Synthetic peptides from P. falciparum sexual stage 25-kDa protein induce antibodies that react with the native protein: the role of IL-2 and conformational structure on immunogenicity of Pfs25.

To identify B-cell epitopes of the Plasmodium falciparum 25-kDa ookinete protein, Pfs25, 41 overlapping synthetic peptides spanning the entire length of the protein were used individually to immunize CAF1 (F1 hybrid of BALB/c female and A/J male) mice. Antipeptide sera were tested for reactivity to live intact zygote/early ookinete (post-fertilization stage) by immunofluorescence, and by Western blot analysis under nonreducing and reducing conditions, immunoprecipitation of 35S-cysteine-labeled antigen, and ELISA using a vaccinia recombinant Pfs25 antigen. Fourteen B-cell epitopes were identified. These peptides were immunogenic only when administered with high-dose recombinant interleukin-2. Antibodies to 11 peptides recognized only the native conformational structure, one peptide induced antibodies that recognized both reduced and native protein, and two other peptides, after primary immunization, made antibodies to denatured Pfs25, but after boosting the antibodies reacted to both denatured and native Pfs25. Anti-sera to peptides in the first (peptide 7) and fourth (peptide 34) epidermal growth factor-like domains of Pfs25 reacted most strongly with zygotes/ookinetes by immunofluorescence assay. The antibodies elicited by immunization with peptide 34 suppressed infectivity of the parasite to mosquitoes. We further observed that the secondary structure of Pfs25 may be important for immunogenicity because monoclonal antibodies (MAbs) 1C7 and 1D2, both transmission-blocking MAbs, protected enzyme cleavage sites in Pfs25 from proteolysis, suggesting that discontinuous segments of Pfs25 may come together to form immunogenic epitopic sites. Thus, definition of B- and T-cell epitopes may be required to construct a Pfs25 vaccine for optimum immunogenicity.