Tissue factor expression in blood cells.
The popular concept of TF serving predominantly as a hemostatic envelope encapsulating the vascular bed, has recently been challenged by the observation that blood of healthy individuals may form TF-induced thrombus under conditions entailing shear stress and activated platelets, corroborating the notion of blood borne TF. Accordingly, small amounts of TF activity is detected in calcium ionophore-stimulated monocytes, whereas it is questionable whether neutrophils and eosinophils express TF. Still there are contradicting reports on TF synthesis and expression in activated platelets, but when using a very sensitive and specific assay for TF activity measurements, we fail to detect TF activity associated with platelets activated with various agonists. However, activated platelets may play a role in decrypting monocyte TF activity in a process entailing transfer of TF to activated platelets in a P-selectin-PSGL-1 reaction whereby inactive TF (encrypted) becomes active through the availability of clusters of phosphatidylserine. Microparticles from plasma of healthy subjects possess weak TF-like activity which is not inactivated by anti-TF antibody. Endothelial cells are well documented to synthesize TF by several agonists in vitro. In contrast, there is little evidence that these cells are capable of synthesizing TF in vivo, and a recent report fails to show that TF on the endothelium may play any role in thrombin generation in a murine endotoxemia model. It may be concluded that monocytes are the only blood cells that synthesize and express TF and which may be the only source for TF-induced thrombosis when the endothelium is intact.
Department of Medical Biology, Faculty of Health Sciences, University of Tromsø, Tromsø, Norway. bjarne.osterud@uit.no
Bjarne
B
Østerud
D000818:Animals; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism; D042783:Endothelial Cells / Q000166:cytology; D019446:Endotoxemia / Q000378:metabolism; D004804:Eosinophils / Q000166:cytology; D006801:Humans; D051379:Mice; D009000:Monocytes / Q000166:cytology / Q000378:metabolism; D009504:Neutrophils / Q000166:cytology / Q000378:metabolism; D019007:P-Selectin / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism; D015539:Platelet Activation; D013917:Thrombin / Q000378:metabolism; D013925:Thromboplastin / Q000378:metabolism*; D013927:Thrombosis / Q000378:metabolism / Q000473:pathology
D019007:P-Selectin; D010718:Phosphatidylserines; D013925:Thromboplastin; D013917:Thrombin