Role of immune serum and complement in stimulation of the metabolic burst of human neutrophils by Plasmodium falciparum

Interaction between human neutrophils (polymorphonuclear leukocytes [PMN]) and Plasmodium falciparum in the natural defense of the host remains to be elucidated. In patients with acute malaria, oxygen consumption (QO2) of PMN at rest and after stimulation by zymosan was significantly increased compared with that in the controls. With 10% immune serum, both QO2 and chemiluminescence of normal PMN were significantly increased after stimulation by a P. falciparum erythrocyte culture. This activation was not observed with a nonparasitized erythrocyte culture and was correlated with parasitemia. Immune serum and complement were required to trigger this metabolic activation of normal PMN. With normal serum or heat-inactivated immune serum, a parasitized erythrocyte culture did not significantly stimulate QO2 or chemiluminescence of normal PMN. The classical complement pathway was essential for this stimulation, whereas the alternate pathway was less involved. Hyperimmune sera from subjects residing in endemic areas were more able to trigger the metabolic burst than were immune sera from subjects from other sources. The use of synchronous cultures showed that PMN were more stimulated by cultures rich in merozoites than by the same cultures which contained only intraerythrocytic forms. Giemsa staining showed granules of hemozoin and occasional merozoites or parasitized erythrocytes within PMN. This increase in production of activated oxygen radicals could damage intra-or extraphagocytic parasitic forms. As P. falciparum is sensitive to oxidant stress and PMN is the phagocyte with the most intense metabolic burst, the role of PMN in defense against malaria should be considered.