Enzootic bovine leukosis is a lethal neoplastic disease caused by bovine leukemia virus
(BLV), belongs to family Retroviridae. The BLV proviral load (PVL) represents the quantity of
BLV genome that has integrated into the host’s genome in BLV-infected cells. Bovine leukocyte
antigen (BoLA) class II allelic polymorphisms are associated with PVLs in BLV-infected cattle. We
sought to identify relationships between BoLA-DRB3 allelic heterozygosity and BLV PVLs among
different cattle breeds. Blood samples from 598 BLV-infected cattle were quantified to determine
their PVLs by real-time polymerase chain reaction. The results were confirmed by a BLV-enzymelinked
immunosorbent assay. Restriction fragment length polymorphism-polymerase chain reaction
identified 22 BoLA-DRB3 alleles. Multivariate negative binomial regression modeling was used to test
for associations between BLV PVLs and BoLA-DRB3 alleles. BoLA-DRB3.2*3, *7, *8, *11, *22, *24, and
*28 alleles were significantly associated with low PVLs. BoLA-DRB3.2*10 was significantly associated
with high PVLs. Some heterozygous allele combinations were associated with low PVLs (*3/*28,
*7/*8, *8/*11, *10/*11, and *11/*16); others were associated with high PVLs (*1/*41, *10/*16, *10/*41,
*16/*27, and *22/*27). Interestingly, the BoLA-DRB3.2*11 heterozygous allele was always strongly
and independently associated with low PVLs. This is the first reported evidence of an association
between heterozygous allelic combinations and BLV PVLs.