Effect of Factor XI or Factor IX Deficiency on an Arterial Occlusion Model.

Factor XI (fXI) and factor IX (fIX) are zymogens of plasma proteases that are required for normal formation and maintenance of a blood clot. Recent work has implicated these proteins in the pathogenesis of vascular thrombosis. Epidemiologic studies indicate that high levels (top 10% of normal distribution) of fXI or fIX are independent risk factors for venous thromboembolism, increasing risk ~2-fold. Recently, it was shown that fXI deficiency protects mice from carotid artery occlusion in a ferric chloride (FeCl3 ) injury model. FeCl₃-induced thrombus formation involves thrombin generation, in addition to platelet activation and von Willebrand factor. We used a modified version of the FeCl₃ model to study the antithrombotic effects of complete fXI or fIX deficiency. In wild type C57Bl/6 mice, carotid artery flow measured by Doppler flow probe is completely blocked within 10 minutes of applying 3.5% FeCl₃ to the vessel. 3.0% FeCl₃ induced occlusion in some (5 of 8) mice by 30 minutes, while no animal treated with 2.5% FeCl₃ experienced occlusion. FXI and fIX deficient mice were fully protected from occlusion induced by 3.5% or 5% FeCl₃. Some fXI (4 of 8) and fIX (4 of 6) deficient animals developed occlusion with 7.5% FeCl₃, while occlusion occurred in all mice at 10% FeCl₃. To put the effect of fXI or fIX deficiency on this model into perspective, it requires a very high dose of heparin (1000 U/kg) to produce similar protection. With 5% FeCl₃, heparin at 200 U/kg only protects 50% of wild type mice from occlusion, despite prolonging the activated partial thromboplastin time beyond the upper limit of the assay (> 500 secs). High dose aspirin (100 mg/kg) did not prevent occlusion induced by 5% FeCl₃, despite producing a nearly complete block of arachidonic acid-induced platelet aggregation in vitro. While fXI and fIX deficiency affect the FeCl₃ model similarly, they have significantly different impacts on a tail bleeding time (TBT) assay. FXI deficient and wild type mice have similar mean TBTs (265 ± 68 and 287 ± 92 secs, respectively), while fIX deficiency causes prolonged bleeding (1561 ± 125 secs, p < 0.01). In comparison, heparin (200 units/kg) causes the TBT to exceed the upper limit of the assay (1800 seconds), while aspirin (30 mg/kg) modestly increases the TBT (~2.2-fold). The data indicate that fXI and fIX are involved in thrombus formation in the FeCl₃ model, and support a growing body of evidence that thrombin formation through the fIX/fXI axis contributes to thrombotic disease. Given the mild bleeding diathesis associated with fXI deficiency, inhibition of fXI may be a useful component of therapy for treating or preventing thrombus formation, and would be associated with a relatively low risk of bleeding.