Angiopoietin-2 is associated with decreased endothelial nitric oxide and poor clinical outcome in severe falciparum malaria

Abstract

Adherence of parasitized erythrocytes to activated endothelium causes microvascular obstruction, tissue ischemia, and clinical complications in severe malaria (SM); however, the mechanisms leading to endothelial activation remain unclear. The angiogenic factors, angiopoietin-2 (Ang-2) and vascular endothelial growth factor (VEGF) are modulators of endothelial activation, with Ang-2 release from Weibel-Palade bodies (WPBs) being regulated by endothelial nitric oxide (NO). We explored the relationships between endothelial NO bioavailability, Ang-2, VEGF, tissue perfusion, and clinical outcomes in SM. We measured plasma Ang-2 and VEGF, together with biomarkers of severity from 146 adults with and without SM, in parallel with longitudinal measures of endothelial function by using reactive hyperemia peripheral arterial tonometry (a measure of endothelial NO bioavailability). Regression was used to relate concentrations of Ang-2/VEGF with malaria disease severity, biomarkers of perfusion, endothelial activation, and parasite biomass. The longitudinal relationship between Ang-2 and endothelial function was assessed by using a mixed-effects model. Ang-2 concentrations were elevated in SM and associated with increased venous lactate, plasma intercellular cell adhesion molecule-1 concentrations, parasite biomass, and mortality. In contrast, VEGF concentrations were inversely associated with these biomarkers. Ang-2 concentrations were significantly better predictors of death than venous lactate (P = 0.03). Recovery of endothelial function was associated with falling concentrations of Ang-2. Ang-2 release from endothelial cells with reduced NO bioavailability is likely to contribute to endothelial activation, sequestered parasite biomass, impaired perfusion, and poor outcome in severe falciparum malaria. Agents that improve endothelial NO, reduce WPB exocytosis, and/or antagonize Ang-2 may have therapeutic roles in SM.

Fig. 1.

Ang-2, VEGF, and lactate concentrations in patients with MSM and SM and HC. (A) Ang-2 plasma concentrations among disease categories (Kruskal–Wallis: P < 0.001). (B) VEGF plasma concentrations among disease categories (Kruskal–Wallis: P = 0.006). (C) Lactate concentrations (P < 0.001). Horizontal lines indicate median for each group. Differences among groups were compared by using the Kruskal–Wallis test. The Mann–Whitney test was used for post hoc pairwise comparisons.

Fig. 2.

Longitudinal course of plasma Ang-2 concentrations (A) and RH-PAT index (B) over time. Mean values (circles) and 95% C.I. (bars) are displayed at each time point. x axis values show time from the start of antimalarial therapy (day 0, 0–12 h; day 1, 13–36 h; day 2, 37–60 h; day 3, 61–84 h; day 4, 85–109 h; day 5–14, >110 h). In a mixed-effects model, the increase in RH-PAT index (27) was associated with the fall in Ang-2 (r = −0.38; P < 0.001).

Fig. 3.

Nonparametric ROC assessing Ang-2 as a prognostic marker for mortality in SM. AUROC, 0.84; 95% C.I., 0.71–0.96.