ANTIBODIES TO NONREPEAT SEQUENCES OF ANTIGEN PFL55 RESA OF PLASMODIUM-FALCIPARUM INHIBIT PARASITE GROWTH IN-VITRO/

Immune responses to the repeat regions of the Plasmodium falciparum an tigen 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 antib odies to the nonrepeat regions of the antigen. In the present study, r abbits were immunized with synthetic peptides corresponding to three d ifferent nonrepeated sequences of antigen Pf155/RESA. The reactivity o f the antibodies with the particular peptides was analyzed by enzyme-l inked immunosorbent assay (ELISA) and that with the parasite antigen, by immunoblotting and immunofluorescence. Although all antisera reacte d strongly with the corresponding synthetic peptides, they reacted onl y weakly with full-length Pf155/RESA in either of the methods used. Th e specificity of the antibodies for Pf155/RESA was confirmed by their failure to stain Pf155/RESA-deficient parasites in erythrocyte membran e immunofluorescence, a method mainly detecting this antigen. Antibodi es to the nonrepeat sequences also efficiently inhibited the mere zoit e invasion in vitro of Pf155/RESA(+) parasites. However, these antibod ies also inhibited Pf155/RESA-deficient parasites, indicating the pres ence of an antigen exhibiting a high degree of homology with Pf155/RES A. The results indicate that nonrepeat sequences of Pf155/RESA are imm unogenic and may serve as targets for parasite-neutralizing antibodies , and, thus, the potential of the antigen as a vaccine candidate is em phasized.