The cattle major histocompatibility complex: is it unique?

Major histocompatibility complex (MHC) class I genes encode highly polymorphic molecules that are expressed on virtually every cell type, and have been identified in all but the most primitive vertebrates. They play a number of crucial roles in the immune response to infectious disease. Most information regarding MHC genes has been generated from humans and mice but, because of the great variability found in the MHC system, it is not always possible to extrapolate from these to other species. Many strategies have evolved to maximise the ability of the MHC to protect individuals and populations against pathogens. Cattle MHC class I genes exhibit a number of unusual features. Evidence from mapping studies, haplotype and phylogenetic analyses suggests the presence of six classical class I loci, in contrast to the more usual two or three, and these are expressed in various combinations of one, two or three on different haplotypes. Although it remains difficult to assign alleles to loci, it appears that none of the loci are expressed on all haplotypes. There is currently limited information relating to polymorphism, but various approaches suggest diversity is high, and may vary between breeds/populations. Functional consequences of variable MHC haplotype composition are discussed. Identifying unique features of the MHC in cattle will lead to new insights into evolution of the immune system.