REDUCED BOVINE LEUKEMIA-VIRUS PROVIRAL LOAD IN GENETICALLY RESISTANT CATTLE

The bovine leukaemia virus (BLV) is an exogenous retrovirus that is cl osely related to the human T cell leukaemia viruses. Genetic resistanc e and susceptibility to persistent lymphocytosis (PL), an advanced sub clinical stage of infection characterized by a polyclonal expansion of the infected B cell population, have been mapped to structural motifs in bovine MHC DRB3 (class II) alleles. To determine whether alleles o f DRB3 influence the number of BLV-infected B cells in peripheral bloo d, seven pairs of Holstein cows naturally infected with BLV were match ed on the basis of DRB3 genotype (resistance or susceptibility to FL), age, and year of seroconversion. Flow cytometry was used to separate B cell populations that then were tested for the presence of provirus by a single-cell PCR methodology. Animals with, the PL-resistance asso ciated DRB3.211 allele had significantly fewer BLV-infected B cells t han did age- and seroconversion-matched cows with DRB3 alleles associa ted with susceptibility to FL. Our results demonstrate that DRB3 or a closely linked gene may play a direct role in controlling the number o f BLV-infected peripheral B cells in vivo. Association of MHC class II alleles with resistance to disease progression in cattle naturally in fected with BLV provides a unique immunogenetic model for the study of host resistance to human and other animal retroviral infections.