38 Regular updates of the numbers of alleles observed at each HLA locus (current numbers are given in Table 2) are recorded in the IMGT/HLA database (http://www.ebi.ac.uk/imgt/hla/), which also provides DNA and amino acid sequences and alignments of HLA alleles and molecules, and nomenclature information.39 This nomenclature has recently been modified substantially according to the allele naming system shown in Fig. 2. The high level of diversity
found at the HLA loci is principally located in exons 2 and 3 for class I genes, and in exon 2 for class II genes. Such exons correspond, at the protein level, to the peptide-binding region (PBR) of the HLA molecules. The mean pairwise DNA sequence differences between HLA alleles are between C646 concentration ∼ 10 and 26 nucleotides, depending on the locus (Table 2 and ref. 40), suggesting a functional relevance. Analysis of the amino acid sequence of HLA molecules shows that allelic variants differ from each other mainly by substitutions in residues contributing to the PBR, in particular in some pockets in the PBR that accommodate side chains of the bound peptides. Hence, peptides eluted from different HLA class I molecules show distinctive amino acid patterns at certain positions, in particular corresponding to Selleck Paclitaxel pockets 2 and 9 of the HLA molecules.41 It is therefore assumed that the polymorphism of
HLA alleles is to a large extent functional because different HLA molecules bind different sets of peptides. A high sequence diversity is therefore required in the PBR of the HLA molecules to bind a high variety of pathogen-derived peptides that are subsequently presented to T-cell receptors. The distribution of HLA alleles in different populations may be a consequence of this functional polymorphism. In many instances the immune response to a particular peptide epitope of a pathogen may depend on the HLA alleles carried by the individual. Individuals heterozygous for HLA alleles may have a wider
peptide binding repertoire and therefore a capability to respond to more pathogen variants, causing selection of heterozygotes. On the other hand, the existence of several different loci both within BCKDHA the class I (A, B and C) and II series (DR, DQ and DP) of molecules may to some extent compensate for the deficits of homozygosity. It should also be noted that there exists a very strong linkage disequilibrium (LD), or non-random association, between HLA alleles at different loci; i.e. some HLA alleles are found together in populations more frequently than expected based on their gene frequencies. For example some alleles of the DRB1 locus demonstrate strong LD with specific alleles at the DQA1 and DQB1 loci. Furthermore, in many populations HLA alleles at one locus with high sequence homology, i.e. DRB1, are in LD with the same alleles at other loci, i.e. DQA1 and DQB1, which may indicate an evolutionary relationship between some alleles, i.e. DRB1.