Severe falciparum malaria (SM) is associated with tissue ischemia related to cytoadherence of parasitized erythrocytes to microvascular endothelium and reduced levels of NO and its precursor, l-arginine. Endothelial function has not been characterized in SM but can be improved by l-arginine in cardiovascular disease. In an observational study in Indonesia, we measured endothelial function using reactive hyperemia–peripheral arterial tonometry (RH-PAT) in 51 adults with SM, 48 patients with moderately severe falciparum malaria (MSM), and 48 controls. The mean RH-PAT index was lower in SM (1.41; 95% confidence interval [CI] = 1.33–1.47) than in MSM (1.82; 95% CI = 1.7–2.02) and controls (1.93; 95% CI = 1.8–2.06; P < 0.0001). Endothelial dysfunction was associated with elevated blood lactate and measures of hemolysis. Exhaled NO was also lower in SM relative to MSM and controls. In an ascending dose study of intravenous l-arginine in 30 more patients with MSM, l-arginine increased the RH-PAT index by 19% (95% CI = 6–34; P = 0.006) and exhaled NO by 55% (95% CI = 32–73; P < 0.0001) without important side effects. Hypoargininemia and hemolysis likely reduce NO bioavailability. Endothelial dysfunction in malaria is nearly universal in severe disease, is reversible with l-arginine, and likely contributes to its pathogenesis. Clinical trials in SM of adjunctive agents to improve endothelial NO bioavailability, including l-arginine, are warranted.

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SummaryNitric oxide (NO)-related activity has been shown to be protective against Plasmodium fakiparum in vitro. It has been hypothesized, however, that excess NO production contributes to the pathogenesis of cerebral malaria. The purpose of this study was to compare markers of NO production [urinary and plasma nitrate + nitrite (NO~)], leukocyte-inducible nitric oxide synthase type 2 (NOS2), and plasma TNF-c~ and IL-10 levels with disease severity in 191 Tanzanian children with and without malaria. Urine NO• excretion and plasma NOx levels (corrected for renal impairment) were inversely related to disease severity, with levels highest in subclinical infection and lowest in fatal cerebral malaria. Results could not be explained by differences in dietary nitrate ingestion among the groups. Plasma levels of IL-10, a cytokine known to suppress NO synthesis, increased with disease severity. Leukocyte NOS2 antigen was detectable in all control children tested and in all those with subclinical infection, but was undetectable in all but one subject with cerebral malaria. This suppression of NO synthesis in cerebral malaria may contribute to pathogenesis. In contrast, high fasting NO x levels and leukocyte NOS2 in healthy controls and asymptomatic infection suggest that increased NO synthesis might protect against clinical disease. NO appears to have a protective rather than pathological role in African children with malaria.

Summary MRL-I/r/I/rHowever, the chromosomal location of the NOS gene was consistent with an insulin-dependent diabetes locus identified in an analysis of nonobese diabetic (NOD) mice. These results suggest that elevated nitric oxide production could be important in the pathogenesis of autoimmunity, and that treatments to block the production of nitric oxide or block its effects might be valuable therapeutically.

We compared interleukin-12 (IL-12) and other cytokine activities during and after an acute clinical episode in a matched-pair case-control study of young African children who presented with either mild or severe Plasmodium falciparum malaria. The acute-phase, pretreatment plasma IL-12 and alpha interferon (IFN-␣) levels, as well as the acute-phase mitogen-stimulated whole-blood production capacity of IL-12, were significantly lower in children with severe rather than mild malaria. IL-12 levels, in addition, showed strong inverse correlations both with parasitemia and with the numbers of circulating malaria pigment-containing neutrophils. Acute-phase plasma tumor necrosis factor (TNF) and IL-10 levels were significantly higher in those with severe malaria, and the concentrations of both of these cytokines were positively correlated both with parasitemia and with the numbers of pigment-containing phagocytes in the blood. Children with severe anemia had the highest levels of TNF in plasma. In all the children, the levels in plasma and production capacities of all cytokines normalized when they were healthy and parasite free. The results indicate that severe but not mild P. falciparum malaria in young, nonimmune African children is characterized by down-regulated IL-12 activity, contrasting markedly with the up-regulation of both TNF and IL-10 in the same children. A combination of disturbed phagocyte functions resulting from hemozoin consumption, along with reduced IFN-␥ responses, may contribute to these differential effects.

Interleukin (IL)-12 and transforming growth factor (TGF)-beta1 regulate the balance between pro- and anti-inflammatory cytokines in animal models of malaria. Since the cytokine balance may be an important determinant of whether a protective or a pathogenic immune response develops, plasma cytokine ratios were examined in Gabonese children with various degrees of malarial severity. Severe disease was characterized by high-density parasitemia and severe anemia. IL-12 and TGF-beta1 were significantly lower, whereas tumor necrosis factor (TNF)-alpha and IL-10 were significantly higher in children with severe malaria. The ratios of TGF-beta1/IL-12 and IL-10/IL-12 were significantly higher in the severe, compared with the mild, malaria group. In contrast, ratios of TGF-beta1/TNF-alpha and IL-10/TNF-alpha were significantly lower in the severe malaria group. These results suggest that the inflammatory cascade in severe malaria is characterized by suppression of the protective effects of TGF-beta1 and IL-12, and that overproduction of TNF-alpha may promote deleterious effects, such as severe anemia.

IntroductionImpaired T cell function in sepsis is associated with poor outcome, but the mechanisms are unclear. In cancer, arginase-expressing myeloid derived suppressor cells (MDSCs) deplete arginine, impair T cell receptor CD3 zeta-chain expression and T cell function and are linked to poor clinical outcome, but their role during acute human infectious disease and in particular sepsis remains unknown. Hypoarginemia is prevalent in sepsis. This study aimed to determine whether neutrophils that co-purify with PBMC express arginase, and if arginine depletion constrains T cell CD3 zeta-chain expression and function in human sepsis.MethodsUsing flow cytometry, cell culture, HPLC, arginase activity and mRNA detection, our study examined whether neutrophils, with reduced buoyant density isolated in the Ficoll interface, metabolise L-arginine and suppress T cell proliferation in sepsis. A total of 35 sepsis patients (23 with septic shock) and 12 hospital controls in a tertiary referral hospital in tropical Australia were evaluated.ResultsOnly sepsis patients had interphase neutrophils, neutrophils co-purifying with mononuclear cells (≤1.077 specific gravity). The percentage of interphase neutrophils in sepsis was proportional to sepsis severity and correlated with plasma IL-6 concentrations. Ex vivo, sepsis-derived interphase neutrophils expressed arginase, metabolised culture L-arginine and suppressed T cell proliferation and CD3 zeta-chain expression. In vivo, in septic shock there was a longitudinal inverse association between interphase neutrophil number and CD3 zeta-chain expression. Depletion or inhibition of interphase neutrophils in vitro restored zeta-chain expression and T cell function.ConclusionsFor the first time during an acute human infection, interphase neutrophils that express arginase were found to circulate in sepsis, in proportion to disease severity. These neutrophil-MDSCs impair T cell CD3 zeta-chain expression and T cell function via L-arginine metabolism, and likely contribute to the T cell dysfunction seen in sepsis. Modulation of neutrophil-MDSC or their downstream effects warrant consideration as targets for novel adjunctive therapies in sepsis.Electronic supplementary materialThe online version of this article (doi:10.1186/cc14003) contains supplementary material, which is available to authorized users.

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