Influence of class II (DRB3) histocompatibility antigens in Italian Holstein Friesian bulls on milk production

The Bovine Major Histocompatibility Complex (BoLA), is a region of 23 chromosome, where series of extremely polymorphic genes are located, encoding important cell surface proteins of the immune response to different antigens. Owing to its high degree of polymorphism, this region may be considered a potential marker for disease resistance and for performance traits in several species of animals. The involvement of BoLA in bovine health has been largely investigated by many authors, like Slob et al. (1982), who analyzed the effect of BoLA Class I on Mastitis, or association studies on Bovine Virus Diarrhoea (Dam and Ostergard 1985) or on Bovine Leukaemia Virus Infection (Lewin and Bernoco 1986, Lewin et al. 1988). On the contrary its involvement in production traits, particularly milk yield, has been less studied. Indeed MHC genes might have a direct effect on production or an indirect effect by influencing production traits through linkage to Quantitative Trait Loci (QTL) or by influencing the health condition of animals. This influence has a great economic importance and for this reason selection programs must lead to get disease resistant animals without penalizing quantitative and qualitative production traits. In accordance with this quest, the present study started with the analysis of Class II BoLA-DRB3 exon 2 polymorphism, on a sample of 119 progeny tested Holstein Friesian bulls. BoLA-DRB3 locus has been chosen because it seems the unique expressed locus on the cell membrane and exon 2 because it is responsible of the MHC-antigen interaction, playing an essential role in the presentation of foreign antigens to immune cells. A total of 18 DRB3 alleles were distinguished by PCR amplification of BoLA-DRB3 exon 2 and by digestion with RsaI, MflI, HaeIII restriction enzymes (PCR-RFLP). All restriction fragment patterns were consistent with restriction sites that were found in previous studies (Van Eijk et al. 1992). The breeding values of the examined animals carried out by ANAFI (Associazione Nazionale Frisona Italiana) with the BLUP-ANIMAL MODEL both for productive and morphological traits. The productive traits considered are: milk, fat, protein yield expressed by Kg, fat and protein percentage. The effect of DRB3 alleles were estimated using least square analysis (LQS) by the regression coefficients in the SAS package, on the bulls' production traits. Rare haplotypes, with frequencies below 0.05, have been put togheter into a unique class. The effect of each allele was expressed as the difference from the mean effect of all alleles in the analysis. A negative effect of DRB3.2*16 allele on fat yield, was found (p=0.054). For the other traits no association with BoLA-DRB3 haplotypes was observed. These results suggest that genes of bovine MHC, directly or indirectly, are associated with milk production performances, but since all the effects on multifactorial traits are small more research is necessary to identify significant associations to be used in marker assisted selection and to determine its biological mechanism.