The Complexity of Bovine Leukemia Virus Oncogenesis

Bovine leukemia virus (BLV) is a retrovirus infecting several bovid species, notably Bos taurus, where it fulfills Koch's postulates for pathogenicity. The virus primarily targets B-lymphocytes, establishing lifelong infections that remain mostly asymptomatic but can progress to lymphocytosis or lymphoma. Transmission occurs through live infected cells via blood, milk, or transplacental routes. Despite a robust antiviral immunity, BLV replicates by producing virions (i.e., the infectious cycle) or inducing mitosis of infected cells (i.e., clonal expansion). The immune system effectively controls the infectious cycle but fails to impede clonal expansion, leading to chronic immune activation and immunosuppression. BLV modifies the transcriptome of the host cell by expressing oncogenic factors (Tax), viral microRNAs and antisense RNAs. Leukemogenesis arises from cumulative alterations of the virus (e.g., 5'-end deletions of the integrated provirus and histone modifications of the LTR promoter) and the host cell (e.g., genomic mutations and favorable chromatin integration). This model underscores a unique persistence strategy, linking chronic infection, immune evasion, and slow multistep oncogenesis in the bovine host.