Cellular immune response in rabbits immunized with purified Visna virus

Visna virus-sensitized lymphocytes were demonstrated in the peripheral blood of rabbits following immunization with purified, concentrated visna virus. Lymphocytes sensitization was maximal at about 2 weeks and fell to control levels in 5 weeks. A delayed-type cutaneous hypersensitivity to the virus was observed at 2 weeks and lasted for at last 9 weeks. Both reactions increased after a booster injection. Although sera from immunized rabbits had precipitating antibodies against visna virus protein P25, their lymphocytes were not stimulated by P25. However, there was a slight reaction to P25 in the skin test. Therefore, in contrast to the humoral immune response, P25 is apparently not the most active viral antigen in the CMI response in rabbits.