Calibrated automated thrombin generation measurement in clotting plasma

Calibrated automated thrombography displays the concentration of thrombin in clotting plasma with or without platelets (platelet-rich plasma/platelet-poor plasma, PRP/PPP) in up to 48 samples by monitoring the splitting of a fluorogenic substrate and comparing it to a constant known thrombin activity in a parallel, non-clotting sample. Thus, the non-linearity of the reaction rate with thrombin concentration is compensated for, and adding an excess of substrate can be avoided. Standard conditions were established at which acceptable experimental variation accompanies sensitivity to pathological changes. The coefficients of variation of the surface under the curve (endogenous thrombin potential) are: within experiment approximately 3%; intra-individual: <5% in PPP, <8% in PRP; interindividual 15% in PPP and 19% in PRP. In PPP, calibrated automated thrombography shows all clotting factor deficiencies (except factor XIII) and the effect of all anticoagulants [AVK, heparin(-likes), direct inhibitors]. In PRP, it is diminished in von Willebrand's disease, but it also shows the effect of platelet inhibitors (e.g. aspirin and abciximab). Addition of activated protein C (APC) or thrombomodulin inhibits thrombin generation and reflects disorders of the APC system (congenital and acquired resistance, deficiencies and lupus antibodies) independent of concomitant inhibition of the procoagulant pathway as for example by anticoagulants.

Thrombin generation and fibrin clot structure

Generation of a hemostatic clot requires thrombin-mediated conversion of fibrinogen to fibrin. Previous in vitro studies have demonstrated that the thrombin concentration present at the time of gelation profoundly influences fibrin clot structure. Clots formed in the presence of low thrombin concentrations are composed of thick fibrin fibers and are highly susceptible to fibrinolysis; while, clots formed in the presence of high thrombin concentrations are composed of thin fibers and are relatively resistant to fibrinolysis. While most studies of clot formation have been performed by adding a fixed amount of purified thrombin to fibrinogen, clot formation in vivo occurs in a context of continuous, dynamic changes in thrombin concentration. These changes depend on the local concentrations of pro- and anti-coagulants and cellular activities. Recent studies suggest that patterns of abnormal thrombin generation produce clots with altered fibrin structure and that these changes are associated with an increased risk of bleeding or thrombosis. Furthermore, it is likely that clot structure also contributes to cellular events during wound healing. These findings suggest that studies explicitly evaluating fibrin formation during in situ thrombin generation are warranted to explain and fully appreciate mechanisms of normal and abnormal fibrin clot formation in vivo.

Evidence of normal thrombin generation in cirrhosis despite abnormal conventional coagulation tests

The role played by coagulation defects in the occurrence of bleeding in cirrhosis is still unclear. This is partly due to the lack of tests that truly reflect the balance of procoagulant and anticoagulant factors in vivo. Conventional coagulation tests such as prothrombin time and activated partial thromboplastin time are inadequate to explore the physiological mechanism regulating thrombin, because they do not allow full activation of the main anticoagulant factor, protein C, whose levels are considerably reduced in cirrhosis. We used a thrombin generation test to investigate the coagulation function in patients with cirrhosis. Thrombin generation measured without thrombomodulin was impaired, which is consistent with the reduced levels of procoagulant factors typically found in cirrhosis. However, when the test was modified by adding thrombomodulin (i.e., the protein C activator operating in vivo), patients generated as much thrombin as controls. Hence, the reduction of procoagulant factors in patients with cirrhosis is compensated by the reduction of anticoagulant factors, thus leaving the coagulation balance unaltered. These findings help clarify the pathophysiology of hemostasis in cirrhosis, suggesting that bleeding is mainly due to the presence of hemodynamic alterations and that conventional coagulation tests are unlikely to reflect the coagulation status of these patients. In conclusion, generation of thrombin is normal in cirrhosis. For a clinical validation of these findings, a prospective clinical trial is warranted where the results of thrombin generation in the presence of thrombomodulin are related to the occurrence of bleeding.

The dynamics of thrombin formation

The central event of the hemostatic process is the generation of thrombin through the tissue factor pathway. This is a highly regulated, dynamic process in which thrombin itself plays many roles, positively and negatively its production and destruction. The hemostatic process is essential to normal physiology and is also the Achilles heel of our aging population. The inappropriate generation of thrombin may lead to vascular occlusion with the consequence of myocardial infarction, stroke, pulmonary embolism, or venous thrombosis. In this review, we summarize our present views regarding the tissue factor pathway by which thrombin is generated and the roles played by extrinsic and intrinsic factor Xa generating complexes in hemostasis and the roles of the stoichiometric and dynamic inhibitors that regulate thrombin generation.

A Systematic Evaluation of the Effect of Temperature on Coagulation Enzyme Activity and Platelet Function

BACKGROUND

Hypothermia is associated with an increased risk of bleeding and is a significant contributing factor to the morbidity and mortality of trauma and complicated surgical procedures. A core temperature of 33 degrees C is associated with a significantly increased risk of death after trauma compared with 37 degrees C. Hypothermia-associated bleeding has been hypothesized to result from dysregulation of enzymatic function, reduced platelet activity, and/or altered fibrinolysis.

METHODS

We systematically evaluated the effects of temperature on isolated pro- and anticoagulant enzyme processes and platelet activation and adhesion. We also evaluated the effects of temperature on complete coagulation systems (activated partial thromboplastin time and an in vitro, cell-based model of coagulation).

RESULTS

Enzyme activities were only slightly reduced at 33 degrees C versus 37 degrees C, and this reduction was not statistically significant (p > 0.05). Platelet activation was also not significantly reduced at 33 degrees C versus 37 degrees C. Conversely, platelet aggregation and adhesion were significantly reduced at 33 degrees C compared with 37 degrees C (p < 0.05). Below 33 degrees C, however, both enzyme activity and platelet function were significantly reduced.

CONCLUSION

Our results suggest that bleeding observed at mildly reduced temperatures (33 degrees - 37 degrees C) results primarily from a platelet adhesion defect, and not reduced enzyme activity or platelet activation. However, at temperatures below 33 degrees C, both reduced platelet function and enzyme activity likely contribute to the coagulopathy.

The pathogenesis of venous limb gangrene associated with heparin-induced thrombocytopenia

BACKGROUND

Platelet-mediated arterial occlusion is a well-recognized cause of limb loss in patients with heparin-induced thrombocytopenia. However, the syndrome of distal ischemic necrosis complicating the deep venous thrombosis (venous limb gangrene) sometimes associated with heparin-induced thrombocytopenia has not been well characterized.

OBJECTIVE

To study the pathogenesis of venous limb gangrene associated with heparin-induced thrombocytopenia.

DESIGN

Characterization (based on descriptive and case-control studies) of a novel syndrome of limb loss and hypothesis testing by analysis of plasma samples.

SETTING

Five university-associated hospitals in one medical community.

PATIENTS

Clinical and laboratory records of 158 patients with heparin-induced thrombocytopenia were reviewed to identify patients with venous limb gangrene (n = 8), limb arterial thrombosis (n = 10), and uncomplicated deep venous thrombosis (n = 58).

MEASUREMENTS

Clinical and laboratory factors associated with venous limb gangrene, including thrombin-antithrombin complexes and vitamin K-dependent procoagulant and anticoagulant factors.

RESULTS

Warfarin treatment was more frequently associated with venous limb gangrene than with limb arterial thrombosis (8 of 8 patients compared with 3 of 10 patients; P = 0.004). The anticoagulant effect of warfarin seemed greater in the 8 patients with venous limb gangrene than in the 58 patients who did not develop gangrene (median International normalized ratio, 5.8 compared with 3.1; P < 0.001). Compared with plasma from controls, plasma from patients with venous limb gangrene had a higher ratio of thrombin-antithrombin complex to protein C activity during warfarin treatment. No hereditable abnormalities of the protein C anticoagulant pathway were seen in any patient.

CONCLUSIONS

Warfarin treatment of deep venous thrombosis associated with heparin-induced thrombocytopenia is a possible cause of venous limb gangrene, perhaps because of acquired failure of the protein C anticoagulant pathway to regulate thrombin generation.

Inhibition of platelet-mediated, tissue factor-induced thrombin generation by the mouse/human chimeric 7E3 antibody. Potential implications for the effect of c7E3 Fab treatment on acute thrombosis and "clinical restenosis"

The murine/human chimeric monoclonal antibody fragment (c7E3 Fab) blocks GPIIb/IIIa and alpha v beta 3 receptors, inhibits platelet aggregation, and decreases the frequency of ischemic events after coronary artery angioplasty in patients at high risk of suffering such events. Although inhibition of platelet aggregation is likely to be the major mechanism of c7E3 Fab's effects, since activated platelets facilitate thrombin generation, it is possible that c7E3 Fab also decreases thrombin generation. To test this hypothesis, the effects of c7E3 Fab and other antiplatelet agents were tested in a thrombin generation assay triggered by tissue factor. c7E3 Fab produced dose-dependent inhibition of thrombin generation, reaching a plateau of 45-50% inhibition at concentrations > or = 15 micrograms/ml. It also inhibited thrombin-antithrombin complex formation, prothrombin fragment F1-2 generation, platelet-derived growth factor and platelet factor 4 release, incorporation of thrombin into clots, and microparticle formation. Antibody 6D1, which blocks platelet GPIb binding of von Willebrand factor, had no effect on thrombin generation, whereas antibody 10E5, which blocks GPIIb/IIIa but not alpha v beta 3 receptors decreased thrombin generation by approximately 25%. Combining antibody LM609, which blocks alpha v beta 3 receptors, with 10E5 increased the inhibition of thrombin generation to approximately 32-41%. The platelets from three patients with Glanzmann thrombasthenia, who lacked GPIIb/IIIa receptors but had normal or increased alpha v beta 3 receptors, supported approximately 21% less thrombin generation than normal platelets. We conclude that thrombin generation initiated by tissue factor in the presence of platelets is significantly inhibited by c7E3 Fab, most likely in part through both GPIIb/IIIa and alpha v beta 3 blockade, and that this effect may contribute to its antithrombotic properties.

Extracellular histones increase plasma thrombin generation by impairing thrombomodulin-dependent protein C activation

BACKGROUND

Histones are basic proteins that contribute to cell injury and tissue damage when released into the extracellular space. They have been attributed a prothrombotic activity, because their injection into mice induces diffuse microvascular thrombosis. The protein C-thrombomodulin (TM) system is a fundamental regulator of coagulation, particularly in the microvasculature, and its activity can be differentially influenced by interaction with several cationic proteins.

OBJECTIVE

To evaluate the effect of histones on the protein C-TM system in a plasma thrombin generation assay and in purified systems.

METHODS

The effect of histones on plasma thrombin generation in the presence or absence of TM was analyzed by calibrated automated thrombinography. Protein C activation in purified systems was evaluated by chromogenic substrate cleavage. The binding of TM and protein C to histones was evaluated by solid-phase binding assay.

RESULTS

Histones dose-dependently increased plasma thrombin generation in the presence of TM, independently of its chondroitin sulfate moiety. This effect was not caused by inhibition of activated protein C activity, but by the impairment of TM-mediated protein C activation. Histones were able to bind to both protein C and TM, but the carboxyglutamic acid domain of protein C was required for their effect. Histones H4 and H3 displayed the highest activity. Importantly, unlike heparin, DNA did not inhibit the potentiating effect of histones on thrombin generation.

CONCLUSIONS

Histones enhance plasma thrombin generation by reducing TM-dependent protein C activation. This mechanism might contribute to microvascular thrombosis induced by histones in vivo at sites of organ failure or severe inflammation.

Identification of patients at low risk for recurrent venous thromboembolism by measuring thrombin generation

CONTEXT

Screening of patients with venous thromboembolism (VTE) for thrombophilic risk factors is common clinical practice. Because of the large number of risk factors, assessing the risk of recurrence in an individual patient is complex. A method covering multicausal thrombophilia is therefore required.

OBJECTIVE

To investigate the relationship between recurrence of VTE and a simple global coagulation assay measuring thrombin generation.

DESIGN, SETTING, AND PARTICIPANTS

Prospective cohort study of 914 patients with first spontaneous VTE who were followed up for an average of 47 months after discontinuation of vitamin K antagonist therapy. The study was conducted at the Department of Internal Medicine I, Medical University of Vienna, Vienna, Austria, between July 1992 and July 2005. Thrombin generation was measured by a commercially available assay system. Patients with a previous or secondary VTE; antithrombin, protein C, or protein S deficiencies; presence of lupus anticoagulant; cancer; or pregnancy were excluded.

MAIN OUTCOME MEASURE

Objectively documented symptomatic recurrent VTE.

RESULTS

Venous thromboembolism recurred in 100 patients (11%). Patients without recurrent VTE had lower thrombin generation than patients with recurrence (mean [SD], 349.2 [108.0] nM vs 419.5 [110.5] nM, respectively; P<.001). Compared with patients who had thrombin generation greater than 400 nM, the relative risk (RR) of recurrence was 0.42 (95% confidence interval [CI], 0.26-0.67; P<.001) in patients with values between 400 nM and 300 nM; for patients with lower values, the RR was 0.37 (95% CI, 0.21-0.66; P = .001). After 4 years, the probability of recurrence was 6.5% (95% CI, 4.0%-8.9%) among patients with thrombin generation less than 400 nM compared with 20.0% (95% CI, 14.9%-25.1%) among patients with higher values (P<.001). Patients with thrombin generation less than 400 nM, representing two thirds of patients, had a 60% lower RR of recurrence than those with greater values (RR, 0.40; 95% CI, 0.27-0.60; P<.001).

CONCLUSION

Measurement of thrombin generation identifies patients at low risk for recurrent VTE.

Elevated numbers of tissue-factor exposing microparticles correlate with components of the metabolic syndrome in uncomplicated type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes is associated with accelerated atherosclerosis. Because cell-derived microparticles support coagulation and inflammation, they may be involved in atherogenesis. We characterized circulating microparticles both in patients with uncomplicated, well-regulated type 2 diabetes and in healthy subjects, as well as their relationship with coagulation and metabolic control.

METHODS AND RESULTS

Microparticles were isolated from plasma, stained with annexin V, cell-specific monoclonal antibodies (MoAbs) and a MoAb directed against tissue factor (TF), and analyzed by flow cytometry. Microparticle numbers and origin were comparable in the two groups, but the median number of TF-positive microparticles was twice as high in patients than in controls (P=0.018). Patients had higher percentages of TF-positive microparticles from T-helper cells (P=0.045), granulocytes (P=0.004), and platelets (P=0.002). Subpopulations of TF-positive microparticles from platelets and T-helper cells exposed granulocytic markers. Correlations were found between the numbers of various TF-positive microparticle subpopulations and body mass index, fasting plasma glucose and insulin, or tumor necrosis factor-alpha and serum HDL cholesterol. Microparticles from patients generated less thrombin in vitro (P=0.007). Microparticle numbers did not correlate with in vivo coagulation markers prothrombin fragment F(1+2) and thrombin-antithrombin complexes.

CONCLUSIONS

TF, possibly of granulocytic origin, is exposed on microparticle subpopulations in asymptomatic patients with well-regulated type 2 diabetes. TF-positive microparticles are associated with components of the metabolic syndrome but not with coagulation. Thus, TF on microparticles may be involved in processes other than coagulation, including transcellular signaling or angiogenesis.

Targeting of Antithrombin in Hemophilia A or B with RNAi Therapy

BACKGROUND

Current hemophilia treatment involves frequent intravenous infusions of clotting factors, which is associated with variable hemostatic protection, a high treatment burden, and a risk of the development of inhibitory alloantibodies. Fitusiran, an investigational RNA interference (RNAi) therapy that targets antithrombin (encoded by SERPINC1), is in development to address these and other limitations.

METHODS

In this phase 1 dose-escalation study, we enrolled 4 healthy volunteers and 25 participants with moderate or severe hemophilia A or B who did not have inhibitory alloantibodies. Healthy volunteers received a single subcutaneous injection of fitusiran (at a dose of 0.03 mg per kilogram of body weight) or placebo. The participants with hemophilia received three injections of fitusiran administered either once weekly (at a dose of 0.015, 0.045, or 0.075 mg per kilogram) or once monthly (at a dose of 0.225, 0.45, 0.9, or 1.8 mg per kilogram or a fixed dose of 80 mg). The study objectives were to assess the pharmacokinetic and pharmacodynamic characteristics and safety of fitusiran.

RESULTS

No thromboembolic events were observed during the study. The most common adverse events were mild injection-site reactions. Plasma levels of fitusiran increased in a dose-dependent manner and showed no accumulation with repeated administration. The monthly regimen induced a dose-dependent mean maximum antithrombin reduction of 70 to 89% from baseline. A reduction in the antithrombin level of more than 75% from baseline resulted in median peak thrombin values at the lower end of the range observed in healthy participants.

CONCLUSIONS

Once-monthly subcutaneous administration of fitusiran resulted in dose-dependent lowering of the antithrombin level and increased thrombin generation in participants with hemophilia A or B who did not have inhibitory alloantibodies. (Funded by Alnylam Pharmaceuticals; ClinicalTrials.gov number, NCT02035605 .).

Inhibition of endotoxin-induced activation of coagulation and fibrinolysis by pentoxifylline or by a monoclonal anti-tissue factor antibody in chimpanzees

Knowledge of the pathogenetic mechanisms responsible for the activation of the coagulation system associated with endotoxemia is important for the development of improved modalities for prevention and treatment. We analyzed the appearance in plasma of TNF, IL-6, and indices of coagulation and fibrinolytic system activation in normal chimpanzees after intravenous infusion of endotoxin. Endotoxin infusion elicited reproducible and dose-dependent elevations in serum TNF and IL-6, as well as marked increases in thrombin generation in vivo as measured by immunoassays for prothrombin activation fragment F1 + 2, thrombin-antithrombin III complexes, and fibrinopeptide A. Activation of the fibrinolytic mechanism was monitored with assays for plasminogen activator activity and plasmin-alpha 2-antiplasmin complexes. To potentially intervene in the molecular pathways elicited by endotoxin, pentoxifylline, an agent that interrupts "immediate early" gene activation by monocytes, or a potent monoclonal antibody that neutralizes tissue factor-mediated initiation of coagulation, were infused shortly before endotoxin. Pentoxifylline markedly inhibited increases in the levels of TNF and IL-6, as well as the effects on coagulation and fibrinolysis. In contrast, the monoclonal antibody to tissue factor completely abrogated the augmentation in thrombin generation, but had no effect on cytokine levels or fibrinolysis. We conclude that the endotoxin-induced activation of coagulation appears to be mediated by the tissue factor-dependent pathway, the fibrinolytic response triggered by endotoxin is not dependent on the generation of thrombin, and that the release of cytokines may be important in mediating the activation of both the coagulation and the fibrinolytic mechanisms in vivo.

Exposure of platelet membrane phosphatidylserine regulates blood coagulation

This article addresses the role of platelet membrane phosphatidylserine (PS) in regulating the production of thrombin, the central regulatory molecule of blood coagulation. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex ("prothrombinase") between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. It is widely accepted that the negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement, although there is not universal agreement on mechanism by which this occurs. Our efforts have led to an alternative view, namely that PS molecules bind to discrete regulatory sites on both factors X(a) and V(a) and allosterically alter their proteolytic and cofactor activities. In this view, exposure of PS on the surface of activated platelet vesicles is a key regulatory event in blood coagulation, and PS serves as a second messenger in this regulatory process. This article reviews our knowledge of the prothrombinase reaction and summarizes recent evidence leading to this alternative viewpoint. This viewpoint suggests a key role for PS both in normal hemostasis and in thrombotic disease.

Cell-derived microparticles generated in patients during cardiopulmonary bypass are highly procoagulant

BACKGROUND

Microparticles from platelets and other cells have been extensively studied and characterized in vitro. Although the level of platelet-derived microparticles is elevated in a variety of diseases, including cardiac surgery, virtually nothing is known about their functions in vivo. The aim of the present study was to investigate the procoagulant properties of microparticles generated in vivo.

METHODS AND RESULTS

In 6 patients at the end of cardiopulmonary bypass, 14.8 x 10(9)/L (median; range, 9.7 to 27.4 x 10(9)/L) platelet-derived microparticles were present in pericardial blood, whereas blood obtained from the systemic circulation contained 1.6 x 10(9)/L (median; range, 0.4 to 8.9 x 10(9)/L) of such microparticles, as determined by flow cytometry. Microparticles stained positively for phosphatidylserine as determined with labeled annexin V. In contrast to systemic blood, pericardial blood contained not only microparticles of platelet origin but also microparticles that originated from erythrocytes, monocytes, or granulocytes, and other hitherto unknown cellular sources. Plasma prepared from pericardial blood and to a lesser extent plasma from systemic blood obtained at the same time, stimulated formation of thrombin in vitro. This activity of pericardial plasma was lost after removal of its microparticles by high-speed centrifugation, whereas the corresponding microparticle pellet was strongly procoagulant. The generation of thrombin in vitro involved a tissue factor/factor VII-dependent and factor XII-independent pathway.

CONCLUSIONS

This study is the first to demonstrate that microparticles generated in vivo can stimulate coagulation.

Evaluation of thrombin generating capacity in plasma from patients with haemophilia A and B

In haemophilia patients, a relationship is usually observed between the clinical expression of the disease and plasmatic factor VIII/factor IX (FVIII/FIX) activity. However, it is known from clinical experience, that some haemophilia patients, despite similar FVIII/FIX plasma levels, could exhibit different bleeding phenotype. After determining preanalytical test conditions, we evaluated the thrombin generation capacity from haemophilia plasma samples in various conditions and the potential usefulness of thrombin generation test (TGT) in haemophilia patients. In a series of 46 haemophilia patients (34 haemophilia A and 12 haemophilia B patients), we found a significant correlation between plasmatic FVIII/FIX levels and endogenous thrombin potential (ETP), peak and time to peak obtained by thrombin generation measurement. In addition, a correlation was found between severe clinical bleeding phenotype and ETP. Our results suggest that TGT could be a promising tool to evaluate haemostasis capacity in patients with haemophilia. Our ex vivo results, obtained 24 hours after FVIII concentrate administration, showed that in patients presenting similar plasmatic FVIIII levels, thrombin generation capacity may be significantly different. These results suggest that in patients with haemophilia, TGT could be useful for individually tailoring prophylactic regimens as well as for adapting clotting factors infusions in surgical situations, in addition to FVIII/FIX plasma clotting activities.

Thrombin generation in patients with cirrhosis: the role of platelets

Coagulation factor defects, thrombocytopenia, and thrombocytopathy are associated with cirrhosis. However, bleeding in patients who have cirrhosis does not entirely correlate with abnormal coagulation tests. Recently, it was shown that because of the concomitant abnormalities of the procoagulant and anticoagulant drives, thrombin generation in plasma patients with cirrhosis is normal when assessed with assays that include thrombomodulin (the main protein C activator). However, thrombin is also generated in vivo as a function of platelets, suggesting that thrombocytopenia and thrombocytopathy might affect thrombin generation in patients with cirrhosis. We addressed this issue using an assay that accounts for the contribution of plasma and platelets. The study showed that platelet-rich plasma with platelets adjusted by dilution of autologous platelet-rich into autologous platelet-poor plasma to a standard count (100 x 10(9)/L) generates as much thrombin in patients with cirrhosis as in controls (1,063 nmol/L vs. 1,167 nmol/L; P value not significant). When platelets were adjusted to correspond to whole-blood counts, patients with cirrhosis generated significantly less thrombin than controls (949 nmol/L vs. 1,239 nmol/L; P < .001). Furthermore, thrombin generation correlated with platelet numbers (rho = 0.50; P < .001). In addition, the amount of thrombin generated as a function of the whole-blood patients' platelet counts increased significantly when the numbers were adjusted to 100 x 10(9)/L (953 nmol/L vs.1,063 nmol/L; P < .001). In conclusion, severe thrombocytopenia may limit thrombin generation in patients with cirrhosis. These findings might justify platelet transfusion in patients with low platelet counts when they bleed spontaneously or before undergoing surgery or liver biopsy. Controlled clinical trials supporting this indication are warranted.

Protein S stimulates inhibition of the tissue factor pathway by tissue factor pathway inhibitor

Tissue factor (TF) plays an important role in hemostasis, inflammation, angiogenesis, and the pathophysiology of atherosclerosis and cancer. In this article we uncover a mechanism in which protein S, which is well known as the cofactor of activated protein C, specifically inhibits TF activity by promoting the interaction between full-length TF pathway inhibitor (TFPI) and factor Xa (FXa). The stimulatory effect of protein S on FXa inhibition by TFPI is caused by a 10-fold reduction of the K(i) of the FXa/TFPI complex, which decreased from 4.4 nM in the absence of protein S to 0.5 nM in the presence of protein S. This decrease in K(i) not only results in an acceleration of the feedback inhibition of the TF-mediated coagulation pathway, but it also brings the TFPI concentration necessary for effective FXa inhibition well within range of the concentration of TFPI in plasma. This mechanism changes the concept of regulation of TF-induced thrombin formation in plasma and demonstrates that protein S and TFPI act in concert in the inhibition of TF activity. Our data suggest that protein S deficiency not only increases the risk of thrombosis by impairing the protein C system but also by reducing the ability of TFPI to down-regulate the extrinsic coagulation pathway.

Thrombin generation, fibrin clot formation and hemostasis

Hemostatic clot formation entails thrombin-mediated cleavage of fibrinogen to fibrin. Previous in vitro studies have shown that the thrombin concentration present during clot formation dictates the ultimate fibrin structure. In most prior studies of fibrin structure, clotting was initiated by adding thrombin to a solution of fibrinogen; however, clot formation in vivo occurs in an environment in which the concentration of free thrombin changes over the reaction course. These changes depend on local cellular properties and available concentrations of pro- and anti-coagulants. Recent studies suggest that abnormal thrombin generation patterns produce abnormally structured clots that are associated with an increased risk of bleeding or thrombosis. Further studies of fibrin formation during in situ thrombin generation are needed to understand fibrin clot formation in vivo.

Platelet- and erythrocyte-derived microparticles trigger thrombin generation via factor XIIa

BACKGROUND

The procoagulant properties of microparticles (MPs) are due to the of the presence of phosphatidylserine (PS) and tissue factor (TF) on their surface. The latter has been demonstrated especially on MPs derived from monocytes.

OBJECTIVES

To investigate the relative contribution of TF and factor (F)XII in initiating coagulation on MPs derived from monocytes, platelets and erythrocytes.

METHODS

Microparticles were isolated from calcium ionophore-stimulated platelets, erythrocytes and monocytic THP-1 cells. MPs were quantified, characterized for cell-specific antigens and analyzed for TF, PS exposure and their thrombin-generating potential.

RESULTS

The MP number was not proportional to PS exposure and the majority of the MPs exposed PS. TF activity was undetectable on platelet- and erythrocyte-derived MPs (< 1 fM nM(-1) PS), whereas monocyte-derived MPs exposed TF (32 fM nM(-1) PS). Platelet-, erythrocyte- and monocyte-derived MPs, but not purified phospholipids, initiated thrombin generation in normal plasma in the absence of an external trigger (lag time < 11 min). Deficiency or inhibition of FVII had no effect on thrombin generation induced by platelet- and erythrocyte-derived MPs, but interfered with monocyte MP-triggered coagulation. Platelet- and erythrocyte-derived MPs completely failed to induce thrombin generation in FXII-deficient plasma. In contrast, monocyte-derived MPs induced similar thrombin generation in normal vs. FXII-deficient plasma.

CONCLUSION

MPs from platelets and erythrocytes not only propagate coagulation by exposing PS but also initiate thrombin generation independently of TF in a FXII-dependent manner. In contrast, monocyte-derived MPs trigger coagulation predominantly via TF.

Thrombin formation

The generation of the enzyme thrombin from its precursor prothrombin is the central event of the blood coagulation process, which is essential to hemostasis and the culprit in thrombosis. Thrombin is produced by a complex series of proteolytic events that are initiated when cryptic tissue factor interacts with plasma factor VIIa to initiate the complex series of events leading to the formation of the blood coagulation enzyme complexes that lead to the efficient generation of the enzyme. During these processes, thrombin contributes to both the generation of the catalysts involved in its ultimate production and to the catalysts that lead to attenuation of its production. Thrombin-catalyzed events both enhance and diminish the process of thrombin generation, which is down-regulated by stoichiometric and dynamic inhibitory processes. The combinations of intensities of activation and inhibition processes provide tight regulation of the hemostatic process, establishing reaction thresholds, essentially leading to an "on/off" switch. This review provides a brief summary of the evolution of knowledge with respect to present-day concepts of thrombin generation via the tissue factor pathway and its regulation.

Venous thromboembolism and hypercoagulability in splenectomized patients with thalassaemia intermedia

Thromboembolic phenomena have been described in patients with thalassaemia intermedia and major, although there are relatively few epidemiological data on the overall frequency of these complications. To obtain more insight into the risk and mechanism of venous thromboembolism in thalassaemia, the aims of this study were: (i) to establish retrospectively the prevalence of thromboembolic events in a large group of adults with thalassaemia intermedia and major during a follow up period of 10 years; (ii) to measure in subgroups of these patients sensitive markers of activation of coagulation and fibrinolysis enzymes; and (iii) to look for possible procoagulant mechanisms. A high prevalence of thromboembolic events was found, particularly in splenectomized patients with thalassaemia intermedia (29%). These patients had high plasma levels of markers of coagulation and fibrinolysis activation. Furthermore, thalassaemic red cells and erythroid precursors from splenectomized patients with thalassaemia intermedia had an enhanced capacity to generate thrombin. To evaluate the role of splenectomy per se on procoagulant activity, we evaluated the capacity to form thrombin in healthy individuals who had been splenectomized for trauma. They produced the same amount of thrombin as non-splenectomized controls. In conclusion, the results of this study show the existence of a hypercoagulable state in splenectomized patients with thalassaemia intermedia and that their red and erythroid cells are capable of acting as activated platelets in thrombin generation.

Statins and Blood Coagulation

The 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase inhibitors (statins) have been shown to exhibit several vascular protective effects, including antithrombotic properties, that are not related to changes in lipid profile. There is growing evidence that treatment with statins can lead to a significant downregulation of the blood coagulation cascade, most probably as a result of decreased tissue factor expression, which leads to reduced thrombin generation. Accordingly, statin use has been associated with impairment of several coagulant reactions catalyzed by this enzyme. Moreover, evidence indicates that statins, via increased thrombomodulin expression on endothelial cells, may enhance the activity of the protein C anticoagulant pathway. Most of the antithrombotic effects of statins are attributed to the inhibition of isoprenylation of signaling proteins. These novel properties of statins, suggesting that these drugs might act as mild anticoagulants, may explain, at least in part, the therapeutic benefits observed in a wide spectrum of patients with varying cholesterol levels, including subjects with acute coronary events. The HMG-CoA reductase inhibitors (statins) have been shown to exhibit several vascular protective effects, including antithrombotic properties, that are not related to changes in lipid profile. Treatment with statins can lead to a significant downregulation of the blood coagulation cascade, most probably as a result of decreased tissue factor expression, which leads to reduced thrombin generation.

An overview of the structure and function of thrombin

The fundamental importance of thrombin in biology and medicine has made it one of the most extensively studied of all proteases. Thrombin performs essential functions in vertebrate biology as the central enzyme involved in blood coagulation and platelet aggregation, and as a mitogen and secretagogue for a variety of cell types. Thrombin is synthesized in the liver and secreted into the general circulation in an inactive zymogen form (prothrombin), a complex multidomain glycoprotein that is activated to yield thrombin at sites of vascular injury by limited proteolysis following upstream activation of the coagulation cascade. Thrombin shares its general architecture and catalytic mechanism with those of pancreatic trypsin, the prototypical digestive serine protease. However, the specificity of thrombin toward substrates and cofactors, as well as its spatiotemporal regulation by effectors and inhibitors, is directed by features of the molecule that distinguish it from relatively nonspecific serine proteases like trypsin. Structural and functional studies have demonstrated the presence of surface loops that partially occlude the active site and make specific contacts with residues adjacent to the scissile bond of substrates. Specificity toward macromolecular substrates and cofactors is additionally enhanced by anion-binding exosites that are spatially distinct from the active site. More than five decades of multidisciplinary research on thrombin have produced an abundance of functional and structural information and provided a robust framework for understanding the role of thrombin in vertebrate biology.