OCCURRENCE OF LYMPHOCYTOSIS IN DAIRY CATTLE HERDS WITH HIGH INCIDENCE OF LYMPHOSARCOMA*

Integration of bovine leukaemia virus at all stages of enzootic bovine leukosis

Integration of bovine leukaemia virus DNA was investigated at all stages of infection in cattle. We report here the detection of integrated proviral DNA in the majority of antibody positive animals without lymphocytosis. In all but one case virus was integrated at a number of different sites. Hybridization analysis failed to detect proviral sequences in animals shown to be BLV antibody-negative by the Agar Gel Immunodiffusion assay. The pattern of integration in leukocytes from animals with persistent lymphocytosis was similar to that in sero-positive animals without lymphocytosis in that multiple sites of integration were evident. As reported by others only one or a few sites of integration were detected in tumours from enzootic bovine leukosis animals. Tumours from different sites in individual animals were either monoclonal or oligoclonal.

Evidence that the spontaneous blastogenesis of lymphocytes from bovine leukemia virus-infected cattle is viral antigen specific

Cattle lymphocytes cultured for 3 days were found to spontaneously incorporate thymidine (3STI). Under optimal conditions of culture, the median magnitude of 3STI activity in lymphocytes from bovine leukemia virus (BLV)-infected cattle was higher than that of BLV-free cattle, but the ranges of the values overlapped. However, the 3STI activity of most BLV-infected cattle was specifically inhibited by serum containing BLV antibodies, whereas the 3STI activity of BLV-free cattle was not. The 3STI inhibitor copurified with immunoglobulin, and its activity could be absorbed with BLV. Rabbit anti-BLV serum inhibited 3STI, but rabbit anti-BLV p25 did not. These results indicate that BLV infection induces or expands a BLV-specific lymphocyte population. Spontaneous blastogenesis may be indicative of an immune response which controls virus spread.