Complete sequence and gene maps of both human and chicken major histocompatibility complex (MHC) regions have recently been reported. The MHCs of chickens and humans differ in their size, organization, complexity and evolution.
The human HLA region comprises 224 genes in 3.6 mb of DNA. Of these, 128 are predicted to be functional and 96 are pseudogenes. It is estimated that 40% of the expressed genes play a role in the immune system. By contrast, the chicken MHC (called the B locus) is 92 kb in size, contains only 19 genes, and the class III region genes are located outside the class I and II region genes. Unexpectedly, proteasome (LMP) genes are absent from the chicken MHC. This feature might help explain why acidic residues are often present at the C-terminus of peptides that bind to chicken classical class I molecules. In humans, which possess proteasome genes in their MHC, hydrophobic or basic residues are preferentially located at the C-terminus.
Another unusual feature of the chicken MHC is the presence of two genes encoding C-type lectins, which resemble the lectinlike natural killer (NK) receptors of mammals. Kaufman et al. speculate that the presence of these genes in the MHC region might explain the association of the chicken MHC and susceptibility to infection by the herpes virus that causes Marek's disease.
In terms of evolution, the human and chicken MHCs couldn't be more different. The size and complexity of the human MHC region permits meiotic recombination to occur resulting in the generation of new MHC haplotypes, whereas the chicken MHC region is small and compact such that meiotic recombination is unlikely to occur, and the co-evolution of MHC genes leads to stable haplotypes.