Tissue factor expression by monocytes: regulation and pathophysiological roles.

The expression of tissue factor (TF) by monocytes/macrophages leads to thrombin generation and contributes to their physiological and pathophysiological roles in wound repair, disseminated intravascular coagulation linked to sepsis, postoperative thrombosis, unstable angina, atherosclerosis, chronic inflammation and cancer. Regulation of TF expression in monocytes is controlled by the transcription factors NF-kappaB and AP-1. In whole blood, the activation of the transcription factors is mediated through the phospholipase A2 pathway. Platelets play a crucial role in the expression of TF activity in monocytes, and granulocytes are mandatory in provoking the platelet effect in a P-selectin-dependent reaction. Although all induced or constitutive TF is expressed on the surface of monocytes, its catalytic activity is only about 10% compared to the activity of lysed cells. This phenomenon has been attributed to the increased availability of anionic phospholipid (phosphatidylserine) after cell lysis. At the surface of viable cells, the transmembrane phospholipid distribution and its regulation may be important for the expression of the catalytic activity of the complex of TF and activated factor VII. Phosphatidylserine pathophysiologically exposed at the outer surface of monocytes may, similar to that for platelet membranes, provide a strong stimulus for thrombin generation.