Thrombomodulin on endothelial cells is a cofactor for thrombin-catalyzed activation of protein C. We have investigated the anticoagulant function of recombinant human soluble thombomodulin (rsTM) in a rat model of arterio-venous (AV)-shunt thrombosis. A bolus injection of rsTM 30 s before the induction of AV-shunt thrombosis inhibited the thrombus formation in a dose-dependent manner. The dose of anticoagulant that inhibited thrombus formation by 50% was 0.4 mg/kg rsTMα, 0.15 mg/kg rsTMβ, and 13 U/kg heparin. Recently, we characterized three monoclonal antibodies (moAbs) against human TM whose epitopes are located in the TM epidermal growth factor-like domain (Nawa et al., 1994). moAb 2A2 inhibited thrombin binding to rsTM, and abolished both TM functions as a cofactor in thrombin-catalyzed activation of protein C and as an anticoagulant by modifying thrombin-induced fibrinogen clotting and platelet aggregation. moAb 1F2 preserved the latter activities as an anticoagulant, but inhibited cofactor activity. moAb 10A3 had no inhibitory effect on either activity. Analysis of the in vivo anticoagulant mechanism of rsTM was facilitated by the availability of these moAbs. After incubation at rsTM/moAb molar ratios of 1:1.25, the effect of the mixtures were examined in the AV-shunt thrombosis model. An injection of 0.8 mg/kg rsTMα or 0.4 mg/kg rsTMβ resulted in a significant reduction on thrombus formation, as expected. moAb 10A3 had no effect on rsTM activity. However, co-injection of rsTM with moAb 1F2 resulted in a significant decrease of the inhibitory activity on thrombus formation. moAb 2A2 essentially abolished the inhibitory effect of rsTM. These moAb effects were observed for both rsTMα and β. These results suggest that the protein C activation accelerated by the injection of rsTM plays an important role for inhibiting thrombus formation in the AV-shunt thrombosis model.
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