Isolation of three distinct parasite populations expressing different circumsporozoite proteins from the Philippine strain of Plasmodium knowlesi.

Approximately 50% of the sporozoites of the Philippine (P) strain of Plasmodium knowlesi were found to react with monoclonal antibodies, produced against sporozoites of the H strain of P. knowlesi, by indirect immunofluorescence. Two of these anti-H strain monoclonal antibodies were used with the P-strain sporozoites in a neutralization assay. They selectively abolished the infectivity of part of the sporozoite population, and the remainder was designated the P2 strain. Monoclonal antibodies produced against sporozoites of the P2 strain identified two different sets of circumsporozoite proteins by Western blot analysis of an extract of these parasites, indicating the presence of two different populations of sporozoites. These two populations were separated by neutralization assays with monoclonal antibodies that recognized the two different sets of circumsporozoite proteins. The corresponding two strains were designated P3A and P3B. By Western blot analysis, two proteins with apparent molecular weights of 41,000 and 51,000 were identified in extracts of sporozoites of the P3A strain, and two proteins with molecular weights of 38,000 and 47,000 were identified in extracts of the P3B sporozoites.