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  • Figure 1. Professional antigen-presenting cells process intracellular and extracellular pathogens differently. In the endogenous pathway, proteins from intracellular pathogens, such as viruses, are degraded by the proteasome and the resulting peptides are shuttled into the endoplasmic reticulum (ER) by TAP proteins. These peptides are loaded onto MHC class I molecules and the complex is delivered to the cell surface, where it stimulates cytotoxic T lymphocytes (CTLs) that kill the infected cells. In contrast, extracellular pathogens are engulfed by phagosomes (exogenous pathway). Inside the phagosome, the pathogen-derived peptides are loaded directly onto MHC class II molecules, which activate helper T cells that stimulate the production of antibodies. But some peptides from extracellular antigens can also be 'presented' on MHC class I molecules. How this cross-presentation occurs has now been explained: it seems that by fusing with the ER, the phagosome gains the machinery necessary to load peptides onto MHC class I molecules. Roy, 2003 ( www )
  • The MHC GENE COMPLEX: The MHC complex contains a number of genes that control several antigens, most of which influence allograft rejection. These antigens (and their genes) can be divided into three major classes: class I , class II and class III . The class I and class II antigens are expressed on cells and tissues whereas as class   III antigens are represented on proteins in serum and other body fluids ( e.g. C4, C2, factor B, TNF). Antigens of class III gene products have no role in graft rejection. Human MHC: Class I MHC: The class I gene complex contains three major loci , B , C and A and other undefined minor loci (Figure 1). Each major locus codes for a polypeptide,  -chain that contains antigenic determinants, is polymorphic (has many alleles) and associates with  -2 microglobulin (  -chain) and expressed the cell surface. Class II MHC: The class II gene complex also contains at least three loci , DP , DQ and DR ; each of these loci codes for  - and one ß‑chain polypeptides which associate together to form the class II antigens . Like the class I antigens, the class II antigens are also polymorphic. DR locus may contain more than one (up to 4) functional  -chain genes. Mouse MHC: The mouse MHC is located on chromosome 17. Class I MHC: It consists of two major loci , K and D . Unlike the human MHC, the mouse class I gene complex loci are not together but they are separated by class II and class III genes. Class II MHC: The class II gene complex contains two loci , A and E each of which code for one  and one ß chain polypeptide, which form one class II molecule . The mouse class II gene complex is also known as the I region and the genes in this complex are referred to as Ir (immune response) genes since they control the magnitude of immune responsiveness of different mouse strains to certain antigens. Products of A and E loci are also termed IA and IE antigens , collectively known as Ia antigens .
  • A number of diseases have been found to occur at a higher frequency in individuals with certain MHC haplotypes. Most prominent among these are ankylosing spondylitis (B27), celiac disease (DR3), Reiter's syndrome (B27) . Other diseases associated with different specificities of the MHC are listed in Table 2. No definite reason is known for this association. However, several hypotheses have been proposed: antigenic similarity between pathogens and MHC, antigenic hypo‑ and hyper‑responsiveness controlled by the class II genes are included among them.
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    • 1. <ul><li>Group of antigens first identified in graft patients </li></ul><ul><li>Important in determining the compatibility of tissues in successful grafting </li></ul><ul><li>Major histocompatibility antigens are glycoproteins found in the membranes of most cells of vertebrate animals </li></ul><ul><li>Function to hold and position antigenic determinants for presentation to T cells </li></ul>Major Histocompatibility Complex (MHC)
    • 2. Historical Background <ul><li>Genes in the MHC were first identified as being important genes in rejection of transplanted tissues </li></ul><ul><li>Genes within the MHC were highly polymorphic </li></ul><ul><li>Studies with inbred strains of mice showed that genes within the MHC were also involved in controlling both humoral and cell-mediated immune responses </li></ul><ul><ul><li>Responder/Non-responder strains </li></ul></ul>
    • 3. Historical Background <ul><li>There were three kinds of molecules encoded by the MHC </li></ul><ul><ul><li>Class I </li></ul></ul><ul><ul><li>Class II </li></ul></ul><ul><ul><li>Class III </li></ul></ul><ul><li>Class I MHC molecules are found on all nucleated cells (not RBCs) </li></ul><ul><li>Class II MHC molecules are found on APC </li></ul><ul><ul><li>Dendritic cells, Macrophages, B cells, other cells </li></ul></ul>
    • 4. Historical Background Class I MHC Class II MHC RBCs APCs Nucleated cells
    • 5. MHC Molecules <ul><li>Major histocompatibility antigen </li></ul><ul><ul><li>Body cell surface proteins coded by a family of highly polymorphic genes </li></ul></ul><ul><ul><li>MHC class I: found on all nucleated cells </li></ul></ul><ul><ul><li>MHC class II: found only on APCs </li></ul></ul><ul><li>T cell receptors recognize antigenic peptide/MHC complexes </li></ul><ul><ul><li>CD4+ T cells: restricted by class II </li></ul></ul><ul><ul><li>CD8+ T cells: restricted by class I </li></ul></ul>
    • 6. Figure 16.11
    • 7. <ul><li>T-independent antigen </li></ul><ul><ul><li>Large antigen molecules with readily accessible, repeating antigenic determinants </li></ul></ul><ul><ul><li>B cells can bind these directly without being processed </li></ul></ul><ul><ul><ul><li>Stimulates B cells to differentiate into a plasma cell and produce antibodies </li></ul></ul></ul>Antigen Processing
    • 8. <ul><li>T-dependent antigens </li></ul><ul><ul><li>Smaller antigens with less accessible antigenic determinants </li></ul></ul><ul><ul><li>B cells require involvement from helper T cells to target these antigens </li></ul></ul><ul><ul><li>Helper T cells are assisted by leukocytes that process the antigen to make the antigenic determinants more accessible </li></ul></ul><ul><ul><ul><li>Processing is different based on whether the antigen is exogenous or endogenous </li></ul></ul></ul>Antigen Processing
    • 9. <ul><li>APC internalizes the invading pathogen and enzymatically digests it into smaller antigenic fragments which are contained within a phagolysosome </li></ul><ul><li>Phagolysosome fuses with a vesicle containing MHCII molecules </li></ul><ul><li>Each fragment binds to the antigen-binding groove of a complementary MHCII molecule </li></ul><ul><li>The fused vesicle then inserts the MHCII-antigen complex into the cytoplasmic membrane so the antigen is presented on the outside of the cell </li></ul>Processing of Exogenous Antigens
    • 10. <ul><li>The intracellular pathogens are also digested into smaller antigenic determinants </li></ul><ul><li>Each fragment binds to a MHCI molecule located in the endoplasmic reticulum membrane </li></ul><ul><li>The membrane is packaged into a vesicle by a Golgi body which is inserted into the cytoplasmic membrane so the antigen is displayed on the cell’s surface </li></ul>Processing of Endogenous Antigens
    • 11. Roy, 2003 ( www ) Endogenous and Exogenous Antigen Presenting Pathways
    • 12. Immune System. In: Encyclopedia of Life Sciences ( www ) Endogenous and Exogenous Antigen Presenting Pathways
    • 13.  
    • 14.  
    • 15. <ul><li>Sites at which grafts are not likely to be rejected </li></ul><ul><li>Different sites are privileged for different reasons </li></ul><ul><ul><li>The brain lacks lymphatic vessels, and its blood vessel walls are impermeable to lymphocytes such as T cells </li></ul></ul><ul><ul><li>Cornea lacks extensive blood vessels </li></ul></ul><ul><ul><li>Eyes and testes contain naturally high levels of immunosuppressive molecules </li></ul></ul><ul><ul><li>Other sites either lack dendritic cells or express low levels of MHC molecules, so antigen processing does not occur </li></ul></ul>Privileged Sites
    • 16. <ul><li>The fetus is not a privileged site but is not rejected </li></ul><ul><li>Rejection is prevented by the many different immunosuppressive mechanisms </li></ul><ul><ul><li>Early embryos do not express MHC class I and II molecules on the placental layer that is in contact with maternal tissues </li></ul></ul><ul><ul><li>Cytokines that enhance MHC expression have no effect on placental cells </li></ul></ul><ul><ul><li>T cells are prevented from functioning in the placenta to reject the fetus </li></ul></ul>Why Fetuses are Not Rejected
    • 17. Immune Evasion Examples Mycobacteria : Inhibits phagolysosome fusion so that it survives within the phagosome Herpes simplex virus : Interferes with TAP transporter (inhibits antigen presentation) Cytomegalovirus : Inhibits proteasome activity and removal of MHC I from ER Epstein-Barr virus : Inhibits proteasome activity; produces IL-10 to inhibit macrophage activation Pox virus : Produces soluble cytokine receptors to inhibit activation of effector cells
    • 18. Structure of Class I MHC NH 2 Alloantigenic sites CHO NH 2 COOH COOH P α1 α2 α3 β2 OH Plasma membrane Disulfide bridge Papain cleavage Cytoplasm NH 2
    • 19. Structure of Class I MHC Peptide-binding Region <ul><li>a “groove” composed of an α-helix on two opposite walls and eight β-pleated sheets forming the floor </li></ul><ul><li>residues lining groove most polymorphic </li></ul><ul><li>peptide in groove 8-10 amino acids long </li></ul><ul><li>specific amino acid on peptide required for “anchor site” in groove </li></ul>
    • 20. Structure of Class II MHC Plasma membrane Cytoplasm CHO CHO CHO NH 2 NH 2 COOH COOH α1 α2 β2 β1
    • 21. Structure of Class II MHC <ul><li>Two polypeptide chains, α and β, of roughly equal length. </li></ul><ul><li>Peptide-binding region – a groove formed from the α1 and β1 domains of the α and β chains – site of polymorphism </li></ul><ul><li>Immunoglobulin-like region – conserved α2 and β2 domains – β2 is site to which CD4 on T cell binds </li></ul>
    • 22. <ul><li>Both have a peptide-binding groove with a wall of two α helices and a floor of eight β-pleated sheets </li></ul><ul><li>Close-ended groove for class I MHC requires an 8-10 amino acid-length peptide to bind; open-ended groove for Class II MHC lets it bind a peptide 13-25 amino acids long, not all of which lie in the groove </li></ul><ul><li>Anchor site rules apply to both classes </li></ul>Peptide-binding grooves for class I and class II MHC are structurally similar
    • 23. The human MHC genes
    • 24. Class I polymorphism Locus Number of alleles (allotypes) HLA - A 218 HLA - B 439 HLA - C 96 There are also HLA - E, HLA - F and HLA - G Relatively few alleles
    • 25. Class II polymorphism Locus Number of alleles (allotypes) HLA - DP A HLA - DP B 12 88 HLA - DQ A HLA - DQ B 17 42 HLA - DR A HLA - DR B1 HLA – DR B3 HLA – DR B4 HLA – DR B5 2 269 30 7 12 There are also HLA - DM and HLA - DO Relatively few alleles
    • 26. Comparison: MHC Class I and II Structure
    • 27. Role of CD4 and CD8 in promoting T-cell responses
    • 28. Costimulation is Necessary for T Cell Activation <ul><li>Engagement of TCR and Ag/MHC in the absence of costimulation can lead to anergy </li></ul><ul><li>Engagement of costimulatory molecules in the absenece of TCR engagement results in no response </li></ul><ul><li>Activation only occurs when both TCR and costimulatory molecules are engaged with their respective ligands </li></ul><ul><li>Downregulation occurs if CTLA-4 interacts with B7 </li></ul><ul><ul><li>CTLA-4 send inhibitory signal </li></ul></ul>
    • 29. T cell selection/education:
    • 30. MHC Restriction: <ul><li>Property of T cells to recognize antigens presented only by self-MHC molecules </li></ul><ul><li>Vital aspect of self/non-self discrimination and hence adaptive immunity </li></ul><ul><li>A marker of T cell that has been positively selected </li></ul><ul><li>Selection occurs in the thymus </li></ul>
    • 31. MHC restriction of immune response
    • 32. MHC-TCR recognition of antigen
    • 33. Dendritic Cell <ul><li>Most potent APC for naïve T cells </li></ul><ul><li>Many long membrane extensions </li></ul><ul><li>Highly variable depending on location </li></ul><ul><ul><li>Langerhan cells in the skin </li></ul></ul><ul><ul><li>Interdigitaing cells in the thymus </li></ul></ul><ul><ul><li>FDC in germinal centers </li></ul></ul><ul><ul><li>Veiled cells in lymphatics </li></ul></ul><ul><ul><li>Blood dendritic cells in circulation </li></ul></ul>
    • 34. Only mature DC activates T cells Activation/Proliferation T cell Mature DC Anergy/Apoptosis/Deletion T cell Immature DC
    • 35. Functions of APCs <ul><li>T cell selection in the thymus (only DCs) </li></ul><ul><li>Trap and capture antigen in the periphery </li></ul><ul><li>Process antigen into peptides </li></ul><ul><li>Store antigens </li></ul><ul><li>Transport antigens to peripheral lymphoid tissues </li></ul><ul><li>Present antigenic peptides to T cells </li></ul><ul><li>Co-stimulate T cells </li></ul>
    • 36. T Cell Activation <ul><li>Requirements: Two signals </li></ul><ul><ul><li>Signal 1: specific recognition of antigen (peptide-MHC complex) via antigen receptor </li></ul></ul><ul><ul><li>Signal 2: costimulatory signals from APC </li></ul></ul><ul><li>Signal 1 alone leads of unresponsiveness </li></ul><ul><ul><li>Anergy, Deletion, Apoptosis </li></ul></ul>
    • 37. Structure of T Cell Receptor CHO CHO CHO CHO Variable region “V” Constant region “C” Hinge “H ” Alpha chain Beta chain Disulfide bridge Transmembrane region Cytoplasmic tail + + +
    • 38. Structure of T Cell Receptor (TCR) <ul><li>Two polypeptide chains, α and β, of roughly equal length </li></ul><ul><li>Both chains consist of a variable (V) and a constant (C) region </li></ul><ul><li>α chain V region has a joining (J) segment </li></ul><ul><li>β chain V region has both a J and diversity (D) segment </li></ul>
    • 39. Structure of T Cell Receptor (continued) <ul><li>Hypervariable regions in V contribute to diversity of TCR </li></ul><ul><li>TCR recognizes portions of MHC molecule and peptide bound in the groove </li></ul><ul><li>Small population of T cells has a TCR comprised of γ and δ chains – γδ TCR specificity differs from αβ TCR </li></ul>
    • 40. Accessory Molecules <ul><li>All are invariant </li></ul><ul><li>Increase adhesion between two engaged cells </li></ul><ul><li>Some show increased expression in response to cytokines </li></ul>
    • 41. Costimulatory Molecules <ul><li>Molecules on T cell and 2 nd cell that engage to deliver 2 nd signal required for activation of T cell </li></ul><ul><li>Most important costimulatory molecules: </li></ul><ul><li>T cell Ligand on 2 nd cell </li></ul><ul><li>CD28 B7-1 (CD80), B7-2 (CD86) </li></ul>
    • 42. Interactions of Th Cell and APC LFA-3 LFA-2 LFA-1 TCR CD4 ICAM-1 Class II MHC B7-1/B7-2 (CD80/CD86 CD28 IL-1 IL-6 TNF-alpha IL-12 IL-15 TNF-beta IFN-gamma GM-CSF IL-4 T helper lymphocyte Antigen- presenting cell peptide
    • 43. Interactions of Tc Cell and Target Cell LFA-1 TCR CD8 ICAM-1 Class I MHC LFA-3 LFA-2 T cytotoxic lymphocyte Target cell peptide
    • 44. Aspects of MHC <ul><li>MHC molecules are membrane-bound. Recognition by T cells requires cell-cell contact. </li></ul><ul><li>Peptide from cytosol associates with class I MHC and is recognized by Tc cells. Peptide from vesicles associates with class II MHC and is recognized by Th cells. </li></ul><ul><li>Although there is a high degree of polymorphism for a species, an individual has maximum of six different class I MHC products and only slightly more class II MHC products </li></ul><ul><li>Mature T cells must have a T cell receptor that recognizes the peptide associated with MHC. This is the second level of control. </li></ul>
    • 45. <ul><li>Each MHC molecule has only one binding site. The different peptides a given MHC molecule can bind all bind to the same site, but only one at a time. </li></ul><ul><li>MHC polymorphism is determined only in the germline. There are no recombinational mechanisms for generating diversity. </li></ul><ul><li>Because each MHC molecule can bind many different peptides, binding is termed degenerate. </li></ul><ul><li>Cytokines (especially interferon- γ) increase level of expression of MHC. </li></ul><ul><li>Alleles for MHC genes are co-dominant . Each MHC gene product is expressed on the cell surface of an individual nucleated cell. </li></ul>
    • 46. HLA and disease association 87.4 37.0 10.4 13.3 15.4 9 9 9 33 26 9 79 52 87 85 B27 B27 B27 CW6 DR3 Ankylsoing spondylitis Reiter’s disease Acute anterior uveitis Psoriasis vulgaris Dermatitis herpetiformis control patients Relative risk Frequency in Associated alleles Disease

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