Groups of Aotus (owl) monkeys were immunized with either the Plasmodiumfalciparum merozoite surfacecoat precursor protein and its processing fragments or a complex of high molecular mass rhoptry proteins and challenged with a lethal infection of the homologous P. falciparum Uganda Palo Alto (FUP) strain. No patent parasitemia could be detected on thick blood films of monkeys immunized with the merozoite surface antigens; however, only one of three monkeys immunized with the rhoptry proteins was partially protected, while two required drug therapy. The experiment clearly demonstrates that the merozoite surface-coat precursor protein can completely protect Aetus monkeys against a lethal infection of the human malaria parasite.

Owl monkeys (Aotus trivirgatus griseimembra) were effectively immunized against a human malaria parasite, Plasmodium falciparum. Two injections of antigen, primarily mature segmenters with fully developed merozoites, mixed with adjuvant (6-O-stearoyl-N-acetylmuramyl-L-alanyl-D-isoglutamine and liposomes) were administered intramuscularly at a 4-week interval. Approximately 2 weeks after the second vaccination, the monkeys were challenged with the homologous strain of P. falciparum. All immunized monkeys survived the challenge. The substitution of Freund's complete adjuvant is an encouraging step toward the development of an effective and safe vaccine for human malaria.

This is the first report of successful immunization of experimental monkeys against a human malaria parasite, Plasmodium falciparum. Of the five owl monkeys (Aotus trivirgatus) used in this pilot study, two served as controls and the other three were immunized with P. falciparum antigen consisting primarily of mature segmenters containing fully developed merozoites. Two injections of antigen emulsified with Freund's complete adjuvant were administered intramuscularly 3 weeks apart. Three weeks after the second vaccination, all monkeys were challenged with the homologous strain of P. falciparum. The control monkeys died with high levels of parasitemia within 2 weeks of challenge. The three immunized monkeys survived and showed strong protection against P. falciparum. These results are encouraging for the possible future development of an effective vaccine against human malaria.

No abstract