Fetal loss in patients with antiphospholipid (aPL) antibodies has been ascribed to thrombosis of placental vessels. However, we have shown that inflammation, specifically activation of complement with generation of the anaphylotoxin C5a, is an essential trigger of fetal injury. In this study, we analyzed the role of the procoagulant molecule tissue factor (TF) in a mouse model of aPL antibody-induced pregnancy loss. We found that either blockade of TF with a monoclonal antibody in wild-type mice or a genetic reduction of TF prevented aPL antibodyinduced inflammation and pregnancy loss. In response to aPL antibody-generated C5a, neutrophils express TF potentiating inflammation in the deciduas and leading to miscarriages. Importantly, we showed that TF in myeloid cells but not fetal-derived cells (trophoblasts) was associated with fetal injury, suggesting that the site for pathologic TF expression is neutrophils. We found that TF expression in neutrophils contributes to respiratory burst and subsequent trophoblast injury
IntroductionThrombosis and inflammation are linked in many clinical conditions. 1 Tissue factor (TF), the major cellular initiator of the coagulation protease cascade, plays important roles in both thrombosis and inflammation. 2 The coagulation cascade is initiated by the complex of TF and factor VIIa (FVIIa). The TF:FVIIa complex activates its substrates factor X and factor IX by limited proteolysis. Activated FX (FXa) then converts prothrombin to thrombin, which cleaves fibrinogen and activates platelets leading to the formation of a hemostatic plug. TF also contributes to inflammation. TF complexes (TF:FVIIa and TF:FVIIa:FXa) induce the expression of TNF-␣, interleukins, and adhesion molecules by cleaving protease activated receptors (PARs). [3][4][5] Monocytes from patients with antiphospholipid (aPL) antibodies express TF and in vitro experiments showed that monocytes incubated with aPL antibodies express TF. 6,7 A variety of inflammatory stimuli, including mitogens, bacterial cell products, components of the complement system, and cytokines, is known to induce the expression of TF on the surface of endothelial cells, monocytes, and neutrophils. 8,9 TF expression on these cells is a characteristic feature of acute and chronic inflammation in conditions such as sepsis, atherosclerosis, Crohn disease, systemic lupus erythematosus, and transplant rejection reactions. 10-14 TF on monocytes and synovial cells promotes leukocyte adhesion and transendothelial migration, potentiating inflammation in joints, 15 while decreased TF activity abrogates the systemic expression of inflammatory mediators in several animal models. 16,17 The antiphospholipid syndrome (APS) is considered a thrombophilic disorder. However, animal studies from our laboratory have shown the importance of inflammation in the pathogenesis of aPL-induced pregnancy loss, a common complication in APS. 18,19 Using a mouse model of APS, we demonstrated that complement activation, through the action of anaphylotoxin C5a, promotes neutrop...