Parasite culture and culture supernatant. P. falciparum asexual stages (3D7 strain) were cultured in vitro in human erythrocytes (blood group O+ or A+) obtained from the Japanese Red Cross Society as previously described (6). To harvest parasite pellets, mature schizonts were purified by using Percoll (GE Healthcare, Camarillo, CA) density gradient centrifugation and further treated with tetanolysin, washed with phosphate-