The bovine major histocompatibility complex: Immunogenetic study of the BoLa-DRB3 locus and disease associations

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Sharif, Shayan

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University of Guelph

Abstract

The bovine major histocompatibility complex class II 'DRB3' gene ('BoLA-DRB3') and its associations with diseases of dairy cattle were investigated. Canadian dairy cows from Holstein (n = 835) and Jersey (n = 66) breeds were genotyped for allelic variants of this locus based on the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique. Cows were monitored for one lactation for occurrence of disease, milk somatic cell counts, production, and the bacterial agents that caused intramammary infection resulting in clinical mastitis. One allele of this locus in Canadian Holsteins ('BoLA-DRB3.2*23') was significantly ('P' <= 0.05) associated with a higher risk of occurrence of severe mastitis (odds ratio of 4.8). Allele '16' was significantly (' P' <= 0.05) associated with lower milk somatic cell score, which is a measure of udder health. These alleles did not adversely affect production traits, such as milk, fat and protein yield, in this population. However, allele ' BoLA-DRB3.28' was associated with increased milk and fat yield, whereas allele *'22' was associated with decreased milk and protein yield. Presence of glutamic acid at position b 74 in pocket 4 of the BoLA-DR antigen binding groove was significantly ('P' <= 0.05 and relative risk of 11) associated with a higher risk of intramammary infections with 'Staphylococcus' species causing clinical mastitis. This residue is present at position b 74 of alleles *'22', *'23' and *' 24'. Other motifs in this pocket, such as the presence of a positively charged residue at position b 13 and arginine at position b 71 showed a tendency ('P' <= 0.1) towards association with an increase in occurrence of ' Staphylococcus' spp. mastitis cases. To shed more light on the underlying biological mechanisms of BoLA-disease associations, transfectant cell lines were established that could constitutively express BoLA allele '23 ', which was associated with occurrence of mastitis, and ' BoLA-DRB3.28' as the control allele which was the most frequent allele in Holsteins. To accomplish this, full length cDNA encoding the DRB and DRA chains of these two alleles were cloned and sequenced. A novel plasmid expression vector was also constructed to facilitate the process of transfection and to reduce costs of transfection. This plasmid comprised two expression cassettes, each one carrying the full length cDNA of BoLA-DRA and DRB chains. The mouse L929 cell line transfected with this plasmid could transcribe the mRNA of BoLA-DRA and DRB chains, as confirmed by reverse transcriptase (RT)-PCR. Moreover, these cells had surface expression of the BoLA-DR molecule, as evidenced by flow cytometry and the population of BoLA-DR positive cells was raised to more than 90% by magnetic cell sorting.

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dairy cattle, bovine major histocompatibility complex, BoLaDR83 locus, milk somatic cell counts, clinical mastitis

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