T lymphocyte responses of sheep to bovine leukaemia virus infection.

Sheep were experimentally infected with bovine leukaemia virus (BLV) by inoculation of peripheral blood lymphocytes (PBL) from BLV infected sheep. Monoclonal antibodies were used to monitor changes in lymphocyte subpopulations in the first few weeks after inoculation. The polymerase chain reaction (PCR) detected BLV DNA in PBL of infected sheep 11-15 days after inoculation, that is, before antibodies to viral structural proteins were detected at 15-39 days post-inoculation. A rise in the number of both B and T lymphocytes coincided with detection of infection by PCR. At this time, an increase in the number of circulation CD8+ lymphocytes resulted in a low CD4: CD8 ratio. It appears that in BLV infection there is a host specific cell-mediated immune response to infected lymphocytes rather than a general immune response to foreign antigens. This response, which is characterized by an increase in the number of circulating CD8+ lymphocytes, precedes seroconversion. There is considerable variation between animals in this cytotoxic T lymphocyte response.