Altered interleukin-2 production by lymphocyte populations from bovine leukemia virus-infected cattle

The effects of bovine leukosis virus (BLV) on the phenotypic and functional characteristics of peripheral blood mononuclear cells were investigated. Whole blood differentials showed that persistently lymphocytotic (BLV+PL) dairy cattle had more lymphocytes and fewer neutrophils than the aleukemic seropositive (BLV+AL) or seronegative (BLV-) animals. Flow cytometric analyses of peripheral blood mononuclear cells indicated that the BLV+PL animals had more B lymphocytes, with a concomitant decrease in CD2 positive cells when compared with the BLV- group. Mononuclear cells from the BLV+AL animals also had fewer CD2 positive cells, but no differences in B lymphocytes were observed when compared with BLV- cattle. Peripheral blood mononuclear cells were used in blastogenesis assays to assess the functional ability of lymphocytes. Lymphocytes from BLV+PL animals had lower proliferative responses to concanavalin A and pokeweed mitogen when compared with cells from the BLV- or BLV+AL groups. The level of spontaneous blastogenesis in the absence of mitogenic stimulation was high for lymphocytes obtained from BLV+AL cattle. Cultures of lymphocytes obtained from BLV+PL animals produced greater amounts of interleukin-2 (IL-2) than BLV+AL and BLV- groups, although no differences were observed in the expression of IL-2 receptors. The development of uncontrolled lymphocytosis in BLV-infected cattle may result from an altered responsiveness to IL-2-regulated B-lymphocyte proliferation.