Bio 151 lec 7 MHCs


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Bio 151 lec 7 MHCs

  1. 1. BIOLOGY 151 LEC 7 Major Histocompatibility Complex (MHC) Parungao-Balolong 2011
  2. 2. Discovery of the MajorHistocompatibility Complex Transplantation experiments in Mice Serologic Studies in HumansStructure and Function of MHCMolecules Class I and Class IIExpression and Regulation of MHCMolecules Parungao-Balolong 2011
  3. 3. INTRO...• MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) • a region of highly polymorphic gene whose products are expressed on the surfaces of a variety of cells • discovered in the 1940s via an artificial transplantation experiments • principal determinants of graft rejection • THUS: individuals who express the same MHC molecules accept tissue grafts from one another, and, individuals who differ at their MHC loci vigorously rejects such grafts Parungao-Balolong 2011
  4. 4. INTRO...MHC: ROLE IN IMMUNE RESPONSE??? 1960s: Benacerraf et al demonstrated that different inbred strains of guinea pigs and mice did or did not produce antibodies in response to immunization with simple polypeptide antigen this immune responsiveness was an autosomal dominant trait mapped to the MHC region genes that controlled such immune response = Ir genes or immune response genes controlled the activation of helper T lymphocytes necessary for antibody response to protein antigens Parungao-Balolong 2011
  5. 5. INTRO...• MHC: ROLE IN IMMUNE RESPONSE??? • 1970s : central role of MHC genes in immune response to protein antigens was explained • demonstrated that antigen specific T lymphocytes do not recognize antigens in free or soluble form but recognize portions of protein antigens that are non-covalently bound to MHC gene products Parungao-Balolong 2011
  6. 6. INTRO...MHC: TYPES OF GENE PRODUCTS MHC Class I molecules MHC Class II moleculesany given T cell recognizes foreign antigen bound to onlyone specific class (I or II)THUS : MHC molecules are integral components of theligands that T cell recognize Parungao-Balolong 2011
  7. 7. IMPORTANCE IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens? 1. MHC molecules are membrane - associated and not secreted : T- lymphocytes can recognize foreign antigens only when bound to surfaces of other cells  This limits T-cell activation such that T cells interact most effectively with other cells that bear MHC-associated antigens and not with soluble antigens (i.e antigen presentation)  The recognition of antigen on a cell surface also serves to localize the effector functions of the activated T cell to the anatomic site of antigen presentation  NOTE: In contrast, antibodies can function in the circulation by binding to and neutralizing soluble antigens Parungao-Balolong 2011
  8. 8. IMPORTANCE IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens? 2. the patterns of antigen association with class I or II MHC molecules determine the kinds of T cells that are stimulated by different forms of antigens  peptide fragments derived from extracellular proteins = binds to class II  endogenously synthesized peptides = associates with class I Parungao-Balolong 2011
  9. 9. IMPORTANCE IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens? 3. the immune response to a foreign protein is determined by the presence or absence of MHC molecules that can bind and present fragments of that proteins to T cells since MHC genes are polymorphic, many different alleles exist within a population and these alleles differ in their ability to bind and present different antigenic determinants of proteins this is how MHC genes control immune responses to protein antigens Parungao-Balolong 2011
  10. 10. IMPORTANCE IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens? 4. Mature T cells in any individual recognize and respond to foreign antigens but are responsive to self proteins this antigen recognition is shaped by the selection of foreign antigen- specific T cells from developing lymphocytes based on their recognition of self MHC molecules with or without bound peptide antigens THUS, a second means by which MHC can influence immune responses to particular antigens is through the role of MHC molecules in shaping the repertoire of mature T cells Parungao-Balolong 2011
  11. 11. DISCOVERY! MURINE MHC George Snell and colleagues used classical genetic techniques to analyze rejection of transplanted tumors and other tissues (grafts) examined the outcome of skin grafts between individual animals using inbred strains of laboratory mice Parungao-Balolong 2011
  12. 12. DISCOVERY! THE EXPERIMENT principle: RECALL.... non-polymorphic: some genes are represented by only one normal nucleic acid sequence variant nucleic acid sequence is an uncommon mutation and may result in a disease state polymorphic: genes may vary at relatively high frequency among normal individuals in the populations polymorphic any individual animal can have the same allele at a genetic locus on both chromosome of the pair (homozygous) or two different alleles one on each chromosome (heterozygous) Parungao-Balolong 2011
  13. 13. DISCOVERY! THE EXPERIMENT inbred mouse strains: produced by repetitive matings of siblings (> 20 generations) every individual animal of a given inbred mouse strain will have identical nucleic acid sequences at all locations on both members of each pair of chromosomes completely homozygous at every genetic locus genetically completely identical to every mouse of the same strain = syngeneic THUS, when a tissue or organ, such as patch of skin, is grafted from one animal to another, two possible outcomes may ensue: grafted skin survives and functions as normal skin or; immune system destroys the graft (graft rejection) Parungao-Balolong 2011
  14. 14. DISCOVERY! THE EXPERIMENT: genetic basis of graft rejection among inbred mice 1. grafts of skin from one animal to itself (isogeneic or isografts) or grafts between animals of the same inbred strain (syngeneic grafts of syngrafts) are usually NEVER rejected 2. grafts between animals of different inbred strains or between outbred mice (allogeneic grafts or allografts) are almost ALWAYS rejected Distinguishes the grafts as FOREIGN: the genes responsible for causing a grafted tissue to be perceived as similar to one’s own tissue or as foreign as called histocompatibility genes differences between foreign and self were attributed to genetic polymorphisms among diffrent histocompatibility alleles Parungao-Balolong 2011
  15. 15. DISCOVERY! THE CONGENEIC MOUSE STRAINS EXPERIMENT differed only by genes responsible for graft rejection (MHC) NOTE: although several different genes could contribute to rejection, a single genetic region is responsible for most rejection phenomena this gene encodes a polymorphic blood group antigen called antigen II or histocompatibility-2 (H-2) JUSTIFICATION initially, MHC congeneic strains were thought to differ at a single locus occasional recombination events occurred within the MHC during interbreeding of different strains, suggesting that the MHC actually contained several different genes, each involved in graft rejection H-2 region is now known to be homologous to genes that determine the fate of grafted tissues in other species (Major Histocompatibility Complex) Parungao-Balolong 2011
  16. 16. DISCOVERY! GENETICS OF GRAFT REJECTION indicated that the products of MHC genes are co- dominantly expressed alleles on both chromosomes of a pair are expressed as a consequence, each parent of a genetic cross between two different strains can reject a graft from the offspring by recognizing MHC alleles inherited from the other parent Parungao-Balolong 2011
  17. 17. DISCOVERY! SEROLOGIC STUDIES IN HUMANS (Dausett et al) development of allogeneic blood transfusion and allogeneic organ transplantation in clinical medicine provided ways to detect and define genes that control rejection in humans OBSERVATIONS: patients who rejected kidneys or had transfusion reactions to WBC often develops circulating antibodies reactive with antigens on the WBC of the blood or organ donor in the presence of complement, the recipient’s serum would lyse lymphocytes obtained from the donor and also lyse lymphocytes obtained from some but not all third parties (individuals other than the blood or organ donor or the recipient) this sera which react against cells of allogeneic individuals are called alloantisera or allosera said to contain alloantibodies whose molecular targets are alloantigens (HLA) HLA or human leukocyte antigens: products of polymorphic genes that distinguish foreign tissues from self HLAs = H-2 in mice = MHC Parungao-Balolong 2011
  18. 18. Structureand Function Larger chain: alpha 1, 2 and 3 domain alpha 3-domain is an immunoglobulin fold and is the attachment point to the membrane alpha 1 and 2 domain forms an 8-stranded β-sheet that serves as a platform for peptide binding. Edges of the peptide binding site are defined by long a - helices, one from a 1 and one from a 2 alpha 3 is paired with β2 microglobulin, which also has a typical Ig fold β2 microglobulin is essential for stability and peptide binding CD8 on TC cells binds to the alpha domain Parungao-Balolong 2011
  19. 19. FUNCTION and PRODUCTION Parungao-Balolong 2011 PRODUCTION The peptides are mainly generated in the cytosol by the proteasome proteasome degrades intracellular proteins into small peptides that are then released into the cytosol The peptides have to be translocated from the cytosol into the (ER) to meet the MHC class I molecule, whose peptide-binding site is in the lumen of the ER FUNCTION display fragments of proteins from within the cell to T cells healthy cells will be ignored while cells containing foreign proteins will be attacked by the immune system Because MHC class I molecules present peptides derived from cytosolic proteins, the pathway of MHC class I presentation is often called the cytosolic or endogenous pathway
  20. 20. The peptide translocation from thecytosol into the lumen of the ER isaccomplished by the transporter TRANSLOCATIONassociated with antigen processing (TAP)= TAP 1 and 2The two subunits form a peptide bindingsite and two ATP binding sites that facethe lumen of the cytosolTAP binds peptides on the cytoplasmicsite and translocates them under ATPconsumption into the lumen of the ERThe MHC class I molecule is then in turnloaded with peptides in the lumen of theERThe peptide-loading process involvesseveral other molecules that form a largemultimeric complex consisting of TAP,tapasin, calreticulin, calnexin, and ERP57 Parungao-Balolong 2011
  21. 21. Once the peptide is loaded ontothe MHC class I molecule, it TRANSLOCATIONleaves the ER through thesecretory pathway to reach thecell surfaceThe transport of the MHC class Imolecules through the secretorypathway involves several post-translational modifications of theMHC moleculeexample: change to the N-glycanregions of the protein, followedby extensive changes to the N-glycans in the Golgi apparatus Parungao-Balolong 2011
  22. 22. GENES AND ISOTYPES Very Less polymorphic polymorphic HLA-A (HLA-A) HLA-E (HLA-E) HLA-B (HLA-B) HLA-F (HLA-F) HLA-C (HLA-C) HLA-G (HLA- G) HLA-K HLA-L Parungao-Balolong 2011
  23. 23. Structureand Function Almost identically sized a and b chains Each chain is divided into two segments, e.g. alpha 1 and 2; beta 1 and 2 alpha 2 and beta 2 are immunoglobulin domains that pair with each other alpha 2 and beta 2 are the point of membrane attachment alpha 1 and beta 1 form the peptide binding domain, conformation quite similar to Class I MHC, except, the ends are open allowing the binding of longer peptides CD4 on TH cells binds to beta 2 domain Parungao-Balolong 2011
  24. 24. FUNCTION found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen- presenting cells (APCs) The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I) MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway Loading of class II molecules: extracellular proteins are endocytosed, digested in lysosomes, and bound by the class II MHC molecule prior to the molecules migration to the plasma membrane Parungao-Balolong 2011
  25. 25. SYNTHESISresult of dimerization of α and β chains, with the assistance of an invariant chain =a special polypeptide involved in the formation and deliverance of MHC class IIproteinThe nascent MHC class II protein in the rough ER has its peptide-binding cleftblocked by the invariant chain (Ii; a trimer) to prevent it from binding cellularpeptides or peptides from the endogenous pathwayThe invariant chain also facilitates MHC class IIs export from the ER in a vesiclewhich fuses with a late endosome containing the endocytosed, degraded proteinsIt is then broken down in stages, leaving only a small fragment called CLIP whichstill blocks the peptide binding cleftAn MHC class II-like structure, HLA-DM, removes CLIP and replaces it with apeptide from the endosomeThe stable MHC class-II is then presented on the cell surface Parungao-Balolong 2011
  27. 27. Characteristic MHC-I pathway MHC-II pathway Polymorphic chain α and β2 Composition of the stable Polymorphic chains α and β, peptide microglobulin, peptide bound to α peptide-MHC complex binds to both chain Dendritic cells, mononuclear Types of antigen presenting cells (APC) All nucleated cells phagocytes, B lymphocytes, some endothelial cells, epithelium of thymus T lymphocytes able to Cytotoxic T lymphocytes Helper T lymphocytes respond (CD8+) (CD4+) cytosolic proteins (mostly synthetized Proteins present in endosomes or Origin of antigenic by the cell; may also enter from the lysosomes (mostly internalized from proteins extracellular medium via phagosomes) extracellular medium) Enzymes responsible for Proteases from endosomes and peptide generation Cytosolic proteasome lysosomes (for instance, cathepsin) Location of loading the Specialized vesicularpeptide on the MHC molecule Endoplasmic reticulum compartment Molecules implicated in transporting the peptides and TAP (transporter associated with loading them on the MHC antigen processing) DM, invariant chain molecules
  28. 28. IMPORTANT ASPECTS OF THE MHC• High polymorphism in MHC for • NO recombination mechanisms a species for creating diversity in MHC• Alleles for MHC genes are co- • Peptide must bind with dominant individual’s MHC to induce immune response• Each MHC gene product is expressed on surface of • MHC molecules are membrane- individual cell bound• Each MHC has ONE peptide • Recognition by Ts requires cell- binding site but each MHC can cell contact bind many different peptide one at a time (Peptide binding is “degenerate”) • Mature Ts must have TCR that recognizes particular MHC Parungao-Balolong 2011
  29. 29. T-CELL RECEPTOR (TCR)• role in immune response• Surface molecule on Ts • Recognize Ag presented in MHC context • Similar to Immunoglobulin• Two types of TCR • α β: predominant in lymphoid tissues • γ δ: enriched at mucosal surfaces Parungao-Balolong 2011
  30. 30. IMPORTANT ASPECTS OF TCR• Each T cell has TCR of only ONE specificity • Allelic exclusion • αβ TCR recognizes Ag only in the context of cell-cell interaction and in correct MHC context • γδ TCR recognizes Ag in MHC-independent manner • Response to certain viral and bacterial Ag Parungao-Balolong 2011
  31. 31. GENETIC BASIS FOR RECEPTOR GENERATION• Accomplished by recombination of V, D and J gene segments • TCR β chain genes have V, D, and J • TCR α chain genes have V and J Parungao-Balolong 2011
  32. 32. TCR AND CD3• TCR is closely associated with CD3 complex • Group of 5 proteins • Commonly called “invariant” chains of TCR• Role of CD3 complex • CD3 necessary for cell surface expression of TCR • transduces signal after Ag interaction with TCR Parungao-Balolong 2011
  34. 34. NICE TO KNOW (from wikipedia though...)• MHC and Sexual Selection• MHC plays a role in the selection of potential mates, via olfaction• MHC genes make molecules that enable the immune system to recognize invaders; in general, the more diverse the MHC genes of the parents the stronger the immune system of the offspring• It would be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with Parungao-Balolong 2011
  35. 35. NICE TO KNOW (from wikipedia though...)• MHC and Sexual Selection• Yamazaki et al. (1976) showed this to be the case for male mice, which show a preference for females of different MHC; similar results have been obtained with fish• Claus Wedekind (1995) determined MHC-dissimilar mate selection tendencies in humans • group of female college students smelled t-shirts that had been worn by male students for two nights, without deodorant, cologne, or scented soaps • An overwhelming number of women preferred the odors of men with dissimilar MHCs to their own • preference was reversed if they were taking oral contraceptives• Rates of early pregnancy loss are lower in couples with dissimilar MHC genes Parungao-Balolong 2011
  36. 36. • Describe the immune ASSIGNMENT response to: : WORK IN • A bacterial infection in your PAIRS arm • A bacterial infection from your intestinal tract • A viral infection • A cancerous cell in your body• Which MHC type is most likely to be involved? Parungao-Balolong 2011