EPCR and Malaria Severity: The Center of a Perfect Storm

Abstract

Severe malaria due to Plasmodium falciparum infection causes nearly half a million deaths per year. The different symptomatology and disease manifestations among patients have hampered understanding of severe malaria pathology and complicated efforts to develop targeted disease interventions. Infected erythrocyte sequestration in the microvasculature plays a critical role in the development of severe disease, and there is increasing evidence that cytoadherent parasites interact with host factors to enhance the damage caused by the parasite. The recent discovery that parasite binding to endothelial protein C receptor (EPCR) is associated with severe disease has suggested new mechanisms of pathology and provided new avenues for severe malaria adjunctive therapy research.

Keywords: EPCR; PfEMP1; Plasmodium; cytoadhesion; malaria; protein C.

Figure 1. The role of EPCR in severe malaria

In healthy microvessels, EPCR promotes a quiescent state in endothelial cells (anti-coagulant and anti-adhesive) by enhancing the activation of protein C by the thrombin-thrombomodulin complex. APC elicits anti-apoptotic, anti-inflammatory and pro-barrier signaling pathways in endothelial cells by cleaving PAR1 at Arg46. Additionally, soluble APC inactivates the coagulation factors FV and FVIII leading to an anti-thrombotic state. In severe malaria, the parasite CIDRα1 domain (yellow) of PfEMP1 blocks the protein C binding site in EPCR. The lack of APC will cause an increase of thrombin formation and eliminate a key negative feedback mechanism for regulating inflammation and coagulation. Thrombin elicits proinflammatory and barrier disruptive signals in endothelial cells by cleaving PAR1 at Arg41 and promotes the release of WPB and their contents (Ang-2, VWF) from activated endothelial cells. Additionally, thrombin cleaves fibrinogen to lead to fibrin deposition. Abbreviations: Ang-2, angiopoietin-2; APC, activated protein C; CIDRα1, cysteine-rich interdomain region α1; EPCR, endothelial protein C receptor; PAR1, protease-activated receptor 1; PfEMP1, P. falciparum erythrocyte membrane protein 1; VWF, von Willebrand factor; WPB, Weibel-Palade bodies.

Figure 2. Schematic representation of the pathogenic events that contribute to severe malaria

(A) Alterations in coagulation. EPCR-binding parasites block the activation of APC. The lack of APC reduces the inactivation of FVIII and FV, and thereby promotes an increase in thrombin generation. Additionally, binding of IEs to endothelial cells induces an upregulation of tissue factor leading to a pro-thrombotic state. Thrombin causes fibrin deposition and enhances microthrombi formation. (B) Parasite factors that mediate loss of barrier properties and impaired vasodilation. Parasite toxins secreted during IE rupture, such as PfHRP-2 and extracellular P. falciparum histones, activate endothelial cells contributing to loss of barrier properties. Rupturing erythrocytes release hemoglobin (Hb) and arginases leading to a decrease in NO bioavailability and impaired vasodilation. (C) Host factors that promote endothelial activation and permeability. Reduced bioavailability of NO and high concentrations of thrombin will increase the release of Ang-2 and VWF from activated endothelial cells. Both thrombin and Ang-2 promote endothelial activation and leakiness, and VWF contributes to a pro-coagulant state by recruiting platelets and coagulation factor (FVIII). Activated endothelial cells present an increased expression of endothelial markers contributing to the recruitment of monocytes. Sequestered monocytes present hemozoin pigment (Hz) due to phagocytosis of IEs and secrete proinflammatory cytokines, such as TNF-α and IFN-γ. (D) Microvascular obstruction. Microcirculatory blockages can result from a combination of sequestration and microvascular congestion. Reduced perfusion may potentiate the effect of inflammatory mediators and endothelial activation and lead to stronger IE binding or greater IE recruitment. Reduced tissue perfusion causes a shift to an anaerobic metabolism and metabolic acidosis. Abbreviations: Ang-2, angiopoietin-2; APC, activated protein C; EPCR, endothelial protein C receptor; IE, infected erythrocytes; IFN-γ, interferon-γ; NO, nitric oxide; PfHRP-2, P. falciparum histidine-rich protein-2; TNF-α, tumor necrosis factor-α; VWF, von Willebrand factor.