BoLA-DRB3 alleles influence proviral load and peripheral blood lymphocyte distribution in bovine leukaemia virus-infected cattle

Background: Bovine leukaemia virus (BLV)-infected Holstein cattle carrying certain bovine leukocyte antigen (BoLA)-DRB3 alleles were previously shown to be resistant to BLV provirus multiplication, while those carrying other alleles were susceptible. This study aimed to determine whether the BoLA-DRB3 alleles carried by BLV-infected cattle could predict proviral load (PVL) and peripheral blood lymphocyte (PBL) count distribution (PVL/PBL distribution).

Methods: Blood samples from Holstein cattle on four dairy farms were tested for the presence of BLV antibodies using a commercial ELISA. The PVL and PBL levels of the BLV-infected cattle were also measured, and genotyping was performed to identify the BoLA-DRB3 alleles they carried, impact of the various BoLA-DRB3 alleles on the PVL/PBL distribution was then investigated.

Results: Of the 316 cattle tested, 114 were positive for BLV. BLV-infected cattle carrying BoLA-DRB3 alleles DRB3*009:02, DRB3*002:01 and DRB3*014:01:01 were classified as resistant (n = 43), those carrying DRB3*012:01 and DRB3*015:01 alleles were classified as susceptible (n = 42) and the remaining cattle were classified as nonsusceptible/nonresistant (n = 29). Multiple regression analysis revealed that PVL was positively correlated (p = 2.1 × 10^-23) with PBL count and age was negatively correlated (p = 1.9 × 10^-6) with PBL count. Cattle with DRB3*014:01:01 tended to have a lower PBL count (p = 0.031).

Limitation: The effects of the BoLA-DRB3 alleles DRB3*002:01, DRB3*009:02, DRB3*012:01 and DRB3*015:01 on PVL/PBL distribution were unclear due to the small numbers of BLV-infected animals carrying these alleles.

Conclusion: The BLV transmission risk in cattle can be estimated by examining their BoLA-DRB3 alleles.