1st workshop on tissue factor activity, expression and quantitation, Nevşehir, Turkey, 15 - 17 August 2014
Tissue factor (TF) is the initiation factor of the extrinsic coagulation pathway and exhibits a distinct tissue distribution. Recently, rather than the extravascular TF concept, blood borne and circulating TF seems to be the point of interest. It is likely that only monocytes express TF since the data is contradictory for platelets and neutrophils. Therefore, there is a proposed theory that TF measured on platelets and neutrophils is acquired from monocytes. Moreover, membrane sheddings of activated cells called microparticles (MPs) are also accepted to be the source of TF in blood circulation. However, the amount of TF provided by MPs in healthy subjects is at inconsiderably low levels. In contrast, production of MPs in several diseases is increased thus they might gain a very procoagulant feature. Time course studies have been done with particulate matter (PM), is the particle component of air polluiton, suggested that acute or chronic exposure to PM may alter TF mediated blood coagulation.
Tissue factor (TF) is a trans membrane glycoprotein that functions as an initiator of blood coagulation and binds plasma factor (F) VII / activated FVII (FVIIa) to form a complex. The complex then activates FIX and FX into corresponding active forms FIXa and FXa. This allows FXa to associate with cofactor FVa to convert prothrombin to thrombin (1). Since TF has long been known to an external factor which is exposed by external tissues such as blood vessels upon injury, the coagulation cascade initiated by TF is named extrinsic pathway. Recent data suggests the other sources of TF (blood borne TF) contributing to blood coagulation especially blood cells expressing TF such as monocytes and TF bearing microparticles. (1)
Since TF is a key factor in blood coagulation and having important roles in hemostasis, thrombogenesis and inflammation, time course measurement of TF activity may contribute to our understanding for underlying mechanisms of inflammatory diseases and thrombosis. By knowing the close relation between inflammation and blood coagulation (2), taking into account the outcomes of such studies would be helpful.
Sources of Tissue Factor
TF in organs, tissues and Saliva
TF is expressed in various organs and tissues such as myocardium, vessels (adventitia), lung, brain, spleen, schwann cells and so on (3). However, generally TF, is located in a hemostatic envelope and ready to activate coagulation upon injury of the vasculature or organ damage. Moreover, recently Berckmans and colleagues showed that saliva also contains TF, 5 fold higher than in the blood (4). In recent years, blood borne TF and circulating TF rather than the concept “hemostatic envelope of TF” has been challenged by several studies.
Blood Borne TF
Tissue factor Expression in blood cells
Circulating monocytes has been reported to express TF when treated with inflammatory cytokines such as interleukin 6 (IL-6), IL-8 (5), C-reactive protein (CRP) (6), P selectin (7), Oxidized low density lipoprotein (OxLDL) (8) or lipopolysaccaride (LPS) (9) however the studies failed to detect significant TF antigen in resting monocytes (10). It has been suggested that monocytes express encrypted TF and only CD14 positive ones possess active form of TF (11). Moreover, upon stimulation, monocytes of some individuals express more TF than monocytes of others and this so called “high and low responder phenomenon“ of monocyte TF activity (12). Platelets are partly responsible for this phenomenon (13). Platelet-rich plasma induced significantly more TF in LPS-stimulated monocytes than platelet-poor plasma (14).
Available data points out controversary results for TF expression on neutrophils and platelets. Expression of TF on neutrophils has been reported by some authors (15, 16) while denied by other investigators (17). It is suggested that TF rich microparticles derived from monocytes acquired by neutrophils (17).
There are several studies suggesting activated platelets by collagen and various agonists results functionally active TF expression (18, 19). Moreover TF mRNA was detected in resting platelets (20). Recently Vignoli et al. showed that platelets of healthy volunters obtained from different blood groups express different TF levels and platelets of 0 blood group express the highest TF compared to A and B groups (21). However, other studies failed to show TF on platelets (10, 22). The contradiction in these results may be explained by first, contamination with leucoytes during platelet isolation or interaction in vivo since Del conde et al. suggested that TF bearing monocyte/macrophage derived microvesicles can bind to activated platelets (23). Second, secretion of TF pathway inhibitor by platelets (24) may mask TF activity. Third, it is proposed that only von willebrand factor plus ristocetin might induce TF activity in platelets (25).
TF bearing microparticles
Microparticles (MPs) are circulating cell fragments derived from cells undergoing activation or apoptosis and the majority of the studies on circulating TF have focused on TF in MPs. Aras et al. investigated TF activity of MPs generated from fibroblats (ionophore activated), smooth muscle cells;SMCs (ionophore activated), human umbilical vein endothelial cells; HUVECs (tumor necrosis factor alpha, TNF-α, activated), CD14 positive monocytes (LPS activated), platelets (ionophore activated) and erytrocytes (ionophore activated). MPs derived from fibroblasts, SMCs, HUVECs, monocytes were positive for TF and TF bearing MPs increased for TNF-α treated HUVECs and LPS treated monocytes (26). Moreover, results of the studies investigated TF activity in MPs of plasma from healthy individuals are contradictory ( 10, 27). Since an elevated levels of MPs has been reported in several diseases such as cardiovascular disease, sickle cell disease, cancer and endotoxemia (28), contribution of TF positive MPs to active coagulation should be considered.
Time course activity of TF
Inflammation as response to infection or trauma, results in a systemic activation of blood coagulation through TF mediated thrombin generation and supressing the activity of physiological anticoagulant mechanisms and inhibiting fibrinolysis (2). Particulate matter (PM) which are small particles as a part of air pollution and are varying in aerodynamic diameter (<10µm; PM10, <2.5 µm; PM2.5 and <0.1 µm; uitrafine particles; UFPs) (29). Several studies have documented inhalation of PM as a risk factor for cardiovascular diseases (29). Exposure to PM is associated with hypercoagulable state which may be the cause of increased TF in lung tissues through inflammatory mediators. We have investigated TF dependent thrombin (measured by calibrated automated thrombogram; CAT in the presence of 1pM TF) generation in healthy human subjects after exposure to PM10 and gas pollutants (CO, NO, NO2 and O3) in different time lags refering to possible direct (0-6h; 0-12h and 0-24h) and indirect effects (24-48h; 48-72h and 72-96h) (30). Thrombin generation was increased within 24-96 h before blood sampling suggest that exposure to air pollution indirectly increases blood thrombogenicity. These indirect effects may be result of air pollution induced synthesis of TF.
In another study, diesel exhaust and PM2.5 exposed to fischer rats for 4 weeks and TF activity and TF dependent thrombin generation in lung tissue were investigated (31). Lung TF activity was decreased after both exposure to diesel exhaust and PM2.5. Although no changes were found in lung thrombin generation measured in the presence of 5pM TF, a significant decrease in lung thrombin generation was observed when triggered with only lung homogenates. This may be explained by increase capacity of thrombomodulin in response to chronic exposure thus significant decrease was seen in only thrombin generation measured in absence of additional TF. Furthermore, UFPs or saline (control) were intratracheally administered to both wild type and FXII deficient mice then blood samples were collected at 4 and 20 hours after administration (32). TF mediated thrombin generation in plasma was measured in the presence of corn trypsin inhibitor. No significant difference found between the control and UFPs administered groups at 4 and 20 hours after administration.
TF is expressed by tissues upon inflammation or a damage. However under several pathologic conditions monocytes may express large amounts of TF contributing to circulating TF pool. Contribution of platelets and neutrophils to this TF pool is unclear but should not be underestimated. Moreover, further studies are needed to elucidate the role MPs in coagulation since the amount of TF bearing MPs in many diseases is increased. The time course studies suggest that not only TF is affected by acute and chronic exposure to PM but also anticoagulant mechanims may be changed.
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