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 2bF8trans mice restrictively expressing human B-domain-deleted FVIII (hBDD FVIII) in platelets on a hemophilic (FVIIInull) mice background. The results showed no statistical difference in clot strength and stability between wild-type (WT) and 2bF8trans 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 2bF8trans mice were more stable than those in FVIIInull mice, while at high pathological shear rates (2,500 s-1), mimicking atherosclerosis, thrombus size and fibrin deposition in 2bF8trans 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 2bF8trans mice. Moreover, a higher ratio of fibrin to platelets was found in thrombus from 2bF8trans mice following laser-induced injury in cremaster arterioles, which might be the underlying mechanism of thrombus stability in 2bF8trans 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.