Antibodies to nonrepeat sequences of antigen Pf155/RESA of Plasmodium falciparum inhibit parasite growth in vitro.

Immune responses to the repeat regions of the Plasmodium falciparum antigen Pf155/RESA have been extensively studied, and antibodies to the repeats are known to interfere with parasite growth both in vitro and in vivo. Less is known with regard to the effect on parasites of antibodies to the nonrepeat regions of the antigen. In the present study, rabbits were immunized with synthetic peptides corresponding to three different nonrepeated sequences of antigen Pf155/RESA. The reactivity of the antibodies with the particular peptides was analyzed by enzyme-linked immunosorbent assay (ELISA) and that with the parasite antigen, by immunoblotting and immunofluorescence. Although all antisera reacted strongly with the corresponding synthetic peptides, they reacted only weakly with full-length Pf155/RESA in either of the methods used. The specificity of the antibodies for Pf155/RESA was confirmed by their failure to stain Pf155/RESA-deficient parasites in erythrocyte membrane immunofluorescence, a method mainly detecting this antigen. Antibodies to the nonrepeat sequences also efficiently inhibited the merozoite invasion in vitro of Pf155/RESA+ parasites. However, these antibodies also inhibited Pf155/RESA-deficient parasites, indicating the presence of an antigen exhibiting a high degree of homology with Pf155/RESA. The results indicate that nonrepeat sequences of Pf155/RESA are immunogenic and may serve as targets for parasite-neutralizing antibodies, and, thus, the potential of the antigen as a vaccine candidate is emphasized.