Plasmodium falciparum:Differential Parasite Growth Inhibition Mediated by Antibodies to the Antigens Pf332 and Pf155/RESA

Plasmodium falciparum -reactive antibodies can inhibit growth of parasite blood stages in vitro by interfering at different stages of parasite development. Antibodies reactive with repeat sequences in Pf155/ring-infected surface antigen (Pf155/RESA) or in antigen Pf332 inhibit parasite growth as determined by a reduction of newly infected erythrocytes. Antibodies to Pf155/RESA have been implicated in merozoite invasion inhibition. Since it was considered unlikely that antibodies to the late stage antigen Pf332 act on merozoite invasion, we investigated at what developmental stage this growth inhibition takes place. Parasites were cultured in the presence of antibodies to repeat sequences of either Pf332 or Pf155/RESA and were examined with regard to the distribution of different stages and their morphology. As expected, antibodies to both antigens decreased the number of ring stages after reinfection. They did not affect the intraerythrocytic development of ring stages or trophozoites. However, antibodies to Pf332 induced high levels of schizonts displaying an abnormal morphology while antibodies to Pf155/RESA induced considerably lower levels of degenerated schizonts. Thus, Pf332-reactive antibodies seem to interfere with the schizont development by blocking the rupture of mature schizonts or, alternatively, by interfering intraerythrocytically with late stage parasites.