Induction of feline leukaemia virus-neutralizing antibodies by immunization with synthetic peptides derived from the FeLV env gene.

The surface glycoprotein, gp70, of feline leukaemia virus (FeLV) contains sites which are important in inducing neutralizing antibodies. Using synthetic peptides corresponding to nucleotide sequence 729-957 (amino acid sequence 243-319) of FeLV-A/Glasgow-1, the antigenicity and immunogenicity of this part of the viral surface glycoprotein were investigated. The region contains two highly conserved domains separated by a variable region (VR4), when compared with FeLV of subgroups B and C, and an epitope known to be involved in virus neutralization is located in the N-terminal conserved domain. Five murine monoclonal antibodies generated by immunization with virus were found to be directed to this domain and four were virus-neutralizing. Polyclonal mouse, rabbit and cat anti-FeLV antisera, which were virus-neutralizing, were directed to B-cell epitopes in the peptides. To determine if those synthetic peptides could induce neutralizing antibodies, rabbits were immunized with the peptides, singly or in combination. Antibody responses were measured by ELISA for anti-peptide, anti-FeLV and anti-gp70 activity, by immunoblotting, by membrane immunofluorescence and by virus-neutralization tests. Virus-neutralizing antibodies were induced by FeLV gp70 peptides and there was a synergistic effect on antibody production when a combination of peptides covering amino acid sequence 243-319 of FeLV-A was used. In a second experiment, six rabbits and six cats were immunized with a combination of two peptides, which covered the above-defined FeLV gp70 sequence. Comparisons were made of the responses to these peptides incorporated into immunostimulating complexes (ISCOMs) via myristic acid tails, inoculated with 'empty' ISCOMs, or with Al(OH)3. Complete Freund's adjuvant (CFA) had a very strong potentiating effect on the induction of antibodies and immunization with peptides incorporated into ISCOMs was superior to immunization using adjuvants other than CFA. It is very promising that not only in rabbits, but also in the natural host of FeLV, the cat, anti-FeLV gp70 (peptides) antibodies could be induced by FeLV gp70 peptides.