Factor XI promotes hemostasis in factor IX‐deficient mice

Essentials Mice lacking factor IX (FIX) or factor XI (FXI) were tested in a saphenous vein bleeding model. FIX‐deficient mice displayed a hemostatic defect and FXI‐deficient mice were similar to wild type mice. Infusion of FXI or over‐expression of FXI in FIX‐deficient mice improved hemostasis. FXI may affect the phenotype of FIX‐deficiency (hemophilia B). Summary Background In humans, deficiency of coagulation factor XI may be associated with a bleeding disorder, but, until recently, FXI‐deficient mice did not appear to have a hemostatic abnormality. A recent study, however, indicated that FXI‐deficient mice show a moderate hemostatic defect in a saphenous vein bleeding (SVB) model. Objectives To study the effect of FXI on bleeding in mice with normal levels of the FXI substrate FIX and in mice lacking FIX (a murine model of hemophilia B). Methods Wild‐type mice and mice lacking either FIX (F9−) or FXI (F11−/−) were tested in the SVB model. The plasma levels of FXI in F11−/− mice were manipulated by infusion of FXI or its active form FXIa, or by overexpressing FXI by the use of hydrodynamic tail vein injection. Results F9− mice showed a significant defect in the SVB model, whereas F11−/− mice and wild‐type mice were indistinguishable. Intravenous infusion of FXI or FXIa into, or overexpression of FXI in, F9− mice improved hemostasis in the SVB model. Overexpression of a FXI variant lacking a FIX‐binding site also improved hemostasis in F9− mice. Conclusions Although we were unable to demonstrate a hemostatic defect in F11−/− mice in the SVB model, our results support the premise that supraphysiological levels of FXI improve hemostasis in F9− mice through FIX‐independent pathways.