MAJOR MAJOR HISTOCOMPATABILITY HISTOCOMPATABILITY

COMPLEXCOMPLEXDr.T.V.Rao MD

Dr.T.V.Rao MD

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MHC - The Major Histocompatibility ComplexMHC - The Major Histocompatibility Complex

1) Originally identified in mice as blood cell antigens by R.A. Gorer and G.D. Snell in 1930s, and defined on the basis of tissue graft rejection - 1980 Nobel prize awarded to Snell.

2) Work by Rolf Zinkernagel and Peter Doherty in the 1970s revealed that it is the complex of MHC molecule plus antigen that is recognized by T cells (MHC restriction of T cell responses) - 1996 Nobel Prize

3) Two classes of MHC molecule: Class I (single MHC chain + 2-microglobulin) and Class II ( chain plus chain) - more Ig-superfamily members (Ig-C domains)

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Major Histocompatibility complex Major Histocompatibility complex (MHC)(MHC)

• The Major Histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success.

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Major Histocompatibility Complex Major Histocompatibility Complex DefinitionDefinition

• A group of genes that code for cell-surface Histocompatibility antigens and are the principal determinants of tissue type and transplant compatibility. They are the most diverse genes in humans and are used to determine if a sample of DNA comes from a specific person.

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• Genes Of MHC Organized In 3 Classes– Class I MHC genes

• Glycoproteins expressed on all nucleated cells

• Major function to present processed Ags to TC

– Class II MHC genes• Glycoproteins expressed on M, B-cells, DCs

• Major function to present processed Ags to TH

– Class III MHC genesClass III MHC genes• Products that include secreted proteins that have immune

functions. Ex. Complement system, inflammatory molecules

MHCMHC

Basis of MHC system

• In man and mouse, as in most species, each class of MHC is represented by more than one locus( polygeny), in man these are called HLA for Human Leucocyte Antigen. The class I loci are HLA-A,-B and -C and the class II loci HLA-DR, -DQ and -DP. All the MHC genes map within a single region of the chromosome (hence the term Complex); the physical map of the MHC of man and mouse has been determined by cloning, mapping and sequencing.

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• Class I MHC Genes Found In Regions A, B and C In Humans (K and D In Mice)

• Class II MHC Genes Found In Regions DR, DP and DQ (IA and IE In Mice)

• Class I and Class II MHC Share Structural Features– Both involved in APC

• Class III MHC Have No Structural Similarity To Class I and II– Ex. TNF, heat shock proteins, complement

components

Class I, II and III MHC

Composition of Class I Composition of Class I MoleculesMolecules

• Class I molecules are composed of two polypeptide chains; one encoded by the BCA region and another (ß2-microglobulin) that is encoded elsewhere. The MHC-encoded polypeptide is about 350 amino acids long and glycosylated, giving a total molecular weight of about 45 kDa. This polypeptide folds into three separate domains called alpha-1, alpha-2 and alpha-3. ß2-microglobulin is a 12 kDa polypeptide that is non-covalently associated with the alpha-3 domain.

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Class II moleculesClass II molecules

• MHC (major Histocompatability complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).

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Humans are very Diversified MHC Plays a Great Role

The loci which specify MHC’s are polymorphic.Many alleles may exist at a locus:

HLA A locus has ~60 allelesHLA B locus ~110 allelesHLA C locus ~40 alleles

The high level of allelism creates diversity within a species (thus restricting allografting) but does not produce diversity within an individual

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What is polygenecity?• Humans have DP, DQ, and DR “regions”

specifying and chains of MHC-II.

• Why are these called “regions”?

MHC Molecules Control the Immune response

• The Major Histocompatibility Complex (MHC) is a set of molecules displayed on cell surfaces that are responsible for lymphocyte recognition and "antigen presentation". The MHC molecules control the immune response through recognition of "self" and "non-self" and, consequently, serve as targets in transplantation rejection

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Peptide Binding to MHC Molecules: Class I vs. IIPeptide Binding to MHC Molecules: Class I vs. II

Class IClass I Class IIClass II

• Generated by proteasome (cytosol) • Generated in endosomes/lysosomes

• Transported to ER by TAP1/2 • Bind in specialized vessicles (MIIC)

• Restricted length, 8-9 residues • Any length, extend from MHC

• Binding energy from N, C termini • Binding energy from backbone

• “Anchor” residues fill specific pockets• “Anchor” residues fill specific pockets

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MHC- I MHC-IIMHC- I MHC-IIMHC-I

Heavy chain (alpha) and

“microglobulin” (beta two)

Heavy is 45 kilodaltons, has three domains + a transmembrane component (40 aa) + a cyto- plasmic tail (30 aa)

The three alpha domains are called: 1, 2, & 3

1 and 2 interact to present processed Ag

Process Ag is optimally a nonomer

MHC-II

An alpha and beta chain, 33 kDA and 28 kDa, respecitvely.

Chains are non-covalently associated.

Each chain has two domains.

1-1 interact to present processed Ag

Processed Ag is optimally 13-18 aa

2 & 2 are part of immunoglobulin super family

Microglobulin (12 kDa) associates non-covalently with 3Microglobulin and 3 are part of immunoglobulin

superfamily

Microglobulin is the only member of the superfamily that does not have a component linking it to a membrane

The Major Histocompatibility Complex1) MHC Class I and Class II genes are encoded in a large

(>4 Mb, >200 genes) tightly linked gene cluster: the MHC

2) The MHC is similarly organized in mice (Ch17) and humans (Ch6) - syntenic

3) Known in humans as HLA (human leukocyte antigen) genes; in mouse as H-2 (histocompatibility group 2) genes

4) Genes are highly polymorphic (many alleles)5) Class I: HLA-A, -B, -C (human); H-2K, -2D, -2L

(mouse)6) Class II: HLA-DP, -DQ, -DR (human), I-A, I-E (mouse)

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Human Human MHC class I: HLA-A, -B, -C

MHC class II: HLA-DR, -DP, -DQ

MouseMouseMHC class I: H2-K, -D, -L

MHC class II H2-A, -E (I-A, I-E)

2-microglobulin is located on a different chromosome.Dr.T.V.Rao MD 16

MHC is represented …MHC is represented …

• In man and mouse, as in most species, each class of MHC is represented by more than one locus(polygeny), in man these are called HLA for Human Leucocyte Antigen. The class I loci are HLA-A,-B and -C and the class II loci HLA-DR, -DQ and -DP. All the MHC genes map within a single region of the chromosome (hence the term Complex); the physical map of the MHC of man and mouse has been determined by cloning, mapping and sequencing.

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Class I and II MHC

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MHC- I & MHC-II

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• Several Hundred Allelic Variants Have Been Identified In Humans

• However, up to 6 MHC I And 12 MHC II Molecules Are Expressed In An Individual

• Enormous Number Of Peptides Needs To Be Presented Using These MHC Molecules

• To Achieve This Task MHC Molecules Are Not Very Specific For Peptides (Unlike TCR and BCR)

• Promiscuous Binding Occurs– A peptide can bind a number of MHC– An MHC molecule can bind numerous peptides

Class I And II SpecificityClass I And II Specificity

• MHC Is One Of The Most Polymorphic Complexes Known

• Alleles Can Differ Up To 20 a/a• Class I Alleles In Humans: 240 A, 470 B, 110 C• Class II Alleles In Humans: HLA-DR 350 , 2 !• HLA-DR

genes vary from 2-9 in different individuals!!!,– 1 gene ( can combine with all products

increasing number of APC molecules)

• DP (2 , 2 ) and DQ (2 , 3 )

Class I And II Diversity And Class I And II Diversity And PolymorphismPolymorphism

MHC I and II belong to Immunoglobulin Supergene Family

• The Class I and Class II MHC molecules belong to a group of molecules known as the Immunoglobulin Supergene Family, which includes immunoglobulin's, T-cell receptors, CD4, CD8, and others

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MHC location on Chromosome 6MHC location on Chromosome 6

• The major major Histocompatibility Histocompatibility complexcomplex is encoded by several genes located on human chromosome 6. Class I molecules are encoded by the BCA region while class II molecules are encoded by the D region. A region between these two on chromosome 6 encodes class III molecules, including some complement components.

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• Expression Is Regulated By Many Cytokines– IFN, IFN, IFN and TNF Increase MHC expression

• Transcription Factors That Increase MHC gene Expression– CIITA (Trans activator), RFX (Trans activator)

• Some Viruses Decrease MHC Expression– CMV, HBV, Ad12

• Reduction Of MHC May Allow For Immune System Evasion

MHC Expression

MHC system important in Rejection of Transplants

• Gene products encoded in the Major Histocompatibility Complex (MHC) were first identified as being important in rejection of transplanted tissues.  Furthermore, genes in the MHC were found to be highly polymorphic (i.e. in the population there were many different allelic forms of the genes)

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Class I MHC proteins• Class I MHC proteins (found on all nucleated cell

surfaces) present antigens to cytotoxic T lymphocytes (CTLs) . Most CTLs possess both T-cell receptors (TCR) and CD8 molecules On their surfaces. These TCRs are able to recognize peptides when they are expressed in complexes with MHC Class I molecules. For the TCR to bind a peptide-MHC complex two conditions must be met. Firstly, the TCR must have a structure which allows it to bind the peptide-MHC complex. Secondly, the accessory molecule CD8, must bind to the alpha-3 domain of the MHC Class I molecule. Due to genetic recombination events each CTL expresses a unique TCR which only binds a specific MHC-peptide complex

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Class I MHC moleculesClass I MHC molecules• Class I MHC molecules

are composed of two polypeptide chains, a long α chain and a short β chain called β2-microglobulin (figure 2).  The α chain has four regions.  First, a cytoplasmic region, containing sites for phosphorylation and binding to cytoskeletal elements

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Class II MHC moleculesClass II MHC molecules• Class II MHC molecules

are composed of two polypeptide chains an α and a β chain of approximately equal length (Figure 6). Both chains have four regions: first, a cytoplasmic region containing sites for phosphorylation and binding to cytoskeletal elements;

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MHC II are Complex in Organization

• Within the MHC there are 5 loci that encode class II molecules, each of which contains a gene for an α chain and at least one gene for a β chain. The loci are designated as HLA-DP, HLA –DQ, HLA-DR, HLA-DM, and HLA-DO. Among these, HLA-DP, HLA –DQ, and HLA-DR are the most important and are most polymorphic..

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MHC is highly polymorphicMHC is highly polymorphic

• The MHC is highly polymorphic. There are a large number of genetic variants (alleles) at each genetic locus. Crucially many of these alleles are represented at significant frequency (> 1%) in the population, and in addition the alleles generally differ from one another by many (up to 30) amino acid substitutions. For example in humans there are more than 200 alleles described at some MHC loci. Such a remarkable degree of polymorphism implies a selective pressure to establish and maintain it.

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MHC PolymorphismMHC Polymorphism

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MHC Polymorphism1) MHC Class I and Class II molecules have

many alleles2) Diversity ensures that a wide range of

peptides can be presented within the population (even if a much more limited set is presented by any individual)

3) For Class II, both and chains are polymorphic (except DR in humans and E in mice), adding more diversity

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Inheritance Of HLA Haplotypes

MHC Polymorphism

4)Collection of Class I + Class II alleles on one chromosome is called the haplotype

5)MHC molecules are expressed co-dominantly: alleles from both chromosomes are expressed in each cell

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Biological role of the MHCBiological role of the MHC

• The products of the MHC play a fundamental role in regulating immune responses. T cells must recognise antigen as a complex with MHC molecules. This requires antigen to be processed by unfolding and proteolytic digestion before it complexes with the MHC molecule

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Biological role of the MHCBiological role of the MHC

• Once formed the complex of antigenic peptide and MHC are generally very stable (half life ~ 24hrs). Thus the biological role of MHC proteins is to bind small peptides and to "present" these at the cell surface for the inspection of T cell antigen receptors.

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MHC-Linked Diseases

• Defects in MHC gene expression lead to immunodeficiencies (MHC molecules are required for both T cell development and activation)

• Some MHC alleles are associated with susceptibility or resistance to autoimmune diseases

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MHC-Linked ImmunodeficienciesBare Lymphocyte Syndromes lead to

loss of MHC molecule expression:• Defects in TAP genes prevent MHC Class I

protein surface expression (even though MHC proteins are normal), so no CD8+ T cells - surprisingly mild immunodeficiency (respiratory and skin infections)

• Defects in TF’s controlling Class II gene expression (CIITA, RFXANK, RFX5, RFXAP) block CD4+ T cell development - result in SCID (severe combined immunodeficiency)

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