A new animal model of thrombophilia confirms that high plasma factor VIII levels are thrombogenic

The thrombotic risk associated with elevated plasma levels of clotting fact or VIII (FVIII) was investigated in a mouse model of thrombophilia. After t he intravenous injection of recombinant human FVIII and/or of purified FVII I-free human von Willebrand factor (VWF), a controlled mild injury was infl icted on the carotid artery of FVB mice by irradiation with filtered green light in combination with intravenous injection of the dye rose bengal. For mation of a platelet-rich thrombus was continuously monitored for 40 min vi a transillumination and the thrombus size was measured via image analysis. Administration of recombinant human FVIII at 40 mu g/kg led to initial FVII I plasma activities equivalent to 250% of normal human plasma FVIII activit y and significantly enhanced thrombus size. Immunohistochemical staining il lustrated the accumulation of FVIII within the thrombi. Human vWF, even at 10 mg/kg, had no effect on thrombus formation. The thrombotic tendency indu ced by FVIII was significantly inhibited by the administration of human VWF in a dose-dependent manner. Separate plasma measurements revealed that hum an FVIII has comparable affinities for human and murine vWF but that human vWF does not effectively bind murine platelets. The inhibition by human vWF of the thrombotic tendency induced by human FVIII could therefore be expla ined by a lack of accumulation of FVIII within the developing thrombus beca use of the reduced affinity of human vWF for murine platelets and the reduc ed occupancy of murine von Willebrand factor by human FVIII after injection of human vWF. These results show that vWF actively participates in FVIII a ccumulation in the arterial thrombus and provide experimental evidence for epidemiological findings that elevated plasma FVIII levels are associated w ith an increased thrombotic risk, also in arteries.