Characteristics of the Thrombus Formation in Transgenic Mice with Platelet-Targeted Factor VIII Expression

Platelet-targeted FVIII gene therapy can efficiently recover bleeding phenotype for hemophilia A (HA), yet characteristics of thrombus formation with this ectopic expression of factor VIII (FVIII) in platelets remain unclear. Here, we generated 2bF8^trans mice restrictively expressing human B-domain-deleted FVIII (hBDD FVIII) in platelets on a hemophilic (FVIII^null) mice background. The results showed no statistical difference in clot strength and stability between wild-type (WT) and 2bF8^trans mice, but with a prolonged reaction time (R-time), by thromboelastography. Fluid dynamics analysis showed that at the shear rates of 500 to 1,500 s^-1, where physiological hemostasis often develops, the thrombi formed in 2bF8^trans mice were more stable than those in FVIII^null mice, while at high pathological shear rates (2,500 s^-1), mimicking atherosclerosis, thrombus size and fibrin deposition in 2bF8^trans mice were less than those in WT mice. Thrombus morphology analysis showed that there was a locally concentrated deposition of fibrin in thrombus at the injured site and fibrin co-localized with activated platelets in 2bF8^trans mice. Moreover, a higher ratio of fibrin to platelets was found in thrombus from 2bF8^trans mice following laser-induced injury in cremaster arterioles, which might be the underlying mechanism of thrombus stability in 2bF8^trans mice at physiological arterial circumstance. These observations suggest that specific morphological features of the thrombi might contribute to the efficacy and safety of platelet-targeted FVIII gene therapy for HA.