Expression of blood coagulation factors on monocytes after exposure to TNF-treated endothelium in a novel whole blood model of arterial flow.

Activated blood monocytes are a major source of tissue factor (TF), the principal initiator of blood coagulation. TF can be shed from the monocyte surface in association with microparticles (MPs) and increased numbers of circulating MPs are indicative of poor clinical outcome in a number of inflammatory disorders, including atherosclerosis. The mechanisms coupling inflammation with aberrant TF production/activity remain obscure but the protease-activated receptor (PAR) family has been implicated. We have previously shown (i) that freshly isolated human monocytes express low levels of cell surface PAR-2, (ii) that cell surface PAR-2 is rapidly upregulated from intracellular stores following mechanical stimulation, and (iii) that PAR-2 stimulation results in elevation of intracellular calcium and cytokine release. Here, we have investigated the expression of PAR-2 on monocytes exposed to TNF-activated endothelial cells both under static conditions and in our newly-established model of arterial flow, using diluted whole blood. Monocyte surface PAR-2 expression was upregulated following static exposure to activated EC and with laminar (atheroprotective) arterial flow there was a further increase in monocyte PAR-2 expression. We have also shown under arterial flow conditions that exposure to TNF-stimulated EC resulted in a significant increase in expression of TF on monocytes compared to that on cells exposed to quiescent EC. These data strongly suggest that direct or indirect interactions with inflamed EC can modulate expression of PAR-2 and TF on the monocyte cell surface.