Tissue factor-bearing microparticles arise from monocyte lipid rafts and can fuse with activated platelets, consolidating all of the membrane-bound coagulation reactions on the platelet surface

Platelets and tissue factor

Tissue factor (TF) is the most important initiator of intravascular coagulation. This article will review published evidence on the contribution of platelets to TF exposure to the circulating blood. The following mechanisms will be discussed: decryption of monocyte TF by platelets, contribution of platelets to TF expression in leukocytes, platelet-derived TF and its procoagulant activity, and activation of circulating TF by platelets.

The role of blood cells and their microparticles in blood coagulation

The transmembrane glycoprotein TF (tissue factor) plays an essential role in haemostasis as the principal initiator of blood coagulation. In this paper, we describe how the circulating blood cells--monocytes, platelets, neutrophils and their microparticles--co-operate in regulating the expression, availability and activity of monocyte-derived TF.

Induction of microparticle- and cell-associated intravascular tissue factor in human endotoxemia

The precise role of intravascular tissue factor (TF) remains poorly defined, due to the limited availability of assays capable of measuring circulating TF procoagulant activity (PCA). As a model of inflammation-associated intravascular thrombin generation, we studied 18 volunteers receiving an infusion of endotoxin. A novel assay that measures microparticle (MP)-associated TF PCA from a number of cellular sources (but not platelets) demonstrated an 8-fold increase in activity at 3 to 4 hours after endotoxin administration (P <.001), with a return to baseline by 8 hours. TF antigen-positive MPs isolated from plasma were visualized by electron microscopy. Interindividual MP-associated TF response to lipopolysaccharide (LPS) was highly variable. In contrast, a previously described assay that measures total (cell and MP-borne) whole-blood TF PCA demonstrated a more modest increase, with a peak in activity (1.3-fold over baseline; P <.000 01) at 3 to 4 hours, and persistence for more than 24 hours. This surprisingly modest increase in whole-blood TF activity is likely explained by a profound although transient LPS-induced monocytopenia. MP-associated TF PCA was highly correlated with whole-blood TF PCA and total number of circulating MPs, and whole-blood TF PCA was highly correlated with TF mRNA levels.

Vascular biology—the role of tissue factor

It is well established that tissue factor (TF) is abundantly present in various extravascular tissues, in the adventitia of blood vessels, and in atheroma. Thus, in the event of plaque rupture or damage to the blood vessel wall, TF is readily exposed to flowing blood, allowing it to form a complex with circulating factor VIIa (FVIIa) in order to activate factor X (FX) both directly, and indirectly via factor IX (FIX). Platelets quickly adhere to the injured site, facilitating localized thrombin formation and subsequent fibrin production. With each new layer of platelets and fibrin that adheres to the injured surface, the exposed TF on the vessel wall, along with the localized circulating factors IX (FIXa) and X (FXa) that it generates, becomes increasingly isolated from the events near the surface of the growing thrombus. The physical blanketing of an injured surface by platelets and fibrin in addition to the release of platelet tissue factor pathway inhibitor (TFPI), prevents FIXa and FXa from diffusing more than a few tens of microns away from the vessel wall, far short of the 3 mm thickness needed for occlusive thrombosis. Thus an alternative FXa-generating mechanism must be involved that allows for the formation of prothrombinase activity far away from the vessel wall near the front of a growing thrombus.