The role of thrombin activatable fibrinolysis inhibitor and factor XI in platelet-mediated fibrinolysis resistance: a thromboelastographic study in whole blood

Potential anticoagulant of traditional chinese medicine and novel targets for anticoagulant drugs

BACKGROUND

Anticoagulants are the main drugs used for the prevention and treatment of thrombosis. Currently, anticoagulant drugs are primarily multitarget heparin drugs, single-target FXa inhibitors and FIIa inhibitors. In addition, some traditional Chinese drugs also have anticoagulant effects, but they are not the main direction of treatment at present. But the anticoagulant drugs mentioned above, all have a common side effect is bleeding. Many other anticoagulation targets are under investigation. With further exploration of coagulation mechanism, how to further determine new anticoagulant targets and how to make traditional Chinese medicine play anticoagulant role have become a new field of exploration.

PURPOSE

The purpose of the study was to summarize the recent research progress on coagulation mechanisms, new anticoagulant targets and traditional Chinese medicine.

METHODS

A comprehensive literature search was conducted using four electronic databases, including PubMed, Embase, CNKI, Wanfang database and ClinicalTrials.gov, from the inception of the study to 28 Feb 2023. Key words used in the literature search were "anticoagulation", "anticoagulant targets", "new targets", "coagulation mechanisms", "potential anticoagulant", "herb medicine", "botanical medicine", "Chinese medicine", "traditional Chinese medicine", "blood coagulation factor", keywords are linked with AND/OR. Recent findings on coagulation mechanisms, potential anticoagulant targets and traditional Chinese medicine were studied.

RESULTS

The active components extracted from the Chinese medicinal herbs, Salvia miltiorrhiza, Chuanxiong rhizoma, safflower and Panax notoginseng have obvious anticoagulant effects and can be used as potential anticoagulant drugs, but the risk of bleeding is unclear. TF/FVIIa, FVIII, FIX, FXI, FXII, and FXIII have all been evaluated as targets in animal studies or clinical trials. FIX and FXI are the most studied anticoagulant targets, but FXI inhibitors have shown stronger advantages.

CONCLUSION

This review of potential anticoagulants provides a comprehensive resource. Literature analysis suggests that FXI inhibitors can be used as potential anticoagulant candidates. In addition, we should not ignore the anticoagulant effect of traditional Chinese medicine, and look forward to more research and the emergence of new drugs.

ProCPU Is Expressed by (Primary) Human Monocytes and Macrophages and Expression Differs between States of Differentiation and Activation

Carboxypeptidase U (CPU, TAFIa, CPB2) is a potent attenuator of fibrinolysis that is mainly synthesized by the liver as its inactive precursor proCPU. Aside from its antifibrinolytic properties, evidence exists that CPU can modulate inflammation, thereby regulating communication between coagulation and inflammation. Monocytes and macrophages play a central role in inflammation and interact with coagulation mechanisms resulting in thrombus formation. The involvement of CPU and monocytes/macrophages in inflammation and thrombus formation, and a recent hypothesis that proCPU is expressed in monocytes/macrophages, prompted us to investigate human monocytes and macrophages as a potential source of proCPU. CPB2 mRNA expression and the presence of proCPU/CPU protein were studied in THP-1, PMA-stimulated THP-1 cells and primary human monocytes, M-CSF-, IFN-γ/LPS-, and IL-4-stimulated-macrophages by RT-qPCR, Western blotting, enzyme activity measurements, and immunocytochemistry. CPB2 mRNA and proCPU protein were detected in THP-1 and PMA-stimulated THP-1 cells as well as in primary monocytes and macrophages. Moreover, CPU was detected in the cell medium of all investigated cell types and it was demonstrated that proCPU can be activated into functionally active CPU in the in vitro cell culture environment. Comparison of CPB2 mRNA expression and proCPU concentrations in the cell medium between the different cell types provided evidence that CPB2 mRNA expression and proCPU secretion in monocytes and macrophages is related to the degree to which these cells are differentiated. Our results indicate that primary monocytes and macrophages express proCPU. This sheds new light on monocytes and macrophages as local proCPU sources.

Activated platelet-based inhibition of fibrinolysis via thrombin-activatable fibrinolysis inhibitor activation system

Our previous real-time imaging studies directly demonstrated the spatiotemporal regulation of clot formation and lysis by activated platelets. In addition to their procoagulant functions, platelets enhanced profibrinolytic potential by augmenting the accumulation of tissue-type plasminogen activator (tPA) and plasminogen, in vivo in a murine microthrombus model, and in vitro in a platelet-containing plasma clot model. To clarify the role of thrombin-activatable fibrinolysis inhibitor (TAFI), which regulates coagulation-dependent anti-fibrinolytic potential, we analyzed tPA-induced clot lysis times in platelet-containing plasma. Platelets prolonged clot lysis times in a concentration-dependent manner, which were successfully abolished by a thrombomodulin-neutralizing antibody or an activated TAFI inhibitor (TAFIaI). The results obtained using TAFI- or factor XIII-deficient plasma suggested that TAFI in plasma, but not in platelets, was essential for this prolongation, though its cross-linkage with fibrin was not necessary. Confocal laser scanning microscopy revealed that fluorescence-labeled plasminogen accumulated on activated platelet surfaces and propagated to the periphery, similar to the propagation of fibrinolysis. Plasminogen accumulation and propagation were both enhanced by TAFIaI, but only accumulation was enhanced by thrombomodulin-neutralizing antibody. Labeled TAFI also accumulated on both fibrin fibers and activated platelet surfaces, which were Lys-binding-site-dependent and Lys-binding-site-independent, respectively. Finally, TAFIaI significantly prolonged the occlusion times of tPA-containing whole blood in a microchip-based flow chamber system, suggesting that TAFI attenuated the tPA-dependent prolongation of clot formation under flow. Thus, activated platelet surfaces are targeted by plasma TAFI, to attenuate plasminogen accumulation and fibrinolysis, which may contribute to thrombogenicity under flow.

Visualizing thrombosis to improve thrombus resolution

The severity, course, and outcomes of thrombosis are determined mainly by the size and location of the thrombus, but studying thrombus structure and composition has been an important but challenging task. The substantial progress in determination of thrombus morphology has become possible due to new intravital imaging methodologies in combination with mechanical thrombectomy, which allows extraction of a fresh thrombus from a patient followed by microscopy. Thrombi have been found to contain various structural forms of fibrin along with platelet aggregates, leukocytes, and red blood cells, many of which acquire a polyhedral shape (polyhedrocytes) as a result of intravital platelet-driven contraction. The relative volume fractions of thrombus components and their structural forms vary substantially, depending on the clinical and pathogenic characteristics. This review summarizes recent research that describes quantitative and qualitative morphologic characteristics of arterial and venous thrombi that are relevant for the pathogenesis, prophylaxis, diagnosis, and treatment of thrombosis.

FVIII/VWF complex displays a greater pro-haemostatic activity than FVIII preparations devoid of VWF: Study in plasma and cell-based models

INTRODUCTION

Plasma-derived FVIII/VWF complex was reported to be less sensitive to inhibitors than FVIII preparations devoid of VWF.

AIM

To compare the efficacy of FVIII/VWF complex (Fanhdi) and five different VWF-free FVIII preparations in restoring thrombin generation and activation of thrombin-activatable fibrinolysis inhibitor (TAFI) in haemophilic plasma, with and without inhibitor, and in cell-based models.

METHODS

Experiments were performed in haemophilic plasma supplemented with inhibitory IgG or in plasma samples obtained from haemophilia A patients without (n = 11) and with inhibitor (n = 12). Thrombin generation was evaluated by calibrated automated thrombography (CAT) under standard conditions, in the presence of activated protein C (APC) or thrombomodulin (TM), and in cell-based models including endothelial cells, either alone or in combination with platelets or tissue factor-expressing blood mononuclear cells. The kinetics of TAFI activation was determined by a two-stage functional assay in the absence and in the presence of APC.

RESULTS

In haemophilic plasma without inhibitor, Fanhdi enhanced thrombin generation and TAFI activation as well as recombinant (2nd-4th generation) and plasma-derived FVIII preparations devoid of VWF. On the contrary, in plasma with inhibitor, Fanhdi displayed a greater ability to restore thrombin generation and TAFI activation under all tested conditions. Notably, in cell-based models including endothelial cells, Fanhdi proved more efficient than all other preparations in improving thrombin generation even in the absence of inhibitor.

CONCLUSION

The greater pro-haemostatic activity of FVIII/VWF complex, either in haemophilic plasma with inhibitor or in the presence of endothelial cells, may offer therapeutic advantages.

Increased thrombin-activatable fibrinolysis inhibitor levels in patients with chronic rhinosinusitis with nasal polyps

BACKGROUND

Chronic rhinosinusitis (CRS) is a heterogeneous chronic inflammatory disease subdivided based on the presence or absence of nasal polyps (NPs). Histologic features of chronic rhinosinusitis with nasal polyps (CRSwNP) include inflammatory cell infiltration and excessive fibrin deposition in NPs. Thrombin-activatable fibrinolysis inhibitor (TAFI) is an enzyme that plays an antifibrinolytic role in the body. The significance of TAFI has been documented in patients with chronic inflammatory diseases, including chronic lung disease; however, it has not been evaluated in the pathogenesis of NPs.

OBJECTIVE

The objective of this study was to evaluate the potential role of TAFI in the pathogenesis of NPs.

METHODS

Nasal lavage fluid was collected from control subjects and patients with CRS. We measured levels of thrombin/anti-thrombin complex (TATc) and TAFI protein using an ELISA.

RESULTS

TATc levels in nasal lavage fluid were significantly increased in patients with CRSwNP and patients with chronic rhinosinusitis without nasal polyps (CRSsNP) compared with control subjects, and TAFI levels in nasal lavage fluid were also significantly increased in patients with CRSwNP compared with those in control subjects and patients with CRSsNP. There was a significant correlation between TATc and TAFI levels in nasal lavage fluid. Interestingly, patients with CRS and asthma showed increased TATc and TAFI levels in nasal lavage fluid compared with those in patients with CRS without asthma, especially patients with CRSwNP.

CONCLUSIONS

Increased TATc and TAFI levels in nasal passages of patients with CRSwNP might participate in fibrin deposition in NPs and might play a role in the pathogenesis of CRSwNP and asthma.

Fibrinolysis in patients with chemotherapy-induced thrombocytopenia and the effect of platelet transfusion

Essentials Bleeding in chemotherapy induced thrombocytopenia (CIT) might be influenced by hyperfibrinolysis. t-PA-thromboelastography is a fast and reliable assay for hyperfibrinolysis in CIT patients. Clots of CIT patients are more susceptible to t-PA induced lysis compared to healthy individuals. Besides platelets, other factors are likely to influence clot lysis in CIT patients.

BACKGROUND

Bleeding events in chemotherapy-induced thrombocytopenic (CIT) patients with similar platelet counts might be influenced by changes in clot lysis potential.

OBJECTIVES

To investigate, in an observational study, thromboelastographic lysis parameters, alterations in clot strength and susceptibility to clot lysis in CIT patients. To identify factors associated with fibrinolytic profiles, and to evaluate the effects of platelet transfusions.

METHODS

Independent determinants of tissue-type plasminogen activator (t-PA)-ROTEM lysis parameters were identified with multivariable linear regression. Clot formation, strength and lysis parameters were compared with the results of healthy individuals. Characteristics of CIT patients with and without hyperfibrinolytic profiles were compared. t-PA-ROTEM results before, 1 hour after and 24 hours after platelet transfusion were compared.

RESULTS

A total of 72 consecutive CIT patients were included. t-PA-ROTEM lysis parameters correlated with changes in fibrinolytic proteins. Clot formation time was longer, maximum clot firmness was weaker and lysis times were shorter than in healthy individuals. CIT patients had low plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor levels, and 40% showed hyperfibrinolytic profiles. Platelet transfusions resulted in less hyperfibrinolytic profiles in many, but not all CIT patients. Patients without hyperfibrinolytic profiles had higher fibrinogen, factor VIII and α2 -antiplasmin levels.

CONCLUSIONS

t-PA-ROTEM can be used as a fast and reliable assay to detect hyperfibrinolytic profiles in CIT patients. CIT patients have weaker clots, which are more susceptible to clot lysis, than healthy individuals. Besides platelets, other factors are likely to influence clot susceptibility to fibrinolysis in CIT patients. The impact of a hyperfibrinolytic t-PA-ROTEM profile on bleeding remains to be investigated.

Blood clot contraction differentially modulates internal and external fibrinolysis

Essentials Clot contraction influences the rate of fibrinolysis in vitro. Internal fibrinolysis is enhanced ∼2-fold in contracted vs. uncontracted blood clots. External fibrinolysis is ∼4-fold slower in contracted vs. uncontracted blood clots. Contraction can modulate lytic resistance and potentially the clinical outcome of thrombosis. SUMMARY: Background Fibrinolysis involves dissolution of polymeric fibrin networks that is required to restore blood flow through vessels obstructed by thrombi. The efficiency of lysis depends in part on the susceptibility of fibrin to enzymatic digestion, which is governed by the structure and spatial organization of fibrin fibers. How platelet-driven clot contraction affects the efficacy of fibrinolysis has received relatively little study. Objective Here, we examined the effects of clot contraction on the rate of internal fibrinolysis emanating from within the clot to simulate (patho)physiological conditions and external fibrinolysis initiated from the clot exterior to simulate therapeutic thrombolysis. Methods Clot contraction was prevented by inhibiting platelet myosin IIa activity, actin polymerization or platelet-fibrin(ogen) binding. Internal fibrinolysis was measured by optical tracking of clot size. External fibrinolysis was determined by the release of radioactive fibrin degradation products. Results and Conclusions Clot contraction enhanced the rate of internal fibrinolysis ∼2-fold. In contrast, external fibrinolysis was ~4-fold slower in contracted clots. This dichotomy in the susceptibility of contracted and uncontracted clots to internal vs. external lysis suggests that the rate of lysis is dependent upon the interplay between accessibility of fibrin fibers to fibrinolytic agents, including clot permeability, and the spatial proximity of the fibrin fibers that modulate the effects of the fibrinolytic enzymes. Understanding how compaction of blood clots influences clot lysis might have important implications for prevention and treatment of thrombotic disorders.

The roles of cellular and molecular components of a hematoma at early stage of bone healing

Bone healing is a complex repair process that commences with the formation of a blood clot at the injured bone, termed hematoma. It has evidenced that a lack of a stable hematoma causes delayed bone healing or non-union. The hematoma at the injured bone constitutes the early healing microenvironment. It appears to dictate healing pathways that ends in a regenerative bone. However, the hematoma is often clinically removed from the damaged site. Conversely, blood-derived products have been used in bone tissue engineering for treating critical sized defects, including fibrin gels and platelet-rich plasma. A second generation of platelet concentrate that is based on leukocyte and fibrin content has also been developed and introduced in market. Conflicting effect of these products in bone repair are reported. We propose that the bone healing response becomes dysregulated if the blood response and subsequent formation and properties of a hematoma are altered. This review focuses on the central structural, cellular, and molecular components of a fracture hematoma, with a major emphasis on their roles in regulating bone healing mechanism, and their interactions with mesenchymal stem cells. New angles towards a better understanding of these factors and relevant mechanisms involved at the beginning of bone healing may help to clarify limited or adverse effects of blood-derived products on bone repair. We emphasize that the recreation of an early hematoma niche with critical compositions might emerge as a viable therapeutic strategy for enhanced skeletal tissue engineering.

Global assays of fibrinolysis

Fibrinolysis is an important and integral part of the hemostatic system. Acting as a balance to blood coagulation, the fibrinolytic system protects the body from unwanted thrombus formation and occlusion of blood vessels. As long as blood coagulation and fibrinolysis remain in equilibrium, response to injury, such as vessel damage, is appropriately regulated. However, alterations in this balance may lead to thrombosis or bleeding. A variety of methods have been proposed to assess fibrinolytic activity in blood or its components, but due to the complexity of the system, the design of a "gold standard" assay that reflects overall fibrinolysis has remained an elusive goal. In this review, we describe the most commonly used methods that have been described, such as thromboelastography (TEG and ROTEM), global fibrinolytic capacity in plasma and whole blood, plasma turbidity methods, simultaneous thrombin and plasmin generation assays, euglobulin clot lysis time and fibrin plate methods. All of these assays have strengths and limitations. We suggest that some methods may be preferable for detecting hypofibrinolytic conditions, whereas others may be better for detecting hyperfibrinolytic states.

Differential Roles for the Coagulation Factors XI and XII in Regulating the Physical Biology of Fibrin

In the contact activation pathway of the coagulation, zymogen factor XII (FXII) is converted to FXIIa, which triggers activation of FXI leading to the activation of FIX and subsequent thrombin generation and fibrin formation. Feedback activation of FXI by thrombin has been shown to promote thrombin generation in a FXII-independent manner and FXIIa can bypass FXI to directly activate FX and prothrombin in the presence of highly negatively charged molecules, such as long-chain polyphosphates (LC polyP). We sought to determine whether activation of FXII or FXI differentially regulate the physical biology of fibrin formation. Fibrin formation was initiated with tissue factor, ellagic acid (EA), or LC polyP in the presence of inhibitors of FXI and FXII. Our data demonstrated that inhibition of FXI decreased the rate of fibrin formation and fiber network density, and increased the fibrin network strength and rate of fibrinolysis when gelation was initiated via the contact activation pathway with EA. FXII inhibition decreased the fibrin formation and fibrin density, and increased the fibrinolysis rate only when fibrin formation was initiated via the contact activation pathway with LC polyP. Overall, we demonstrate that inhibition of FXI and FXII distinctly alter the biophysical properties of fibrin.

Imaging analyses of coagulation-dependent initiation of fibrinolysis on activated platelets and its modification by thrombin-activatable fibrinolysis inhibitor

Using intravital confocal microscopy, we observed previously that the process of platelet phosphatidylserine (PS) exposure, fibrin formation and lysine binding site-dependent plasminogen (plg) accumulation took place only in the centre of thrombi, not at their periphery. These findings prompted us to analyse the spatiotemporal regulatory mechanisms underlying coagulation and fibrinolysis. We analysed the fibrin network formation and the subsequent lysis in an in vitro experiment using diluted platelet-rich plasma supplemented with fluorescently labelled coagulation and fibrinolytic factors, using confocal laser scanning microscopy. The structure of the fibrin network formed by supplemented tissue factor was uneven and denser at the sites of coagulation initiation regions (CIRs) on PS-exposed platelets. When tissue-type plasminogen activator (tPA; 7.5 nM) was supplemented, labelled plg (50 nM) as well as tPA accumulated at CIRs, from where fibrinolysis started and gradually expanded to the peripheries. The lysis time at CIRs and their peripheries (50 µm from the CIR) were 27.9 ± 6.6 and 44.4 ± 9.7 minutes (mean ± SD, n=50 from five independent experiments) after the addition of tissue factor, respectively. Recombinant human soluble thrombomodulin (TMα; 2.0 nM) attenuated the CIR-dependent plg accumulation and strongly delayed fibrinolysis at CIRs. A carboxypeptidase inhibitor dose-dependently enhanced the CIR-dependent fibrinolysis initiation, and at 20 µM it completely abrogated the TMα-induced delay of fibrinolysis. Our findings are the first to directly present crosstalk between coagulation and fibrinolysis, which takes place on activated platelets' surface and is further controlled by thrombin-activatable fibrinolysis inhibitor (TAFI).

Reduced fibrinolytic resistance in patients with factor XI deficiency. Evidence of a thrombin-independent impairment of the thrombin-activatable fibrinolysis inhibitor pathway

Essentials Plasma of factor XI-deficient patients (FXI-dp) displays enhanced fibrinolysis. We investigated the role of thrombin activatable fibrinolysis inhibitor (TAFI) in 18 FXI-dp. FXI-dp generated less activated TAFI (TAFIa) on clotting challenge and were resistant to TAFIa. TAFI activation and TAFIa resistance correlated with bleeding score and bleeding phenotype.

SUMMARY

Background Factor XI (FXI) deficiency, a rare disorder with unpredictable bleeding, has been associated with reduced fibrinolytic resistance as a result of abnormal fibrin density. Objective We investigated the involvement of thrombin-activatable fibrinolysis inhibitor (TAFI) in the increased lysability of FXI-deficient (FXI-def) clots and the role of thrombin. Patients/Methods Eighteen patients with FXI deficiency (1-58%) and 17 matched controls were investigated for fibrinolytic resistance to t-PA, thrombin generation, TAFI activation and response to TAFIa. Results When clotting was induced by 0.5 pm tissue factor (TF), FXI-def plasmas displayed less thrombin and TAFIa generation and shorter lysis time than controls. A 100-fold higher TF concentration (to bypass FXI) abolished the difference in thrombin generation but not in lysis time between patients and controls. Normalization of FXI levels by a FXI concentrate increased thrombin generation but had no effect on the lysis time of FXI-def plasma. Moreover, when clots were induced by purified thrombin and high concentrations of FXa inhibitor, FXI-def plasma still generated less TAFIa and displayed a shorter lysis time than controls. Finally, upon TAFIa addition, the lysis time of FXI-def plasma was prolonged significantly less than that of control plasma, suggesting a TAFIa resistance. TAFIa generation and TAFIa resistance were correlated with the bleeding score, displaying a considerable capacity to discriminate between patients with and without bleeding. Conclusions TAFI pathway impairment, largely caused by a hitherto unknown TAFIa resistance, appears to be one main cause of decreased fibrinolytic resistance in FXI deficiency and might be clinically useful for assessing the bleeding risk of FXI-def patients.

[Basic carboxypeptidases of blood: significance for coagulology]

This review considers the basic metallocarboxypeptidases of human blood and their role in coagulologic disorders. In includes information on the history of the discovery and biological characteristics of potential enzymes-regulators of the fibrinolytic process: carboxypeptidase U and carboxypeptidase N. Certain attention is paid to the biochemical mechanisms and the main modern concepts of the antifibrinolytic effects of these enzymes.

Low Cerebral Oxygenation Levels during Resuscitation in Out-of-hospital Cardiac Arrest Are Associated with Hyperfibrinolysis

BACKGROUND

The authors investigated whether patients with out-of-hospital cardiac arrest with an initial low cerebral oxygen level during cardiopulmonary resuscitation are more prone to develop hyperfibrinolysis than patients with normal cerebral oxygenation levels and which part of the fibrinolytic system is involved in this response.

METHODS

In 46 patients, hyperfibrinolysis was diagnosed immediately upon emergency department admission using rotational thromboelastometry and defined as a lysis more than 15%. Simultaneously, initial cerebral tissue oxygenation was measured using near-infrared spectroscopy, and oxygen desaturation was defined as a tissue oxygenation index (TOI) of 50% or less. Blood sample analysis included markers for hypoperfusion and fibrinolysis.

RESULTS

There was no difference in prehospital cardiopulmonary resuscitation duration between patients with or without hyperfibrinolysis. An initial TOI of 50% or less was associated with more clot lysis (91% [17 to 100%; n = 16]) compared with patients with a normal TOI (6% [4 to 11%]; n = 30; P < 0.001), with lower levels of plasminogen (151.6 ± 61.0 vs. 225.3 ± 47.0 μg/ml; P < 0.001) and higher levels of tissue plasminogen activator (t-PA; 18.3 ± 7.4 vs. 7.9 ± 4.7 ng/ml; P < 0.001) and plasminogen activator inhibitor-1 (19.3 ± 8.9 vs. 12.1 ± 6.1 ng/ml; P = 0.013). There were no differences in (activated) protein C levels among groups. The initial TOI was negatively correlated with t-PA (r = -0.69; P < 0001). Mortality rates were highest in patients with hyperfibrinolysis.

CONCLUSION

Activation of the fibrinolytic system is more common in out-of-hospital cardiac arrest patients with an initial cerebral tissue oxygenation value of 50% or less during resuscitation and is linked to increased levels of t-PA rather than involvement of protein C.

Peroxynitrite may affect fibrinolysis via the reduction of platelet-related fibrinolysis resistance and alteration of clot structure

We tested the hypothesis that in vitro peroxynitrite (ONOO(-), a product of activated inflammatory cells) may affect fibrinolysis in human blood through the reduction of platelet-related fibrinolysis resistance. It was found that ONOO(-) (25-300 µM) accelerated lysis of platelet-fibrin clots (in PRP) dose-dependently, whereas fibrinolysis of platelet-free clots was slightly inhibited by ≥ 1000 µM stressor. Concentrations of ONOO(-) affecting the lysis of platelet-rich clots, inhibited clot retraction (CR) in a dose-dependent manner. Thromboelastometry (ROTEM) measurements performed in PRP showed that treatment with ONOO(-) (threshold conc. 100 µM) prolongs clotting time, and reduces alpha angle, and clot formation velocity parameters indicating for reduced thrombin formation rate. In PRP, ONOO(-) (threshold conc. 100 µM) reduced the collagen-evoked exposure of phosphatidylserine (PS) on platelets' plasma membrane, the shedding of platelet-derived microparticles (PMP), and inhibited platelet-dependent thrombin generation (measured in artificial system), dose-dependently. As judged by confocal microscopy, similar ONOO(-) concentrations altered the architecture of clots formed in collagen-treated PRP. Clots formed in the presence of ONOO(-) were less dense and were composed of thicker fibers, which make them more susceptible to lysis. In platelet-depleted plasma, ONOO(-) (up to milimolar concentration) did not alter clot structure. Blockage of PS exposed on platelets resulted in an alteration of clot architecture toward more prone to lysis. ONOO(-), at lysis-affecting concentrations, inhibited the collagen-evoked secretion of fibrinolytic inhibitors from platelets. We conclude that physiologically relevant ONOO(-) concentrations may accelerate the lysis of platelet-fibrin clots predominantly via downregulation of platelet-related mechanisms including: platelet secretion, clot retraction, platelet procoagulant response, and the alteration in clot architecture associated with it.

Assessment of Fibrinolysis in Sepsis Patients with Urokinase Modified Thromboelastography

INTRODUCTION

Impairment of fibrinolysis during sepsis is associated with worse outcome. Early identification of this condition could be of interest. The aim of this study was to evaluate whether a modified point-of-care viscoelastic hemostatic assay can detect sepsis-induced impairment of fibrinolysis and to correlate impaired fibrinolysis with morbidity and mortality.

METHODS

This single center observational prospective pilot study was performed in an adult Intensive Care Unit (ICU) of a tertiary academic hospital. Forty consecutive patients admitted to the ICU with severe sepsis or septic shock were included. Forty healthy individuals served as controls. We modified conventional kaolin activated thromboelastography (TEG) adding urokinase to improve assessment of fibrinolysis in real time (UK-TEG). TEG, UK-TEG, plasminogen activator inhibitor (PAI)-1, thrombin-activatable fibrinolysis inhibitor (TAFI), d-dimer, DIC scores and morbidity (rated with the SOFA score) were measured upon ICU admission. Logistic regression was used to calculate odds ratios (ORs) and 95% confidence intervals (95% CIs) of mortality at ICU discharge.

RESULTS

UK-TEG revealed a greater impairment of fibrinolysis in sepsis patients compared to healthy individuals confirmed by PAI-1. TAFI was not different between sepsis patients and healthy individuals. 18/40 sepsis patients had fibrinolysis impaired according to UK-TEG and showed higher SOFA score (8 (6-13) vs 5 (4-7), p = 0.03), higher mortality (39% vs 5%, p = 0.01) and greater markers of cellular damage (lactate levels, LDH and bilirubin). Mortality at ICU discharge was predicted by the degree of fibrinolysis impairment measured by UK-TEG Ly30 (%) parameter (OR 0.95, 95% CI 0.93-0.98, p = 0.003).

CONCLUSIONS

Sepsis-induced impairment of fibrinolysis detected at UK-TEG was associated with increased markers of cellular damage, morbidity and mortality.

Formation of blood clot on biomaterial implants influences bone healing

The first step in bone healing is forming a blood clot at injured bones. During bone implantation, biomaterials unavoidably come into direct contact with blood, leading to a blood clot formation on its surface prior to bone regeneration. Despite both situations being similar in forming a blood clot at the defect site, most research in bone tissue engineering virtually ignores the important role of a blood clot in supporting healing. Dental implantology has long demonstrated that the fibrin structure and cellular content of a peri-implant clot can greatly affect osteoconduction and de novo bone formation on implant surfaces. This article reviews the formation of a blood clot during bone healing in relation to the use of platelet-rich plasma (PRP) gels. It is implicated that PRP gels are dramatically altered from a normal clot in healing, resulting in conflicting effect on bone regeneration. These results indicate that the effect of clots on bone regeneration depends on how the clots are formed. Factors that influence blood clot structure and properties in relation to bone healing are also highlighted. Such knowledge is essential for developing strategies to optimally control blood clot formation, which ultimately alter the healing microenvironment of bone. Of particular interest are modification of surface chemistry of biomaterials, which displays functional groups at varied composition for the purpose of tailoring blood coagulation activation, resultant clot fibrin architecture, rigidity, susceptibility to lysis, and growth factor release. This opens new scope of in situ blood clot modification as a promising approach in accelerating and controlling bone regeneration.

Placental prothrombin mRNA levels in APC resistance (APCR) women with increased placental fibrin deposition

We investigated the link between the mRNA of the procoagulant prothrombin in the placental tissue with the increased placental fibrin deposition associated with activated protein C resistance (APCR). Women with APCR were not found to produce higher levels of prothrombin transcript compared to women with a normal APC ratio. This indicates that accumulated fibrin in the placenta is not the consequence of too much production of the procoagulant prothrombin transcript, but may be associated with altered function of other haemostatic factors interacting with APC in the placenta.

In vitro evaluation of clot quality and stability in a model of severe thrombocytopenia: effect of fibrinogen, factor XIII and thrombin-activatable fibrinolysis inhibitor

BACKGROUND

The treatment options in severe thrombocytopenia (platelet count ≤20×10(9)/L) are limited. The aim of this study was to investigate ways of improving blood clotting and stability in reconstituted thrombocytopenia.

MATERIALS AND METHODS

Thrombocytopenia (platelets [16±4]×10(9)/L) was created by differential centrifugation of normal blood followed by reconstitution of whole blood which was subjected to clotting in a rotation thromboelastometer by CaCl2 and tissue factor, and to fibrinolysis by tissue plasminogen activator (tPA). In separate experiments, blood was diluted by 40% with TRIS/saline solution. Blood was treated with fibrinogen (fib), factor XIII (FXIII), and thrombin-activatable fibrinolysis inhibitor (TAFI).

RESULTS

The maximum clot firmness of thrombocytopenic blood was approximately 2-fold less than that of intact blood. Supplementation of blood with fib and FXIII improved clot formation. In the presence of tPA, among fib, FXIII and TAFI, only fib stimulated clot propagation whereas each of these agents increased clot strength. There was a synergistic effect when fib was added together with FXIII or TAFI. Fibrinolysis was inhibited by TAFI and to a greater extent by TAFI + FXIII. Fourty percent dilution of blood reduced clot strength and increased susceptibility to tPA. Clot strength was increased by the treatments in the following order: fib/FXIII/TAFI > fib/TAFI > fib > TAFI > FXIII. In the presence of tPA, TAFI and FXIII lysed the clots significantly more slowly. This effect was stronger when blood was treated with the combination of fib/FXIII/TAFI. Doubling the fib concentration, alone or together with other agents, did not improve clot strength or stability.

DISCUSSION

Augmentation of clot formation and anti-fibrinolysis by combining fib, FXIII and TAFI may be beneficial for the treatment of patients with severe thrombocytopenia especially when complicated by haemodilution following introduction of fluids to compensate for massive blood loss.

Differential profiles of thrombin inhibitors (heparin, hirudin, bivalirudin, and dabigatran) in the thrombin generation assay and thromboelastography in vitro

Thrombin is a central enzyme in hemostasis and thrombosis, and a proven target for anticoagulant therapies. We compared four marketed and representative thrombin inhibitors, heparin, hirudin, bivalirudin, and dabigatran, in in-vitro spike-in assays that covered their therapeutic ranges. The assays employed were low tissue factor (1 pmol/l)-triggered thrombin generation assay (TGA) with plasma and 1:8000 Recombiplastin-triggered thromboelastography (TEG) with whole blood, with or without tissue plasminogen activator (tPA)-induced fibrinolysis. The three direct thrombin inhibitors (DTIs) prolonged TGA lag time and TEG clotting time (R) with a potency stack-ranking of hirudin>dabigatran approximately equal to bivalirudin. Heparin had the most steep concentration-response curve for both parameters. In TGA, 1-2 μmol/l dabigatran or hirudin resulted in complete inhibition on peak, slope, and endogenous thrombin potential, whereas bivalirudin had no effect on these parameters up to 10 μmol/l. All three DTIs, but not heparin, displayed a paradoxical increase in peak and slope in the low concentration range. In TEG, whereas all four agents reduced clot strength (maximal amplitude) in synergy with tPA, hirudin was the only DTI that reduced maximal amplitude appreciably without tPA. Dabigatran had the strongest potentiating effect on tPA-induced fibrinolytic activity (Ly30). With regard to the effects on coagulation and clot strength (lag time, R, and maximal amplitude) in the respective therapeutic range, dabigatran elicited the most modest changes. In summary, our observations highlight the distinct features of each agent in thrombin generation, coagulation, and fibrinolysis. The contrasts between the agents are consistent with their known properties and are informative on efforts to define the optimal profiles of new anticoagulants.

Insights into thrombin activatable fibrinolysis inhibitor function and regulation

Fibrinolysis is initiated when the zymogen plasminogen is converted to plasmin via the action of plasminogen activators. Proteolytic cleavage of fibrin by plasmin generates C-terminal lysine residues capable of binding both plasminogen and the plasminogen activator, thereby stimulating plasminogen activator-mediated plasminogen activation and propagating fibrinolysis. This positive feedback mechanism is regulated by activated thrombin activatable fibrinolysis inhibitor (TAFIa), which cleaves C-terminal lysine residues from the fibrin surface, thereby decreasing its cofactor activity. TAFI can be activated by thrombin alone, but the rate of activation is accelerated when in complex with thrombomodulin. Plasmin is also known to activate TAFI. TAFIa has no known physiologic inhibitors and consequently, its primary regulatory mechanism involves its intrinsic thermal instability. The rate of TAFI activation and stability of the active form, TAFIa, function in maintaining its concentration above the threshold value required to down-regulate fibrinolysis. Although all methods to quantify TAFI or TAFIa have their limitations, epidemiologic studies have indicated that elevated TAFI levels are correlated with an increased risk of venous thrombosis. Major efforts have been made to develop TAFI inhibitors that can either directly interfere with TAFIa activity or impair its activation. However, the anti-inflammatory properties of TAFIa might complicate the development and application of a TAFIa inhibitor that aims to increase the efficiency of thrombolytic therapy.

α(1,3)-Fucosyltransferases FUT4 and FUT7 control murine susceptibility to thrombosis

The α(1,3)-fucosyltransferases, types IV and VII (FUT4 and FUT7, respectively), are required for the synthesis of functional selectin-type leukocyte adhesion molecule ligands. The selectins and their ligands modulate leukocyte trafficking, and P-selectin and its ligand, P-selectin glycoprotein ligand-1, can modulate hemostasis and thrombosis. Regulation of thrombosis by FUT4 and/or FUT7 activity was examined in mouse models of carotid artery thrombosis and collagen/epinephrine-induced thromboembolism. Mice lacking both FUT4 and FUT7 (Fut(-/-) mice) had a shorter time to occlusive thrombus formation in the injured carotid artery and a higher mortality due to collagen/epinephrine-induced pulmonary thromboemboli. Mice lacking P-selectin or P-selectin glycoprotein ligand-1 did not have a prothrombotic phenotype. Whole blood platelet aggregation was enhanced, and plasma fibrinogen content, clot weight, and clot strength were increased in Fut(-/-) mice, and in vitro clot lysis was reduced compared with wild type. Fut4(-/-), but not Fut7(-/-), mice had increased pulmonary thromboembolism-induced mortality and decreased thromboemboli dissolution in vivo. These data show that FUT4 and FUT7 activity regulates thrombosis in a P-selectin- and P-selectin glycoprotein ligand-1-independent manner and suggest that FUT4 activity is important for thrombolysis.

Effect of warfarin treatment on thrombin activatable fibrinolysis inhibitor (TAFI) activation and TAFI-mediated inhibition of fibrinolysis

BACKGROUND

Severe clotting deficiencies are associated with enhanced in vitro fibrinolysis due to insufficient thrombin activatable fibrinolysis inhibitor (TAFI) activation. Because oral anticoagulant therapy (OAT) with warfarin causes a partial deficiency of vitamin K-dependent factors, its effect on clot lysability remains unclear.

OBJECTIVES

To evaluate plasma and blood fibrinolytic capacity in patients under stable OAT (n = 221) as compared with controls (n = 132).

METHODS

Fibrinolysis resistance of plasma (turbidimetry) and blood (thromboelastography) clots was calculated as the lysis time of tissue factor-induced clots exposed to 30 and 100 ng mL(-1) t-PA, respectively.

RESULTS

Plasma PAI-1 was similar in the two groups, whereas TAFI was slightly lower in patients. OAT plasma clots lysed faster than controls (P = 0.001). The addition of the TAFIa inhibitor PTCI reduced lysis time by 14% in OAT and 34% in controls, and the difference between the groups disappeared. Similar data were obtained with blood clots. Thrombin and TAFIa generation in OAT plasma amounted to roughly 50% of controls, supporting a reduced thrombin-dependent TAFI activation. Clot resistance of OAT plasma was normalized by Ba-citrate plasma eluate or prothrombin but not by BaSO(4) serum eluate, rFVIIa or FX. Surprisingly, circulating levels of TAFIa and its inactive derivative TAFIai were higher in warfarin patients (P < 0.0001) and correlated with plasmin-antiplasmin (P = 0.0001) but not with prothrombin F(1) (+) (2) .

CONCLUSIONS

OAT enhances both plasma and blood fibrinolysis by reducing thrombin-dependent TAFI activation, a phenomenon largely determined by low prothrombin levels. At variance with in vitro data, 'basal' in vivo TAFIa/ai levels seem related to plasmin rather than thrombin generation.

Factor XI: hemostasis, thrombosis, and antithrombosis

Coagulation factor FXI (FXI), a plasma serine protease zymogen, has important roles in both intrinsic and extrinsic coagulation pathways and bridges the initiation and amplification phases of plasmatic hemostasis. Recent studies have provided new insight into the molecular structure and functional features of FXI and have demonstrated distinct structural and biological differences between activated factor XII (FXIIa)-mediated FXI activation and tissue factor/thrombin-mediated FXI activation. The former is important in thrombosis; the latter is more essential in hemostasis. Activated partial thromboplastin tine (aPTT) artificially reflects FXIIa-initiated intrinsic coagulation pathway in vitro. Conversely, FXIIa-inhibited diluted thromboplastin time assay may reflect tissue factor/thrombin-mediated FXI activation in vivo. Further explication of the genetic mutations of FXI deficiency has improved the understanding of the structure-function relationship of FXI. Besides its procoagulant activity, the antifibrinolytic activity of FXI was well documented in a wealth of literature. Finally, the new emerging concept of inhibiting FXI as a novel antithrombotic approach with an improved benefit-risk ratio has been supported through observations from human FXI deficiency and various animal models. Large- and small-molecule FXI inhibitors have shown promising antithrombotic effects. The present review summarizes the recent advancements in the molecular physiology of FXI and the molecular pathogenesis of FXI deficiency and discusses the evidence and progress of FXI-targeting antithrombotics development.

Thrombin activatable fibrinolysis inhibitor: at the nexus of fibrinolysis and inflammation

TAFI (thrombin activatable fibrinolysis inhibitor) is the precursor of a basic carboxypeptidase (TAFIa) with strong antifibrinolytic and anti-inflammatory activity. Compelling evidence indicates that thrombin, either alone or in complex with thrombomodulin, is the main physiological activator of TAFI. For this reason derangements of thrombin formation, whatever the cause, may influence the fibrinolytic process too. Experimental models of thrombosis suggest that TAFI may participate in thrombus development and persistence under certain circumstances. In several models of pharmacological thrombolysis, the administration of TAFI inhibitors along with the fibrinolytic agent leads to a marked improvement of thrombus lysis, underscoring the potential of TAFI inhibitors as adjuvants for thrombolytic therapy. The role of TAFI in inflammatory diseases is more complex as it may serve as a defense mechanism, exacerbate the disease, or have no influence, depending on the nature of the model and the role played by the mediators controlled by TAFIa. Finally, the numerous clinical studies in patients with thrombotic disease support the idea that increased levels of TAFI and/or the enhancement of TAFI activation may represent a new risk factor for venous and arterial thrombosis.

Clot lysis time in platelet-rich plasma: method assessment, comparison with assays in platelet-free and platelet-poor plasmas, and response to tranexamic acid

Fibrinolysis dysfunctions cause bleeding or predisposition to thrombosis. Platelets contain several factors of the fibrinolytic system, which could up or down regulate this process. However, the temporal relationship and relative contributions of plasma and platelet components in clot lysis are mostly unknown. We developed a clot lysis time (CLT) assay in platelet-rich plasma (PRP-CLT, with and without stimulation) and compared it to a similar one in platelet-free plasma (PFP) and to another previously reported test in platelet-poor plasma (PPP). We also studied the differential effects of a single dose of tranexamic acid (TXA) on these tests in healthy subjects. PFP- and PPP-CLT were significantly shorter than PRP-CLT, and the three assays were highly correlated (p < 0.0001). PFP- and PPP-, but more significantly PRP-CLT, were positively correlated with age and plasma PAI-1, von Willebrand factor, fibrinogen, LDL-cholesterol, and triglycerides (p < 0.001). All these CLT assays had no significant correlations with platelet aggregation/secretion, platelet counts, and pro-coagulant tests to explore factor X activation by platelets, PRP clotting time, and thrombin generation in PRP. Among all the studied variables, PFP-CLT was independently associated with plasma PAI-1, LDL-cholesterol, and triglycerides and, additionally, stimulated PRP-CLT was also independently associated with plasma fibrinogen. A single 1 g dose of TXA strikingly prolonged all three CLTs, but in contrast to the results without the drug, the lysis times were substantially shorter in non-stimulated or stimulated PRP than in PFP and PPP. This standardized PRP-CLT may become a useful tool to study the role of platelets in clot resistance and lysis. Our results suggest that initially, the platelets enmeshed in the clot slow down the fibrinolysis process. However, the increased clot resistance to lysis induced by TXA is overcome earlier in platelet-rich clots than in PFP or PPP clots. This is likely explained by the display of platelet pro-fibrinolytic effects. Focused research is needed to disclose the mechanisms for the relationship between CLT and plasma cholesterol and its potential pathophysiologic and clinical relevance.

Thrombin activatable fibrinolysis inhibitor

Thrombin activatable fibrinolysis inhibitor (TAFI) was discovered two decades ago as a consequence of the identification of an unstable carboxypeptidase (CPU), which was formed upon thrombin activation of the respective pro-enzyme (proCPU). The antifibrinolytic function of the activated form (TAFIa, CPU) is directly linked to its capacity to remove C-terminal lysines from the surface of the fibrin clot. No endogenous inhibitors have been identified, but TAFIa activity is regulated by its intrinsic temperature-dependent instability with a half-life of 8 to 15 min at 37 °C. A variety of studies have demonstrated a role for TAFI/TAFIa in venous and arterial diseases. In addition, a role in inflammation and cell migration has been shown. Since an elevated level of TAFIa it is a potential risk factor for thrombotic disorders, many inhibitors, both at the level of activation or at the level of activity, have been developed and were proven to exhibit a profibrinolytic effect in animal models. Pharmacologically active inhibitors of the TAFI/TAFIa system may open new ways for the prevention of thrombotic diseases or for the establishment of adjunctive treatments during thrombolytic therapy.