Major Histocompatibility Complex (MHC)

Prof. Dr. Sabiha PitImmunology

GIM Block, Session 2009 / 2010

The major histocompatibility complex

• The principal functions of T lymphocytes are: - defense against intracellular microbes. - activation and interaction of other cells (infected host cells, dendritic cells, macrophages and B cells).

• T cells are able to interact with these cells because - the antigen receptor on T cells can only recognize antigens that are displayed on these cells.

• T cells can recognize cell-associated antigens.

• Cell-associated antigens which are recognized by the T cells are specialized proteins which are encoded by genes in a locus called the major histocompatibility complex (MHC).

There are 2 main types of MHC gene products:

• Class I MHC molecules

• Class II MHC molecules

Properties of MHC genes

• Human MHC molecules are called human leucocyte antigens (HLA) and are equivalent to the H-2 molecules of mice.

• The 2 types of polymorphic MHC genes, namely the class I and Class II MHC genes, encode 2 groups of structurally distinct but homologous proteins.- Class I MHC molecules present peptides to CD8+

cytolytic T lymphocytes (CTLs)- Class II MHC molecules present peptides to CD4+ helper T cells.

Properties of MHC genes

• MHC genes are the most polymorphic genes present in the genome.

- eg. For some HLA loci, more than 250 alleles have been identified by serological tests.

- Molecular sequencing has shown a single serologically defined HLA allele may actually consist of multiple variants that differ slightly – polymorphism is even greater than that predicted from serological studies.

Allelic Polymorphism

There are a large number of genetic variants (alleles) at each genetic locus.

Many of these alleles are represented at significant frequency (>1%) in the population.

The alleles generally differ from one another by many (up to 30) amino acid substitutions.

Properties of MHC genes• MHC genes are co-dominantly expressed in each individual.

- Each individual expresses both the MHC alleles that are inherited from the 2 parents.

- This maximises the number of MHC molecules available to bind to peptides for presentation to T cells.

• The set of MHC alleles present on each chromosome is called an MHC haplotype. eg the haplotype of an individual could be HLA-A2, HLA-B5, HLA-DR3 and so on.

• An individual has 2 HLA haplotypes.- In humans, certain HLA alleles at different loci are inherited together more frequently than would be predicted by random assortment, a phenomenon called linkage disequilibrium.

Properties of the MHC molecules

• Each MHC molecule consists of :- an extracellular peptide-binding cleft (groove). - a pair of immunoglobulin (Ig)–like domains and is anchored to the cell by transmembrane and cytoplasmic domains.

Class I MHC molecules are composed of a polymorphic α (heavy) chain noncovalently attached to a non-polymorphic polypeptide called β2 microglobulin.Peptide binding cleft of Class I molecules is formed by the α1 and α2 of the α chain.Ig-like domains of Class I molecules contain the binding sites for T cell coreceptors CD8

Common structure• Extracellular peptide-binding cleft

•A nonpolymorphic Ig-like region- Transmembrane region- Cytoplasmic region

STRUCTURE OF CLASS I MHCMOLECULE eg HLA-B27X-ray Crystallography

Common structure•Extracellularpeptide-binding cleft

•A nonpolymorphic Ig-like region- Transmembrane region- Cytoplasmic region

Class II MHC molecules contain two MHC-encoded polymorphic chains, an α chain and β chain.Peptide binding cleft of Class II molecules is formed by the α1 and β1 of the two chains.Ig-like domains of Class II molecules contain the binding sites for T cell coreceptors CD4

STRUCTURE OF CLASS II MHCMOLECULE eg. HLA-DR1X-ray Crystallography

Major Histocompatability Complex (MHC)

Features of class I and Class II MHC molecules

Characteristics of Peptide-MHC interactions

• MHC molecules show a broad specificity for peptide binding and the fine specificity of antigen recognition resides largely in the antigen receptors of T lymphocytes.

• Each Class I or Class II molecule has a single peptide-binding cleft that can accommodate many different peptides.- If a T cell specific for one peptide is stimulated by

APC presenting the peptide, the response is inhibited by the addition of an excess of other, structurally similar peptides.

Antigen competition for T cells

A T cell recognizes a peptide presented by one MHC molecule.

An excess of a different peptidethat binds to the same MHC molecule competitively inhibits the presentation of the peptidethat the T cell recognizes.

Characteristics of Peptide-MHC interactions

• The peptides that binds to MHC molecules share structural features that promote this interaction.- MHC Class I molecules can accommodate peptides that are 8 to 11 residues long.- MHC Class II molecules can accommodate peptides 10-30 residues long.

Characteristics of Peptide-MHC interactions

• The MHC molecules of an individual do not discriminate between foreign peptides (eg those derived from microbial antigens) and peptides derived from the antigens of that individual (self antigens).

Genomic organization of the MHC

• In human, the MHC is located on the short arm of the chromosome 6 and β microglobulin is encoded by a gene on chromosome 15.

• The human MHC occupies a large segment of DNA, extending about 3500 kilobases (kb).

Genomic organization of the MHC

• Class I genes: HLA-A, HLA-B, HLA-C• Class I-like genes:

- HLA-E, HLA-F, HLA-G, HLA-J, HLA-X. - HLA-H does not appear to be involved in the immune system.- These resemble Class I but exhibit little or no

polymorphism.- Some encode proteins that are expressed in association with β2 microglobulin and are called Class IB molecules.

Genomic organization of the MHC

Class II MHC genes• HLA-DP, HLA-DQ, HLA-DR• Within the Class II locus are genes that code for several

proteins that play critical roles in antigen processing.- eg. Transporter associated with antigen processing (TAP)- Proteosomes – degrades proteins to peptides that are

subsequently presented by the MHC Class I molecules.

Class III MHC genes code for• Several components of the complement system.• Three structurally related cytokine: tumor necrosis factor,

lymphotoxin and lymphotoxin-β.• Some heat shock proteins.

MAP OF THE HUMAN MHCClass I: HLA-A, HLA-B, HLA-CClass I – like molecules: HLA-E, HLA-F, HLA-G, HLA-J and HLA-X

Class II: HLA-DP, HLA-DQ, HLA-DR proteosome genes

TAP genesClass III: Complement proteins- C4,C2, factor B

Tumor necrosis factor (TNF), LT (lymphotoxin), LTβ (lymphotoxin β

Expression of MHC molecules

• Class I molecules are constitutively expressed on virtually all nucleated cells.

• Class II molecules are normally expressed on only dendritic cells, B lymphocytes, macrophages and a few other cell types (endothelial cells and thymic epithelial cells).

Expression of MHC molecules

Cell Types MHC Class I MHC Class IIT cells +++ Varies, inducible in

some speciesB cells +++ ++Macrophages +++ +Dendritic cells +++ x 10 +++ x 10Granulocytes ++ -Endothelium ++ -Hepatocytes + -

Expression of MHC molecules

• The expression of MHC molecules is increased by cytokines produced during both the innate and adaptive immune responses.- On most cell types, the interferon IFNα, IFNβ and IFNγ {also tumor necrosis factor (TNF) and lymphotoxin (LT)} increase the level of expression of class I molecules- IFNγ is the principal cytokine involved in stimulating expression of class II molecules, in antigen presenting cells such as macrophages.

Enhancement of class II MHC expression by IFNγ

IFNγ produced by NK cells during innate immune reactions to microbes or by T cells during adaptive immune reactions,stimulates Class II MHC expression on antigen-presenting cells (APC) and thus enhances the activation of CD4+ T cells.

IFNγ has similar effect on the expression of Class II MHC molecules and the activation of CD8+ T cells.

Importance of MHC

Regulation of immune response- The products of the MHC play a fundamental

role in regulating immune response. - T cells must recognize antigen as a complex with

MHC. Parentage testing. Association with diseases. Major transplantation antigen : HLA matching. Forensic identification testing.

Haplotype of the off-springs HLA-A1, B37 HLA- A1, B37 HLA- A9, B40 HLA-A9, B40 HLA-A3, B5 HLA-A3, B5 HLA-A28,B8 HLA-28, B8

HaplotypeHLA-A3, B5HLA-A28,B8

HaplotypeHLA- A1,B37HLA-A9,B40

MHC inheritance

Father Mother

Antigen HLA: HLA - A 2 10 Antigen HLA : HLA- A 9 10HLA - B 5 12 HLA - B 8 16

X

Haplotype: HLA-A 2, B 5 Haplotype: HLA -A 9, B 16 HLA-A10, B12 HLA -A10, B 8

HLA antigen: HLA antigen HLA antigen HLA - A 2 9 HLA - A 2 10 HLA - A 10HLA – B 5 16 HLA – B 5 8 HLA – B 8 12

Haplotype Haplotype Haplotype HLA-A2, B5 HLA-A2, B5 HLA-A10, B8HLA-A9, B16 HLA-A10, B8 HLA-A10, B12

Association of HLA with certain diseases

Certain HLA alleles are found more frequently in patients with specific diseases than in the normal population.

A relative risk can be calculated which expresses the increased likelihood of an individual possessing the associated allele developing the disease relative to an individual not having the allele.

Association of HLA with certain diseases

Disease HLA allele Relative risk

Ankylosing spondylitis B27 87

Reiter’s syndrome B27 40

Acute anterior uveitis B27 8

Behcet’s disease B5 3.3

Insulin-dependent DM DR3 5.7

SLE DR2 3

Grave’s disease DR3 4

Sjogren’s syndrome DR3 6

HLA-typing

Classical tests- Microcytotoxicity - MHC Class I.- Mixed lymphocyte culture (mixed lymphocyte reaction) - MHC Class II.

Molecular techniques- Polymerase chain reaction.- DNA sequence analysis.