MHC - Major Histocompatibility Complex Notes

MHC Molecules and Their Discovery

Discovery of MHC

The Major Histocompatibility Complex (MHC) was initially identified through experiments involving the rejection of transplanted grafts, particularly tumors, between members of the same species, primarily in mice. Peter Gorer identified a strain-specific antigen II, later known as H-2, by raising antibodies to mouse red blood cells in rabbits. His work demonstrated that the rejection of an albino tumor in C57 mice was linked to the presence of this H-2 antigen.

George Snell further developed this field by identifying most mouse strains and introducing the term 'histocompatibility antigen' to describe antigens that provoke graft rejection. Snell's extensive mouse experiments revealed that differences at the H-2 locus resulted in the strongest rejection responses. Jean Dausset (humans) and Baruj Benacerraf (humans) also contributed to this field. In 1980, Jean Dausset, George Snell, and Baruj Benacerraf were awarded the Nobel Prize in Physiology or Medicine for the discovery of the major histocompatibility complex genes.

What is the MHC?

The Major Histocompatibility Complex (MHC) is a single genetic region encoding polymorphic class I and class II molecules. It represents a cluster of genes found in vertebrates, with some differences observed between mammals, birds, and reptiles. MHC plays a crucial role in discriminating between self and non-self and participates in the development of humoral and cellular immunity. MHC molecules, present on cell surfaces, present both self and non-self antigens to cells of the immune system, specifically T-cells.

Definitions

• Locus: A specific location of a gene or DNA sequence on a chromosome.
• Polymorphic: Existing in a variety of different shapes/forms. Variability at a gene locus in which variants occur at a frequency of > 1%.
• Allele: Variants of a polymorphic gene (one of a pair) at a particular locus.
• Haplotype: A linked combination of alleles at adjacent locations (loci) on a chromosome that are inherited together. Individuals express two haplotypes (maternal/paternal).

MHC Interaction with T Cell Receptor

MHC molecules display peptides and interact with the T cell receptor (TCR).

MHC Complex and Genes

The MHC complex is a collection of genes located on chromosome 6 in humans and chromosome 17 in mice. In humans, it is referred to as the Human Leukocyte Antigen (HLA), while in mice, it is known as the H-2 Complex. MHC genes are organized into regions encoding three classes of molecules: MHC class I, MHC class II, and MHC class III.

Each human typically expresses 3 types of MHC class I molecules (A, B, C) and 3 types of MHC class II molecules (DR, DP, DQ), resulting in 6 MHC class I and 6 MHC class II alleles, one from each parent. In some individuals, the DR region can have up to 3 loci, leading to the expression of 9 MHC genes. This is however very rare.

Location of MHC genes

MHC genes are located on Chromosome 6 (in humans) and Chromosome 17 (in mice).

Polymorphism and Diversity

MHC is highly polymorphic, with many polymorphic genes, such as HLA B. This polymorphism is important for diversity. If everyone had the same MHC allele during a viral outbreak, the entire population could be at risk. Different alleles result in varied immune responses, leading to a spectrum of survival outcomes. For example, HIV progression is associated with HLA B35, while HLA B57 is associated with slower progression. The specific combination of MHC genes can offer selective advantages.

Expression and Function of MHC Molecules

Expression of MHC Molecules

MHC class I molecules are found on all nucleated cells, while MHC class II molecules are primarily found on "professional antigen-presenting cells" such as dendritic cells, macrophages, and B cells.

Endogenous vs. Exogenous Pathway

MHC class I molecules bind peptides derived from endogenous antigens (those originating from within the cell). These antigens are processed in the cytoplasm and presented to CD8+ T cells. CD8 on a T cell binds MHC Class I.

MHC class II molecules bind peptides derived from exogenous antigens (those originating from outside the cell). These antigens are internalized via endocytosis and processed within the endocytic pathway before being presented to CD4+ T cells. CD4 on a T cell binds MHC Class II.

Antigen Presentation

MHC Class I antigen presentation involves endogenous antigens, e.g., when a virus uses a cell to produce viral proteins. MHC Class II antigen presentation involves exogenous antigens.

Structure of MHC Molecules

MHC Class I Structure

The structure of MHC Class I molecules include $\alpha$ and $\beta$ chains, which generate helices and peptide-binding clefts.

• The $\alpha$ chain is a polymorphic transmembrane glycoprotein with a molecular weight of 45 kDa, containing 3 globular domains.
• The peptide cleft is formed by a pair of $\alpha$-helices ($\alpha$1 and $\alpha$2) on a floor of anti-parallel $\beta$ strands.
• $\beta$2-microglobulin ($\beta$2m), with a molecular weight of 12kDa, is non-MHC encoded, non-transmembrane, and non-covalently bound to the $\alpha$-chain.
• The $\alpha$3 domain and $\beta$2m exhibit structural and amino acid sequence homology with Ig constant domains.
• Association of the $\alpha$-chain with $\beta$2m is essential for the expression of Class I molecules on cell membranes.

MHC Class II Structure

MHC Class II molecules consist of two transmembrane anchored glycoproteins: an $\alpha$ chain (34 kDa) and a $\beta$ chain (29 kDa).

• They do not contain $\beta$2-microglobulin.
• The peptide cleft is formed by a pair of $\alpha$-helices on a floor of anti-parallel $\beta$ strands.
• The $\alpha$2 and $\beta$2 domains have structural and amino acid sequence homology with Ig constant domains.

Peptide Binding

MHC binding clefts accommodate short peptides blocked at both ends (MHC Class I) and peptides with longer sequences (MHC Class II).

MHC-Peptide Anchors

Peptides are held in position within MHC molecules through anchor residues. MHC Class I molecules have constant anchor residues per haplotype (e.g., H-2K vs. H-2D) with a highly conserved C-terminal and variable N-terminal. MHC Class II molecules have longer peptides with more anchor residues and sit higher in the binding cleft.

Peptide Binding by Class I and Class II MHC Molecules

Regulation and Function of MHC Genes

Regulation of MHC Gene Expression

The expression of MHC genes is regulated by cytokines such as IFN$\alpha$, IFN$\beta$, IFN$\gamma$, and TNF, which increase MHC expression. Transcription factors like CIITA and RFX are also involved in increasing MHC gene expression.

Some viruses, such as CMV and Hepatitis B virus, decrease MHC expression to evade the immune system. Reduction of MHC expression may contribute to immune evasion.

MHC Class III Genes

MHC Class III genes encode structurally and functionally diverse proteins within the MHC region but do not encode a receptor. These proteins include complement components, tumor necrosis factors ($\alpha$ & $\beta$), and transporter molecules for MHC class II polypeptides. These gene products do not directly participate in antigen presentation to the TCR.

MHC Restriction

MHC restriction refers to the requirement for antigen-presenting cells (APCs) to express MHC molecules that the T cell recognizes as self in order for that T cell to respond to the antigen presented by that APC.

Zinkernagel and Doherty Experiment

Zinkernagel and Doherty demonstrated that antigen recognition by CD8 T cells exhibits MHC restriction. H-2k mice were infected with the lymphocytic choriomeningitis (LCM) virus to induce CD8 T cells specific for the virus.

Spleen cells from this LCM-infected mouse (containing H-2k CD8 T cells) were added to target cells (antigen-presenting cells) of a different H-2b haplotype that were either infected or not with the LCM virus to allow presentation of peptides on the surface.

Killing of the target cells by CD8+ T cells was measured by the release of 51Cr into the culture supernatant and occurred only if the target cells had the same MHC haplotype as the CD8+ T cells.

In essence, for effective killing, the peptide presented must be the same for each haplotype.

Learning Objectives

• Describe the structure and function of class I and II major histocompatibility molecules (MHC).
• Explain the concept of MHC restriction.