Monoclonal antibodies identify the protective antigens of sporozoites of Plasmodium knowlesi.

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Abstract
Nine monoclonal antibodies against surface antigens of sporozoites of the simian malaria parasite P. knowlesi were produced by fusion of the plasmacytoma cells with spleen cells of a mouse immunized with the parasites. Immunoprecipitation of extracts of [35S]methionine-labeled sporozoites with 7 of the monoclonals identified the same 3 polypeptides with apparent MW of 52,000 (Pk52), 50,000 (Pk50) and 42,000 (Pk42). These antigens also were recognized by serum of a rhesus monkey immunized with and protected against P. knowlesi sporozoites. Pulse-chase experiments indicated that the higher MW proteins are precursors of Pk42. As shown by trypsin treatment of viable sporozoites, Pk42 is a surface antigen whereas Pk52 and Pk50 appear to be intracellular. Three of the monoclonal antibodies also reacted with a membrane antigen of sporozoites of another simian malaria, P. cynomolgi, and 1 monoclonal antibody reacted with sporozoites of human malaria, P. falciparum. When assayed for sporozoite neutralizing activity, most of the antibodies and their Fab fragments, which recognize Pk52, Pk50 and Pk42, abolished parasite infectivity.