Defective TAFI activation in hemophilia A mice is a major contributor to joint bleeding.

Joint bleeds are common in congenital hemophilia but rare in acquired hemophilia A (aHA) for reasons unknown. To identify key mechanisms responsible for joint-specific bleeding in congenital hemophilia, bleeding phenotypes after joint injury and tail transection were compared in aHA wild-type (WT) mice (receiving an anti-factor VIII [FVIII] antibody) and congenital HA (FVIII^-/-) mice. Both aHA and FVIII^-/- mice bled severely after tail transection, but consistent with clinical findings, joint bleeding was notably milder in aHA compared with FVIII^-/- mice. Focus was directed to thrombin-activatable fibrinolysis inhibitor (TAFI) to determine its potentially protective effect on joint bleeding in aHA. Joint bleeding in TAFI^-/- mice with anti-FVIII antibody was increased, compared with WT aHA mice, and became indistinguishable from joint bleeding in FVIII^-/- mice. Measurements of circulating TAFI zymogen consumption after joint injury indicated severely defective TAFI activation in FVIII^-/- mice in vivo, consistent with previous in vitro analyses in FVIII-deficient plasma. In contrast, notable TAFI activation was observed in aHA mice, suggesting that TAFI protected aHA joints against bleeding. Pharmacological inhibitors of fibrinolysis revealed that urokinase-type plasminogen activator (uPA)-induced fibrinolysis drove joint bleeding, whereas tissue-type plasminogen activator-mediated fibrinolysis contributed to tail bleeding. These data identify TAFI as an important modifier of hemophilic joint bleeding in aHA by inhibiting uPA-mediated fibrinolysis. Moreover, our data suggest that bleed protection by TAFI was absent in congenital FVIII^-/- mice because of severely defective TAFI activation, underscoring the importance of clot protection in addition to clot formation when considering prohemostatic strategies for hemophilic joint bleeding. © 2018 by The American Society of Hematology.

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