Suppressive Role of Tissue Factor Pathway Inhibitor-α in Platelet-Dependent Fibrin Formation under Flow Is Restricted to Low Procoagulant Strength

Targeting tissue factor pathway inhibitor with concizumab to improve hemostasis in patients with Glanzmann thrombasthenia: an in vitro study

BACKGROUND

Glanzmann thrombasthenia (GT) is caused by an inherited defect of platelet αIIbβ3 integrin. Concizumab, a monoclonal antibody specific for tissue factor pathway inhibitor, abolishes its anticoagulant effect.

OBJECTIVES

To evaluate the in vitro ability of concizumab to improve hemostasis in GT.

METHODS

The effects of concizumab were evaluated in whole blood or platelet-rich plasma from GT patients (n = 5-9) using a thrombin generation assay, rotational thromboelastometry (ROTEM), a global fibrinolytic capacity assay, and a flow chamber assay (Total Thrombus formation Analysis System). Washed platelets (WPs) and 20 nM recombinant activated factor VII (rFVIIa) were included for comparison.

RESULTS

The lag time in the thrombin generation assay was significantly longer (+85%; P < .0001) in GT patients than in controls. WPs, rFVIIa, and concizumab each significantly improved thrombin generation profiles. The ROTEM clotting time (CT) was significantly longer in GT patients than in controls (677 seconds vs 523 seconds; P = .03). However, CT improved after adding WPs, rFVIIa, or concizumab. Under flow, occlusive thrombi were present in all healthy controls after 10 minutes, whereas platelet-fibrin depositions were not seen in GT patients. Subocclusive or occlusive thrombi formed when GT blood was mixed with WPs, rFVIIa, or concizumab. Clots in GT platelet-rich plasma were more susceptible to fibrinolysis and were improved by WPs, rFVIIa, or concizumab.

CONCLUSION

Concizumab enhanced thrombin generation, decreased the ROTEM CT, improved thrombus formation under flow, and reduced clot lysis. Our results demonstrate the potential of concizumab for subcutaneous prophylaxis in GT patients.

Concizumab improves clot formation in hemophilia A under flow

BACKGROUND

Inhibition of tissue factor pathway inhibitor (TFPI) is an emerging therapeutic strategy for treatment of hemophilia. Concizumab is a monoclonal antibody that binds TFPI and blocks its inhibition of factor (F)Xa thereby extending the initiation of coagulation and compensating for lack of FVIII or FIX.

OBJECTIVES

The objective of this in vitro study was to evaluate how concizumab affects clot formation in hemophilia A under flow.

METHODS

Blood was collected from normal controls or people with hemophilia A. An anti-FVIII antibody was added to normal controls to simulate hemophilia A with inhibitory antibodies to FVIII. Whole blood and recombinant activated FVII (rFVIIa, 25 nM) or concizumab (200, 1000, and 4000 ng/mL) were perfused at 100 s-1 over a surface micropatterned with tissue factor (TF) and collagen-related peptide. Platelet and fibrin(ogen) accumulation were measured by confocal microscopy. Static thrombin generation in plasma was measured in response to rFVIIa and concizumab.

RESULTS

Concizumab (1000 and 4000 ng/mL) and rFVIIa both rescued (93%-101%) total platelet accumulation, but only partially rescued (53%-63%) fibrin(ogen) incorporation to normal control levels in simulated hemophilia A. Results using congenital hemophilia A blood confirmed effects of rFVIIa and concizumab. While these 2 agents had similar effect on clot formation under flow, concizumab enhanced thrombin generation in plasma under static conditions to a greater extent than rFVIIa.

CONCLUSION

TFPI inhibition by concizumab enhanced activation and aggregation of platelets and fibrin clot formation in hemophilia A to levels comparable with that of rFVIIa.

Inhibition of platelet-surface-bound proteins during coagulation under flow I: TFPI

Blood coagulation is a self-repair process regulated by activated platelet surfaces, clotting factors, and inhibitors. Tissue factor pathway inhibitor (TFPI) is one such inhibitor, well known for its inhibitory action on the active enzyme complex comprising tissue factor (TF) and activated clotting factor VII. This complex forms when TF embedded in the blood vessel wall is exposed by injury and initiates coagulation. A different role for TFPI, independent of TF:VIIa, has recently been discovered whereby TFPI binds a partially cleaved form of clotting factor V (FV-h) and impedes thrombin generation on activated platelet surfaces. We hypothesized that this TF-independent inhibitory mechanism on platelet surfaces would be a more effective platform for TFPI than the TF-dependent one. We examined the effects of this mechanism on thrombin generation by including the relevant biochemical reactions into our previously validated mathematical model. Additionally, we included the ability of TFPI to bind directly to and inhibit platelet-bound FXa. The new model was sensitive to TFPI levels and, under some conditions, TFPI could completely shut down thrombin generation. This sensitivity was due entirely to the surface-mediated inhibitory reactions. The addition of the new TFPI reactions increased the threshold level of TF needed to elicit a strong thrombin response under flow, but the concentration of thrombin achieved, if there was a response, was unchanged. Interestingly, we found that direct binding of TFPI to platelet-bound FXa had a greater anticoagulant effect than did TFPI binding to FV-h alone, but that the greatest effects occurred if both reactions were at play. The model includes activated platelets' release of FV species, and we explored the impact of varying the FV/FV-h composition of the releasate. We found that reducing the zymogen FV fraction of this pool, and thus increasing the fraction that is FV-h, led to acceleration of thrombin generation.

Inhibition of platelet adhesion, thrombus formation, and fibrin formation by a potent αIIbβ3 integrin inhibitor from ticks

BACKGROUND

Ticks puncture the skin of their hosts and secrete saliva, containing antiplatelet proteins, into the blood. Here, we studied disagregin, a potent platelet-inhibiting protein derived from the salivary glands of Ornithodoros moubata, an African soft tick. Whereas conventional αIIbβ3 antagonists contain an Arg-Gly-Asp (RGD) sequence for platelet integrin binding, disagregin contains an Arg-Glu-Asp (RED) sequence, hypothesizing a different mode of inhibitory action.

OBJECTIVES

We aimed to compare the inhibitory effects of disagregin and its RGD variant (RGD-disagregin) on platelet activation and to unravel the molecular basis of disagregin-αIIbβ3 integrin interactions.

METHODS

Disagregin and RGD-disagregin were synthesized by tert-butyloxycarbonyl -based solid-phase peptide synthesis. Effects of both disagregins on platelet aggregation were assessed by light transmission aggregometry in human platelet-rich plasma. Whole-blood thrombus formation was investigated by perfusing blood over collagen I with and without tissue factor at a high wall-shear rate (1000 s-1) in the presence of disagregin, RGD-disagregin, or eptifibatide.

RESULTS

Disagregin showed inhibition of collagen- and ADP-induced platelet aggregation with half maximal inhibitory concentration values of 64 and 99 nM, respectively. This resembled the complete antiaggregatory effect of eptifibatide. Multiparameter assessment of thrombus formation showed highly suppressed platelet adhesion and aggregate formation with both disagregins, in contrast to eptifibatide treatment, which incompletely blocked aggregation under flow. Fibrin formation under flow was delayed by both disagregin and RGD-disagregin (P < .01) and eptifibatide (P < .05).

CONCLUSIONS

Both αIIbβ3-blocking disagregins have a strong potential to suppress collagen-tissue factor-mediated platelet adhesion, thrombus formation, and fibrin formation. Both disagregins can be seen as potential new αIIbβ3 inhibitors.

Platelet-primed interactions of coagulation and anticoagulation pathways in flow-dependent thrombus formation

In haemostasis and thrombosis, platelet, coagulation and anticoagulation pathways act together to produce fibrin-containing thrombi. We developed a microspot-based technique, in which we assessed platelet adhesion, platelet activation, thrombus structure and fibrin clot formation in real time using flowing whole blood. Microspots were made from distinct platelet-adhesive surfaces in the absence or presence of tissue factor, thrombomodulin or activated protein C. Kinetics of platelet activation, thrombus structure and fibrin formation were assessed by fluorescence microscopy. This work revealed: (1) a priming role of platelet adhesion in thrombus contraction and subsequent fibrin formation; (2) a surface-independent role of tissue factor, independent of the shear rate; (3) a mechanism of tissue factor-enhanced activation of the intrinsic coagulation pathway; (4) a local, suppressive role of the anticoagulant thrombomodulin/protein C pathway under flow. Multiparameter analysis using blood samples from patients with (anti)coagulation disorders indicated characteristic defects in thrombus formation, in cases of factor V, XI or XII deficiency; and in contrast, thrombogenic effects in patients with factor V-Leiden. Taken together, this integrative phenotyping approach of platelet–fibrin thrombus formation has revealed interaction mechanisms of platelet-primed key haemostatic pathways with alterations in patients with (anti)coagulation defects. It can help as an important functional add-on whole-blood phenotyping.

Localized endothelial-based control of platelet aggregation and coagulation under flow: A proof-of-principle vessel-on-a-chip study

BACKGROUND

In the intact vessel wall, endothelial cells form a barrier between the blood and the remaining vascular structures, serving to maintain blood fluidity and preventing platelet activation and fibrin clot formation. The spatiotemporal space of this inhibition is largely unknown.

OBJECTIVE

To assess the local inhibitory roles of a discontinuous endothelium, we developed a vessel-on-a-chip model, consisting of a microfluidic chamber coated with the thrombogenic collagen and tissue factor (TF), and covered with patches of human endothelial cells. By flow perfusion of human blood and plasma, the heterogeneous formation of platelet aggregates and fibrin clots was monitored by multicolor fluorescence microscopy.

RESULTS

On collagen/TF coatings, a coverage of 40% to 60% of human umbilical vein endothelial cells resulted in a strong overall delay in platelet deposition and fibrin fiber formation under flow. Fibrin formation colocalized with the deposited platelets, and was restricted to regions in between endothelial cells, thus pointing to immediate local suppression of the clotting process. Fibrin kinetics were enhanced by treatment of the cells with heparinase III, partially disrupting the glycocalyx, and to a lesser degree by antagonism of the endothelial thrombomodulin. Co-coating of purified thrombomodulin and collagen had a similar coagulation-suppressing effect as endothelial thrombomodulin.

CONCLUSIONS

In this vessel-on-a-chip system with patches of endothelial cells on thrombogenic surfaces, the coagulant activity under flow is regulated by: (a) the residual exposure of trigger (collagen/TF), (b) the endothelial glycocalyx, and (c) to a lesser degree the endothelial thrombomodulin.

A mathematical model of coagulation under flow identifies factor V as a modifier of thrombin generation in hemophilia A

BACKGROUND

The variability in bleeding patterns among individuals with hemophilia A, who have similar factor VIII (FVIII) levels, is significant and the origins are unknown.

OBJECTIVE

To use a previously validated mathematical model of flow-mediated coagulation as a screening tool to identify parameters that are most likely to enhance thrombin generation in the context of FVIII deficiency.

METHODS

We performed a global sensitivity analysis (GSA) on our mathematical model to identify potential modifiers of thrombin generation. Candidates from the GSA were confirmed by calibrated automated thrombography (CAT) and flow assays on collagen-tissue factor (TF) surfaces at a shear rate of 100 per second.

RESULTS

Simulations identified low-normal factor V (FV) (50%) as the strongest modifier, with additional thrombin enhancement when combined with high-normal prothrombin (150%). Low-normal FV levels or partial FV inhibition (60% activity) augmented thrombin generation in FVIII-inhibited or FVIII-deficient plasma in CAT. Partial FV inhibition (60%) boosted fibrin deposition in flow assays performed with whole blood from individuals with mild and moderate FVIII deficiencies. These effects were amplified by high-normal prothrombin levels in both experimental models.

CONCLUSIONS

These results show that low-normal FV levels can enhance thrombin generation in hemophilia A. Further explorations with the mathematical model suggest a potential mechanism: lowering FV reduces competition between FV and FVIII for factor Xa (FXa) on activated platelet surfaces (APS), which enhances FVIII activation and rescues thrombin generation in FVIII-deficient blood.

Microfluidic hemophilia models using blood from healthy donors

BACKGROUND

Microfluidic clotting assays permit drug action studies for hemophilia therapeutics under flow. However, limited availability of patient samples and Inter-donor variability limit the application of such assays, especially with many patients on prophylaxis.

OBJECTIVE

To develop approaches to phenocopy hemophilia using modified healthy blood in microfluidic assays.

METHODS

Corn trypsin inhibitor (4 µg/mL)-treated healthy blood was dosed with either anti-factor VIII (FVIII; hemophilia A model) or a recombinant factor IX (FIX) missense variant (FIX-V181T; hemophilia B model). Treated blood was perfused at 100 s-1 wall shear rate over collagen/tissue factor (TF) or collagen/factor XIa (FXIa).

RESULTS

Anti-FVIII partially blocked fibrin production on collagen/TF, but completely blocked fibrin production on collagen/FXIa, a phenotype reversed with 1 µmol/L bispecific antibody (emicizumab), which binds FIXa and factor X. As expected, emicizumab had no significant effect on healthy blood (no anti-FVIII present) perfused over collagen/FXIa. The efficacy of emicizumab in anti-FVIII-treated healthy blood phenocopied the action of emicizumab in the blood of a patient with hemophilia A perfused over collagen/FXIa. Interestingly, a patient-derived FVIII-neutralizing antibody reduced fibrin production when added to healthy blood perfused over collagen/FXIa. For low TF surfaces, reFIX-V181T (50 µg/mL) fully blocked platelet and fibrin deposition, a phenotype fully reversed with anti-TFPI.

CONCLUSION

Two new microfluidic hemophilia A and B models demonstrate the potency of anti-TF pathway inhibitor, emicizumab, and a patient-derived inhibitory antibody. Using collagen/FXIa-coated surfaces resulted in reliable and highly sensitive hemophilia models.

Clinical Protocol to Prevent Thrombogenic Effect of Liver-Derived Mesenchymal Cells for Cell-Based Therapies

The efficacy of mesenchymal stem cell infusion is currently tested in numerous clinical trials. However, therapy-induced thrombotic consequences have been reported in several patients. The aim of this study was to optimize protocols for heterologous human adult liver-derived progenitor cell (HHALPC) infusion, in order to eliminate acute thrombogenesis in liver-based metabolic or acute decompensated cirrhotic (ADC) patients. In rats, thrombotic effects were absent when HHALPCs were infused at low cell dose (5 × 10⁶ cells/kg), or at high cell dose (5 × 10⁷ cells/kg) when combined with anticoagulants. When HHALPCs were exposed to human blood in a whole blood perfusion assay, blocking of the tissue factor (TF) coagulation pathway suppressed fibrin generation and platelet activation. In a Chandler tubing loop model, HHALPCs induced less explosive activation of coagulation with blood from ADC patients, when compared to blood from healthy controls, without alterations in coagulation factor levels other than fibrinogen. These studies confirm a link between TF and thrombogenesis, when TF-expressing cells are exposed to human blood. This phenomenon however, could be controlled using either a low, or a high cell dose combined with anticoagulants. In clinical practice, this points to the suitability of a low HHALPC dose infusion to cirrhotic patients, provided that platelet and fibrinogen levels are monitored.

Association of TFPI polymorphisms rs8176592, rs10931292, and rs10153820 with venous thrombosis: A meta-analysis

BACKGROUND

Tissue factor pathway inhibitor (TFPI) polymorphisms are known to be involved in venous thrombosis; however, any correlation between the TFPI polymorphisms rs8176592, rs10931292, and rs10153820 and venous thrombosis remains controversial. This meta-analysis aimed to elucidate the relationship between these TFPI polymorphisms and the susceptibility to venous thrombosis.

METHODS

A literature search for relevant studies was conducted in PubMed, Embase, Cochrane Library, China National Knowledge Infrastructure (CNKI), and Wanfang Med Online databases. Odds ratios (ORs) and their corresponding 95% confidence intervals (95% CIs) were calculated using fixed-effect/random-effect models by the STATA 12.0 software. Sources of heterogeneity were analyzed by subgroup analysis.

RESULTS

Eleven case-control studies involving 3740 subjects (1362 venous thrombosis patients and 2378 healthy controls) were included. The TFPI rs8176592 polymorphism was associated with increased risk of venous thrombosis in the whole population, while no significant association was found between rs10931292/rs10153820 and venous thrombosis. In subgroup analysis based on ethnicity, an increased risk was observed with rs8176592 polymorphism in Asians (Recessive model, OR = 1.48, 95% CI = 1.06-2.07, P = .023). An increased risk associated with rs10931292 was identified in non-Asians (Recessive model, OR = 1.42, 95% CI = 1.03-1.97, P = .033). No significant association was found in either Asians or non-Asians with the rs10153820 polymorphism. In subgroup analysis based on source of controls, increased risks were identified in the hospital-based group with rs8176592 polymorphism and in the population-based group with rs10931292 polymorphism, whereas decreased risk was identified in the hospital-based group with the rs10931292 and rs10153820 polymorphisms.

CONCLUSION

Meta-analysis suggested that different TFPI polymorphisms may have different associations with venous thrombosis. TFPI rs8176592 polymorphism may increase the risk of venous thrombosis, especially in Asians and hospital-based patients. The TFPI rs10931292 polymorphism may increase the venous thrombosis risk for both non-Asians and population-based patients. Moreover, rs10931292 and rs10153820 polymorphisms of TFPI may decrease the risk of venous thrombosis for hospital-based patients.

Integrating platelet and coagulation activation in fibrin clot formation

Platelets interact with the coagulation system in a multitude of ways, not only during the phases of thrombus formation, but also in specific areas within a formed thrombus. This review discusses current concepts of platelet control of thrombin generation, fibrin formation and structure, and anticoagulation. Indicated are how combined signalling via the platelet receptors for collagen (glycoprotein VI) and thrombin induces the secretion of (anti)coagulation factors, as well as surface exposure of phosphatidylserine, thereby catalysing thrombin generation. This procoagulant platelet response is also facilitated by the adhesive complexes glycoprotein Ib-V-IX and integrin αIIbβ3. In the buildup of a platelet-fibrin thrombus, the extrinsic, tissue factor-driven coagulation pathway is predominant in early stages, while the intrinsic, factor XII pathway seems to promote at later time points. Already early generation of thrombin enforces platelet responses and stimulates intra-thrombus heterogeneity with patches of loosely aggregated, contracted, and phosphatidylserine-exposing platelets. Fibrin actively formed on the surface of activated platelets supports thrombus growth, but also captures thrombin. The fibrin distribution in a thrombus appears to rely on the local procoagulant trigger and the blood flow rate. Clinical studies support the importance of the platelet-coagulation interplay, by showing beneficial effects of combination therapy in the secondary prevention of cardiovascular disease.