Preclinical evaluation of a synthetic Plasmodium falciparum MAP malaria vaccine in Aotus monkeys and mice.
Multiple antigen peptides (MAPs) containing epitopes of the major surface protein of the malaria sporozoite, the circumsporozoite (CS) protein, have been shown in previous studies to elicit antibody-mediated protection against sporozoite challenge in experimental murine and simian hosts. For the preparation for a phase I trial of a P. falciparum (T1B)4 MAP, which contains T and B cell epitopes from the CS repeat region, pre-clinical immunogenicity and adjuvant formulation studies were carried out in mice and Aotus monkeys. The (T1B)4 MAP was found to be immunogenic in three different species of owl monkeys, Aotus nancymae, A. vociferans and A. nigriceps. Optimal antibody responses were obtained in A. nancymae immunized s.c. with (T1B)4 MAP emulsified in Freund's, in which peak titers of over 10(6) were obtained in individual monkeys. MAP immunized A. vociferans also developed high levels of anti-sporozoite antibodies, although the kinetics and the magnitude of the response differed from A. nancymae. (T1B)4 MAP adsorbed to alum (aluminum hydroxide), a formulation that is acceptable for human use, was less immunogenic in naive A. nancymae, as well as A. nigriceps. The injection of MAPs/alum, however, significantly enhanced antibody responses in sporozoite-primed monkeys, suggesting that the administration of the MAP vaccine may be an effective means to increase the low levels of antibody present in individuals living in malaria endemic areas. The addition of a co-adjuvant QS-21, a purified saponin, significantly increased the immunogenicity of the alum-adsorbed MAP in both mice and monkeys, providing a vaccine formulation suitable for phase I trials in human volunteers.