Spontaneously proliferating lymphocytes from bovine leukaemia virus-infected, lymphocytotic cattle are not the virus-expressing lymphocytes, as these cells are delayed in G(0)/G(1) of the cell cycle and are spared from apoptosis.

Bovine leukaemia virus (BLV) is in the family of oncogenic retroviruses which includes human T cell leukaemia virus (HTLV). BLV infects B lymphocytes and induces a non-neoplastic persistent lymphocytosis (PL) of B lymphocytes in cattle. A characteristic of BLV- and HTLV-induced disease is spontaneous lymphocyte proliferation of cultured peripheral blood mononuclear cells (PBMC). To investigate the role of virus expression on lymphocyte survival and proliferation, we evaluated cell cycle position, apoptosis and virus expression on a single-cell basis of cultured PBMC from BLV-infected PL cattle, BLV-infected non-PL cattle and uninfected cattle. Results demonstrated that the majority of bovine B lymphocytes spontaneously entered G(2)/M of the cell cycle and died by apoptosis by 24 h post-culture, regardless of BLV infection or PL status. The spontaneous proliferation that characterizes PL cattle was primarily due to a small population of surviving B lymphocytes, but T lymphocytes also contributed. Viral protein expression was detectable in only 5-15% of cultured PBMC from PL cattle and the majority of these lymphocytes were delayed in cell cycle and spared from apoptosis. Unexpectedly, we determined that only 3% of the spontaneously proliferating lymphocytes expressed viral proteins. Previous reports show that spontaneous proliferation decreases when virus expression is suppressed. Together with our results, this suggests that virus expression by one population of B lymphocytes promotes proliferation of another population of B lymphocytes that does not express virus. This may be due to an effect of virus on CD4 T lymphocytes, as depletion of CD4 T lymphocytes significantly decreased spontaneous proliferation.