Endosomal NADPH-oxidase is critical for induction of the tissue factor gene in monocytes and endothelial cells

Redox Pathogenesis in Rheumatic Diseases

Despite being some of the most anecdotally well-known roads to pathogenesis, the mechanisms governing autoimmune rheumatic diseases are not yet fully understood. The overactivation of the cellular immune system and the characteristic development of autoantibodies have been linked to oxidative stress. Typical clinical manifestations, such as joint swelling and deformities and inflammation of the skin and internal organs, have also been connected directly or indirectly to redox mechanisms. The differences in generation and restraint of oxidative stress provide compelling evidence for the broad variety in pathology among rheumatic diseases and explain some of the common triggers and discordant manifestations in these diseases. Growing evidence of redox mechanisms in pathogenesis has provided a broad array of new potential therapeutic targets. Here, we explore the mechanisms by which oxidative stress is generated, explore its roles in autoimmunity and end-organ damage, and discuss how individual rheumatic diseases exhibit unique features that offer targets for therapeutic interventions.

Targeting the tissue factor coagulation initiation complex prevents antiphospholipid antibody development

ABSTRACT

Antiphospholipid antibodies (aPL) in primary or secondary antiphospholipid syndrome (APS) are a major cause for acquired thrombophilia, but specific interventions preventing autoimmune aPL development are an unmet clinical need. Although autoimmune aPL cross react with various coagulation regulatory proteins, lipid-reactive aPL, including those derived from patients with COVID-19, recognize the endolysosomal phospholipid lysobisphosphatidic acid presented by the cell surface-expressed endothelial protein C receptor. This specific recognition leads to complement-mediated activation of tissue factor (TF)-dependent proinflammatory signaling and thrombosis. Here, we show that specific inhibition of the TF coagulation initiation complex with nematode anticoagulant protein c2 (NAPc2) prevents the prothrombotic effects of aPL derived from patients with COVID-19 in mice and the aPL-induced proinflammatory and prothrombotic activation of monocytes. The induction of experimental APS is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, and NAPc2 suppresses monocyte endosomal reactive oxygen species production requiring the TF cytoplasmic domain and interferon-α secretion from dendritic cells. Latent infection with murine cytomegalovirus causes TF cytoplasmic domain-dependent development of persistent aPL and circulating phospholipid-reactive B1 cells, which is prevented by short-term intervention with NAPc2 during acute viral infection. In addition, treatment of lupus prone MRL-lpr mice with NAPc2, but not with heparin, suppresses dendritic-cell activation in the spleen, aPL production and circulating phospholipid-reactive B1 cells, and attenuates lupus pathology. These data demonstrate a convergent TF-dependent mechanism of aPL development in latent viral infection and autoimmune disease and provide initial evidence that specific targeting of the TF initiation complex has therapeutic benefits beyond currently used clinical anticoagulant strategies.

Modulation of the activation of endothelial nitric oxide synthase and nitrosative stress biomarkers by aspirin triggered lipoxins: A possible mechanism of action of aspirin in the antiphospholipid syndrome

PROBLEM

Antiphospholipid syndrome (APS) is characterized by the clinical manifestation of vascular thrombosis (VT) or pregnancy morbidity (PM) and antiphospholipid antibodies (aPL) that can modify the nitric oxide production. Low-dose aspirin is used in the prevention and treatment of diverse alterations of pregnancy. One of the mechanisms of action of aspirin is to induce the production of aspirin-triggered-lipoxins (ATL). The aim of this study was to evaluate the modulatory effect of ATL over the activation of endothelial nitric oxide synthase (eNOS) and nitrosative stress biomarkers induced by aPL.

METHODS

We used polyclonal IgG and sera from women with aPL and PM/VT or VT only, and from women with PM only and positive for non-criteria aPL (SN-OAPS). In these sera, biomarkers of nitrosative stress (nitrites and nitrotyrosine) were measured. The protein expression of nitrotyrosine and the phosphorylation of eNOS (at Ser1177) were estimated in human umbilical vein endothelial cells (HUVECs) stimulated with polyclonal IgG with or without ATL.

RESULTS

Women with SN-OAPS showed increased circulating levels of nitrites and nitrotyrosine. Likewise, polyclonal IgG from either SN-OAPS or VT patients stimulated nitrotyrosine expression in HUVECs. ATL decreased the nitrotyrosine expression induced by polyclonal IgG from the SN-OAPS group. ATL also recovered the reduced eNOS phosphorylation at Ser1177 in HUVECs stimulated with polyclonal IgG from women with PM/VT or SN-OAPS.

CONCLUSIONS

Increased nitrosative stress present in serum of women with SN-OAPS is associated with IgG-mediated impaired endothelial NO synthesis in endothelial cells. ATL prevent these cellular changes.

Interaction between endothelial cell-derived extracellular vesicles and monocytes: A potential link between vascular thrombosis and pregnancy-related morbidity in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disease driven by a wide group of autoantibodies primarily directed against phospholipid-binding proteins (antiphospholipid antibodies). APS is defined by two main kinds of clinical manifestations: vascular thrombosis and pregnancy-related morbidity. In recent years, in vitro and in vivo assays, as well as the study of large groups of patients with APS, have led some authors to suggest that obstetric and vascular manifestations of the disease are probably the result of different pathogenic mechanisms. According to this hypothesis, the disease could be differentiated into two parallel entities: Vascular APS and obstetric APS. Thus, vascular APS is understood as an acquired thrombophilia in which a generalised phenomenon of endothelial activation and dysfunction (coupled with a triggering factor) causes thrombosis at any location. In contrast, obstetric APS seems to be due to an inflammatory phenomenon accompanied by trophoblast cell dysfunction. The recent approach to APS raises new issues; for instance, the mechanisms by which a single set of autoantibodies can lead to two different clinical entities are unclear. This review will address the monocyte, a cell with well-known roles in haemostasis and pregnancy, as a potential participant in vascular thrombosis and pregnancy-related morbidity in APS. We will discuss how in a steady state the monocyte-endothelial interaction occurs via extracellular vesicles (EVs), and how antiphospholipid antibodies, by inducing endothelial activation and dysfunction, may disturb this interaction to promote the release of monocyte-targeted procoagulant and inflammatory messages.

Disseminated intravascular coagulation and its immune mechanisms

Disseminated intravascular coagulation (DIC) is a syndrome triggered by infectious and noninfectious pathologies characterized by excessive generation of thrombin within the vasculature and widespread proteolytic conversion of fibrinogen. Despite diverse clinical manifestations ranging from thrombo-occlusive damage to bleeding diathesis, DIC etiology commonly involves excessive activation of blood coagulation and overlapping dysregulation of anticoagulants and fibrinolysis. Initiation of blood coagulation follows intravascular expression of tissue factor or activation of the contact pathway in response to pathogen-associated or host-derived, damage-associated molecular patterns. The process is further amplified through inflammatory and immunothrombotic mechanisms. Consumption of anticoagulants and disruption of endothelial homeostasis lower the regulatory control and disseminate microvascular thrombosis. Clinical DIC development in patients is associated with worsening morbidities and increased mortality, regardless of the underlying pathology; therefore, timely recognition of DIC is critical for reducing the pathologic burden. Due to the diversity of triggers and pathogenic mechanisms leading to DIC, diagnosis is based on algorithms that quantify hemostatic imbalance, thrombocytopenia, and fibrinogen conversion. Because current diagnosis primarily assesses overt consumptive coagulopathies, there is a critical need for better recognition of nonovert DIC and/or pre-DIC states. Therapeutic strategies for patients with DIC involve resolution of the eliciting triggers and supportive care for the hemostatic imbalance. Despite medical care, mortality in patients with DIC remains high, and new strategies, tailored to the underlying pathologic mechanisms, are needed.

Oxidative Stress-Related Mechanisms in SARS-CoV-2 Infections

The COVID-19 pandemic caused relatively high mortality in patients, especially in those with concomitant diseases (i.e., diabetes, hypertension, and chronic obstructive pulmonary disease (COPD)). In most of aforementioned comorbidities, the oxidative stress appears to be an important player in their pathogenesis. The direct cause of death in critically ill patients with COVID-19 is still far from being elucidated. Although some preliminary data suggests that the lung vasculature injury and the loss of the functioning part of pulmonary alveolar population are crucial, the precise mechanism is still unclear. On the other hand, at least two classes of medications used with some clinical benefits in COVID-19 treatment seem to have a major influence on ROS (reactive oxygen species) and RNS (reactive nitrogen species) production. However, oxidative stress is one of the important mechanisms in the antiviral immune response and innate immunity. Therefore, it would be of interest to summarize the data regarding the oxidative stress in severe COVID-19. In this review, we discuss the role of oxidative and antioxidant mechanisms in severe COVID-19 based on available studies. We also present the role of ROS and RNS in other viral infections in humans and in animal models. Although reactive oxygen and nitrogen species play an important role in the innate antiviral immune response, in some situations, they might have a deleterious effect, e.g., in some coronaviral infections. The understanding of the redox mechanisms in severe COVID-19 disease may have an impact on its treatment.

The role of NADPH oxidases in infectious and inflammatory diseases

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) are enzymes that generate superoxide or hydrogen peroxide from molecular oxygen utilizing NADPH as an electron donor. There are seven enzymes in the NOX family: NOX1-5 and dual oxidase (DUOX) 1-2. NOX enzymes in humans play important roles in diverse biological functions and vary in expression from tissue to tissue. Importantly, NOX2 is involved in regulating many aspects of innate and adaptive immunity, including regulation of type I interferons, the inflammasome, phagocytosis, antigen processing and presentation, and cell signaling. DUOX1 and DUOX2 play important roles in innate immune defenses at epithelial barriers. This review discusses the role of NOX enzymes in normal physiological processes as well as in disease. NOX enzymes are important in autoimmune diseases like type 1 diabetes and have also been implicated in acute lung injury caused by infection with SARS-CoV-2. Targeting NOX enzymes directly or through scavenging free radicals may be useful therapies for autoimmunity and acute lung injury where oxidative stress contributes to pathology.

Pathogenic lipid-binding antiphospholipid antibodies are associated with severity of COVID-19

BACKGROUND

Coronavirus disease 19 (COVID-19)-associated coagulopathy is a hallmark of disease severity and poor prognosis. The key manifestations of this prothrombotic syndrome-microvascular thrombosis, stroke, and venous and pulmonary clots-are also observed in severe and catastrophic antiphospholipid syndrome. Antiphospholipid antibodies (aPL) are detectable in COVID-19 patients, but their association with the clinical course of COVID-19 remains unproven.

OBJECTIVES

To analyze the presence and relevance of lipid-binding aPL in hospitalized COVID-19 patients.

METHODS

Two cohorts of 53 and 121 patients from a single center hospitalized for PCR-proven severe acute respiratory syndrome-coronavirus 2 infection were analyzed for the presence of aPL and clinical severity of COVID-19.

RESULTS

We here demonstrate that lipid-binding aPL are common in COVID-19. COVID-19 patients with lipid-binding aPL have higher median concentrations of C-reactive protein and D-dimer, and are more likely to have a critical clinical course and fatal outcome. Lipid-binding aPL isolated from COVID-19 patients target the recently described cell surface complex of lysobisphosphatidic acid (LBPA) with the protein C receptor (EPCR) to induce prothrombotic and inflammatory responses in monocytes and endothelial cells. We show that B1a cells producing lipid-reactive aPL of the IgG isotype circulate in the blood of COVID-19 patients. In vivo, COVID-19 aPL accelerate thrombus formation in an experimental mouse model dependent on the recently delineated signaling pathway involving EPCR-LBPA.

CONCLUSIONS

COVID-19 patients rapidly expand B1a cells secreting pathogenic lipid-binding aPL with broad thrombotic and inflammatory effects. The association with markers of inflammation and coagulation, clinical severity, and mortality suggests a causal role of aPL in COVID-19-associated coagulopathy.

Microparticles: An Alternative Explanation to the Behavior of Vascular Antiphospholipid Syndrome

Antiphospholipid syndrome is an autoimmune disease characterized by the persistent presence of antiphospholipid antibodies, along with occurrence of vascular thrombosis and pregnancy morbidity. The variety of antiphospholipid antibodies and their related mechanisms, as well as the behavior of disease in wide groups of patients, have led some authors to propose a differentiation of this syndrome into two independent entities: vascular and obstetric antiphospholipid syndrome. Thus, previous studies have discussed whether specific autoantibodies may be responsible for this differentiation or, in contrast, how the same antibodies are able to generate two different clinical presentations. This discussion is yet to be settled. The capability of serum IgG from patients with vascular thrombosis to trigger the biogenesis of endothelial cell-derived microparticles in vitro is one of the previously discussed differences between the clinical entities of antiphospholipid syndrome. These vesicles constitute a prothrombotic mechanism as they can directly lead to clot activation in murine models and recalcified human plasma. Nevertheless, other indirect mechanisms by which microparticles can spread a procoagulant phenotype could be critical to understanding their role in antiphospholipid syndrome. For this reason, questions regarding the cargo of microparticles, and the signaling pathways involved in their biogenesis, are of interest in attempting to explain the behavior of this autoimmune disease.

Induction of tissue factor expression by anti-β2-glycoprotein I is mediated by tumor necrosis factor α

Antiphospholipid antibodies (aPL) are heterogeneous and there is evidence that binding specificity determines which cellular effects they can trigger. We have therefore hypothesised that the induction of tissue factor (TF) in monocytes and endothelial cells by aPL depends on their binding specificity. To further investigate this, we have analyzed the ability of three human monoclonal aPL with distinctly different binding specificities to induce transcription and cell surface expression of TF in monocytes and endothelial cells. Results with human monoclonal aPL were validated with IgG-fractions obtained from patients with APS. We confirmed previous results that a lipid reactive human monoclonal aPL rapidly induced TF transcription and cell surface expression in monocytes and endothelial cells. A monoclonal aPL reactive against β2 glycoprotein I (β2GPI) induced TF with a delayed time course. This was fully dependent on the induction of tumor necrosis factor alpha (TNFα) secretion as capture of TNFα by adalimumab prevented TF induction. This pattern was confirmed with patient IgG fractions. Both lipid reactive and anti-β2GPI induced TF transcription. Unexpectedly, this activity of anti-β2GPI was mediated fully by TNFα which was secreted in response to incubation with anti-β2GPI. The role of TNFα in mediating TF induction by anti-β2GPI may have wider implications for APS pathogenesis.

Thrombotic complications of COVID-19 may reflect an upregulation of endothelial tissue factor expression that is contingent on activation of endosomal NADPH oxidase

The high rate of thrombotic complications associated with COVID-19 seems likely to reflect viral infection of vascular endothelial cells, which express the ACE2 protein that enables SARS-CoV-2 to invade cells. Various proinflammatory stimuli can promote thrombosis by inducing luminal endothelial expression of tissue factor (TF), which interacts with circulating coagulation factor VII to trigger extrinsic coagulation. The signalling mechanism whereby these stimuli evoke TF expression entails activation of NADPH oxidase, upstream from activation of the NF-kappaB transcription factor that drives the induced transcription of the TF gene. When single-stranded RNA viruses are taken up into cellular endosomes, they stimulate endosomal formation and activation of NADPH oxidase complexes via RNA-responsive toll-like receptor 7. It is therefore proposed that SARS-CoV-2 infection of endothelial cells evokes the expression of TF which is contingent on endosomal NADPH oxidase activation. If this hypothesis is correct, hydroxychloroquine, spirulina (more specifically, its chromophore phycocyanobilin) and high-dose glycine may have practical potential for mitigating the elevated thrombotic risk associated with COVID-19.

New insights into antiphospholipid-related endothelial dysfunction by assessment of vascular glycocalyx layer: results from a preliminary cross-sectional study

Introduction

Antiphospholipid syndrome (APS) is associated with greater atherothrombotic risk and endothelial dysfunction, suggesting that endothelial glycocalyx is impaired in this disease.

Objectives

The aim was to investigate the endothelial glycocalyx and the relationship between glycocalyx markers, endothelial dysfunction parameters and atherosclerotic markers in APS.

Methods

A total of 15 primary arterial APS patients and healthy controls were included in the study. Glycocalyx was assessed in both groups by sublingual sidestream dark field imaging and syndecan-1 plasma level. Endothelial function was evaluated by brachial artery flow-mediated dilatation (FMD) and early atherosclerosis by carotid intima media thickness (IMT). Thrombotic profile was also performed by measuring the plasma level of the tissue factor (TF).

Results

APS patients had significantly increased syndecan-1 plasma level 38.6 ± 5.0 pg/ml vs. 19.1 ± 3.5 pg/ml; p < 0.01 and a reduced glycocalyx thickness 0.26 ± 0.03 µm vs. 0.75 ± 0.07 µm; p < 0.01 compared with control. FMD was impaired in APS patients compared with control, 5.68% ± 0.42 vs. 8.29 ± 0.30, p < 0.01, respectively. IMT was significantly increased in APS patients compared with control, 0.52 ± 0.13 mm vs. 0.40 ± 0.06 mm, p < 0.01, respectively. Soluble TF, thiobarbituric acid-reactive substances levels were increased in the sera from APS patients compared with control.

Conclusions

This preliminary study supports, for the first time, that in APS patients endothelial glycocalyx is impaired, which could lead to thrombosis, endothelial dysfunction and early atherosclerosis.

Oxidative stress in endothelial cells induced by the serum of women with different clinical manifestations of the antiphospholipid syndrome

Introducción.

El síndrome antifosfolípido se caracteriza por la presencia persistente de anticuerpos antifosfolípidos y manifestaciones clínicas de trombosis o morbilidad gestacional, las cuales se asocian con estrés oxidativo y disfunción endotelial.

Objetivo.

Evaluar los marcadores de estrés oxidativo en células endoteliales, inducidos por el suero de mujeres con diferentes manifestaciones clínicas del síndrome antifosfolípido y analizar la capacidad antioxidante de los sueros.

Materiales y métodos.

Se incluyeron 48 mujeres que fueron clasificadas así: presencia de anticuerpos antifosfolípidos y criterios clínicos de morbilidad gestacional, trombosis vascular o ambas. Como grupos control se incluyeron mujeres negativas para anticuerpos antifosfolípidos. En un modelo in vitro de células endoteliales estimuladas con los sueros de las mujeres del estudio, se determinaron algunos marcadores de estrés oxidativo por citometría de flujo. También, se analizó la capacidad antioxidante de los sueros incluidos.

Resultados.

Los sueros de los grupos de mujeres con síndrome antifosfolípido que presentaban trombosis, con morbilidad gestacional o sin ella, generaron un incremento significativo (p<0,05 y p<0,001) en los marcadores de estrés oxidativo endotelial, en contraste con el control de suero humano normal. No se observaron diferencias en el efecto de los sueros de los diferentes grupos de estudio sobre la lipoperoxidación endotelial. Tampoco se encontró diferencia en la actividad antioxidante de los sueros.

Conclusión.

El estrés oxidativo mitocondrial en el endotelio se asocia con la presencia de trombosis. Sin embargo, cuando esta se asocia con morbilidad gestacional, también se genera estrés oxidativo intracelular.

NADPH oxidase in the vasculature: Expression, regulation and signalling pathways; role in normal cardiovascular physiology and its dysregulation in hypertension

The last 20-25 years have seen an explosion of interest in the role of NADPH oxidase (NOX) in cardiovascular function and disease. In vascular smooth muscle and endothelium, NOX generates reactive oxygen species (ROS) that act as second messengers, contributing to the control of normal vascular function. NOX activity is altered in response to a variety of stimuli, including G-protein coupled receptor agonists, growth-factors, perfusion pressure, flow and hypoxia. NOX-derived ROS are involved in smooth muscle constriction, endothelium-dependent relaxation and smooth muscle growth, proliferation and migration, thus contributing to the fine-tuning of blood flow, arterial wall thickness and vascular resistance. Through reversible oxidative modification of target proteins, ROS regulate the activity of protein tyrosine phosphatases, kinases, G proteins, ion channels, cytoskeletal proteins and transcription factors. There is now considerable, but somewhat contradictory evidence that NOX contributes to the pathogenesis of hypertension through oxidative stress. Specific NOX isoforms have been implicated in endothelial dysfunction, hyper-contractility and vascular remodelling in various animal models of hypertension, pulmonary hypertension and pulmonary arterial hypertension, but also have potential protective effects, particularly NOX4. This review explores the multiplicity of NOX function in the healthy vasculature and the evidence for and against targeting NOX for antihypertensive therapy.

Neutrophils-Important Communicators in Systemic Lupus Erythematosus and Antiphospholipid Syndrome

Systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS) are two autoimmune diseases that can occur together or separately. Insights into the pathogenesis have revealed similarities, such as development of autoantibodies targeting subcellular antigens as well as a shared increased risk of cardiovascular morbidity, potentially due to mutual pathologic mechanisms. In this review, we will address the evidence implicating neutrophils in the pathogenesis of these conditions, highlighting their shared features. The neutrophil is the most abundant leukocyte, recognized for its role in infectious and inflammatory diseases, but dysregulation of neutrophil effector functions, including phagocytosis, oxidative burst and formation of neutrophil extracellular traps (NETs) may also contribute to an autoimmune process. The phenotype of neutrophils in SLE and APS differs from neutrophils of healthy individuals, where neutrophils in SLE and APS are activated and prone to aggregate. A specific subset of low-density neutrophils with different function compared to normal-density neutrophils can also be found within the peripheral blood mononuclear cell (PBMC) fraction after density gradient centrifugation of whole blood. Neutrophil phagocytosis is required for regular clearance of cell remnants and nuclear material. Reactive oxygen species (ROS) released by neutrophils during oxidative burst are important for immune suppression and impairment of ROS production is seen in SLE. NETs mediate pathology in both SLE and APS via several mechanisms, including exposure of autoantigens, priming of T-cells and activation of autoreactive B-cells. NETs are also involved in cardiovascular events by forming a pro-thrombotic scaffolding surface. Lastly, neutrophils communicate with other cells by producing cytokines, such as Interferon (IFN) -α, and via direct cell-cell contact. Physiological neutrophil effector functions are necessary to prevent autoimmunity, but in SLE and APS these are altered.

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

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

Hydroxychloroquine reverses the prothrombotic state in a mouse model of antiphospholipid syndrome: Role of reduced inflammation and endothelial dysfunction

Antiphospholipid antibodies (aPL) promote endothelial dysfunction, inflammation and procoagulant state. We investigated the effect of hydroxychloroquine (HCQ) on prothrombotic state and endothelial function in mice and in human aortic endothelial cells (HAEC). Human aPL were injected to C57BL/6 mice treated or not with HCQ. Vascular endothelial function and eNOS were assessed in isolated mesenteric arteries. Thrombosis was assessed both in vitro by measuring thrombin generation time (TGT) and tissue factor (TF) expression and in vivo by the measurement of the time to occlusion in carotid and the total thrombosis area in mesenteric arteries. TGT, TF, and VCAM1 expression were evaluated in HAEC. aPL increased VCAM-1 expression and reduced endothelium dependent relaxation to acetylcholine. In parallel, aPL shortened the time to occlusion and extended thrombus area in mice. This was associated with an overexpression of TF and an increased TGT in mice and in HAEC. HCQ reduced clot formation as well as TGT, and improved endothelial-dependent relaxations. Finally, HCQ increased the p-eNOS/eNOS ratio. This study provides new evidence that HCQ improves procoagulant status and vascular function in APS by modulating eNOS, leading to an improvement in the production of NO.

Complement C5 but not C3 is expendable for tissue factor activation by cofactor-independent antiphospholipid antibodies

The complement and coagulation cascades interact at multiple levels in thrombosis and inflammatory diseases. In venous thrombosis, complement factor 3 (C3) is crucial for platelet and tissue factor (TF) procoagulant activation dependent on protein disulfide isomerase (PDI). Furthermore, C5 selectively contributes to the exposure of leukocyte procoagulant phosphatidylserine (PS), which is a prerequisite for rapid activation of monocyte TF and fibrin formation in thrombosis. Here, we show that monoclonal cofactor-independent antiphospholipid antibodies (aPLs) rapidly activate TF on myelomonocytic cells. TF activation is blocked by PDI inhibitor and an anti-TF antibody interfering with PDI binding to TF, and requires C3 but unexpectedly not C5. Other prothrombotic, complement-fixing antibodies, for example, antithymocyte globulin, typically induce TF activation dependent on C5b-7-mediated PS exposure on the outer membrane of monocytes. We show that aPLs directly induce procoagulant PS exposure independent of C5. Accordingly, mice deficient in C3, but not mice deficient in C5, are protected from in vivo thrombus formation induced by cofactor-independent aPLs. Only immunoglobulin G (IgG) fractions with cofactor-independent anticardiolipin reactivity from patients with antiphospholipid syndrome (APS) induce complement-independent monocyte PS exposure and PDI-dependent TF activation. Neither a human monoclonal aPL directed against β2-glycoprotein I (β2GPI) nor patient IgG with selective reactivity to β2GPI rapidly activated monocyte TF. These results indicate that inhibitors of PDI and TF, but not necessarily clinically available drugs targeting C5, have therapeutic benefit in preventing thrombosis associated with APS caused by pathogenic aPLs primarily reactive with lipid, independent of β2GPI.

Pathogenesis of antiphospholipid syndrome: recent insights and emerging concepts

Introduction: Even though our understanding of the antiphospholipid syndrome (APS) has improved tremendously over the last decades, we are still not in a position to replace symptomatic anticoagulation by pathogenesis based causal treatments. Areas covered: Recent years have provided further insights into pathogenetically relevant mechanisms. These include a differentiation of pathogenic subtypes of antiphospholipid antibodies (aPL), novel mechanisms modulating disease activity, for example, extracellular vesicles and microRNA, and novel players in pathogenesis, for example, neutrophils and neutrophil extracellular traps (NETs). Expert commentary: It is evident that aPL induce a proinflammatory and procoagulant state and recent data suggest that different aPL species activate different signaling pathways which sometimes converge into a common cellular response. This implies that presence of more than one aPL species may disproportionally increase the risk for the major manifestations of APS, that is, thrombosis and fetal loss. Further delineation of the pathogenic mechanisms will hopefully provide clues to causal rather than symptomatic treatments of APS.

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

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

Pathophysiological insights into the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is characterized by venous and/or arterial thrombosis and severe pregnancy morbidity in presence of antiphospholipid antibodies (aPL). While there is compelling evidence that aPL cause the clinical manifestations of APS, the underlying mechanisms are still a matter of scientific debate. This is mainly related to the broad heterogeneity of aPL. There are three major types of aPL: The first one binds to (anionic) phospholipids, e.g. cardiolipin, in absence of other factors (cofactor independent aPL). The second type binds to phospholipids only in presence of protein cofactors, e.g. ß2-glycoprotein I (ß2GPI) (cofactor dependent aPL). The third type binds to cofactor proteins directly without need for phospholipids. It is widely believed that cofactor independent aPL (type 1) are associated with infections and, more importantly, non-pathogenic, while pathogenic aPL belong to the second and in particular to the third type. This view, in particular with regard to type 1 aPL, has not been undisputed and novel research data have shown that it is in fact untenable. We summarize the available data on the pathogenetic role of aPL and the implications for diagnosis of APS and future research.

Antiphospholipid antibody-induced cellular responses depend on epitope specificity : implications for treatment of antiphospholipid syndrome

Essentials Antiphospholipid antibodies (aPL) are heterogeneous and induce different cellular responses. We analyzed signaling events induced by different monoclonal and patient aPL in monocytes. Two major signaling pathways involving either NADPH-oxidase or LRP8 were identified. Our data suggest that these two pathways mediate the majority of aPL effects on monocytes.

SUMMARY

Background Antiphospholipid antibodies (aPLs) contribute to the pathogenesis of the antiphospholipid syndrome (APS) by induction of an inflammatory and procoagulant state in different cell types, and several signaling pathways have been described. Objectives To investigate whether signaling depends on the epitope specificity of aPLs. Methods Cellular effects of three human monoclonal aPLs with distinctly different epitope specificities were analyzed in vitro. Expression of tumor necrosis factor-α mRNA by mouse and human monocytes was the major readout. Analysis included cells from genetically modified mice, and the use of specific inhibitors in human monocytes. Data were validated with IgG isolated from 20 APS patients. Results Cofactor-independent anticardiolipin aPLs activated monocytes by induction of endosomal NADPH oxidase. Activation could be blocked by hydroxychloroquine (HCQ). Anti-β2 -glycoprotein I aPL activated monocytes by interacting with LDL receptor-related protein 8 (LRP8). This could be blocked by rapamycin. Analysis of 20 APS patients' IgG showed that all IgG fractions activated the same two pathways as the monoclonal aPL, depending on their epitope patterns as determined by ELISA. Monocyte activation by APS IgG could be blocked completely by HCQ and/or rapamycin, suggesting that in most, if not all, APS patients there is no other relevant signaling pathway. Conclusions aPLs activate two major proinflammatory signal transduction pathways, depending on their epitope specificity. HCQ and rapamycin, either alone or in combination, completely suppress signaling by APS IgG. These observations may provide a rationale for specific treatment of APS patients according to their aPL profile.

Cofactor-independent antiphospholipid antibodies activate the NLRP3-inflammasome via endosomal NADPH-oxidase: implications for the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an autoimmune disease characterised by thromboembolic events and/or pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Here we show that three cofactor independent human monoclonal aPL can induce transcription of NLRP3 and caspase-1 resulting in inflammasome activation specific for NLRP3. This depends fully on activation of endosomal NADPH-oxidase-2 (NOX2) by aPL. Activation of NOX2 and subsequent inflammasome activation by aPL are independent from TLR2 or TLR4. While endosomal superoxide production induces caspase-1 and NLRP3 transcription, it does not affect prae-IL-1β transcription. Therefore, release of IL-1β occurs only after activation of additional pathways like TLR7/8 or TLR2. All effects exerted by the monoclonal aPL can be reproduced with IgG fractions of APS patients proving that the monoclonal aPL are representative for the APS. IgG fractions of healthy controls or patients suffering from systemic lupus erythematosus have no effect. In a mouse model of the APS we can show inflammasome activation in vivo. Furthermore, mononuclear cells isolated from patients with the APS show an increased expression of caspase-1 and NLRP3 which is accompanied by a three-fold increased serum concentration of IL-1β suggesting chronic inflammasome activation in APS patients. In summary, we provide further evidence that endosomal NOX2 can be activated by cofactor independent aPL. This leads to induction of the NLRP3 inflammasome. Our data indicate that cofactor independent aPL might contribute significantly to the pathogenesis of the APS.

Endosomal Redox Signaling in the Antiphospholipid Syndrome

PURPOSE OF REVIEW

It is well established that the antiphospholipid syndrome (APS) is caused by antiphospholipid antibodies (aPL). While several underlying mechanisms have been described in the past, many open questions remain. Here, we will review data on endosomal signaling and, in particular, redox signaling in APS.

RECENT FINDINGS

Endosomal redox signaling has been implicated in several cellular processes including signaling of proinflammatory cytokines. We have shown that certain aPL can activate endosomal NADPH-oxidase (NOX) in several cell types followed by induction of proinflammatory and procoagulant cellular responses in vitro. Involvement of endosomes in aPL signaling has also been reported by others. In wild-type mice but not in NOX-deficient mice, aPL accelerate venous thrombus formation underscoring the relevance of endosomal NOX. Furthermore, hydroxychloroquine (HCQ) inhibits activation of endosomal NOX and prevents thrombus formation in aPL-treated mice. Endosomal redox signaling is an important novel mechanism involved in APS pathogenesis. This makes endosomes a potential target for future treatment approaches of APS.

Hydroxychloroquine inhibits proinflammatory signalling pathways by targeting endosomal NADPH oxidase

OBJECTIVES

Hydroxychloroquine (HCQ) has been used for decades to treat patients with rheumatic diseases, for example, systemic lupus erythematosus (SLE), rheumatoid arthritis or the antiphospholipid syndrome (APS). We hypothesise that HCQ might target endosomal NADPH oxidase (NOX), which is involved in the signal transduction of cytokines as well as antiphospholipid antibodies (aPL).

METHODS

For in vitro experiments, monocytic cells were stimulated with tumour necrosis factor α (TNFα), interleukin-1β (IL-1β) or a human monoclonal aPL and the activity of NOX was determined by flow cytometry. The expression of genes known to be induced by these stimuli was quantified by quantitative reverse transcription PCR. Live cell imaging was performed by confocal laser scanning microscopy. Finally, the effects of HCQ on NOX-induced signal transduction were analysed in an in vivo model of venous thrombosis.

RESULTS

HCQ strongly reduces or completely prevents the induction of endosomal NOX by TNFα, IL-1β and aPL in human monocytes and MonoMac1 cells. As a consequence, induction of downstream genes by these stimuli is reduced or abrogated. This effect of HCQ is not mediated by direct interference with the agonists but by inhibiting the translocation of the catalytic subunit of NOX2 (gp91phox) into the endosome. In vivo, HCQ protects mice from aPL-induced and NOX2-mediated thrombus formation.

CONCLUSIONS

We describe here a novel mechanism of action of HCQ, that is, interference with the assembly of endosomal NOX2. Since endosomal NOX2 is involved in many inflammatory and prothrombotic signalling pathways, this activity of HCQ might explain many of its beneficial effects in rheumatic diseases including the APS.

Antiphospholipid Antibodies: Their Origin and Development

Antiphospholipid antibodies (aPL) are a hallmark of the antiphospholipid syndrome (APS), which is the most commonly acquired thrombophilia. To date there is consensus that aPL cause the clinical manifestations of this potentially devastating disorder. However, there is good evidence that not all aPL are pathogenic. For instance, aPL associated with syphilis show no association with the manifestations of APS. While there has been intensive research on the pathogenetic role of aPL, comparably little is known about the origin and development of aPL. This review will summarize the current knowledge and understanding of the origin and development of aPL derived from animal and human studies.

Cofactor-independent human antiphospholipid antibodies induce venous thrombosis in mice

Essentials Cofactor-independent antiphospholipid antibodies (CI-aPL) are generally considered non-pathogenic. We analyzed the effects of human monoclonal CI-aPL in a mouse model of venous thrombosis. As shown in vitro, CI-aPL induce a procoagulant state in vivo by activation of endosomal NADPH-oxidase. Contrary to common belief, CI-aPL induce venous thrombosis in vivo.

SUMMARY

Background There is general consensus that the antiphospholipid syndrome (APS) is caused by antiphospholipid antibodies (aPL) with antibodies against β2-glycoprotein-I being the most relevant. aPL that bind phospholipids in the absence of protein cofactors are generally considered pathogenetically irrelevant. We showed that cofactor-independent human monoclonal aPL isolated from APS patients induce proinflammatory and procoagulant cellular responses by activating endosomal NADPH-oxidase 2 (NOX2). Similar aPL were detected in all IgG fractions from APS patients analyzed. Objectives We aimed to clarify if cofactor-independent aPL can be thrombogenic in vivo and, if so, whether these effects are mediated via activation of NOX2. Methods Two cofactor-independent human monoclonal aPL, HL5B and RR7F, were tested in a mouse model of venous thrombosis. Genetically modified mice and in vitro assays were used to delineate the mechanisms underlying thrombus induction. Results HL5B and RR7F dramatically accelerate thrombus formation in this mouse model. Thrombus formation depends on tissue factor activation. It cannot be induced in NOX2-deficient mice. Bone marrow chimeras of C57BL/6J mice reconstituted with NOX2-deficient bone marrow showed that leukocyte activation plays a major role in thrombus formation. Neither TLR4 signaling nor platelet activation by our aPL is required for venous thrombus formation. Conclusions Cofactor-independent aPL can induce thrombosis in vivo. This effect is mainly mediated by leukocyte activation, which depends on the previously described signal transduction via endosomal NOX2. Because most APS patients have been shown to harbor aPL with similar activity, our data are of general relevance for the APS.

Antiphospholipid antibodies in a large population-based cohort: genome-wide associations and effects on monocyte gene expression

The antiphospholipid syndrome (APS) is characterised by venous and/or arterial thrombosis and pregnancy morbidity in women combined with the persistent presence of antiphospholipid antibodies (aPL). We aimed to identify genetic factors associated with the presence of aPL in a population based cohort. Furthermore, we wanted to clarify if the presence of aPL affects gene expression in circulating monocytes. Titres of IgG and IgM against cardiolipin, β2glycoprotein 1 (anti-β2GPI), and IgG against domain 1 of β2GPI (anti-domain 1) were determined in approx. 5,000 individuals from the Gutenberg Health Study (GHS) a population based cohort of German descent. Genotyping was conducted on Affymetrix Genome-Wide Human SNP 6.0 arrays. Monocyte gene expression was determined in a subgroup of 1,279 individuals by using the Illumina HT-12 v3 BeadChip. Gene expression data were confirmed in vitro and ex vivo by qRT-PCR. Genome wide analysis revealed significant associations of anti-β2GPI IgG and APOH on chromosome 17, which had been previously identified by candidate gene approaches, and of anti-domain1 and MACROD2 on chromosome 20 which has been listed in a previous GWAS as a suggestive locus associated with the occurrence of anti-β2GPI antibodies. Expression analysis confirmed increased expression of TNFα in monocytes and identified and confirmed neuron navigator 3 (NAV3) as a novel gene induced by aPL. In conclusion, MACROD2 represents a novel genetic locus associated with aPL. Furthermore, we show that aPL induce the expression of NAV3 in monocytes and endothelial cells. This will stimulate further research into the role of these genes in the APS.

NF-κB is activated from endosomal compartments in antiphospholipid antibodies-treated human monocytes

BACKGROUND

The antiphospholipid antibody syndrome (APS) is an autoimmune disease associated with arterial or venous thrombosis and/or recurrent fetal loss and is caused by pathogenic antiphospholipid antibodies (aPLA). We recently demonstrated that Toll-like receptor 2 (TLR2) and CD14 contribute to monocyte activation of aPLA.

OBJECTIVE

To study the mechanisms of cell activation by aPLA, leading to pro-coagulant and pro-inflammatory responses.

METHODS AND RESULTS

For this study, we used purified antibodies from the plasmas of 10 different patients with APS and healthy donors. We demonstrate that aPLA, but not control IgG, co-localizes with TLR2 and TLR1 or TLR6 on human monocytes. Blocking antibodies to TLR2, TLR1 or TLR6, but not to TLR4, decreased TNF and tissue factor (TF) responses to aPLA. Pharmacological and siRNA approaches revealed the importance of the clathrin/dynamin-dependent endocytic pathway in cell activation by aPLA. In addition, soluble aPLA induced NF-κB activation, while bead-immobilized aPLA beads, which cannot be internalized, were unable to activate NF-κB. Internalization of aPLA in monocytes and NF-κB activation were dependent on the presence of CD14.

CONCLUSION

We show that TLR2 and its co-receptors, TLR1 and TLR6, contribute to the pathogenicity of aPLA, that aPLA are internalized via clathrin- and CD14-dependent endocytosis and that endocytosis is required for NF-κB activation. Our results contribute to a better understanding of the APS and provide a possible therapeutic approach.

Endo-PDI is required for TNFα-induced angiogenesis

Protein disulfide isomerase (PDI) and its homologs are oxidoreductases facilitating protein folding in the ER. Endo-PDI (also termed ERp46) is highly expressed in endothelial cells. It belongs to the PDI family but its physiological function is largely unknown. We studied the role of Endo-PDI in endothelial angiogenic responses. Stimulation of human umbilical vein endothelial cells (with TNFα (10ng/ml) increased ERK1/2 phosphorylation. This effect was largely attenuated by Endo-PDI siRNA, whereas JNK and p38 MAP kinase phosphorylation was Endo-PDI independent. Similarly, TNFα-stimulated NF-κB signaling determined by IκBα degradation as well as TNFα-induced ICAM expression was unaffected by Endo-PDI siRNA. The action of Endo-PDI was not mediated by extracellular thiol exchange or cell surface PDI as demonstrated by nonpermeative inhibitors and PDI-neutralizing antibody. Moreover, exogenously added PDI failed to restore ERK1/2 activation after Endo-PDI knockdown. This suggests that Endo-PDI acts intracellularly potentially by maintaining the Ras/Raf/MEK/ERK pathway. Indeed, knockdown of Endo-PDI attenuated Ras activation measured by G-LISA and Raf phosphorylation. ERK activation influences gene expression by the transcriptional factor AP-1, which controls MMP-9 and cathepsin B, two proteases required for angiogenesis. TNFα-stimulated MMP-9 and cathepsin B induction was reduced by silencing of Endo-PDI. Accordingly, inhibition of cathepsin B or Endo-PDI siRNA blocked the TNFα-stimulated angiogenic response in the spheroid outgrowth assays. Moreover ex vivo tube formation and in vivo Matrigel angiogenesis in response to TNFα were attenuated by Endo-PDI siRNA. In conclusion, our study establishes Endo-PDI as a novel, important mediator of AP-1-driven gene expression and endothelial angiogenic function.

Receptors involved in cell activation by antiphospholipid antibodies

The antiphospholipid syndrome (APS) is an autoimmune disease associated with arterial or venous thrombosis and/or recurrent fetal loss and is caused by pathogenic antiphospholipid antibodies (aPLA). The plasma protein β2-glycoprotein 1 (β2GP1) has been identified as a major target of aPLA associated with APS. Cell activation by aPLA appears to be a major pathogenic cause in the pathogenesis of APS. Receptors, co-receptors and accessory molecules are known to assist the pathogenic effects of aPLA. Members of the TLR family and the platelet receptor apolipoprotein E receptor 2' (apoER2'), a receptor belonging to the low-density lipoprotein receptor (LDL-R) family, as well as GPIbα, were identified as putative candidates for aPLA recognition. CD14, a co-receptor for TLR2 and TLR4, and annexin A2, a ubiquitous Ca2+ -binding protein that is essential for actin-dependent vesicle transport, could serve as important accessory molecules in mediating the pathogenic effects of aPLA. Finally, complement activation has been reported in association with the pathogenicity of APS. The relative contribution of these different mechanisms in the pathogenesis of APS is controversial. Here, we review the various in vivo and in vitro models that have been used to investigate the pathogenic mechanisms of aPLA in APS.