The simian malaria
Plasmodium knowlesi
provides many favourable features as an experimental model; it can be grown
in vivo
or
in vitro
. Parasites of defined variant specificity and stage of development are readily obtained and both the natural host and a highly susceptible host are available for experimental infection and vaccination trials. Proteins synthesized by erythrocytic
P. knowlesi
parasites are characteristic of the developmental stage, as are the alterations that the parasite induces in the red cell surface. Erythrocytic merozoites are anatomically and biochemically complex, their surface alone is covered by at least eight distinct polypeptides. Immune serum from merozoite-immunized rhesus recognizes many parasite components, especially those synthesized by schizonts. All of the merozoite surface components and some of the schizont-infected red cell surface antigens are recognized by such immune sera. Rhesus monkeys rendered immune by repeated infection may by contrast recognize comparatively few antigens; a positive correlation was established for these ‘ naturally ’ immunized monkeys between protection and antibody directed against a 74000 molecular mass antigen. Im m unization with this purified antigen confers partial protection. O ther putative protective antigens have been identified by monoclonal antibodies that inhibit merozoite invasion of red cells
in vitro
. The antigens recognized by inhibitory monoclonal antibodies are synthesized exclusively by schizonts and are processed, at the time ofschizont rupture and merozoite release, to smaller molecules that are present on the merozoite surface. The multiplicity of protective antigens is clearly demonstrated by the fact that seven distinct merozoite surface antigens are recognized by three different inhibitory monoclonals. None of the protective antigens identified are variant or strain specific.
