- MAJOR HISTOCOMPATABILITY COMPLEX

MHC - Large complex of tightly linked genes that encodes molecules involved in many aspects of the immune response.

The HLA (human leucocyte associated) complex is located on chromosome 6.

The H-2 (mice) complex is located on chromosome 17.

The MHC is termed:
DLA in dogs
XLA in Xenopus etc.

To illustrate the size of the human HLA complex it is interesting to note that it is roughly the same size as the entire genome of E. coli.

The MHC is both multigenic and multiallelic (polymorphic):

At each classical MHC locus there are multiple allelic forms possible. At the two most polymorphic regions (Class I and Class II) there are well over 100 alleles at each loci.

The genes in the MHC are very tightly-linked. Because there is such a low frequency of recombination, the MHC genes are inherited in 2 sets, (one from each parent). Each of these sets is referred to as a haplotype. Therefore, siblings have a 1/4 probability of having the same MHC.

In outbred populations, individuals are usually heterozygous at most loci. Therefore, most individuals possess 2 different allelic forms of each of the MHC genes.

The MHC genes are co-dominantly expressed so an individual will express both maternal and paternal alleles in the same cell. Theoretically, there are more possible combinations of alleles than there are members of the species.

In inbred mice, each H-2 locus is homozygous, because the maternal and paternal haplotypes are identical. Certain inbred strains are prototypes H-2^a, H-2^b, H-2^d, H-2^k.

The superscript letter refers to the entire set of inherited alleles w/o referring to each allele individually.

It is possible for several different strains to have the same haplotype CBA, C3H, and AKR are all H-2^k but they have different background genes.

H-2^k X H-2^b

F1 will be H-2^k/b

Congenic Mouse Strains
Two strains are congenic if they are gentically identical except at a single genetic locus or region. Such mice have
been very useful in elucidating the contribution of MHC genes to specific diseases.

Three classes of molecules are encoded within the MHC:

Class I - present on almost all nucleated cells (interesting exceptions include sperm and the cells of the trophoblast).
Class II- present on Antigen Presenting Cells (macrophages, B cells, and dendritic cells).
Class III- are not surface molecules, but instead are various proteins typically which have some immunological role (C2,C4, Tumor necrosis factor alpha and beta, various HSPs)

Human HLA region [Highly Simplified version !]

Protein Products: DPa and DPb Complement HLA-B (a)
DQa and DQb TNF a & b HLA-C (a)
DRa and DRb HSP proteins HLA-A (a)

Class: [class II] [class III] [class I]

Mouse H-2 region [Highly Simplified version]

Protein Products: H-2K IAa and IAb Complement H-2D a
IEa and IEb C2, C4 H-2L a
TNF
HSPs

Class: [class I] [class II] [class III] [class I]

The MHC is the most polymorphic gene complex known. The diversity of the MHC proteins (in particular the
Class I and Class II MHC proteins) is due to this polymorphism.

Class I Molecules

Mouse [ H-2K,H-2D,H-2L]
Human [HLA-A, HLA-B, HLA-C]

The class I MHC molecules are found on most nucleated cells. However, expression does vary between different cell types.

(mouse over image to see protein chains)

The highest levels occur on lymphocytes while very low levels occur on liver hepatocytes. This may be one
reason that liver transplants are among the most successful organ transplants.

(See Structure of Class I MHC protein in text )

The protein is a heterodimer, but only one chain (the a chain) is encoded by the MHC.

The other component of the Class I MHC protein, Beta-2 microglobulin (b2- microglobulin), is encoded by a gene outside of the MHC. This protein has a mw of `12,000D. It is a small globular protein, very similar in structure to an antibody constant region domain. NOT inserted into the cell membrane. Essential for stabilizing the structure of the a chain. If b2- microglobulin is not expressed, Class I MHC is not expressed.

The alpha chain has a mw of ~45,000 D
There are three extracellular globular domains (designated a1, a2, a3), a transmembrane domain and a cytoplasmic domain.

The Beta2- microglobulin and alpha 3 domains are associated by hydophobic interactions (NOT covalently bound to each other).

The alpha 3 domain and Beta-2microglobulin show a lot of homology with Ig constant region domains.

All individuals of the same species possess the same Beta-2 microglobulin, the alpha-3 domain is also pretty constant, but the alpha 1 and alpha 2 domains show extensive polymorphism. The alpha-1 and alpha-2 regions are
recognized as foreign when tissue is transplanted between different individuals of the same species.

X Ray crystallographic analysis was published in1987 (Pamela Bjorkman). The complete analysis took her 8 years due to the challenge of obtaining a homogeneous preparation of the Class I alpha chain and the difficulty and complexity of the analysis.
With todays's technology, if one has a purified crystalized protein, X Ray crystallographic analysis can be completed in 6 hours!

The peptide binding site is formed by the alpha 1 and alpha 2 domains of the alpha chain. The cleft is formed by a floor of 8 anti-parallel Beta strands while the sides of the cleft are formed by 2 alpha helices. The cleft is closed on each end and it has been demonstrated that the peptide anchors at each of its ends and bows out in the middle.
The peptides which fit the cleft are between 8-10 amino acids in length, with the nonamer (9 amino acids) being the most common.

CLASS II MHC

Mouse [ I-A, I-E]
Human [DR, DQ, DP]

These proteins are found only on the surface of antigen presenting cells. The Class II protein is a heterodimer, composed of 2 distinct polypeptide chains which are designated alpha and beta. Both chains are encoded by genes in the Class II MHC region and both are inserted into the membrane of the APC.

Both polypeptide chains have 2 extracellular domains (designated alpha 1, alpha 2 & beta 1, beta 2), a short hydrophobic transmembrane section and a hydrophilic cytoplasmic domain inside the cell. The alpha 1 and beta 1domains are highly variable between different allelic forms of the alpha and beta chain respectively.

X-Ray crystallography (taking a total of 10 years to complete) has proven that the alpha 1 and beta 1 domains together form the peptide binding cleft of the Class II MHC protein. The peptide binding cleft is very similar to the cleft observed for Class I MHC proteins (in fact, as you can see in the text the two clefts are actually superimposable). There are a few differences, however. The Class II MHC peptide binding cleft is open on both ends. Therefore, the processed peptide lays within the cleft somewhat like a hot dog in a bun. Rather than being anchored to the cleft at both ends (as in the Class I MHC cleft), the peptide is attached by various amino acid side chains along its length.

The peptides which "fit" the cleft range in size from 13-24 amino acids in length.

Since APCs are nucleated cells, *APCs possess both Class I and Class II MHC on their surface.

The sequence divergence among alleles of the MHC Class I and Class II molecules within a species are as great or greater than the divergence normally seen in proteins between species. This variability may be due to a high rate of gene conversion. Gene conversion occurs when short DNA sequences insert into recipient DNA sequences. Gene conversion often utilizes short sequences from pseudogenes and such pseudogenes are common within the MHC complex.

Even though Class I MHC proteins are found on the surface of almost all nucleated cells, the level of expression varies between different cell types. Lymphocytes probably express the highest levels of Class I while hepatocytes of the liver have only low level expression.

Class II MHC proteins are expressed only on a limited # of cell types.

Question that arises: How is the expression of MHC genes regulated?

At one level there are specific transcription factors which bind to promoter sequences 5' to each alpha and beta chain gene. When these transcription factors are present in high enough quantity, RNA polymerase is able to transcribe the gene.

Certain cytokines are known to impact the synthesis of these transcription factors. All interferons are able to upregulate CLass I MHC expression on cells. IFN gamma in particular induces the synthesis of a specific transcription factor that binds to the promoter sequence flanking the class I genes. The binding of the transcription factor to the promoter sequence appears to coordinately up-regulate transcription of the genes encoding both the class I alpha chain and beta-2 microglobulin, Interestingly, the expression of Class II MHC molecules in B cells is down-regulated by IFN gamma, whereas IFNg up-regulates the expression of Class II MHC on macrophages and dendritic cells. This effect relates to the ability of IFN-gamma to function in down-regulation of the humoral (antibody) response.

Corticosteroids and prostaglandins have long been known to be immuno suppressive. It turns out that both are able to decrease the expression of Class II MHC proteins.

Class I MHC protein expression is also influenced by various viruses. Specifically, CMV (cytomegalovirus), HBV (hepatitis B), and Ad12 (adenovirus 12) can decrease Class I MHC expression.

CMV
There is a viral peptide which binds to beta-2-microglobulin preventing Class I assembly and transport to the plasma membrane.

Ad12
Causes a pronounced decrease in the transcription of TAP1 and TAP2 which are required to transport processed peptides from the cytoplasm into the RER.

Of course the down regulation of Class I MHC proteins helps these viruses to evade the immune response since viral peptides are presented to CD8 Tc cells by Class I MHC proteins.

MHC haplotype has been shown to strongly influence immune responsiveness. This influence is strongest in the class II MHC region reflecting the central role of class II MHC molecules in presenting antigen to T helper lymphocytes.

There are two explanations which have been given for the variability observed in immune responsiveness between different haplotypes.

The Determinant Selection Model states that different Class II MHC molecules differ in their ability to bind processed antigen. In other words, individuals may simply lack MHC molecules for certain peptides.

The Holes in the Repertoire Model states that and individual may not have T cells with receptors that can recognize particular peptide-MHC complexes. This is entirely likely due to the phenomenon of negative selection which occurs during thymic education.

These two models are not mutually exclusive and both have been shown to be correct.

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