Thrombomodulin alfa attenuates the procoagulant effect and cytotoxicity of extracellular histones through the promotion of protein C activation

Extracellular histones are reported to increase thrombin generation in the plasma and induce endothelial cell death in vitro. These effects of histones were suggested to involve histone-induced inhibition of TM-dependent activated protein C (APC) generation. Therefore, we hypothesized that TM alfa, a recombinant human soluble TM, attenuates these effects of histones by promoting the generation of APC. In the present study, we investigated the effects of TM alfa on the histone-induced decrease in APC generation, an increase in thrombin generation, and endothelial cell death in vitro. APC generation was investigated using a chromogenic substrate based assay. Thrombin generation in plasma was studied by using a calibrated automated thrombogram method. Histone cleavage was detected by western blot analysis. Histone-induced endothelial cell death was evaluated by the trypan blue exclusion test. Histones decreased APC generation and increased thrombin generation in the presence of endothelial cells. TM alfa increased APC generation and decreased thrombin generation in the presence of histones and endothelial cells. TM alfa with thrombin and protein C cleaved histone H3, and attenuated histone-induced endothelial cell death. Antithrombin, an endogenous thrombin inhibitor, and gabexate mesilate, a synthetic protease inhibitor, inhibited thrombin generation, decreased APC generation, and did not have any effect on histone H3 cleavage or histone-induced endothelial cell death. TM alfa attenuated the histone-induced increase in thrombin generation and endothelial cell death by promoting APC generation in vitro. •Histones were reported to increase thrombin generation and endothelial cell death.•The effect of thrombomodulin alfa on histones was investigated in this study.•Thrombomodulin alfa increased protein C activation that was inhibited by histones.•Thrombomodulin alfa attenuated the procoagulant effect and cytotoxicity of histones.