Blood coagulation, inflammation, and malaria

Abstract

Malaria remains a highly prevalent disease in more than 90 countries and accounts for at least 1 million deaths every year. Plasmodium falciparum infection is often associated with a procoagulant tonus characterized by thrombocytopenia and activation of the coagulation cascade and fibrinolytic system; however, bleeding and hemorrhage are uncommon events, suggesting that a compensated state of blood coagulation activation occurs in malaria. This article (i) reviews the literature related to blood coagulation and malaria in a historic perspective, (ii) describes basic mechanisms of coagulation, anticoagulation, and fibrinolysis, (iii) explains the laboratory changes in acute and compensated disseminated intravascular coagulation (DIC), (iv) discusses the implications of tissue factor (TF) expression in the endothelium of P. falciparum infected patients, and (v) emphasizes the procoagulant role of parasitized red blood cells (RBCs) and activated platelets in the pathogenesis of malaria. This article also presents the Tissue Factor Model (TFM) for malaria pathogenesis, which places TF as the interface between sequestration, endothelial cell (EC) activation, blood coagulation disorder, and inflammation often associated with the disease. The relevance of the coagulation-inflammation cycle for the multiorgan dysfunction and coma is discussed in the context of malaria pathogenesis.

FIG. 1

Coagulation cascade and its regulation. A) Pro-coagulant mechanisms. TF: a critical initiator of coagulation. Formation of a complex with Factor VIIa (FVIIa) leads to activation of FIX and FX. FXa in the presence of phosphatidyl serine and Ca²⁺ (prothrombinase complex) amplifies the coagulation cascade through conversion of prothrombin to thrombin, resulting in platelet aggregation, fibrin formation, and inflammation. Thrombin also activates FXI to XIa, which activates FIX to FIXa. FIXa in the presence of phosphatidyl serine and Ca²⁺ converts FX to FXa, consolidating the coagulation cascade. pRBC, parasitized red blood cells. B) Anticoagulant mechanism. TF pathway inhibitor (TFPI) binds to FXa and inhibits FVIIa/TF complex. Protein C is activated by thrombin (in the presence of thrombomodulin and EPCR), and APC inhibits the coagulation cascade through cleavage of cofactors FVa and FVIIIa. Antithrombin in the presence of heparin sulphate specifically interacts with and inhibits FXa and thrombin. Heparin cofactor II (in the presence of dermatan sulphate) inhibits thrombin. C). Pro- and antifibrinolytic mechanism. PAI-1, plasminogen activator inhibitor-1. The zymogen plasminogen is converted to the active serine protease, plasmin, through the action primarily of two-chain tissue plasminogen activator (tc-tPA) or two-chain urokinase (tc-uPA). Both tPA and uPA can be inhibited by plasminogen activator inhibitor-1 (PAI), while plasmin is inhibited by its major inhibitor, α₂-antiplasmin, and to a lesser extent by α₂-macroglobulin (not shown). For details, see Supplementary Material (Section B, “Tissue Factor and the Blood Coagulation Cascade”).

FIG. 2

Compensated and decompensated responses and modulation by the coagulation cascade. Activation of coagulation cascade as a consequence of inflammation is an essential part of the host defense of the body. This physiologic response is triggered in an effort to contain the invading entity and to keep the consequent inflammatory response to a limited area. An exaggerated or uncontrolled response, however, may lead to a situation in which coagulation and microthrombosis contribute to disease. This is illustrated by the occurrence of systemic coagulation activation in combination with microvascular failure, which results from the systemic inflammatory response to severe infection or sepsis, and that contributes to multiple organ dysfunction. Modified from Ruf, 2004 [108].

FIG. 3

The coagulation-inflammation cycle. Diagrammatic representation of activation of coagulation and inflammation in response to an underlying disorder (e.g., infection). Infection is potentially associated with induction of pro-inflammatory cytokines and TF expression. Exposure of TF-bearing inflammatory cells (e.g., EC, monocytes) to blood results in thrombin generation, platelet aggregation, and conversion of fibrinogen to fibrin. Thrombin and other activated coagulation factors activate protease-activated receptors on inflammatory cells, inducing release of proinflammatory cytokines, which will further modulate coagulation through induction of TF, on one hand, and prevention of fibrinolysis through decrease in PAI-1 and thrombomodulin function, on the other. The vicious cycle between coagulation-inflammation leads to microvascular thrombosis, EC activation, increased vascular permeability, ROS generation, metabolic stress, apoptosis and ultimately to organ dysfunction. For details, see text and Supplementary Material (Section C, “Coagulation-inflammation cycle and the Relevance for Multiorgan Dysfunction and Coma in Malaria and Sepsis”).

FIG. 4

TF and the interface of pathologic features observed in malaria. Sequestration is associated with endothelial cell activation (estimated by an increase in ICAM-1, VCAM-1, E-selectin, and TF expression). TF expression is needed for activation of the coagulation cascade, which promotes thrombocytopenia, increase in TAT levels, and D-dimers. Coagulation factors also induce secretion of inflammatory cytokines (TNF-α, IL-1β, and IL-6), which in turn induce TF expression in different cell types promoting and sustaining a coagulation-inflammation cycle (for details, see text). TF, Tissue Factor.

FIG. 5

TF staining of EC associated with sequestration in a pediatric CM case. IHC was performed with anti-TF monoclonal antibodies. A) Regular light showing positive staining in the EC. B) Polarized light that detects hemozoin, indicative of sequestration. Staining was negative in the absence of primary antibody. For details, see text and [82].

FIG. 6

The TF model of human CM: Initiation: Normal, quiescent endothelium does not express TF in the absence of biologic stimulation. According to this model, sequestration and/or sequestration-associated events (e.g. cytokines, fibrin, hypoxia, apoptosis, proinflammatory molecules released by pRBC which activate TLRs such as GPI and plasmodium DNA-containing hemozoin) primarily induces EC activation in the microvessels of the brain and in other vascular beds. This contributes to TF expression, at sequestration sites, and/or possibly paracrinally. Monocytes may also be a source of TF in malaria. Mechanistically, TF initiates the coagulation cascade through binding to coagulation FVIIa and the substrate FIX and FX (extrinsic Xase). In this ternary initiation complex, FIXa and FXa are generated. Amplification: FXa, FVa, and prothrombin assemble in the pRBC surface and/or activated platelets, with formation of the prothrombinase complex leading to explosive thrombin formation and amplification of the coagulation cascade. Thrombin thus formed promotes fibrin deposition and induces platelet aggregation. Thrombin also activates FXI to FXIa (not shown), which activates FIX to FIXa. FIXa, FVIIIa, and FX assemble in the membrane of activated platelets or pRBC with formation of the intrinsic Xnase complex required for production of additional FXa owing to feedback inhibition of the FVIIa/TF complex by TFPI(not shown). Therefore, the TF model of human CM proposes that initiation of blood coagulation by TF expression and the amplification phase supported by pRBC (and/or activated platelets) - particularly at sequestration sites where the concentration of pRBC-derived phosphatydilserine is presumably very high - are critical for disease pathogenesis. Coagulation-inflammation: TF/VIIa, TF/FVIIa/FXa, and thrombin activate protease-activated receptors (PARs) in different cell types including mononuclear cells and EC at sequestration sites and/or paracrinally. PAR activation in EC is accompanied by upregulation of molecules (e.g., ICAM-1, VCAM-1, E-selectin, COX-2, NO synthase) and production of proinflammatory cytokines (e.g., TNF-α, IL-1β, IL-6) reportedly found in CM. Because cytokines act synergistically with coagulation factors and induce upregulation of TF and adhesion molecules, they both are critical to perpetuate the inflammatory response that promotes increased interaction of monocytes, platelets, and/or pRBC with activated EC. Hypoxia, GPI, DNA-containing hemozoin and other events may also contribute to the coagulation-inflammation cycle. The result is a convergence of signals leading to exacerbated TF expression that sustains the coagulation-inflammatory cycle. This cycle may leads to microvascular thrombosis, additional EC activation, increased vascular permeability, ROS generation, metabolic stress, apoptosis and ultimately to organ dysfunction (e.g. SARS) that is often associated with poor prognosis in malaria. In extreme cases, death occurs. For details, see text.