1. The MHC complex: genetics, function and disease association Lecturer: Adelheid Cerwenka, PhD, D080, Innate Immunity Sources: Janeway: Immunobiology, 5th edition Kuby: Immunology, 4th edition Klein/Horejsi:Immunology 2nd edition

2. Only complementary surfaces fit together

3. MHC-structure Major Histocompatibility Complex (MHC): linked cluster of genes, which products play a role in intercellular recognition between self and nonself. The MHC is a region of multiple loci that play major roles in determining, whether transplanted tissue is accepted as self (histocompatible) or rejected as foreign (histoincompatible)

4. The concept of Histocompatibility A skin-graft transplanted from A donor to a genetically identical recipient is accepted, to a genetically disparate recipient is rejected

5. MHC = Major Histocombitibiliy Complex Minor Histocompatibility Antigens: proteins, which are cell surface expressed and their peptides are loaded into MHC molecules MHC is a generic name HLA = Human Leucocyte Antigen, eg SLA = Swine Leucocyte Antigen Mouse: MHC has an historical name = H2 (H-2) stands for histocompatibility 2 Nomenclature

6. Introduction Structure of MHC I and II molecules Genetic organisation of the MHC Polymorphisms of MHC alleles MHC and disease Table of contents

7. 1.) Cell cell contact via cell surface receptors: cell surface proteins have been classified as CDs (=cluster of differentiation) CD2 DC T cell MHC TCR B7 CD28 2.) Cell to cell contact via soluble mediators such as cytokines (interleukins-IL) or chemokines (CCR, CXCR) DC T cell MHC TCR B7 CD28 IL-12 IFN-  Communication of cells in the body

8. Host defense Against intracellular infection by viruses Against intracellular infection by mycobacteria

9. MHC class I molecules present antigen derived from proteins in the cytosol

10. MHC class II molecules present antigen originating in intracellular vesicles

11. MHC molecules on the cell surface display peptide fragments

12. Structure of MHC class I Computer graphic representation and ribbon diagramms of of the human MHC class I molecule HLA-A2. Heterodimer:  chain (43 kDa): polymorphic  2-microglobin (12 kDa): non- polymorphic, non-covalently bound  1 and  2: peptide binding, cleft formed by single structure  3: transmembrane

13. Structure of MHC class II Computer graphic representation and ribbon diagramms of of the human MHC class II molecule, HLA-DRI Heterodimer, 2 transmembrane chains:  chain (34 kDa) b-chain (29 kDa)  1 and  1: peptide binding, not joined by covalent bond  2 and b2 : transmembrane Peptide binding groove is the MHC class II molecules is open at both ends

14. Peptide binding sites and binding sites for CD4 or CD8 on MHC class I and MHC class II The binding sites for CD4 and CD8 on MHC class II molecules or MHC class I lie in the immunoglobulin domain, nearest to the membrane Base of  2 domain (green)  chain (purple)  chain (white)  2- Microglobuline (purple)  Chain (white) Base of  3 domain (green)

15. Peptides bind to MHC I molecules through structurally related anchor molecules Free amino and carboxy termini are stabilizing contacts Peptides eluted from two different MHC class I molecules are shown. Anchor residues in green: Not identical but related: eg: F and Y are both aromatic amino acids V, L and I are large hydrophobic amino acids MHC class I without peptide instable Pockets in the MHC molecules are lined by polymorphic amino acids.

16. Peptides that bind MHC class II are variable in length and anchor residues lie at various distances from the ends of the peptide Peptides that bind to mouse MHC II A k allele, or human MHC II HLA-DR3 Peptides that bind to MHC class II are at least 13-17 AA long, Ends of peptides are not conserved. Ends do not bind, binding pockets more permissive Blue: negatively charged residue D, aspartic acid, E glutamic acid, green: hydrophobic residues

17. The expression of MHC molecules differs between tissues MHC class I: Expressed on all nucleated cells MHC class II: Expressed on surface of APCs (antigen presenting cells) Viruses can infect all types of cells Plasmodia (malaria) live in red blood cells

18. Regulation of MHC class I expression Expression of MHC class I regulated by sequences upstream of the coding part. MHC enhancer segment: enhancer A, IRE interferon response element, enhancer B MHC class I expression can be regulated by Interferon (IFN-  ). IFN-  also induces the key components of the intracellular machinery that enables peptides to be loaded onto MHC class I molecules

19.  T cells are not restricted by classical MHC molecules They may be specialized to bind certain types of ligands (heatshock proteins, mycobacterial lipid antigens) directly or presented by non-classical MHC molecules. T cells bearing a  T cell receptor

20. MHC class I and II molecules have different structure, different distribution on cells in the body, and different function Peptides, that bind to MHC class I or II are derived of different compartments and are of different length The expression of MHC class I molecules can be regulated by interferon- . Conclusion: Structure of MHC molecules

21. Genetic organisation of MHC

22. MHC diversity MHC is polygenic means that it contains several different MHC class I and class II genes MHC is polymorphic (poly=many Morphic=shape, structure): means that there are multiple variants of a gene within a population as a whole

23. Genetic organisation of the MHC Mouse chromosome 17 Human chromosome 6

24. Detailed map of the human MHC MHC class IB genes =Non-classical MHC Molecules =Non-conventional MHC Class I molecules

25. Ligands of inhibitory (HLA-G) or activating (MIC) Natural Killer cell receptors Presentation of non-conventional peptides to ?? Cells: In mice, the H-2M locus encodes a nonconventional MHC class I molecule that present peptides that have a formylated methionin (eg also found in prokaryotic organisms such as mycobacterium tuberculosis, listeria, Salmonella) Presentation of lipid antigens (CD1) Function of non-conventional MHC molecules

26. MHC class I receptors on human Natural killer cells Receptors……………………………Ligands effect KIR receptors (Killer immunoglobulin receptors)…HLA-C mostly inhib. NKG2A/CD94………………………..HLA-E mostly inhib. NKG2D……………………………….MIC activ.

27. 11 22 33  m 11 22 33 11 22 11 22 Classical MHC I human MICA, B ULBP-human RAE-1- like human NKG2D-ligands mouse NKG2D-ligands RAE-1, H60 MHC class I-like ligands for the activating receptor NKG2D

28. MHC class I related chain (MIC): ligands for human NKG2D 11 22 33 polymorphic MIC = non-conventional MHC molecule Expression absent from healthy tissue,overexpressed on tumors and in the gut epithelium A soluble form of MICA is found in the serum of cancer patients Expression induced by heat shock, viral infection and bacteria

29. Lymphomas expressing mouse homologues of MIC molecules (RAE-1) are rejected Lymphoma cells +RAE-1

30. Polymorphism of MHC genes The figures are the numbers of alleles currently officially assigned by the WHO 100 different class I or class II alleles in mice H-2 complex: theoretical diversity is: 100 (K) x 100 (IA  )x 100 (IEa) x 100 (IEb) x 100 (D)=10 12 Linkage disequilibrium occurs in human

31. Expression of MHC alleles is co dominant 4 possible combinations of haplotypes are found in the offspring, there being one chance in four that an individual will share both haplotypes with a sibling.

32. Diversity of MHC molecules expressed by an individual Polygeny the presence of several different related genes With similar function ensures that each individual produces a number of different MHC molecules

33. Allelic variation occurs at specific sites within MHC molecules Allelic variability is clustered at specific sites within domains

34. Gene conversion and new alleles Sequences can be transferred from one gene to a similar but different gene by a process know as gene conversion. This can occur by a misalignment of two paired homologous chromosomes When there are many copies of similar genes arrayed in tandem. Polymorphisms have been actively selected during evolution.

35. MHC restriction The antigen specific T cell receptor recognizes a complex of antigenic peptide and MHC.

36. History: MHC restriction Zinkernagel and Dohety 1975, JEM, 141:502

37. Many T cells respond to superantigens Superantigens (produced by bacteria and viruses) can bind independently to MHC class II molecules and TCR, binding to the V  domain of the TCR. Stapphylococcal enterotoxins (SE) cause food poisoning and toxic shock syndrome

38. Conclusion: Polymorphism of MHC Extensive polymorphism can extend the range of antigens to which the immune system can respond. It is an advantage for the survival of the species It has evolved to outflank evasive strategies of pathogens. Pathogens are clever: they can evade detection or can suppress host responses. Exposure to select for expression of particular MHC alleles: strong association of HLA-B53 with recovery from malaria Why not more MHC loci? For maintenance of self-tolerance

39. Cheetah were bred from limited breeding stock: limited polymorphism. Disadvantage for survival?

40. MHC-dependent mate preferences in humans ??

41. MHC and transplantation

42. Mating of inbred mouse strains with different MHC haplotypes

43. Various MHC molecules expressed on antigen presenting cells of a heterozygous H-2 k/d mouse Diversity generated by these mechanisms presumably increases the number of antigenic peptides that can be presented and thus is advantageous to the organism.

44. Skin transplantion between between different mouse strains with same or different MHC haplotype

45. T cells (CD4 and CD8 T cells) can transfer allograft rejection (1950. Mitchison) Nude mice (have no T cells) even accept xenografts

46. Even complete matching does not ensure graft survival 1.) HLA typing not precise, complex polymorphisms, only siblings inherit the same haplotypes 2.) Minor histocompatibility antigens exist, peptides from polymorphic proteins presented by the MHC molecules on the graft. Although MHC genotype can be matched, polymorphism in any other gene can graft rejection.

47. Minor H antigens

48. 2 different ways of graft recognition

49. Initiation of graft rejection: Dynamics of graft rejection

50. Hyper acute graft rejection Preexisting antibody against donor graft antigens can cause hyperacute graft rejection

51. Mixed lymphocyte reaction Allogeneic bone marrow transplantion: often graft versus host disease (rashes, diarrhea, pneumonitis). Also because of minor H anitgen difference with siblings. Tests with MLR (mixed lymphocyte reaction).

52. Effect of antigen matching on the survival of kidney grafts

53. Tissues successfully transplanted

54. Pregnancy: The fetus is an allograft that is tolerated repeatedly. Fetus carries parental MHC and minor H antigens that differ from the mother. Trophoblast and immunosuppressive cytokines (low MHC class I) protects fetus

56. Conclusion: MHC and transplantation Most transplants need generalized immunosuppression (toxic) MHC matching often not sufficient for graft survival (minor H antigens) Tolerance to fetus is the key for a species to survive

57. Autoimmune disease Viral disease Neurologic disorders Allergic reactions MHC and disease association

58. MHC genes and Pathogen defence

59. Population studies show association of susceptibility to IDDM with HLA genotype Certain HLA genotype are frequently found in diabetic patients DR3/4 tight linkage to DQ  Affected siblings share 2 HLA haplotypes much more frequently than expected

60. Position of the DQ  chain affects susceptibility to insulin-dependent diabetes mellitus AA 57 forms a salt bridge Across the peptide binding cleft of DQ Possible explanation: 1.) Allelic variants of MHC molecules differ in ability to present the autoantigenic peptides to autoreactive T cells 2.) Shaping of the T cell repertoire

61. Both inherited and environmental factors play a role in the induction of autoimmune disease Inbred mice show uniform susceptibility to autoimmune disease But also other independly segregating disease susceptibility loci have been defined Also amount of self antigen transcribed in the thymus plays a role Significant associations of HLA Alleles with increased risk for various diseases

62. In the fight against viruses and tumors: high MHC I expression on target cells: good or bad ?? NK cell CD8 cell Tumor cell: lots of MHC I No lysis Lysis NK cell CD8 cell Tumor cell: little MHC I Lysis No Lysis