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

Tine Wyseure,Esther J Cooke,Paul J Declerck,Niels Behrendt,Joost C.M Meijers,Annette Von Drygalski,Laurent O Mosnier

doi:10.1182/blood-2018-01-828434

Abstract

AbstractJoint 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.

Highlights

Patients with severe factor VIII (FVIII) or FIX deficiency experience frequent joint bleeding that results in hemophilic joint disease, a major cause of joint deterioration and disability in the hemophilia population.[1]
Similar tail bleeding was observed when the inhibitory anti-FVIII antibody was administered to WT C57Bl/6J mice (Figure 2A-C), indicating that there were no apparent differences between C57Bl/6J and BALB/c strains, FVIII2/2 C57BL/6J mice were not tested
Only minimal joint bleeding was observed in anti-FVIII antibody– treated WT C57Bl/6J mice (D2 Hct, 40% 6 4%; Figure 1A) or

Summary

Introduction

Patients with severe factor VIII (FVIII) or FIX deficiency (hemophilia A [HA] or B) experience frequent joint bleeding that results in hemophilic joint disease, a major cause of joint deterioration and disability in the hemophilia population.[1]. Factor replacement therapy limits the incidence of joint bleeds to a certain degree but cannot prevent the progression of hemophilic joint disease, even when prophylaxis is initiated very early in life.[2,3,4] the development of inhibitory alloantibodies in ;30% of patients with hemophilia complicates lifelong factor replacement therapy and increases susceptibility to frequent joint bleeds.[5]. FVIII, and these type 2 inhibitors typically do not accomplish full inhibition of FVIII.[6] This divergent clinical bleeding pattern provides unique opportunities to interrogate the key mechanism(s) responsible for joint-specific bleeding in congenital HA. Such mechanisms may extend well beyond the initial clot formation and involve factors that modulate vascular bed–specific clot stability and/or fibrinolysis

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