In vitro inhibition of intracellular growth of Plasmodium falciparum by immune sera.

Beginning at the ring stage, synchronized cultures of Plasmodium falciparum were grown in suspension for 22-32 hours. Intracellular growth was assayed by measuring cellular uptake and incorporation into protein of 35S-methionine. Low concentrations (2%) of serum from immune humans and Aotus monkeys were found to inhibit the uptake of the 35S-methionine. The amount of inhibition for a given serum was often inversely related to its indirect fluorescent antibody test titer. Inhibition occurred during the trophozoite stage and was not obtained with a clone lacking the erythrocyte modifications referred to as knobs. Thus, a sensitive new assay is described which allows detection of factors in immune primate sera which can affect maturation of P. falciparum within the erythrocyte. These serum factors are likely to be antibodies which react with antigens expressed at the trophozoite stage on the surface of K+-infected erythrocytes.