Vaccination of cats with attenuated feline immunodeficiency virus proviral DNA vaccine expressing gamma interferon

A feline immunodeficiency virus (FIV) provirus with a vif gene deletion (FIV ΔvifATG;γ) that coexpresses feline gamma interferon (IFN-γ) was tested as a proviral DNA vaccine to extend previous studies showing efficacy with an FIV-pPPRΔvif DNA vaccine. Cats were vaccinated with either FIVΔvifATGγ or FIV-pPPRΔvif proviral plasmid DNA or with both FIV-pPPRΔvif DNA and a feline IFN-γ expression plasmid (pCDNA-IFNγ). A higher frequency of FIV-specific T-cell proliferation responses was observed in cats immunized with either FIVΔvifATGγ or FIV-pPPRΔvif plus pCDNA-IFNγ, while virus-specific cytotoxic-T-lymphocyte responses were comparable between vaccine groups. Antiviral antibodies were not observed postvaccination. Virus-specific cellular and humoral responses were similar between vaccine groups after challenge with a biological FIV isolate (FIV-PPR) at 13 weeks postimmunization. All vaccinated and unvaccinated cats were infected after FIV-PPR challenge and exhibited similar plasma virus loads. Accordingly, inclusion of plasmids containing IFN-γ did not enhance the efficacy of FIV-pPPRΔvif DNA immunization. Interestingly, the lack of protection associated with FIV-pPPRΔvif DNA immunization contrasted with findings from a previous study and suggested that multiple factors, including timing of FIV-pPPRΔvif inoculations and challenge, as well as route of challenge virus delivery, may significantly impact vaccine efficacy.