The Major Histocompatibility Complex (MHC)

Transcription

1 Advanced molecular immunology-2011 The Major Histocompatibility Complex (MHC) Youmin Kang

2 Activation IFN-g IL-4 IL-2 YYYYYY YYYYY B cells

3 MHC class II pathway

4 Discovery of MHC MHC Class I pathway

5 Genetic basis of transplant rejection Inbred mouse strains - all genes are identical Transplantation of skin between strains showed that rejection or acceptance was dependent upon the genetics of each strain ACCEPTED Skin from an inbred mouse grafted onto the same strain of mouse REJECTED Skin from an inbred mouse grafted onto a different strain of mouse

6 Immunological basis of graft rejection 6 months Primary rejection of strain skin e.g. 10 days Naïve mouse Secondary rejection of strain skin e.g. 3 days Primary rejection of strain skin e.g. 10 days Transplant rejection is due to an antigen-specific immune response with immunological memory.

7 Immunogenetics of graft rejection Parental strains A X B F1 hybrid (one set of alleles from each parent) A x B A x B ACCEPTED A REJECTED Mice of strain (A x B) are immunologically tolerant to A or B skin Skin from (A x B) mice carry antigens that are recognised as foreign by parental strains B

8 T cells respond to MHC antigens Graft rejection in vivo is mediated by infiltrating T lymphocytes The in-vitro correlate of graft rejection is the MIXED LYMPHOCYTE REACTION Responder cells from an MHC-A mouse T + Irradiated stimulator cells from an MHC-A mouse T cells do not respond T Responder cells from an MHC-A mouse T + Irradiated stimulator cells from an MHC-B mouse T cells respond T T T T T T T MHC antigens are involved in the activation of T cells

9 MHC directs the response of T cells to foreign antigens Graft rejection is an unnatural immune response MHC antigens PRESENT foreign antigens to T cells Cells that present antigen are ANTIGEN PRESENTING CELLS T Y Ag T Y TYY T T Y Y T Blocking anti-mhc antibody Y Y Y T Y Ag Anti response No anti response

10 MHC Every vertebrate animals contain MHC Humans MHC HLA (Human Leukocyte Antigen) Mouse MHC H-2 Rat MHC H-1 Chimpanzee MHC ChLA Swine MHC-SLA

11 MHC complex

12 MHC molecules MHC class I Peptide MHC class II Peptide binding groove Cell Membrane

13 Overall structure of MHC class I molecules MHC-encoded -chain of 43kDa m -chain anchored to the cell membrane Peptide antigen in a groove formed from a pair of -helicies on a floor of anti-parallel strands 2-microglobulin, 12kDa, non-mhc encoded, non-transmembrane, non covalently bound to - chain 3 domain & 2m have structural & amino acid sequence homology with Ig C domains Ig GENE SUPERFAMILY

14 MHC class I molecule structure Peptide -chain 2-microglobulin Chains Structures

15 Overall structure of MHC class II molecules MHC-encoded, -chain of 34kDa and a -chain of 29kDa and chains anchored to the cell membrane No -2 microglobulin Peptide antigen in a groove formed from a pair of -helicies on a floor of anti-parallel strands 2 & 2 domains have structural & amino acid sequence homology with Ig C domains Ig GENE SUPERFAMILY

16 Structure of MHC class II molecules 1 and 2 domains form two segmented - helicies on eight anti-parallel -strands to form an antigen-binding cleft. Chains Structures Properties of the inner faces of the helicies and floor of the cleft determine which peptides bind to the MHC molecule

17 MHC class II molecule structure Peptide -chain -chain Cleft is made of both and chains

18 Cleft geometry MHC class I MHC class II Peptide is held in the cleft by non-covalent forces

19 Cleft geometry -chain -chain Peptide 2-M Peptide -chain MHC class I accommodate peptides of 8-10 amino acids MHC class II accommodate peptides of >13 amino acids

20 Genes encode the MHC

21 Differential distribution of MHC molecules Tissue MHC class I MHC class II T cells +++ +/- B cells Macrophages Other APC Epithelial cells of thymus Neutrophils Hepatocytes + - Kidney + - Brain + - Erythrocytes - - Cell activation affects the level of MHC expression The pattern of expression reflects the function of MHC molecules: Class I is involved in anti-viral immune responses Class II involved in activation of other cells of the immune system

22 A flexible binding site? A binding site that is flexible at an early, intracellular stage of maturation Formed by folding the MHC molecules around the peptide. Floppy Compact Allows a single type of MHC molecule to bind many different peptides bind peptides with high affinity form stable complexes at the cell surface Export only molecules that have captured a peptide to the cell surface

23 Peptides can be eluted from MHC molecules Purify stable MHCpeptide complexes Fractionate and microsequence peptides Acid elute peptides

24 Peptides can be eluted from MHC molecules

25 Eluted peptides from MHC molecules have different sequences but contain motifs A common sequence in a peptide antigen that binds to an MHC molecule is called a MOTIF Amino acids common to many peptides tether the peptide to structural features of the MHC molecule ANCHOR RESIDUES Tethering amino acids need not be identical but must be related Y & F are aromatic V, L & I are hydrophobic Side chains of anchor residues bind into POCKETS in the MHC molecule N T Y Q R T R L V S Y F P E I H I K Y Q A V T T L S Y I P S A K I R G Y V Y Q Q L S I I N F E K L A P G N Y P A L Different types of MHC molecule bind peptides with different patterns of conserved amino acids C

26 MHC molecules can bind peptides of different length Arched peptide S Y I P S S A K I A Y K S I P S I MHC molecule MHC molecule Complementary anchor residues & pockets provide the broad specificity of a particular type of MHC molecule for peptides Peptide sequence between anchors can vary Number of amino acids between anchors can vary

27 Peptide antigen binding to MHC class II molecules Negatively charged Hydrophobic I I S P D N N Q L T L F K L S D S N T K Y F H K D G R I K Y T L N A T K Y G N M T E D H V N H L L Q N A P P E V T G K V F T V L Y G H G F L T G A A I R L L L T F N S P Y T S R T S P A D V Y T Y K K S N P I I R T Y K V P E T S L S Y I A S G F R Q G G A S Q Y Y L R E P F S W A E L Y P T T D N Y V V Anchor residues are not localised at the N and C termini Ends of the peptide are in extended conformation and may be trimmed Motifs are less clear than in class I-binding peptides Pockets are more permissive

28 MHC-binding peptides Each human usually expresses: 3 types of MHC class I (A, B, C) and 3 types of MHC class II (DR, DP,DQ) The number of different T cell antigen receptors is estimated to be 1,000,000,000,000,000 Each of which may potentially recognise a different peptide antigen How can 6 invariant molecules have the capacity to bind to 1,000,000,000,000,000 different peptides?

29 MHC molecules Adopt a flexible floppy conformation until a peptide binds Fold around the peptide to increase stability of the complex Use a small number of anchor residues to tether the peptide this allows different sequences between anchor different lengths of peptide

30 MHC molecules are targets for immune evasion by pathogens T cells can only be activated by interaction between the antigen receptor and peptide antigen in an MHC molecule Without T cells there can be no effective immune response There is strong selective pressure on pathogens to evade the immune response The MHC has evolved two strategies to prevent evasion by pathogens More than one type of MHC molecule in each individual Extensive differences in MHC molecules between individuals

31 Example: If MHC X was the only type of MHC molecule MHC XX Pathogen that evades MHC X Survival of individual threatened Population threatened with extinction

32 Example: If each individual could make two MHC molecules, MHC X and Y Pathogen that evades MHC X but has sequences that bind to MHC Y MHC XX MHC YY MHC XY Impact on the individual depends upon genotype Population survives

33 Example: If each individual could make two MHC molecules, MHC X and Y and the pathogen mutates Pathogen that evades MHC X but has sequences that bind to MHC Y.until it mutates to evade MHC Y MHC XX MHC YY MHC XY Survival of individual threatened Population threatened with extinction The number of types of MHC molecule can not be increased ad infinitum

34 Populations need to express variants of each type of MHC molecule The rate of replication by pathogenic microorganisms is faster than human reproduction In a given time a pathogen can mutate genes more frequently than humans and can easily evade changes in MHC molecules The number of types of MHC molecules are limited To counteract the flexibility of pathogens: The MHC has developed many variants of each type of MHC molecule These variants may not necessarily protect all individuals from every pathogen, but will protect the population from extinction

35 Variant MHC molecules protect the population Pathogen that evades MHC X and Y but binds to the variant MHC X R and MHC Y R MHC XX MHC YY MHC XY XX XX R XY R YX R YY R YY X R X R From 2 MHC types and 2 variants. 10 different genotypes Y R Y R X R Y R XY MHC YY R MHC XX R Variants of each type of MHC molecule increase the resistance of the population from rapidly mutating or newly encountered pathogens without increasing the number of types of MHC molecule

36 Molecular basis of MHC types and variants POLYGENISM( 多元 ) Several MHC class I and class II genes encoding different types of MHC molecule with a range of peptide-binding specificities. POLYMORPHISM( 多态 ) Variation >1% at a single genetic locus in a population of individuals MHC genes are the most polymorphic known The type and variant MHC molecules do not vary in the lifetime of the individual The diversity in MHC molecules exists at the population level This sharply contrast diversity in T and B cell antigen receptors which exists within the individual

37 Simplified map of the HLA region DP DM LMP/TAP DQ 1 3 DR 4 5 B C A MHC Class II Class III MHC Class I Polygeny CLASS I: 3 types HLA-A, HLA-B, HLA-C (sometimes called class Ia genes) CLASS II: 3 types HLA-DP HLA-DQ HLA-DR. 3 extra DR genes in some individuals can allow 3 extra HLA-DR molecules Maximum of 9 types of antigen presenting molecule allow interaction with a wide range of peptides.

38 Map of the Human MHC from the Human Genome Project 3,838,986 bp 224 genes on chromosome 6 The MHC sequencing consortium Nature 401,

39 Simplified map of the mouse MHC Chromosome 17 K M LMP/TAP A E D L Class I Class II Class III Class I Similar organisation to the human MHC except: one class I gene is translocated relative to human MHC 2 pairs of genes encoding class II molecules no alternative class II chains

40 No of polymorphisms Polymorphism in MHC Class I genes Variation >1% at a single genetic locus in a population of individuals Each polymorphic variant is called an allele In the human population, nearly 1000 MHC class I alleles have been identified - some are null alleles, synonyms or differ in regions outside the coding region Class I 998 alleles (657 in July 2000) A B C E F G Data from October 2003

41 No of polymorphisms Polymorphism in MHC Class II genes In the human population, over 1,200 MHC alleles have been identified - some are null alleles, synonyms or differ in regions outside the coding region 356 Class II 668 alleles (492 in July 2000) A B1 B2-9 A1 B1 A1 B1 DR A B A B DQ DP DM DO Data from October 2003

42 No of serologicallydefined antigens Diversity of MHC Class I and II antigens Because so many MHC class I & II alleles are null, or contain synonymous mutations, the diversity of MHC molecules that can be identified by antibodies, i.e. SEROLOGICALLY, is considerably fewer than that by DNA sequencing Class I - ~100 antigens Class II - ~40 antigens A B C DR DQ DP Data from October 2003

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44 Allelic polymorphism is concentrated in the peptide antigen binding site Class I m Class II (HLA-DR) Polymorphism in the MHC affects peptide antigen binding Allelic variants may differ by 20 amino acids

45 Most polymorphisms are point mutations 30 HLA-DP allele sequences between Nucleotides 204 and 290 (amino acids 35-68) Polymorphic nucleotides encode amino acids associated with the peptide binding site E-A Y-F A-V Silent A-D A-E DPB1*01011 TAC GCG CGC TTC GAC AGC GAC GTG GGG GAG TTC CGG GCG GTG ACG GAG CTG GGG CGG CCT GCT GCG GAG TAC TGG AAC AGC CAG AAG GAC ATC CTG GAG GAG DPB1* A DPB1* T- -T A- -A DPB1* T- -T AC -A DPB1*0202 CT- -T AG DPB1*0301 -T- -T A- -A- --C C DPB1*0401 -T DPB1*0402 -T- -T A- -A DPB1*0501 CT- -T AG DPB1*0601 -T- -T A- -A- --C C DPB1*0801 -T- -T A- -A DPB1*0901 -T- -T A- -A- --C DPB1*1001 -T- -T A- -A DPB1* A C DPB1* A C DPB1* DPB1*1401 -T- -T A- -A- --C C DPB1* A C DPB1*1601 -T- -T A- -A DPB1*1701 -T- -T A- -A- --C DPB1*1801 -T- -T A- -A DPB1*1901 -T- -T AG DPB1* T- -T A- -A- --C C DPB1* T- -T A- -A- --C C DPB1*2101 CT- -T AG DPB1*2201 CT- -T AG DPB1*2301 -T- -T DPB1*2401 -T AG DPB1*2501 -T- -T A- -A C DPB1* A DPB1* I-L

46 How diverse are MHC molecules in the population? IF each individual had 6 types of MHC the alleles of each MHC type were randomly distributed in the population any of the 1,200 alleles could be present with any other allele ~6 x unique combinations In reality MHC alleles are NOT randomly distributed in the population Alleles segregate with lineage and race Group of alleles HLA-A1 HLA- A2 HLA- A3 HLA- A28 HLA- A36 Frequency (%) CAU AFR ASI

47 Diversity of MHC molecules in the individual DP DQ DR 1 B C A Polygeny HAPLOTYPE 1 DP DQ DR 1 B C A Variant alleles polymorphism HAPLOTYPE 2 DP DQ DR 1 B C A Additional set of variant alleles on second chromosome MHC molecules are CODOMINANTLY ( 共显性 )expressed Two of each of the six types of MHC molecule are expressed Genes in the MHC are tightly LINKED and usually inherited in a group The combination of alleles on a chromosome is an MHC HAPLOTYPE

48 Inheritance of MHC haplotypes DP-1,2 DQ-3,4 DR-5,6 B-7,8 C-9,10 A-11,12 DP-9,8 DQ-7,6 DR-5,4 B-3,2 C-1,8 A-9,10 Parents DP DQ DR DP DQ DR X DP DQ DR DP DQ DR B C B C B C B C A A A A Children DP-1,9 DQ-3,7 DR-5,5 B-7,3 C-9,1 A-11,9 DP-1,8 DQ-3,6 DR-5,4 B-7,2 C-9,8 A-11,10 DP-2,8 DQ-4,6 DR-6,4 B-8,2 C-10,8 A-12,10 DP-2,9 DQ-4,7 DR-6,5 B-8,3 C-10,10 A-12,9 DP DQ DR DP DQ DR DP DQ DR DP DQ DR DP DQ DR DP DQ DR DP DQ DR DP DQ DR B C B C B C B C B C B C B C B C A A A A A A A A

49 Errors in the inheritance of haplotypes generate polymorphism in the MHC by gene conversion and recombination A B C A B C A B C Multiple distinct but closely related MHC genes A B C During meiosis chromosomes misalign A B C Chromosomes separate after meiosis DNA is exchanged between haplotypes GENE CONVERSION A B C RECOMBINATION between haplotypes In both mechanisms the type of MHC molecule remains the same, but a new allelic variant may be generated

50 Polymorphism and polygenism in the MHC provides the diversities for various antigens To protect the population from pathogens evading the immune system

51 MHC Associated Problems Transplantation Autoimmune disease Infection Vaccine: non-responder

52 Genetic barriers to transplantation Autologous( 自体同源 ): in the same individual isologous: between genetically Identical individuals, i.e., identical twins (inbred animals) homologous: between individuals of the same species heterologous: between individuals different species

53 Graft versus host disease 移植物抗宿主反应

54 Graft versus host disease

55 A clinically relevant application of MHC genetics: Matching of transplant donors and recipients The diversity and complexity of the MHC locus and its pattern of inheritance explains: The need to match the MHC of the recipient of a graft with the donor The difficulty of finding an appropriate match from unrelated donors The ~20% chance of finding a match in siblings

56 How many loci should I match? Locus HLA-A HLA-B HLA-C HLA-DRB1 DRB3, 4 and 5 HLA-DQA1 and B1 HLA-DPA1 and B1 Recommendation Yes Yes Yes Yes Unknown Unclear Unclear

57 How many loci should be typed? Locus HLA-A HLA-B HLA-C HLA-DRB1 DRB3, 4 and 5 HLA-DQA1 and B1 HLA-DPA1 and B1 Recommendation Yes, allele level Yes, allele level Yes, allele level Yes, allele level Yes, for linkage DQB1 for linkage Unclear

58 Methods to type HLA Complement killing Mixed lymphocyte reaction (MLR) Gene chip

59 Serological tissue typing: the scheme

60 Tissue typing by the mixed lymphocyte reaction (MLR)

61 Tissue typing: the MLR scheme

62 Lymphocyte proliferation during MLR

63 Mechanisms of graft rejection IL2, TNF-, IFN-g TNF-, NO Inflammation IL2, IFN-g IL2, IL4, IL5 lysis ADCC lysis rejection

64 Examples in medical DNA diagnostics HLA typing Chip DQA1 of the chip as an example HLA-DQA1: 22 alleles Sequence-specific oligos out of 13 polymorphic regions of exon 2 and 3 of the HLA-DQA1-gene on the chip combination of 3 oligonucleotides per allele allow the typing of the 22 alleles

65 Allel controls a b c Oligo variants = Cy5-control = Perfect match = 1 mismatch a b c

66 One Amplicon multiple internal solid phase primer HLA - DQA haplotypes

67 Diseases Related to Specific HLAs 风湿热 爱迪生氏病 /( 肾上腺性 ) 青铜色皮肤病 甲状腺机能亢进 何杰金病, 淋巴肉芽肿病

68 Association with AIDS Protection to AIDS is assciated with HLA-B*27 and -B*57 allotypes whereas susceptibility to AIDS progression is observed in patients with HLA-B*35 specificities Carrington, M. and Bontrop, R.E. (2002) Effects of MHC class I on HIV/SIV disease in primates. AIDS 16, s105 s114

69 Evidence of HIV-1 Adaptation to HLA-Restricted Immune Responses at a Population Level HLA-B*5101 TAFTIPSI TAFTIPST fold decreases in CTL activity SCIENCE VOL MAY 2002

70 MHC-I resistance gene products and relevant epitopes in HIV-1- and SIV-infected primates

71 Three pathways for antigen process

72 CD4 T-CELL CD3 CD8 T-CELL CD3 CD4 TCR CD8 TCR MHC CLASS II aa peptide MHC CLASS I aa peptide 2 m ANTIGEN PRESENTING CELL ANTIGEN PRESENTING CELL

73 Thank you