▪ Serum contained antibodies that recognized cells from blood or kidney donors ▪ Multiparous women have circulating antibodies that recognized paternal cells Proteins recognized by these antibodies called human leukocyte antigens (HLA) ▪ Leukocyte because antibodies tested by binding to leukocytes of other individuals ▪ Antigens because molecules recognized by antibodies ▪ Subsequent analyses show: inheritance of particular genes (HLA alleles) is major determinant of graft acceptance or rejection ▪ HLA proteins have essentially identical structures ▪ HLA genes determine fate of grafted tissues & are present in all mammalian species encoding MHC molecules 21  PROTEINS ENCODED BY MHC (HLA) Class I locus ▪ HLA-A, HLA-B, and HLA-C Class II locus ▪ 2 separate genes encoding α chain and β chain of HLA-DP ▪ 2 encoding HLA-DQ (DQα and DQβ) ▪ 1 or 2 for HLA-DR [DRβ (HLA-DRB1 always and sometimes HLA-DRB3, HLA-DR4, or HLA-DR5) and 1 for DRα ▪ MHC haplotype: set of MHC genes present on each chromosome ▪ Every person inherits two haplotypes, one from each parent ▪ Genes in MHC haplotype tightly linked and inherited together ▪ Chance that two siblings will inherit identical sets of HLA alleles is 25% 22 INHERITANCE & EXPRESSION OF OF MHC Each haplotype contains loci encoding: ▪ all Class I molecules (HLA-A, B, C, E, F and G) ▪ Inherit 1 from each parent ▪ all Class II molecules (HLA-DP, DQ, DR, DM and DO) ▪ Inherit 1 from each parent ▪ Every person expresses six class I HLA alleles (one allele of HLA-A, -B, and -C from each parent) and usually six or seven class II HLA alleles (one allele of HLA-DQ and HLA-DP and one or two of HLA-DR from each parent) ▪ High polymorphism Co-dominant expression ▪ Protein products of both parental alleles at given locus expressed equally 23 IMPLICATION OF POLYMORPHISM - 1 Physiologic function of MHC 🡺 display peptide antigens for recognition by T cells Total # of HLA alleles with different amino acid sequences >14,000 ▪ >10,500 class I and 3500 class II molecules ▪ >3500 variants for HLA-B locus alone 24  IMPLICATION OF POLYMORPHISM - 2 ▪ Called histocompatibility molecules because they determine if grafted tissue (histo) is compatible or incompatible with host's immune system. ▪ Human MHC 🡺 human leukocyte antigen (HLA) complex (Chromosome 6) ▪ The reaction to allogeneic MHC antigens on another individual’s cells is one of the strongest immune responses known ▪ Principal targets of transplant rejection = antigens of allografts encoded in MHC Pre-transplantation: must determine patient’s HLA type and match with donor Strongest determinant HLA-DR > HLA-B > HLA-A 25  Contrast cellular expression of MHC Class I vs MHC Class II molecules Diagram and Label Class I and a Class II MHC WHAT DOES molecule as it appears on the surface of the cell. THE MHC Label MOLECULE 1. polypeptide chains of the molecules LOOK LIKE? 3. 2. beta 2 macroglobulin peptide binding groove 4. bound peptide 5. anchor residues and pockets 6. binding sites for TCR 7. binding sites for CD4 or CD8 26 WHICH CELLS EXPRESS WHICH MHC? 27 MHC I STRUCTURE 1. composed of three subunits: transmembrane heavy chain, small β-2-microglobulin subunit and the presenting antigenic peptide α 1 and α 2 domains form peptide binding groove and are regions responsible for high polymorphism of MHC class I molecules. α3 domain: invariant portion that binds CD8 β-2-microglobulin is the smaller, 11-13 kDa subunit, with no transmembrane domain (ENCODED ON 15q21.1) o It is non-covalently attached to the MHC class I heavy chain on the cell surface. o β -2-microglobulin is encoded by a gene outside of the MHC cluster and it is structurally also immunoglobulin-like 2. assembled in the endoplasmic reticulum 3. dependent on the presence of all three subunits for proper cell surface expression as a trimer 4. Accommodates peptides of 8–11 residues 28  MHC II STRUCTURE 1. composed of transmembrane α and β subunits α 1 and β1: associate to form the peptide-binding groove α 2 and β 2: associate to form the CD4 binding site β 2 domain: invariant portion that binds CD4 2. assembled in the endoplasmic reticulum 2. MHC class II αβ exhibit a tendency to self-associate forming (αβ)2 "dimers of dimers“ heterodimer 3. Accommodates peptides of 10–30 residues+ 29  PEPTIDE BINDING BY MHC ▪ MHC polymorphism is largely confined to the peptide binding cleft ▪ α chain for class I genes ▪ β chains for class II genes ▪ Different MHC isoforms bind different peptides ▪ “reason” for polymorphisms and co-dominant expression pattern ▪ Peptide binding is “selective”, not antigen specific like antigen receptors on B and T cells ▪ One isoform can bind many different peptides with same overall shape ▪ Non-polymorphic areas bind CD4 (Class II) or CD8 (Class I) co-receptors 30 FEATURES OF PEPTIDE BINDING TO MHC 31 FEATURES OF PEPTIDE BINDING TO MHC, CON’T 32 PROTEIN ANTIGENS CAPTURE D BY ANTIGEN- PRESENTI NG CELLS 33 CAPTURE, TRANSPORT , AND PRESENTAT ION OF PROTEIN ANTIGENS BY DENDRITIC CELLS 34  SUMMARY: FEATURES OF ANTIGENS RECOGNIZED BY T CELLS Features of Antigens Recognized by T Cells Explanation Most T cells recognize peptides and no other 1 Only peptides bind to MHC molecules. molecules. T cells recognize linear peptides and not Linear peptides bind clefts of MHC molecules 🡺 protein 2 conformational determinants of protein antigens. conformation is lost during generation of these peptides Most TCRs recognize only peptide-MHC complexes T cells recognize cell-associated and not soluble 3 MHC molecules are membrane proteins that display stably antigens. bound peptides on cell surfaces Pathways of assembly of MHC molecules ensure Class II molecules display peptides derived from + + CD4 and CD8 T cells preferentially recognize extracellular proteins taken up into vesicles in APCs 4 antigens sampled from the extracellular and cytosolic Class I molecules present peptides from cytosolic pools, respectively. proteins CD4 and CD8 bind nonpolymorphic regions of class II and class I MHC, respectively 35  ARE HAPTENS RECOGNIZED BY T CELLS? Antigen presentation on B cells to helper T cells ▪ Hapten-specific B cells bind antigen through hapten determinant ▪ Endocytose the hapten-carrier conjugate ▪ Present peptides derived from carrier protein to carrier-specific helper Tcells ▪ 🡺 the two cooperating lymphocytes recognize different epitopes of same complex antigen ▪ Hapten is responsible for efficient internalization of carrier protein into B cell ▪ Explains why hapten and carrier must be physically linked 36 https://www.creative-diagnostics.com/Hapten.htm  A MODEL FOR T CELL RECOGNITION OF A PEPTIDE-MHC COMPLEX The electron micrograph (courtesy of J. W. Uhr) shows a B cell and T cell bound to each other. MHC molecules are membrane proteins on APCs that display peptide antigens for 37 th Cellular and Molecular Basis of Medicine, 9 Ed. Figure 6-1 recognition by T lymphocytes.  WHAT ARE THE Identify STEPS IN THE MHC the key steps in exogenous PATHWAYS OF and endogenous antigen PEPTIDE processing, including the role of TAP, proteasomes, DEGRADATION AND chaperones, invariant PRESENTATION? chain/CLIP, and HLA-DM 38  OVERVIEW OF ANTIGEN PROCESSING AND PRESENTATION (WATCH ON YOUR OWN) One way of rationalizing the development of two different pathways is that each ultimately stimulates the population of T cells that is most effective in eliminating that type of antigen. How a microbe or its products enter an APC strongly influences the nature of the immune response. 39 http://highered.mheducation.com/sites/0072507470/student_view0/chapter22/animation__antigen_processing.html OVERVIEW OF ANTIGEN PROCESSING AND PRESENTATION PATHWAYS 40 CLASS II MHC PATHWAY: PROCESSING OF INTERNALIZED PROTEIN ANTIGENS Exogenous antigens ▪ Foreign molecules that are ingested (endocytosed, phagocytosis) by APCs ▪ They are then are fragmented/ degraded and extruded as antigens carried on MHC II for presentation to CD4+ Th cells 41 CLASS I MHC PATHWAY: PROCESSING OF INTRACELLULAR/CYTOSOLIC PROTEIN Endogenous antigen ▪ Internally generated molecules that become presented on the cell surface in complex with MHC I. ▪ May result from exogenous viral or bacterial infections that have altered the host cell. 42 FEATURES OF THE PATHWAYS OF ANTIGEN PROCESSING TAP: transporter associated with antigen processing DM: HLA-DM CLIP: class II invariant chain peptide = invariant chain (Ii) 43  JUST WHEN YOU HAD IT DOWN, Describe HERE’S A NEW WAY cellular/molecular OF ACTIVATING mechanism and significance of cross-presentation in CD8+ activation of CD8+ T cells T CELLS 44  CROSS PRESENTATION (AKA CROSS-PRIMING) Professional APCs (typically dendritic cells) can uptake exogenous antigen and display it on MHC I 1. Cells infected with intracellular microbes, such as viruses, are ingested (captured) by professional APCs, and the antigens of the infectious microbes are broken down and presented in association with the APC MHC molecules. 2. In most cases, the term cross-presentation (or cross-priming) is applied to CD8+ T cells-cytotoxic T lymphocytes (CTLs)-recognizing class I MHC-associated antigens that have been derived exogenously (as shown) 3. The same cross-presenting APC may display class II MHC-associated antigens from the microbe for recognition by CD4+ helper T cells. 45  CROSS PRESENTATION: WHAT HAPPENS WHEN AN INFECTED CELL IS INGESTED-1? A cell infected with an intracellular microbe, such as a virus or parasite is ingested (endocytosed, phagocytosis) by a professional APC (pAPC)… ▪ Inside the infected cell itself, it is attempting to process the viral antigens being produced via its class I MHC pathway, however, many viruses are ‘clever’ and can shut down or diminish this pathway inside the cells they infect. This makes the cell basically a virus factory with no way to elicit an effective immune response. ▪ In addition, the infected cell is not capable on its own to infect another cell. 46 CROSS PRESENTATION: WHAT HAPPENS WHEN AN INFECTED CELL IS INGESTED-2? ▪ This leaves the cell, with its intracellular invader, as being seen as an exogenous antigen by pAPCs at this point since there is a distress signal coming from it, therefore, allowing a pAPC to ingest it ▪ More about this distress signal in later lecture (CMI) ▪ The infected cell is uptaken into the pAPC along with viral antigens that are inside of it. ▪ Once the pAPC recognizes that the foreign proteins coming from the ingested cell are viral, this leads to ▪ The pAPC processing and presenting these viral proteins via its own MHC I pathway (antigen transfers from endocytic pathway to cytosol🡺 proteosomal processing) 47 CROSS PRESENTATION: WHAT HAPPENS WHEN AN INFECTED CELL IS INGESTED-3? pAPC presents these viral proteins via MHC I to CD8+ 🡺 Cross Presentation CD8+ T cells are activated and become CTLs: cytoxic T lymphocytes that can kill other infected cells in circulation. The same cross-presenting pAPC may display class II MHC-associated antigens from the microbe for recognition by CD4+ Th cells. NOTE: If the normal pathway for exogenous antigen was followed here, an MHC II processing and presenting of ingested antigen from the infected cell by the pAPC would take place 🡺 CD4+ helper response. This response would not eliminate other virally-infected cells in circulation that need help. 48 WHAT COMES NEXT? WHAT HAPPENS ONCE CD4+ AND CD8+ T CELLS RECOGNIZE MICROBES AND ARE ACTIVATED VIA MHC-ASSOCIAT ED ANTIGEN PRESENTATION ? 49  1. When antigens enter through the skin, in what organs ON YOUR are they concentrated? What cell type(s) plays an important role in this process of antigen capture? 2. What are MHC molecules? What are human MHC OWN: molecules called? How were MHC molecules discovered, and what is their function? THOUGHT 3. What are the differences between the antigens that are displayed by class I and class II MHC molecules? 4. Describe the sequence of events by which class I and QUESTION class II MHC molecules acquire antigens for display. 5. Which subsets of T cells recognize antigens presented S by class I and class II MHC molecules? What molecules on T cells contribute to their specificity for either class I or class II MHC–associated peptide antigens? 50 REMEMBER THE BIG PICTURE 51