Inhibition of tissue factor limits the growth of venous thrombus in the rabbit

Exploring the Mechanism of Chuanxiong Rhizoma against Thrombosis Based on Network Pharmacology, Molecular Docking and Experimental Verification

Chuanxiong rhizoma (CX) has been utilized for centuries as a traditional herb to treat blood stasis syndromes. However, the pharmacological mechanisms are still not completely revealed. This research was aimed at exploring the molecular mechanisms of CX treatment for thrombosis. Network pharmacology was used to predict the potential anti-thrombosis mechanism after correlating the targets of active components with targets of thrombosis. Furthermore, we verified the mechanism of using CX to treat thrombosis via molecular docking and in vitro experiments. Network pharmacology results showed that a total of 18 active ingredients and 65 targets of CX treatment for thrombosis were collected, including 8 core compounds and 6 core targets. We revealed for the first time that tissue factor (TF) had a close relationship with most core targets of CX in the treatment of thrombosis. TF is a primary coagulation factor in physiological hemostasis and pathological thrombosis. Furthermore, core components of CX have strong affinity for core targets and TF according to molecular docking analysis. The in vitro experiments indicated that Ligustilide (LIG), the representative component of CX, could inhibit TF procoagulant activity, TF mRNA and protein over-expression in a dose-dependent manner in EA.hy926 cells through the PI3K/Akt/NF-κB signaling pathway. This work demonstrated that hemostasis or blood coagulation was one of the important biological processes in the treatment of thrombosis with CX, and TF also might be a central target of CX when used for treating thrombosis. The inhibition of TF might be a novel mechanism of CX in the treatment of thrombosis.

Apolipoprotein L Domain Containing 1 Inhibits Tissue Factor to Impede Thrombus Formation in a Rat Model of Deep Vein Thrombosis via Activating PI3K/Akt Pathway

BACKGROUND

Deep venous thrombosis (DVT) is one of the major health problems worldwide. Apolipoprotein L domain containing 1 (APOLD1) was reported to be downregulated in DVT. The present study intended to investigate whether APOLD1 affects thrombus formation in a rat model of DVT.

METHODS

The rat model of DVT was established by inferior vena cava (IVC) stenosis. At 6 hr, 12 hr, 24 hr, and 48 hr after IVC stenosis, the gross IVC with thrombus was dissected and observed. Then, the rats were preinjected with the lentiviral overexpression vector, APOLD1-LVs, 1 hr before IVC stenosis, to evaluate the influence of APOLD1 on thrombosis in rats. The serum levels of D-dimer and TAT as well as the content of TF in IVC tissues were detected by enzyme-linked immunosorbent assay (ELISA).

RESULTS

IVC stenosis resulted in thrombus formation in rats, increased serum levels of D-dimer and TAT, and decreased APOLD1 expression. APOLD1 overexpression inhibited in vivo thrombosis, reduced serum levels of D-dimer, and downregulated tissue factor (TF) activity and level. APOLD1 overexpression also increased p-PI3K and p-Akt protein levels.

CONCLUSIONS

APOLD1 suppresses thrombus formation in a rat model of DVT via downregulating TF expression by activating the PI3K/Akt pathway.

Screening of a Library for Factor VIIa Inhibitors

Background:

As an alternative to the anticoagulant’s strategy using direct or indirect anti-Xa drugs, considering other targets upstream in the coagulation cascade such as anti-Factor VIIa could represent an effective and safer strategy in coagulation and pathological angiogenesis.

Objective:

The objective of the study was to assess a high technology methodology composed of virtual screening, anticoagulant, and anti-angiogenesis assays to identify potent small-molecule FVIIa inhibitors.

Methods:

Chemical databanks were screened to select molecules bearing functional groups that could fit into the active site of FVIIa, which were then tested. Ligands assigned with the lowest scores were retained and then biologically assessed.

Results:

From the 500 molecules considered, 8 chemical structures revealed to be effective compounds in vitro and to inhibit angiogenesis in the chick chorioallantoic membrane (CAM) model.

Conclusion:

New potent small-molecule FVIIa inhibitors have been identified; further biochemical and chemical developments would be investigated directly from the selected scaffolds.

Extracellular Vesicles Linking Inflammation, Cancer and Thrombotic Risks

Extracellular vesicles (EVs) being defined as lipid-bilayer encircled particles are released by almost all known mammalian cell types and represent a heterogenous set of cell fragments that are found in the blood circulation and all other known body fluids. The current nomenclature distinguishes mainly three forms: microvesicles, which are formed by budding from the plasma membrane; exosomes, which are released, when endosomes with intraluminal vesicles fuse with the plasma membrane; and apoptotic bodies representing fragments of apoptotic cells. Their importance for a great variety of biological processes became increasingly evident in the last decade when it was discovered that they contribute to intercellular communication by transferring nucleotides and proteins to recipient cells. In this review, we delineate several aspects of their isolation, purification, and analysis; and discuss some pitfalls that have to be considered therein. Further on, we describe various cellular sources of EVs and explain with different examples, how they link cancer and inflammatory conditions with thrombotic processes. In particular, we elaborate on the roles of EVs in cancer-associated thrombosis and COVID-19, representing two important paradigms, where local pathological processes have systemic effects in the whole organism at least in part via EVs. Finally, we also discuss possible developments of the field in the future and how EVs might be used as biomarkers for diagnosis, and as vehicles for therapeutics.

Inflammatory Monocyte Counts Determine Venous Blood Clot Formation and Resolution

BACKGROUND

Monocytes are thought to be involved in venous thrombosis but the role of individual monocyte subpopulations on thrombus formation, clot inflammation, and degradation is an important unresolved issue. We investigate the role of inflammatory Ly6Chi monocytes in deep vein thrombosis and their potential therapeutic impact.

METHODS

Frequencies and compositions of blood monocytes were analyzed by flow cytometry in CCR2-/- (C-C chemokine receptor type 2) and wild-type mice of different ages and after treatment with the NR4A1 (nuclear receptor group 4 family A member 1, Nur77) agonist CnsB (cytosporone B). TF (tissue factor) sufficient and deficient Ly6Chi monocytes were adoptively transferred into aged CCR2-/- mice. Thrombus formation and size were followed by ultrasound over a 3-week period after surgical reduction of blood flow (stenosis) in the inferior vena cava.

RESULTS

Reduced numbers of peripheral monocytes in aged (>30 w) CCR2-/- mice are accompanied by reduced thrombus formation after inferior vena cava ligation. Reducing the number of inflammatory Ly6Chi monocytes in wild-type mice by CsnB treatment before ligation, similarly suspends clotting, while later treatment (d1 or d4) reduces thrombus growth and accelerates resolution. We describe how changes in inflammatory monocyte numbers affect the gradual differentiation of monocytes in thrombi and show that only tissue factor-competent Ly6Chi monocytes restore thrombosis in aged CCR2-/- mice.

CONCLUSIONS

We conclude that the number of inflammatory Ly6Chi monocytes controls deep vein thrombosis formation, growth, and resolution and can be therapeutically manipulated with a NR4A1 agonist at all disease stages.

Can we use biomarkers of coagulation to predict which patients with foot and ankle injury will develop deep vein thrombosis?

BACKGROUND

Our aim was to determine whether plasma levels of Tissue Factor (TF), Vascular Cell Adhesion Molecule 1 (VCAM-1), Interleukin 6 (IL-6) or D-dimer after foot and ankle injury could predict which patients would develop deep vein thrombosis (DVT).

METHODS

Patients aged 18-60 years with acute foot and ankle injury had venous blood sample to measure TF, VCAM-1, IL-6 and D-dimer within 3 days of injury. Patients had bilateral lower limb venous ultrasound to assess for DVT on discharge from clinic.

RESULTS

21 of 77 patients were found to have DVT (27%). There was no statistically significant association between levels of TF, VCAM-1, IL-6 or D-dimer and subsequent development of DVT.

CONCLUSION

Tissue Factor (TF), Vascular Cell Adhesion Molecule-1 (VCAM-1), Interleukin-6 (IL-6) and D-dimer levels were not associated with development deep vein thrombosis in patients with acute foot and ankle injury.

Soluble endothelial cell molecules and circulating endothelial cells in patients with venous thromboembolism

: To evaluate the plasma levels of soluble endothelial cell molecules in patients with venous thromboembolism (VTE) out of the acute phase as compared with healthy individuals. We also investigated the possible associations of the soluble endothelial cell molecules among them, as well as with other clinical and laboratory data, including the numbers of circulating endothelial cells (CEC), circulating endothelial progenitor cells (CEP), and CEC expressing activation-related [cluster of differentiation (CD)54 and CD62E] and procoagulant (CD142) markers. In total, 15 patients with VTE and 20 normal individuals were studied. The CEC and CEP were quantified and characterized by flow cytometry. The soluble molecules studied included P-selectin, E-selectin, intercellular cell adhesion molecule 1, vascular cell adhesion molecule 1 and tissue factor (ELISA), and von Willebrand factor antigen (immunoturbidimetry). VTE patients had significantly higher levels of vascular cell adhesion molecule 1 and von Willebrand factor antigen and lower levels of soluble E-selectin than controls. They also showed significantly higher numbers of CEC, as of activated/procoagulant CEC and lower numbers of CEP, compared with controls. We did not find any correlation between the levels of soluble molecules and the numbers of endothelial cell in circulation, but there was with several clinical and laboratory data in VTE patients. Our results would suggest that in VTE patients, the endothelium remains activated and in some hypercoagulable state. The levels of soluble endothelial cell molecules did not seem to be directly related to the numbers of CEC and CEP neither reflected the number of activated CEC, which may be because of the different function that surface and soluble molecules may have.

The role of leukocytes in thrombosis

In recent years, the traditional view of the hemostatic system as being regulated by a coagulation factor cascade coupled with platelet activation has been increasingly challenged by new evidence that activation of the immune system strongly influences blood coagulation and pathological thrombus formation. Leukocytes can be induced to express tissue factor and release proinflammatory and procoagulant molecules such as granular enzymes, cytokines, and damage-associated molecular patterns. These mediators can influence all aspects of thrombus formation, including platelet activation and adhesion, and activation of the intrinsic and extrinsic coagulation pathways. Leukocyte-released procoagulant mediators increase systemic thrombogenicity, and leukocytes are actively recruited to the site of thrombus formation through interactions with platelets and endothelial cell adhesion molecules. Additionally, phagocytic leukocytes are involved in fibrinolysis and thrombus resolution, and can regulate clearance of platelets and coagulation factors. Dysregulated activation of leukocyte innate immune functions thus plays a role in pathological thrombus formation. Modulation of the interactions between leukocytes or leukocyte-derived procoagulant materials and the traditional hemostatic system is an attractive target for the development of novel antithrombotic strategies.

Association between micro particle-tissue factor activity, factor VIII activity and recurrent VTE in patients with acute pulmonary embolism

Studies on the association between microparticle expressing tissue factor (MP-TF) activity, FVIII activity (FVIII:C) and recurrent VTE yielded inconclusive results. We studied these associations in patients diagnosed with acute pulmonary embolism. Plasma levels of MP-TF and FVIII activity were measured in 277 patients with a first and 72 patients with a recurrent VTE. All patients were categorized based on the quintiles of MP-TF and FVIII activity in those with a single VTE. For both markers, odds ratios (ORs) for recurrent VTE were computed using patients in the lowest quintile as a reference group. No association was observed between MP-TF activity and recurrent VTE, with an OR of 1.4 (95 % CI 0.7–2.9) in the highest quintile of MP-TF activity. Compared with the reference group, patients in the highest quintile of FVIII:C were at increased risk of recurrent VTE, OR 4.2 (95 % CI 1.4–12.2). MP-TF activity was not associated with recurrent VTE whereas high FVIII:C levels were associated with a 4-fold increased risk of VTE recurrence. Future prospective studies are necessary to explore the potential of FVIII:C as a tool for risk stratification, either by itself or in combination with other pro-thrombotic markers.

CD235a (Glycophorin-A) Is the Most Predictive Value Among Different Circulating Cellular Microparticles in Thrombocytopenic Human Immunodeficiency Virus Type 1

BACKGROUND

This study was conducted to assess different cellular microparticles (MPs) in thrombocytopenic human immunodeficiency virus type 1 and their significance as disease activity markers.

METHODS

Thirty-five thrombocytopenic human immunodeficiency diseases and 25 healthy controls with matched age and sex were selected. Viral load was quantitated by COBAS real-time polymerase reaction (PCR) assessment of absolute T-cell subsets CD4, CD8 as a disease progress marker. Platelet MPs, platelet-derived monocyte MPs (CD42a, CD61), erythrocyte MP (CD235a), monocytic MP (CD14), and platelet activity MPs (CD62P, PAC-1) were assessed by multicolor flow cytometry FACSCalibur, while platelet functions were assessed by platelet function analyzer (PFA-100). CD42a, CD61, and platelet activity index represented by PAC-1 and CD62.

RESULTS

P-selectin in HIV-infected patient samples were significantly greater (P < 0.001) than among controls. There was a negative correlation between the proportion of PAC-1 and CD62 P-selectin-positive MPs and levels of CD4(+) T-cell counts (r = -0.403, P = 0.016; r = -0.438, P = 0.008), respectively. There was a negative correlation between collagen-ADP and levels of CD4(+) T-cell counts (r = -0.368, P = 0.03). There was a significant high expression level of CD14 monocyte MPs in patients than controls (P < 0.0001), overexpression of CD235a (P < 0.0001), and no correlation between CD14 and CD4, whereas there was a significant negative correlation with CD235a (r = -0.394, P = 0.019). A linear regression analysis of CD4 as a disease progression marker with other variable indicators in HIV patients showed that CD235a could be the most sensitive predictor similar to CD4.

CONCLUSION

Different cellular MPs and platelets activated in HIV patients could have a role in thrombotic events in these patients.

Spatial aspects of blood coagulation: two decades of research on the self-sustained traveling wave of thrombin

In a number of experimental studies, it has been demonstrated that the forefront of blood coagulation can propagate in the manner of a signal relay. These data strongly support the concept that the formation of a blood clot is governed by a self-sustained traveling wave of thrombin. The present review critically appraises the experimental data obtained in recent decades concerning the self-sustained spatial propagation of thrombin. Open questions regarding the experimental detection of the self-sustained propagation of thrombin are discussed.

The extrinsic coagulation cascade and tissue factor pathway inhibitor in macrophages: a potential therapeutic opportunity for atherosclerotic thrombosis

OBJECTIVES

The coagulation protease cascade plays the central requisite role in initiation of arterial atherothrombosis. However, the relative participation of the extrinsic as compared to the intrinsic pathway is incompletely resolved. We have investigated in vivo the relative importance of the extrinsic and intrinsic pathways to define which is more essential to atherothrombosis and therefore the preferable prophylactic therapeutic target. We further addressed which type of plaque associated macrophage population is associated with the thrombotic propensity of atherosclerotic plaques.

METHODS

Both photochemical injury and ferric chloride vascular injury models demonstrated arterial thrombosis formation in ApoE deficient mice. We found that direct interference with the extrinsic pathway, but not the intrinsic pathway, markedly diminished the rate of thrombus formation and occlusion of atherosclerotic carotid arteries following experimental challenge. To explore which plaque macrophage subtype may participate in plaque thrombosis in regard to expression tissue factor pathway inhibitor (TFPI), bone marrow derived macrophages of both M and GM phenotypes expressed tissue factor (TF), but the level of TFPI was much greater in M- type macrophages, which exhibited diminished thrombogenic activity, compared to type GM-macrophages.

RESULTS AND CONCLUSIONS

Our works support the hypothesis that the TF-initiated and direct extrinsic pathway provides the more significant contribution to arterial plaque thrombogenesis. Activation of the TF driven extrinsic pathway can be influenced by differing colony-stimulating factor influenced macrophage TFPI-1 expression. These results advance our understanding of atherothrombosis and identify potential therapeutic targets associated with the extrinsic pathway and with macrophages populating arterial atherosclerotic plaques.

Enhanced effect of inhibition of thrombin on endothelium in murine endotoxaemia: specific inhibition of thrombocytopenia

INTRODUCTION

In systemic endotoxaemia, bacterial lipopolysaccharide causes the rapid expression of tissue factor (TF) and disseminated intravascular coagulation and in animal models, anticoagulants limit pathology and promote survival. Recent studies have emphasised the importance of TF expressed by mononuclear cells for initiating thrombin generation during endotoxaemia and suggested that endothelial cell TF is of little relevance. However, the precise importance of endothelium for intravascular thrombin generation has not been established. In this study, we compared the effect of equivalent levels of hirudin tethered to either endothelium or platelets and monocytes.

MATERIALS AND METHODS

CD31-Hir-Tg mice express a vesicle-targeted, membrane-tethered hirudin fusion protein on endothelium, platelets and monocytes. Bone marrow chimeras between these mice and C57BL/6 were generated The level of intravascular hirudin expressed during endotoxaemia was quantified by inhibition studies using an anti-hirudin antibody and reference to the circulating thrombin anti-thrombin complexes generated in control mice given soluble hirudin.

RESULTS AND CONCLUSIONS

Antibody inhibition studies indicated that individual chimeras expressed similar levels of hirudin fusion protein on endothelium alone as on platelets and leukocytes combined and accordingly, the levels of thrombin anti-thrombin complexes and fibrinogen in each chimera were similar, indicating equivalent inhibition of thrombin generation. However, mice with hirudin on endothelium alone developed significantly less thrombocytopenia. These results suggest a hitherto unrecognized role of endothelium in thrombin-dependent platelet sequestration during endotoxaemia. The data have implications for the development of therapeutic strategies based on targeted anticoagulation to limit disseminated intravascular coagulation.

Encryption and decryption of tissue factor

Tissue factor (TF) is a transmembrane cofactor that binds and promotes the catalytic activity of factor (F) VIIa. The TF/VIIa complex activates FX by limited proteolysis to initiate blood coagulation and helps provide the thrombin burst that is important for a stable thrombus. TF is present both in the extravascular compartment, where it functions as a hemostatic envelope, and the intravascular compartment, where it contributes to thrombus formation, particularly when endothelial disruption is minimal. The regulation of its cofactor function appears to differ in the two compartments. Intravascular TF derives predominately from leucocytes, with either monocytes or neutrophils implicated in different models of thrombosis. This TF exists mostly in a non-coagulant or cryptic form and acute events lead to local decryption of TF and FX activation. A variety of experimental observations imply that decryption of leucocyte surface TF involves both a dithiol/disulfide switch and exposure of phosphatidylserine. The dithiol/disulfide switch appears to involve the Cys186-Cys209 disulfide bond in the membrane-proximal domain of TF, although this has not been demonstrated in vivo. Activation of a purinergic receptor or complement has recently been observed to decrypt TF on myeloid cells and a dithiol/disulfide switch and the oxidoreductase, protein disulfide isomerase, have been implicated in both systems. The molecular mechanism of action of protein disulfide isomerase in TF encryption/decryption, though, remains to be determined.

Revisited role of microparticles in arterial and venous thrombosis

Microparticles (MPs) represent a heterogeneous population of submicronic vesicles that are released in response to cell activation or apoptosis. MPs harbor a large repertoire of cell surface receptors and mRNA and biological activities representative of their parent cells and related to their involvement in many biological functions. Although MP generation is a physiological response, a dramatic increase in circulating MPs is detectable in a variety of thrombosis-associated disorders compared with healthy individuals. In this review, we will discuss a new vision of MPs as complex and ambivalent structures that express both activators and inhibitors of coagulation but also convey fibrinolytic properties. After summarizing our current knowledge about the role of MPs in venous and arterial thrombosis, this review will explore how this new vision of MPs influences their definition as emergent biomarkers in thrombotic diseases. Among the studies that have aimed to establish a link between thrombosis and MPs, a few studies have demonstrated a predictive value of MPs. So far, it is unclear whether this limited causative association is the result of current technical concerns and limited standardization or has to be integrated into a more complex vision of the role of MPs as key systems for regulating the balance between coagulation and fibrinolysis.

Elevated plasma factor VIII enhances venous thrombus formation in rabbits: contribution of factor XI, von Willebrand factor and tissue factor

Elevated plasma levels of factor VIII (FVIII) are associated with increased risk of deep venous thrombosis. The aim of this study is to elucidate how elevated FVIII levels affect venous thrombus formation and propagation in vivo. We examined rabbit plasma FVIII activity, plasma thrombin generation, whole blood coagulation, platelet aggregation and venous wall thrombogenicity before and one hour after an intravenous infusion of recombinant human FVIII (rFVIII). Venous thrombus induced by the endothelial denudation of rabbit jugular veins was histologically assessed. Thrombus propagation was evaluated as indocyanine green fluorescence intensity. Argatroban, a thrombin inhibitor, and neutralised antibodies for tissue factor (TF), factor XI (FXI), and von Willebrand factor (VWF) were infused before or after thrombus induction to investigate their effects on venous thrombus formation or propagation. Recombinant FVIII (100 IU/kg) increased rabbit plasma FVIII activity two-fold and significantly enhanced whole blood coagulation and total plasma thrombin generation, but did not affect initial thrombin generation time, platelet aggregation and venous wall thrombogenicity. The rFVIII infusion also increased the size of venous thrombus 1 hour after thrombus induction. Argatroban and the antibodies for TF, FXI or VWF inhibited such enhanced thrombus formation and all except TF suppressed thrombus propagation. In conclusion, elevated plasma FVIII levels enhance venous thrombus formation and propagation. Excess thrombin generation by FXI and VWF-mediated FVIII recruitment appear to contribute to the growth of FVIII-driven venous thrombus.

Protein anticoagulants targeting factor VIIa-tissue factor complex: a comprehensive review

Anticoagulants are pivotal for the treatment of debilitating thromboembolic and associated disorders. Current anticoagulants such as heparin and warfarin are non-specific and have a narrow therapeutic window. These limitations have provided the impetus to develop new anticoagulant therapies/strategies that target specific factors in the blood coagulation cascade, ideally those located upstream in the clotting process. Factor VIIa (FVIIa) presents an attractive target as it, in complex with tissue factor (TF), acts as the prima ballerina for the formation of blood clot. A comprehensive review delineating the structure-activity relationship of protein/peptide anticoagulants targeting FVIIa or TF-FVIIa complex is absent in the literature. In this article, we have addressed this deficit by appraising the peptide/protein anticoagulants that target FVIIa/TF-FVIIa complex. Further, the current status of these anticoagulants, with regard to their performance in different clinical trials has also been presented. Lastly, the unexplored domains of these unique proteins have also been highlighted, which will facilitate further translational research in this paradigm, to improve strategies to counter and treat thromboembolic disorders.

Inhibitors of the initiation of coagulation

Initiation of blood coagulation occurs mainly through tissue factor (TF) that becomes exposed to blood following vascular injury. Cell-associated TF binds to the serine protease FVIIa and initiates a cascade of amplified zymogen activation reactions leading to thrombus formation. As TF-FVIIa directed inhibitors might achieve anticoagulant efficacy without significantly interfering with normal haemostasis, the TF-FVIIa complex is an interesting target in thrombosis-related disease. Various approaches have been used to inhibit the TF-FVIIa complex including active site-inhibited FVIIa, TF antibodies, tissue factor pathway inhibitor (TFPI), naturally occurring inhibitors, peptide exosite inhibitors and active site inhibitors. Several experimental studies using these inhibitors have displayed promise. However, none of these TF/FVIIa inhibitors has reached clinical testing. Further studies are required to evaluate the clinical efficacy of these novel inhibitors.

Myeloid cell tissue factor does not contribute to venous thrombogenesis in an electrolytic injury model

INTRODUCTION

Tissue factor (TF) is a potent initiator of the extrinsic coagulation cascade. The role and source of TF in venous thrombotic disease is not clearly defined. Our study objective was to identify the contribution of myeloid cell TF to venous thrombogenesis in mice.

MATERIALS AND METHODS

The mouse electrolytic inferior vena cava model was used to induce thrombosis. The following groups of mice were used (1) TF(flox/flox)LysMCre(+) mice that have reduced TF expression in myeloid cells, (2) TF(flox/flox)LysMCre(-) littermate controls, (3) Wild type mice given a monoclonal anti-mouse TF antibody (1H1) to inhibit TF activity, and (4) Wild type mice given rat IgG. Evaluations at baseline, day 2, and day 6 post thrombosis included thrombus weight, vein wall inflammatory cell migration, vein wall TF mRNA, and plasma D-dimer levels.

RESULTS

Inhibition of TF significantly decreased thrombus weight 2days post venous thrombosis. In contrast, TF(flox/flox)LysMCre(+) had no change in thrombus weight when compared to littermate controls. The absence of myeloid cell TF did not affect infiltration of neutrophils or monocytes into the vein wall. TF mRNA expression in the vein wall decreased at 2days but then returned to baseline levels by 6days post thrombosis. D-dimer levels peaked at 2days post thrombosis in mice with or without myeloid cell TF.

CONCLUSIONS

TF is important in the formation of venous thrombi in the macrovasculature. However, TF expression by myeloid cells does not significantly contribute to venous thrombogenesis in this model.

Tissue factor, blood coagulation, and beyond: an overview

Emerging evidence shows a broad spectrum of biological functions of tissue factor (TF). TF classical role in initiating the extrinsic blood coagulation and its direct thrombotic action in close relation to cardiovascular risks have long been established. TF overexpression/hypercoagulability often observed in many clinical conditions certainly expands its role in proinflammation, diabetes, obesity, cardiovascular diseases, angiogenesis, tumor metastasis, wound repairs, embryonic development, cell adhesion/migration, innate immunity, infection, pregnancy loss, and many others. This paper broadly covers seminal observations to discuss TF pathogenic roles in relation to diverse disease development or manifestation. Biochemically, extracellular TF signaling interfaced through protease-activated receptors (PARs) elicits cellular activation and inflammatory responses. TF diverse biological roles are associated with either coagulation-dependent or noncoagulation-mediated actions. Apparently, TF hypercoagulability refuels a coagulation-inflammation-thrombosis circuit in “autocrine” or “paracrine” fashions, which triggers a wide spectrum of pathophysiology. Accordingly, TF suppression, anticoagulation, PAR blockade, or general anti-inflammation offers an array of therapeutical benefits for easing diverse pathological conditions.

Microparticles in hemostasis and thrombosis

Blood contains microparticles (MPs) derived from a variety of cell types, including platelets, monocytes, and endothelial cells. In addition, tumors release MPs into the circulation. MPs are formed from membrane blebs that are released from the cell surface by proteolytic cleavage of the cytoskeleton. All MPs are procoagulant because they provide a membrane surface for the assembly of components of the coagulation protease cascade. Importantly, procoagulant activity is increased by the presence of anionic phospholipids, particularly phosphatidylserine (PS), and the procoagulant protein tissue factor (TF), which is the major cellular activator of the clotting cascade. High levels of platelet-derived PS(+) MPs are present in healthy individuals, whereas the number of TF(+), PS(+) MPs is undetectable or very low. However, levels of PS(+), TF(+) MPs are readily detected in a variety of diseases, and monocytes appear to be the primary cellular source. In cancer, PS(+), TF(+) MPs are derived from tumors and may serve as a useful biomarker to identify patients at risk for venous thrombosis. This review will summarize our current knowledge of the role of procoagulant MPs in hemostasis and thrombosis.

Role of tissue factor in venous thrombosis

Venous thromboembolism (VTE) is a leading cause of morbidity and mortality worldwide. However, the mechanisms by which clots are formed in the deep veins have not been determined. Tissue factor (TF) is the primary initiator of the coagulation cascade and is essential for hemostasis. Under pathological conditions, TF is released into the circulation on small-membrane vesicles termed microparticles (MPs). Recent studies suggest that elevated levels of MP TF may trigger thrombosis. This review provides an overview of the role of TF in VTE.

Tissue factor and thrombosis: The clot starts here

Thrombosis, or complications from thrombosis, currently occupies the top three positions in the cardiovascular causes of morbidity and mortality in the developed world. There are a limited number of safe and effective drugs to prevent and treat thrombosis. Animal models of thrombosis are necessary to better understand the complex components and interactions involved in the formation of a clot. Tissue factor (TF) is required for the initiation of blood coagulation and likely plays a key role in both arterial and venous thrombosis. Understanding the role of TF in thrombosis may permit the development of new antithrombotic drugs. This review will focus on the role of TF in in vivo models of thrombosis.

Disturbed blood flow induces erosive injury to smooth muscle cell-rich neointima and promotes thrombus formation in rabbit femoral arteries

BACKGROUND

Plaque erosion is a cause of atherothrombosis that preferentially occurs on smooth muscle cell (SMC)- and proteoglycan-rich rather than lipid-rich plaques. However, its underlying mechanisms remain unknown.

OBJECTIVE

To determine whether disturbed blood flow induces erosive injury and thrombus formation on SMC-rich neointima.

METHODS

Three weeks after balloon injury, SMC-rich neointima with increased tissue factor (TF) activity developed in rabbit femoral arteries that were narrowed with a vascular occluder to disturb blood flow after stenosis. Neointimal injury and thrombus formation were assessed at 15, 30, and 180 min after the vascular narrowing.

RESULTS

Endothelial detachment, platelet adhesion and neointimal cell apoptosis became evident at the post-stenotic regions of all femoral arteries (n = 5) within 15 min of narrowing. Mural thrombi composed of platelet and fibrin developed after 30 min, and then occlusive thrombi were generated in three out of five vessels after 180 min. The identical vascular narrowing of normal femoral arteries also induced endothelial detachment with small platelet thrombi at post-stenotic regions, but fibrin and occlusive thrombi did not develop. Computational simulation analysis indicated that oscillatory shear stress contributes to the development of erosive damage to the neointima.

CONCLUSIONS

These results suggest that disturbed post-stenotic blood flow can induce erosive injury in SMC-rich plaques and promote thrombus formation that results in vascular events.

Analysis of tissue factor positive microparticles

There has recently been intense interest in the clinical measurement of tissue factor (TF)-positive microparticles (MPs) in clinical disease states. This interest has been driven by the demonstration of an putative role for circulating TF-positive MPs in animal models of thrombus propagation. Both immunological and functional assays for MP-TF have been described. While each approach has its own advantages and drawbacks, neither has yet been truly established as the 'gold standard'. Heterogeneity of TF-bearing MPs, such as the variable co-expression of surface phosphatidylserine, may determine not only their procoagulant potential, but also additional properties including rate of clearance from the circulation.

Tissue factor in cardiovascular disease pathophysiology and pharmacological intervention

Tissue factor (TF) is the major trigger of the coagulation cascade and thereby crucially involved in the maintenance of vascular hemostasis. By binding factor VIIa, the resulting TF:VIIa complex activates the coagulation factors IX and X ultimately leading to fibrin and clot formation. In the vessel wall, TF expression and activity is detectable in vascular smooth muscle cells and fibroblasts and, at a much lower level, in endothelial cells and can be induced by various stimuli including cytokines. In addition, TF is found in the bloodstream in circulating cells such as monocytes, in TF containing microparticles, and as a soluble splicing isoform. Besides its well-known extracellular role as a trigger of coagulation, TF also functions as a transmembrane receptor, and TF-dependent intracellular signaling events regulate the expression of genes involved in cellular responses such as proliferation and migration. TF indeed appears to be involved in the pathogenesis of neointima formation and tumor growth, and increased levels of TF have been detected in patients with cardiovascular risk factors or coronary artery disease as well as in those with cancer. Therefore, pharmacological or genetic inhibition of TF may be an attractive target for the treatment of cardiovascular disease and cancer. Different strategies for inhibition of TF have been developed such as inhibition of TF synthesis and blockade of TF action. Clinical applications of such strategies need to be tested in appropriate trials, in particular for evaluating the advantages of targeted versus systemic delivery of the inhibitors.

Tissue factor and cardiovascular disease: quo vadis?

The coagulation cascade represents a system of proteases responsible to maintain vascular integrity and to induce rapid clot formation after vessel injury. Tissue factor (TF), the key initiator of the coagulation cascade, binds to factor VIIa and thereby activates factor IX and factor X, resulting in thrombus formation. Different stimuli enhance TF gene expression in endothelial and vascular smooth muscle cells. In addition to these vascular cells, TF has recently been detected in the bloodstream in circulating cells such as leukocytes and platelets, as a component of microparticles, and as a soluble, alternatively spliced form of TF. Various cardiovascular risk factors like hypertension, diabetes, and dyslipidemia, increase levels of TF. In line with this observation, enhanced vascular TF expression occurs during atherogenesis, particularly in patients with acute coronary syndromes. (Circ J 2010; 74: 3 - 12).

The role of tissue factor and tissue factor pathway inhibitor in health and disease states

OBJECTIVE

To review the veterinary and human literature on the role of tissue factor (TF) and tissue factor pathway inhibitor (TFPI) in health and disease states.

DATA SOURCES

Original research articles and scientific reviews from both human and veterinary literature were searched for relevance to TF and TFPI.

HUMAN DATA SYNTHESIS

Interest in both TF and TFPI has grown widely over the last several years. The impact TF plays in coagulation, inflammation, angiogenesis, tumor metastasis, and cellular signaling has become apparent. Treatment with TFPI for severe sepsis has been examined and is still currently under investigation. Inhibition of the TF pathway is being studied as an aid in the treatment of neoplasia. The important physiologic and pathophysiologic role these molecules play has only begun to be understood.

VETERINARY DATA SYNTHESIS

There is a paucity of publications that discuss the importance of TF and TFPI in veterinary medicine. An enhanced understanding of the TF pathway in human medicine, in experimental animal models treating sepsis with TFPI, and in animal models demonstrating the proangiogenic properties of TF provides relevance to veterinary medicine.

CONCLUSION

It is apparent that TF and TFPI are important in health and disease. An enhanced understanding of the physiologic and pathophysiologic roles of these factors provides better insight into coagulation, inflammation, angiogenesis, disseminated intravascular coagulation, and tumor metastasis. This greater understanding may provide for the development of therapeutics for sepsis, disseminated intravascular coagulation, and neoplasia.

Contributions of extravascular and intravascular cells to fibrin network formation, structure, and stability

Fibrin is essential for hemostasis; however, abnormal fibrin formation is hypothesized to increase thrombotic risk. We previously showed that in situ thrombin generation on a cell's surface modulates the 3-dimensional structure and stability of the fibrin network. Currently, we compared the abilities of extravascular and intravascular cells to support fibrin formation, structure, and stability. Extravascular cells (fibroblasts, smooth muscle) supported formation of dense fibrin networks that resisted fibrinolysis, whereas unstimulated intravascular (endothelial) cells produced coarse networks that were susceptible to fibrinolysis. All 3 cell types produced a fibrin structural gradient, with a denser network near, versus distal to, the cell surface. Although fibrin structure depended on cellular procoagulant activity, it did not reflect interactions between integrins and fibrin. These findings contrasted with those on platelets, which influenced fibrin structure via interactions between beta3 integrins and fibrin. Inflammatory cytokines that induced prothrombotic activity on endothelial cells caused the production of abnormally dense fibrin networks that resisted fibrinolysis. Blocking tissue factor activity significantly reduced the density and stability of fibrin networks produced by cytokine-stimulated endothelial cells. Together, these findings indicate fibrin structure and stability reflect the procoagulant phenotype of the endogenous cells, and suggest abnormal fibrin structure is a novel link between inflammation and thrombosis.

Tissue factor in the antiphospholipid syndrome

Antiphospholipid (aPL) antibodies are clinically important acquired risk factors for thrombosis and pregnancy loss and are thought to have a direct prothrombotic effect in vivo. Data suggest that a major mechanism by which aPL antibodies contribute to thrombophilia is the upregulation of tissue factor (TF) (CD142) on blood cells and vascular endothelium. TF is the physiological trigger of normal blood coagulation and thrombosis in many hypercoagulable conditions. This article reviews the physiology of TF, the molecular regulation of TF expression and the effects of aPL antibodies on intravascular TF regulation and expression. Inhibition of TF and the pathways by which aPL antibodies induce TF expression are potentially attractive therapeutic targets in the antiphospholipid syndrome.

Circulating tissue factor procoagulant activity is elevated in stable moderate to severe chronic obstructive pulmonary disease

INTRODUCTION

Chronic obstructive pulmonary disease (COPD) patients have increased risk for cardiovascular mortality and venous thromboembolism. Tissue factor (TF) is the physiological initiating mechanism for blood coagulation and is pro-inflammatory.

METHODS

We have studied circulating blood-borne TF-procoagulant activity (TF-PCA), plasma coagulation factors (F) VIIa and FVIII, and thrombin-antithrombin (TAT) complexes in 11 stable, moderate-severe COPD patients, 10 free of exacerbation for >3 weeks.

RESULTS

TF-PCA was increased in COPD patients (52.3+/-5.6 U/ml, (SE)) compared to control subjects (20.7+/-1.5, n=45, p<0.0001). TAT levels were increased (COPD patients: 2.99+/-0.65 ug/l; control subjects: 1.31+/-0.13, n=53, p<0.0001), indicating enhanced thrombin generation. Plasma FVIIa (the activated form of FVII) was higher in COPD (83+/-11 mU/ml; controls, 64+/-5 mU/ml, n=20) but did not reach statistical significance. Plasma FVIIc and FVIII were not increased. TF-PCA levels were inversely related to plasma FVIIa (r=-0.80, p=0.003) and FVIIc (r=-0.76, p=0.007).

CONCLUSIONS

Blood-borne TF-PCA is elevated and constitutes a prothrombotic and proinflammatory state in stable but moderate-severe COPD, and may contribute to the increased risk for vascular events.

Recent advances in clot biology and assessment of clotting

Since the first description of the haemostatic process by Morawitz in 1904, knowledge about the haemostasis mechanism has undergone substantial modifications. Increasing knowledge of enzymology, purification and characterisation of coagulation proteins led to the introduction of the waterfall or cascade model of coagulation. However, these models were based on in vitro studies in the presence of artificial phospholipids and the absence of cells. Two pathways to achieve the formation of a haemostatic fibrin plug were identified, the so-called `extrinsic system' involving both factors present in the circulation and from the extravascular space, and the `intrinsic system' using only factors present in the circulation. However, with increasing knowledge about the interaction between factors from the two systems, the relevance of this model was questioned. The availability of recombinant FVIIa has made further research of the role of FVII/FVIIa and TF feasible, resulting in the current concept of haemostasis according to which the process principally occurs on two cell surfaces, the TF-bearing cell and the thrombin-activated platelet. A limited amount of thrombin is generated by the FVIIa-TF complex on the TF-bearing cell resulting in activation of platelets, FIX, FVIII and FV. The further and full thrombin generation then takes place on the activated platelet surface. The most frequently used assays for evaluation of the global haemostatic capacity are the prothrombin time (PT) and the activated partial thromboplastin time (APTT). The PT measures the formation of a fibrin clot in the presence of an abundance of TF thereby principally reflecting the initial thrombin generation dependent especially on FVII, FV and FX, while the APTT mimics the processes on the activated platelet surface involving FVIII, FIX, FXl, FV, FX and prothrombin. For more specific analyses, assay systems measuring the level of various coagulation factors are available. Platelet function is measured by platelet count and bleeding time. The platelet aggregation response to different agonists can be measured in special aggregometers. The usefulness of these techniques in evaluating a potential bleeding risk is, however, doubtful.

Platelet tissue factor: how did it get there and is it important?

Recently, the presence of functionally active tissue factor (TF) in platelets has been reported by several groups. In this location, TF is postulated to play an important role in the propagation phase of thrombus formation. Although the existence of platelet TF still remains controversial to some extent, a review of the current literature proposes at least three distinct sources of "platelet-associated TF" in those laboratories that have reported its presence: (1) TF that is taken up in the form of circulating microparticles, usually derived from monocytes; (2) TF stored in the alpha-granules of platelets that may have been taken up and/or endogenously synthesized; and (3) TF that is synthesized and expressed on the plasma membrane of mature platelets. These pathways are not mutually exclusive, and the dominant mechanism may depend on the state of platelet activation and, possibly, on other host factors that differ in physiological hemostasis versus pathological thrombosis. This brief review will summarize the state-of-the-art understanding on the origins and possible role of platelet TF.

Tissue factor activity is increased in a combined platelet and microparticle sample from cancer patients

BACKGROUND

Cancer patients have an increased risk of thrombosis. Tissue factor (TF) antigen and TF activity associated with microparticles in plasma are elevated in patients with various types of cancer. Of these two measurements, TF activity is considered superior to TF antigen levels because the activity more closely reflects the ability of TF to initiate coagulation. Recent studies showed that platelets also express TF.

OBJECTIVE

To determine the level of TF activity associated with a combined platelet and microparticle sample from cancer patients (n = 20) and healthy individuals (n = 23).

METHODS

TF activity was measured using a two step chromogenic assay and soluble P-selectin was measured by ELISA in healthy controls and metastatic cancer patients.

RESULTS

We determined the composition of a combined platelet and microparticle sample. The sample consisted of platelets, large microparticles (30-200 nm) and membrane debris. We compared the TF activity of a combined platelet and microparticle sample from cancer patients with that from healthy individuals. We found that TF activity in a combined platelet and microparticle sample from cancer patients was higher than in samples from healthy individuals (21.5+/-12.3 pM (n = 20) versus 8.6+/-6.8 pM (n = 23), mean+/-SD, p < 0.001). Cancer patients also had a higher level of soluble P-selectin compared with controls (18.9+/-5.5 ng/mL versus 13.2+/-2.3 ng/mL, p < 0.001).

CONCLUSION

This study indicates that measurement of TF activity in a combined platelet and microparticle sample can be used as a simple assay to determine the level of circulating TF.

Individual propensity for thrombosis: comparison of venous and arterial circulations

INTRODUCTION

The pathogenesis of venous thrombosis has been attributed to complex interaction between environmental and inherited variables. A basal predisposition for venous thrombophilia independent of environmental variables has not been previously defined experimentally. Both to address the existence of an individual propensity to venous thrombosis and to establish an animal model in which variables governing this propensity could be tested, we provoked venous thrombi in a cohort of pigs of uniform size and age. We furthermore sought to determine whether the thrombotic propensity in the venous circulation is associated with similar propensity for arterial thrombosis.

MATERIALS AND METHODS

Bilateral iliac venous stents were deployed and 2 h later, thrombi were harvested and weighed. The thrombotic response was compared to carotid arterial thrombi generated by crush injury within the same pig. Venous and arterial thrombus platelet deposition were measured by scintillation detection of autologous (111)In-platelet content.

RESULTS

In a cohort of 27 pigs, venous thrombus weights and platelet content varied over greater trrhan 10-fold range from least to greatest responders. There was strong intra-individual correlation of thrombus platelet deposition (r=0.86; p=0.008) and thrombus weights (r=0.68; p=0.015) between stented iliac vein pairs. Venous thrombosis correlated with whole blood platelet counts but not carotid platelet-rich thrombus formation.

CONCLUSIONS

The wide variation in venous thrombotic response to a standardized injury appears to represent an intrinsic propensity of the individual. The poor correlation with arterial thrombosis implies unique mechanisms responsible for this propensity in arteries and veins.

Inhibitors of Factor VIIa/tissue factor

The formation of the proteolytic complex composed of the serine protease Factor VIIa and the cell-associated glycoprotein tissue factor (FVIIa/TF) initiates a cascade of amplified zymogen activation reactions leading to thrombus formation. The critical role of the coagulation cascade in pathological thrombosis has been the basis for significant efforts to design selective inhibitors of the protease components as new anticoagulant alternatives for the treatment of thrombotic diseases. However, for the new generation of anticoagulant drugs in development that primarily target protease complexes distal from FVIIa/TF, the differential between efficacy and safety as defined by bleeding is unresolved. Targeting the FVIIa/TF complex has several theoretical advantages that exploit the amplified nature of the coagulation cascade. However, progress on the development of clinical-stage FVIIa/TF-based anticoagulants has not been as successful to date. This review summarizes recent efforts in the discovery of synthetic inhibitors of FVIIa/TF.

Role of the extrinsic pathway of blood coagulation in hemostasis and thrombosis

Hemostasis requires both platelets and the coagulation system. At sites of vessel injury, bleeding is minimized by the formation of a hemostatic plug consisting of platelets and fibrin. The traditional view of the regulation of blood coagulation is that the initiation phase is triggered by the extrinsic pathway, whereas amplification requires the intrinsic pathway. The extrinsic pathway consists of the transmembrane receptor tissue factor (TF) and plasma factor VII/VIIa (FVII/FVIIa), and the intrinsic pathway consists of plasma FXI, FIX, and FVIII. Under physiological conditions, TF is constitutively expressed by adventitial cells surrounding blood vessels and initiates clotting. In addition so-called blood-borne TF in the form of cell-derived microparticles (MPs) and TF expression within platelets suggests that TF may play a role in the amplification phase of the coagulation cascade. Under pathologic conditions, TF is expressed by monocytes, neutrophils, endothelial cells, and platelets, which results in an elevation of the levels of circulating TF-positive MPs. TF expression within the vasculature likely contributes to thrombosis in a variety of diseases. Understanding how the extrinsic pathway of blood coagulation contributes to hemostasis and thrombosis may lead to the development of safe and effective hemostatic agents and antithrombotic drugs.

Hemostatic effect of activated factor VII without promotion of thrombus growth in melagatran-anticoagulated primates

INTRODUCTION

Pharmacological enhancement of coagulation using activated prothrombin complex concentrate (APCC) or activated factor VII (FVIIa) might be useful hemostatic approaches to bleeding emergencies during anticoagulant therapy. However, any such intervention should not increase thrombotic risk. We therefore investigated their hemostatic and prothrombotic potential during propagation of large arterial-type thrombin in anticoagulated baboons.

MATERIALS AND METHODS

High dose melagatran, a competitive inhibitor of thrombin (0.6 mg/kg/h), or inactivated FVIIa (FVIIai), a competitive inhibitor of FVIIa (2 mg/kg) were used for anticoagulation. APCC or FVIIa were administered to melagatran-anticoagulated animals only. Primary hemostasis was assessed as template bleeding time (BT). Thrombus formation was quantified as fibrin deposition (FD) and platelet deposition (PLD) in synthetic vascular grafts that were deployed for 40 min into arteriovenous shunts.

RESULTS

Melagatran (n=11) prolonged BT to 279% (95% CI +/-140%; P<0.019), reduced FD to 33% [+/-8%; P<0.001]; and PLD to 39% [+/-11%; P<0.001] of untreated controls. FVIIai (n=3) prolonged BT (222% [+/-51%; P<0.010]) without inhibiting thrombus propagation. APCC (n=10) reduced the antithrombotic effect of melagatran (FD 52% [+/-9%; P<0.002], PLD 61% [+/-17%; P=0.028] versus melagatran alone) at a dose (250 U/kg) that had no effect on the BT (327% [+/-150%; P=0.607]. Meanwhile, FVIIa (n=12) normalized the BT to 115% (+/-32%; P<0.05) at a dose (270 microg/kg) that was not yet prothrombotic (FD 26% [+/-4%; P<0.001], PLD 39% [+/-9%; P=0.970]).

CONCLUSION

Administration of FVIIa during antithrombotic treatment with direct thrombin inhibitors might support hemostasis before promoting the intraluminal expansion of thrombi.

Mechanism of action, development and clinical experience of recombinant FVIIa

Recombinant FVIIa has been developed for treatment of bleedings in hemophilia patients with inhibitors, and has been found to induce hemostasis even during major surgery such as major orthopedic surgery. Recombinant FVIIa is being produced in BHK cell cultures and has been shown to be very similar to plasma-derived FVIIa. The use of rFVIIa in hemophilia treatment is a new concept of treatment and is based on the low affinity binding of FVIIa to the surface of thrombin activated platelets demonstrated in a cell-based in vitro model. By the administration of pharmacological doses of exogenous rFVIIa the thrombin generation on the platelet surface at the site of injury is enhanced independently of the presence of FVIII/FIX. As a result of the increased and rapid thrombin formation, a tight fibrin hemostatic plug is being formed. A tight fibrin structure has been found to be more resistant to fibrinolytic degradation thereby helping to maintain hemostasis. The general mechanism of action of pharmacological doses of rFVIIa shown to induce hemostasis not only in hemophilia, but also in patients with platelet defects, and with profuse bleedings triggered by extensive surgery or trauma, may very well be the capacity of generating a tight fibrin hemostatic plug through the increased thrombin generation. Such a fibrin plug will help to resist the overwhelming mostly local release of fibrinolytic activity triggered by the vast tissue damage occurring in extensive trauma. A release of fibrinlytic activity locally has also been demonstrated to occur in the gastrointestinal tract as well as during profuse postpartum bleedings. Pharmacological doses of rFVIIa have in fact, also been shown to induce hemostasis in such cases.

Flow Effects on Coagulation and Thrombosis

Thrombosis occurs in a dynamic rheological field that constantly changes as the thrombus grows to occlusive dimensions. In the initiation of thrombosis, flow conditions near the vessel wall regulate how quickly reactive components are delivered to the injured site and how rapidly the reaction products are disseminated. Whereas the delivery and removal of soluble coagulation factors to the vessel is thought to occur via classic convection-diffusion phenomena, the movement of cells and platelets to the injured wall is strongly augmented by flow-dependent cell-cell collisions that enhance their ability to interact with the wall. In addition, increased shear conditions have been shown to activate platelets, alter the cellular localization of proteins such as tissue factor (TF) and TF pathway inhibitor, and regulate gene production. In the absence of high shearing forces, red cells, leukocytes, and platelets can form stable aggregates with each other or cells lining the vessel wall, which, in addition to altering the biochemical makeup of the aggregate or vessel wall, effectively increases the local blood viscosity. Thus, hemodynamic forces not only regulate the predilection of specific anatomic sites to thrombosis, but they strongly influence the biochemical makeup of thrombi and the reaction pathways involved in thrombus formation.

The role of inflammation in early and late venous thrombosis: Are there clinical implications?

Venous thrombosis is associated with a significant inflammatory response. Inflammatory cells, adhesion molecules (especially selectins), cytokines, and procoagulant microparticles appear to be associated with the thrombogenic process. Once thrombus forms, inflammatory cells are important to thrombus resolution along with fibrinolytic agents and proinflammatory mediators. Collagen and elastin breakdown by the DVT renders the vein wall stiff and non-compliant. Rapid and complete thrombus resolution should lessen vein wall damage and lessen or prevent the development of chronic venous insufficiency. Understanding the basic biology of thrombogenesis and thrombus resolution is important, as novel therapies to both prevent and treat venous thrombosis and hasten thrombus resolution should result from a better understanding of the basic biological mechanisms.