Expression of exogenous tissue factor pathway inhibitor in vivo suppresses thrombus formation in injured rabbit carotid arteries

OBJECTIVES: The aim of the present study was to test the hypothesis that retrovirus-mediated in vivo tissue factor pathway inhibitor (TFPI) gene transfer to the arterial wall would efficiently inhibit thrombosis without causing significant changes in systemic hemostatic variables. BACKGROUND: Acute coronary syndromes (unstable angina and acute myocardial infarction) are usually caused by atherosclerotic plaque rupture, with consequent activation of the coagulation cascade and circulating platelets. Tissue factor (TF) exposure represents an early event in this pathophysiologic sequence, leading to activation of the extrinsic coagulation pathway and thrombin formation. Tissue factor pathway inhibitor is a naturally occurring inhibitor of the extrinsic pathway. METHODS: In the present study, the gene coding for rabbit TFPI was inserted in a retroviral vector under control of a tetracycline-inducible promoter. Replication-defective, infectious, recombinant retroviruses were used to transfect rabbit carotid arteries with either TFPI or a reporter gene--green fluorescent protein (GFP). RESULTS: Retroviral-mediated arterial gene transfer of TFPI resulted in potent inhibition of intravascular thrombus formation in stenotic and injured rabbit carotid arteries, whereas transfection of the contralateral carotid artery with GFP had no effect on thrombosis. No significant changes in systemic hemostatic variables (prothrombin time and partial thromboplastin time) were observed when thrombosis was inhibited. CONCLUSIONS: These data suggest that retroviral-mediated transfection of the arterial wall with TFPI might represent an attractive approach for the treatment of thrombotic disorders.     

组织因子与急性冠脉综合征研究进展

组织因子作为FⅦ/FⅦa的细胞膜表面受体,是外源性凝血系统的关键因子,组织因子通过介导凝血激活形成血栓。动脉粥样硬化斑块破裂处血栓形成是急性冠脉综合征的主要原因,其临床后果的严重性决定于血栓的范围和进展。急性冠脉综合征时循环单核细胞和微颗粒表达组织因子,促进全身的促凝活性。动脉粥样硬化斑块中巨噬细胞、平滑肌细胞、血管内皮细胞表达组织因子,不稳定性斑块中组织因子表达和活性较稳定性斑块更高。组织因子通路抑制物是内源性组织因子抑制物,对调剂血栓形成有重要作用。现就目前组织因子与急性冠脉综合征研究进展作一综述。     

Tissue factor in acute coronary syndromes

Thrombosis at the site of atherosclerotic plaque disruption is the principal cause of acute coronary syndromes. The severity of the clinical consequences is determined by the extent and the progression of the thrombus that are caused by local and systemic factors. In atherosclerotic lesions mediators induce tissue factor (TF) in macrophages, smooth muscle cells, and endothelial cells. Procoagulant microparticles in the lipid core further enhance the thrombogenicity of the plaque. In addition, in acute coronary syndromes circulating monocytes and microparticles express TF and, thereby, contribute to systemic procoagulant activity. As a regulatory mechanism surface-bound, endogenous tissue factor pathway inhibitor-1 (TFPI) inhibits TF activity by translocation of the quaternary complex TF-FVIIa-FXa-TFPI into glycosphingolipid-rich microdomains more efficiently than exogenously added TFPI. This inhibition occurs not only in endothelial cells but also on circulating monocytes and presumably microparticles. Because therapeutic thrombolysis in acute myocardial infarction degrades TFPI, a prothrombotic state due to unopposed TF activity may occur. Several studies have demonstrated a contribution of local and bloodborne TF to thrombus formation; a direct relationship with the clinical outcome, however, awaits further studies. This article discusses the current understanding of the role of TF and its regulation by TFPI in acute coronary syndromes.     

Effects of lipids and lipoproteins on thrombosis and rheology

Atherosclerotic plaque rupture and erosions precipitate thrombus formation and may lead to an acute ischemic syndrome. Lipids and lipoproteins modulate the expression and/or function of thrombotic, fibrinolytic and rheologic factors, and thereby influence hemostasis and potential tissue damage resulting from vascular injury. Triglyceride-enriched lipoproteins are accompanied by elevations in factor VII clotting activity, plasminogen activator inhibitor (PAI-1) and viscosity of blood and plasma. Low density lipoprotein (LDL) promotes platelet activation and tissue factor expression and LDL levels correlate with levels of vitamin K dependent coagulation factors and fibrinogen. Conversely, LDL inhibits tissue factor pathway inhibitor (TFPI) which limits activation of the extrinsic coagulation pathway. High density lipoprotein (HDL) has anti-atherothrombotic properties that result from inhibition of platelet and erythrocyte aggregation, reduced blood viscosity and suppression of tissue factor activity and PAI-1 activity and antigen levels. The effects of lipids and lipoproteins on hemostasis and rheology may have important implications for the clinical sequelae following plaque disruption and erosion.     

New Potential Therapeutic Modalities: Tissue Factor Pathway Inhibitor

Tissue Factor Pathway Inhibitor (TFPI) is an endogenous inhibitor of the extrinsic pathway of coagulation. TFPI is a serine protease inhibitor that inhibits factor Xa directly and the factor VIIa/tissue factor catalytic complex in a Xa dependent fashion. TFPI is a multivalent inhibitor with three tandemly arranged Kunitz-type protease inhibitor domains, that appears to provide the protein with distinct functional properties, which make the inhibitor well suited for a regulatory role. TFPI is synthesized and secreted by endothelial cells. Circulating TFPI is lipoprotein associated. Control of thrombus formation in the microvasculature is likely to lead to the development of therapeutic modalities for the treatment of severe sepsis, especially when the thrombotic process is known to be a stimulus for multiple organ failure. A large pool of endogenous TFPI is found bound to the endothelium, while a small pool is stored in platelets.The concept of utilizing TFPI to protect the integrity of microvasculature, inhibit the initiation and propagation of multiple organ failure through the reduction of microthrombi formation, was examined in several models of sepsis, with and without severe DIC. There was significant survival benefit by the administration of recombinant TFPI to septic animals, in bacteremia and peritonitis models, several hours after the onset of symptoms. TFPI was found to exhibit anti-inflammatory activities as demonstrated by the decrease of IL-6 and IL-8 levels in the treated animals. TFPI was reported to bind endotoxin in vitro and reduce Fas levels in vivo suggesting the attenuation of apoptosis, in the target microvascular endothelial cells.TFPI was tested for safety and tolerability in normal human volunteers and more recently in patients with severe sepsis. The human clinical data suggest that TFPI is safe and well tolerated. TFPI is currently in phase II studies in severe sepsis patients.     

Plasma tissue factor and tissue factor pathway inhibitor levels in patients with disseminated intravascular coagulation

We measured the plasma levels of tissue factor (TF) and tissue factor pathway inhibitor (TFPI) in patients with disseminated intravascular coagulation (DIC) to examine the relationship between TFPI and vascular endothelial cell injury. Plasma TF (273 ± 90 pg/ml) and TFPI (252 ± 125 ng/ml) levels were significantly increased in patients with DIC compared with non-DIC patients. Plasma TF antigen level was significantly increased in pre-DIC patients (285 ± 85 pg/ml), while the plasma TFPI level (152 ± 54 ng/ml) was not markedly increased in such a state. The plasma TF/TFPI ratio was high in the pre-DIC patients (2.10 ± 0.90), and low in the DIC patients (1.40 ± 0.87) and healthy volunteers (0.84 ± 0.26). There was no significant difference between the DIC patients with a good outcome and those with a poor outcome in terms of plasma TF levels, although the plasma TFPI level in the DIC patients with a good outcome (289 ± 133 ng/ml) was significantly higher than that in those with a poor outcome (187 ± 75 ng/ml). During the clinical course of DIC, plasma TF antigen was increased first, and an increase of the plasma TFPI level followed the increase in plasma TF level. These findings suggest that plasma TFPI is released from vascular endothelial cells and it may reflect vascular endothelial cell injury. It is conceivable that TF and TFPI may play an important role in the onset of DIC. © 1996 Wiley-Liss, Inc.     

Elevated TFPI in malignant disease: relation to cancer type and hypercoagulation

We have previously reported high levels of the coagulation inhibitor TFPI in the blood of patients with gastrointestinal cancer. TFPI is not an acute-phase reactant, but high levels have also been reported in patients with septicaemia and disseminated intravascular coagulation (DIC). To study its relationship with other types of malignancy, TFPI activity was first determined in plasma samples from 214 patients with various malignancies. In a second cohort of 83 patients, total and free TFPI antigen, protein C, antithrombin, fibrin monomer and D-dimer were also measured. Elevated TFPI activity and antigens were found in about half of the patients with solid tumours. In contrast, elevated TFPI was rare in haematological malignancies (12%). In the 18 patients with acute nonlymphocytic leukaemia (ANLL), elevated free TFPI was found only in patients who also had DIC. No correlation was found between TFPI levels and fibrin monomer or D-dimer levels. Only four out of 20 patients with solid tumours had normal levels of fibrin monomer and D-dimer, yet three out of these four had elevated TFPI. In conclusion, elevated TFPI in ANLL is related to the coexistence of DIC. In solid tumour disease increased TFPI may reduce protective fibrin formation, but the pathogenic mechanism is as yet unknown.     

Plasma tissue factor and tissue factor pathway inhibitor levels in patients with disseminated intravascular coagulation

We measured the plasma levels of tissue factor (TF) and tissue factor pathway inhibitor (TFPI) in patients with disseminated intravascular coagulation (DIC) to examine the relationship between TFPI and vascular endothelial cell injury. Plasma TF (273 ± 90 pg/ml) and TFPI (252 ± 125 ng/ml) levels were significantly increased in patients with DIC compared with non-DIC patients. Plasma TF antigen level was significantly increased in pre-DIC patients (285 ± 85 pg/ml), while the plasma TFPI level (152 ± 54 ng/ml) was not markedly increased in such a state. The plasma TF/TFPI ratio was high in the pre-DIC patients (2.10 ± 0.90), and low in the DIC patients (1.40 ± 0.87) and healthy volunteers (0.84 ± 0.26). There was no significant difference between the DIC patients with a good outcome and those with a poor outcome in terms of plasma TF levels, although the plasma TFPI level in the DIC patients with a good outcome (289 ± 133 ng/ml) was significantly higher than those with a poor outcome (187 ± 75 ng/ml). During the clinical course of DIC, plasma TF antigen was increased first, and an increase of the plasma TFPI level followed the increase in plasma TF level. These findings suggest that plasma TFPI is released from vascular endothelial cells and it may reflect vascular endothelial cell injury. It is conceivable that TF and TFPI may play an important role in the onset of DIC. Am. J. Hematol. 55:169–174, 1997. © 1997 Wiley-Liss, Inc.     

Decreased plasma tissue factor pathway inhibitor in women taking combined oral contraceptives

Use of combined oral contraceptives (OC) is associated with a significant risk of thrombosis. The mechanisms of this effect are not clearly defined. Tissue factor pathway inhibitor (TFPI) is a circulating anti-coagulant that inhibits the earliest steps in activation of the extrinsic coagulation pathway. It plays a central role in control of coagulation but its contribution to the thrombotic risk associated with OC has not been assessed. Plasma TFPI antigen and activity, factor VIIa, prothrombin fragments 1&2, von Willebrand antigen, fibrinogen, and low density lipoprotein cholesterol were measured by standard assays in women taking OC (aged 16 to 45 years, n = 40) and age-matched women not taking OC (controls, n = 40). Plasma TFPI antigen did not vary significantly across the menstrual cycle in controls. Women on OC had a 25% reduction in plasma TFPI antigen (median 51.0 ng/ml; 95% confidence intervals [CI] 37.5 to 85.5; control 68.0 ng/ml, CI 61.0 to 95.0; P < 0.001) and a 29% reduction in TFPI activity (78.5 U/ml, CI 57.5 to 107.5; control 111.0 U/ml, CI 79.5 to 171.0; P < 0.001) compared to controls. Plasma factor VIIa activity and prothrombin fragments 1&2 were also significantly increased in women using OC (both P < 0.001), indicating activation of the extrinsic coagulation pathway. These results demonstrate that normal cyclic variations in estrogen and/or progesterone do not significantly alter plasma TFPI levels. However, estrogens and/or progestogens in OC result in activation of the extrinsic coagulation pathway and significantly reduce plasma TFPI, its major circulating inhibitor. Reduced plasma TFPI levels may underlie the thrombotic effects of OC.     

Increased truncated form of plasma tissue factor pathway inhibitor levels in patients with disseminated intravascular coagulation

To evaluate that the relationship between the truncated form of tissue factor pathway inhibitor (TFPI) and the stage of disseminated intravascular coagulation (DIC), we measured the plasma levels of tissue factor (TF) antigen and the intact and truncated forms of TFPI antigens in 41 patients with DIC, 12 with pre-DIC, and 20 with non-DIC. The plasma TF and total TFPI antigen levels were significantly higher in patients with DIC than in non-DIC patients. Plasma levels of intact TFPI antigen in the pre-DIC groups were significantly lower than in the non-DIC and DIC groups. The truncated form of TFPI antigen levels in DIC patients were significantly increased compared with those in non-DIC and pre-DIC patients. The fact that the intact form of TFPI was decreased in pre-DIC patients compared with that in non-DIC patients, suggests that it is consumed in the pre-DIC state and that hypercoagulability occurs in pre-DIC patients. The increased level of the truncated form of TFPI in DIC patients may be attributed to proteolysis of the intact form of TFPI in these patients. The increased level of the truncated form of TFPI may be a useful index for the diagnosis of DIC. Am. J. Hematol. 60:94–98, 1999. © 1999 Wiley-Liss, Inc.     

Tissue factor pathway inhibitor in health and disease

The knowledge of tissue factor pathway inhibitor (TFPI) has greatly expanded during the last few years, and TFPI is now established as a coagulation inhibitor of great importance. Its main role seems to be inhibition of small amounts of tissue factor, which probably is essential for maintaining a normal hemostatic balance. The acceleration of TFPI's inhibitory effect by heparin, the TFPI release caused by heparin injection, and TFPI's heparin affinity may greatly contribute to the anticoagulant properties of the endothelium, and may be particularly important for the outcome of vascular injury. The information provided by one single measurement of plasma TFPI in a patient is difficult to interpret, but serial measurements during the course of a disease may signal the prognosis. Recombinant TFPI has proved effective in the treatment of experimental disseminated intravascular coagulation, sepsis, and thrombosis. Whether TFPI or TFPI derivatives will be as effective in the treatment of patients remains to be determined.     

Increased levels of tissue factor pathway inhibitor may reflect disease activity and play a role in thrombotic tendency in Behçet's disease

Tissue factor pathway inhibitor (TFPI) is a Kunitz‐type proteinase inhibitor that has a crucial role in haemostasis and is primarily synthesized in the vascular endothelium. We investigated plasma total TFPI, antiphospholipid antibodies, and some other coagulation and fibrinolytic system parameters in 30 patients with Behçet's disease and 15 controls by the enzyme‐linked immunosorbent assay method. TFPI levels were significantly higher in the Behçet's group (119 ± 57.5 ng/ml) compared with the control group (74.8 ± 31.5) (P < 0.009). We also noted a statistical significance in TFPI levels between patients with active disease (n:16) (139 ± 55) and patients without activation (n:14) (96 ± 53) (P < 0.03), whereas inactive patients lacked any significance when compared with the control group (P < 0.29). Other parameters disclosed no statistical significance between patients and control group except for elevated fibrinogen and plasminogen activator inhibitor‐1 levels in the patient group (P < 0.003). Increased levels of TFPI may reflect a defensive mechanism like in other diseases characterized by thrombotic tendency and represent a parameter of disease activity. Am. J. Hematol. 68:225–230, 2001. © 2001 Wiley‐Liss, Inc.     

The role of tissue factor pathway inhibitor in atherosclerosis and arterial thrombosis

Tissue factor pathway inhibitor (TFPI) is the main inhibitor of tissue factor (TF)-mediated coagulation. In atherosclerotic plaques TFPI co-localizes with TF, where it is believed to play an important role in attenuating TF activity. Findings in animal models such as TFPI knockout models and gene transfer models are consistent on the role of TFPI in arterial thrombosis as they reveal an active role for TFPI in attenuating arterial thrombus formation. In addition, ample experimental evidence exists indicating that TFPI has inhibitory effects on both smooth muscle cell migration and proliferation, both which are recognized as important pathological features in atherosclerosis development. Nonetheless, the clinical relevance of these antithrombotic and atheroprotective effects remains unclear. Paradoxically, the majority of clinical studies find increased instead of decreased TFPI antigen and activity levels in atherothrombotic disease, particularly in atherosclerosis and coronary artery disease (CAD). Increased TFPI levels in cardiovascular disease might result from complex interactions with established cardiovascular risk factors, such as hypercholesterolemia, diabetes and smoking. Moreover, it is postulated that increased TFPI levels reflect either the amount of endothelial perturbation and platelet activation, or a compensatory mechanism for the increased procoagulant state observed in cardiovascular disease. In all, the prognostic value of plasma TFPI in cardiovascular disease remains to be established. The current review focuses on TFPI in clinical studies of asymptomatic and symptomatic atherosclerosis, coronary artery disease and ischemic stroke, and discusses potential atheroprotective actions of TFPI.     

Tissue factor, the blood, and the arterial wall

Thrombogenic tissue factor (TF) on cell-derived microparticles is present in the circulating blood of patients with acute coronary syndromes. Recently, we reported that leukocytes transfer TF-positive particles to platelet thrombi, making them capable of triggering and propagating thrombus growth. This observation changes the original dogma that vessel-wall injury and exposure of tissue factor within the vasculature to blood is sufficient for the occurrence of arterial thrombosis. The transfer of TF-positive leukocyte particles is dependent on the interaction of CD15 and TF with platelet thrombi. The inhibition of TF transfer and TF activity suggests a novel therapeutic approach to the prevention of thrombosis that may prove to be effective in disorders associated with increased blood TF.     

Inhibition of restenosis by tissue factor pathway inhibitor: in vivo and in vitro evidence for suppressed monocyte chemoattraction and reduced gelatinolytic activity

Activation of inflammatory and procoagulant mechanisms is thought to contribute significantly to the initiation of restenosis, a common complication after balloon angioplasty of obstructed arteries. During this process, expression of tissue factor (TF) represents one of the major physiologic triggers of coagulation that results in thrombus formation and the generation of additional signals leading to vascular smooth muscle cell (VSMC) proliferation and migration. In this study, we have investigated the mechanisms by which inhibition of coagulation at an early stage through overexpression of tissue factor pathway inhibitor (TFPI), an endogenous inhibitor of TF, might reduce restenosis. In a rabbit femoral artery model, percutaneous delivery of TFPI using a recombinant adenoviral vector resulted in a significant reduction of the intimamedia ratio 21 days after injury. Investigating several markers of inflammation and coagulation, we found reduced neointimal expression of monocyte chemoattractant protein-1 (MCP-1), lesional monocyte infiltration, and expression of vascular TF, matrix metalloproteinase-2 (MMP-2), and MMP-9. Moreover, overexpression of TFPI suppressed the autocrine release of platelet-derived growth factor BB (PDGF-BB), MCP-1, and MMP-2 in response to factors VIIa and Xa from VSMCs in vitro and inhibited monocyte TF activity. These results suggest that TFPI exerts its action in vivo through not only thrombotic, but also nonthrombotic mechanisms.     

Probable regulation of factor VIIa–tissue factor and prothrombinase by factor Xa–TFPI and TFPI in vivo

Given that factor VIIa–tissue factor (TF) probably initiates coagulation in vivo , this study investigated the relationship between plasma concentrations of factor VIIa and prothrombin fragment 1+2 in plasma (the latter as an index of prothrombinase activity in vivo ). The relationships between these two parameters and the concentrations of tissue factor pathway inhibitor (TFPI) and factor Xa–TFPI in plasma were also investigated. TFPI inactivates factor Xa in a reaction accelerated by heparin, whereas factor Xa–TFPI inactivates factor VIIa–TF and prothrombinase. Established enzyme-linked immunosorbent assays (ELISAs) were used to quantify TFPI and prothrombin fragment 1+2, whereas we developed an ELISA to quantify factor Xa–TFPI using affinity purified rabbit (anti-human TFPI)-IgG and chicken anti-(human factor Xa–TFPI)-IgY as the capture and detector antibodies, respectively. Plasma factor VIIa was quantified using truncated tissue factor. The concentrations of factor VIIa and prothrombin fragment 1+2 increased in parallel in the plasmas of up to 145 healthy adults assayed ( P  = 0.007), as did the concentrations of factor VIIa and TFPI ( P  = 0.0039), and prothrombin fragment 1+2 and TFPI ( P  = 0.013). In contrast, there was an inverse relationship between the concentrations of free factor Xa–TFPI and factor VIIa ( P  <0.0001) and free factor Xa–TFPI and prothrombin fragment 1+2 ( P =0.0095). These results are consistent with factor Xa–TFPI regulating factor VIIa–tissue factor and prothrombinase in vivo .     

Tissue factor pathway inhibitor (TFPI) in disseminated intravascular coagulation: low levels of the activated factor X-TFPI complex.

Tissue factor pathway inhibitor (TFPI) fractions and coagulation markers were determined in 26 patients with disseminated intravascular coagulation (DIC). Thrombin-antithrombin complex and fibrin monomer values were markedly elevated in all patients (P < 0.01). The median TFPI activity level (2.1 nmol/l) was lower than in normal controls (2.6 nmol/l; P < 0.01). The median free TFPI level was within the normal reference range, but seven patients had levels above and nine patients had levels below normal range. The median activated factor X (FXa)-TFPI complex level in patients (0.13 nmol/l) was lower than in controls (0.18 nmol/l; P < 0.01). Only one patient had a FXa-TFPI complex level above the normal range, while eight patients had levels below. In conclusion, TFPI activity, free TFPI antigen and FXa-TFPI complex levels vary considerably in DIC. Activation of coagulation may increase TFPI levels, as reported by other workers and supported by a positive correlation between tissue factor and free TFPI in the present material. A negative correlation between fibrin monomer and free TFPI (r = -0.46, P = 0.019) might indicate that hyperactive coagulation leads to consumption of TFPI. Subnormal FXa-TFPI levels in DIC, possibly caused by consumption, may imply a reduced capacity to inactivate the triggering factor VIIa-tissue factor complex.     

Clinical significance of hemostatic markers and thrombomodulin in systemic lupus erythematosus: evidence for a prothrombotic state

Systemic lupus erythematosus (SLE) is a systemic autoimmune disorder with overwhelming thrombotic states. The precise pathogenetic mechanisms underlying the prethrombotic state in SLE is not fully understood, but interactions between the antiphospholipid antibodies and antigen targets on the coagulation components have been incriminated to play fundamental roles. To evaluate this issue, 34 women with antiphospholipid antibody negative SLE were investigated for molecular markers of blood coagulation and fibrinolytic activity: prothrombin fragment1+2 (PF1+2), thrombin-antithrombin complex (TAT), plasmin-alpha2-antiplasmin inhibitor complex (PAP), and tissue factor pathway inhibitor (TFPI). We also analysed plasma soluble thrombomodulin (sTM) levels. SLE disease activity was determined using the SLE Disease Activity Index (SLEDAI). Concentrations of TAT, PAP, PF1+2 and sTM were significantly elevated (P<0.0001, P=0.0002, P<0.0001, and P<0.0001, respectively), while TFPI antigen levels were found to be reduced (P<0.0001) in patients with SLE compared to the control group. In patients with active SLE, anti-ds DNA levels were correlated positively with plasma TAT (P<0.05), PF1+2 (P<0.05), and sTM (P<0.01) concentrations and negatively with plasma TFPI levels (P<0.05). SLEDAI scores were correlated positively with plasma TAT (P<0.01), PF1+2 (<0.01), and sTM (P<0.01) levels. This study illustrates that both a prethrombotic state and a compensatory fibrinolytic process secondary to subclinical intravascular coagulation might coexist in SLE with elevated sTM levels, indicating impaired endothelial functions.     

Physiological function of tissue factor pathway inhibitor and interaction with heparins

Tissue factor pathway inhibitor (TFPI) is now recognized as a major physiological anticoagulant. Its main role is to modulate factor VIIa/tissue factor catalytic activity. Another important role is to potentiate the effect of heparins. TFPI is released from the vascular endothelium after injection of either unfractionated heparin (UFH) or low-molecular-weight heparins (LMWHs), which may then provide high concentrations of TFPI at sites of tissue damage and ongoing thrombosis. In dilute prothrombin-time-based assays, released TFPI contributes approximately one-third to the anticoagulant effect of heparin, the remaining being accounted for by antithrombin. Released TFPI, but not plasma TFPI, contains the basic carboxy-terminal tail which is important for the anticoagulant effect. UFH and LMWH exert differential effects on intravascular TFPI. UFH, but not LMWH, given in therapeutic doses, is associated with a progressive depletion of TFPI, which is associated with a strong rebound activation of coagulation after cessation of treatment. Such depletion may explain the apparent superior efficacy of LMWH observed in clinical trials.     

Platelet microparticles and vascular cells interactions: a checkpoint between the haemostatic and thrombotic responses

Described 40 years ago as cell dust, microparticles (MPs) are now considered a key component in the haemostatic response. Owing to their plasma membrane reactivity, platelets are believed to constitute the main source of circulating procoagulant microparticles and behave as true sensors for the haemostatic response. Erythrocytes, leukocytes and endothelial cells are also able to shed MPs in the blood flow, their respective contribution varying with the pathophysiologic circumstances and extent of the cellular damage. The catalytic properties of MPs rely on a procoagulant anionic phospholipid, phosphatidylserine, made accessible at the outer leaflet following plasma membrane remodelling and on the eventual presence of tissue factor (TF). Under resting conditions, most membrane-bound TF is encrypted. Although able to bind to FVIIa, it does not trigger blood coagulation. Under prothrombotic conditions, TF decryption would occur through intricate pathways involving platelets, monocytes, endothelial cells and derived MPs. P-selectin/P-selectin glycoprotein Ligand-1 (PSGL-1) interactions and reactive oxygen species would promote TF decryption in cell-MP aggregates. At sites of endothelium injury, the swift recruitment of TF+-MPs through P-selectin/PSGL-1 interactions enables the concentration of TF activity above a threshold allowing coagulation to be triggered. Another crucial feature in the initiation of blood coagulation, possibly tuned by MPs, is the balance between TF and TFPI. In specific pathophysiologic contents with elevated levels of circulating TF+-MPs, accessible TFPI at the MP surface would be overwhelmed. Beyond their procoagulant properties demonstrated in vitro, a number of pieces of evidence points to procoagulant MPs as efficient effectors in the haemostatic response, and as pathogenic markers of thrombotic disorders and vascular damage. This review will focus on the pathophysiological significance of platelet-derived MPs and their interaction with vascular cells.