Protective immunity against bovine leukaemia virus (BLV) induced in carrier sheep by inoculation with a vaccinia virus-BLV env recombinant: association with cell-mediated immunity.

The effects of vaccination of sheep with a recombinant vaccinia virus (rVV) expressing the bovine leukaemia virus (BLV) envelope glycoprotein (gp60) were studied by determining BLV titres in peripheral blood leukocytes after vaccination and challenge. The proliferation of BLV was suppressed markedly, not only when rVV was inoculated prior to challenge with BLV, but also when it was inoculated after challenge. These results indicate that vaccination with rVV induces protective immunity that can suppress the growth of BLV in carrier animals. Since rVV induced a strong anti-BLV delayed-type hypersensitivity response without producing detectable levels of binding or neutralizing antibodies, and there was no apparent correlation between the humoral immune response and BLV proliferation, a cell-mediated immune response was assumed to play a major role in protective immunity.