Impact of <i>BoLA‐DRB3</i> Polymorphisms on Clonality of Bovine Leukaemia Virus‐Infected Cells of Cattle With Lymphoma

Bovine leukaemia virus (BLV) infects cattle, integrates into the host genome as a provirus, and induces a persistent infection that remains asymptomatic but can cause leukaemia/lymphoma. Most BLV-infected cell clones are created by massive depletion, and a few of these infected cell clones expand through the mitotic cycle, leading to the onset of lymphoma. Bovine lymphocyte antigen (BoLA)-DRB3 polymorphism is associated with susceptibility to BLV-induced leukemogenesis. However, whether BoLA-DRB3 polymorphism affects the clonality of BLV-infected cells in vivo remains unknown. Here, we investigated whether lymphoma integration sites have specific features in cattle with varying susceptibility to lymphoma. Genomic DNA was extracted from 99 BLV-infected Holstein cattle with lymphoma in a nationwide survey across Japan, and the integration sites were analysed using BLV proviral DNA-capture sequencing, which we had previously developed. Among the integration sites identified in 99 animals, no identical sites were confirmed. Comparison of integration sites between cattle with susceptible and resistant BoLA-DRB3 alleles showed no significant differences in the distribution of integration sites on the chromosome and in the genes and intergenic regions. With respect to the orientation of the proviruses, or proviral structures between individuals with resistance or susceptibility to lymphoma. In contrast, resistant animals showed a significantly higher proportion of monoclonal cell types than susceptible animals. In summary, the BoLA-DRB3 polymorphism affects BLV clonality; for example, there is massive depletion and clonal expansion of infected cell clones in fully transformed clones obtained from BLV-infected cattle with lymphoma.