Effects of plasma and serum on the in vitro expression of bovine leukemia virus

Bovine leukemia virus (BLV) is difficult to detect in freshly isolated peripheral blood mononuclear cells (PBMCs) from infected cattle. The lack of viral expression has been attributed to a plasma blocking factor capable of inhibiting the in vitro expression of the BLV p24 antigen.

The purpose of this study was to investigate the effects of plasma and serum on the in vitro synthesis of the p24 antigen within short-term cultures of PBMCs from BLV-seropositive cows. BLV p24 antigen was detected in the cell lysate using an antigen capture enzyme-linked immunosorbent assay.

Antigen synthesis was suppressed in PBMCs cultured in plasma from both seropositive and seronegative cows. Serum from infected cows suppressed p24 expression, but to a lesser extent than any of the plasmas. Addition of phytohemagglutinin-P to the cultures resulted in the detection of more p24 in PBMCs cultured in serum, but had no effect on p24 expression within cells cultured in plasma. The reduction in p24 was not related to lower cell viability, nor was it influenced by the effects of the anticoagulant citrate-phosphate dextrose on calcium-dependent lymphocyte activation. No significant difference was observed in the potency of the p24 inhibition between cells grown in autologous and homologous serum, or between cells grown in autologous and homologous plasma from seropositive cows.

Blocking activity is present in the plasma of seronegative cows, and in the plasma and serum of BLV-seropositive cows. This activity may be important in regulating viral latency and provirus expression in vivo.