A role for Toll-like receptor mediated signals in neutrophils in the pathogenesis of the anti-phospholipid syndrome

Anti-phospholipid autoantibodies in human diseases

Anti-phospholipid autoantibodies are a group of antibodies that can specifically bind to anionic phospholipids and phospholipid protein complexes. Recent studies have reported elevated serum anti-phospholipid autoantibody levels in patients with antiphospholipid syndrome, systemic lupus erythematosus, rheumatoid arthritis, metabolic disorders, malaria, SARS-CoV-2 infection, obstetric diseases and cardiovascular diseases. However, the underlying mechanisms of anti-phospholipid autoantibodies in disease pathogenesis remain largely unclear. Emerging evidence indicate that anti-phospholipid autoantibodies modulate NETs formation, monocyte activation, blockade of apoptotic cell phagocytosis in macrophages, complement activation, dendritic cell activation and vascular endothelial cell activation. Herein, we provide an update on recent advances in elucidating the effector mechanisms of anti-phospholipid autoantibodies in the pathogenesis of various diseases, which may facilitate the development of potential therapeutic targets for the treatment of anti-phospholipid autoantibody-related disorders.

Low-density granulocytes are related to shorter pregnancy duration but not to interferon alpha protein blood levels in systemic lupus erythematosus

BACKGROUND

An increased risk of pregnancy complications is seen in women with systemic lupus erythematosus (SLE), but the specific immunopathological drivers are still unclear. Hallmarks of SLE are granulocyte activation, type I interferon (IFN) overproduction, and autoantibodies. Here we examined whether low-density granulocytes (LDG) and granulocyte activation increase during pregnancy, and related the results to IFNα protein levels, autoantibody profile, and gestational age at birth.

METHODS

Repeated blood samples were collected during pregnancy in trimesters one, two, and three from 69 women with SLE and 27 healthy pregnant women (HC). Nineteen of the SLE women were also sampled late postpartum. LDG proportions and granulocyte activation (CD62L shedding) were measured by flow cytometry. Plasma IFNα protein concentrations were quantified by single molecule array (Simoa) immune assay. Clinical data were obtained from medical records.

RESULTS

Women with SLE had higher LDG proportions and increased IFNα protein levels compared to HC throughout pregnancy, but neither LDG fractions nor IFNα levels differed during pregnancy compared to postpartum in SLE. Granulocyte activation status was higher in SLE relative to HC pregnancies, and it was increased during pregnancy compared to after pregnancy in SLE. Higher LDG proportions in SLE were associated with antiphospholipid positivity but not to IFNα protein levels. Finally, higher LDG proportions in trimester three correlated independently with lower gestational age at birth in SLE.

CONCLUSION

Our results suggest that SLE pregnancy results in increased peripheral granulocyte priming, and that higher LDG proportions late in pregnancy are related to shorter pregnancy duration but not to IFNα blood levels in SLE.

Antiphospholipid Syndrome in Pregnancy: New and Old Pathogenetic Mechanisms

The antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized, according to the Sydney criteria, by the persistent presence of autoantibodies directed against phospholipid-binding proteins associated with thrombosis and/or obstetrical complications. The most frequent complications in obstetric antiphospholipid syndrome are recurrent pregnancy losses and premature birth due to placental insufficiency or severe preeclampsia. In recent years, vascular APS (VAPS) and obstetric APS (OAPS) have been described as two different clinical entities. In VAPS, antiphospholipid antibodies (aPL) interfere with the mechanisms of coagulation cascade and the 'two hit hypothesis' has been suggested to explain why aPL positivity does not always lead to thrombosis. OAPS seems to involve additional mechanisms, such as the direct action of anti-β2 glycoprotein-I on trophoblast cells that can lead to a direct placental functional damage. Furthermore, new actors seem to play a role in the pathogenesis of OAPS, including extracellular vesicles, micro-RNAs and the release of neutrophil extracellular traps. The aim of this review is to investigate the state-of-the-art antiphospholipid syndrome pathogenesis in pregnancy, in order to provide a comprehensive overview of both old and new pathogenetic mechanisms involved in this complex disease.

From Co-Infections to Autoimmune Disease via Hyperactivated Innate Immunity: COVID-19 Autoimmune Coagulopathies, Autoimmune Myocarditis and Multisystem Inflammatory Syndrome in Children

Neutrophilia and the production of neutrophil extracellular traps (NETs) are two of many measures of increased inflammation in severe COVID-19 that also accompany its autoimmune complications, including coagulopathies, myocarditis and multisystem inflammatory syndrome in children (MIS-C). This paper integrates currently disparate measures of innate hyperactivation in severe COVID-19 and its autoimmune complications, and relates these to SARS-CoV-2 activation of innate immunity. Aggregated data include activation of Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD) receptors, NOD leucine-rich repeat and pyrin-domain-containing receptors (NLRPs), retinoic acid-inducible gene I (RIG-I) and melanoma-differentiation-associated gene 5 (MDA-5). SARS-CoV-2 mainly activates the virus-associated innate receptors TLR3, TLR7, TLR8, NLRP3, RIG-1 and MDA-5. Severe COVID-19, however, is characterized by additional activation of TLR1, TLR2, TLR4, TLR5, TLR6, NOD1 and NOD2, which are primarily responsive to bacterial antigens. The innate activation patterns in autoimmune coagulopathies, myocarditis and Kawasaki disease, or MIS-C, mimic those of severe COVID-19 rather than SARS-CoV-2 alone suggesting that autoimmunity follows combined SARS-CoV-2-bacterial infections. Viral and bacterial receptors are known to synergize to produce the increased inflammation required to support autoimmune disease pathology. Additional studies demonstrate that anti-bacterial antibodies are also required to account for known autoantigen targets in COVID-19 autoimmune complications.

Targeting thromboinflammation in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a systemic autoimmune disease, where persistent presence of antiphospholipid antibodies (aPL) leads to thrombotic and obstetric complications. APS is a paradigmatic thromboinflammatory disease. Thromboinflammation is a pathophysiological mechanism coupling inflammation and thrombosis, which contributes to the pathophysiology of cardiovascular disease. APS can serve as a model to unravel mechanisms of thromboinflammation and the relationship between innate immune cells and thrombosis. Monocytes are activated by aPL into a proinflammatory and procoagulant phenotype, producing proinflammatory cytokines such as tumor necrosis factor α, interleukin 6, as well as tissue factor. Important cellular signaling pathways involved are the NF-κB-pathway, mammalian target of rapamycin (mTOR) signaling, and the NOD-, LRR-, and pyrin domain-containing protein 3 inflammasome. All of these may serve as future therapeutic targets. Neutrophils produce neutrophil extracellular traps in response to aPL, and this leads to thrombosis. Thrombosis in APS also stems from increased interaction of neutrophils with endothelial cells through P-selectin glycoprotein ligand-1. NETosis can be targeted not only with several experimental therapeutics, such as DNase, but also through the redirection of current therapies such as defibrotide and the antiplatelet agent dipyridamole. Activation of platelets by aPL leads to a procoagulant phenotype. Platelet-leukocyte interactions are increased, possibly mediated by increased levels of soluble P-selectin and soluble CD40-ligand. Platelet-directed future treatment options involve the inhibition of several platelet receptors activated by aPL, as well as mTOR inhibition. This review discusses mechanisms underlying thromboinflammation in APS that present targetable therapeutic options, some of which may be generalizable to other thromboinflammatory diseases.

Mechanisms of immunothrombosis and vasculopathy in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune thrombophilia propelled by circulating antiphospholipid antibodies that herald vascular thrombosis and obstetrical complications. Antiphospholipid antibodies recognize phospholipids and phospholipid-binding proteins and are not only markers of disease but also key drivers of APS pathophysiology. Thrombotic events in APS can be attributed to various conspirators including activated endothelial cells, platelets, and myeloid-lineage cells, as well as derangements in coagulation and fibrinolytic systems. Furthermore, recent work has especially highlighted the role of neutrophil extracellular traps (NETs) and the complement system in APS thrombosis. Beyond acute thrombosis, patients with APS can also develop an occlusive vasculopathy, a long-term consequence of APS characterized by cell proliferation and infiltration that progressively expands the intima and leads to organ damage. This review will highlight known pathogenic factors in APS and will also briefly discuss similarities between APS and the thrombophilic coagulopathy of COVID-19.

A systematic review of the association between anti-β-2 glycoprotein I antibodies and APS manifestations

Anti-β-2 glycoprotein I antibodies (anti-B2GPI) are often cited as the major pathogenically relevant antibody in antiphospholipid syndrome (APS), but it is unclear if there is clinical evidence to support this theory. We performed a systematic review to determine if immunoglobulin G anti-B2GPI positivity was independently associated with thrombotic and/or obstetric manifestations of APS. We searched MEDLINE, EMBASE, The Cochrane Library, and clinicaltrials.gov electronic databases through April 2020 for prospective studies that met prespecified design criteria. Of 4758 articles identified through computer-assisted search, 4 studies examining obstetric outcomes and 2 studies examining thrombotic outcomes were included for qualitative assessment. The presence of anti-B2GPI had only a weak independent association with thrombosis and was, at best, inconsistently associated with obstetric complications. A quantitative assessment could not be performed because of study heterogeneity. The overall quality of the evidence was very low. Although anti-B2GPI are commonly thought to mediate APS manifestations, clinical evidence is lacking with very low-quality data to support a weak association with thrombosis.

NETs in APS: Current Knowledge and Future Perspectives

PURPOSE OF REVIEW

Antiphospholipid syndrome (APS) is a thrombo-inflammatory disease that is primarily treated with anticoagulation. Better understanding the inflammatory aspects of APS could lead to safer, more effective, and more personalized therapeutic options. To this end, we sought to understand recent literature related to the role of neutrophils and, in particular, neutrophil extracellular traps (NETs) in APS.

RECENT FINDINGS

Expression of genes associated with type I interferons, endothelial adhesion, and pregnancy regulation are increased in APS neutrophils. APS neutrophils have a reduced threshold for NET release, which likely potentiates thrombotic events and perhaps especially large-vein thrombosis. Neutrophil-derived reactive oxygen species also appear to play a role in APS pathogenesis. There are new approaches for preventing and disrupting NETs that could potentially be leveraged to reduce the risk of APS-associated thrombosis. Neutrophils and NETs contribute to APS pathophysiology. More precisely understanding their roles at a mechanistic level should help identify new therapeutic targets for inhibiting NET formation, enhancing NET dissolution, and altering neutrophil adhesion. Such approaches may ultimately lead to better clinical management of APS patients and thereby reduce the chronic burden of this disease.

Increased Adhesive Potential of Antiphospholipid Syndrome Neutrophils Mediated by β2 Integrin Mac-1

OBJECTIVE

While the role of antiphospholipid antibodies in activating endothelial cells has been extensively studied, the impact of these antibodies on the adhesive potential of leukocytes has received less attention. This study was undertaken to investigate the extent to which antiphospholipid syndrome (APS) neutrophils adhere to resting endothelial cells under physiologic flow conditions and the surface molecules required for that adhesion.

METHODS

Patients with primary APS (n = 43), patients with a history of venous thrombosis but negative test results for antiphospholipid antibodies (n = 11), and healthy controls (n = 38) were studied. Cells were introduced into a flow chamber and perfused across resting human umbilical vein endothelial cells (HUVECs). Surface adhesion molecules were quantified by flow cytometry. Neutrophil extracellular trap release (NETosis) was assessed in neutrophil-HUVEC cocultures.

RESULTS

Upon perfusion of anticoagulated blood through the flow chamber, APS neutrophils demonstrated increased adhesion as compared to control neutrophils under conditions representative of either venous (n = 8; P < 0.05) or arterial (n = 15; P < 0.0001) flow. At the same time, APS neutrophils were characterized by up-regulation of CD64, CEACAM1, β2 -glycoprotein I, and activated Mac-1 on their surface (n = 12-18; P < 0.05 for all markers). Exposing control neutrophils to APS plasma or APS IgG resulted in increased neutrophil adhesion (n = 10-11; P < 0.0001) and surface marker up-regulation as compared to controls. A monoclonal antibody specific for activated Mac-1 reduced the adhesion of APS neutrophils in the flow-chamber assay (P < 0.01). The same monoclonal antibody reduced NETosis in neutrophil-HUVEC cocultures (P < 0.01).

CONCLUSION

APS neutrophils demonstrate increased adhesive potential, which is dependent upon the activated form of Mac-1. In patients, this could lower the threshold for neutrophil-endothelium interactions, NETosis, and possibly thrombotic events.

Geometrical reorganization of Dectin-1 and TLR2 on single phagosomes alters their synergistic immune signaling

Receptors of innate immune cells function synergistically to detect pathogens and elicit appropriate immune responses. Many receptor pairs also appear "colocalized" on the membranes of phagosomes, the intracellular compartments for pathogen ingestion. However, the nature of the seemingly receptor colocalization and the role it plays in immune regulation are unclear, due to the inaccessibility of intracellular phagocytic receptors. Here, we report a geometric manipulation technique to directly probe the role of phagocytic receptor "colocalization" in innate immune regulation. Using particles with spatially patterned ligands as phagocytic targets, we can decouple the receptor pair, Dectin-1 and Toll-like receptor (TLR)2, to opposite sides on a single phagosome or bring them into nanoscale proximity without changing the overall membrane composition. We show that Dectin-1 enhances immune responses triggered predominantly by TLR2 when their centroid-to-centroid proximity is <500 nm, but this signaling synergy diminishes upon receptor segregation beyond this threshold distance. Our results demonstrate that nanoscale proximity, not necessarily colocalization, between Dectin-1 and TLR2 is required for their synergistic regulation of macrophage immune responses. This study elucidates the relationship between the spatial organization of phagocytic receptors and innate immune responses. It showcases a technique that allows spatial manipulation of receptors and their signal cross-talk on phagosomes inside living cells.

Antifungal Drugs Influence Neutrophil Effector Functions

There is a growing body of evidence for immunomodulatory side effects of antifungal agents on different immune cells, e.g., T cells. Therefore, the aim of our study was to clarify these interactions with regard to the effector functions of polymorphonuclear neutrophils (PMN). Human PMN were preincubated with fluconazole (FLC), voriconazole (VRC), posaconazole (POS), isavuconazole (ISA), caspofungin (CAS), micafungin (MFG), conventional amphotericin B (AMB), and liposomal amphotericin B (LAMB). PMN then were analyzed by flow cytometry for activation, degranulation, and phagocytosis and by dichlorofluorescein assay to detect reactive oxygen species (ROS). Additionally, interleukin-8 (IL-8) release was measured by enzyme-linked immunosorbent assay. POS led to enhanced activation, degranulation, and generation of ROS, whereas IL-8 release was reduced. In contrast, ISA-pretreated PMN showed decreased activation signaling, impaired degranulation, and lower generation of ROS. MFG caused enhanced expression of activation markers but impaired degranulation, phagocytosis, generation of ROS, and IL-8 release. CAS showed increased phagocytosis, whereas degranulation and generation of ROS were reduced. AMB led to activation of almost all effector functions besides impaired phagocytosis, whereas LAMB did not alter any effector functions. Independent from class, antifungal agents show variable influence on neutrophil effector functions in vitro Whether this is clinically relevant needs to be clarified.

Elevated plasma level of soluble triggering receptor expressed on myeloid cells-1 is associated with inflammation activity and is a potential biomarker of thrombosis in primary antiphospholipid syndrome

Background

Soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) is an innate-immune receptor found in blood. Its presence reflects innate immune cell activation. We sought to investigate plasma sTREM-1 levels in patients with primary antiphospholipid syndrome (PAPS).

Methods

A cross-sectional, case-control design was used. Plasma sTREM-1 levels were analyzed by enzyme-linked immunosorbent assay (ELISA) in consecutive patients diagnosed with PAPS or asymptomatic antiphospholipid antibody (APLA) carriers and controls.

Results

The study cohort included 33 patients with PAPS, 10 asymptomatic APLA carriers, and 73 controls. Mean plasma sTREM-1 levels were significantly higher in patients with PAPS (299.2 ± 146.7 pg/ml) and thrombotic PAPS-ever (current and past thrombotic event) (327.2 ± 151.3 pg/ml) compared with controls (230.2 ± 85.5 pg/ml; p = 0.006 and p = 0.003, respectively), patients with thrombotic PAPS compared with patients with past obstetric APS (195.12 ± 58.52 pg/ml, p = 0.01) and APLA carriers (215.8 ± 51.6 pg/ml, p = 0.02), patients with current thrombotic PAPS (429.5 ± 227.5 pg/ml) compared with patients with past thrombotic PAPS (289.5 ± 94.65 pg/ml, p = 0.01), and patients with PAPS who had ever had a stroke or venous thromboembolic event compared with patients who had not (p = 0.007 and p = 0.02, respectively). On receiver operator characteristic curve analysis, plasma sTREM-1 levels differentiated patients with current thrombotic PAPS from asymptomatic APLA carriers and controls, with an area under the curve of 0.7292 (p = 0.0014) and 0.88 (p < 0.0001), respectively. Multivariate regression analysis to identify sTREM-1 predictors (thrombotic PAPS-ever, age, and sex) yielded an independent association of sTREM-1 levels with thrombotic PAPS (p < 0.0001).

Conclusions

Plasma sTREM-1 levels are significantly elevated in patients with thrombotic PAPS. Levels of sTREM-1 might serve as a biomarker for thrombosis in patients with PAPS.

Neutrophils and redox stress in the pathogenesis of autoimmune disease

Polymorphonuclear leukocytes, or neutrophils, are specialist phagocytic cells of the innate immune system. Their primary role is host defence against micro-organisms, which they kill via phagocytosis, followed by release of reactive oxygen species (ROS) and proteolytic enzymes within the phagosome. ROS are generated via the action of the NADPH oxidase (also known as NOX2), in a process termed the 'Respiratory Burst'. This process consumes large amounts of oxygen, which is converted into the highly-reactive superoxide radical O₂^- and H₂O₂. Subsequent activation of myeloperoxidase (MPO) generates secondary oxidants and chloroamines that are highly microbiocidal in nature, which together with proteases such as elastase and gelatinase provide a toxic intra-phagosomal environment able to kill a broad range of micro-organisms. However, under certain circumstances such as during an auto-immune response, neutrophils can be triggered to release ROS and proteases extracellularly causing damage to host tissues, modification of host proteins, lipids and DNA and dysregulation of oxidative homeostasis. This review describes the range of ROS species produced by human neutrophils with a focus on the implications of neutrophil redox products in autoimmune inflammation.

Effects of Toll-like receptor 4 on β2-glycoprotein I-induced splenic T cell subsets differentiation

Our previous study demonstrated that beta 2-glycoprotein I (β2GPI) stimulation promotes bone marrow derived dendritic cells (BMDCs) maturation and T cell proliferation in a Toll-like receptor 4 (TLR4) dependent manner. However, β2GPI induced T cell differentiation and the role of TLR4 in this process have rarely been reported. In the present study, we focused on the differentiation of splenic T cells in β2GPI immunized Balb/c, C3H/HeN and C3H/HeJ mice. According to our results, Th2 dominated differentiation was observed in β2GPI immunized Balb/c and C3H/HeN mice than in those treated with normal saline (NS), namely the up-regulated levels of Th2 markers GATA3 and IL-4 (p < 0.05). Meanwhile, reduced Th1 markers T-bet and IFN-γ, and Treg marker Foxp3 were observed in β2GPI immunized mice (p < 0.05). C3H/HeJ mice have the same gene background with C3H/HeN mice except a functional mutant in TLR4 gene. However, the described Th2 differentiation was not detected in these TLR4 deficient mice, indicating the importance of TLR4 in immune response against β2GPI. In addition, we found that β2GPI-induced Th2 differentiation could be strengthened by cytokines secreted by dendritic cells (DCs) and DCs-T cells interaction. However, DCs-T cells contact was indispensable during this process because of its unique role in suppressing Th1 function. Furthermore, this Th2 biased differentiation pattern was more noticeable in mice received 4 times β2GPI immunization than those received 2 times, suggesting the amplifying effects of anti-β2GPI Ab on β2GPI induced Th2 response. These findings may partly explain the immune imbalance in APS patient through the view angle of T cell differentiation and anti-β2GPI antibody production.

Diagnosis and management of the antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy complications in the presence of persistent antiphospholipid antibodies (APLA). Laboratory diagnosis of APLA depends upon the detection of a lupus anticoagulant, which prolongs phospholipid-dependent anticoagulation tests, and/or anticardiolipin (aCL) and anti-β2-glycoprotein-1 (β2GPI) antibodies. APLA are primarily directed toward phospholipid binding proteins. Pathophysiologic mechanisms underlying thrombosis and pregnancy loss in APS include APLA induced cellular activation, inhibition of natural anticoagulant and fibrinolytic systems, and complement activation, among others. There is a high rate of recurrent thrombosis in APS, especially in triple positive patients (patients with lupus anticoagulant, aCL and anti-β2GPI antibodies), and indefinite anticoagulation with a vitamin K antagonist is the standard of care for thrombotic APS. There is currently insufficient evidence to recommend the routine use of direct oral anticoagulants (DOAC) in thrombotic APS. Aspirin with low molecular weight or unfractionated heparin may reduce the incidence of pregnancy loss in obstetric APS. Recent insights into the pathogenesis of APS have led to the identification of new potential therapeutic interventions, including anti-inflammatory and immunomodulatory therapies. Additional research is needed to better understand the effects of APLA on activation of signaling pathways in vascular cells, to identify more predictive biomarkers that define patients at greatest risk for a first or recurrent APLA-related clinical event, and to determine the safety and efficacy of DOACs and novel anti-inflammatory and immune-modulatory therapies for refractory APS.

Neutrophil-derived chemokines on the road to immunity

During recent years, it has become clear that polymorphonuclear neutrophils are remarkably versatile cells, whose functions go far beyond phagocytosis and killing. In fact, besides being involved in primary defense against infections-mainly through phagocytosis, generation of toxic molecules, release of toxic enzymes and formation of extracellular traps-neutrophils have been shown to play a role in finely regulating the development and the evolution of inflammatory and immune responses. These latter neutrophil-mediated functions occur by a variety of mechanisms, including the production of newly manufactured cytokines. Herein, we provide a general overview of the chemotactic cytokines/chemokines that neutrophils can potentially produce, either under inflammatory/immune reactions or during their activation in more prolonged processes, such as in tumors. We highlight recent observations generated from studying human or rodent neutrophils in vitro and in vivo models. We also discuss the biological significance of neutrophil-derived chemokines in the context of infectious, neoplastic and immune-mediated diseases. The picture that is emerging is that, given their capacity to produce and release chemokines, neutrophils exert essential functions in recruiting, activating and modulating the activities of different leukocyte populations.

Release of neutrophil extracellular traps by neutrophils stimulated with antiphospholipid antibodies: a newly identified mechanism of thrombosis in the antiphospholipid syndrome

OBJECTIVE

Antiphospholipid antibodies (aPL), especially those targeting β2 -glycoprotein I (β2 GPI), are well known to activate endothelial cells, monocytes, and platelets, with prothrombotic implications. In contrast, the interaction of aPL with neutrophils has not been extensively studied. Neutrophil extracellular traps (NETs) have recently been recognized as an important activator of the coagulation cascade, as well as an integral component of arterial and venous thrombi. This study was undertaken to determine whether aPL activate neutrophils to release NETs, thereby predisposing to the arterial and venous thrombosis inherent in the antiphospholipid syndrome (APS).

METHODS

Neutrophils, sera, and plasma were prepared from patients with primary APS (n = 52) or from healthy volunteers and characterized. No patient had concomitant systemic lupus erythematosus.

RESULTS

Sera and plasma from patients with primary APS had elevated levels of both cell-free DNA and NETs, as compared to healthy volunteers. Freshly isolated neutrophils from patients with APS were predisposed to high levels of spontaneous NET release. Further, APS patient sera, as well as IgG purified from APS patients, stimulated NET release from control neutrophils. Human aPL monoclonal antibodies, especially those targeting β2 GPI, also enhanced NET release. The induction of APS NETs was abrogated with inhibitors of reactive oxygen species formation and Toll-like receptor 4 signaling. Highlighting the potential clinical relevance of these findings, APS NETs promoted thrombin generation.

CONCLUSION

Our findings indicate that NET release warrants further investigation as a novel therapeutic target in APS.

Delineating the deranged immune system in the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is a systemic autoimmune disease that is characterized serologically by the presence of antiphospholipid antibodies (aPL) and clinically by vascular thrombosis and obstetric complications. The protein β2 glycoprotein I (β2GPI) is identified as the most important autoantigen in this syndrome. Activation of endothelial cells, thrombocytes and placental tissue by anti-β2GPI antibodies relates to the clinical manifestations of APS. This review describes genetic and environmental factors in relation to APS and summarizes the current knowledge on abnormalities in components of both the innate and adaptive immune system in APS. The role of dendritic cells, T-cells, B-cells, monocytes, neutrophils and NK-cells as well as the complement system in APS are discussed. Several gaps in our knowledge on the pathophysiology of APS are identified and a plea is made for future extensive immune cell profiling by a systems medicine approach in order to better unravel the pathogenesis of APS, to gain more insight in the role of the immune system in APS as well as having the potential to reveal biomarkers or novel therapeutic targets.

Neutrophil Depletion Attenuates Placental Ischemia-Induced Hypertension in the Rat

Preeclampsia is characterized by reduced placental perfusion with placental ischemia and hypertension during pregnancy. Preeclamptic women also exhibit a heightened inflammatory state and greater number of neutrophils in the vasculature compared to normal pregnancy. Since neutrophils are associated with tissue injury and inflammation, we hypothesized that neutrophils are critical to placental ischemia-induced hypertension and fetal demise. Using the reduced uteroplacental perfusion pressure (RUPP) model of placental ischemia-induced hypertension in the rat, we determined the effect of neutrophil depletion on blood pressure and fetal resorptions. Neutrophils were depleted with repeated injections of polyclonal rabbit anti-rat polymorphonuclear leukocyte (PMN) antibody (antiPMN). Rats received either antiPMN or normal rabbit serum (Control) on 13.5, 15.5, 17.5, and 18.5 days post conception (dpc). On 14.5 dpc, rats underwent either Sham surgery or clip placement on ovarian arteries and abdominal aorta to reduce uterine perfusion pressure (RUPP). On 18.5 dpc, carotid arterial catheters were placed and mean arterial pressure (MAP) was measured on 19.5 dpc. Neutrophil-depleted rats had reduced circulating neutrophils from 14.5 to 19.5 dpc compared to Control, as well as decreased neutrophils in lung and placenta on 19.5 dpc. MAP increased in RUPP Control vs Sham Control rats, and neutrophil depletion attenuated this increase in MAP in RUPP rats without any effect on Sham rats. The RUPP-induced increase in fetal resorptions and complement activation product C3a were not affected by neutrophil depletion. Thus, these data are the first to indicate that neutrophils play an important role in RUPP hypertension and that cells of the innate immune system may significantly contribute to pregnancy-induced hypertension.

Role of Toll-like receptors 2 and 4 in mediating endothelial dysfunction and arterial remodeling in primary arterial antiphospholipid syndrome

OBJECTIVE

To assess the role of Toll-like receptors (TLRs) in antiphospholipid antibody (aPL)-mediated vascular abnormalities in patients with primary arterial antiphospholipid syndrome (APS).

METHODS

Forty-eight subjects participated in the study. Arterial function and structure and TLR pathway activation were determined in patients with primary arterial APS and matched controls. The pathogenic effects of aPL isolated from patients were assessed in wild-type (WT) and TLR-knockout mice.

RESULTS

APS patients had endothelial dysfunction, arterial stiffening, and hypertrophy, as evidenced by decreased brachial artery endothelium-dependent flow-mediated dilation (FMD) and increased aortic pulse wave velocity and carotid intima-media thickness (IMT), as compared with controls. Plasma samples from APS patients revealed decreased nitric oxide (NO) availability and a pro-oxidative, proinflammatory, and prothrombotic state illustrated by a decrease in nitrite and an increase in lipid peroxidation, tumor necrosis factor α levels, and tissue factor (TF) levels. Furthermore, TLR pathway activation was found in APS patients with increased TLR-2 and TLR-4 messenger RNA expression and increased protein levels of the activated TLR transduction protein interleukin-1 receptor-associated kinase 1 in peripheral blood mononuclear cells. Moreover, agonist-stimulated cell-surface expression of TLR-2 and TLR-4 in circulating monocytes was higher in APS patients than in controls. These changes were positively associated with IMT and negatively associated with FMD. Finally, aPL injection decreased mesenteric endothelium-dependent relaxation and increased TF expression in WT mice but not in TLR-2- or TLR-4-knockout mice.

CONCLUSION

This translational study supports the notion that TLR-2 and TLR-4 play a role in mediating vascular abnormalities in patients with primary arterial APS. TLRs thus constitute a promising pharmacologic target for preventing cardiovascular complications in APS.

Effects of TLR4 on β2-glycoprotein I-induced bone marrow-derived dendritic cells maturation

Our previous study has demonstrated that Toll-like receptor 4 (TLR4) can contribute to anti-β2-glycoprotein I/β2-glycoprotein I (anti-β2GPI/β2GPI)-induced tissue factor (TF) expression in human acute monocytic leukemia cell line THP-1. However, the role of TLR4 in the activation of autoimmune response in antiphospholipid syndrome (APS) has rarely been reported. In this study, we focused on the role of TLR4 in β2GPI-induced maturation of bone marrow-derived dendritic cells (BMDCs). iDCs from C3H/HeN mice stimulated with β2GPI were more mature than that from C3H/HeJ mice, yields of CD11c(+)MHCII(+)DCs, CD11c(+)CD80(+)DCs and CD11c(+)CD86(+)DCs and production of some pro-inflammatory cytokines in iDCs from C3H/HeN were higher than those from C3H/HeJ (p<0.05). Moreover, the ability of β2GPI-treated iDCs from C3H/HeJ to stimulate proliferation of allogeneic mixed lymphocytes was lower than that of iDCs from C3H/HeN. In conclusion, our results indicate that TLR4 may play a significant role in β2-glycoprotein I-induced BMDCs maturation.

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.

Circulating anti-beta2-glycoprotein I antibodies are associated with endothelial dysfunction, inflammation, and high nitrite plasma levels in patients with intermittent claudication

Our aim is to investigate a possible association of circulating anti-beta2-glycoprotein I antibodies (ABGPI) with the endothelial dysfunction, nitric oxide bioactivity dysregulation, and the inflammatory status that surrounds peripheral arterial disease. We carried out an observational translational study, including 50 male patients with intermittent claudication and a healthy control group of 10 male subjects, age and sex matched with the cases. Flow-mediated arterial dilatation (FMAD) was assessed as a surrogate of endothelial dysfunction, and C-reactive protein (hsCRP) was determined as a marker of inflammation. Nitrite plasma levels were measured by colorimetric analysis. Circulating ABGPI titer was detected with indirect immunofluorescence. Titers <1 : 10 represented the reference range and the lower detection limit of the test. Circulating ABGPI titer ≥1 : 10 was detected in 21 (42%) patients and in none of the control subjects (P < 0.01). Patients with ABGPI titer ≥1 : 10 had a lower FMAD (P = 0.01). The CRP levels were higher in patients with ABGPI titer ≥1 : 10 (P = 0.04). The nitrite plasma levels were higher in patients with ABGPI titer ≥1 : 10 (P < 0.01). These data suggest that these circulating ABGPI may collaborate in the development of atherosclerosis; however, further prospective studies are required to establish a causal relationship.

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.

An ultrastructural analysis of platelets, erythrocytes, white blood cells, and fibrin network in systemic lupus erythematosus

The study suggests that patients with systemic lupus erythematosus (SLE) present with distinct inflammatory ultrastructural changes such as platelets blebbing, generation of platelet-derived microparticles, spontaneous formation of massive fibrin network and fusion of the erythrocytes membranes. Lupoid platelets actively interact with other inflammatory cells, particularly with white blood cells (WBCs), and the massive fibrin network facilitates such an interaction. It is possible that the concerted actions of platelets, erythrocytes and WBC, caught in the inflammatory fibrin network, predispose to pro-thrombotic states in patients with SLE.