Longitudinal study on the in vitro immune response to Plasmodium falciparum in Sudan.

Inhibition of Plasmodium falciparum in vitro by human immune serum provides needed information in understanding antimalarial immune mechanisms. Longitudinal, dry season-to-wet season changes in antimalarial activities were studied in sera isolated from 62 individuals living in an area of hyperendemic but unstable malaria. Highly synchronous cultures of P. falciparum were used to distinguish and quantitate two antimalarial activities, merozoite invasion inhibition, and intraerythrocytic parasite retardation. In 54% of the individuals, intraerythrocytic parasite retardation activity increased significantly, nearly threefold, in wet-season sera as compared with dry-season sera. Merozoite invasion inhibition activity was moderate and did not change seasonally. Merozoite invasion inhibition was, however, correlated to parasite-specific immunoglobulin G titers and total serum immunoglobulin G concentrations. These results confirm earlier studies which demonstrate two antimalarial activities in Sudanese sera and provide evidence that intraerythrocytic parasite retardation activity plays a role in antimalarial immunity.