Immunization of monkeys with a 140 kilodalton merozoite surface protein of Plasmodium knowlesi malaria: appearance of alternate forms of this protein.

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Abstract
The merozoite is the invasive stage of the malaria parasite which is released by rupture of the schizont-infected erythrocyte. A monoclonal antibody against a 140 kilodalton (kDa) merozoite surface antigen of Plasmodium knowlesi was used to characterize and to purify this antigen. It was shown by pulse-chase metabolic labeling of mature schizonts that the 140 kDa merozoite antigen was the processed product of a 143 kDa schizont component, and that processing occurred at the time of erythrocyte rupture. Antiserum, prepared by immunizing a rabbit with the 143/140 kDa antigen purified by immunoaffinity chromatography with the monoclonal antibody, strongly inhibited invasion of erythrocytes in vitro; Fab fragments prepared from purified rabbit IgG were inactive at blocking invasion, suggesting that agglutination of merozoites was the mechanism of invasion inhibition. The purified 143/140 kDa antigen was used in Freund's adjuvant to immunize four rhesus monkeys. Two of the immunized animals developed fulminating infections on challenge with 10(4) schizonts, as did the three control animals. The remaining two immunized animals controlled their infections and developed chronic low-grade parasitemias. The animals which were partially protected were those that had developed anti-143/140 kDa antibodies capable of blocking invasion in vitro. Parasites were isolated from the chronic stage of infection (V5 population) and were compared with the original parasite population used for challenge (P population). Inhibition of invasion, immunofluorescence, and immunoprecipitation with anti-143/140 kDa monoclonal antibody, with immune rabbit, and with monkey sera showed that the 143/140 kDa surface antigen had been replaced by multiple cross-reacting alternate antigenic forms of the molecule in the V population. Thus, specific immune response directed against a purified merozoite surface antigen resulted in the replacement of this antigen by variant or mutant forms.