The susceptibility of the human malaria parasite, Plasmodium falciparum, to killing in vitro by macrophage secretory products was investigated. The effect of 02 radicals and tumor necrosis factor on parasite viability was assessed both morphologically and by following the uptake of [3H]hypoxanthine. H202 produced by the interaction of glucose and glucose oxidase was found to reduce viability; this effect was reversed by the addition of exogenous catalase. Further studies indicated that the catalase level within the erythrocyte was not altered upon parasite invasion. 02 radicals produced during the xanthine-xanthine oxidase interaction also killed P. falciparum. The addition of various 02 radical scavengers (including catalase) did not reverse this effect; therefore, it was not possible to determine which of the 02 radicals were involved in the killing process. Samples from three different sources containing tumor necrosis factor, a nonspecific soluble mediator derived from Mycobacterium bovis BCG-activated macrophages treated with endotoxin, also killed the parasite. There was no evidence that tumor necrosis factor or the products of the xanthine-xanthine oxidase interaction caused damage to the erythrocyte membrane that could be implicated as an important aspect of the killing process. These findings all strongly suggest that such macrophage products play an important role in immunity to malaria. * Corresponding author. was assessed both morphologically and by measuring the uptake of [3H]hypoxanthine over a 24-h period after treatment. MATERIALS AND METHODS Parasites. P. falciparum (Wellcome/Liverpool strain) was maintained by routine cultivation procedures (26), using RPMI 1640 medium supplemented with 15% ARh+ human serum and N-tris(hydroxymethyl)methyl-2-aminoethane sul

In a recent study (1), we demonstrated a role for the macrophage (MO) type 3 complement receptor (CR3) in serum-independent binding of Leishmania donovani promastigotes to murine resident peritoneal Mo (RPM). Earlier studies had also established a role for CR3 in serum-independent binding of the yeast wall product zymosan to murine and human MO (2) and to human neutrophils (3). In all three studies, binding of the parasite/particle was inhibited by mAbs (M1/70, Mot, OKM1, MN-41, and anti-Leu-15) directed against the a-chain of CR3 . In the neutrophil system it was argued that binding of zymosan to CR3 involved direct lectin-like recognition processes not mediated by complement, as binding was inhibited by an anti-CR3 mAb (OKM1) not specifically directed against the iC3b binding site, but not inhibited by Fab anti-C3 (3). In the MO system it was postulated that complement proteins secreted by MO mediate local iC3b-opsonization of zymosan and promastigotes, both of which are good activators of the alternative complement pathway (1, 2). This was supported by three independent lines of evidence . First, binding of zymosan to MO was also reduced after treatment of M¢ with cyclohexamide, an inhibitor of protein synthesis (2). Second, the potent inhibition of promastigote binding to RPM using MI/70 was completely mimicked with Fab anti-C3 or the nucleophile sodium salicyl hydroxamate (1). The latter is a potent inhibitor of the covalent binding of activated C3 to the activator surface (4), thus providing strong evidence that cleaved C3, in the degraded form iC3b, on the surface of promastigotes mediates binding to CR3. Finally, under assay conditions identical to those used in binding assays, it was shown (2) by SDS polyacrylamide gel analysis that MO-derived C3 (iC3b) can be deposited on zymosan in the absence of exogenous complement . In this study, the evidence for local iC3b-oponization is extended by direct visualization of Mo-derived C3 on the surface of L. donovani promastigotes using an anti-C3 antibody and a protein A-gold conjugate in electron microscope sections .

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A study has been made of the damage incurred by normal and Plasmodium falciparum-infected human erythrocytes following exposure to a variety of oxidant-generating systems. Hydrogen peroxide, produced by the glucose-glucose oxidase system, increased methaemoglobin formation within normal erythrocytes while normal levels of oxyhaemoglobin were maintained. Exposure to products of the xanthine-xanthine oxidase interaction did not have the same effect. Malondialdehyde measurements indicated that the host cell membranes of parasitized cells had undergone lipid peroxidation even before exposure to the oxidant-generating systems. Lipid peroxidation of normal and parasitized cell membranes was increased upon exposure to reagent-grade hydrogen peroxide and alloxan: this increase was not observed following exposure to the two enzyme-substrate systems that generated reactive oxygen intermediates. In addition, the effects of parasitism on intracellular levels of catalase and superoxide dismutase were assessed. Normal and parasitized erythrocytes were found to possess similar levels of these enzymes, which protect against oxidant-induced damage. It was therefore concluded that the increased susceptibility of infected cells to oxidant damage was probably not related to any decrease in the function of these enzymes.