Pharmacologic targeting of coagulation factors XII and XI by monoclonal antibodies reduces thrombosis in nitinol stents under flow

BACKGROUND

Cardiovascular implantable devices, such as vascular stents, are critical for the treatment of cardiovascular diseases. However, their success is dependent on robust and often long-term antithrombotic therapies. Yet, the current standard-of-care therapies often pose significant bleeding risks to patients. Coagulation factor (F)XI and FXII have emerged as potentially safe and efficacious targets to safely reduce pathologic thrombin generation in medical devices.

OBJECTIVES

To study the efficacy of monoclonal antibody-targeting FXII and FXI of the contact pathway in preventing vascular device-related thrombosis.

METHODS

The effects of inhibition of FXII and FXI using function-blocking monoclonal antibodies were examined in a nonhuman primate model of nitinol stent-related thrombosis under arterial and venous flow conditions.

RESULTS

We found that function-blocking antibodies of FXII and FXI reduced markers of stent-induced thrombosis in vitro and ex vivo. However, FXI inhibition resulted in more effective mitigation of thrombosis markers under varied flow conditions.

CONCLUSION

This work provides further support for the translation of contact pathway of coagulation inhibitors for their adjunctive clinical use with cardiovascular devices.

Biology of factor XI

ABSTRACT

Unique among coagulation factors, the coagulation factor XI (FXI) arose through a duplication of the gene KLKB1, which encodes plasma prekallikrein. This evolutionary origin sets FXI apart structurally because it is a homodimer with 2 identical subunits composed of 4 apple and 1 catalytic domain. Each domain exhibits unique affinities for binding partners within the coagulation cascade, regulating the conversion of FXI to a serine protease as well as the selectivity of substrates cleaved by the active form of FXI. Beyond serving as the molecular nexus for the extrinsic and contact pathways to propagate thrombin generation by way of activating FIX, the function of FXI extends to contribute to barrier function, platelet activation, inflammation, and the immune response. Herein, we critically review the current understanding of the molecular biology of FXI, touching on some functional consequences at the cell, tissue, and organ level. We conclude each section by highlighting the DNA mutations within each domain that present as FXI deficiency. Together, a narrative review of the structure-function of the domains of FXI is imperative to understand the etiology of hemophilia C as well as to identify regions of FXI to safely inhibit the pathological function of activation or activity of FXI without compromising the physiologic role of FXI.

Factor XI as a therapeutic target in neuroinflammatory disease

PURPOSE OF REVIEW

This review summarizes the pathophysiology and potential therapeutic options for treatment of multiple sclerosis, a common neuronal demyelinating disorder affecting 2.2 million people worldwide. As an autoimmune disorder, multiple sclerosis is associated with neuroinflammation and increased permeability of the blood-brain barrier (BBB), although the cause linking multiple sclerosis with compromised barrier function remains ill-defined. It has been previously shown that coagulation factors, including thrombin and fibrin, exacerbate the inflammatory processes and permeability of the BBB.

RECENT FINDINGS

Increased levels of the coagulation factor (F) XII have been found in patients presenting with relapsing-remitting multiple sclerosis, with a deleterious role for FXII being validated in murine model of multiple sclerosis, experimental autoimmune encephalitis (EAE). Recent work has uncovered a role for the major substrate activated by FXII and thrombin, FXI, in the disorder of EAE. The study found that pharmacological targeting of FXI decreased clinical symptoms, lymphocyte invasion, and white matter destruction in a multiple sclerosis model.

SUMMARY

This review emphasizes the role of FXII and FXI in regulating barrier function and the immune response in neuroinflammation. These new findings broaden the potential for therapeutic utility of FXI inhibitors beyond thrombosis to include neuroinflammatory diseases associated with compromised BBB function, including multiple sclerosis.

Factor XI Inhibition for the Prevention of Catheter-Associated Thrombosis in Patients With Cancer Undergoing Central Line Placement: A Phase 2 Clinical Trial

BACKGROUND

Despite the ubiquitous utilization of central venous catheters in clinical practice, their use commonly provokes thromboembolism. No prophylactic strategy has shown sufficient efficacy to justify routine use. Coagulation factors FXI (factor XI) and FXII (factor XII) represent novel targets for device-associated thrombosis, which may mitigate bleeding risk. Our objective was to evaluate the safety and efficacy of an anti-FXI mAb (monoclonal antibody), gruticibart (AB023), in a prospective, single-arm study of patients with cancer receiving central line placement.

METHODS

We enrolled ambulatory cancer patients undergoing central line placement to receive a single dose of gruticibart (2 mg/kg) administered through the venous catheter within 24 hours of placement and a follow-up surveillance ultrasound at day 14 for evaluation of catheter thrombosis. A parallel, noninterventional study was used as a comparator.

RESULTS

In total, 22 subjects (n=11 per study) were enrolled. The overall incidence of catheter-associated thrombosis was 12.5% in the interventional study and 40.0% in the control study. The anti-FXI mAb, gruticibart, significantly prolonged the activated partial thromboplastin time in all subjects on day 14 compared with baseline (P<0.001). Gruticibart was well tolerated and without infusion reactions, drug-related adverse events, or clinically relevant bleeding. Platelet flow cytometry demonstrated no difference in platelet activation following administration of gruticibart. T (thrombin)-AT (antithrombin) and activated FXI-AT complexes increased following central line placement in the control study, which was not demonstrated in our intervention study. CRP (C-reactive protein) did not significantly increase on day 14 in those who received gruticibart, but it did significantly increase in the noninterventional study.

CONCLUSIONS

FXI inhibition with gruticibart was well tolerated without any significant adverse or bleeding-related events and resulted in a lower incidence of catheter-associated thrombosis on surveillance ultrasound compared with the published literature and our internal control study. These findings suggest that targeting FXI could represent a safe intervention to prevent catheter thrombosis.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT04465760.

Activation of coagulation FXI promotes endothelial inflammation and amplifies platelet activation in a nonhuman primate model of hyperlipidemia

Background

Hyperlipidemia is associated with chronic inflammation and thromboinflammation. This is an underlying cause of several cardiovascular diseases, including atherosclerosis. In diseased blood vessels, rampant thrombin generation results in the initiation of the coagulation cascade, activation of platelets, and endothelial cell dysfunction. Coagulation factor (F) XI represents a promising therapeutic target to reduce thromboinflammation, as it is uniquely positioned at an intersection between inflammation and thrombin generation.

Objectives

This study aimed to investigate the role of FXI in promoting platelet and endothelial cell activation in a model of hyperlipidemia.

Methods

Nonhuman primates (NHPs) were fed a standard chow diet (lean, n = 6) or a high-fat diet (obese, n = 8) to establish a model of hyperlipidemia. Obese NHPs were intravenously administered a FXI blocking antibody (2 mg/kg) and studied at baseline and at 1, 7, 14, 21, and 28 days after drug administration. Platelet activation and inflammatory markers were measured using fluorescence-activated cell sorting or enzyme-linked immunosorbent assay. Molecular imaging was used to quantify vascular cell adhesion molecule 1 (VCAM-1) expression at the carotid bifurcation.

Results

Obese NHPs demonstrated increased sensitivity for platelet P-selectin expression and phosphatidylserine exposure in response to platelet GPVI or PAR agonists compared with lean NHPs. Obese NHPs exhibited elevated levels of C-reactive protein, cathepsin D, and myeloperoxidase compared with lean NHPs. Following pharmacological inhibition of FIX activation by FXIa, platelet priming for activation by GPVI or PAR agonists, C-reactive protein levels, and endothelial VCAM-1 levels were reduced in obese NHPs.

Conclusion

FXI activation promotes the proinflammatory phenotype of hyperlipidemia by priming platelet activation and inciting endothelial cell dysfunction.

Targeting the Contact Pathway of Coagulation for the Prevention and Management of Medical Device-Associated Thrombosis

Hemorrhage remains a major complication of anticoagulants, with bleeding leading to serious and even life-threatening outcomes in rare settings. Currently available anticoagulants target either multiple coagulation factors or specifically coagulation factor (F) Xa or thrombin; however, inhibiting these pathways universally impairs hemostasis. Bleeding complications are especially salient in the medically complex population who benefit from medical devices. Extracorporeal devices-such as extracorporeal membrane oxygenation, hemodialysis, and cardiac bypass-require anticoagulation for optimal use. Nonetheless, bleeding complications are common, and with certain devices, highly morbid. Likewise, pharmacologic prophylaxis to prevent thrombosis is not commonly used with many medical devices like central venous catheters due to high rates of bleeding. The contact pathway members FXI, FXII, and prekallikrein serve as a nexus, connecting biomaterial surface-mediated thrombin generation and inflammation, and may represent safe, druggable targets to improve medical device hemocompatibility and thrombogenicity. Recent in vivo and clinical data suggest that selectively targeting the contact pathway of coagulation through the inhibition of FXI and FXII can reduce the incidence of medical device-associated thrombotic events, and potentially systemic inflammation, without impairing hemostasis. In the following review, we will outline the current in vivo and clinical data encompassing the mechanism of action of drugs targeting the contact pathway. This new class of inhibitors has the potential to herald a new era of effective and low-risk anticoagulation for the management of patients requiring the use of medical devices.

Substrates, Cofactors, and Cellular Targets of Coagulation Factor XIa

Coagulation factor XI (FXI) has increasingly been shown to play an integral role in several physiologic and pathological processes. FXI is among several zymogens within the blood coagulation cascade that are activated by proteolytic cleavage, with FXI converting to the active serine protease form (FXIa). The evolutionary origins of FXI trace back to duplication of the gene that transcribes plasma prekallikrein, a key factor in the plasma kallikrein-kinin system, before further genetic divergence led to FXI playing a unique role in blood coagulation. While FXIa is canonically known for activating the intrinsic pathway of coagulation by catalyzing the conversion of FIX into FIXa, it is promiscuous in nature and has been shown to contribute to thrombin generation independent of FIX. In addition to its role in the intrinsic pathway of coagulation, FXI also interacts with platelets, endothelial cells, and mediates the inflammatory response through activation of FXII and cleavage of high-molecular-weight kininogen to generate bradykinin. In this manuscript, we critically review the current body of knowledge surrounding how FXI navigates the interplay of hemostasis, inflammatory processes, and the immune response and highlight future avenues for research. As FXI continues to be clinically explored as a druggable therapeutic target, understanding how this coagulation factor fits into physiological and disease mechanisms becomes increasingly important.

Revised model of the tissue factor pathway of thrombin generation: Role of the feedback activation of FXI

BACKGROUND

Biochemical reaction networks are self-regulated in part due to feedback activation mechanisms. The tissue factor (TF) pathway of blood coagulation is a complex reaction network controlled by multiple feedback loops that coalesce around the serine protease thrombin.

OBJECTIVES

Our goal was to evaluate the relative contribution of the feedback activation of coagulation factor XI (FXI) in TF-mediated thrombin generation using a comprehensive systems-based analysis.

MATERIALS AND METHODS

We developed a systems biology model that improves the existing Hockin-Mann (HM) model through an integrative approach of mathematical modeling and in vitro experiments. Thrombin generation measured using in vitro assays revealed that the feedback activation of FXI contributes to the propagation of thrombin generation based on the initial concentrations of TF or activated coagulation factor X (FXa). We utilized experimental data to improve the robustness of the HM model to capture thrombin generation kinetics without a role for FXI before including the feedback activation of FXI by thrombin to construct the extended (ext.) HM model.

RESULTS AND CONCLUSIONS

Using the ext.HM model, we predicted that the contribution of positive feedback of FXI activation by thrombin can be abolished by selectively eliminating the inhibitory function of tissue factor pathway inhibitor (TFPI), a serine protease inhibitor of FXa and TF-activated factor VII (FVIIa) complex. This prediction from the ext.HM model was experimentally validated using thrombin generation assays with function blocking antibodies against TFPI and plasmas depleted of FXI. Together, our results demonstrate the applications of combining experimental and modeling techniques in predicting complex biochemical reaction systems.

Abstract 133: Coagulation FXI Regulates Endothelial Cell Barrier Function By Inducing VE-cadherin Proteolysis In An ADAM10-dependent Manner

Introduction: Proteolysis of vascular endothelial (VE)-cadherin is a biomarker for disruption of endothelial cell (EC) barrier function, with elevated levels of soluble VE-cadherin associated with atherosclerosis and sepsis. Factor XI (FXI), known for its role in the coagulation cascade, also plays a regulatory role in inflammation. While a relationship between FXI activity, VE-cadherin expression levels, and endothelial barrier function has been observed, the mechanisms by which this process is mediated are unknown.

Hypothesis: Activated FXI (FXIa) regulates EC barrier function by inducing VE-cadherin proteolysis.

Methods: Human umbilical vein endothelial cells were stimulated with FXIa (30 and 5 nM) for six hours. VE-cadherin proteolysis and expression levels were analyzed by Western blot and immunofluorescence. ECs permeability was quantified by measuring the flux of Evans blue-albumin across the ECs. Immunoprecipitation and Western blot analysis were used to study FXIa-PAI-1 or -very low density lipoprotein receptor (VLDLR) interactions.

Results: FXIa (5 nM) generated a VE-cadherin C-terminal fragment and decreased VE-cadherin expression on ECs; this process was reversed by the serine protease inhibitor, PPACK. Thrombin, kallikrein, or FXIIa did not induce VE-cadherin proteolysis. FXIa increased ECs permeability, yet did not induce the expression of VCAM-1. In contrast, while VCAM-1 expression was upregulated by tumor necrosis factor alpha or vascular endothelial growth factor, neither of these induced VE-cadherin proteolysis. A pharmacological inhibitor of a disintegrin and metalloproteinase 10 (ADAM10) abrogated the effects of FXIa on ECs, including proteolysis of VE-cadherin. Similarly, the low density lipoprotein antagonist, receptor-associated protein (RAP), inhibited FXIa-induced VE-cadherin proteolysis by preventing FXIa complex formation with PAI-1 and VLDLR on ECs.

Conclusion: These results suggest that FXIa disrupts VE-cadherin-mediated EC permeability by increasing ADAM10 activity through complex formation and internalization by PAI-1 and VLDLR. It remains to be seen whether this pathway represents a druggable target to protect EC barrier function in inflammatory diseases by inhibiting FXIa.

Ibrutinib Inhibits BMX-Dependent Endothelial VCAM-1 Expression In Vitro and Pro-Atherosclerotic Endothelial Activation and Platelet Adhesion In Vivo

Introduction

Inflammatory activation of the vascular endothelium leads to overexpression of adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1), contributing to the pro-thrombotic state underpinning atherogenesis. While the role of TEC family kinases (TFKs) in mediating inflammatory cell and platelet activation is well defined, the role of TFKs in vascular endothelial activation remains unclear. We investigated the role of TFKs in endothelial cell activation in vitro and in a nonhuman primate model of diet-induced atherosclerosis in vivo.

Methods and Results

In vitro, we found that ibrutinib blocked activation of the TFK member, BMX, by vascular endothelial growth factors (VEGF)-A in human aortic endothelial cells (HAECs). Blockade of BMX activation with ibrutinib or pharmacologically distinct BMX inhibitors eliminated the ability of VEGF-A to stimulate VCAM-1 expression in HAECs. We validated that treatment with ibrutinib inhibited TFK-mediated platelet activation and aggregation in both human and primate samples as measured using flow cytometry and light transmission aggregometry. We utilized contrast-enhanced ultrasound molecular imaging to measure platelet GPIbα and endothelial VCAM-1 expression in atherosclerosis-prone carotid arteries of obese nonhuman primates. We observed that the TFK inhibitor, ibrutinib, inhibited platelet deposition and endothelial cell activation in vivo.

Conclusion

Herein we found that VEGF-A signals through BMX to induce VCAM-1 expression in endothelial cells, and that VCAM-1 expression is sensitive to ibrutinib in vitro and in atherosclerosis-prone carotid arteries in vivo. These findings suggest that TFKs may contribute to the pathogenesis of atherosclerosis and could represent a novel therapeutic target.

Cross-Talk between the Complement Pathway and the Contact Activation System of Coagulation: Activated Factor XI Neutralizes Complement Factor H

Complement factor H (CFH) is the major inhibitor of the alternative pathway of the complement system and is structurally related to beta2-glycoprotein I, which itself is known to bind to ligands, including coagulation factor XI (FXI). We observed reduced complement activation when FXI activation was inhibited in a baboon model of lethal systemic inflammation, suggesting cross-talk between FXI and the complement cascade. It is unknown whether FXI or its activated form, activated FXI (FXIa), directly interacts with the complement system. We explored whether FXI could interact with and inhibit the activity of CFH. We found that FXIa neutralized CFH by cleavage of the R341/R342 bonds. FXIa reduced the capacity of CFH to enhance the cleavage of C3b by factor I and the decay of C3bBb. The binding of CFH to human endothelial cells was also reduced after incubating CFH with FXIa. The addition of either short- or long-chain polyphosphate enhanced the capacity of FXIa to cleave CFH. FXIa also cleaved CFH that was present on endothelial cells and in the secretome from blood platelets. The generation of FXIa in plasma induced the cleavage of CFH. Moreover, FXIa reduced the cleavage of C3b by factor I in serum. Conversely, we observed that CFH inhibited FXI activation by either thrombin or FXIIa. Our study provides, to our knowledge, a novel molecular link between the contact pathway of coagulation and the complement system. These results suggest that FXIa generation enhances the activity of the complement system and thus may potentiate the immune response.

Factor XII plays a pathogenic role in organ failure and death in baboons challenged with Staphylococcus aureus

Activation of coagulation factor (F) XI promotes multiorgan failure in rodent models of sepsis and in a baboon model of lethal systemic inflammation induced by infusion of heat-inactivated Staphylococcus aureus. Here we used the anticoagulant FXII-neutralizing antibody 5C12 to verify the mechanistic role of FXII in this baboon model. Compared with untreated control animals, repeated 5C12 administration before and at 8 and 24 hours after bacterial challenge prevented the dramatic increase in circulating complexes of contact system enzymes FXIIa, FXIa, and kallikrein with antithrombin or C1 inhibitor, and prevented cleavage and consumption of high-molecular-weight kininogen. Activation of several coagulation factors and fibrinolytic enzymes was also prevented. D-dimer levels exhibited a profound increase in the untreated animals but not in the treated animals. The antibody also blocked the increase in plasma biomarkers of inflammation and cell damage, including tumor necrosis factor, interleukin (IL)-1β, IL-6, IL-8, IL-10, granulocyte-macrophage colony-stimulating factor, nucleosomes, and myeloperoxidase. Based on clinical presentation and circulating biomarkers, inhibition of FXII prevented fever, terminal hypotension, respiratory distress, and multiorgan failure. All animals receiving 5C12 had milder and transient clinical symptoms and were asymptomatic at day 7, whereas untreated control animals suffered irreversible multiorgan failure and had to be euthanized within 2 days after the bacterial challenge. This study confirms and extends our previous finding that at least 2 enzymes of the contact activation complex, FXIa and FXIIa, play critical roles in the development of an acute and terminal inflammatory response in baboons challenged with heat-inactivated S aureus.

Pharmacological targeting of coagulation factor XI mitigates the development of experimental atherosclerosis in low-density lipoprotein receptor-deficient mice

BACKGROUND

Human coagulation factor (F) XI deficiency, a defect of the contact activation system, protects against venous thrombosis, stroke, and heart attack, whereas FXII, plasma prekallikrein, or kininogen deficiencies are asymptomatic. FXI deficiency, inhibition of FXI production, activated FXI (FXIa) inhibitors, and antibodies to FXI that interfere with FXI/FXII interactions reduce experimental thrombosis and inflammation. FXI inhibitors are antithrombotic in patients, and FXI and FXII deficiencies are atheroprotective in apolipoprotein E-deficient mice.

OBJECTIVES

Investigate the effects of pharmacological targeting of FXI in experimental models of atherogenesis and established atherosclerosis.

METHODS AND RESULTS

Low-density lipoprotein receptor-knockout (Ldlr-/- ) mice were administered high-fat diet (HFD) for 8 weeks; concomitantly, FXI was targeted with anti-FXI antibody (14E11) or FXI antisense oligonucleotide (ASO). 14E11 and FXI-ASO reduced atherosclerotic lesion area in proximal aortas when compared with controls, and 14E11 also reduced aortic sinus lesions. In an established disease model, in which therapy was given after atherosclerosis had developed, Ldlr-/- mice were fed HFD for 8 weeks and then administered 14E11 or FXI-ASO weekly until 16 weeks on HFD. In this established disease model, 14E11 and FXI-ASO reduced atherosclerotic lesion area in proximal aortas, but not in aortic sinus. In cultures of human endothelium, FXIa exposure disrupted VE-Cadherin expression and increased endothelial lipoprotein permeability. Strikingly, we found that 14E11 prevented the disruption of VE-Cadherin expression in aortic sinus lesions observed in the atherogenesis mouse model.

CONCLUSION

Pharmacological targeting of FXI reduced atherogenesis in Ldlr-/- mice. Interference with the contact activation system may safely reduce development or progression of atherosclerosis.

Role of platelets in regulating activated coagulation factor XI activity

Factor XI (FXI) has been shown to bind platelets, but the functional significance of this observation remains unknown. Platelets are essential for hemostasis and play a critical role in thrombosis, whereas FXI is not essential for hemostasis but promotes thrombosis. An apparent functional contradiction, platelets are known to support thrombin generation, yet platelet granules release protease inhibitors, including those of activated FXI (FXIa). We aim to investigate the secretory and binding mechanisms by which platelets could support or inhibit FXIa activity. The presence of platelets enhanced FXIa activity in a purified system and increased coagulation Factor IX (FIX) activation by FXIa and fibrin generation in human plasma. In contrast, platelets reduced the activation of FXI by activated coagulation factor XII (FXIIa) and the activation of FXII by kallikrein (PKa). Incubation of FXIa with the platelet secretome, which contains FXIa inhibitors, such as protease nexin-II, abolished FXIa activity, yet in the presence of activated platelets, the secretome was not able to block the activity of FXIa. FXIa variants lacking the anion-binding sites did not alter the effect of platelets on FXIa activity or interaction. Western blot analysis of bound FXIa [by FXIa-platelet membrane immunoprecipitation] showed that the interaction with platelets is zinc dependent and, unlike FXI binding to platelets, not dependent on glycoprotein Ib. FXIa binding to the platelet membrane increases its capacity to activate FIX in plasma likely by protecting it from inhibition by inhibitors secreted by activated platelets. Our findings suggest that an interaction of FXIa with the platelet surface may induce an allosteric modulation of FXIa.

Development of Coagulation Factor XII Antibodies for Inhibiting Vascular Device-Related Thrombosis

Introduction

Vascular devices such as stents, hemodialyzers, and membrane oxygenators can activate blood coagulation and often require the use of systemic anticoagulants to selectively prevent intravascular thrombotic/embolic events or extracorporeal device failure. Coagulation factor (F)XII of the contact activation system has been shown to play an important role in initiating vascular device surface-initiated thrombus formation. As FXII is dispensable for hemostasis, targeting the contact activation system holds promise as a significantly safer strategy than traditional antithrombotics for preventing vascular device-associated thrombosis.

Objective

Generate and characterize anti-FXII monoclonal antibodies that inhibit FXII activation or activity.

Methods

Monoclonal antibodies against FXII were generated in FXII-deficient mice and evaluated for their binding and anticoagulant properties in purified and plasma systems, in whole blood flow-based assays, and in an in vivo non-human primate model of vascular device-initiated thrombus formation.

Results

A FXII antibody screen identified over 400 candidates, which were evaluated in binding studies and clotting assays. One non-inhibitor and six inhibitor antibodies were selected for characterization in functional assays. The most potent inhibitory antibody, 1B2, was found to prolong clotting times, inhibit fibrin generation on collagen under shear, and inhibit platelet deposition and fibrin formation in an extracorporeal membrane oxygenator deployed in a non-human primate.

Conclusion

Selective contact activation inhibitors hold potential as useful tools for research applications as well as safe and effective inhibitors of vascular device-related thrombosis.

Evaluation of the Effect of Crosslinking Method of Poly(Vinyl Alcohol) Hydrogels on Thrombogenicity

PURPOSE

Crosslinked poly(vinyl alcohol) (PVA) is a biomaterial that can be used for multiple cardiovascular applications. The success of implanted biomaterials is contingent on the properties of the material. A crucial consideration for blood-contacting devices is their potential to incite thrombus formation, which is dependent on the material surface properties. The goal of this study was to quantify the effect of different crosslinking methods of PVA hydrogels on in vitro thrombogenicity.

METHODS

PVA was manufactured using three different crosslinking methods: 30% sodium trimetaphosphate (STMP), three 24 h freeze-thaw cycles (FT), and 2% glutaraldehyde-crosslinked (GA) to produce STMP-PVA, FT-PVA and GA-PVA, respectively. Expanded polytetrafluoroethylene (ePTFE) was used as a clinical control. As markers of thrombus formation, the degree of coagulation factor (F) XII activation, fibrin formation, and platelet adhesion were measured.

RESULTS

The GA-PVA material increased FXII activation in the presence of cofactors compared to vehicle and increase platelet adhesion compared to other PVA surfaces. The STMP-PVA and FT-PVA materials had equivalent degrees of FXII activation, fibrin formation and platelet adhesion.

CONCLUSION

This work supports crosslinker dependent thrombogenicity of PVA hydrogels and advances our understanding of how the manufacturing of a PVA hydrogel affects its hemocompatibility.

Antibody inhibition of contact factor XII reduces platelet deposition in a model of extracorporeal membrane oxygenator perfusion in nonhuman primates

BACKGROUND

The contact factor XII (FXII) activates upon contact with a variety of charged surfaces. Activated FXII (FXIIa) activates factor XI, which activates factor IX, resulting in thrombin generation, platelet activation, and fibrin formation. In both in vitro and in vivo rabbit models, components of medical devices, including extracorporeal oxygenators, are known to incite fibrin formation in a FXII-dependent manner. Since FXII has no known role in hemostasis and its inhibition is therefore likely a safe antithrombotic approach, we investigated whether FXII inhibition also reduces accumulation of platelets in extracorporeal oxygenators.

OBJECTIVES

We aimed to determine the effect of FXII inhibition on platelet deposition in perfused extracorporeal membrane oxygenators in nonhuman primates.

METHODS

A potent FXII neutralizing monoclonal antibody, 5C12, was administered intravenously to block contact activation in baboons. Extracorporeal membrane oxygenators were temporarily deployed into chronic arteriovenous access shunts. Radiolabeled platelet deposition in oxygenators was quantified in real time using gamma camera imaging. Biochemical assays were performed to characterize the method of action of 5C12.

RESULTS

The anti-FXII monoclonal antibody 5C12 recognized both the alpha and beta forms of human and baboon FXII by binding to the protease-containing domain, and inhibited FXIIa activity. Administration of 5C12 to baboons reduced platelet deposition and fibrin formation in the extracorporeal membrane oxygenators, in both the presence and absence of systemic low-dose unfractionated heparin. The antiplatelet dose of 5C12 did not cause measurable increases in template bleeding times in baboons.

CONCLUSIONS

FXII represents a possible therapeutic and safe target for reducing platelet deposition and fibrin formation during medical interventions including extracorporeal membrane oxygenation.

Contact Activation Inhibitor and Factor XI Antibody, AB023, Produces Safe, Dose-Dependent Anticoagulation in a Phase 1 First-In-Human Trial

Objective- Factor XI (FXI) contributes to thrombotic disease while playing a limited role in normal hemostasis. We generated a unique, humanized anti-FXI antibody, AB023, which blocks factor XIIa-mediated FXI activation without inhibiting FXI activation by thrombin or the procoagulant function of FXIa. We sought to confirm the antithrombotic activity of AB023 in a baboon thrombosis model and to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics in healthy adult subjects. Approach and Results- In a primate model of acute vascular graft thrombosis, AB023 reduced platelet and fibrin accumulation within the grafts by >75%. To evaluate the safety of AB023, we performed a first-in-human study in healthy adult volunteers without any serious adverse events. Overall, 10 of 21 (48%) subjects experienced 20 treatment-emergent adverse events, with 7 of 16 (44%) subjects following active treatment and 3 of 5 (60%) subjects following placebo. AB023 did not increase bleeding or prothrombin times. Anticoagulation was verified by a saturable ≈2-fold prolongation of the partial thromboplastin time for over 1 month after the highest dose. Conclusions- AB023, which inhibits contact activation-initiated blood coagulation in vitro and experimental thrombus formation in primates, produced a dose-dependent duration of limited anticoagulation without drug-related adverse effects in a phase 1 trial. When put in context with earlier observations suggesting that FXI contributes to venous thromboembolism and cardiovascular disease, although contributing minimally to hemostasis, our data further justify clinical evaluation of AB023 in conditions where contact-initiated FXI activation is suspected to have a pathogenic role. Clinical Trial Registration- URL: http://www.clinicaltrials.gov . Unique identifier: NCT03097341.

Factor XII Activation Promotes Platelet Consumption in the Presence of Bacterial-Type Long-Chain Polyphosphate In Vitro and In Vivo

Objective- Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood promotes thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo. Here, we sought to determine whether presence of long-chain polyP or bacteria in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner. Approach and Results- Long-chain polyP promoted platelet P-selectin expression, microaggregate formation, and platelet consumption in flowing whole blood in a contact activation pathway-dependent manner. Moreover, long-chain polyP promoted local fibrin formation on collagen under shear flow in a FXI-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the blood flow in a FXI- and FXII-dependent manner. In a murine model, long-chain polyP promoted platelet deposition and fibrin generation in lungs in a FXII-dependent manner. In a nonhuman primate model of bacterial sepsis, pre-treatment of animals with an antibody blocking FXI activation by FXIIa reduced lethal dose₁₀₀ Staphylococcus aureus-induced platelet and fibrinogen consumption. Conclusions- This study demonstrates that bacterial-type long-chain polyP promotes platelet activation in a FXII-dependent manner in flowing blood, which may contribute to sepsis-associated thrombotic processes, consumptive coagulopathy, and thrombocytopenia.

The Safety and Efficacy of Novel Agents Targeting Factors XI and XII in Early Phase Human Trials

Although anticoagulation without hemorrhage is a primary aim, this vision has remained as yet out of reach. Even despite the superior safety profile of the direct oral anticoagulants, hemorrhage remains a major risk of anticoagulation. Selective inhibition of the contact pathway of coagulation, specifically coagulation factor XI (FXI) and/or factor XII (FXII), has now substantial epidemiologic and preclinical data supporting the notion that these factors contribute to pathologic thrombosis and are yet primarily dispensable for in vivo hemostasis. In this way, targeting FXI and FXII may revolutionize the future anticoagulation landscape. Several drugs are under development for this purpose, including: ISIS 416858, a FXI antisense oligonucleotide which impairs hepatic synthesis of FXI; MAA868, a monoclonal antibody that binds the procoagulant enzymatic site of both zymogen and activated FXI (FXIa); BAY 1213790, a monoclonal antibody that binds the procoagulant enzymatic site of FXIa only; and AB023, a monoclonal antibody that inhibits activated FXII-mediated activation of FXI, along with two small molecules in clinical trials. Each of these drugs have demonstrated favorable safety profiles in their phases 1 and 2 studies to date, with preclinical data also supporting efficacy of abrogating thrombosis in various animal models. Other benefits of some of these drugs include once-monthly dosing and safety in patients with renal or hepatic impairment, while others offer quickly metabolized parenteral options, thus providing more convenient and widely available anticoagulation options. Though still far from the marketplace, drugs targeting FXI and FXII have the potential to usher in a new era of anticoagulation therapy.

Endothelial PAI-1 (Plasminogen Activator Inhibitor-1) Blocks the Intrinsic Pathway of Coagulation, Inducing the Clearance and Degradation of FXIa (Activated Factor XI)

Objective- Activation of coagulation FXI (factor XI) by FXIIa (activated factor XII) is a prothrombotic process. The endothelium is known to play an antithrombotic role by limiting thrombin generation and platelet activation. It is unknown whether the antithrombotic role of the endothelium includes sequestration of FXIa (activated factor XI) activity. This study aims to determine the role of endothelial cells (ECs) in the regulation of the intrinsic pathway of coagulation. Approach and Results- Using a chromogenic assay, we observed that human umbilical veins ECs selectively blocked FXIa yet supported kallikrein and FXIIa activity. Western blotting and mass spectrometry analyses revealed that FXIa formed a complex with endothelial PAI-1 (plasminogen activator inhibitor-1). Blocking endothelial PAI-1 increased the cleavage of a chromogenic substrate by FXIa and the capacity of FXIa to promote fibrin formation in plasma. Western blot and immunofluorescence analyses showed that FXIa-PAI-1 complexes were either released into the media or trafficked to the early and late endosomes and lysosomes of ECs. When baboons were challenged with Staphylococcus aureus to induce a prothrombotic phenotype, an increase in circulating FXIa-PAI-1 complex levels was detected by ELISA within 2 to 8 hours postchallenge. Conclusions- PAI-1 forms a complex with FXIa on ECs, blocking its activity and inducing the clearance and degradation of FXIa. Circulating FXIa-PAI-1 complexes were detected in a baboon model of S. aureus sepsis. Although ECs support kallikrein and FXIIa activity, inhibition of FXIa by ECs may promote the clearance of intravascular FXIa. Visual Overview- An online visual overview is available for this article.

Abstract 037: Coagulation Factor XII Promotes Platelet Consumption in the Presence of Microbial Polyphosphate Under Shear Flow

Background: Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood can promote thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo .

Aim: Determine whether presence of long-chain polyP in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner.

Methods/Results: The addition of long-chain polyP to human whole blood promoted platelet P-selectin expression, microaggregate formation and platelet consumption in the bloodstream under shear in a FXII-dependent manner. Moreover, long-chain polyP accelerated thrombus formation on immobilized collagen surfaces under shear flow in a thrombin generation-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the bloodstream. Inhibiting contact activation of coagulation using established and novel agents reduced fibrin formation on collagen as well as platelet consumption in the bloodstream distal to the site of thrombus formation. In vivo , FXII deficiency was protective against long-chain polyP occlusive lung thrombus formation in mice. Lastly, in a non-human primate model of sepsis, pretreatment of animals with an antibody blocking FXI activation by FXIIa (14E11) diminished LD 100 S. aureus -induced platelet and fibrinogen consumption.

Conclusions: This study demonstrates that bacterial-type long-chain polyP promotes FXII-mediated thrombin generation and platelet activation in the flowing blood and could contribute to sepsis-associated thrombotic processes, consumptive coagulopathy and thrombocytopenia.

Abstract 091: Coagulation Factor XII Promotes Platelet Consumption in the Presence of Microbial Polyphosphate Under Shear Flow

Background: Terminal complications of bacterial sepsis include development of disseminated intravascular consumptive coagulopathy. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway in flowing whole blood can promote thrombin generation and platelet activation and consumption distal to thrombus formation ex vivo and in vivo .

Aim: Determine whether presence of long-chain polyP in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner.

Methods/Results: The addition of long-chain polyP to human whole blood promoted platelet P-selectin expression, microaggregate formation and platelet consumption in the bloodstream under shear in a FXII-dependent manner. Moreover, long-chain polyP accelerated thrombus formation on immobilized collagen surfaces under shear flow in a thrombin generation-dependent manner. Distal to the site of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the bloodstream. Inhibiting contact activation of coagulation using established and novel agents reduced fibrin formation on collagen as well as platelet consumption in the bloodstream distal to the site of thrombus formation. In vivo , FXII deficiency was protective against long-chain polyP occlusive lung thrombus formation in mice. Lastly, in a non-human primate model of sepsis, pretreatment of animals with an antibody blocking FXI activation by FXIIa (14E11) diminished LD 100 S. aureus -induced platelet and fibrinogen consumption.

Conclusions: This study demonstrates that bacterial-type long-chain polyP promotes FXII-mediated thrombin generation and platelet activation in the flowing blood and could contribute to sepsis-associated thrombotic processes, consumptive coagulopathy and thrombocytopenia.

Abstract 362: Plasminogen Activator Inhibitor 1 Forms a Complex With FXIa on the Surface of Endothelial Cells, Blocking Its Activity and Inducing the Clearance, Internalization and Degradation of FXIa

Background: The activation of coagulation factor (F) XI by activated coagulation FXII (FXIIa) is a prothrombotic process. The endothelium is known to play an antithrombotic role by preventing thrombin generation and platelet activation. It is unknown whether the antithrombotic role of the endothelium includes regulation of activated FXI (FXIa) activity.

Hypothesis: Endothelial cells (ECs) express anticoagulant properties that block the procoagulant activity of FXIa.

Methods: We used a chromogenic assay to measure FXIa, kallikrein or FXIIa activity on cultured human umbilical vein ECs (HUVECs). To detect FXIa-inhibitor complexes in either the supernatant (SN) or cell lysate (CL), HUVECs were exposed to FXIa for 2 hrs at 4°C, then washed and lysed, followed by immunoprecipitation and western blot (WB) using an anti-FXIa mAb to the catalytic domain. To assess FXIa binding and internalization, HUVECs were exposed to FXIa for 2 hrs a 4C, washed and incubated at 37C for 5 min to 2 hrs, followed by fixation, and immunostaining with an anti-FXI antibody.

Results: HUVECs block the activity of FXIa but not kallikrein or FXIIa activity. The catalytic domain of FXIa (30 kDa) formed a covalent bond with one inhibitor generating one band at 80 kDa. The formation of FXIa-inhibitor complexes appeared in the SN when ECs were transferred to 37C. Mass spectrometry analysis of the 80kDa band revealed that FXIa forms a complex with plasminogen activator inhibitor 1 (PAI-1). WB for PAI-1 from the SN and CL detected a band at 80 kDa, while the expression of PAI-1 on the HUVEC surface decreased after incubation with FXIa. FXIa bound to the EC surface at 4C. After 5 mins at 37C, FXIa was internalized and localized beneath the plasma membrane, and after 30 mins to 2 hr at 37C, FXIa accumulated in the early and late endosomes and lysosomes. The serine protease inhibitor PPACK blocked FXIa-PAI-1 complex formation and internalization of FXIa. A blocking anti-PAI-1 antibody increased the cleavage of the chromogenic substrate by FXIa and the capacity of FXIa to promote fibrin formation on ECs in recalcified plasma.

Summary: FXIa forms a complex with PAI-1 on the surface of ECs blocking its activity. Inhibition of FXIa on the endothelium may support internalization and clearance of FXIa from the circulation.

Removal of the C-Terminal Domains of ADAMTS13 by Activated Coagulation Factor XI induces Platelet Adhesion on Endothelial Cells under Flow Conditions

Platelet recruitment to sites of vascular injury is mediated by von Willebrand factor (VWF). The shear-induced unraveling of ultra-large VWF multimers causes the formation of a “stringlike” conformation, which rapidly recruits platelets from the bloodstream. A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) regulates this process by cleaving VWF to prevent aberrant platelet adhesion; it is unclear whether the activity of ADAMTS13 itself is regulated. The serine proteases α-thrombin and plasmin have been shown to cleave ADAMTS13. Based on sequence homology, we hypothesized that activated coagulation factor XI (FXIa) would likewise cleave ADAMTS13. Our results show that FXIa cleaves ADAMTS13 at the C-terminal domains, generating a truncated ADAMTS13 with a deletion of part of the thrombospondin type-1 domain and the CUB1-2 domains, while α-thrombin cleaves ADAMTS13 near the CUB1-2 domains and plasmin cleaves ADAMTS13 at the metalloprotease domain and at the C-terminal domain. Using a cell surface immunoassay, we observed that FXIa induced the deletion of the CUB1-2 domains from ADAMTS13 on the surface of endothelial cells. Removal of the C-terminal domain of ADAMTS13 by FXIa or α-thrombin caused an increase in ADAMTS13 activity as measured by a fluorogenic substrate (FRETS) and blocked the ability of ADAMTS13 to cleave VWF on the endothelial cell surface, resulting in persistence of VWF strands and causing an increase in platelet adhesion under flow conditions. We have demonstrated a novel mechanism for coagulation proteinases including FXIa in regulating ADAMTS13 activity and function. This may represent an additional hemostatic function by which FXIa promotes local platelet deposition at sites of vessel injury.

Abstract 613: In Vitro Microvessels to Study the Platelet-Endothelium Interface

Background: Under normal conditions, endothelial cells (ECs) govern blood flow dynamics including providing a barrier between blood and tissue and regulating platelet aggregation and thrombin generation in the bloodstream. In turn, blood components, primarily platelets and coagulation factors such as thrombin, regulate EC barrier integrity. The breakdown of EC barrier function is a hallmark of a variety of vascular diseases. In sepsis, for example, the dysfunction of vascular ECs has been correlated with poorer outcomes due to hemorrhage and multi-organ failure associated with consumption of platelets and coagulation factors into clots within the microcirculation, a condition termed disseminated intravascular coagulation (DIC).

Aim: Develop an endothelialized flow chamber to study the platelet-endothelium interface.

Methods and Results: We developed a 3D-chamber with a perfuseable cylindrical microvessel embedded in an extracellular matrix (ECM) material. This model allows for the study of the role of thrombin generation and platelet aggregation in endothelial barrier leak development and repair in healthy as well as inflamed microvessels. Incorporation of subendothelial matrix proteins in these 3D-microvessel devices expands the capacity of the microfluidic studies to investigate blood cell extravasation and enables the control of physical parameters such as transmural pressure and interstitial flow through the ECM.

Conclusion: This model may provide insight into the pathophysiology of different disease states and serve as an expedient platform for therapy design and testing.

The platelet-endothelium interface under shear flow. Diagram ( A ) and an experimental prototype ( B ) of a 3D-perfuseable device. Microvessel phenotype (following treatment with vehicle or 10 ng/mL TNFα) pre- and post- perfusion with recalcified whole blood for 33 min as visualized by differential interference contrast, DIC, ( C ) and fluorescence microscopy ( D ).

Abstract 342: Coagulation Factor XII Promotes Platelet Consumption in the Presence of Microbial Polyphosphate Under Shear Flow

Background: Terminal complications of bacterial sepsis include development of consumptive coagulopathy referred to as disseminated intravascular coagulation. Bacterial constituents, including long-chain polyphosphates (polyP), have been shown to activate the contact pathway of coagulation in plasma. Recent work shows that activation of the contact pathway is capable of promoting thrombin generation and platelet activation and consumption in whole blood distal to thrombus formation under shear ex vivo and in vivo .

Aim: Test the hypothesis that the presence of long-chain polyP in the bloodstream promotes platelet activation and consumption in a coagulation factor (F)XII-dependent manner.

Methods and Results: Presence of long-chain polyP in whole blood promoted platelet aggregation on immobilized collagen surfaces under shear flow. Long-chain polyP enhanced fibrin formation and shortened clotting times of plasma and whole blood. The addition of long-chain polyP promoted platelet P-selectin expression, microaggregate formation and platelet consumption in the bloodstream under shear in a FXII-dependent manner. Moreover, long-chain polyP accelerated thrombus formation on immobilized collagen surfaces under shear flow. Distal to the sites of thrombus formation, platelet consumption was dramatically enhanced in the presence of long-chain polyP in the bloodstream. Inhibiting contact activation of the coagulation pathway reduced fibrin formation on collagen as well as platelet consumption in the bloodstream distal to the site of thrombus formation.

Conclusions: This study demonstrates that bacterial-type long-chain polyP promotes FXII-mediated thrombin generation and platelet activation in the flowing blood and could exaggerate sepsis-associated thrombotic processes, consumptive coagulopathy and thrombocytopenia.

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

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

Activated protein C inhibits neutrophil extracellular trap formation in vitro and activation in vivo

Activated protein C (APC) is a multifunctional serine protease with anticoagulant, cytoprotective, and anti-inflammatory activities. In addition to the cytoprotective effects of APC on endothelial cells, podocytes, and neurons, APC cleaves and detoxifies extracellular histones, a major component of neutrophil extracellular traps (NETs). NETs promote pathogen clearance but also can lead to thrombosis; the pathways that negatively regulate NETosis are largely unknown. Thus, we studied whether APC is capable of directly inhibiting NETosis via receptor-mediated cell signaling mechanisms. Here, by quantifying extracellular DNA or myeloperoxidase, we demonstrate that APC binds human leukocytes and prevents activated platelet supernatant or phorbol 12-myristate 13-acetate (PMA) from inducing NETosis. Of note, APC proteolytic activity was required for inhibiting NETosis. Moreover, antibodies against the neutrophil receptors endothelial protein C receptor (EPCR), protease-activated receptor 3 (PAR3), and macrophage-1 antigen (Mac-1) blocked APC inhibition of NETosis. Select mutations in the Gla and protease domains of recombinant APC caused a loss of NETosis. Interestingly, pretreatment of neutrophils with APC prior to induction of NETosis inhibited platelet adhesion to NETs. Lastly, in a nonhuman primate model of Escherichia coli-induced sepsis, pretreatment of animals with APC abrogated release of myeloperoxidase from neutrophils, a marker of neutrophil activation. These findings suggest that the anti-inflammatory function of APC at therapeutic concentrations may include the inhibition of NETosis in an EPCR-, PAR3-, and Mac-1-dependent manner, providing additional mechanistic insight into the diverse functions of neutrophils and APC in disease states including sepsis.

Utility of microfluidic devices to study the platelet-endothelium interface

The integration of biomaterials and understanding of vascular biology has led to the development of perfusable endothelialized flow models, which have been used as valuable tools to study the platelet-endothelium interface under shear. In these models, the parameters of geometry, compliance, biorheology, and cellular complexity are varied to recapitulate the physical biology of platelet recruitment and activation under physiologically relevant conditions of blood flow. In this review, we summarize the mechanistic insights learned from perfusable microvessel models and discuss the potential utility as well as challenges of endothelialized microfluidic devices to study platelet function in the bloodstream in vitro.

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.

Nucleic acids as cofactors for factor XI and prekallikrein activation: Different roles for high-molecular-weight kininogen

The plasma zymogens factor XI (fXI) and prekallikrein (PK) are activated by factor XIIa (fXIIa) during contact activation. Polyanions such as DNA and RNA may contribute to thrombosis and inflammation partly by enhancing PK and fXI activation. We examined PK and fXI activation in the presence of nucleic acids, and determine the effects of the cofactor high molecular weight kininogen (HK) on the reactions. In the absence of HK, DNA and RNA induced fXI autoactivation. Proteases known to activate fXI (fXIIa and thrombin) did not enhance this process appreciably. Nucleic acids had little effect on PK activation by fXIIa in the absence of HK. HK had significant but opposite effects on PK and fXI activation. HK enhanced fXIIa activation of PK in the presence of nucleic acids, but blocked fXI autoactivation. Thrombin and fXIIa could overcome the HK inhibitory effect on autoactivation, indicating these proteases are necessary for nucleic acid-induced fXI activation in an HK-rich environment such as plasma. In contrast to PK, which requires HK for optimal activation, fXI activation in the presence of nucleic acids depends on anion binding sites on the fXI molecule. The corresponding sites on PK are not necessary for PK activation. Our results indicate that HK functions as a cofactor for PK activation in the presence of nucleic acids in a manner consistent with classic models of contact activation. However, HK has, on balance, an inhibitory effect on nucleic acid-supported fXI activation and may function as a negative regulator of fXI activation.

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.

Colocalization of neutrophils, extracellular DNA and coagulation factors during NETosis: Development and utility of an immunofluorescence-based microscopy platform

BACKGROUND

Neutrophils, the most populous innate immune cell type, are the first responders to sites of infection and inflammation. Neutrophils can release their DNA to form extracellular traps (NETs), webs of DNA and granular proteases that contribute to pathogen clearance and promote thrombus formation. At present, the study of NETs is in part limited to the qualitative analysis of fluorescence microscopy-based images, thus quantification of the interactions between NETs and coagulation factors remains ill-defined.

AIM

Develop a quantitative method to measure the spatial distribution of DNA and colocalization of coagulation factor binding to neutrophils and NETs utilizing fluorescence-based microscopy.

APPROACH

Human neutrophils were purified from peripheral blood, bound to fibronectin and treated with the PKC-activator phorbol myristate acetate (PMA) to induce neutrophil activation and NETs formation. Samples were incubated with purified coagulation factors or plasma before staining with a DNA-binding dye and coagulation factor-specific antibodies. The spatial distribution of DNA and coagulation factors was imaged via fluorescence microscopy and quantified via a custom-built MATLAB-based image analysis algorithm. The algorithm first established global thresholding parameters on a training set of fluorescence image data and then systematically quantified intensity profiles across treatment conditions. Quantitative comparison of treatment conditions was enabled through the normalization of fluorescent intensities using the number of cells per image to determine the percent and area of DNA and coagulation factor binding per cell.

RESULTS

Upon stimulation with PMA, NETs formation resulted in an increase in the area of DNA per cell. The coagulation factor fibrinogen bound to both the neutrophil cell body as well as NETs, while prothrombin, FX and FVIIa binding was restricted to the neutrophil cell body. The Gla domain of FX was required to mediate FX-neutrophil binding. Activated protein C (APC), but not Gla-less APC, bound to neutrophil cell bodies and NETs in a punctate manner. Neither FXIIa nor FXIa were found to bind either neutrophil cell bodies or NETs. Fibrinogen binding was dependent on extracellular DNA, while FX and APC required phosphatidylserine exposure for binding to activated neutrophils.

CONCLUSIONS

We have developed a quantitative measurement platform to define the spatial localization of fluorescently-labeled coagulation factor binding to neutrophils and extracellular DNA during NETosis.

Group V secretory phospholipase A2 impairs endothelial protein C receptor-dependent protein C activation and accelerates thrombosis in vivo

BACKGROUND

Endothelial protein C receptor (EPCR) must be bound to a molecule of phosphatidylcholine (PC) to be fully functional, i.e. to interact with protein C/activated protein C (APC) properly. PC can be replaced with other lipids, such as lysophosphatidylcholine or platelet-activating factor, by the action of group V secretory phospholipase A2 (sPLA2-V), an enzyme that is upregulated in a variety of inflammatory conditions. Studies in purified systems have demonstrated that the substitution of PC notably impairs EPCR function in a process called EPCR encryption.

OBJECTIVES

To analyze whether sPLA2-V was able to regulate EPCR-dependent protein C activation in vivo, and its impact on thrombosis and the hemostatic system.

METHODS

Mice were transfected with sPLA2-V by hydrodynamic gene delivery. The effects on thrombosis were studied with the laser carotid artery occlusion model, and APC generation capacity was measured with ELISA. Global hemostasis was analyzed with thromboelastometry.

RESULTS

We found that sPLA2-V overexpression in mice significantly decreased their ability to generate APC. Furthermore, a murine carotid artery laser thrombosis model revealed that higher sPLA2-V levels were directly associated with faster artery thrombosis.

CONCLUSIONS

sPLA2-V plays a thrombogenic role by impairing the ability of EPCR to promote protein C activation.

Factor XII promotes blood coagulation independent of factor XI in the presence of long-chain polyphosphates

BACKGROUND

Inorganic polyphosphates (polyP), which are secreted by activated platelets (short-chain polyP) and accumulate in some bacteria (long-chain polyP), support the contact activation of factor XII (FXII) and accelerate the activation of FXI.

OBJECTIVES

The aim of the present study was to evaluate the role of FXI in polyP-mediated coagulation activation and experimental thrombus formation.

METHODS AND RESULTS

Pretreatment of plasma with antibodies that selectively inhibit FXI activation by activated FXII (FXIIa) or FIX) activation by activated FXI (FXIa) were not able to inhibit the procoagulant effect of long or short-chain polyP in plasma. In contrast, the FXIIa inhibitor, corn trypsin inhibitor, blocked the procoagulant effect of long and short polyP in plasma. In a purified system, long polyP significantly enhanced the rate of FXII and prekallikrein activation and the activation of FXI by thrombin but not by FXIIa. In FXI-deficient plasma, long polyP promoted clotting of plasma in an FIX-dependent manner. In a purified system, the activation of FXII and prekallikrein by long polyP promoted FIX activation and prothombin activation. In an ex vivo model of occlusive thrombus formation, inhibition of FXIIa with corn trypsin inhibitor but not of FXI with a neutralizing antibodies abolished the prothrombotic effect of long polyP.

CONCLUSIONS

We propose that long polyP promotes FXII-mediated blood coagulation bypassing FXI. Accordingly, some polyp-containing pathogens may have evolved strategies to exploit polyP-initiated FXII activation for virulence, and selective inhibition of FXII may improve the host response to pathogens.

Is EPCR a multi-ligand receptor? Pros and cons

In the last decade, the endothelial cell protein C/activated protein C receptor (EPCR) has received considerable attention. The role initially attributed to EPCR, i.e. the enhancement of protein C (PC) activation by the thrombin-thrombomodulin complex on the surface of the large vessels, although important, did not go beyond the haemostasis scenario. However, the discovery of the cytoprotective, anti-inflammatory and anti-apoptotic features of the activated PC (APC) and the required involvement of EPCR for APC to exert such actions did place the receptor in a privileged position in the crosstalk between coagulation and inflammation. The last five years have shown that PC/APC are not the only molecules able to interact with EPCR. Factor VII/VIIa (FVII/VIIa) and factor Xa (FXa), two other serine proteases that play a central role in haemostasis and are also involved in signalling processes influencing wound healing, tissue remodelling, inflammation or metastasis, have been reported to bind to EPCR. These observations have paved the way for an exploration of unsuspected new roles for the receptor. This review aims to offer a new image of EPCR in the light of its extended panel of ligands. A brief update of what is known about the APC-evoked EPCR-dependent cell signalling mechanisms is provided, but special care has been taken to assemble all the information available about the interaction of EPCR with FVII/VIIa and FXa.

Blocking endothelial protein C receptor (EPCR) accelerates thrombus development in vivo

The endothelial protein C receptor (EPCR) plays an anticoagulant role by improving protein C activation. Although low levels of activated protein C (APC) constitute a thrombosis risk factor, the relationship between modulating EPCR function and thrombosis has not been addressed so far. Monoclonal antibodies (mAb) against murine EPCR were raised, and their ability to block protein C/APC binding was tested. The ferric chloride carotid artery injury model in mice was chosen to test the effect of anti-EPCR mAb on thrombus formation. The time to total occlusion of the vessel was analysed in three groups, given an isotype control mAb (IC), a blocking (RCR-16) or a non-blocking (RCR-20) anti-EPCR mAb. RCR-16 prevented the interaction between protein C/APC and EPCR as demonstrated by surface plasmon resonance and flow cytometry, and inhibited the activation of protein C on the endothelium. IC and RCR-20 were unable to induce such effects. In vivo , RCR-16 shortened the time to total vessel occlusion with respect to IC [13.4 +/- 1.0 (mean +/- SD) and 17.8 +/- 3.2 minutes, respectively, p<0.001]. Occlusive thrombi lasting for more than one hour were observed in all RCR-16-treated animals, but only in 43% of IC-treated ones. Results with RCR-20 were indistinguishable from those observed with IC. For the first time, a direct relationship between blocking EPCR and thrombosis is demonstrated. Blocking anti-EPCR autoantibodies can predispose to thrombosis episodes and may constitute a new therapeutic target.

Binding of factor VIIa to the endothelial cell protein C receptor reduces its coagulant activity

BACKGROUND

Endothelial cell protein C receptor (EPCR) binds protein C through its gamma-carboxyglutamic acid (Gla) domain and enhances its thrombin-thrombomodulin complex-dependent activation. So far, only protein C/activated protein C has been shown to interact with EPCR. Factor VII (FVII), the coagulation trigger upon tissue factor (TF) interaction, is a serine protease whose Gla domain is highly homologous to the Gla domain of protein C.

OBJECTIVES

To characterize the binding of FVII/FVIIa to EPCR and its functional consequences.

METHODS AND RESULTS

We demonstrated by surface plasmon resonance (SPR) that FVII/FVIIa binds to EPCR through its Gla domain. At therapeutic concentrations, FVIIa reduced the activation of protein C by 40%. Soluble EPCR (sEPCR) was also able to prolong dose-dependently the clotting time induced by the FVIIa-TF complex. SPR and amidolytic experiments showed that FVIIa is able to interact simultaneously with TF and EPCR, thus ruling out the possibility that the effect of EPCR on clotting time was due to the inhibition of the binding between FVIIa and TF. sEPCR inhibited dose-dependently the activation of FX by the FVIIa-TF complex. Notably, blocking the binding site of EPCR on the endothelial surface increased the generation of FXa 2-fold.

CONCLUSIONS

EPCR binds to FVII/FVIIa and inhibits the procoagulant activity of the FVIIa-TF complex.