ABSTRACT
BACKGROUND: Tissue factor pathway inhibitor (TFPI) regulates tissue factor (TF)-triggered coagulation. Humans and mice express transcripts encoding for multi-distributed (endothelial, platelet, and plasma) 3-Kunitz domain TFPIα and endothelial membrane-anchored 2-Kunitz TFPIß. Mice express a third transcript, γ, that encodes plasma lipoprotein-associated 2-Kunitz TFPI. In humans, proteolysis of α and/or ß produces plasma lipoprotein-associated 2-Kunitz TFPI at lower levels. In clinical trials, monoclonal antibodies that target all TFPI isoforms extend coagulation and correct bleeding in hemophilia patients but with some thrombosis risks. OBJECTIVES: Determine the impact of TFPI isoform-specific deletions on promoting clotting in hemophilia mice. METHODS: Engineered TFPI isoform-specific, hemophilia (FVIII-null) mice were evaluated for clotting. RESULTS: Mice expressing any single TFPI isoform are healthy. Thrombin generation assays identified TFPIγ as the dominant anticoagulation isoform in mouse plasma. Hemostasis was assessed by serial bleeding times from a tail vein laceration. Repeatedly, after a clot forms, it is manually disrupted; the number of clots/disruptions occurring over a 15-minute period is reported. C57BL/6 and hemophilia mice clot on average 25.6 versus 5.4 times, respectively. On a hemophilia background, TFPIß or TFPIγ-specific deletion improves clotting to 14.6 and 15.2 times, respectively (p<0.0001). TFPIα-specific deletion is without impact, clotting 5.1 times. Heterozygous deletion of TFPIß is effective, clotting 11.8 times (p<0.0001). Heterozygous deletion of TFPIα or TFPIγ alone is ineffective clotting 3.0 and 6.1 times, respectively; but heterozygous TFPIαγ deletion improves clotting to 11.2 times (p<0.001). CONCLUSION: In hemophilia mice, endothelial TFPIß and plasma γ-derived 2-Kunitz TFPI individually contribute more to bleeding than total TFPIα.
ABSTRACT
BACKGROUND: Previous studies have reported marked interindividual variation in factor VIII (FVIII) clearance in patients with hemophilia (PWH) and proposed a number of factors that influence this heterogeneity. OBJECTIVES: To investigate the importance of the clearance rates of endogenous von Willebrand factor (VWF) compared with those of other FVIII half-life modifiers in adult PWH. METHODS: The half-life of recombinant FVIII was determined in a cohort of 61 adult PWH. A range of reported modifiers of FVIII clearance was assessed (including plasma VWF:antigen and VWF propeptide levels; VWF-FVIII binding capacity; ABO blood group; and nonneutralizing anti-FVIII antibodies). The FVIII-binding region of the VWF gene was sequenced. Finally, the effects of variation in FVIII half-life on clinical phenotype were investigated. RESULTS: We demonstrated that heterogeneity in the clearance of endogenous plasma VWF is a key determinant of variable FVIII half-life in PWH. Both ABO blood group and age significantly impact FVIII clearance. The effect of ABO blood group on FVIII half-life in PWH is modulated entirely through its effect on the clearance rates of endogenous VWF. In contrast, the age-related effect on FVIII clearance is, at least in part, VWF independent. In contrast to previous studies, no major effects of variation in VWF-FVIII binding affinity on FVIII clearance were observed. Although high-titer immunoglobulin G antibodies (≥1:80) were observed in 26% of PWH, these did not impact FVIII half-life. Importantly, the annual FVIII usage (IU/kg/y) was significantly (p = .0035) increased in patients with an FVIII half-life of <12 hours. CONCLUSION: Our data demonstrate that heterogeneity in the half-life of FVIII concentrates in patients with hemophilia A is primarily attributable to variability in the clearance of endogenous VWF.
