Protection by vaccination against bovine leukemia virus infection in sheep

Four preparations were tested as potential vaccines to protect sheep against bovine leukemia virus (BLV) infection. Purified glycoprotein (gp) 51 and protein (p) 24 antigens from the virus and glutaraldehyde-fixed fetal lamb kidney (FLK) cells chronically infected with BLV or sheep fibroblasts transformed with BLV (SF-28 cells) were used to inoculate 12 sheep. Each vaccine was given 3 times 2 and 4 weeks apart to 3 sheep. Six sheep vaccinated with gp51 antigen or fixed FLK cells developed complement-fixing antibody against gp51; the titers ranged from 1:8 to 1:128 at the time of virus challenge exposure at postinoculation week 9. Although the 6 sheep inoculated with p24 antigen or fixed SF-28 cells developed antibody against the respective inoculum, none of these sheep had gp51 antibody at the time of challenge exposure. All 12 vaccinated and 2 control sheep were challenge exposed with BLV-infected lymphocytes, and cells from the sheep subsequently were tested for infection by syncytium assay. Sheep inoculated with gp51 antigen or FLK cells were protected, but sheep inoculated with p24 antigen or SF-28 cells became infected. The cytotoxic activity of lymphocytes from protected and nonprotected sheep was not different from that of normal sheep. Seemingly, purified gp51 antigen or fixed FLK cells were capable of preventing BLV infection in sheep. Humoral immune responses to gp51 appears to have an important role in protection against BLV infection, whereas cytotoxic activity of lymphocytes does not.