Distinct Localization of Coagulation Factor VIII, Von Willebrand Factor and Factor VIIIa-Mimetic Bispecific Antibody Contributing to Thrombus Formation Under Whole Blood Flow Conditions

Background

Factor VIII (FVIII) is an essential factor for coagulation system in the intrinsic pathway. Due to the short survival of FVIII in the plasma circulation, it requires von Willebrand factor (VWF) as a carrier protein to maintain the optimal level for hemostasis. VWF also plays an important role in primary hemostasis by bridging platelets to exposed subendothelial collagens, especially under high shear flow environment. Since VWF carries FVIII, it is conceivable that VWF takes FVIII to the sites of vascular injury. However, the role of FVIII at the local sites under flow conditions is not fully understood despite of the fact that increased level of FVIII is associated with the risk of venous thrombosis and the deficiency of FVIII is the pathology of the bleeding disorder, hemophilia A. The treatment of hemophilia A largely depends on the infusion of FVIII concentrates, which is often complicated by the development of the inhibitor. Recently, bispecific antibody（ACE910）that mimics the role of FVIIIa by recognizing FIXa and FX has been developed and is currently under clinical trial. This antibody theoretically works regardless of the presence of devastating inhibitors against FVIII. Furthermore, it could also improve the clinical outcome of the other bleeding disorders, such as von Willebrand disease (VWD).

Aim

To analyze the role of FVIII and VWF, and impact of ACE910 at the sites of vascular injury under various shear conditions, we have developed the flow-mediated thrombosis model using flow chamber system.

Method

Whole blood obtained from healthy donors, hemophilia A and VWD patients were perfused into the collagen coated flow chamber under high (2,500s-1) or low shear (50s-1) flow conditions with/without FVIII concentrate, FVIII/VWF concentrate and ACE910. Formed thrombus was fixed and immunostaining was performed with phalloidin (Platelet), anti-FVIII antibody (FVIII) and anti-thrombin antibody (Thrombin). For the detection of ACE910, anti-human IgG or anti-ACE antibody (rAQ8 or rAJ540) were used. Size of thrombi and distribution of platelet, FVIII, thrombin and ACE910 were analyzed.

Result

1) Under high shear flow, thrombus formation of VWD blood was significantly impaired while blood from Hemophilia A demonstrated nearly normal thrombus formation. Addition of FVIII/VWF but not FVIII concentrate to the blood of these patients rescued the impaired thrombus formation. ACE910 enhanced the thrombus formation of blood from both VWD and hemophilia A. Under low shear flow, blood from both hemophilia A and VWD demonstrated decreased thrombus formation. FVIII, FVIII/VWF concentrates and ACE910 improved the size of thrombus. 2) Localization of FVIII was evaluated with thrombin as a marker for the activation of coagulation. Platelets and thrombin demonstrated complete co-localization and intensity of thrombin staining was associated with thrombus size. VWF localized mainly outer layer of thrombus and FVIII localized in and around thrombus. At high shear condition, FVIII and VWF mostly existed with platelets. By contrast, FVIII and VWF demonstrated less co-localization with platelets under low shear condition. ACE910 demonstrated similar tendency to FVIII localization although ACE910 did not appear around thrombus.

Conclusion

We have developed the flow chamber system to evaluate the extent of thrombogenesis under various shear environment. VWF showed dominant role under high shear conditions while FVIII plays a key role under low shear conditions. FVIII, VWF and ACE910 demonstrated distinct localization. Interestingly, the distribution of FVIII was broader than VWF and platelet. FVIII localized to platelets presumably prior to its activation and contributed for the subsequent thrombin generation at local sites. Finally, ACE910 demonstrated consistent enhancement of thrombus formation of blood from both hemophilia A and VWD and, therefore, is prompted for the treatment of these bleeding disorders.