Abstract

The major merozoite surface Ag (gp195) of Plasmodium falciparum has been shown to protect monkeys against parasite infection, and gp195-based synthetic peptides and recombinant polypeptides have been evaluated as potential malaria vaccines. A major problem in developing a gp195-based recombinant vaccine has been the difficulty in obtaining a recombinant polypeptide that is immunologically equivalent to the native protein. In this study, the carboxyl-terminal processing fragment (p42) of gp195 was produced in yeast and in a baculovirus recombinant system. Immunologic analyses indicated that the secreted baculovirus p42 (BVp42) expressed native, disulfide-dependent conformational epitopes, whereas these epitopes were poorly represented in the intracellular yeast p42. BVp42, but not yeast p42, was also recognized by the majority of gp195-specific antibodies of animals immunized with purified native gp195, indicating that the anti-gp195 response of these animals was focused on conformational determinants of the p42 processing fragment. Sera against native gp195 of congenic mice of diverse H-2 haplotypes recognized the BVp42 polypeptide, demonstrating that a genetically heterogeneous population is capable of responding to p42 epitopes. BVp42 was highly immunogenic and induced high titers of antibodies that were cross-reactive with purified native gp195 in an ELISA and also reacted with schizonts and merozoites by immunofluorescence. Anti-BVp42 antibodies completely inhibited the in vitro growth of the malaria parasite, whereas anti-yeast p42 antibodies had no effect. These results indicate that native, conformational epitopes of p42 are critical for the induction of gp195-specific, parasite growth-inhibitory antibodies and that the BVp42 polypeptide efficiently induces antibodies specific for these native determinants.