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

Abstract

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.

Antibodies to a non-repeat region ofPlasmodium falciparumantigen Pf155/RESA in individuals from malaria-endemic areas

“…In the present study we show that af®nity-puri®ed human antibodies reactive with Pf155/RESA non-repeat sequences as presented in MAP14 and 15 also have the capacity to inhibit parasite growth in vitro. In contrast to most antibodies to Pf155/RESA repeat sequences [44], the rabbit antibodies to non-repeat sequences of the antigen were shown also to inhibit the in vitro growth of parasites de®cient in Pf155/RESA [18], indicating cross-reactivity with some other antigen. Whether the corresponding human antibodies show a similar inhibitory effect on Pf155/RESA-de®cient parasites remains to be tested.…”

“…We have recently also shown that rabbit antibodies to non-repeat sequences in the N-terminal part of Pf155/RESA, including antibodies to MAP14 and 15, inhibit parasite growth in vitro [18,43]. However, antibodies to the cytoadherence related motifs in Pf155/RESA as presented in MAP16 and 18 were without effect in this respect [43].…”

“…Furthermore, B cell epitopes, frequently recognized by antibodies from individuals living in malaria-endemic areas, have been detected in the N-terminal non-repeat region of Pf155/RESA [17,22,42]. Rabbit antibodies induced by synthetic peptides corresponding to certain sequences in this region have the capacity to inhibit P. falciparum growth in vitro [18,43], indicating that this region of the antigen may also constitute an important target for immune responses during malaria infection. We now report that individuals living in malaria-endemic regions show a high level of seroreactivity to sequences in the N-terminal non-repeat region of Pf155/RESA.…”

“…Duplicates were set up of all samples and the percentages of parasitaemia were determined from the mean of 20 000 erythrocytes analysed. Experiments where the parasitaemia in the absence of antibodies had increased twice or more from the time zero parasitaemia were included in the analysis [18].…”

“…B and T cell epitopes have been described in a segment within the N-terminal non-repeat region of Pf155/RESA residue 170±220 [17,19±22]. Also, rabbit antibodies to sequences in non-repeated regions of the antigen have been demonstrated to inhibit P. falciparum growth in vitro [18].…”