Variation in the number of expressed MHC genes in different cattle class Ihaplotypes

Analysis of cattle major histocompatibility complex (MHC) (BoLA) class I ge ne expression using serological and biochemical methods has demonstrated a high level of polymorphism. However, analysis of class I cDNA sequences has failed to produce conclusive evidence concerning the number and nature of expressed genes. Such information is essential for detailed studies of catt le immune responses, and to increase our understanding of the mechanisms of MHC evolution. In this study a selective breeding programme has been used to generate a number of MHC homozygous cattle expressing common serological ly defined class I specificities. Detailed analysis of five class I haploty pes was carried out, with transcribed class I genes identified and characte rized by cDNA cloning, sequence analysis, and transfection/expression studi es. Surface expression of the gene products ton lymphocytes) was confirmed using monoclonal antibodies of defined BoLA specificity. Phylogenetic analy sis of available transcribed cattle MHC class I sequences revealed complex evolutionary relationships including possible evidence for recombination. T he study of individual haplotypes suggests that certain groupings of relate d sequences may correlate with loci, but overall it was not possible to def ine the origin of individual alleles using this approach. The most striking finding of this study is that none of the cattle class I genes is consiste ntly expressed, and that in contrast to human, haplotypes differ from one a nother in both the number and composition of expressed classical class I ge nes.