International Histocompatibility Workshops

The Major Histocompatibility Complex (MHC), known as the Human Leukocyte Antigen (HLA) system in humans, is the most polymorphic region in humans. Since the identification of what came to be named the MHC in 1958, there are now, as of Nov 2018, are a total of 20,272 HLA and related alleles described – see http://hla.alleles.org/alleles/index.html. The extensive polymorphism of the HLA region is believed to have been driven by the evolutionary pressure to detect and mount an immune response to infectious pathogens.

This huge advance in our knowledge of the MHC system has been achieved largely due to the very early appreciation of the scope of work necessary to elucidate the HLA system and the willingness of the earlier researchers in this field to collaborate. This international group of investigators were willing to share reagents and unpublished data. The first HLA antigens were defined by individual groups using their own reagents, antisera and cell panels, identified locally. An exchange of reagents was necessary to compare antisera to standardize the definition of antigens and to establish a common nomenclature. The researchers agreed to come together in a workshop to create the opportunity to exchange reagents for mutual study.

The first Histocompatibility Testing Workshop was organized by Dr Bernard Amos and held in his laboratory at Duke University in Durham, North Carolina in 1964. It involved scientists from several countries. It was at this workshop that Paul Terasaki introduced the lymphocytotoxicity test for serologic typing and crossmatching, describing the first positive leukocyte antibody crossmatch test associated with hyperacute renal graft rejection.

This first workshop proved very successful, leading to more extensive collaborations in the following years.

In addition to providing a mechanism for exchanging reagents, the International HLA Workshops have also been on the forefront of promoting new technology and disseminating both reagents and technical skills worldwide. This has been an invaluable resource for stimulating immunogenetics research and facilitating rapid translation of new technology and knowledge to patient care.

Structure and Function of MHC

In humans, the MHC molecules are arranged into two classical sets – MHC class I and II both with different functions.

HLA class I molecules are heterodimeric, membrane bound glycoproteins made up of a polymorphic 43 kDa α or heavy chain, in non-covalent association with a non-polymorphic β2 microglobulin (12 kDa) protein. The class I molecule is anchored to the cell membrane by the α-chain. The α-chain is made up of three extra cellular domains, α1, α2 and α3, a transmembrane region and a cytoplasmic chain.

The α3 domain and β2 microglobulin are proximal to the cell membrane and have a folded structure that resembles that of an immunoglobulin C domain. The α1 and α2 domains are distal to the cell membrane and fold together into a structure consisting of a floor of anti-parallel β pleated sheets flanked by a pair of anti-parallel α helix side walls. The folding of the α1 and α2 domains creates a groove into which peptides can bind. Peptides typically of 8 – 10 amino acids in length bind into this groove through a series of hydrogen bonds and ionic interactions at each end of the peptide. The vast majority of the polymorphism of the HLA class I gene codes for amino acids that line this peptide binding groove and therefore have a direct impact on the nature of the peptide that can bind.

The main function of HLA class I molecules is to present cytosolically derived (intracellular) peptides to the T cell receptor of CD8+ cytotoxic T cells. Where the peptides are pathogen derived, this can lead to an immune response resulting in the killing of the infected cell.

HLA class II molecules are heterodimeric membrane bound glycoproteins made up of two non-covalently associated polymorphic chains, α (34 kDa) and β (29 kDa). Both chains contribute two domains each, α1 and α2 and β1 and β2 respectively. The α2 and β2 domains are proximal to the cell membrane and have a folded structure that resembles that of an immunoglobulin C domain. The α1 and β1 domains are distal from the cell surface and fold together into a structure consisting of a floor of anti-parallel β pleated sheets flanked by a pair of anti-parallel α helix side walls. The folding of the α1 and β1 domains creates a groove into which peptides can bind. The groove is open at both ends which allowing long peptides to bind. Peptides bound by HLA class II molecules are typically longer than peptide bound by HLA class I molecules. The vast majority of the polymorphism of the HLA class II gene codes for amino acids that line this peptide binding groove, mainly in the β chain but also in the α chain.

The main function of HLA class II molecules is to present extracellular derived peptide to the T cell receptor of primarily CD4+ helper T cells thereby eliciting an immune response, including B cell activation for production of antibodies and cytotoxic T cell activation.

MHC Genetics

MHC genes are encoded on the short arm of chromosome 6 (6p21.31). The MHC is the most polygenic region in humans with over 220 genetic loci identified. The MHC genes are located in clusters, with the genes that code for the class II α and β chains centromeric of the gene that codes for the class I α chain. The gene that codes for the class I β chain is located outside of the MHC on chromosome 15. The class I and II genes are separated by a cluster of class III genes which code for several components of the complement system and for other molecules involved in the immune response such as TNF.

Generation of MHC Diversity

The huge diversity of the HLA system has been driven evolutionarily by survival advantage to disease, the so-called Pathogen Mediated Selection theory. Different populations in different parts of the world have been exposed to different pathogens, thus driving the survival of certain HLA types which confer protection, over other HLA types. Without HLA diversity, a single disease would be able to wipe out a whole population.

Proposed types of Pathogen Mediated Selection theory include:

• The Heterozygous advantage theory which proposes that heterozygous individuals will be able to present a wider array of peptides to T cells
• The rare allele theory which proses that pathogen may evolve to overcome the common alleles but not a given rare allele
• The fluctuating selection hypothesis which states that HLA diversity is generated as the frequency of various pathogens in the population fluctuate over time

The mechanism by which diversity is generated include point mutation and gene conversion.  de novo MHC sequences are mostly likely generated by point mutations. New haplotypes on the other hand are likely generated by gene conversion in which transferring of sections of DNA within and across MHC loci takes place.