Experimentally, it has been demonstrated that susceptible kittens can be protected from FeLV
infection following passive immunisation with high titred antisera against FeLV [Hoover et
al., 1977]. Once persistent viraemia has become established, treatment with virus neutralizing
monoclonal antibodies to FeLV is ineffective [Weijer et al., 1986].
Most cats that overcome FeLV viraemia exhibit high antibody titres to the virus (ELISA or VN) [Lutz et al., 1980a; Russel & Jarrett, 1978]; antibodies are directed against all components of the virus [Lutz et al., 1980a]. In most – but not all – cats that overcame viraemia, virus neutralising antibodies can be detected [Flynn et al., 2002]. Since not all immune cats develop high antibody titres, it was concluded that cytotoxic T-lymphocytes (CTLs) are also important in FeLV immunity [Lutz et al., 1980a]. Indeed, it was demonstrated recently that CTLs specific for FeLV appear before virus neutralising antibodies and that following adoptive transfer of FeLV specific CTLs stimulated in vitro, the viral load in FeLV viraemic cats could be lowered, consistent with an important role for CTLs in FeLV immunity [Flynn et al., 2002].