Efficacy evaluation of prime-boost protocol: canarypoxvirus-based feline immunodeficiency virus (FIV) vaccine and inactivated FIV-infected cell vaccine against heterologous FIV challenge in cats.

OBJECTIVE

To evaluate the immunogenicity and prophylactic efficacy of immunization schemes employing a recombinant canarypoxvirus ('ALVAC')-based feline immunodeficiency virus (FIV) vaccine alone or in combination with an inactivated FIV-infected cell vaccine against homologous and heterologous FIV challenges in cats.

METHODS

Specific pathogen-free cats were given a total of three immunizations with subtype A vaccines and challenged 4 weeks after the final immunization with 50 median animal infectious doses (ID50) of FIV-Petaluma, a subtype A isolate. Following the initial challenge, protected cats received a second challenge with 75 ID50 of FIV-Bangston, a subtype B isolate. FIV-specific humoral and cell-mediated responses were measured to determine the immune correlates of protection.

RESULTS

Two of three cats immunized with the ALVAC FIV recombinants alone were protected from homologous FIV challenge in the presence of FIV-specific cytotoxic T-lymphocyte (CTL) responses but in the absence of FIV-specific humoral responses. All three cats immunized with the ALVAC-FIV recombinant and boosted with FIV-infected cell vaccine were also protected from homologous FIV challenge in the presence of both FIV-specific CTL and humoral responses. Partial to full protection was observed in ALVAC-FIV/FIV-infected cell vaccine-immunized cats against a heterologous FIV challenge given 8 months after the initial challenge. Two out of three cats had transient infection and the remaining cat had no sign of FIV infection at a dose at which all three control cats were readily infected.

CONCLUSIONS

Immunization schemes employing ALVAC-based FIV vaccines in combination with inactivated FIV-infected cell vaccine generate protective immune responses that can cross-react with FIV isolates that are genetically distinct from the vaccine strains.