Coagulation Factor XII Plays an Important Role in Procoagulant Activity of Apoptotic Cells

Apoptosis can be induced in a variety of pathological disorders, including inflammation, autoimmune diseases, and chemotherapy. When cells undergo apoptosis, they express phosphatidylserine (PS) on cell membrane surface and thus become procoagulant. Although it has been known that the procoagulant activity of apoptotic cells are tightly associated with thrombotic disorders, such as atherothrombosis and Trousseau syndrome, the mechanisms by which apoptotic cells activate the coagulation system and enhance blood clotting are largely unknown. In this study we investigated which coagulation factor(s) is involved in this process. Using western blotting and chromogenic substrate assay, we found that incubation with apoptotic cells induced by Dexamethasone (DXMS), but not with viable cells, resulted in rapid cleavage and activation of FXII. Moreover, apoptotic cells-mediated FXII activation was significantly increased in the presence of prekallikrein (PK) and high molecular weight kininogen (HK), other two components of plasma contact system. However, incubation of apoptotic cells did not cause dramatic changes of other coagulation factors, suggesting a selective association of FXII activation with apoptotic cells. Activation of FXII by apoptotic cells was markedly inhibited by a specific anti-kallikrein antibody, indicating the activation of the contact system by apoprotic cells. Flow cytometric measurement showed that FXII bound to apoptotic cells in a concentration-dependent manner, which was inhibited by annexin V and PS liposome. A surface plasmon resonance assay showed a direct binding of FXII to PS (KD=3.9E-9 M). When challenged by apoptotic cells, clotting time of plasma from FXII-knockout mice was significantly prolonged, which was reversed by replenishment with human FXII. Moreover, an inhibitory anti-FXII antibody completely prevented apoptotic cells-induced intrinsic tenase complex formation. Consistently, apoptotic cells significantly increased thrombin production in normal plasma, which were attenuated by PS blocker annexin V, an inhibitory anti-FXII antibody, and the deficiency of FXII, respectively. Addition of human FXII to XII-deficient plasma recovered thrombin generation. As evaluated by ELISA, the levels of thrombin-antithrombin complex in circulation were significantly increased when apoptotic cells were intravenously injected into wild-type mice, but not in FXII-knockout mice. In conclusion, FXII plays an important role in apoptotic cells-mediated procoagulant activity.