Abstract 609: Heterozygous Tissue Factor Deficiency Protects Against Factor VII Activation And Consumption In A Murine Endotoxemia Model

Abstract

Background: At least 1 in 25,000 humans carry F3 null alleles predicted to cause heterozygous tissue factor (TF) deficiency. Mice with heterozygous TF deficiency ( F3 +/- ) exhibit increased bleeding times and reduced thrombus formation in some models, and are protected against lethal thrombosis in penetrant thrombophilia. TF supports the activation of factor VII (FVII) in vitro , an effect that is evident in vivo in the extreme case of low-TF mice expressing ~1% of basal TF levels. Aim: To determine the impact of heterozygous TF deficiency on FVII activation in vivo . Methods: Citrated plasma was isolated via inferior vena cava blood draw from F3 +/- and wildtype littermate mice at 10-11 weeks of age. To induce coagulation activation through endotoxemia, mice were challenged with intraperitoneal injection of 8 mg/kg LPS for 6 hours. Plasma TF was determined by ELISA. FVII and FVIIa levels were measured by FVII activity and Staclot VIIa-rTF clotting assays, respectively. n≥10 mice in all experiments. Results: F3 +/- mice have significantly reduced plasma TF antigen levels compared to wildtype littermate controls under basal conditions (6.5 pg/mL vs 11.6 pg/mL, p<0.001), differences that persist following endotoxemic challenge (9.9 pg/mL vs 14.8 pg/mL, p<0.001). At baseline, F3 +/- mice had similar circulating levels of zymogen FVII and FVIIa compared to controls. However, the levels of FVIIa correlated with FVII activity levels in F3 +/- mice (r 2 =0.66, p=0.003), whereas no correlation between FVIIa and FVII activity levels was observed in controls (r 2 =0.17, p=0.23). During endotoxemia, F3 +/- mice maintained significantly higher total FVII (85.6% vs 66.1%, p<0.05) and FVIIa (52.9% vs 35.4%, p<0.05) as compared to controls. Conclusions: FVII and FVIIa levels are unaffected by heterozygous TF deficiency in healthy mice, but FVIIa levels are dependent on the amount of FVII when TF is limiting. When TF expression and coagulation activation are augmented as during endotoxemic challenge, F3 +/- mice are protected against aberrant FVII activation and consumption. These studies suggest that human genetic variation influencing TF dosage could influence FVII activation and disease outcomes.