New locations of intravascular tissue factor: indications.

Tissue factor (TF) is the major initiator of blood coagulation, and a mediator of inflammation, angiogenesis, and carcinogenesis. According to recent evidence, preformed TF and inducible expression of the protein is observed in several blood components. TF is apparently constitutively expressed on circulating microparticles, and can be exposed within minutes on the cell membrane of activated eosinophils and platelets, and, potentially, on neutrophils. Expression in monocytes and neutrophils largely requires transcriptional activation of the TF gene. Eosinophils appear to harbour the highest concentration of preformed TF among all blood components under resting conditions. TF expression in eosinophil progenitor cells is substantially higher than in precursors of other granulocyte fractions. Eosinophil TF promotes transendothelial migration, which documents that presynthesized TF in blood supports functions apart from coagulation. It is still an open question how the intravascular TF is activated to trigger initiation of coagulation. TF activation in different blood components is likely to be differentially regulated according to the (patho)physiologic context.