Peptide mimics of a conformationally constrained protective epitopes of respiratory syncytial virus fusion protein

Aims: To identify peptides that mimic (mimotopes) conformational and protective epitopes of RSV fusion protein and to assess their efficacy as immunogens and potential vaccines. Material and methods: An 8-mer solid- phase (TG resin) library was screened with a neutralising and protective RSV fusion protein specific monoclonal antibodies (Mab-19). After selection of positive beads, reactive sequences were identified by microsequencing and 8- mer peptides were synthesised. Improvement of binding was analysed by amino acid replacement using the SPOTs method. Results: Mabs were not able to bind to the free and soluble peptides, nor did these peptides induce anti-RSV specific antibodies. However, several peptides re-synthesised on a TG resin (to produce de-protected 8-mer peptides linked to the resin) or as SPOTs reacted specifically. Therefore it was critical to be able to reproduce this conformation in order to use these mimotopes as immunogens and potential vaccines. Using C-terminal constrained versions of the mimotopes, strong binding of one of the Mabs to the peptides was demonstrated by surface- plasmon resonance. Immunisation of Balb/c mice with these peptide-mimics produced anti-sera that: (1) reacted specifically with RSV; (2) inhibited the binding of the Mab to the virus; (3) neutralised RSV in vitro with high titres (range: 80-640); and (4) reduce significantly the vital load in the lungs of mice challenged with RSV (P < 0.01). Conclusions: This report demonstrates for the first time that: (1) a protective epitope of the conserved RSV fusion protein can be mimicked by synthetic peptides: and (2) immunisations with these mimotopes induced specific anti-RSV neutralising antibodies and reduced vital load in vivo. These results represent a novel concept for the development of a vaccine against RSV.