Absence of hematopoietic tissue factor pathway inhibitor mitigates bleeding in mice with hemophilia. Proc Natl Acad Sci U S A. 2012;109:3927-3931. [PMID: 22355108 DOI: 10.1073/pnas.1119858109]

Deletion of tissue factor pathway inhibitor isoform beta or gamma, but not alpha, improves clotting in hemophilic mice

BACKGROUND

Tissue factor pathway inhibitor (TFPI) regulates tissue factor-triggered coagulation. Humans and mice express transcripts encoding for multidistributed (endothelial, platelet, and plasma) 3-Kunitz domain TFPIα and endothelial membrane-anchored 2-Kunitz TFPIβ. Mice express a third transcript, γ, that encodes plasma lipoprotein-associated 2-Kunitz TFPI. In humans, proteolysis of α and/or β produces plasma lipoprotein-associated 2-Kunitz TFPI at lower levels. In clinical trials, monoclonal antibodies that target all TFPI isoforms extend coagulation and correct bleeding in hemophilic patients but with some thrombosis risks.

OBJECTIVES

To determine the impact of TFPI isoform-specific deletions on promoting clotting in hemophilic mice.

METHODS

Engineered TFPI isoform-specific, hemophilic (factor VIII-null) mice were evaluated for clotting.

RESULTS

Mice expressing any single TFPI isoform were healthy. Thrombin generation assays identified TFPIγ as the dominant anticoagulation isoform in mouse plasma. Hemostasis was assessed by serial bleeding times from a tail vein laceration. Repeatedly, after a clot forms, it was manually disrupted; the number of clots/disruptions occurring over a 15-minute period were reported. C57BL/6 and hemophilic mice clot on average 25.6 vs 5.4 times, respectively. On a hemophilia background, TFPIβ or TFPIγ-specific deletion improved clotting to 14.6 and 15.2 times, respectively (P < .0001). TFPIα-specific deletion was without impact, clotting 5.1 times. Heterozygous deletion of TFPIβ was effective, clotting 11.8 times (P < .0001). Heterozygous deletion of TFPIα or TFPIγ alone was ineffective, clotting 3.0 and 6.1 times, respectively, but heterozygous TFPIαγ deletion improved clotting to 11.2 times (P < .001).

CONCLUSION

In hemophilic mice, endothelial TFPIβ and plasma γ-derived 2-Kunitz TFPI individually contribute more to bleeding than total TFPIα.

Relationship between plasma tissue Factor Pathway Inhibitor (TFPI) levels, thrombin generation and clinical risk of bleeding in patients with severe haemophilia A or B

INTRODUCTION

Bleeding severity in severe haemophilic patients, with low thrombin generation (TG) capacity, can vary widely between patients, possibly reflecting differences in tissue factor pathway inhibitor (TFPI) level.

AIM

To compare free TFPI (fTFPI) levels in patients with severe haemophilia A (sHA) and severe haemophilia B (sHB) and to investigate in these patients as a whole the relationships between bleeding and TG potential, between TG potential and fTFPI level and between fTFPI level and bleeding tendency.

METHODS

Data on bleeding episodes retrospectively recorded during follow-up visits over 5-10 years were collected and used to calculate the annualised joint bleeding rate (AJBR). fTFPI levels and basal TG parameters were determined in platelet-poor plasma (PPP) and platelet-rich plasma (PRP) using calibrated automated tomography (CAT).

RESULTS

Mean fTFPI levels did not differ significantly between sHA (n = 34) and sHB (n = 19) patients. Mean values of endogenous thrombin potential (ETP) and thrombin peak (peak) in PPP and PRP were two-fold higher when fTFPI levels < 9.4 versus > 14.3 ng/mL. In patients treated on demand, ETP and peak in PRP were doubled when AJBR was≤ 4.9 $ \le 4.9$ , AJBR being halved in patients with a low fTFPI level (9.4 ng/mL). In patients on factor prophylaxis, no association was found between TG parameters and either fTFPI level or AJBR.

CONCLUSION

In patients treated on demand, bleeding tendency was influenced by fTFPI levels, which in turn affected basal TG potential. In patients on prophylaxis, bleeding tendency is probably determined primarily by the intensity of this treatment.

Polyphosphate as an antithrombotic target and hemostatic agent

Polyphosphate (PolyP) is a polymer comprised of linear phosphate units connected by phosphate anhydride bonds. PolyP exists in a diverse range of eukaryotes and prokaryotes with varied chain lengths ranging from six to thousands of phosphate units. Upon activation, human platelets and neutrophils release short-chain PolyP, along with other components, to initiate the coagulation pathway. Long-chain PolyP derived from cellular or bacterial organelles exhibits higher proinflammatory and procoagulant effects compared to short-chain PolyP. Notably, PolyP has been identified as a low-hemorrhagic antithrombotic target since neutralizing plasma PolyP suppresses the thrombotic process without impairing the hemostatic functions. As an inorganic polymer without uniform steric configuration, PolyP is typically targeted by cationic polymers or recombinant polyphosphatases rather than conventional antibodies, small-molecule compounds, or peptides. Additionally, because of its procoagulant property, PolyP has been incorporated in wound-dressing materials to facilitate blood hemostasis. This review summarizes current studies on PolyP as a low-hemorrhagic antithrombotic target and the development of hemostatic materials based on PolyP.

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.

Phenotypic and genetic analyses of four cases of coagulation factor XII deficiency

OBJECTIVES

To identify the clinical phenotypic and molecular pathogeneses of four cases of coagulation factor XII deficiency and to deepen the cognition of this disease.

METHODS

Coagulation tests were performed through one stage of coagulation on a STAGO coagulation analyser. Coagulation factor XII antigen was detected using enzyme-linked immunosorbent assay. The species conservatism and structural change of mutant proteins were analysed using MegAlign and PYMOL. Meanwhile, missense variants and a novel splice site variant were identified using PolyPhen2 and NetGene2.

RESULTS

The four cases had an observably prolonged activated partial thromboplastin time but without obvious bleeding tendency. Their coagulation factor XII activity (FⅫ:C) and antigen (FXII:Ag) were greatly reduced. Six mutations were detected: NM_000505.4:c.398-1G>A, NP_000496.2:p.(Pro182Leu), NP_000496.2:p.(Ser479Ter), NP_000496.2:p.(Cys559Arg), NC_000005.10:g.7217_7221delinsGTCTA and NM_000505.4:c.1681-1G>A. The first five are newly discovered mutations. The two missense mutation sites were highly conservative, and their protein secondary structure changes may occur not only on the mutation sites but also on other domains. In silico analysis revealed that NP_000496.2:p.(Pro182Leu) may be BENIGN, NP_000496.2:p.(Cys559Arg) may be damaging, and that NM_000505.4:c.398-1G>A and NM_000505.4:c.1681-1G>A are crucial for splicing.

CONCLUSION

We found six types of mutations, of which five were novel. The two missense mutation sites might be closely related to the function of coagulation factor XII. The mutations were the primary culprits of factor XII deficiency.

Regulation of coagulation by tissue factor pathway inhibitor: Implications for hemophilia therapy

Tissue factor pathway inhibitor (TFPI) is an alternatively spliced anticoagulant protein that primarily dampens the initiation phase of coagulation before thrombin is generated. As such, TFPI's actions are localized to cells expressing TF and to sites of injury, where it is an important regulator of bleeding in hemophilia. The major splice isoforms TFPIα and TFPIβ localize to different sites within and surrounding the vasculature. Both forms directly inhibit factor Xa (FXa) via their Kunitz 2 domain and inhibit TF-FVIIa via their Kunitz 1 domain in a tight complex primarily localized to cells. By forming complexes localized to distinct cellular microenvironments and engaging additional cell surface receptors, TFPI alters cellular trafficking and signaling pathways driven by coagulation proteases of the TF pathway. TFPIα, which circulates in complex with FV and protein S, also serves an inhibitor of FXa independent of the TF initiation complex and prevents the formation of an active prothrombinase. This regulation of thrombin generation in the context of vessel injury is effectively blocked by antibodies to Kunitz 2 domain of TFPI and exploited as a therapy to restore efficient hemostasis in hemophilia.

Knockdown and Knockout of Tissue Factor Pathway Inhibitor in Zebrafish

Tissue Factor Pathway Inhibitor (TFPI) is an anticoagulant that inhibits factor VIIa and Xa in the blood coagulation pathways. TFPI contains three Kunitz domains, K1, K2, and K3. K1 and K2 inhibit factor VIIa and Xa, respectively. However, the regulation of TFPI is poorly studied. Since zebrafish has become an alternate model to discover novel actors in hemostasis, we hypothesized that TFPI regulation could be studied using this model. As a first step, we confirmed the presence of tfpia in zebrafish using RT-PCR. We then performed piggyback knockdowns of tfpia and found increased coagulation activity in tfpia knockdown. We then created a deletion mutation in tfpia locus using CRISPR/Cas9 method. The tfpia homozygous deletion mutants showed increased coagulation activities similar to that found in tfpia knockdown. Taken together, our data suggest that tfpia is a negative regulator for zebrafish coagulation, and silencing it leads to thrombotic phenotype. Also, the zebrafish tfpia knockout model could be used for reversing this thrombotic phenotype to identify antithrombotic novel factors by the genome-wide piggyback knockdown method.

The contribution of TFPIα to the hemostatic response to injury in mice

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an essential regulator of coagulation, limiting thrombin generation and preventing thrombosis. In humans and mice, TFPIα is the sole isoform present in platelets.

OBJECTIVE

Here, we asked whether TFPIα, because of its release from platelets at sites of injury, has a unique role in limiting the hemostatic response.

METHODS

TFPIα-mutant (TfpiΔα/Δα ) mice were generated by introducing a stop codon in the C-terminus. Platelet accumulation, platelet activation, and fibrin accumulation were measured following penetrating injuries in the jugular vein and cremaster muscle arterioles, and imaged by fluorescence and scanning electron microscopy. Time to bleeding cessation was recorded in the jugular vein studies.

RESULTS

TfpiΔα/Δα mice were viable and fertile. Plasma TFPI levels were normal in the TfpiΔα/Δα mice, no TFPI protein or activity was present in their platelets and thrombin-antithrombin complex levels were indistinguishable from Tfpi+/+ littermates. There was a small, but statistically significant reduction in the time to bleeding cessation following jugular vein puncture injury in the TfpiΔα/Δα mice, but no measurable changes in platelet or fibrin accumulation or in hemostatic plug architecture following injury of the micro- or macrovasculature.

CONCLUSION

Loss of TFPIα expression does not produce a global prothrombotic state in mice. Platelet TFPIα is expected to be released or displayed in a focal manner at the site of injury, potentially accumulating to high concentrations in the narrow gaps between platelets. If so, the data from the vascular injury models studied here indicate this is not essential for a normal hemostatic response in mice.

Thrombin generation abnormalities in commonly encountered platelet function disorders

INTRODUCTION

Studies of thrombin generation (TG) with platelet-rich plasma (PRP) and platelet-poor plasma (PPP) have provided insights on bleeding disorders. We studied TG for a cohort with commonly encountered platelet function disorders (PFD).

METHODS

Participants included 40 controls and 31 with PFD due to: nonsyndromic dense granule (DG) deficiency (PFD-DGD, n = 9), RUNX1 haploinsufficiency (n = 6) and aggregation defects from other, uncharacterized causes (n = 16). TG was tested with PRP and PPP samples. As DG store ADP and polyphosphate that enhance platelet-dependent TG, PFD-DGD PRP TG was tested for correction with ADP, polyphosphate and combined additives. Tissue factor pathway inhibitor (TFPI), platelet factor V (FV), and platelet TFPI and ANO6 transcript levels were also evaluated. Findings were tested for associations with TG endpoints and bleeding.

RESULTS

PFD samples had impaired PRP TG, but also impaired PPP TG, with strong associations between their PRP and PPP TG endpoints (P ≤ .005). PFD-DGD PRP TG endpoints showed associations to PPP TG endpoints but not to DG counts, and were improved, but not fully corrected, by adding polyphosphate and agonists. PFD participants had increased plasma TFPI and reduced platelet TFPI (P ≤ .02) but normal levels of platelet FV, and platelet TFPI and ANO6 transcripts levels. PFD plasma TFPI levels showed significant association to several PPP TG endpoints (P ≤ .04). Several PFD PRP TG endpoints showed significant associations to bleeding symptoms, including wound healing problems and prolonged bleeding from minor cuts (P ≤ .04).

CONCLUSION

TG is impaired in commonly encountered PFD, with their PRP TG findings showing interesting associations to symptoms.

Intrauterine lethality in Tfpi gene disrupted mice is differentially suppressed during mid- and late-gestation by platelet TFPIα overexpression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein required for murine embryonic development. Intrauterine lethality of Tfpi-/- mice occurs at mid- and late gestation, the latter of which is associated with severe cerebrovascular defects. Megakaryocytes produce only the TFPIα isoform, which is stored within platelets and released upon activation.

OBJECTIVES

To examine biological activities of platelet TFPIα (pTFPIα) by characterizing effects of pTFPIα overexpression in Tfpi-/- mice.

METHODS

Transgenic mice overexpressing pTFPIα were generated and crossed onto the Tfpi-/- background. Genetic and histological analyses of embryos were performed to investigate the function of pTFPIα during embryogenesis.

RESULTS

The transgene (Tg) increased pTFPIα 4- to 5-fold without altering plasma TFPI in adult Tfpi+/+ and Tfpi+/- mice but did not rescue Tfpi-/- mice to wean. Analyses of the impact of pTFPIα overexpression on Tfpi-/- survival, however, were complicated by linkage between the Tg integration site and the endogenous Tfpi locus on chromosome 2. Strain-specific genetic interactions also modulated Tfpi-/- embryonic survival. After accounting for these underlying genetic factors, pTFPIα overexpression completely suppressed mid-gestational lethality of Tfpi-/- embryos but had no effect on development of cerebrovascular defects during late gestation resulting in their lack of survival to wean.

CONCLUSIONS

pTFPIα overexpression rescued Tfpi-/- embryos from mid-gestational but not late gestational lethality. The prevalence of underlying genetic factors complicating analyses within our study illustrates the importance of meticulously characterizing transgenic mouse models to avoid spurious interpretation of results.

Heterogeneity in Bleeding Tendency and Arthropathy Development in Individuals with Hemophilia

People with hemophilia (PWH) have an increased tendency to bleed, often into their joints, causing debilitating joint disease if left untreated. To reduce the incidence of bleeding events, PWH receive prophylactic replacement therapy with recombinant factor VIII (FVIII) or FIX. Bleeding events in PWH are typically proportional to their plasma FVIII or IX levels; however, in many PWH, bleeding tendency and the likelihood of developing arthropathy often varies independently of endogenous factor levels. Consequently, many PWH suffer repeated bleeding events before correct dosing of replacement factor can be established. Diagnostic approaches to define an individual's bleeding tendency remain limited. Multiple modulators of bleeding phenotype in PWH have been proposed, including the type of disease-causing variant, age of onset of bleeding episodes, plasma modifiers of blood coagulation or clot fibrinolysis pathway activity, interindividual differences in platelet reactivity, and endothelial anticoagulant activity. In this review, we summarize current knowledge of established factors modulating bleeding tendency and discuss emerging concepts of additional biological elements that may contribute to variable bleeding tendency in PWH. Finally, we consider how variance in responses to new gene therapies may also necessitate consideration of patient-specific tailoring of treatment. Cumulatively, these studies highlight the need to reconsider the current "one size fits all" approach to treatment regimens for PWH and consider therapies guided by the bleeding phenotype of each individual PWH at the onset of therapy. Further characterization of the biological bases of bleeding heterogeneity in PWH, combined with the development of novel diagnostic assays to identify those factors that modulate bleeding risk in PWH, will be required to meet these aspirations.

A Molecular Revolution in the Treatment of Hemophilia

For decades, the monogenetic bleeding disorders hemophilia A and B (coagulation factor VIII and IX deficiency) have been treated with systemic protein replacement therapy. Now, diverse molecular medicines, ranging from antibody to gene to RNA therapy, are transforming treatment. Traditional replacement therapy requires twice to thrice weekly intravenous infusions of factor. While extended half-life products may reduce the frequency of injections, patients continue to face a lifelong burden of the therapy, suboptimal protection from bleeding and joint damage, and potential development of neutralizing anti-drug antibodies (inhibitors) that require less efficacious bypassing agents and further reduce quality of life. Novel non-replacement and gene therapies aim to address these remaining issues. A recently approved factor VIII-mimetic antibody accomplishes hemostatic correction in patients both with and without inhibitors. Antibodies against tissue factor pathway inhibitor (TFPI) and antithrombin-specific small interfering RNA (siRNA) target natural anticoagulant pathways to rebalance hemostasis. Adeno-associated virus (AAV) gene therapy provides lasting clotting factor replacement and can also be used to induce immune tolerance. Multiple gene-editing techniques are under clinical or preclinical investigation. Here, we provide a comprehensive overview of these approaches, explain how they differ from standard therapies, and predict how the hemophilia treatment landscape will be reshaped.

Maintaining extraembryonic expression allows generation of mice with severe tissue factor pathway inhibitor deficiency

Tissue factor pathway inhibitor (TFPI) is a serine protease with multiple anticoagulant activities. The Kunitz1 (K1) domain of TFPI binds the active site of factor VIIa and is required for inhibition of tissue factor (TF)/factor VIIa catalytic activity. Mice lacking TFPI K1 domain die in utero. TFPI is highly expressed on trophoblast cells of the placenta. We used genetic strategies to selectively ablate exon 4 encoding TFPI K1 domain in the embryo, while maintaining expression in trophoblast cells. This approach resulted in expected Mendelian frequency of TFPI K1 domain-deficient mice. Real-time polymerase chain reaction confirmed 95% to 99% genetic deletion and a similar reduction in transcript expression. Western blotting confirmed the presence of a truncated protein instead of full-length TFPI. Mice with severe TFPI K1 deficiency exhibited elevated thrombin-antithrombin (TAT) levels, frequent fibrin deposition in renal medulla, and increased susceptibility to TF-induced pulmonary embolism. They were fertile, and most lived normal life spans without any overt thrombotic events. Of 43 mice observed, 2 displayed extensive brain ischemia and infarction. We conclude that in contrast to complete absence of TFPI K1 domain, severe deficiency is compatible with in utero development, adult survival, and reproductive functions in mice. Inhibition of TFPI activity is being evaluated as a means of boosting thrombin generation in hemophilia patients. Our results show that in mice severe reduction of TFPI K1 activity is associated with a prothrombotic state without overt developmental outcomes. We note fibrin deposits in the kidney and rare cases of brain ischemia.

Choosing a Mouse Model of Venous Thrombosis

Murine models are widely used valuable tools to study deep vein thrombosis. Leading experts in venous thrombosis research came together through the American Venous Forum to develop a consensus on maximizing the utility and application of available mouse models of venous thrombosis. In this work, we provide an algorithm for model selection, with discussion of the advantages, disadvantages, and applications of the main mouse models of venous thrombosis. Additionally, we provide a detailed surgical description of the models with guidelines to validate surgical technique.

Serum lipoprotein(a) and risk of hemorrhagic stroke among incident peritoneal dialysis patients: a large study from a single center in China

Background: This retrospective study investigated whether baseline serum lipoprotein(a) (Lp(a)) may predict subsequent stroke in patients under chronic peritoneal dialysis (PD). Methods: Eight hundred and sixty incident PD patients, treated from 1 November 2005 to 28 February 2017, were enrolled, and followed until discontinuation of PD, death, or 31 May 2017. Hemorrhagic or ischemic stroke was the primary outcome. The population was stratified by baseline serum Lp(a) tertile. The risk of each stroke subtype was analyzed using the Cox proportional hazard models. Adjustments were made for: age; gender; history of stroke and hypertension; systolic blood pressure; lipid-lowering, antiplatelet and antihypertensive medications; laboratory profiles including hemoglobin, serum albumin, calcium, triglyceride, total and low-density lipoprotein cholesterol; and apolipoprotein A1. Results: Among the 860 participants, 19.3% and 4.1% had diabetes mellitus and a history of stroke, respectively. The median baseline serum Lp(a) was 328 (172-585) mg/L. After 28 (14-41) months of follow-up, 33 (3.84%) and 12 (1.40%) patients developed hemorrhagic and ischemic stroke, respectively. Participants in the highest Lp(a) tertile had a significantly lower risk of hemorrhagic stroke compared with those in the lowest tertile (hazard ratio (HR) 0.3, 95% confidence interval (CI) 0.1-0.86; p = .026); the rates of ischemic stroke were comparable among the tertiles. Each 10 mg/L rise in serum Lp(a) was associated with a 2% (95% CI 0.96-1; p = .033) lower risk of hemorrhagic stroke. Conclusions: Among patients with incident PD, a higher serum Lp(a) level may predict a lower risk of hemorrhagic stroke.

Defective fibrin deposition and thrombus stability in Bambi-/- mice are mediated by elevated anticoagulant function

BACKGROUND

Bone morphogenetic and activin membrane-bound inhibitor (BAMBI) is a transmembrane protein related to the type I transforming growth factor- β (TGF-β) receptor family that is present on both platelets and endothelial cells (ECs). Bambi-deficient mice exhibit reduced hemostatic function and thrombus stability characterized by an increased embolization.

OBJECTIVE

We aimed to delineate how BAMBI influences endothelial function and thrombus stability.

METHODS

Bambi-deficient mice were subjected to the laser-induced thrombosis model where platelet and fibrin accumulation was evaluated. Expression of thrombomodulin and tissue factor pathway inhibitor (TFPI) was also assessed in these mice.

RESULTS

Thrombus instability in Bambi-/- mice was associated with a profound defect in fibrin deposition. Injection of hirudin into Bambi+/+ mice prior to thrombus formation recapitulated the Bambi-/- thrombus instability phenotype. In contrast, hirudin had no additional effect upon thrombus formation in Bambi-/- mice. Deletion of Bambi in ECs resulted in mice with defective thrombus stability caused by decreased fibrin accumulation. Increased levels of the anticoagulant proteins TFPI and thrombomodulin were detected in Bambi-/- mouse lung homogenates. Endothelial cells isolated from Bambi-/- mouse lungs exhibited enhanced ability to activate protein C due to elevated thrombomodulin levels. Blocking thrombomodulin and TFPI in vivo fully restored fibrin accumulation and thrombus stability in Bambi-/- mice.

CONCLUSIONS

We demonstrate that endothelial BAMBI influences fibrin generation and thrombus stability by modulating thrombomodulin and TFPI anticoagulant function of the endothelium; we also highlight the importance of these anticoagulant proteins in the laser-induced thrombosis model.

Measurement of plasma and platelet tissue factor pathway inhibitor, factor V and Protein S in people with haemophilia

INTRODUCTION

Tissue factor pathway inhibitor (TFPI) is a naturally occurring anticoagulant found in plasma, where it circulates bound to lipoproteins, factor V (FV) or Protein S (PS), and in platelets. Therapeutic agents targeting TFPI are under development for the treatment of haemophilia A and haemophilia B.

AIM

To begin to understand how TFPI, FV and PS interact to modulate haemophilia bleeding.

METHODS

Plasma and platelet antigen concentrations of these factors were determined in 73 people with haemophilia A and 18 with haemophilia B. Using multiple regression models, these were compared to the same analytes measured in 224 male blood donors.

RESULTS

There were no differences in plasma or platelet TFPI, FV or PS concentrations between haemophilia types or severities. However, compared to blood donors, people with haemophilia had approximately one-third lower plasma PS, 9% lower plasma TFPIα, 50% higher platelet FV and 26% lower platelet Protein S.

CONCLUSION

Together, the presented data suggest that individuals with haemophilia may have a compensatory procoagulant response of both plasma and platelet proteins to the decreased concentrations of FVIII or FIX.

Paradigm shift for the treatment of hereditary haemophilia: Towards precision medicine

Patients with haemophilia A (HA) or B (HB) experience spontaneous limb- or life-threatening bleedings which are prevented by regular prophylactic intravenous infusions of the deficient coagulation factor (FVIII or FIX). Prophylaxis with subcutaneous long-acting non-factor products that improve in vivo thrombin generation is now under intensive investigation (concizumab, fitusiran) or successfully employed (emicizumab) in haemophilia patients. Both haemophilia patients with/without inhibitors take advantage of non-factor products employed alone. In those who also need bypassing agents (or FVIII concentrates) for breakthrough bleeds, thromboembolic events and/or thrombotic microangiopathy may occur. By enhancing thrombin generation, prothrombotic mutations co-segregating with FVIII/FIX gene mutations may trigger thrombotic episodes in HA patients carrying acquired thrombogenic factors (e.g. venous catheters). A thorough knowledge of individual needs increasingly contributed to improve comprehensive care and personalize treatments in haemophilia. Integrating genomics, lifestyle and environmental data is expected to be key to: 1) identify which haemophilia patients are less likely to benefit from a given intervention; 2) define optimal dosing and scheduling of bypassing agents (or FVIII) to employ in combination with non-factor products; 3) establish tests to monitor in vivo thrombin generation; 4) improve communication and deliver results to individuals. As individual outcomes will be improved and the risk of adverse events minimized, non-factor products will come into wider use within the haemophilia community, and patients will hopefully have no more risks of breakthrough bleeds. The risks of a normal life for a "former haemophilia patient" is likely to change the treatment landscape and the structure of haemophilia Centers.

A systems pharmacokinetic/pharmacodynamic model for concizumab to explore the potential of anti-TFPI recycling antibodies

Concizumab is a humanized monoclonal antibody in clinical investigation directed against membrane-bound and soluble tissue factor pathway inhibitor (mTFPI and sTFPI) for treatment of hemophilia. Concizumab displays a non-linear pharmacokinetic (PK) profile due to mTFPI-mediated endocytosis and necessitates a high dose and frequent dosing to suppress the abundant sTFPI, a negative regulator of coagulation. Recycling antibodies that can dissociate bound mTFPI/sTFPI in endosomes for degradation and rescue antibody from degradation have a potential in reducing the dose by extending antibody systemic persistence and sTFPI suppression. We developed a systems PK/pharmacodynamics (PD) model with nested endosome compartments to simulate the effect of decreased antibody binding to mTFPI/sTFPI in endosomes on antibody clearance and sTFPI suppression for exploring the potential of anti-TFPI recycling antibodies in reducing the dose. A dynamic model-building strategy was taken. A reduced PK/PD model without the endosome compartments was developed to optimize unknown target turnover parameters using concizumab PK data. The optimized parameters were then employed in the systems PK/PD model for simulations. The obtained systems PK/PD model adequately described the PK of concizumab in rabbits, monkeys, and humans and the PD in humans. The systems PK/PD model predicted that an anti-TFPI recycling antibody with a 100-fold higher mTFPI/sTFPI dissociation constant in endosomes than concizumab can extend sTFPI suppression from 12 days to 1 month. Thus, the systems PK/PD model provides a quantitative platform for guiding the engineering and translational development of anti-TFPI recycling antibodies.

Tissue factor pathway inhibitor primes monocytes for antiphospholipid antibody-induced thrombosis

Antiphospholipid antibodies (aPLs) with complex lipid and/or protein reactivities cause complement-dependent thrombosis and pregnancy complications. Although cross-reactivities with coagulation regulatory proteins contribute to the risk for developing thrombosis in patients with antiphospholipid syndrome, the majority of pathogenic aPLs retain reactivity with membrane lipid components and rapidly induce reactive oxygen species-dependent proinflammatory signaling and tissue factor (TF) procoagulant activation. Here, we show that lipid-reactive aPLs activate a common species-conserved TF signaling pathway. aPLs dissociate an inhibited TF coagulation initiation complex on the cell surface of monocytes, thereby liberating factor Xa for thrombin generation and protease activated receptor 1/2 heterodimer signaling. In addition to proteolytic signaling, aPLs promote complement- and protein disulfide isomerase-dependent TF-integrin β1 trafficking that translocates aPLs and NADPH oxidase to the endosome. Cell surface TF pathway inhibitor (TFPI) synthesized by monocytes is required for TF inhibition, and disabling TFPI prevents aPL signaling, indicating a paradoxical prothrombotic role for TFPI. Myeloid cell-specific TFPI inactivation has no effect on models of arterial or venous thrombus development, but remarkably prevents experimental aPL-induced thrombosis in mice. Thus, the physiological control of TF primes monocytes for rapid aPL pathogenic signaling and thrombosis amplification in an unexpected crosstalk between complement activation and coagulation signaling.

Choosing a mouse model of venous thrombosis: a consensus assessment of utility and application

Murine models are widely used valuable tools to study deep vein thrombosis (VT). Leading experts in VT research came together through the American Venous Forum to develop a consensus on maximizing the utility and application of available mouse models of VT. In this work, we provide an algorithm for model selection, with discussion of the advantages, disadvantages, and applications of the main mouse models of VT. Additionally, we provide a detailed surgical description of the models with guidelines to validate surgical technique.

Assessing Blood Clotting and Coagulation Factors in Mice

The mammalian blood coagulation system was designed to restrict blood loss due to injury as well as keep the blood fluid within the blood vessels of the organism. Blood coagulation activity in inbred mouse strains varies widely among strains, suggesting that many genomic variants affect hemostasis. Some of these molecules have been discovered and characterized; however, many are still unknown. Genetically modified mouse technologies are providing a plethora of new mouse models for investigating the regulation of blood coagulation. Here we provide a protocol for the tail bleeding time as a primary assessment of in vivo blood coagulation, as well as in vitro methods such as the prothrombin time, activated partial thromboplastin time, and thrombin generation assay. We also provide protocols for the assessment of the activities of specific known factors involved in blood coagulation. © 2019 by John Wiley & Sons, Inc.

New therapies using nonfactor products for patients with hemophilia and inhibitors

Regular prophylaxis with factor VIII (FVIII) or FIX products to prevent bleeding in patients with severe hemophilia A (HA) and HB, respectively, results in marked suppression of the onset of arthropathy and contributes greatly to improvements in quality of life. Some issues remain with the use of clotting factor replacement therapy, however. The need for multiple IV infusions is associated with a substantial mental and physical burden, and the hemostatic effect of bypassing agents (BPAs) in patients with inhibitor is inconsistent. The development of subcutaneous products with prolonged hemostatic efficiency, irrespective of the presence of inhibitors, has been a longtime wish for patients. A new class of therapeutic agents that act by enhancing coagulation (emicizumab) and inhibiting anticoagulant pathways (fitusiran and concizumab) have been established, and clinical trials using these nonfactor products are ongoing. The current findings have demonstrated that prophylaxis by nonfactor products supports marked reductions of bleeding episodes in hemophilia patients with or without inhibitor. Emicizumab has already been approved for use internationally. Some concerns are evident, however. Thrombotic microangiopathy and thromboembolism have occurred in 5 emicizumab-treated patients receiving repeated infusions of activated prothrombin complex concentrates, and a sinus vein thrombosis has occurred in a fitusiran-treated patient receiving repeated infusions of FVIII product. Moreover, reliable techniques to monitor hemostatic function in patients receiving nonfactor products with concomitant BPA or FVIII/FIX therapies require further assessment. These novel therapeutic agents have promising hemostatic properties, although wider experience in hemophilia centers is warranted to establish appropriate therapeutic strategies.

Suppressing protein Z-dependent inhibition of factor Xa improves coagulation in hemophilia A

Essentials Protein Z (PZ) catalyzes PZ-dependent proteinase inhibitor (ZPI) inactivation of factor (F)Xa. Gene-deletion of PZ or ZPI improves coagulation in hemophilia (FVIII knockout) mice. A PZ blocking antibody enhances thrombin generation in human hemophilia plasma. Suppression of the PZ/ZPI pathway may ameliorate the phenotype of severe hemophilia. SUMMARY: Background Hemostasis requires a balance between procoagulant and anticoagulant factors. Hemophiliacs bleed because of a procoagulant deficiency. Targeted reduction in the activity of endogenous anticoagulant pathways is currently being investigated as a means of improving hemostasis in hemophilia. Protein Z (PZ) is a cofactor that serves as a catalyst for PZ-dependent protease inhibitor (ZPI) inactivation of activated factor X at phospholipid surfaces. Objectives To evaluate the effects of PZ or ZPI gene deletion in hemophilic mice, and of blocking PZ in human hemophilic plasma. Methods A tail vein rebleeding assay (TVRB) was developed on the basis of the serial disruption of clots forming over a period of 15 min following tail vein laceration in an anesthetized mouse. Wild-type (WT)/FVIII knockout FVIIIKO, PZ knockout PZKO/FVIIIKO and ZPI knockout ZPIKO/FVIIIKO mice were evaluated in this model, and their plasmas were tested in thrombin generation assays. A mAb against PZ was evaluated in human hemophilic plasma thrombin generation assays. Results The numbers of clot disruptions (mean ± standard error of the mean) in the TVRB were: 4.0 ± 0.9 for WT/FVIIIKO mice; 23.8 ± 1.1 for WT/FVIIIKO mice supplemented with 100% FVIII; 15.2 ± 1.1 for PZKO/FVIIIKO mice; and 14.7 ± 1.2 for ZPIKO/FVIIIKO mice. Thrombin generation in PZKO/FVIIIKO and ZPIKO/FVIIIKO mouse plasmas was similar to that in FVIIIKO plasma supplemented with ~ 15% recombinant FVIII. A mAb against PZ added to human hemophilic plasma enhanced thrombin generation to an extent similar to the addition of ~ 15% FVIII. Conclusions Blockade of the PZ/ZPI system may be sufficient to ameliorate the phenotype of severe hemophilia.

New therapies for hemophilia

Hemophilia A (HA) and hemophilia B (HB) are the most common severe bleeding disorders. Replacement therapy, providing the missing coagulation factor, has been the mainstay of treatment both prophylactically and to treat bleeding. Despite widespread availability of safe and effective replacement therapy, patients with HA and HB continue to experience a tremendous burden of treatment, breakthrough bleeding, and progressive joint disease, as well as high rates of inhibitor development. These remaining challenges are now being addressed by incredible advances in bioengineering. Recombinant bioengineering has led to replacement therapies with easier modes of administration, decreased immunogenicity, increased efficacy, and extended half-lives. Emicizumab, a bispecific antibody that acts as a substitutive therapy for HA, has been approved for patients with and without inhibitors. Novel compounds are in development to exploit the natural balance of hemostasis by targeting the natural anticoagulants protein C, protein S, tissue factor pathway inhibitor, and antithrombin. The substitution and rebalancing therapies provide an opportunity for steady-state hemostatic control without exposure to immunogenic clotting factor proteins. As such, they may have broader applications outside those being investigated in the clinical trial programs.

Re-evaluation of mouse tissue factor pathway inhibitor and comparison of mouse and human tissue factor pathway inhibitor physiology

Essentials Mouse models are often used to define roles of tissue factor pathway inhibitor (TFPI) in man. TFPI isoform-specific KOs reveal unexpected differences between mouse and human TFPI physiology. Mouse plasma contains 20 times more TFPI than man, derived from TFPIγ, a form not found in man. TFPIγ null mice, expressing only TFPI isoforms α and β, may better reflect the human situation. SUMMARY: Background Mouse models can provide insight into the pathophysiology of human thrombosis and hemostasis. Tissue factor pathway inhibitor (TFPI) regulates coagulation through protein S (PS)-enhanced factor (F) Xa inhibition and FXa-dependent inhibition of FVIIa/tissue factor (TF) activity. TFPI is expressed as isoforms α and β in man, and α, β and γ in the mouse. Objective Assess the reliability of extending TFPI-related studies in mice to humans. Method Compare mouse and human TFPI physiology using a variety of methods. Results Mouse TFPI and human TFPI are similar in regard to: (i) the mechanisms for FVIIa/TF and FXa inhibition; (ii) TFPIα is a soluble form and TFPIβ is glycosyl phosphatidyl inositol (GPI) membrane anchored; (iii) the predominant circulating form of TFPI in plasma is lipoprotein-associated; (iv) low levels of TFPIα circulate in plasma and increase following heparin treatment; and (v) TFPIα is the isoform in platelets. They differ in that: (i) mouse TFPI circulates at a ~20-fold higher concentration; (ii) mouse lines with isolated isoform deletions show this circulating mouse TFPI is derived from TFPIγ; (iii) sequences homologous to the mouse TFPIγ exon are present in many species, including man, but in primates are unfavorable for splicing; and (iv) tandem mass spectrometry (MS/MS) detects sequences for TFPI isoforms α and β in human plasma and α and γ in mouse plasma. Conclusion To dissect the pathophysiological roles of human TFPIα and TFPIβ, studies in TFPIγ null mice, expressing only α and β, only α or only β should better reflect the human situation.

Anti-tissue factor pathway inhibitor (TFPI) therapy: a novel approach to the treatment of haemophilia

Novel approaches to the treatment of haemophilia are needed due to the limitations of the current standard of care, factor replacement therapy. Aspirations include lessening the treatment burden and effectively preventing joint damage. Treating haemophilia by restoring thrombin generation may be an effective approach. A promising target for restoring thrombin generation is tissue factor pathway inhibitor (TFPI), a multivalent Kunitz-type serine protease inhibitor that regulates tissue factor-induced coagulation via factor Xa-dependent feedback inhibition of the tissue factor-factor VIIa complex. Inhibition of TFPI reverts the coagulation process to a more primitive state evolutionarily, whilst regulation by other natural inhibitors is preserved. An aptamer and three monoclonal antibodies directed against TFPI have been investigated in clinical trials. As well as improving thrombin generation in the range associated with mild haemophilia, anti-TFPI therapies have the advantage of subcutaneous administration. However, the therapeutic window needs to be defined along with the potential for complications due to the novel mechanism of action. This review provides an overview of TFPI, its role in normal coagulation, the rationale for TFPI inhibition, and a summary of anti-TFPI therapies, previously or currently in development.

Attempting to remedy sub-optimal medication adherence in haemophilia: The rationale for repeated ultrasound visualisations of the patient's joint status

Haemophilia is marked by joint bleeding (haemarthrosis) leading to cartilage damage (arthropathy). Lifelong prophylaxis-initiated after the first bleeding episode-leads to a dramatic decrease in arthropathy in haemophilia patients. However, adherence to continuous intravenous administrations of factor VIII (FVIII) or FIX products is challenging, and patients potentially suffer from breakthrough bleedings while on prophylaxis. Newer FVIII/FIX products with enhanced convenience attributes and/or easier infusion procedures are intended to improve adherence. However, pharmacokinetic data should be harmonised with information from individual attitudes and treatment needs, to tailor intravenous dosing and scheduling in patients who receive extended half-life products. Nor is there sound evidence as to how subcutaneous non-FVIII/FIX replacement approaches (concizumab; emicizumab; fitusiran) or single intravenous injections of adeno-associated viral vectors (when employing gene therapy) will revolutionize adherence in haemophilia. In rheumatoid arthritis, repeated ultrasound examination of a patient's major joints is a valuable tool to educate patients and parents to understand the disease and provide an objective framework for clinicians to acknowledge patient's adherence. Joint ultrasound examination in haemophilia significantly correlates with cartilage damage, effusion, and synovial hypertrophy evaluated by magnetic resonance imaging. Furthermore, in patients with haemophilia undergoing prophylaxis with an extended half-life product for a ≈ 2.8 year period, a significant continued improvement in joint health is detected at the physical examination. This provides the rationale for studies on repeated ultrasound examinations of joint status to attempt to remedy sub-optimal medication adherence and help identify which approach is most suited on which occasion and for which patient.

Novel therapeutics for hemophilia and other bleeding disorders

Hemophilia and von Willebrand disease are the most common congenital bleeding disorders. Treatment of these disorders has focused on replacement of the missing coagulation factor to prevent or treat bleeding. New technologies and insights into hemostasis have driven the development of many promising new therapies for hemophilia and von Willebrand disease. Emerging bypass agents including zymogen-like factor IXa and Xa molecules are in development and a bispecific antibody, emicizumab, demonstrated efficacy in a phase 3 trial in people with hemophilia A and inhibitors. Tissue factor pathway inhibitor, the protein C/S system, and antithrombin are targets of novel compounds in development to alter the hemostatic balance and new approaches using modified factor VIII molecules are being tested for prevention and eradication of inhibitor antibodies in hemophilia A. The first recombinant von Willebrand factor (VWF) product has been approved and has unique VWF multimer content and does not contain factor VIII. These new approaches may offer better routes of administration, improved dosing regimens, and better efficacy for prevention and treatment of bleeding in congenital bleeding disorders.

Inhibition of Tissue Factor Pathway Inhibitor (TFPI) as a Treatment for Haemophilia: Rationale with Focus on Concizumab

Replacement therapy with missing factor (F) VIII or IX in haemophilia patients for bleed management and preventative treatment or prophylaxis is standard of care. Restoration of thrombin generation through novel mechanisms has become the focus of innovation to overcome limitations imposed by protein replacement therapy. Tissue factor pathway inhibitor (TFPI) is a multivalent Kunitz-type serine protease inhibitor that regulates tissue factor (TF)-induced coagulation through a FXa-dependent feedback inhibition of the TF.FVIIa complex in plasma and on endothelial surfaces. Concizumab is a monoclonal, humanised antibody, specific for the second Kunitz domain of TFPI that binds and inhibits FXa, abolishing the inhibitory effect of TFPI. Concizumab restored thrombin generation in FVIII and FIX deficient plasmas and decreased blood loss in a rabbit haemophilia model. Phase 1 single and multiple dose escalation studies in haemophilia patients demonstrated a dose dependent decrease in TFPI levels and a pro-coagulant effect with increasing d-dimers and prothrombin fragment 1 + 2. A dose dependent increase in peak thrombin and endogenous thrombin potential was observed with values in the normal range when plasma TFPI levels were nearly undetectable. A few haemophilia patients in the highest dose cohorts with complete inhibition of plasma TFPI showed a decreased fibrinogen concentration with normal levels of anti-thrombin and platelets and no evidence of thrombosis. Pharmacokinetic parameters were influenced by binding to the target (TFPI), demonstrating target mediated drug disposition. A trend towards decreasing bleeding tendency was observed and this preventative effect is being studied in Phase 2 studies with additional data gathered to improve our understanding of the therapeutic window and potential for thrombosis.

The Potential Close Future of Hemophilia Treatment - Gene Therapy, TFPI Inhibition, Antithrombin Silencing, and Mimicking Factor VIII with an Engineered Antibody

Summary Hemophilia is one of the best researched monogenic diseases. Hemophilia A will affect approximately 1:5,000 male live births. In recent decades, great progress has been made with the introduction of recombinant proteins in the 1990s for therapy and prophylaxis, securing adequate availability and, with the introduction of the prophylaxis concept, reducing the negative impact of hemophilia on morbidity (especially arthropathy). Despite this progress, there are still challenges to overcome to secure adequate prophylaxis and treatment: for the time being, causal pharmacological hemophilia prophylaxis and therapy requires repeated i.v. application on a regular basis. Although this approach leads to a reduced comorbidity, it does not yet represent an optimized approach with continuous reversal of the hemophilic defect, which would be the ideal solution. This review summarizes the very new treatment strategies for the treatment of hemophilia A and B.

Targeting anticoagulant protein S to improve hemostasis in hemophilia

Improved treatments are needed for hemophilia A and B, bleeding disorders affecting 400 000 people worldwide. We investigated whether targeting protein S could promote hemostasis in hemophilia by rebalancing coagulation. Protein S (PS) is an anticoagulant acting as cofactor for activated protein C and tissue factor pathway inhibitor (TFPI). This dual role makes PS a key regulator of thrombin generation. Here, we report that targeting PS rebalances coagulation in hemophilia. PS gene targeting in hemophilic mice protected them against bleeding, especially when intra-articular. Mechanistically, these mice displayed increased thrombin generation, resistance to activated protein C and TFPI, and improved fibrin network. Blocking PS in plasma of hemophilia patients normalized in vitro thrombin generation. Both PS and TFPIα were detected in hemophilic mice joints. PS and TFPI expression was stronger in the joints of hemophilia A patients than in those of hemophilia B patients when receiving on-demand therapy, for example, during a bleeding episode. In contrast, PS and TFPI expression was decreased in hemophilia A patients receiving prophylaxis with coagulation factor concentrates, comparable to osteoarthritis patients. These results establish PS inhibition as both controller of coagulation and potential therapeutic target in hemophilia. The murine PS silencing RNA approach that we successfully used in hemophilic mice might constitute a new therapeutic concept for hemophilic patients.

Tissue Factor: An Essential Mediator of Hemostasis and Trigger of Thrombosis

Tissue factor (TF) is the high-affinity receptor and cofactor for factor (F)VII/VIIa. The TF-FVIIa complex is the primary initiator of blood coagulation and plays an essential role in hemostasis. TF is expressed on perivascular cells and epithelial cells at organ and body surfaces where it forms a hemostatic barrier. TF also provides additional hemostatic protection to vital organs, such as the brain, lung, and heart. Under pathological conditions, TF can trigger both arterial and venous thrombosis. For instance, atherosclerotic plaques contain high levels of TF on macrophage foam cells and microvesicles that drives thrombus formation after plaque rupture. In sepsis, inducible TF expression on monocytes leads to disseminated intravascular coagulation. In cancer patients, tumors release TF-positive microvesicles into the circulation that may contribute to venous thrombosis. TF also has nonhemostatic roles. For instance, TF-dependent activation of the coagulation cascade generates coagulation proteases, such as FVIIa, FXa, and thrombin, which induce signaling in a variety of cells by cleavage of protease-activated receptors. This review will focus on the roles of TF in protective hemostasis and pathological thrombosis.

High fat diet-induced obesity exacerbates hematopoiesis deficiency and cytopenia caused by 5-fluorouracil via peroxisome proliferator-activated receptor γ

The aim of the present study was to investigate the influence of a high-fat diet (HFD) on hematopoietic system recovery under stress condition caused by 5-fluorouracil (5-Fu) and to evaluate the alleviating benefit of proliferator-activated receptor γ (PPAR-γ) inhibition on aggravation of 5-Fu-induced toxicity under HFD. Survival rates of HFD or normal diet (ND) mice were monitored after 5-Fu injection. Hematopoietic stem cells (HSCs) and the progenitor cells in bone marrow were detected at various time points after 5-Fu administration by flow cytometry. Genes expressed in stem cell and platelet proliferation was tested by reverse transcription polymerase chain reaction. The blood cell profile and coagulation function were determined with an ADVIA 120 hematology system and rotational thromboelastometry, respectively. None of HFD mice remained alive after 5-Fu injection, whereas 87.4% of ND mice survived. HFD accelerated 5-Fu-induced myelosuppression and cytopenia in the circulation, reflected by the worse recovery of HSCs and myeloid progenitor cells. In HFD mice, the number of platelets significantly decreased, and the Vcam-1/Vla-4 signal pathway was downregulated in bone marrow. Meanwhile, mice receiving HFD exhibited dysfunction of blood coagulation with increased clotting formation time. However, pretreatment of HFD mice with bisphenol A diglycidyl ether (BADGE), the PPAR-γ antagonist, reversed the acute lethal toxicity and bone marrow suppression of 5-Fu and accelerated the recovery of HSCs and platelets. In the light of these results, HFD could exacerbate 5-Fu-induced bone marrow toxicity and cytopenia. BADGE treatment offers a potential strategy to alleviate the aggravated hematopoiesis under an HFD.

Correlates of plasma and platelet tissue factor pathway inhibitor, factor V, and Protein S

Background

Plasma Tissue Factor Pathway Inhibitor (TFPI) circulates bound to factor V (fV) and Protein S (PS). Estrogen therapy decreases plasma TFPI and PS. TFPI, fV, and PS circulate within platelets, and are released upon activation to modulate thrombus formation.

Objective

Identify factors affecting the concentrations of plasma and platelet TFPI, fV, and PS.

Methods

Blood samples were obtained from 435 healthy individuals. Plasma total TFPI, TFPIɑ, fV, and PS, and platelet TFPI, fV, and PS were quantified. Correlations between these protein concentrations and age, gender, race, and estrogen use were established.

Results

In males, only plasma fV increased with age, while in females, all plasma analytes increased with age. Males had higher plasma total TFPI, TFPIα, and PS than females. The platelet proteins in either sex remained relatively stable with increasing age. Platelet TFPI and PS were comparable in both sexes, while platelet fV was higher in females. Estrogen use was associated with decreased plasma total TFPI and TFPIα, and platelet PS, but not with platelet TFPI concentration. Racial differences in plasma and platelet proteins were observed, some of which were larger than inter-individual differences observed within racial groups. TFPI, fV and PS concentrations correlated in plasma, while only fV and PS correlated in platelets.

Conclusions

Plasma and platelet TFPI, fV and PS differ in their: (i) in vivo association; (ii) demographic correlates; and (iii) alteration by estrogen therapies. Therefore, the plasma and platelet pools of these proteins may modulate hemostasis and thrombosis via different biochemical pathways.

Selective factor VIII activation by the tissue factor-factor VIIa-factor Xa complex

Safe and effective antithrombotic therapy requires understanding of mechanisms that contribute to pathological thrombosis but have a lesser impact on hemostasis. We found that the extrinsic tissue factor (TF) coagulation initiation complex can selectively activate the antihemophilic cofactor, FVIII, triggering the hemostatic intrinsic coagulation pathway independently of thrombin feedback loops. In a mouse model with a relatively mild thrombogenic lesion, TF-dependent FVIII activation sets the threshold for thrombus formation through contact phase-generated FIXa. In vitro, FXa stably associated with TF-FVIIa activates FVIII, but not FV. Moreover, nascent FXa product of TF-FVIIa can transiently escape the slow kinetics of Kunitz-type inhibition by TF pathway inhibitor and preferentially activates FVIII over FV. Thus, TF synergistically primes FIXa-dependent thrombin generation independently of cofactor activation by thrombin. Accordingly, FVIIa mutants deficient in direct TF-dependent thrombin generation, but preserving FVIIIa generation by nascent FXa, can support intrinsic pathway coagulation. In ex vivo flowing blood, a TF-FVIIa mutant complex with impaired free FXa generation but activating both FVIII and FIX supports efficient FVIII-dependent thrombus formation. Thus, a previously unrecognized TF-initiated pathway directly yielding FVIIIa-FIXa intrinsic tenase complex may be prohemostatic before further coagulation amplification by thrombin-dependent feedback loops enhances the risk of thrombosis.

Targeting TFPI for hemophilia treatment

Hemophilia is a severe bleeding disorder treated by infusion of the missing blood coagulation protein, factor VIII or factor IX. The discovery and characterization of the anticoagulant protein tissue factor pathway inhibitor (TFPI) led to the realization that inhibition of TFPI activity could restore functional hemostasis through the extrinsic blood coagulation pathway in a manner that does not require the activity of factors VIII or IX. There are currently several therapeutic agents that inhibit TFPI in development for treatment of hemophilia. A comprehensive understanding of TFPI structure, biochemistry, and cellular expression is necessary to understand how it modulates bleeding in hemophilia and the physiological impact of therapeutic agents targeting TFPI.

Platelet full length TFPI-α in healthy volunteers is not affected by sex or hormonal use

BACKGROUND

Only 10% of plasma TFPIα (TFPI) exists in the full length form, the rest circulates as a C-terminally truncated form. However, blood platelets exclusively contain full length TFPI, which is released at the site of injury upon platelet activation, and which could play an important local regulatory role in thrombin generation and prevention of thrombosis.

METHODS

The anticoagulant activities of full length and truncated TFPI were investigated using thrombin generation assays. Blood samples were obtained from 30 healthy volunteers (10 male subjects, 10 female subjects, and 10 females using oral contraceptives). Platelet TFPI was released in platelet rich plasma and in platelet isolates using convulxin or thrombin, and measured by free TFPI ELISA and thrombin generation assays.

RESULTS

Full length TFPI and platelet TFPI were much more potent inhibitors of thrombin generation than truncated TFPI, which was virtually inactive. Although mean plasma TFPI antigen levels decreased from men (0.30 nM) to women (0.20 nM) to women using oral contraceptives (0.11 nM), no relevant differences were found in platelet TFPI among those subgroups.

CONCLUSIONS

Platelets release similar amounts of TFPI regardless of plasma TFPI concentrations and is unaffected by sex or oral contraceptive use. We speculate that platelet TFPI is important to prevent systemic coagulation and thrombosis and restrict thrombus formation to the site of the growing platelet plug. The stable contribution of platelet TFPI to the anticoagulant potential in plasma is likely to become particularly relevant under conditions of low plasma TFPI levels in combination of oral contraceptives use.

Platelet-Derived Short-Chain Polyphosphates Enhance the Inactivation of Tissue Factor Pathway Inhibitor by Activated Coagulation Factor XI

Introduction

Factor (F) XI supports both normal human hemostasis and pathological thrombosis. Activated FXI (FXIa) promotes thrombin generation by enzymatic activation of FXI, FIX, FX, and FV, and inactivation of alpha tissue factor pathway inhibitor (TFPIα), in vitro. Some of these reactions are now known to be enhanced by short-chain polyphosphates (SCP) derived from activated platelets. These SCPs act as a cofactor for the activation of FXI and FV by thrombin and FXIa, respectively. Since SCPs have been shown to inhibit the anticoagulant function of TFPIα, we herein investigated whether SCPs could serve as cofactors for the proteolytic inactivation of TFPIα by FXIa, further promoting the efficiency of the extrinsic pathway of coagulation to generate thrombin.

Methods and Results

Purified soluble SCP was prepared by size-fractionation of sodium polyphosphate. TFPIα proteolysis was analyzed by western blot. TFPIα activity was measured as inhibition of FX activation and activity in coagulation and chromogenic assays. SCPs significantly accelerated the rate of inactivation of TFPIα by FXIa in both purified systems and in recalcified plasma. Moreover, platelet-derived SCP accelerated the rate of inactivation of platelet-derived TFPIα by FXIa. TFPIα activity was not affected by SCP in recalcified FXI-depleted plasma.

Conclusions

Our data suggest that SCP is a cofactor for TFPIα inactivation by FXIa, thus, expanding the range of hemostatic FXIa substrates that may be affected by the cofactor functions of platelet-derived SCP.

The hemostatic role of factor XI

Coagulation factor (F)XI has been described as a component of the early phase of the contact pathway of blood coagulation, acting downstream of factor XII. However, patients deficient in upstream members of the contact pathway, including FXII and prekallikrein, do not exhibit bleeding complications, while FXI-deficient patients sometimes experience mild bleeding, suggesting FXI plays a role in hemostasis independent of the contact pathway. Further complicating the picture, bleeding risk in FXI-deficient patients is difficult to predict because bleeding symptoms have not been found to correlate with FXI antigen levels or activity. However, recent studies have emerged to expand our understanding of FXI, demonstrating that activated FXI is able to activate coagulation factors FX, FV, and FVIII, and inhibit the anti-coagulant tissue factor pathway inhibitor (TFPI). Understanding these activities of FXI may help to better diagnose which FXI-deficient patients are at risk for bleeding. In contrast to its mild hemostatic activities, FXI is known to play a significant role in thrombosis, as it is a demonstrated independent risk factor for deep vein thrombosis, ischemic stroke, and myocardial infarction. Recent translational approaches have begun testing FXI as an antithrombotic, with one promising clinical study showing that an anti-sense oligonucleotide against FXI prevented venous thrombosis in elective knee surgery. A better understanding of the varied and complex role of FXI in both thrombosis and hemostasis will help to allow better prediction of bleeding risk in FXI-deficient patients and also informing the development of targeted agents to inhibit the thrombotic activities of FXI while preserving hemostasis.

Tissue Factor Pathway Inhibitor: Multiple Anticoagulant Activities for a Single Protein

Tissue factor (TF) pathway inhibitor (TFPI) is an anticoagulant protein that inhibits early phases of the procoagulant response. Alternatively spliced isoforms of TFPI are differentially expressed by endothelial cells and human platelets and plasma. The TFPIβ isoform localizes to the endothelium surface where it is a potent inhibitor of TF-factor VIIa complexes that initiate blood coagulation. The TFPIα isoform is present in platelets. TFPIα contains a stretch of 9 amino acids nearly identical to those found in the B-domain of factor V that are well conserved in mammals. These amino acids provide exosite binding to activated factor V, which allows for TFPIα to inhibit prothrombinase during the initiation phase of blood coagulation. Endogenous inhibition at this point in the coagulation cascade was only recently recognized and has provided a biochemical rationale to explain the pathophysiological mechanisms underlying several clinical disorders. These include the east Texas bleeding disorder that is caused by production of an altered form of factor V with high affinity for TFPI and a paradoxical procoagulant effect of heparins. In addition, these findings have led to ideas for pharmacological targeting of TFPI that may reduce bleeding in hemophilia patients.

New products for the treatment of haemophilia

Current developments in haemophilia therapy are directed at two therapeutic targets: reduction of injection frequency and reduction or bypassing of inhibitors. A variety of new molecules addressing these aims are now completing clinical trials and are ready to enter clinical use. First amongst these are modified Factor VIII (FVIII) and Factor IX (FIX) molecules with extended half-lives. FIX modifications have achieved 5-fold prolongation of half-life whilst effects on FVIII have been more modest, at less than two-fold. We now face the problem of integrating these into clinical practice. Other approaches have generated chemically modified FVIII molecules with altered activation profiles. An alternative way of correcting the haemophilia defect is to reduce the activity of natural anticoagulants in an attempt to restore the balance of haemostasis. These methods are also giving promising results but, as with all new approaches, it will be some while before they all find their place in practice.

New insights into the biology of tissue factor pathway inhibitor

Tissue factor pathway inhibitor (TFPI) dampens the initiation of blood coagulation by inhibiting two potent procoagulant complexes, tissue factor-factor VIIa (TF-FVIIa) and early forms of prothrombinase. TFPI isoforms, TFPIα and TFPIβ, result from alternative splicing of mRNA, producing distinct C-terminal ends of the two proteins. Both isoforms inhibit TF-FVIIa, but only TFPIα can inhibit early forms of prothrombinase by binding of its positively charged C-terminus with high affinity to the acidic B-domain exosite of FVa, which is generated upon activation by FXa. TFPIα and TFPIβ are produced in cultured human endothelial cells, while platelets contain only TFPIα. Knowledge of the anticoagulant mechanisms and tissue expression patterns of TFPIα and TFPIβ have improved our understanding of the phenotypes observed in different mouse models of TFPI deficiency, the east Texas bleeding disorder, and the development of pharmaceutical agents that block TFPI function to treat hemophilia.

A balance between TFPI and thrombin-mediated platelet activation is required for murine embryonic development

Tissue factor pathway inhibitor (TFPI) is a critical anticoagulant protein present in endothelium and platelets. Mice lacking TFPI (Tfpi(-/-)) die in utero from disseminated intravascular coagulation. They are rescued by concomitant tissue factor (TF) deficiency, demonstrating that TFPI modulates TF function in vivo. Recent studies have found TFPI inhibits prothrombinase activity during the initiation of coagulation and limits platelet accumulation during thrombus formation, implicating TFPI in modulating platelet procoagulant activity. To examine whether altered platelet function would compensate for the lack of TFPI and rescue TFPI-null embryonic lethality, Tfpi(+/-) mice lacking the platelet thrombin receptor, protease activated receptor 4 (PAR4; Par4(-/-)), or its coreceptor, PAR3, were mated. PAR3 deficiency did not rescue Tfpi(-/-) embryos, but >40% of expected Tfpi(-/-):Par4(-/-) offspring survived to adulthood. Adult Tfpi(-/-):Par4(-/-) mice did not exhibit overt thrombosis. However, they had focal sterile inflammation with fibrin(ogen) deposition in the liver and elevated plasma thrombin-antithrombin complexes, indicating activation of coagulation at baseline. Tfpi(-/-):Par4(-/-) mice have platelet and fibrin accumulation similar to Par4(-/-) mice following venous electrolytic injury but were more susceptible than Par4(-/-) mice to TF-induced pulmonary embolism. In addition, ∼30% of the Tfpi(-/-):Par4(-/-) mice were born with short tails. Tfpi(-/-):Par4(-/-) mice are the first adult mice described that lack TFPI with unaltered TF. They demonstrate that TFPI physiologically modulates thrombin-dependent platelet activation in a manner that is required for successful embryonic development and identify a role for TFPI in dampening intravascular procoagulant stimuli that lead to thrombin generation, even in the absence of thrombin-mediated platelet activation.

Endogenous tissue factor pathway inhibitor has a limited effect on host defence in murine pneumococcal pneumonia

Streptococcus (S.) pneumoniae is the most common causative pathogen in community-acquired pneumonia. Coagulation and inflammation interact in the host response to infection. Tissue factor pathway inhibitor (TFPI) is a natural anticoagulant protein that inhibits tissue factor (TF), the main activator of inflammation-induced coagulation. It was the objective of this study to investigate the effect of endogenous TFPI levels on coagulation, inflammation and bacterial growth during S. pneumoniae pneumonia in mice. The effect of low endogenous TFPI levels was studied by administration of a neutralising anti-TFPI antibody to wild-type mice, and by using genetically modified mice expressing low levels of TFPI, due to a genetic deletion of the first Kunitz domain of TFPI (TFPIK1(-/-)) rescued with a human TFPI transgene. Pneumonia was induced by intranasal inoculation with S. pneumoniae and samples were obtained at 6, 24 and 48 hours after infection. Anti-TFPI reduced TFPI activity by ~50 %. Homozygous lowTFPI mice and heterozygous controls had ~10 % and ~50 % of normal TFPI activity, respectively. TFPI levels did not influence bacterial growth or dissemination. Whereas lung pathology was unaffected in all groups, mice with ~10 % (but not with ~50 %) of TFPI levels displayed elevated lung cytokine and chemokine concentrations 24 hours after infection. None of the groups with low TFPI levels showed an altered procoagulant response in lungs or plasma during pneumonia. These data argue against an important role for endogenous TFPI in the antibacterial, inflammatory and procoagulant response during pneumococcal pneumonia.