The MHC complex: genetics, function and disease association

Lecturer: Adelheid Cerwenka, PhD, D080, Innate Immunity

Sources: Janeway: Immunobiology, 5th editionKuby: Immunology, 4th editionKlein/Horejsi:Immunology 2nd edition

Only complementary surfaces fit together

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)

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

• 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

• Introduction

• Structure of MHC I and II molecules

• Genetic organisation of the MHC

• Polymorphisms of MHC alleles

• MHC and disease

Table of contents

1.) Cell cell contact via cell surface receptors:cell surface proteins have been classified as CDs (=cluster of differentiation)

CD2

DCT cellMHCTCR

B7CD28

2.) Cell to cell contact via soluble mediators such as cytokines (interleukins-IL) or chemokines (CCR, CXCR)

DCT cellMHCTCR

B7CD28

IL-12

IFN-

Communication of cells in the body

Host defense

Against intracellular infection by viruses Against intracellular infection by mycobacteria

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

MHC class II molecules present antigen originating in intracellular vesicles

MHC molecules on the cell surface display peptide fragments

Structure of MHC class I

Computer graphic representationand ribbon diagramms of of the human MHC class I moleculeHLA-A2.

Heterodimer: chain (43 kDa): polymorphic2-microglobin (12 kDa): non-polymorphic, non-covalently bound

1 and 2: peptide binding, cleft formed by single structure3: transmembrane

Structure of MHC class II

Computer graphic representationand 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 bond2 and b2 : transmembrane

Peptide binding groove is the MHC class II molecules is open at both ends

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 of2 domain(green)

chain(purple)

chain (white)

2-Microglobuline(purple)

Chain (white)

Base of 3 domain(green)

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.

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 Ak allele, or human MHC II HLA-DR3Peptides 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 permissiveBlue: negatively charged residue D, aspartic acid, E glutamic acid, green: hydrophobic residues

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 cellsPlasmodia (malaria)live in red blood cells

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 BMHC 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

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

• 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

Genetic organisation of MHC

MHC diversity

MHC is polygenicmeans that it containsseveral different MHC class I and class II genes

MHC is polymorphic(poly=manyMorphic=shape, structure):

means that there are multiple variants of a gene within a population as a whole

Genetic organisation of the MHC

Mouse chromosome 17

Human chromosome 6

Detailed map of the human MHC

MHC class IB genes=Non-classical MHC Molecules=Non-conventional MHC Class I molecules

• 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

MHC class I receptors on human Natural killer cells

Receptors……………………………Ligands effectKIR receptors (Killer immunoglobulin receptors)…HLA-C mostly inhib.

NKG2A/CD94………………………..HLA-E mostly inhib.NKG2D……………………………….MIC activ.

1 2

3 m

1 2

3 m1 2 1 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

MHC class I related chain (MIC): ligands for human NKG2D

1 2

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

Lymphomas expressing mouse homologues of MIC molecules (RAE-1) are rejected

Lymphoma cells

Lymphoma cells+RAE-1

Polymorphism of MHC genes

The figures are the numbers of alleles currently officially assigned by the WHO100 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)=1012

Linkage disequilibrium occurs in human

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.

Diversity of MHC molecules expressed by an individual

Polygeny the presence of several different related genesWith similar function ensures that each individual produces a number of different MHC molecules

Allelic variation occurs at specific sites within MHC molecules

Allelic variability is clustered at specific sites within domains

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.

MHC restriction

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

History: MHC restriction

Zinkernagel and Dohety1975, JEM, 141:502

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

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

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

MHC-dependent mate preferences in humans ??

MHC and transplantation

Mating of inbred mouse strains with different MHC haplotypes

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.

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

T cells (CD4 and CD8 T cells) can transfer allograft rejection (1950. Mitchison)

Nude mice (have no T cells) even accept xenografts

Even complete matching does not ensure graft survival

1.) HLA typing not precise, complex polymorphisms, only siblings inherit the same haplotypes2.) 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.

Minor H antigens

2 different ways of graft recognition

Initiation of graft rejection: Dynamics of graft rejection

Hyper acute graft rejection

Preexisting antibody against donor graft antigens can cause hyperacute graft rejection

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).

Effect of antigen matching on the survival of kidney grafts

Tissues successfully transplanted

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

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

• Autoimmune disease

• Viral disease

• Neurologic disorders

• Allergic reactions

MHC and disease association

MHC genes and Pathogen defence

Population studies show association of susceptibility to IDDM with HLA genotype

Certain HLA genotype are frequently found in diabetic patientsDR3/4 tight linkage to DQ

Affected siblings share 2 HLA haplotypes much more frequently than expected

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 cells2.) Shaping of the T cell repertoire

• 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

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