In the face of natural infection the efficacy of passive immunity acquired via colostrum from FIV-infected or vaccinated queens is not known. Experimentally, it has been demonstrated that susceptible kittens can be protected from FIV infection following passive transfer of antibody, indicating that antibodies may be protective [Hohdatsu et al. 1993; Pu et al. 1995] in response to challenge with laboratory-adapted isolates of FIV. However, passive transfer of antibody may not protect kittens against infection with virulent field isolates and indeed there is a report of enhanced infection in experimental cats following the passive transfer of antibodies from cats immunised with an experimental vaccine, indicating that a fine balance may exist between neutralising and enhancing antibodies (Siebelink et al. 1995).
Cats infected with FIV are persistently infected in spite of mounting antibody and cellmediated immune responses. CD8+ FIV-specific cytotoxic T cells (CTL) can be detected in the blood within one week of infection [Beatty et al. 1996]. Coincident with the peak in virus load, anti-FIV antibodies, including virus-neutralizing antibodies, appear in the plasma [Fevereiro et al. 1991]. In general, anti-FIV antibodies are detectable from 2-4 weeks post infection, although seroconversion may be delayed in cats exposed to low doses of virus [Hosie and Jarrett 1990]. In experimentally infected cats, it was shown that antibodies recognising Env appeared earlier than antibodies against the Gag protein p24 [Rimmelzwaan et al. 1994]. A population of CD8+ T cells termed CD8low [Willett et al. 1993] has been observed in early FIV infection with some isolates; these cells act as a marker of immune activation by more virulent strains of FIV and may contribute functionally to the non-cytolytic activity against FIV mediated by CD8+ T cells [Flynn et al. 2002].