Studies of fibrin formation and fibrinolytic function in patients with the antiphospholipid syndrome

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.

The Pathophysiology of The Antiphospholipid Syndrome: A Perspective From The Blood Coagulation System

The antiphospholipid syndrome (APS), a systemic autoimmune disease characterized by a hypercoagulability associated to vascular thrombosis and/or obstetric morbidity, is caused by the presence of antiphospholipid antibodies such as lupus anticoagulant, anti-β-2-glycoprotein 1, and/or anticardiolipin antibodies. In the obstetrical APS, antiphospholipid antibodies induce the production of proinflammatory cytokines and tissue factor by placental tissues and recruited neutrophils. Moreover, antiphospholipid antibodies activate the complement system which, in turn, induces a positive feedback leading to recruitment of neutrophils as well as activation of the placenta. Activation of these cells triggers myometrial contractions and cervical ripening provoking the induction of labor. In thrombotic and obstetrical APS, antiphospholipid antibodies activate endothelial cells, platelets, and neutrophils and they may alter the multimeric pattern and concentration of von Willebrand factor, increase the concentration of thrombospondin 1, reduce the inactivation of factor XI by antithrombin, increase the activation of factor XII, and reduce the activity of tissue plasminogen activator with the subsequent production of plasmin. All these effects result in less permeable clots, denser, thinner, and with more branched fibrin fibers which are more difficult to lysate. As a consequence, thrombosis, the defining clinical criterion of APS, complicates the clinical course of the patient.

Fibrin Network Porosity and Endo-/Exogenous Thrombin Cross-talk

The earliest assessment of fibrin network porosity used a liquid permeation system and confocal 3D microscopy, which was later replaced by scanning electron microscopy. Although the methods have extensively been applied in studies of health or disease, there remains debate on the choice of a proper clotting trigger. In this review, we assess published data and convey our opinions with regard to several issues. First, when the coagulation process is initiated by recombinant tissue factor (rTF) and phospholipids, the fibrin network porosity is regulated by the endogenous thrombin based on enzymatic activations of multiple coagulants. If purified thrombin (1.0 IU/mL) is employed as the clotting trigger, fibrin network porosity may be affected by exogenous thrombin, which directly polymerizes fibrinogen in plasma, and additionally by endogenous thrombin stemming from a "positive feedback loop" action of the added thrombin. Second, with use of either endogenous or exogenous thrombin, the concentration and clotting property of available fibrinogen both influence the fibrin network porosity. Third, in the assay systems in vitro, exogenous thrombin but not rTF-induced endogenous thrombin seems to be functional enough to activate factor XIII, which then contributes to a decrease in the fibrin network porosity. Fourth, fibrin network porosity determines the transport of fibrinolytic components into/through the clots and therefore serves as an indicator of the fibrinolysis potential in plasma.

Fibrinolysis in Venous Thromboembolism

Fibrinolysis is of paramount importance in maintaining or regaining the patency of veins and pulmonary arteries obstructed by thrombi. Growing experimental and clinical evidence indicates that impaired fibrinolysis mediated by multiple complex mechanisms is involved in venous thromboembolism (VTE). Global plasma fibrin clot lysis markers, especially clot lysis time, have been reported to predict recurrent deep-vein thrombosis and pulmonary embolism. The current overview summarizes available data linking fibrinolysis to VTE and its long-term sequelae.

Impaired Fibrinolysis in the Antiphospholipid Syndrome

The pathogenesis of the antiphospholipid syndrome (APS) is complex and involves the persistent presence of antiphospholipid antibodies (aPL) in the bloodstream causing a prothrombotic condition. aPL induce excessive activation of the endothelium, monocytes, and platelets in consort with aberrations in hemostasis/clotting, fibrinolytic system, and complement activation. Impaired fibrinolysis has been found in APS patients with thrombotic as well as obstetric manifestations. Increased levels of plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor, together with the presence of aPL against annexin-2, tissue-type plasminogen activator, and plasminogen contribute to the compromised fibrinolytic activity in these patients. Furthermore, unfavorably altered fibrin morphology, less amenable to fibrinolysis, has been proposed as a novel prothrombotic mechanism in APS. This review aims to summarize the present knowledge of the mechanisms involved in impaired fibrinolysis in APS patients. We also present a case from clinical practice as an illustration of fibrinolysis impairment in APS patients from a real-life setting.

Thrombotic antiphospholipid syndrome: A practical guide to diagnosis and management

Thrombotic antiphospholipid syndrome (APS) is characterised by venous, arterial and/or small vessel thrombosis in the context of persistently positive antiphospholipid antibodies (aPL). The diagnosis and management of thrombotic APS continues to prove challenging for clinicians. We provide a practical guide to the diagnosis of APS including who to test for aPL and which tests to do. We also consider clinical practice points on the management of venous, arterial and small vessel thrombosis, in the context of first and recurrent thrombotic events. Non-criteria manifestations of APS are reviewed. An approach to recurrent thrombosis and anticoagulant-refractory APS is discussed, with options including increasing the anticoagulation intensity of vitamin K antagonists, switching to low-molecular-weight-heparin, the use of fondaparinux and/or the addition of antiplatelet treatment. Adjunctive options such as vitamin D, hydroxychloroquine and statins are also addressed.

Diagnostic Accuracy in Acute Venous Thromboembolism: Comparing D-Dimer, Thrombin Generation, Overall Hemostatic Potential, and Fibrin Monomers

Introduction  For acute venous thromboembolism (VTE), a biomarker with higher specificity than D-dimer would be of great clinical use. Thrombin generation and overall hemostatic potential (OHP) reflect the hemostatic balance by globally assessing multiple coagulation factors and inhibitors. These tests discriminate between healthy controls and patients with a prothrombotic tendency but have yet to be established as clinical biomarkers of VTE.

Objective  This study compares endogenous thrombin potential (ETP) and OHP to D-dimer and fibrin monomers (FM) in outpatients with suspected VTE.

Methods  A cross-sectional diagnostic study where 954 patients with suspected pulmonary embolism or deep venous thrombosis were recruited consecutively from the medical emergency department at Karolinska University Hospital. D-dimer, FM, OHP, and ETP were analyzed in a subpopulation of 60 patients with VTE and 98 matched controls without VTE. VTE was verified either by ultrasonography or computed tomography and clinical data were collected from medical records.

Results  Compared with healthy controls, both VTE and non-VTE patients displayed prothrombotic profiles in OHP and ETP. D-dimer, FM, ETP area under the curve (AUC), and ETP T _lag were significantly different between patients with VTE and non-VTE. The largest receiver-operating characteristic AUCs for discrimination between VTE and non-VTE, were found in D-dimer with 0.94, FM 0.77, and ETP AUC 0.65. No useful cutoff could be identified for the ETP or the OHP assay.

Conclusion  Compared with D-dimer, neither ETP nor OHP were clinically viable biomarkers of acute venous thrombosis. The data indicated that a large portion of the emergency patients with suspected VTE were in a prothrombotic state.

Antiphospholipid antibodies and extracellular vesicles in pregnancy

Antiphospholipid antibodies (aPL) are autoantibodies that target phospholipid-binding proteins, such as β2 glycoprotein I (β2GPI), and can induce thrombosis systemically, as well as increase the risk of obstetric complications such as recurrent miscarriage and preeclampsia. Due to the expression of β2GPI by placental trophoblasts, aPL readily target the maternal-fetal interface during pregnancy and many studies have investigated the deleterious effects of aPL on placental trophoblast function. This review will focus on studies that have examined the effects of aPL on the production and modification of extracellular vesicles (EVs) from trophoblasts, as EVs are a key mode of feto-maternal communication in both normal and pathological pregnancy. A more comprehensive understanding of the effects of aPL on the quantity and cargo of EVs extruded by the human placenta may contribute to our current knowledge of how aPL induce both systemic and obstetric disease.

The antiphospholipid syndrome - often overlooked cause of vascular occlusions?

The antiphospholipid syndrome (APS) was fully recognized as a clinical entity in the early 1980s. Still, more than 30 years later, the epidemiology of APS is not well described, and furthermore, APS remains a challenge in terms of both diagnostic issues and clinical praxis involving a wide range of specialties. To date, there are no diagnostic criteria for APS. The present classification criteria rely on a combination of clinical manifestations and persistently positive tests for antiphospholipid antibodies (aPL). Clinical symptoms comprise vascular thrombosis, which can affect any vascular bed, including venous, microvascular and arterial vessels, and a set of pregnancy morbidities including early and late miscarriages, foetal death and preeclampsia. APS is more frequent among patients with other autoimmune diseases, and it is especially common in systemic lupus erythematosus (SLE). Importantly, APS symptoms can present in almost any medical specialty, but general knowledge and most previous clinical studies have essentially been confined to haematology, rheumatology and obstetrics/gynaecology. However, recent data demonstrate a relatively high prevalence of aPL also in patients from the general population who suffer from vascular occlusions or pregnancy complications. It is important that these patients are recognized by the general health care since APS is a treatable condition. This review aims to summarize the present knowledge on the history, pathogenesis, clinical manifestations and treatment of APS in order to urge a wide range of clinicians to consider comprehensive assessment of all patients where the diagnosis APS may be conceivable.

Antiphosphatidylserine/prothrombin complex antibodies as a determinant of prothrombotic plasma fibrin clot properties in patients with antiphospholipid syndrome

BACKGROUND

Antiphosphatidylserine/prothrombin complex (aPS/PT) antibodies are recognized as a marker for antiphospholipid syndrome (APS). Dense and poorly lysable fibrin clots occur in thrombotic APS. Compact clots predict thromboembolism, but determinants of the unfavorable clot phenotype remain unknown in APS. We hypothesized that elevated aPS/PT antibodies determine unfavorable clot features.

METHODS

In a cohort study involving 124 consecutive patients with thrombotic APS, we measured at baseline plasma fibrin clot permeability (Ks ), efficiency of fibrinolysis (clot lysis time, CLT), and turbidity (off anticoagulation) along with immunoglobulin (Ig)G/IgM aPS/PT. During follow-up, symptomatic thromboembolic events were recorded.

RESULTS

Elevated IgG and IgM aPS/PT antibodies >30 international enzyme units (UI) were detected in 54.8% and 42.7% of APS patients, including 76.2% and 54% of lupus anticoagulant- (LA, n = 63) positive patients, respectively. Elevated IgG and IgM aPS/PT antibodies predicted low Ks (lower quartile, <6 × 10-9  cm2 ; odds ratio [OR] = 5.93, 95% confidence interval [CI] 2.09-16.82 and OR = 11.79, 95% CI 4.10-33.92) and prolonged CLT (top quartile, ≥116 min; OR = 4.85, 95% CI 2.42-25.07 and OR = 6.04, 95% CI 2.42-15.07). No such associations were observed for anticardiolipin or β2-glycoprotein I antibodies or LA presence. During follow-up (median 72.5, range 66-83 months), thromboembolic events observed in 32 (26.7%, 4.6%/year) patients were independently predicted by IgG aPS/PT antibodies >30 UI (hazard ratio [HR] = 3.04, 95% CI 1.20-8.88) and low Ks (HR = 3.00, 95% CI 1.41-6.50).

CONCLUSIONS

We identified aPS/PT antibodies as a determinant of denser and poorly lysable plasma fibrin clot formation in APS patients. The association of elevated aPS/PT antibodies with thromboembolism in APS could be at least in part mediated by prothrombotic clot properties.

Improving fibrinolysis in venous thromboembolism: impact of fibrin structure

Introduction. Fibrinolysis is of key importance in maintaining vessel patency. Impaired fibrinolysis associated with more compact fibrin structure has been shown in patients with venous thromboembolism (VTE), including deep-vein thrombosis and pulmonary embolism (PE). Currently, recombinant or modified plasminogen activators are the only commonly available thrombolytic agents. However, they are fraught with side effects and suboptimal effectiveness. Areas covered. Based on the available literature, the current evidence linking fibrinolysis with VTE and potential therapeutic targets among fibrinolysis proteins are presented. Expert opinion. Prolonged clot lysis time has been reported as a new predictor of first-time and recurrent VTE, including PE. Anticoagulant therapy, including non-vitamin K antagonist oral anticoagulants, has a favorable impact on fibrinolysis in VTE patients. Several VTE risk factors are also related to lower efficiency of fibrinolysis and their treatment improve fibrinolysis, in part by alterations to fibrin properties. There is an increasing number of studies aiming at developing novel profibrinolytic therapeutic agents for treatment of VTE patients, mostly targeting the antifibrinolytic proteins, i.e. antiplasmin, plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor.

Complement in the Pathophysiology of the Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Complement is a system of enzymes and regulatory proteins of the innate immune system that plays a key role in the inflammatory response to pathogenic stimuli. The complement and coagulation pathways are closely linked, and expanding data indicate that complement may be activated in patients with aPL and function as a cofactor in the pathogenesis of aPL-associated clinical events. Complement activation by aPL generates C5a, which induces neutrophil tissue factor-dependent procoagulant activity. Beta-2-glycoprotein I, the primary antigen for pathogenic aPL, has complement regulatory effects in vitro. Moreover, aPL induce fetal loss in wild-type mice but not in mice deficient in specific complement components (C3, C5). Antiphospholipid antibodies also induce thrombosis in wild type mice and this effect is attenuated in C3 or C6 deficient mice, or in the presence of a C5 inhibitor. Increased levels of complement activation products have been demonstrated in sera of patients with aPL, though the association with clinical events remains unclear. Eculizumab, a terminal complement inhibitor, has successfully been used to treat catastrophic APS and prevent APS-related thrombotic microangiopathy in the setting of renal transplant. However, the mechanisms of complement activation in APS, its role in the pathogenesis of aPL related complications in humans, and the potential of complement inhibition as a therapeutic target in APS require further study.

Differences in plasma fibrin clot composition in patients with thrombotic antiphospholipid syndrome compared with venous thromboembolism

The prothrombotic fibrin clot phenotype has been reported in patients with thrombotic antiphospholipid syndrome (APS) and venous thromboembolism (VTE). Protein composition of plasma fibrin clots in APS has not been studied. We evaluated 23 patients with thrombotic APS, 19 with VTE alone, and 20 well-matched controls. A proteomic analysis of fibrin clots generated from citrated plasma was based on liquid chromatography-mass spectrometry. Plasma levels of thrombospondin-1 (TSP1), apolipoprotein(a), A-I, and B-100, complement components (C)3a, C5b-C9, histidine-rich glycoprotein (HRG), and prothrombin were evaluated using immunoenzymatic tests. In plasma fibrin clots of APS patients, compared with VTE subjects and controls, we identified decreased amounts of (pro)thrombin, antithrombin-III, apolipoprotein A-I, and HRG with no differences in plasma levels of antithrombin, prothrombin, along with lower plasma HRG and apolipoprotein A-I. In APS patients, plasma HRG positively correlated with amounts of clot-bound HRG, while apolipoprotein A-I was inversely associated with clot-bound levels of this protein. The most predominant proteins within the clots of APS patients were bone marrow proteoglycan, C5-C9, immunoglobulins, apolipoprotein B-100, platelet-derived proteins, and TSP1. Our study is the first to demonstrate differences in the protein composition of fibrin clots generated from plasma of thrombotic APS patients versus those with VTE alone.

Prothrombotic Fibrin Clot Phenotype in Patients with Deep Vein Thrombosis and Pulmonary Embolism: A New Risk Factor for Recurrence

Prothrombotic fibrin clot phenotype, involving faster formation of dense meshwork composed of thinner and highly branched fibers that are relatively resistant to plasmin-induced lysis, has been reported in patients with not only myocardial infarction or stroke, but also venous thromboembolism (VTE), encompassing deep vein thrombosis (DVT), and/or pulmonary embolism (PE). Prothrombotic fibrin clot phenotype, in particular prolonged clot lysis time, is considered a novel risk factor for VTE as well as venous thrombosis at unusual location, for example, cerebral sinus venous thrombosis, retinal vein obstruction, and Budd-Chiari syndrome. Growing evidence from observational studies indicates that abnormal fibrin clot properties can predict recurrent DVT and PE and they are involved in serious complications of VTE, for example, thromboembolic pulmonary hypertension and postthrombotic syndrome. The purpose of this article is to review our current understanding of the role of fibrin clot structure and function in venous thrombosis with emphasis on clinical issues ranging from prognosis to therapy.

Fibrin structure in organized thrombotic material removed during pulmonary artery endarterectormy: the effect of vessel calibre

Pulmonary endarterectomy (PEA) is a curative therapeutic approach in patients with chronic thromboembolic pulmonary hypertension (CTEPH). The location-dependent structural differences of thrombotic material found in pulmonary arteries in CTEPH are poorly investigated. We present the case of a 47-year-old woman with antiphospholipid syndrome, diabetes mellitus and abnormal fibrin phenotype, who underwent PEA for CTEPH. Intravascular material removed bilaterally during PEA (from lobar, segmental and sub-segmental arteries) has been studied using light and scanning electron microscopy (SEM). Light microscopy showed tighter fibrous network in the portions of intraluminal thrombotic material facing the vessel wall, which contained collagen and fibrin fibers, and abundant cells. Cells, evaluated by immunostaining, were present in the whole removed material. Tissue factor expression was also observed with the highest values in the portions of intravascular material facing the vessel wall. In the main pulmonary arteries, SEM images revealed thick fibers of fibrous proteins loosly meshed and few erythrocytes and platelets between them (both dysmorphic "wedged" and fresh cells were present). In the fibrotic layers, containing mainly collagen and fibrin, removed from the lobar/segmental pulmonary arteries we found a stepwise increase in fiber density with decreasing vessel calibre, followed by denser fibrous networks composed of thinner fibers. Elastic fibers in the lobar and segmental arteries were aligned along the blood flow vector. These findings demonstrate differences in the structure of endarterectomized PEA material dependent on the vessel calibre and might contribute to understanding of CTEPH pathophysiology.