Association of BoLA class II haplotypes with subclinical progression of bovine leukaemia virus infection in Holstein-Friesian cattle.

The influence of bovine lymphocyte antigen (BoLA) complex polymorphism on subclinical progression of bovine leukaemia virus (BLV) infection was investigated in 41 Holstein-Friesian cows from two herds in Italy. All cows were seropositive for BLV and 22 had persistent lymphocytosis (PL). BoLA-A specificities were defined by serology, and class II haplotypes were defined based on restriction fragment length polymorphism (RFLP) and polymerase chain reaction (PCR)-RFLP analysis of DQ and DR genes. Chi-square analysis revealed a significant and absolute association of haplotype DQA3A-DQB3A-DRB22A-DRB3.211 with resistance to PL (P chi 2 = 0.028, relative risk (RR) = 0.061). Consistent with this observation, multiple regression analysis revealed that animals carrying this haplotype had lower lymphocyte counts (P = 0.0057). By contrast, haplotype DQA12-DQB12-DRB23A-DRB3.28 was associated with susceptibility to PL (P chi 2 = 0.043, RR = 9.625) and increased lymphocyte counts (P = 0.0537). These results confirm the association of haplotype DQA3A-DQB3A-DRB22A-DRB3.211 with resistance to PL, and substantiate earlier findings of haplotype DQA12-DQB12-DRB23A-DRB3.28 as a risk factor for subclinical progression to PL in BLV-infected Holstein-Friesian cattle.