The killing of blood-stage malaria parasites in vivo has been attributed to reactive intermediates of oxygen (ROI) and of nitrogen (RNI). However, in the case of the latter, this contention is challenged by recent observations that parasitemia was not exacerbated in nitric oxide synthase (NOS) knockout (KO) (NOS2 ؊/؊
or NOS3؊/؊ ) mice or in mice treated with NOS inhibitors. We now report that the time course shows that Plasmodium chabaudi parasitemia in NADPH oxidase KO (p47 phox؊/؊ ) mice also was not exacerbated, suggesting a minimal role for ROI-mediated killing of blood-stage parasites. It is possible that the production of protective antibodies during malaria may mask the function of ROI and/or RNI. However, parasitemia in B-cell-deficient J H ؊/؊ ؋ NOS2 ؊/؊ or J H ؊/؊ ؋ p47 phox؊/؊ mice was not exacerbated. In contrast, the magnitude of peak parasitemia was significantly enhanced in p47 phox؊/؊ mice treated with the xanthine oxidase inhibitor allopurinol, but the duration of patent parasitemia was not prolonged. Whereas the time course of parasitemia in NOS2 ؊/؊ ؋ p47 phox؊/؊ mice was nearly identical to that seen in normal control mice, allopurinol treatment of these double-KO mice also enhanced the magnitude of peak parasitemia. Thus, ROI generated via the xanthine oxidase pathway contribute to the control of ascending P. chabaudi parasitemia during acute malaria but alone are insufficient to suppress parasitemia to subpatent levels. Together, these results indicate that ROI or RNI can contribute to, but are not essential for, the suppression of parasitemia during blood-stage malaria.The adaptive immune response to blood-stage malaria parasites is dependent on both cell-and antibody-mediated immune mechanisms. Accumulating evidence indicates that activated CD4 ϩ T cells release factors, including gamma interferon, which induce downstream mechanisms to kill parasites (reviewed in references 32, 34, and 52). Observations of fragmented DNA and condensed chromatin suggest that apoptotic and/or cytotoxic mechanisms are responsible for the killing of blood-stage malaria parasites (40). Both reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI), especially nitric oxide (NO), have been suggested as possible malaria-parasite-killing agents (9, 50).Based on observations that different ROI-generating chemicals, such as tert-butyl hydroperoxide, alloxan, and phenylhydrazine, and ROI enzyme-producing substrate systems had the ability to kill murine and human malarial parasites, Clarke et al. proposed that ROI function to kill malaria parasites (9, 11). ROI are utilized in the body as oxidative, cytotoxic agents and are produced by phagocytic cells during the oxidative burst induced by infection (44). The majority of ROI, hydrogen peroxide (H 2 O 2 ) or superoxide ions (O 2 Ϫ ), are produced via two pathways, involving phagocyte NADPH oxidase or hypoxanthine metabolism (3, 24). In the phagocyte NADPH oxidase pathway, a complex of four structural proteins, flavocytochrome b 558 Ϫ can function ...
