A persistent foamy virus infection was established in rabbits and its effect on the cell-mediated (CMI) and humoral immune response was studied. Virus was consistently isolated from viable, peripheral blood mononuclear cells collected 7 to 164 day after inoculation, by co-cultivation, with cell monolayers. The response of leukocytes from infected rabbits to in vitro stimulation with phytohaemagglutinin was studied by two techniques: incorporation of 3H-thymidine and production of the lymphokine, immune interferon. Both parameters of the cell-mediated immune response were depressed in leukocytes collected from foamy virus infected rabbits during the first 2 weeks of the infection. This depression in the cell-mediated immune response was not observed after 14 days p.i. Since primary or reactivation infections with herpes viruses are common following immunosuppression, it was interesting to note that one of the rabbits persistently infected with foamy virus developed a herpes virus infection. The humoral response in infected rabbits appears to be unaffected since the virus infection did not alter the development of antibodies to sheep erythrocytes. Transient depression of the cell-mediated immune response in foamy virus infections may be important in initiating persistence and demonstrates for the first time that foamy viruses can, indeed, have adverse effects.
