Sequence of two cattle MHC class I cDNAs associated with BoLA A10 specificity

BoLA class I allele diversity and polymorphism in a herd of cattle

Major histocompatibility complex class I genes are among the most polymorphic genes characterized. The high level of polymorphism is essential for generating host immune responses. In humans, three distinct genomic loci encode human leukocyte antigen (HLA) class I genes, allowing individuals to express up to six different HLA class I molecules. In cattle, the number of distinct genomic loci are currently at least six, and the number of different bovine leukocyte antigens (BoLA) class I molecules that are expressed in individual animals are variable. The extent of allele variation within the cattle population is unknown. In this study, the number and variety of BoLA class I sequences expressed by 36 individuals were determined from full-length BoLA class I cDNA clones. Twenty distinct BoLA class I alleles were identified, with only four being previously reported. The number of expressed BoLA class I alleles in individual animals ranged between one and four, with none of the animals having an identical complement of BoLA class I molecules. Variation existed in the number of BoLA class I alleles expressed as well as the composition of expressed alleles, however, several BoLA class I alleles were found in multiple individual animals. Polymorphic amino acid sites were analyzed for positive and negative selection using the ADAPTSITE program. In the antigen recognition sites (ARS), there were eight positions that were predicted to be under positive selection and three positions that were predicted to be under negative selection from 62 positions. In contrast, for non-antigen recognition sites (non-ARS), there were three positions that were predicted to be under positive selection and 20 that were predicted to be under negative selection from 278, indicating that positive selection of amino acids occurs at a greater frequency within the antigen recognition sites.

Generation and maintenance of diversity in the cattle MHC class I region

Major histocompatibility complex (MHC) class I genes play a crucial role in the immune defence against intracellular pathogens. An important evolutionary strategy is to generate and maintain a high level of diversity in these genes. Humans express three highly polymorphic classical MHC class I genes (HLA-A, HLA-B and HLA-C). In contrast, some species, for example rat and rhesus macaque, maintain diversity by generation of haplotypes that vary considerably with regard to the number and combination of transcribed genes. Cattle appear to use both strategies. We show that various combinations of six apparently classical genes, three of which are highly polymorphic, are transcribed on different haplotypes. Although additional sequences were identified in both cDNA and gDNA, it was not possible to assign them to any of these defined genes. Most were highly divergent or were non-classical class I genes. Thus, we found little evidence for frequent duplication and deletion of classical class I genes as reported in some other species. However, the maintenance of class I diversity in cattle may involve limited gene shuffling and deletion, possibly as a result of unequal crossing-over within the class I region.

The E5 protein of BPV-4 interacts with the heavy chain of MHC class I and irreversibly retains the MHC complex in the Golgi apparatus

BPV-4 E5 inhibits transcription of the bovine MHC class I heavy chain (HC) gene, increases degradation of HC and downregulates surface expression of MHC class I by retaining the complex in the Golgi apparatus (GA). Here we report that transcription inhibition can be alleviated by interferon treatment and the degradation of HC can be reversed by treatment with inhibitors of proteasomes and lysosomes. However, the inhibition of transport of MHC class I to the cell surface is irreversible. We show that E5 is capable of physically interacting with HC. Together with the inhibition of the vacuolar ATPase (due to the interaction between E5 and 16k subunit c), the interaction between E5 and HC is likely to be responsible for retention of MHC class I in the GA. C-terminus deletion mutants of E5 are incapable of either downregulating surface MHC class I or interacting with HC, establishing that the C-terminus domain of E5 is important in the inhibition of MHC class I.

Haplotype characterization of transcribed ovine major histocompatibility complex (MHC) class I genes

The ovine major histocompatibility complex (MHC) remains poorly characterized compared with those of other livestock species. Molecular genetic analysis of the bovine MHC has revealed considerable haplotype and allelic diversity that earlier serological analysis had not detected. To develop cellular and molecular tools to support development of vaccines against intracellular pathogens of sheep, we have undertaken a molecular genetic analysis of four distinct ovine MHC haplotypes carried by two heterozygous Blackface rams. We have identified 12 novel class I transcripts and used a class I sequence-specific genotyping system to assign each of these transcripts to individual haplotypes. Using a combination of phylogenetic analysis, haplotype and transcript expression data, we identified at least four distinct polymorphic class I MHC loci, three of which appear together in a number of combinations in individual haplotypes. The haplotypes were further characterized at the highly polymorphic Ovar-DRB1 locus, allowing selection of the progeny of the two founder rams for the establishment of an MHC-defined resource population.

Serological and molecular diversity in the cattle MHC class I region

Information on major histocompatibility complex (MHC) diversity in cattle is important to aid our understanding of immune responses and may contribute to maintenance of healthy cattle populations. Equally, understanding the mechanisms involved in generating this diversity may shed light on the complex nature of mammalian MHC evolution. The aim of this study was to assess molecular and serological variation within cattle MHC class I molecules and to study the mechanisms generating diversity. To address this aim, sequence variation was examined in 12 serologically assigned alleles from three putative loci and correlated with monoclonal antibody (mAb) binding data. The results demonstrate that both alloantisera and mAbs often fail to distinguish gene products that differ by a significant number of amino acids. Conversely, some mAbs could distinguish alleles differing by only one or two amino acids. Examination of the sequences demonstrates sharing of motifs between alleles, some encoded at distinct loci, supporting the occurrence of interlocus recombination within the cattle MHC class I region. The implications of this for MHC sequence diversity, and functional capability, are discussed.

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.