Activation of factor XI by products of prothrombin activation

Thrombin activation of the factor XI dimer is a multistaged process for each subunit

BACKGROUND

Factor (F)XI can be activated by proteases, including thrombin and FXIIa. The interactions of these enzymes with FXI are transient in nature and therefore difficult to study.

OBJECTIVES

To identify the binding interface between thrombin and FXI and understand the dynamics underlying FXI activation.

METHODS

Crosslinking mass spectrometry was used to localize the binding interface of thrombin on FXI. Molecular dynamics simulations were applied to investigate conformational changes enabling thrombin-mediated FXI activation after binding. The proposed trajectory of activation was examined with nanobody 1C10, which was previously shown to inhibit thrombin-mediated activation of FXI.

RESULTS

We identified a binding interface of thrombin located on the light chain of FXI involving residue Pro520. After this initial interaction, FXI undergoes conformational changes driven by binding of thrombin to the apple 1 domain in a secondary step to allow migration toward the FXI cleavage site. The 1C10 binding site on the apple 1 domain supports this proposed trajectory of thrombin. We validated the results with known mutation sites on FXI. As Pro520 is conserved in prekallikrein (PK), we hypothesized and showed that thrombin can bind PK, even though it cannot activate PK.

CONCLUSION

Our investigations show that the activation of FXI is a multistaged procedure. Thrombin first binds to Pro520 in FXI; thereafter, it migrates toward the activation site by engaging the apple 1 domain. This detailed analysis of the interaction between thrombin and FXI paves a way for future interventions for bleeding or thrombosis.

Impact of genetic structural variants in factor XI deficiency: identification, accurate characterization, and inferred mechanism by long-read sequencing

BACKGROUND

Congenital factor XI (FXI) deficiency is a probably underestimated coagulopathy that confers antithrombotic protection. Characterization of genetic defects in F11 is mainly focused on the identification of single-nucleotide variants and small insertion/deletions because they represent up to 99% of the alterations accounting for factor deficiency, with only 3 gross gene defects of structural variants (SVs) having been described.

OBJECTIVES

To identify and characterize the SVs affecting F11.

METHODS

The study was performed in 93 unrelated subjects with FXI deficiency recruited in Spanish hospitals over a period of 25 years (1997-2022). F11 was analyzed by next-generation sequencing, multiplex ligand probe amplification, and long-read sequencing.

RESULTS

Our study identified 30 different genetic variants. Interestingly, we found 3 SVs, all heterozygous: a complex duplication affecting exons 8 and 9, a tandem duplication of exon 14, and a large deletion affecting the whole gene. Nucleotide resolution obtained by long-read sequencing revealed Alu repetitive elements involved in all breakpoints. The large deletion was probably generated de novo in the paternal allele during gametogenesis, and despite affecting 30 additional genes, no syndromic features were described.

CONCLUSION

SVs may account for a high proportion of F11 genetic defects implicated in the molecular pathology of congenital FXI deficiency. These SVs, likely caused by a nonallelic homologous recombination involving repetitive elements, are heterogeneous in both type and length and may be de novo. These data support the inclusion of methods to detect SVs in this disorder, with long-read-based methods being the most appropriate because they detect all SVs and achieve adequate nucleotide resolution.

A site on factor XII required for productive interactions with polyphosphate

BACKGROUND

During plasma contact activation, factor XII (FXII) binds to surfaces through its heavy chain and undergoes conversion to the protease FXIIa. FXIIa activates prekallikrein and factor XI (FXI). Recently, we showed that the FXII first epidermal growth factor-1 (EGF1) domain is required for normal activity when polyphosphate is used as a surface.

OBJECTIVES

The aim of this study was to identify amino acids in the FXII EGF1 domain required for polyphosphate-dependent FXII functions.

METHODS

FXII with alanine substitutions for basic residues in the EGF1 domain were expressed in HEK293 fibroblasts. Wild-type FXII (FXII-WT) and FXII containing the EGF1 domain from the related protein Pro-HGFA (FXII-EGF1) were positive and negative controls. Proteins were tested for their capacity to be activated, and to activate prekallikrein and FXI, with or without polyphosphate, and to replace FXII-WT in plasma clotting assays and a mouse thrombosis model.

RESULTS

FXII and all FXII variants were activated similarly by kallikrein in the absence of polyphosphate. However, FXII with alanine replacing Lys73, Lys74, and Lys76 (FXII-Ala73,74,76) or Lys76, His78, and Lys81 (FXII-Ala76,78,81) were activated poorly in the presence of polyphosphate. Both have <5% of normal FXII activity in silica-triggered plasma clotting assays and have reduced binding affinity for polyphosphate. Activated FXIIa-Ala73,74,76 displayed profound defects in surface-dependent FXI activation in purified and plasma systems. FXIIa-Ala73,74,76 reconstituted FXII-deficient mice poorly in an arterial thrombosis model.

CONCLUSION

FXII Lys73, Lys74, Lys76, and Lys81 form a binding site for polyanionic substances such as polyphosphate that is required for surface-dependent FXII function.

Allosteric modulation of exosite 1 attenuates polyphosphate-catalyzed activation of factor XI by thrombin

BACKGROUND

Polyphosphate (polyP) promotes feedback activation of factor (F) XI by thrombin by serving as a template. The contribution of thrombin's exosites to these interactions is unclear.

OBJECTIVES

To determine the contribution of thrombin exosites 1 and 2 to polyP-induced potentiation of FXI activation by thrombin.

METHODS

The affinities of α-thrombin; K109E/110E-thrombin, an exosite 1 variant, or R93E-thrombin, an exosite 2 variant; FXI; and FXIa for polyP-70 were quantified using surface plasmon resonance in the absence or presence of exosite ligands. FXI was activated with α-thrombin or thrombin variants in the absence or presence of polyP-70 and exosite ligands.

RESULTS

α-Thrombin, K109/110E-thrombin, FXI, and FXIa bound polyP-70, whereas R93E-thrombin exhibited minimal binding. Exosite 1 and exosite 2 ligands attenuated thrombin binding to polyP-70. PolyP-70 accelerated the rate of FXI activation by α-thrombin and K109E/110E-thrombin but not R93E-thrombin up to 1500-fold in a bell-shaped, concentration-responsive manner. Exosite 1 and exosite 2 ligands had no impact on FXI activation by thrombin in the absence of polyP-70; however, in its presence, they attenuated activation by 40% to 65%.

CONCLUSION

PolyP-70 binds FXI and thrombin and promotes their interaction. Exosite 2 ligands attenuate activation because thrombin binds polyP-70 via exosite 2. Attenuation of FXI activation by exosite 1 ligands likely reflects allosteric modulation of exosite 2 and/or the active site of thrombin because exosite 1 is not directly involved in FXI activation. Therefore, allosteric modulation of thrombin's exosites may represent a novel strategy for downregulating FXI activation.

SARS-CoV-2 Spike Proteins and Cell-Cell Communication Inhibits TFPI and Induces Thrombogenic Factors in Human Lung Microvascular Endothelial Cells and Neutrophils: Implications for COVID-19 Coagulopathy Pathogenesis

In SARS-CoV-2-infected humans, disease progression is often associated with acute respiratory distress syndrome involving severe lung injury, coagulopathy, and thrombosis of the alveolar capillaries. The pathogenesis of these pulmonary complications in COVID-19 patients has not been elucidated. Autopsy study of these patients showed SARS-CoV-2 virions in pulmonary vessels and sequestrated leukocytes infiltrates associated with endotheliopathy and microvascular thrombosis. Since SARS-CoV-2 enters and infects target cells by binding its spike (S) protein to cellular angiotensin-converting enzyme 2 (ACE2), and there is evidence that vascular endothelial cells and neutrophils express ACE2, we investigated the effect of S-proteins and cell-cell communication on primary human lung microvascular endothelial cells (HLMEC) and neutrophils expression of thrombogenic factors and the potential mechanisms. Using S-proteins of two different SARS-CoV-2 variants (Wuhan and Delta), we demonstrate that exposure of HLMEC or neutrophils to S-proteins, co-culture of HLMEC exposed to S-proteins with non-exposed neutrophils, or co-culture of neutrophils exposed to S-proteins with non-exposed HLMEC induced transcriptional upregulation of tissue factor (TF), significantly increased the expression and secretion of factor (F)-V, thrombin, and fibrinogen and inhibited tissue factor pathway inhibitor (TFPI), the primary regulator of the extrinsic pathway of blood coagulation, in both cell types. Recombinant (r)TFPI and a thiol blocker (5,5'-dithio-bis-(2-nitrobenzoic acid)) prevented S-protein-induced expression and secretion of Factor-V, thrombin, and fibrinogen. Thrombomodulin blocked S-protein-induced expression and secretion of fibrinogen but had no effect on S-protein-induced expression of Factor-V or thrombin. These results suggests that following SARS-CoV-2 contact with the pulmonary endothelium or neutrophils and endothelial-neutrophil interactions, viral S-proteins induce coagulopathy via the TF pathway and mechanisms involving functional thiol groups. These findings suggest that using rTFPI and/or thiol-based drugs could be a viable therapeutic strategy against SARS-CoV-2-induced coagulopathy and thrombosis.

Contact and intrinsic coagulation pathways are activated and associated with adverse clinical outcomes in COVID-19

Coagulation activation is a prominent feature of severe acute respiratory syndrome coronavirus 2 (COVID-19) infection. Activation of the contact system and intrinsic pathway has increasingly been implicated in the prothrombotic state observed in both sterile and infectious inflammatory conditions. We therefore sought to assess activation of the contact system and intrinsic pathway in individuals with COVID-19 infection. Baseline plasma levels of protease:serpin complexes indicative of activation of the contact and intrinsic pathways were measured in samples from inpatients with COVID-19 and healthy individuals. Cleaved kininogen, a surrogate for bradykinin release, was measured by enzyme-linked immunosorbent assay, and extrinsic pathway activation was assessed by microvesicle tissue factor-mediated factor Xa (FXa; MVTF) generation. Samples were collected within 24 hours of COVID-19 diagnosis. Thirty patients with COVID-19 and 30 age- and sex-matched controls were enrolled. Contact system and intrinsic pathway activation in COVID-19 was demonstrated by increased plasma levels of FXIIa:C1 esterase inhibitor (C1), kallikrein:C1, FXIa:C1, FXIa:α1-antitrypsin, and FIXa:antithrombin (AT). MVTF levels were also increased in patients with COVID-19. Because FIXa:AT levels were associated with both contact/intrinsic pathway complexes and MVTF, activation of FIX likely occurs through both contact/intrinsic and extrinsic pathways. Among the protease:serpin complexes measured, FIXa:AT complexes were uniquely associated with clinical indices of disease severity, specifically total length of hospitalization, length of intensive care unit stay, and extent of lung computed tomography changes. We conclude that the contact/intrinsic pathway may contribute to the pathogenesis of the prothrombotic state in COVID-19. Larger prospective studies are required to confirm whether FIXa:AT complexes are a clinically useful biomarker of adverse clinical outcomes.

Anti-HK antibody reveals critical roles of a 20-residue HK region for Aβ-induced plasma contact system activation

Alzheimer's disease (AD) is a neurodegenerative disorder and the leading cause of dementia. Vascular abnormalities and neuroinflammation play roles in AD pathogenesis. Plasma contact activation, which leads to fibrin clot formation and bradykinin release, is elevated in many AD patients, likely due to the ability of AD's pathogenic peptide β-amyloid (Aβ) to induce its activation. Since overactivation of this system may be deleterious to AD patients, the development of inhibitors could be beneficial. Here, we show that 3E8, an antibody against a 20-amino acid region of high molecular weight kininogen's (HK) domain 6, inhibits Aβ-induced intrinsic coagulation. Mechanistically, 3E8 inhibits contact system activation by blocking the binding of prekallikrein (PK) and factor XI (FXI) to HK, thereby preventing their activation and the continued activation of factor XII (FXII). The 3E8 antibody can also disassemble HK/PK and HK/FXI complexes in normal human plasma in the absence of a contact system activator due to its strong binding affinity for HK, indicating its prophylactic ability. Furthermore, the binding of Aβ to both FXII and HK is critical for Aβ-mediated contact system activation. These results suggest that a 20-amino acid region of HK's domain 6 plays a critical role in Aβ-induced contact system activation, and this region may provide an effective strategy to inhibit or prevent contact system activation in related disorders.

Milvexian, an orally bioavailable, small-molecule, reversible, direct inhibitor of factor XIa: In vitro studies and in vivo evaluation in experimental thrombosis in rabbits

BACKGROUND

Milvexian (BMS-986177/JNJ-70033093) is an orally bioavailable factor XIa (FXIa) inhibitor currently in phase 2 clinical trials.

OBJECTIVES

To evaluate in vitro properties and in vivo characteristics of milvexian.

METHODS

In vitro properties of milvexian were evaluated with coagulation and enzyme assays, and in vivo profiles were characterized with rabbit models of electrolytic-induced carotid arterial thrombosis and cuticle bleeding time (BT).

RESULTS

Milvexian is an active-site, reversible inhibitor of human and rabbit FXIa (Ki 0.11 and 0.38 nM, respectively). Milvexian increased activated partial thromboplastin time (APTT) without changing prothrombin time and potently prolonged plasma APTT in humans and rabbits. Milvexian did not alter platelet aggregation to ADP, arachidonic acid, or collagen. Milvexian was evaluated for in vivo prevention and treatment of thrombosis. For prevention, milvexian 0.063 + 0.04, 0.25 + 0.17, and 1 + 0.67 mg/kg+mg/kg/h preserved 32 ± 6*, 54 ± 10*, and 76 ± 5%* of carotid blood flow (CBF) and reduced thrombus weight by 15 ± 10*, 45 ± 2*, and 70 ± 4%*, respectively (*p < .05; n = 6/dose). For treatment, thrombosis was initiated for 15 min and CBF decreased to 40% of control. Seventy-five minutes after milvexian administration, CBF averaged 1 ± 0.3, 39 ± 10, and 66 ± 2%* in groups treated with vehicle and milvexian 0.25 + 0.17 and 1 + 0.67 mg/kg+mg/kg/h, respectively (*p < .05 vs. vehicle; n = 6/group). The combination of milvexian 1 + 0.67 mg/kg+mg/kg/h and aspirin 4 mg/kg/h intravenous did not increase BT versus aspirin monotherapy.

CONCLUSIONS

Milvexian is an effective antithrombotic agent with limited impact on hemostasis, even when combined with aspirin in rabbits. This study supports inhibition of FXIa with milvexian as a promising antithrombotic therapy with a wide therapeutic window.

Structural insights into the activation of blood coagulation factor XI zymogen by thrombin: A computational molecular dynamics study

Activation of human blood coagulation factor XI zymogen to factor XIa plays a significant role in the upstream coagulation pathway, in which factor XIa activates factor IX zymogen. The mechanistic details of the proteolytic activation of factor XI by the activating enzyme thrombin are not well-understood at atomic level. In this study, we employed a combination of molecular docking and microsecond time-scale molecular dynamics simulations to identify the key regions of interaction between fXI and thrombin. The activating complex between the substrate and enzyme was modeled to represent the initial acylation step of the serine-protease hydrolysis mechanism. The proposed solution structural complex, fIX:fIIa, obtained from 3 microseconds of MD refinement, suggests that the activation of factor XI is mediated by thrombin's anion binding exosite-II interactions with A3 and A4 domains. We predict that the two positively charged arginine residues (Arg409 and Arg413) in the exosite-2 region, the β- and γ-insertion loops of thrombin play an important structural role in the initial activating complex between fXI and thrombin.

DNase 1 Protects From Increased Thrombin Generation and Venous Thrombosis During Aging: Cross-Sectional Study in Mice and Humans

Background Human aging is associated with increased risk of thrombosis, but the mechanisms are poorly defined. We hypothesized that aging induces peroxide-dependent release of neutrophil extracellular traps that contribute to thrombin generation and thrombosis. Methods and Results We studied C57BL6J mice and littermates of glutathione peroxidase-1 transgenic and wild-type mice at young (4 month) and old (20 month) ages and a healthy cohort of young (18-39 years) or middle-aged/older (50-72 years) humans. In plasma, we measured thrombin generation potential and components of neutrophil extracellular traps (cell-free DNA and citrullinated histone). Aged wild-type mice displayed a significant increase in thrombin generation that was decreased in aged glutathione peroxidase-1 transgenic mice. Both aged wild-type and aged glutathione peroxidase-1 transgenic mice demonstrated similar elevation of plasma cell-free DNA compared with young mice. In contrast, plasma levels of citrullinated histone were not altered with age or genotype. Release of neutrophil extracellular traps from neutrophils in vitro was also similar between young and aged wild-type or glutathione peroxidase-1 transgenic mice. Treatment of plasma or mice with DNase 1 decreased age-associated increases in thrombin generation, and DNase 1 treatment blocked the development of experimental venous thrombi in aged C57BL6J mice. Similarly, thrombin generation potential and plasma cell-free DNA, but not citrullinated histone, were higher in middle-aged/older humans, and treatment of plasma with DNase 1 reversed the increase in thrombin generation. Conclusions We conclude that DNase 1 limits thrombin generation and protects from venous thrombosis during aging, likely by hydrolyzing cell-free DNA.

The evolution of factor XI and the kallikrein-kinin system

Factor XI (FXI) is the zymogen of a plasma protease (FXIa) that contributes to hemostasis by activating factor IX (FIX). In the original cascade model of coagulation, FXI is converted to FXIa by factor XIIa (FXIIa), a component, along with prekallikrein and high-molecular-weight kininogen (HK), of the plasma kallikrein-kinin system (KKS). More recent coagulation models emphasize thrombin as a FXI activator, bypassing the need for FXIIa and the KKS. We took an evolutionary approach to better understand the relationship of FXI to the KKS and thrombin generation. BLAST searches were conducted for FXI, FXII, prekallikrein, and HK using genomes for multiple vertebrate species. The analysis shows the KKS appeared in lobe-finned fish, the ancestors of all land vertebrates. FXI arose later from a duplication of the prekallikrein gene early in mammalian evolution. Features of FXI that facilitate efficient FIX activation are present in all living mammals, including primitive egg-laying monotremes, and may represent enhancement of FIX-activating activity inherent in prekallikrein. FXI activation by thrombin is a more recent acquisition, appearing in placental mammals. These findings suggest FXI activation by FXIIa may be more important to hemostasis in primitive mammals than in placental mammals. FXI activation by thrombin places FXI partially under control of the vitamin K-dependent coagulation mechanism, reducing the importance of the KKS in blood coagulation. This would explain why humans with FXI deficiency have a bleeding abnormality, whereas those lacking components of the KKS do not.

Activated Factor XI is Increased in Plasma in Response to Surgical Trauma but not to Recombinant Activated FVII-Induced Thrombin Formation

Aim: Feedback activation of factor XI (FXI) by thrombin is believed to play a critical role in the amplification phase of thrombin generation and to contribute to thrombosis development and hemostasis. However, the activation of FXI by thrombin has been shown in vitro to require a cofactor. In this study, the role of thrombin in activated FXI (FXIa) formation in vivo is investigated.

Methods: The study population comprised probands in whom coagulation activation was triggered by low-dose (15 µg/kg) recombinant activated factor VII (rFVIIa, n =89), of whom 34 with (VTE+) and 45 without a history of venous thromboembolism (VTE−), and patients undergoing major orthopedic surgeries ( n =45). FXIa was quantified via an enzyme capture assay using a monoclonal FXI-specific antibody. Thrombin formation was monitored using an oligonucleotide-based enzyme capture assay and the thrombin activation markers prothrombin fragment 1＋2 (F1＋2) and thrombin–antithrombin complex (TAT).

Results: In the rFVIIa cohort, FXIa and thrombin remained below their lower limit of quantification of 3.48 and 1.06 pmol/L, respectively. By contrast, during the surgeries, median FXIa levels increased from 3.69 pmol/L pre-operatively to 9.41 pmol/L mid-operatively ( P =4·10 ⁻⁴ ) and remained significantly elevated 24 h thereafter, with 9.38 pmol/L ( P =0.001). Peak levels of F1+2 were comparable in the VTE+, VTE−, and surgery cohort (235, 268, and 253 pmol/L), whereas peak TAT levels were higher in the surgery cohort (53.1, 33.9, and 147.6 pmol/L).

Conclusions: Under in vivo conditions, the activation of FXI requires specific local features that are present at the wounded site including potential cofactors of thrombin.

Clot activators and anticoagulant additives for blood collection. A critical review on behalf of COLABIOCLI WG-PRE-LATAM

In the clinical laboratory, knowledge of and the correct use of clot activators and anticoagulant additives are critical to preserve and maintain samples in optimal conditions prior to analysis. In 2017, the Latin America Confederation of Clinical Biochemistry (COLABIOCLI) commissioned the Latin American Working Group for Preanalytical Phase (WG-PRE-LATAM) to study preanalytical variability and establish guidelines for preanalytical procedures to be applied by clinical laboratories and health care professionals. The aim of this critical review, on behalf of COLABIOCLI WG-PRE-LATAM, is to provide information to understand the mechanisms of the interactions and reactions that occur between blood and clot activators and anticoagulant additives inside evacuated tubes used for laboratory testing. Clot activators - glass, silica, kaolin, bentonite, and diatomaceous earth - work by surface dependent mechanism whereas extrinsic biomolecules - thrombin, snake venoms, ellagic acid, and thromboplastin - start in vitro coagulation when added to blood. Few manufacturers of evacuated tubes state the type and concentration of clot activators used in their products. With respect to anticoagulant additives, sodium citrate and oxalate complex free calcium and ethylenediaminetetraacetic acid chelates calcium. Heparin potentiates antithrombin and hirudin binds to active thrombin, inactivating the thrombin irreversibly. Blood collection tubes have improved continually over the years, from the glass tubes containing clot activators or anticoagulant additives that were prepared by laboratory personnel to the current standardized evacuated systems that permit more precise blood/additive ratios. Each clot activator and anticoagulant additive demonstrates specific functionality, and both manufacturers of tubes and laboratory professional strive to provide suitable interference-free sample matrices for laboratory testing. Both manufacturers of in vitro diagnostic devices and laboratory professionals need to understand all aspects of venous blood sampling so that they do not underestimate the impact of tube additives on laboratory testing.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

The synthetic LPS binding peptide 19-2.5 interferes with clotting and prevents degradation of high molecular weight kininogen in plasma

Sepsis and septic shock are life-threatening conditions and remain an important medical problem, emphasizing the need to identify novel therapeutic approaches. Coagulation dysfunction, hypotension, disturbed microcirculation and multiorgan failure occur frequently. These severe conditions result from an overwhelming inflammatory response, induced by pathogen and damage associated molecular patterns (PAMPs and DAMPs) released into the bloodstream. In the present study, we demonstrated that the synthetic Lipopolysaccharid (LPS)-binding peptide 19-2.5 interferes with the activation of the coagulation and contact system. Moreover, binding of LPS to high molecular weight kininogen (HK), one of the major LPS carrier in blood, could be prevented by the peptide. Thus, peptide 19-2.5 might represent a promising target in the treatment of endotoxemia and sepsis, not only by its anti-inflammatory potential, but also by the anticoagulant effect, together with its ability to prevent degradation of HK.

Protease activity in single-chain prekallikrein

Prekallikrein (PK) is the precursor of the trypsin-like plasma protease kallikrein (PKa), which cleaves kininogens to release bradykinin and converts the protease precursor factor XII (FXII) to the enzyme FXIIa. PK and FXII undergo reciprocal conversion to their active forms (PKa and FXIIa) by a process that is accelerated by a variety of biological and artificial surfaces. The surface-mediated process is referred to as contact activation. Previously, we showed that FXII expresses a low level of proteolytic activity (independently of FXIIa) that may initiate reciprocal activation with PK. The current study was undertaken to determine whether PK expresses similar activity. Recombinant PK that cannot be converted to PKa was prepared by replacing Arg371 with alanine at the activation cleavage site (PK-R371A, or single-chain PK). Despite being constrained to the single-chain precursor form, PK-R371A cleaves high-molecular-weight kininogen (HK) to release bradykinin with a catalytic efficiency ∼1500-fold lower than that of kallikrein cleavage of HK. In the presence of a surface, PK-R371A converts FXII to FXIIa with a specific activity ∼4 orders of magnitude lower than for PKa cleavage of FXII. These results support the notion that activity intrinsic to PK and FXII can initiate reciprocal activation of FXII and PK in solution or on a surface. The findings are consistent with the hypothesis that the putative zymogens of many trypsin-like proteases are actually active proteases, explaining their capacity to undergo processes such as autoactivation and to initiate enzyme cascades.

Rapid serum tube technology overcomes problems associated with use of anticoagulants

Introduction

Failure to obtain complete blood clotting in serum is a common laboratory problem. Our aim was to determine whether snake proth-rombin activators are effective in clotting blood and producing quality serum for analyte measurement in anticoagulated patients.

Materials and methods

Whole blood clotting was studied in a total of 64 blood samples (41 controls, 20 Warfarin patients, 3 anticoagulated patients using snake venom prothrombin activator (OsPA)) with plain tubes. Coagulation was analysed using a visual assay, Hyland-Clotek and thromboelastography. Healthy control blood was spiked with a range of anticoagulants to determine the effectiveness of OsPa-induced clotting. A paired analysis of a Dabigatran patient and a control investigated the effectiveness of the OsPA clotting tubes. Biochemical analytes (N = 31) were determined for 7 samples on chemistry and immunoassay analysers and compared with commercial tubes.

Results

Snake venom prothrombin activators efficiently coagulated blood and plasma spiked with heparin and commonly used anticoagulants. Clotting was observed in the presence of anticoagulants whereas no clotting was observed in BDRST tubes containing 3 U/mL of heparin. Snake venom prothrombin activator enhanced heparinised blood clotting by shortening substantially the clotting time and improving significantly the strength of the clot. Comparison of 31 analytes from the blood of five healthy and two anticoagulated participants gave very good agreement between the analyte concentrations determined.

Conclusions

Our results showed that the snake venom prothrombin activators OsPA and PtPA efficiently coagulated recalcified and fresh bloods with or without added anticoagulants. These procoagulants produced high quality serum for accurate analyte measurement.

The contact system at the crossroads of various key patho- physiological functions: Update on present understanding, laboratory exploration and future perspectives

The contact system initiates the intrinsic pathway of coagulation and is started by Factor XII activation, which then activates prekallicrein to kallicrein and Factor XI to Factor XIa and, in the presence of high molecular weight kininogen, forms a "contact phase activation loop", that amplifies Factor XII activation. FXII deficiency is not associated with bleeding tendencies, but when the blood clots, the thrombus is less dense, thus favoring antithrombotic protection. Activated Factor XII inhibition emerges as an efficient target for preventing thrombo-embolic diseases without inducing a hemorrhagic risk. Activated Factor XII exhibits other activities, in that it can activate complement and provoke inflammation, contributing to innate immunity. It also stimulates fibrinolysis through uPA activation from scu-PA. Among the other components of the contact phase, Factor XI has a more important role in coagulation pathways and can directly activate FX, FVIII and FV, in a FIX independent pathway. Its deficiency is associated with a mild bleeding diathesis ("pseudo-hemophilia" or hemophilia C), with a variable incidence among kindreds. Recently, the occurrence of thrombotic events the same day following infusion of immunoglobulin concentrates has been demonstrated to be caused by the presence of trace amounts of activated Factor XI, pointing out the key role of this factor for thrombogenicity. Prekallicrein can be activated at the endothelial surface in the presence of high molecular weight kininogen, whose cleavage generates bradykinins and contributes to vessel tonicity and inflammation. The contact phase, through its activation loop, is then an important physiological system, which can initiate and regulate various biological functions and is at the crossroads of various biological activities. Many of the body's physiological functions are intimately linked between them, making the global approach of special usefulness for understanding the interactions which can result from any abnormality of one of them. New pharmaceutical drugs targeting a defined activity need to be investigated for all the possible interferences or side effects. In this article we aim to present and summarize the present understanding of contact phase system activation and regulation, its involvement in various physiological functions, and the laboratory tools for its exploration.

Thromboinflammation: challenges of therapeutically targeting coagulation and other host defense mechanisms

Thrombosis with associated inflammation (thromboinflammation) occurs commonly in a broad range of human disorders. It is well recognized clinically in the context of superficial thrombophlebitis (thrombosis and inflammation of superficial veins); however, it is more dangerous when it develops in the microvasculature of injured tissues and organs. Microvascular thrombosis with associated inflammation is well recognized in the context of sepsis and ischemia-reperfusion injury; however, it also occurs in organ transplant rejection, major trauma, severe burns, the antiphospholipid syndrome, preeclampsia, sickle cell disease, and biomaterial-induced thromboinflammation. Central to thromboinflammation is the loss of the normal antithrombotic and anti-inflammatory functions of endothelial cells, leading to dysregulation of coagulation, complement, platelet activation, and leukocyte recruitment in the microvasculature. α-Thrombin plays a critical role in coordinating thrombotic and inflammatory responses and has long been considered an attractive therapeutic target to reduce thromboinflammatory complications. This review focuses on the role of basic aspects of coagulation and α-thrombin in promoting thromboinflammatory responses and discusses insights gained from clinical trials on the effects of various inhibitors of coagulation on thromboinflammatory disorders. Studies in sepsis patients have been particularly informative because, despite using anticoagulant approaches with different pharmacological profiles, which act at distinct points in the coagulation cascade, bleeding complications continue to undermine clinical benefit. Future advances may require the development of therapeutics with primary anti-inflammatory and cytoprotective properties, which have less impact on hemostasis. This may be possible with the growing recognition that components of blood coagulation and platelets have prothrombotic and proinflammatory functions independent of their hemostatic effects.

A mechanism for hereditary angioedema with normal C1 inhibitor: an inhibitory regulatory role for the factor XII heavy chain

The plasma proteins factor XII (FXII) and prekallikrein (PK) undergo reciprocal activation to the proteases FXIIa and kallikrein by a process that is enhanced by surfaces (contact activation) and regulated by the serpin C1 inhibitor. Kallikrein cleaves high-molecular-weight kininogen (HK), releasing the vasoactive peptide bradykinin. Patients with hereditary angioedema (HAE) experience episodes of soft tissue swelling as a consequence of unregulated kallikrein activity or increased prekallikrein activation. Although most HAE cases are caused by reduced plasma C1-inhibitor activity, HAE has been linked to lysine/arginine substitutions for Thr³⁰⁹ in FXII (FXII-Lys/Arg309). Here, we show that FXII-Lys/Arg309 is susceptible to cleavage after residue 309 by coagulation proteases (thrombin and FXIa), resulting in generation of a truncated form of FXII (δFXII). The catalytic efficiency of δFXII activation by kallikrein is 15-fold greater than for full-length FXII. The enhanced rate of reciprocal activation of PK and δFXII in human plasma and in mice appears to overwhelm the normal inhibitory function of C1 inhibitor, leading to increased HK cleavage. In mice given human FXII-Lys/Arg309, induction of thrombin generation by infusion of tissue factor results in enhanced HK cleavage as a consequence of δFXII formation. The effects of δFXII in vitro and in vivo are reproduced when wild-type FXII is bound by an antibody to the FXII heavy chain (HC; 15H8). The results contribute to our understanding of the predisposition of patients carrying FXII-Lys/Arg309 to angioedema after trauma, and reveal a regulatory function for the FXII HC that normally limits PK activation in plasma.

Plasma contact factors as therapeutic targets

Direct oral anticoagulants (DOACs) are small molecule inhibitors of the coagulation proteases thrombin and factor Xa that demonstrate comparable efficacy to warfarin for several common indications, while causing less serious bleeding. However, because their targets are required for the normal host-response to bleeding (hemostasis), DOACs are associated with therapy-induced bleeding that limits their use in certain patient populations and clinical situations. The plasma contact factors (factor XII, factor XI, and prekallikrein) initiate blood coagulation in the activated partial thromboplastin time assay. While serving limited roles in hemostasis, pre-clinical and epidemiologic data indicate that these proteins contribute to pathologic coagulation. It is anticipated that drugs targeting the contact factors will reduce risk of thrombosis with minimal impact on hemostasis. Here, we discuss the biochemistry of contact activation, the contributions of contact factors in thrombosis, and novel antithrombotic agents targeting contact factors that are undergoing pre-clinical and early clinical testing.

Factor XI promotes hemostasis in factor IX-deficient mice

Essentials Mice lacking factor IX (FIX) or factor XI (FXI) were tested in a saphenous vein bleeding model. FIX-deficient mice displayed a hemostatic defect and FXI-deficient mice were similar to wild type mice. Infusion of FXI or over-expression of FXI in FIX-deficient mice improved hemostasis. FXI may affect the phenotype of FIX-deficiency (hemophilia B).

SUMMARY

Background In humans, deficiency of coagulation factor XI may be associated with a bleeding disorder, but, until recently, FXI-deficient mice did not appear to have a hemostatic abnormality. A recent study, however, indicated that FXI-deficient mice show a moderate hemostatic defect in a saphenous vein bleeding (SVB) model. Objectives To study the effect of FXI on bleeding in mice with normal levels of the FXI substrate FIX and in mice lacking FIX (a murine model of hemophilia B). Methods Wild-type mice and mice lacking either FIX (F9- ) or FXI (F11-/- ) were tested in the SVB model. The plasma levels of FXI in F11-/- mice were manipulated by infusion of FXI or its active form FXIa, or by overexpressing FXI by the use of hydrodynamic tail vein injection. Results F9- mice showed a significant defect in the SVB model, whereas F11-/- mice and wild-type mice were indistinguishable. Intravenous infusion of FXI or FXIa into, or overexpression of FXI in, F9- mice improved hemostasis in the SVB model. Overexpression of a FXI variant lacking a FIX-binding site also improved hemostasis in F9- mice. Conclusions Although we were unable to demonstrate a hemostatic defect in F11-/- mice in the SVB model, our results support the premise that supraphysiological levels of FXI improve hemostasis in F9- mice through FIX-independent pathways.

Next-generation rapid serum tube technology using prothrombin activator coagulant: fast, high-quality serum from normal samples

Background

Incomplete blood clotting or latent clotting in serum is a common laboratory problem, especially for patients on anticoagulant therapy or when serum tubes are centrifuged before clotting is completed. We describe a novel approach to producing high-quality serum using snake venom prothrombin activator complex (OsPA) as an additive in blood collection tubes for non-anticoagulated (normal) individuals.

Methods

Plasma clotting assays were performed using a Hyland-Clotek instrument. Blood clotting was visually observed, and thromboelastography was also performed to determine the important parameters of coagulation. Thrombin generation was assayed using the chromogenic substrate S-2238, and biochemical analytes in the serum were determined on chemistry and immunoassay analysers. Fibrinogen was determined by either ELISA or Clauss fibrinogen assay.

Results

We initially showed that OsPA had strong coagulation activity in clotting not only recalcified citrated plasma and recalcified citrated whole blood, but also fresh whole blood in a clinical setting. The use of TEG clearly showed improved speed of clotting and generation of a firmer clot. We also showed that the use of OsPA to produce serum did not interfere with the determination of commonly measured biochemical analytes. The underlying clotting mechanism involves a burst of thrombin production at the initial stages of the clotting process upon contact with prothrombin in blood.

Conclusions

These results demonstrate rapid generation of high-quality serum, contributing to faster turnaround times with standardised quality samples, for accurate analyte determinations in normal individuals.

Peptidoglycan induces disseminated intravascular coagulation in baboons through activation of both coagulation pathways

Anthrax infections exhibit progressive coagulopathies that may contribute to the sepsis pathophysiology observed in fulminant disease. The hemostatic imbalance is recapitulated in primate models of late-stage disease but is uncommon in toxemic models, suggesting contribution of other bacterial pathogen-associated molecular patterns (PAMPs). Peptidoglycan (PGN) is a bacterial PAMP that engages cellular components at the cross talk between innate immunity and hemostasis. We hypothesized that PGN is critical for anthrax-induced coagulopathies and investigated the activation of blood coagulation in response to a sterile PGN infusion in primates. The PGN challenge, like the vegetative bacteria, induced a sepsis-like pathophysiology characterized by systemic inflammation, disseminated intravascular coagulation (DIC), organ dysfunction, and impaired survival. Importantly, the hemostatic impairment occurred early and in parallel with the inflammatory response, suggesting direct engagement of coagulation pathways. PGN infusion in baboons promoted early activation of contact factors evidenced by elevated protease-serpin complexes. Despite binding to contact factors, PGN did not directly activate either factor XII (FXII) or prekallikrein. PGN supported contact coagulation by enhancing enzymatic function of active FXII (FXIIa) and depressing its inhibition by antithrombin. In parallel, PGN induced de novo monocyte tissue factor expression in vitro and in vivo, promoting extrinsic clotting reactions at later stages. Activation of platelets further amplified the procoagulant state during PGN challenge, leading to DIC and subsequent ischemic damage of peripheral tissues. These data indicate that PGN may be a major cause for the pathophysiologic progression of Bacillus anthracis sepsis and is the primary PAMP behind the pathogen-induced coagulopathy in late-stage anthrax.

Modeling thrombosis in silico: Frontiers, challenges, unresolved problems and milestones

Hemostasis is a complex physiological mechanism that functions to maintain vascular integrity under any conditions. Its primary components are blood platelets and a coagulation network that interact to form the hemostatic plug, a combination of cell aggregate and gelatinous fibrin clot that stops bleeding upon vascular injury. Disorders of hemostasis result in bleeding or thrombosis, and are the major immediate cause of mortality and morbidity in the world. Regulation of hemostasis and thrombosis is immensely complex, as it depends on blood cell adhesion and mechanics, hydrodynamics and mass transport of various species, huge signal transduction networks in platelets, as well as spatiotemporal regulation of the blood coagulation network. Mathematical and computational modeling has been increasingly used to gain insight into this complexity over the last 30 years, but the limitations of the existing models remain profound. Here we review state-of-the-art-methods for computational modeling of thrombosis with the specific focus on the analysis of unresolved challenges. They include: a) fundamental issues related to physics of platelet aggregates and fibrin gels; b) computational challenges and limitations for solution of the models that combine cell adhesion, hydrodynamics and chemistry; c) biological mysteries and unknown parameters of processes; d) biophysical complexities of the spatiotemporal networks' regulation. Both relatively classical approaches and innovative computational techniques for their solution are considered; the subjects discussed with relation to thrombosis modeling include coarse-graining, continuum versus particle-based modeling, multiscale models, hybrid models, parameter estimation and others. Fundamental understanding gained from theoretical models are highlighted and a description of future prospects in the field and the nearest possible aims are given.

Prochemerin cleavage by factor XIa links coagulation and inflammation

Chemerin is a chemoattractant and adipokine that circulates in blood as inactive prochemerin (chem163S). Chem163S is activated by a series of C-terminal proteolytic cleavages resulting in diverse chemerin forms with different levels of activity. We screened a panel of proteases in the coagulation, fibrinolytic, and inflammatory cascades to identify those that process prochemerin in plasma. Factor XIa (FXIa) cleaved chem163S, generating a novel chemerin form, chem162R, as an intermediate product, and chem158K, as the final product. Processing at Arg162 was not required for cleavage at Lys158 or regulation of chemerin bioactivity. Contact phase activation of human platelet-poor plasma by kaolin led to cleavage of chem163S, which was undetectable in FXI-depleted plasma and markedly enhanced in platelet-rich plasma (PRP). Contact phase activation by polyphosphate in PRP resulted in 75% cleavage of chem163S. This cleavage was partially inhibited by hirudin, which blocks thrombin activation of FXI. After activation of plasma, levels of the most potent form of chemerin, chem157S, as well as inactive chem155A, increased. Plasma levels of chem163S in FXI-deficient patients were significantly higher compared with a matched control group (91 ± 10 ng/mL vs 58 ± 3 ng/mL, n = 8; P < .01) and inversely correlated with the plasma FXI levels. Thus FXIa, generated on contact phase activation, cleaves chem163S to generate chem158K, which can be further processed to the most active chemerin form, providing a molecular link between coagulation and inflammation.

Single-chain factor XII: a new form of activated factor XII

PURPOSE OF REVIEW

Exposure of blood to foreign surfaces induces reciprocal conversion of the plasma proteins factor XII (fXII) and plasma prekallikrein (PPK) to the proteases α-fXIIa and α-kallikrein. This process, called contact activation, has a range of effects on host defence mechanisms, including promoting coagulation. The nature of the triggering mechanism for contact activation is debated. One hypothesis predicts that fXII has protease activity, either intrinsically or upon surface-binding, that initiates contact activation. We tested this by assessing the proteolytic activity of a recombinant fXII variant that cannot be converted to α-fXIIa.

RECENT FINDINGS

The proteolytic activity of fXII-T (for 'triple' mutant), a variant with alanine substitutions for arginine at activation cleavage sites (Arg334, Arg344, and Arg353) was tested with known α-fXIIa substrates. FXII-T activates PPK in solution, and the reaction is enhanced by polyphosphate, an inducer of contact activation released from platelets. In the presence of polyphosphate, fXII-T converts fXII to α-fXIIa, and also converts the coagulation protein factor XI to its active form.

SUMMARY

The findings support the hypothesis that contact activation is initiated through activity intrinsic to single-chain fXII, and indicate that preexisting α-fXIIa is not required for induction of contact activation.

An update on factor XI structure and function

Factor XI (FXI) is the zymogen of a plasma protease, factor XIa (FXIa), that contributes to thrombin generation during blood coagulation by proteolytic activation of several coagulation factors, most notably factor IX (FIX). FXI is a homolog of prekallikrein (PK), a component of the plasma kallikrein-kinin system. While sharing structural and functional features with PK, FXI has undergone adaptive changes that allow it to contribute to blood coagulation. Here we review current understanding of the biology and enzymology of FXI, with an emphasis on structural features of the protein as they relate to protease function.

Inhibition of Factors XI and XII for Prevention of Thrombosis Induced by Artificial Surfaces

Exposure of blood to a variety of artificial surface induces contact activation, a process that contributes to the host innate response to foreign substances. On the foreign surface, the contact factors, factor XII (FXII), and plasma prekallikrein undergo reciprocal conversion to their fully active protease forms (FXIIa and α-kallikrein, respectively) by a process supported by the cofactor high-molecular-weight kininogen. Contact activation can trigger blood coagulation by conversion of factor XI (FXI) to the protease FXIa. There is interest in developing therapeutic inhibitors to FXIa and FXIIa because these activated factors can contribute to thrombosis in certain situations. Drugs targeting these proteases may be particularly effective in thrombosis triggered by exposure of blood to the surfaces of implantable medical devices. Here, we review clinical data supporting roles for FXII and FXI in thrombosis induced by medical devices, and preclinical data suggesting that therapeutic targeting of these proteins may limit surface-induced thrombosis.

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.

The Intrinsic Pathway of Coagulation as a Target for Antithrombotic Therapy

Plasma coagulation in the activated partial thromboplastin time assay is initiated by sequential activation of coagulation factors XII, XI, and IX. While this series of proteolytic reactions is not an accurate model for hemostasis in vivo, there is mounting evidence that factor XI and factor XII contribute to thrombosis, and that inhibiting them can produce an antithrombotic effect with a small effect on hemostasis. This article discusses the contributions of components of the intrinsic pathway to thrombosis in animal models and humans, and results of early clinical trials of drugs targeting factors IX, XI, and XII.

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.

Proteolytic properties of single-chain factor XII: a mechanism for triggering contact activation

When blood is exposed to variety of artificial surfaces and biologic substances, the plasma proteins factor XII (FXII) and prekallikrein undergo reciprocal proteolytic conversion to the proteases αFXIIa and α-kallikrein by a process called contact activation. These enzymes contribute to host-defense responses including coagulation, inflammation, and fibrinolysis. The initiating event in contact activation is debated. To test the hypothesis that single-chain FXII expresses activity that could initiate contact activation, we prepared human FXII variants lacking the Arg353 cleavage site required for conversion to αFXIIa (FXII-R353A), or lacking the 3 known cleavage sites at Arg334, Arg343, and Arg353 (FXII-T, for "triple" mutant), and compared their properties to wild-type αFXIIa. In the absence of a surface, FXII-R353A and FXII-T activate prekallikrein and cleave the tripeptide S-2302, demonstrating proteolytic activity. The activity is several orders of magnitude weaker than that of αFXIIa. Polyphosphate, an inducer of contact activation, enhances PK activation by FXII-T, and facilitates FXII-T activation of FXII and FXI. In plasma, FXII-T and FXII-R353A, but not FXII lacking the active site serine residue (FXII-S544A), shortened the clotting time of FXII-deficient plasma and enhanced thrombin generation in a surface-dependent manner. The effect was not as strong as for wild-type FXII. Our results support a model for induction of contact activation in which activity intrinsic to single-chain FXII initiates αFXIIa and α-kallikrein formation on a surface. αFXIIa, with support from α-kallikrein, subsequently accelerates contact activation and is responsible for the full procoagulant activity of FXII.

A new manufacturing process to remove thrombogenic factors (II, VII, IX, X, and XI) from intravenous immunoglobulin gamma preparations

Coagulation factors (II, VII, IX, X, and particularly XIa) remaining in high concentrations in intravenous immunoglobulin (IVIG) preparations can form thrombi, causing thromboembolic events, and in serious cases, result in death. Therefore, manufacturers of biological products must investigate the ability of their production processes to remove procoagulant activities. Previously, we were able to remove coagulation factors II, VII, IX, and X from our IVIG preparation through ethanol precipitation, but factor XIa, which plays an important role in thrombosis, remained in the intermediate products. Here, we used a chromatographic process using a new resin that binds with high capacity to IgG and removes procoagulant activities. The procoagulant activities were reduced to low levels as determined by the thrombin generation assay: <1.56 mIU/mL, chromogenic FXIa assay: <0.16 mIU/mL, non-activated partial thromboplastin time (NaPTT): >250 s, FXI/FXIa ELISA: <0.31 ng/mL. Even after spiking with FXIa at a concentration 32.5 times higher than the concentration in normal specimens, the procoagulant activities were below the detection limit (<0.31 ng/mL). These results demonstrate the ability of our manufacturing process to remove procoagulant activities to below the detection limit (except by NaPTT), suggesting a reduced risk of thromboembolic events that maybe potentially caused by our IVIG preparation.

Why factor XI deficiency is a clinical concern

INTRODUCTION

Inherited fXI deficiency has been an enigma since its discovery in 1953. The variable and relatively mild symptoms in patients with even the most severe form of the disorder seem out of step with the marked abnormalities in standard clotting assays. Indeed, the contribution of factor XI to hemostasis in an individual is not adequately assessed by techniques available in modern clinical laboratories.

AREAS COVERED

We discuss clinical studies, genetic/genomic analyses, and advances in laboratory medicine that are reshaping our views on the role of factor XI in pathologic coagulation. We review how the disorder associated with factor XI deficiency has contributed to changes in blood coagulation models, and discuss the complex genetics of the deficiency state and its relationship to bleeding. Finally, we cover new laboratory approaches that may distinguish deficient patients who are prone to bleeding from those without such predisposition. Expert commentary: Advances in understanding the biology of factor XI have led to modifications in treatment of factor XI-deficient patients. Factor replacement is used more judiciously, and alternative approaches are gaining favor. In the future, better laboratory tests may allow us to target therapy to those patients who would benefit most.

The susceptibility of plasma coagulation factor XI to nitration and peroxynitrite action

Coagulation factor XI is present in blood plasma as the zymogen, like other serine proteases of hemostatic system, but as the only coagulation factor forms 140-160kDa homodimers. Its activation is induced by thrombin, and a positive feedback increases the generation of the extra thrombin. Experimental and clinical observations confirm protective roles of factor XI deficiencies in certain types of thromboembolic disorders. Thromboembolism still causes serious problems for modern civilization. Diseases associated with the blood coagulation system are often associated with inflammation and oxidative stress. Peroxynitrite is produced from nitric oxide and superoxide in inflammatory diseases. The aim of the current study is to evaluate effects of nitrative stress triggered by peroxynitrite on coagulation factor XI in human plasma employing biochemical and bioinformatic methods. The amidolytic assay shows increase in factor XI activity triggered by peroxynitrite. Peroxynitrite interferes factor XI by nitration and fragmentation, which is demonstrated by immunoprecipitation followed by western blotting. Nitrated factor XI is even present in control blood plasma. The results suggest possible modifications of factor XI on the molecular level. Computer simulations show tyrosine residues as targets of peroxynitrite action. The modifications induced by peroxynitrite in factor XI might be important in thrombotic disorders.

Limiting prothrombin activation to meizothrombin is compatible with survival but significantly alters hemostasis in mice

Thrombin-mediated proteolysis is central to hemostatic function but also plays a prominent role in multiple disease processes. The proteolytic conversion of fII to α-thrombin (fIIa) by the prothrombinase complex occurs through 2 parallel pathways: (1) the inactive intermediate, prethrombin; or (2) the proteolytically active intermediate, meizothrombin (fIIa(MZ)). FIIa(MZ) has distinct catalytic properties relative to fIIa, including diminished fibrinogen cleavage and increased protein C activation. Thus, fII activation may differentially influence hemostasis and disease depending on the pathway of activation. To determine the in vivo physiologic and pathologic consequences of restricting thrombin generation to fIIa(MZ), mutations were introduced into the endogenous fII gene, resulting in expression of prothrombin carrying 3 amino acid substitutions (R157A, R268A, and K281A) to limit activation events to yield only fIIa(MZ) Homozygous fII(MZ) mice are viable, express fII levels comparable with fII(WT) mice, and have reproductive success. Although in vitro studies revealed delayed generation of fIIa(MZ) enzyme activity, platelet aggregation by fII(MZ) is similar to fII(WT) Consistent with prior analyses of human fIIa(MZ), significant prolongation of clotting times was observed for fII(MZ) plasma. Adult fII(MZ) animals displayed significantly compromised hemostasis in tail bleeding assays, but did not demonstrate overt bleeding. More notably, fII(MZ) mice had 2 significant phenotypic advantages over fII(WT) animals: protection from occlusive thrombosis after arterial injury and markedly diminished metastatic potential in a setting of experimental tumor metastasis to the lung. Thus, these novel animals will provide a valuable tool to assess the role of both fIIa and fIIa(MZ) in vivo.

Factor XI Deficiency Alters the Cytokine Response and Activation of Contact Proteases during Polymicrobial Sepsis in Mice

Sepsis, a systemic inflammatory response to infection, is often accompanied by abnormalities of blood coagulation. Prior work with a mouse model of sepsis induced by cecal ligation and puncture (CLP) suggested that the protease factor XIa contributed to disseminated intravascular coagulation (DIC) and to the cytokine response during sepsis. We investigated the importance of factor XI to cytokine and coagulation responses during the first 24 hours after CLP. Compared to wild type littermates, factor XI-deficient (FXI^-/-) mice had a survival advantage after CLP, with smaller increases in plasma levels of TNF-α and IL-10 and delayed IL-1β and IL-6 responses. Plasma levels of serum amyloid P, an acute phase protein, were increased in wild type mice 24 hours post-CLP, but not in FXI^-/- mice, supporting the impression of a reduced inflammatory response in the absence of factor XI. Surprisingly, there was little evidence of DIC in mice of either genotype. Plasma levels of the contact factors factor XII and prekallikrein were reduced in WT mice after CLP, consistent with induction of contact activation. However, factor XII and PK levels were not reduced in FXI^-/- animals, indicating factor XI deficiency blunted contact activation. Intravenous infusion of polyphosphate into WT mice also induced changes in factor XII, but had much less effect in FXI deficient mice. In vitro analysis revealed that factor XIa activates factor XII, and that this reaction is enhanced by polyanions such polyphosphate and nucleic acids. These data suggest that factor XI deficiency confers a survival advantage in the CLP sepsis model by altering the cytokine response to infection and blunting activation of the contact (kallikrein-kinin) system. The findings support the hypothesis that factor XI functions as a bidirectional interface between contact activation and thrombin generation, allowing the two processes to influence each other.

The Fragment 1 Region of Prothrombin Facilitates the Favored Binding of Fragment 12 to Zymogen and Enforces Zymogen-like Character in the Proteinase

Thrombin is produced from the C-terminal half of prothrombin following its proteolytic activation. The N-terminal half, released as the propiece Fragment 12 (F12), is composed of an N-terminal γ-carboxyglutamate domain (Gla) followed by two kringles (K1 and K2). The propiece plays essential roles in regulating prothrombin activation and proteinase function. The latter results from the ability of F12 to reversibly bind to the (pro)catalytic domain through K2 with high affinity and highly favorable thermodynamic constants when it is a zymogen in comparison to proteinase. Such discrimination is lost for K2 binding after proteolytic removal of the N-terminal Gla-K1 region of F12. The Ca(2+)-stabilized structure of the Gla domain is not required for F12 to bind the zymogen form more favorably. Enhanced binding to zymogen versus proteinase correlates with the ability of the propiece to enforce zymogen-like character in the proteinase. This is evident in variants of meizothrombin, an intermediate of prothrombin activation that contains the propiece covalently attached. This phenomenon is also independent of the Gla domain. Thus, the presence of K1 in covalent linkage with K2 in the propiece governs the ability of K2 to bind the (pro)catalytic domain in favor of zymogen, thereby enforcing zymogen-like character in the proteinase.

A comparison of the effects of factor XII deficiency and prekallikrein deficiency on thrombus formation

Studies with animal models implicate the plasma proteases factor XIIa (FXIIa) and α-kallikrein in arterial and venous thrombosis. As congenital deficiencies of factor XII (FXII) or prekallikrein (PK), the zymogens of FXIIa and α-kallikrein respectively, do not cause bleeding disorders, inhibition of these enzymes may have therapeutic benefit without compromising hemostasis. The relative contributions of FXIIa and α-kallikrein to thrombosis in animal models are not clear. We compared mice lacking FXII or PK to wild type mice in established models of arterial thrombosis. Wild type mice developed carotid artery occlusion when the vessel was exposed to a 3.5% solution of ferric chloride (FeCl3). FXII-deficient mice were resistant to occlusion at 5% FeCl3 and partially resistant at 10% FeCl3. PK-deficient mice were resistant at 3.5% FeCl3 and partially resistant at 5% FeCl3. Mice lacking high molecular weight kininogen, a cofactor for PK activation and activity, were also partially resistant to thrombosis at 5% FeCl3. Induction of carotid artery thrombosis with Rose Bengal was delayed in FXII-deficient mice compared to wild type or PK-deficient animals. In human plasma supplemented with silica, DNA or collagen to induce contact activation, an antibody to the FXIIa active site was more effective at preventing thrombin generation than an antibody to the α-kallikrein active site. Similarly, the FXIIa antibody was more effective at reducing fibrin formation in human blood flowing through collagen coated-tubes. The findings suggest that inhibitors of FXIIa will have more potent anti-thrombotic effects than inhibitors of α-kallikrein.

Hypercoagulability and the risk of myocardial infarction and ischemic stroke in young women

BACKGROUND

Myocardial infarction (MI) and ischemic stroke (IS) are acute forms of arterial thrombosis and share some, but not all, risk factors, indicating different pathophysiological mechanisms.

OBJECTIVE

This study aims to determine if hypercoagulability has a differential effect on the risk of MI and IS.

PATIENTS AND METHODS

We reviewed the results from the Risk of Arterial Thrombosis in Relation to Oral Contraceptives study, a population-based case-control study involving young women (< 50 years) with MI, non-cardioembolic IS and healthy controls. From these data, relative odds ratios (ORIS /ORMI ) and their corresponding confidence intervals for all prothrombotic factors that were studied in both subgroups were calculated.

RESULTS

Twenty-nine prothrombotic risk factors were identified as measures of hypercoagulability. Twenty-two of these risk factors (21/29, 72%) had a relative odds ratios > 1; for 12 (41%), it was > 2; and for 5 (17%), it was > 2.75. The five risk factors with the largest differences in associations were high levels of activated factor XI (FXI) and FXII, kallikrein, the presence of lupus anticoagulans, and a genetic variation in the FXIII gene.

CONCLUSION

In young women, prothrombotic factors are associated more with the risk of IS than with MI risk, suggesting a different role of hypercoagulability in the mechanism leading to these two diseases.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

The mechanism underlying activation of factor IX by factor XIa

Factor XI (fXI) is the zymogen of a plasma protease, factor XIa (fXIa), that contributes to thrombin generation during blood coagulation by proteolytic conversion of factor IX (fIX) to factor IXaβ (fIXaβ). There is considerable interest in fXIa as a therapeutic target because it contributes to thrombosis, while serving a relatively minor role in hemostasis. FXI/XIa has a distinctly different structure than other plasma coagulation proteases. Specifically, the protein lacks a phospholipid-binding Gla-domain, and is a homodimer. Each subunit of a fXIa dimer contains four apple domains (A1 to A4) and one trypsin-like catalytic domain. The A3 domain contains a binding site (exosite) that largely determines affinity and specificity for the substrate fIX. After binding to fXIa, fIX undergoes a single cleavage to form the intermediate fIXα. FIXα then rebinds to the A3 domain to undergo a second cleavage, generating fIXaβ. The catalytic efficiency for the second cleavage is ~7-fold greater than that of the first cleavage, limiting fIXα accumulation. Residues at the N-terminus and C-terminus of the fXIa A3 domain likely form the fIX binding site. The dimeric conformation of fXIa is not required for normal fIX activation in solution. However, monomeric forms of fXI do not reconstitute fXI-deficient mice in arterial thrombosis models, indicating the dimer is required for normal function in vivo. FXI must be a dimer to be activated normal by the protease fXIIa. It is also possible that the dimeric structure is an adaptation that allows fXI/XIa to bind to a surface through one subunit, while binding to its substrate fIX through the other.