Preliminary studies on vaccination of rhesus monkeys with irradiated sporozoites of Plasmodium knowlesi and characterization of surface antigens of these parasites.

Studies were conducted to develop an effective method of inducing protection against sporozoite-induced malaria in a primate system and to obtain information regarding the surface membrane antigens of sporozoites. Immunization of rhesus monkeys was performed with gamma-irradiated sporozoites of Plasmodium knowlesi. Levels of antisporozoite antibodies were monitored by immunofluorescence, sporozoite neutralization, and the circumsporozoite precipitate reaction, and appeared to correlate well with protection. Only the intravenous route was effective in inducing both protection and antisporozoite antibodies. Immunization with sporozoites mixed with Freund's complete adjuvant failed completely to induce protection and resulted in a minimal antibody response. Mechanisms of resistance to sporozoites probably involve the interaction of the host's immune system with the parasite's surface antigen(s). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of surface-labelled, partially purified sporozoites followed by autoradiography revealed the presence of a small number of labelled proteins in the extract. Immunoprecipitation with specific antisera to P. berghei detected primarily one of these membrane components, with an apparent molecular weight of 41 000. The molecular weight of this main surface antigen in sporozoites of P. berghei was different from that in sporozoites of P. knowlesi.