Active and Passive Immunization of Cats With Inactivated Feline Oncornaviruses 2

The humoral Immune responses of young (<1 mo) and older (>6 mo) cats to killed and live feline leukemia virus (FeLV), feline sarcoma virus (FeSV), and the feline oncornavirus associated cell membrane antigen (FOCMA) were evaluated. in addition, the biologic responses to oncogenic feline oncornavlrus challenge were analyzed in cats actively immunized with inactivated Gardner-Arnstein FeSV (GA-FeSV) or passively immunized with either the Rickard FeLV (R-FeLV) or GA-FeSV. Kittens were immunized during the first month of life with UV- or formalin-inactivated GA-FeSV and challenged at 5–6 weeks of age with a known oncogenic dose of GA-FeSV. Passive immunization was evaluated in kittens that suckled mothers immunized during pregnancy with purified GA-FeSV or R-FeLV. Kittens during the first month of life were capable of producing humoral antibody against FOCMA and keyhole-limpet hemocyanin. Apparently linear disease responses to live R-FeLV from susceptibility (viremia with no FOCMA antibody) to resistance (no viremia and high FOCMA antibody level) were observed in cats from birth to 4 months of age. Newborn kittens failed to develop FOCMA and virus-neutralizing (VN) antibody (<1:2) following challenge, whereas adult cats responded with significant levels of FOCMA and VN antibody 2 months after infection. Thirty-three young kittens immunized with UV- or formalin-inactivated GA-FeSV failed to develop significant levels of VN antibody (≤2) in contrast to 7 immunized adult female cats that responded with significant levels (mean titer, 1:14). No evidence of protection from oncogenic challenge was observed in actively immunized kittens. Moreover, the results suggested that vaccinated kittens were more susceptible to oncogenic virus challenge than were nonvaccinated cats. Actively immunized cats had a shorter survival time, greater primary tumor size, faster rate of tumor growth, and a greater extent of tumor metastases. in addition, significantly fewer immunized cats responded to FOCMA and viral envelope antigens than did nonimmunized cats. With the use of the same criteria, definitive evidence of protection from oncogenic virus challenge was observed in kittens that had been nursed by dams vaccinated during pregnancy.