Evidence for factor IX‐independent roles for factor XIa in blood coagulation

Summary Background Factor XIa is traditionally assigned a role in FIX activation during coagulation. However, recent evidence suggests this protease may have additional plasma substrates. Objective To determine whether FXIa promotes thrombin generation and coagulation in plasma in the absence of FIX, and to determine whether FXI‐deficiency produces an antithrombotic effect in mice independently of FIX. Methods FXIa, FXIa variants and anti‐FXIa antibodies were tested for their effects on plasma coagulation and thrombin generation in the absence of FIX, and for their effects on the activation of purified coagulation factors. Mice with combined FIX and FXI deficiency were compared with mice lacking either FIX or FXI in an arterial thrombosis model. Results In FIX‐deficient plasma, FXIa induced thrombin generation, and anti‐FXIa antibodies prolonged clotting times. This process involved FXIa‐mediated conversion of FX and FV to their active forms. Activation of FV by FXIa required the A3 domain on the FXIa heavy chain, whereas activation of FX did not. FX activation by FXIa, unlike FIX activation, was not a calcium‐dependent process. Mice lacking both FIX and FXI were more resistant to ferric chloride‐induced carotid artery occlusion than FXI‐deficient or FIX‐deficient mice. Conclusion In addition to its predominant role as an activator of FIX, FXIa may contribute to coagulation by activating FX and FV. As the latter reactions do not require calcium, they may make important contributions to in vitro clotting triggered by contact activation. The reactions may be relevant to FXIa's roles in hemostasis and in promoting thrombosis.