Elimination of fibrin γ-chain cross-linking by FXIIIa increases pulmonary embolism arising from murine inferior vena cava thrombi

A novel method to quantify fibrin-fibrin and fibrin-α2-antiplasmin cross-links in thrombi formed from human trauma patient plasma

BACKGROUND

The widespread use of the antifibrinolytic agent, tranexamic acid (TXA), interferes with the quantification of fibrinolysis by dynamic laboratory assays such as clot lysis, making it difficult to measure fibrinolysis in many trauma patients. At the final stage of coagulation, factor (F)XIIIa catalyzes the formation of fibrin-fibrin and fibrin-α2-antiplasmin (α2AP) cross-links, which increases clot mechanical strength and resistance to fibrinolysis.

OBJECTIVES

Here, we developed a method to quantify fibrin-fibrin and fibrin-α2AP cross-links that avoids the challenges posed by TXA in determining fibrinolytic resistance in conventional assays.

METHODS

Fibrinogen alpha (FGA) chain (FGA-FGA), fibrinogen gamma (FGG) chain (FGG-FGG), and FGA-α2AP cross-links were quantified using liquid chromatography-mass spectrometry (LC-MS) and parallel reaction monitoring in paired plasma samples from trauma patients prefibrinogen and postfibrinogen replacement. Differences in the abundance of cross-links in trauma patients receiving cryoprecipitate (cryo) or fibrinogen concentrate (Fg-C) were analyzed.

RESULTS

The abundance of cross-links was significantly increased in trauma patients postcryo, but not Fg-C transfusion (P < .0001). The abundance of cross-links was positively correlated with the toughness of individual fibrin fibers, the peak thrombin concentration, and FXIII antigen (P < .05).

CONCLUSION

We have developed a novel method that allows us to quantify fibrin cross-links in trauma patients who have received TXA, providing an indirect measure of fibrinolytic resistance. Using this novel approach, we have avoided the effect of TXA and shown that cryo increases fibrin-fibrin and fibrin-α2AP cross-linking when compared with Fg-C, highlighting the importance of FXIII in clot formation and stability in trauma patients.

"What makes blood clots break off?" A Back-of-the-Envelope Computation Toward Explaining Clot Embolization

PURPOSE

One in four deaths worldwide is due to thromboembolic disease; that is, one in four people die from blood clots first forming and then breaking off or embolizing. Once broken off, clots travel downstream, where they occlude vital blood vessels such as those of the brain, heart, or lungs, leading to strokes, heart attacks, or pulmonary embolisms, respectively. Despite clots' obvious importance, much remains to be understood about clotting and clot embolization. In our work, we take a first step toward untangling the mystery behind clot embolization and try to answer the simple question: "What makes blood clots break off?"

METHODS

To this end, we conducted experimentally-informed, back-of-the-envelope computations combining fracture mechanics and phase-field modeling. We also focused on deep venous clots as our model problem.

RESULTS

Here, we show that of the three general forces that act on venous blood clots-shear stress, blood pressure, and wall stretch-induced interfacial forces-the latter may be a critical embolization force in occlusive and non-occlusive clots, while blood pressure appears to play a determinant role only for occlusive clots. Contrary to intuition and prior reports, shear stress, even when severely elevated, appears unlikely to cause embolization.

CONCLUSION

This first approach to understanding the source of blood clot bulk fracture may be a critical starting point for understanding blood clot embolization. We hope to inspire future work that will build on ours and overcome the limitations of these back-of-the-envelope computations.

Management of anticoagulation and factor XIII replacement in a patient with severe factor XIII deficiency and recurrent venous thromboembolic disease: case report and review of literature

Background

Thrombotic events in congenital factor (F)XIII deficiency are extremely rare. To our knowledge, we describe the first case of severe congenital FXIII deficiency associated with recurrent venous thrombotic events.

Key Clinician Question

How to deal with anticoagulation treatment in patients with severe FXIII deficiency?

Clinical Approach

The patient was treated with rivaroxaban and plasma-derived FXIII substitution therapy as prophylaxis without bleeding complications. We aimed at FXIII trough levels of 50% during the loading doses of rivaroxaban, then 30% during the maintenance dose of rivaroxaban, and finally 20% during the long-term use of prophylactic dose of rivaroxaban.

Conclusion

Treatment of thrombotic events with rivaroxaban in patients with severe bleeding disorders seems to be safe, requiring an adaptation of the intensity of the replacement therapy.

Fibrin clot properties in cardiovascular disease: from basic mechanisms to clinical practice

Fibrinogen conversion into insoluble fibrin and the formation of a stable clot is the final step of the coagulation cascade. Fibrin clot porosity and its susceptibility to plasmin-mediated lysis are the key fibrin measures, describing the properties of clots prepared ex vivo from citrated plasma. Cardiovascular disease (CVD), referring to coronary heart disease, heart failure, stroke, and hypertension, has been shown to be associated with the formation of dense fibrin networks that are relatively resistant to lysis. Denser fibrin mesh characterized acute patients at the onset of myocardial infarction or ischaemic stroke, while hypofibrinolysis has been identified as a persistent fibrin feature in patients following thrombotic events or in those with stable coronary artery disease. Traditional cardiovascular risk factors, such as smoking, diabetes mellitus, hyperlipidaemia, obesity, and hypertension, have also been linked with unfavourably altered fibrin clot properties, while some lifestyle modifications and pharmacological treatment, in particular statins and anticoagulants, may improve fibrin structure and function. Prospective studies have suggested that prothrombotic fibrin clot phenotype can predict cardiovascular events in short- and long-term follow-ups. Mutations and splice variants of the fibrinogen molecule that have been proved to be associated with thrombophilia or increased cardiovascular risk, along with fibrinogen post-translational modifications, prothrombotic state, inflammation, platelet activation, and neutrophil extracellular traps formation, contribute also to prothrombotic fibrin clot phenotype. Moreover, about 500 clot-bound proteins have been identified within plasma fibrin clots, including fibronectin, α2-antiplasmin, factor XIII, complement component C3, and histidine-rich glycoprotein. This review summarizes the current knowledge on the mechanisms underlying unfavourable fibrin clot properties and their implications in CVD and its thrombo-embolic manifestations.

Fibrin clot properties and thrombus composition in cirrhosis

Patients with cirrhosis frequently acquire profound hemostatic alterations, which may affect thrombus quality and composition-factors that determine the susceptibility to embolization and fibrinolysis. In this narrative review, we describe in vitro studies on fibrin clot formation and quantitative and qualitative changes in fibrinogen in patients with cirrhosis, and describe recent findings on the composition of portal vein thrombi in patients with cirrhosis. Patients with mild cirrhosis have increased thrombin generation capacity and plasma fibrinogen levels, which may be balanced by delayed fibrin polymerization and decreased factor XIII levels. With progressing illness, plasma fibrinogen levels decrease, but thrombin generation capacity remains elevated. Fibrinogen is susceptible to posttranslational protein modifications and is, for example, hypersialylated and carbonylated in patients with cirrhosis. Despite changes in thrombin generation, factor XIII levels and the fibrinogen molecule, fibrin fiber thickness, and density are normal in patients with cirrhosis. Paradoxically, fibrin clot permeability in patients with cirrhosis is decreased, possibly because of posttranslational protein modifications. Most patients have normal fibrinolytic potential. We have recently demonstrated that portal vein thrombosis is likely a misnomer as the material that may obstruct the cirrhotic portal vein frequently consists of a thickened portal vein wall, rather than a true thrombus. Patients with cirrhosis often have thrombocytopenia and anemia, which may also affect clot stability and composition, but the role of cellular components in clot quality in cirrhosis has not been extensively studied. Finally, we summarize abstracts on fibrin formation and clot quality that were presented at the ISTH 2022 meeting in London.

Blood clot contraction: Mechanisms, pathophysiology, and disease

A State of the Art lecture titled "Blood Clot Contraction: Mechanisms, Pathophysiology, and Disease" was presented at the International Society on Thrombosis and Haemostasis (ISTH) Congress in 2022. This was a systematic description of blood clot contraction or retraction, driven by activated platelets and causing compaction of the fibrin network along with compression of the embedded erythrocytes. The consequences of clot contraction include redistribution of the fibrin-platelet meshwork toward the periphery of the clot and condensation of erythrocytes in the core, followed by their deformation from the biconcave shape into polyhedral cells (polyhedrocytes). These structural signatures of contraction have been found in ex vivo thrombi derived from various locations, which indicated that clots undergo intravital contraction within the blood vessels. In hemostatic clots, tightly packed polyhedrocytes make a nearly impermeable seal that stems bleeding and is impaired in hemorrhagic disorders. In thrombosis, contraction facilitates the local blood flow by decreasing thrombus obstructiveness, reducing permeability, and changing susceptibility to fibrinolytic enzymes. However, in (pro)thrombotic conditions, continuous background platelet activation is followed by platelet exhaustion, refractoriness, and impaired intravital clot contraction, which is associated with weaker thrombi predisposed to embolization. Therefore, assays that detect imperfect in vitro clot contraction have potential diagnostic and prognostic values for imminent or ongoing thrombosis and thrombotic embolism. Collectively, the contraction of blood clots and thrombi is an underappreciated and understudied process that has a pathogenic and clinical significance in bleeding and thrombosis of various etiologies. Finally, we have summarized relevant new data on this topic presented during the 2022 ISTH Congress.

Fibrinogen, Fibrin, and Fibrin Degradation Products in COVID-19

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is the highly pathogenic and highly transmissible human coronavirus that is the causative agent for the worldwide COVID-19 pandemic. COVID-19 manifests predominantly as a respiratory illness with symptoms consistent with viral pneumonia, but other organ systems (e.g., kidney, heart, brain) can also become perturbed in COVID-19 patients. Accumulating data suggest that significant activation of the hemostatic system is a common pathological manifestation of SARS-CoV-2 infection. The clotting protein fibrinogen is one of the most abundant plasma proteins. Following activation of coagulation, the central coagulation protease thrombin converts fibrinogen to fibrin monomers, which selfassemble to form a matrix, the primary structural component of the blood clot. Severe COVID-19 is associated with a profound perturbation of circulating fibrinogen, intra- and extravascular fibrin deposition and persistence, and fibrin degradation. Current findings suggest high levels of fibrinogen and the fibrin degradation product D-dimer are biomarkers of poor prognosis in COVID-19. Moreover, emerging studies with in vitro and animal models indicate fibrin(ogen) as an active player in COVID-19 pathogenesis. Here, we review the current literature regarding fibrin(ogen) and COVID-19, including possible pathogenic mechanisms and treatment strategies centered on clotting and fibrin(ogen) function.

Cryoprecipitate transfusion in trauma patients attenuates hyperfibrinolysis and restores normal clot structure and stability: Results from a laboratory sub-study of the FEISTY trial

BACKGROUND

Fibrinogen is the first coagulation protein to reach critical levels during traumatic haemorrhage. This laboratory study compares paired plasma samples pre- and post-fibrinogen replacement from the Fibrinogen Early In Severe Trauma studY (FEISTY; NCT02745041). FEISTY is the first randomised controlled trial to compare the time to administration of cryoprecipitate (cryo) and fibrinogen concentrate (Fg-C; Riastap) in trauma patients. This study will determine differences in clot strength and fibrinolytic stability within individuals and between treatment arms.

METHODS

Clot lysis, plasmin generation, atomic force microscopy and confocal microscopy were utilised to investigate clot strength and structure in FEISTY patient plasma.

RESULTS

Fibrinogen concentration was significantly increased post-transfusion in both groups. The rate of plasmin generation was reduced 1.5-fold post-transfusion of cryo but remained unchanged with Fg-C transfusion. Plasminogen activator inhibitor 1 activity and antigen levels and Factor XIII antigen were increased post-treatment with cryo, but not Fg-C. Confocal microscopy analysis of fibrin clots revealed that cryo transfusion restored fibrin structure similar to those observed in control clots. In contrast, clots remained porous with stunted fibres after infusion with Fg-C. Cryo but not Fg-C treatment increased individual fibre toughness and stiffness.

CONCLUSIONS

In summary, our data indicate that cryo transfusion restores key fibrinolytic regulators and limits plasmin generation to form stronger clots in an ex vivo laboratory study. This is the first study to investigate differences in clot stability and structure between cryo and Fg-C and demonstrates that the additional factors in cryo allow formation of a stronger and more stable clot.

Composition and Organization of Acute Ischemic Stroke Thrombus: A Wealth of Information for Future Thrombolytic Strategies

During the last decade, significant progress has been made in understanding thrombus composition and organization in the setting of acute ischemic stroke (AIS). In particular, thrombus organization is now described as highly heterogeneous but with 2 preserved characteristics: the presence of (1) two distinct main types of areas in the core—red blood cell (RBC)-rich and platelet-rich areas in variable proportions in each thrombus—and (2) an external shell surrounding the core composed exclusively of platelet-rich areas. In contrast to RBC-rich areas, platelet-rich areas are highly complex and are mainly responsible for the thrombolysis resistance of these thrombi for the following reasons: the presence of platelet-derived fibrinolysis inhibitors in large amounts, modifications of the fibrin network structure resistant to the tissue plasminogen activator (tPA)-induced fibrinolysis, and the presence of non-fibrin extracellular components, such as von Willebrand factor (vWF) multimers and neutrophil extracellular traps. From these studies, new therapeutic avenues are in development to increase the fibrinolytic efficacy of intravenous (IV) tPA-based therapy or to target non-fibrin thrombus components, such as platelet aggregates, vWF multimers, or the extracellular DNA network.

Fibrinogen and Factor XIII in Venous Thrombosis and Thrombus Stability

As the third most common vascular disease, venous thromboembolism is associated with significant mortality and morbidity. Pathogenesis underlying venous thrombosis is still not fully understood. Accumulating data suggest fibrin network structure and factor XIII-mediated crosslinking are major determinants of venous thrombus mass, composition, and stability. Understanding the cellular and molecular mechanisms mediating fibrin(ogen) and factor XIII production and function and their ability to influence venous thrombogenesis and resolution may inspire new anticoagulant strategies that target these proteins to reduce or prevent venous thrombosis in certain at-risk patients. This article summarizes fibrinogen and factor XIII biology and current knowledge of their function during venous thromboembolism.

Cross-linking by tissue transglutaminase-2 alters fibrinogen-directed macrophage proinflammatory activity

BACKGROUND

The blood coagulation factor fibrin(ogen) can modulate inflammation by altering leukocyte activity. Analyses of fibrin(ogen)-mediated proinflammatory activity have largely focused on leukocyte integrin binding activity revealed by conversion of fibrinogen to a stabilized fibrin polymer by blood coagulation enzymes. In addition to coagulation enzymes, fibrinogen is a substrate for tissue transglutaminase-2 (TG2), a widely expressed enzyme that produces unique fibrinogen Aα-γ chain cross-linked products.

OBJECTIVES

We tested the hypothesis that TG2 dependent cross-linking alters the proinflammatory activity of surface-adhered fibrinogen.

METHODS

Mouse bone marrow-derived macrophages (BMDMs) were cultured on tissue culture plates coated with fibrinogen or TG2-cross-linked fibrinogen (10 µg/ml) and then stimulated with lipopolysaccharide (LPS, 1 ng/ml) or vehicle for various times.

RESULTS

In the absence of LPS stimulation, TG2-cross-linked fibrin(ogen) enhanced inflammatory gene induction (e.g., Tnfα) compared with unmodified fibrinogen. LPS stimulation induced mitogen-activated protein kinase phosphorylation, IκBα degradation, and expression of proinflammatory cytokines (e.g., tumor necrosis factor α) within 60 min. This initial cellular activation was unaffected by unmodified or TG2-cross-linked fibrinogen. In contrast, LPS induction of interleukin-10 mRNA and protein and STAT3 phosphorylation was selectively attenuated by TG2-cross-linked fibrinogen, which was associated with enhanced proinflammatory cytokine secretion by LPS-stimulated BMDMs at later time points (6 and 24 h).

CONCLUSIONS

The results indicate that atypical cross-linking by TG2 imparts unique proinflammatory activity to surface-adhered fibrinogen. The results suggest a novel coagulation-independent mechanism controlling fibrinogen-directed macrophage activation.

Why fibrin biomechanical properties matter for hemostasis and thrombosis

Polymeric fibrin displays unique structural and biomechanical properties that contribute to its essential role of generating blood clots that stem bleeds. The aim of this review is to discuss how the fibrin clot is formed, how protofibrils make up individual fibrin fibers, what the relationship is between the molecular structure and fibrin biomechanical properties, and how fibrin biomechanical properties relate to the risk of thromboembolic disease. Fibrin polymerization is driven by different types of bonds, including knob-hole interactions displaying catch-slip characteristics, and covalent crosslinking of fibrin polypeptides by activated factor XIII. Key biophysical properties of fibrin polymer are its visco-elasticity, extensibility and resistance to rupture. The internal packing of protofibrils within fibers changes fibrin biomechanical behavior. There are several methods to analyze fibrin biomechanical properties at different scales, including AFM force spectroscopy, magnetic or optical tweezers and rheometry, amongst others. Clinically, fibrin biomechanical characteristics are key for the prevention of thromboembolic disorders such as pulmonary embolism. Future studies are needed to address unanswered questions regarding internal molecular structure of the fibrin polymer, the structural and molecular basis of its remarkable mechanical properties and the relationship of fibrin biomechanical characteristics with thromboembolism in patients with deep vein thrombosis and ischemic stroke.

Novel venous thromboembolism mouse model to evaluate the role of complete and partial factor XIII deficiency in pulmonary embolism risk

BACKGROUND

Venous thrombosis (VT) and pulmonary embolism (PE), collectively venous thromboembolism (VTE), cause high mortality and morbidity. Factor XIII (FXIII) crosslinks fibrin to enhance thrombus stability and consequently may influence PE risk. Elucidating mechanisms contributing to PE is limited by a lack of models that recapitulate human PE characteristics.

OBJECTIVE

We aimed to develop a mouse model that permits embolization of red blood cell (RBC)- and fibrin-rich VT and determine the contribution of FXIII to PE risk.

METHODS AND RESULTS

In a thrombin-infusion PE model, F13a+/+ , F13a+/- , and F13a-/-  mice had similar incidence of microthrombi in the lungs; however, thrombi were small, with low RBC content (≤7%), unlike human PEs (~70%). To identify a model producing PE consistent with histological characteristics of human PE, we compared mouse femoral vein electrolytic injury, femoral vein FeCl3 injury, and infrarenal vena cava (IVC) stasis models of VT. Electrolytic and FeCl3  models produced small thrombi with few RBCs (5% and 4%, respectively), whereas IVC stasis produced large thrombi with higher RBC content (68%) that was similar to human PEs. After IVC stasis and ligature removal (de-ligation) to permit thrombus embolization, compared to F13a+/+ mice, F13a+/-  and F13a-/-  mice had similar and increased PE incidence, respectively.

CONCLUSIONS

Compared to thrombin infusion-, electrolytic injury-, and FeCl3 -based models, IVC stasis produces thrombi that are more histologically similar to human thrombi. IVC stasis followed by de-ligation permits embolization of existing RBC- and fibrin-rich thrombi. Complete FXIII deficiency increases PE incidence, but partial deficiency does not.