| Summary: | 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 (<i>BoLA</i>) class II allelic polymorphisms are associated with PVLs in BLV-infected cattle. We sought to identify relationships between <i>BoLA-DRB3</i> 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-enzyme-linked immunosorbent assay. Restriction fragment length polymorphism-polymerase chain reaction identified 22 <i>BoLA-DRB3</i> alleles. Multivariate negative binomial regression modeling was used to test for associations between BLV PVLs and <i>BoLA-DRB3</i> alleles. <i>BoLA-DRB3.2*3</i>, <i>*7</i>, <i>*8</i>, <i>*11</i>, <i>*22</i>, <i>*24</i>, and <i>*28</i> alleles were significantly associated with low PVLs. <i>BoLA-DRB3.2*10</i> was significantly associated with high PVLs. Some heterozygous allele combinations were associated with low PVLs <i>(*3/*28</i>, <i>*7/*8</i>, <i>*8/*11</i>, <i>*10/*11</i>, and <i>*11/*16)</i>; others were associated with high PVLs <i>(*1/*41</i>, <i>*10/*16</i>, <i>*10/*41</i>, <i>*16/*27</i>, and <i>*22/*27)</i>. Interestingly, the <i>BoLA-DRB3.2*11</i> 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.
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