A potential role for plasma uric acid in the endothelial pathology of Plasmodium falciparum malaria

Abstract

Background: Inflammatory cytokinemia and systemic activation of the microvascular endothelium are central to the pathogenesis of Plasmodium falciparum malaria. Recently, 'parasite-derived' uric acid (UA) was shown to activate human immune cells in vitro, and plasma UA levels were associated with inflammatory cytokine levels and disease severity in Malian children with malaria. Since UA is associated with endothelial inflammation in non-malaria diseases, we hypothesized that elevated UA levels contribute to the endothelial pathology of P. falciparum malaria.

Methodology/principal findings: We measured levels of UA and soluble forms of intercellular adhesion molecule-1 (sICAM-1), vascular cell adhesion molecule-1 (sVCAM-1), E-selectin (sE-Selectin), thrombomodulin (sTM), tissue factor (sTF) and vascular endothelial growth factor (VEGF) in the plasma of Malian children aged 0.5-17 years with uncomplicated malaria (UM, n = 487) and non-cerebral severe malaria (NCSM, n = 68). In 69 of these children, we measured these same factors once when they experienced a malaria episode and twice when they were healthy (i.e., before and after the malaria transmission season). We found that levels of UA, sICAM-1, sVCAM-1, sE-Selectin and sTM increase during a malaria episode and return to basal levels at the end of the transmission season (p<0.0001). Plasma levels of UA and these four endothelial biomarkers correlate with parasite density and disease severity. In children with UM, UA levels correlate with parasite density (r = 0.092, p = 0.043), sICAM-1 (r = 0.255, p<0.0001) and sTM (r = 0.175, p = 0.0001) levels. After adjusting for parasite density, UA levels predict sTM levels.

Conclusions/significance: Elevated UA levels may contribute to malaria pathogenesis by damaging endothelium and promoting a procoagulant state. The correlation between UA levels and parasite densities suggests that parasitized erythrocytes are one possible source of excess UA. UA-induced shedding of endothelial TM may represent a novel mechanism of malaria pathogenesis, in which activated thrombin induces fibrin deposition and platelet aggregation in microvessels. This protocol is registered at clinicaltrials.gov (NCT00669084).

Figure 1. Plasma UA levels increase during an acute episode of P. falciparum malaria and further increase with disease severity.

A. UA levels were quantified in paired plasma samples from 69 Malian children before and after the 2009 malaria transmission season, and at their first episode of malaria in the interim. B. UA levels were quantified in plasma samples from Malian children with uncomplicated (UM, n = 487) or non-cerebral severe malaria (NCSM, n = 68). Boxplots show the median, interquartile range, with outliers shown as open circles beyond the range. Data points are displayed by density –.

Figure 2. Correlations between plasma levels of UA and those of sICAM-1 or sTM during an acute episode of P. falciparum malaria.

The levels of UA, sICAM-1 and sTM were quantified in plasma samples from 487 Malian children at their first episode of uncomplicated P. falciparum malaria and their relationships analyzed. A. The graph shows a positive linear correlation between UA and sICAM-1 levels, but not at the lowest and highest UA levels measured. B. The graph shows a positive linear correlation between UA and sTM levels, but not at the lowest and highest UA levels measured. The blue line in each graph is the loess smooth using the default values , showing a moving average line with 95% pointwise confidence intervals.