Immunogenicity and efficacy trials in Aotus nancymai monkeys with model compounds representing parts of a 75-kD merozoite surface antigen of Plasmodium falciparum.

We tested the ability of a recombinant DNA-encoded fragment (C7Ag) of a Plasmodium falciparum merozoite protein (p75) and of two carrier-free peptide models (28-mer and 76-mer) to stimulate boostable antibody responses in Aotus nancymai monkeys. In addition, we evaluated protection against challenge with the Uganda Palo Alto (FUP) strain of this parasite. The data indicate that C7Ag elicited a strong and boostable IgG antibody response in all the monkeys immunized. However, studies with the peptide models demonstrated that various animals produce antibodies to different portions of this structure. When the post-boost sera from monkeys immunized with C7Ag were analyzed for reactivity against two major portions of C7Ag, most of the antibody response was observed against the disulfide-bonded 76-residue region that forms a conformational immunogenic epitope. In the same sera, antibody levels against the charged helical region modeled with a 28-mer were generally low. Immunization with synthetic peptides revealed that the 76-mer stimulated an antibody response almost as strong as C7Ag, with substantial cross-reactivity against the parasite antigen. The 28-mer evoked a response that was not efficient or uniform, and showed little reactivity with the authentic parasite antigen. Aotus nancymai was shown to be susceptible to infection with the Uganda Palo Alto strain of P. falciparum; however, maximum parasitemia varied markedly in both immunized and control monkeys. Statistical analysis failed to recognize differences in maximum parasitemia between the vaccine and control groups. The variation in maximum parasitemia suggests that the FUP strain in this species of Aotus is a poor model for the detection of differences in efficacy based on maximum parasitemia. This initial study with structures based on parts of the 75-kD merozoite surface antigen of P. falciparum indicated that both the recombinant-produced protein C7 and the 76-mer synthetic peptide, when combined with a Syntex adjuvant formulation, were safe and immunogenic in A. nancymai monkeys. However, the data emphasize the problems of using animal models to evaluate the potential effects of immunogens in humans.