Up-regulation of tumor necrosis factor α mRNA is associated with bovine-leukemia virus (BLV) elimination in the early phase of infection

Protective immune responses were analyzed in eight sheep vaccinated with BLV envelope peptides and experimentally infected with bovine-leukemia virus (BLV). Five of eight peptide-immunized sheep showed a high T-cell proliferative response to the BLV peptides and all of these were protected from the infection. The other three peptide-immunized sheep showed no T-cell proliferative responses to any BLV antigens similar to control sheep, though they also exhibited resistance to BLV challenge. To investigate other mechanisms which suppress BLV expansion in these non-responding sheep, we measured the levels of the cytokine expressions before, and after, BLV challenge using competitive reverse-transcriptase polymerase chain-reaction systems. It was revealed that the expression of tumor necrosis factor α (TNFα) was higher in BLV-resistant sheep than in BLV-susceptible sheep. Thus, TNFα expression rather than specific T-cell activity may play an important role in the protective mechanism against BLV infection, at least during the primary viremia phase.