In vitro infection of cells of the monocytic/macrophage lineage with bovine leukaemia virus

The oncogenic retrovirus bovine leukaemia virus (BLV) primarily infects B c ells. Most infected animals remain asymptomatic for long periods of time be fore an increase in circulating B cells or localized tumours can be observe d. This long clinical latency period may be explained by cells of the monoc yte/macrophage lineage (M/M) becoming infected and acting as a reservoir fo r the virus, as shown for other retroviruses (human immunodeficiency virus- 1, feline immunodeficiency virus). M/M cells in different stages of differe ntiation (HL-60, THP-1, U-937, J774, BGM, PM2, primary macrophages of sheep and cows) were cultured with BLV produced by permanently infected donor ce lls (FLKBLV and BLV-bat(2)). Donor cells were inhibited from multiplying by either irradiation or treatment with mitomycin C. In other experiments, su pernatant from donor cells containing virus was used. In co-culture with th e donor cells, the less differentiated monocytic cells showed severe cellul ar changes such as differentiation, vacuolization, cell lysis and membrane blebbing; apoptosis was a frequent phenomenon. Budding and extracellular vi ruses were also observed. The more differentiated macrophage cells, althoug h they showed less signs of infection by microscopy, had a complete BLV pro tein profile, as seen by Western blotting; bands corresponding to p24CA (Ga g) and its precursors were clearly seen. In addition, gp51SU was identified by syncytia formation assays. It is concluded that M/M cells may be infect ed by BLV, the consequences of the infection differing according to the typ e of cell.