Detection of bovine leukaemia virus RNA sequences in non-cultivated peripheral lymphocytes by in situ hybridization with 3H-labelled viral cDNA.

Bovine leukaemia virus (BVL) in circulating lymphatic cells of the blood is repressed and has to be activated for its expression. After in situ hybridization of BLV-3HcDNA with a specific activity of 3.9 X 10 dpm/micrograms to noncultivated blood cells from 5 leukotic cattle, in 4 of these animals from 0.96% to 1.5% lymphocytes were found to contain BLV-specific RNA sequences. Non-hybridized lymphocytes as well as calf thymus cells, used as controls, gave only 0.47% and 0.39% labelled cells, respectively. After short-term cultivation of white blood cells from these leukotic animals the percentage of labelled cells increased. During the short-term cultivation the number of hybridizing cells from one and the same animal and from all animal, when compared with each other, varied to a high extent. The highest values of BLV-RNA sequences-containing cells from all 5 animals at different cultivation times were within the range of from 2.4 to 16.2%.