Localization of protective 143/140 kDa antigens of Plasmodium knowlesi by the use of antibodies and ultracryomicrotomy.

Immune sera from mice immunized with the 143/140 kDa protein have been shown to partially block erythrocyte invasion by P. knowlesi merozoites. Therefore, immunoelectron microscopy utilizing ultracryomicrotomy, antibody to 143/140 kDa protein, and protein A gold particles were used to determine the precise localization of this protein in malarial parasites. Gold particles were not seen associated with young trophozoites but appeared in the parasite cytoplasm as the parasites grew to multi-nucleate schizonts. In presegmenter-schizonts, gold particles were associated with the well-developed endoplasmic reticulum, the parasite plasma membrane, and the parasitophorous vacuole membrane. The surface of merozoites was covered with gold particles. Maurer's clefts, which appeared in Plasmodium infected erythrocytes, were also associated with gold particles. These observations suggest that 143/140 kDa protective malarial proteins may be synthesized in the endoplasmic reticulum of P. knowlesi schizonts before being transported to the surface of the schizonts and merozoites. Shedding of the merozoite surface coat may be responsible for the presence of the 143/140 kDa proteins in the parasitophorous vacuole and Maurer's clefts.