[Studies on RESA in Plasmodium falciparum culture medium]

P. falciparum grown in the complete RPMI 1640 medium (containing about 10% normal rabbit serum) for 24 h was collected at about 5% parasitemia. The medium was centrifuged at 500g for 20 min and the supernatant was then centrifuged once more at 40,000g for 30 min. Ten serum samples from falciparum malaria patients with anti-RESA antibodies collected from Yunnan Province were mixed with different dilutions of the culture supernatant, incubated at 37 degrees C for 2h and then at 4 degrees C overnight, and used for conducting RESA-IFA test. The titers of RESA-IFA test of all samples were reduced as compared with controls. When heated supernatant was used in the test, the inhibitory activity remained unchanged. The results indicate that RESA in the supernatants was soluble and heat-stable. To establish the origin of the reacting antigens in the inhibition experiments, the soluble fraction of sonicated merozoite-enriched collection and sonicates of ghost made from normal O phenotype erythrocytes were used in the inhibition RESA-IFA test with sera containing anti-RESA antibodies. The results showed that merozoite-enriched preparations were fully inhibitory at the concentration of 3.1 micrograms/ml, while no inhibition was observed at 0.2 micrograms/ml. In contrast, similar extracts made from normal erythrocyte ghosts were not inhibitory until concentrations about 30 times higher than that of merozoite antigen.