Factor V (FV) links procoagulant amplification to anticoagulant feedback, but how FV limits tissue factor-initiated coagulation are not fully defined. We hypothesized that procofactor FV downregulates factor X (FX) activation by tissue factor:factor VIIa (TF:FVIIa), independently of tissue factor pathway inhibitor α (TFPIα), and that this mechanism is especially important in hemophilia. Thrombin generation was measured in FV/FVIII‑immunodepleted plasma and synthetic plasma while titrating FV, with TFPIα removed, blocked, or re-added. TF:FVIIa activation of FX was measured on phosphatidylserine‑containing or phosphatidylserine‑free liposomes with antibodies against the FV light chain and C2 domain. FV and TFPIα levels modulated thrombin generation in plasma from people with hemophilia A. In TF‑initiated coagulation lacking TFPIα, thrombin generation peaked at 2 nM FV and decreased as FV increased; at 20 nM FV (normal concentration), peak thrombin and thrombin generation rate were reduced by up to 50-80%, with larger effects at low FVIII. In purified TF:FVIIa assays, FV reduced FX activation by ~80% at physiologic concentration and inhibited FX activation on TF-expressing fibroblasts. Increasing PS content enhanced FX activation and increased the FV-sensitive component, while blocking the FV light chain or C2 domain partially relieved inhibition. In hemophilia A plasma, higher FV was associated with longer lag time and time-to-peak, and lower peak thrombin independent of TFPIα. Thus, FV is an endogenous anticoagulant that inhibits TF-initiated coagulation by limiting FX activation by TF:FVIIa through a membrane-dependent mechanism. This mechanism refines models of coagulation initiation and may help explain how FV variation contributes to bleeding and thrombosis.
Jewell et al. (Fri,) studied this question.