The significance of human monocyte thrombomodulin during membrane vesiculation and after stimulation by lipopolysaccharide.

Thrombomodulin acts as an essential membrane cofactor of thrombin in the triggering of the natural anticoagulant protein C pathway responsible for the inactivation of procoagulant cofactors VIIIa and Va. Because monocytes play a critical role in the coupling between infection/inflammation and thrombosis, the fate of monocyte thrombomodulin was assessed at the cell plasma membrane and on derived microparticles. A significant basal level of thrombomodulin activity was measured on unstimulated monocytes and microparticles. Lipopolysaccharide treatment resulted in increased thrombomodulin activity on monocytes (approximately 40%) and microparticles (approximately 80%), whereas tissue factor and prothrombinase activities were strongly expressed on both. Flow cytometry detection of thrombomodulin antigen confirmed its presence on unstimulated monocytes and microparticles. A decrease (approximately 30%) in thrombomodulin labelling was noticed on stimulated monocytes. Labelling of microparticles shed from stimulated and unstimulated monocytes remained unchanged, only an increased proportion of microparticles (approximately 20%) was observed. The absence of early down-regulation of thrombomodulin following monocyte stimulation suggests that it fulfils an important regulatory function of membrane-associated procoagulant activities. This would be of particular significance at the surface of microparticles having the ability to diffuse and concentrate by adherence at inflammatory sites.