Βeta-2-glycoprotein I exerts antithrombotic function through its domain V in mice.

Little is known about the physiological role of beta-2-glycoprotein I (ß2GPI) despite it being the major auto-antigen in the antiphospholipid syndrome. A systematic study of the role of ß2GPI in thrombus formation in vivo has not been performed to date. Herein, we report that ß2GPI deficient (-/-) mice have enhanced thrombus formation compared to wild type (WT) mice in a laser-induced arteriole and venule model of thrombosis. Furthermore, neutrophil accumulation and elastase activity was enhanced in thrombi of ß2GPI -/- compared with WT mice. The antithrombotic function of ß2GPI is dependent on its fifth domain (domain V); intravenous administration of the ß2GPI domain deletion mutant lacking domain V (human recombinant domain I-IV) had no effect on platelet and fibrin thrombus size in ß2GPI -/- or WT mice. On the contrary, intravenous administration of human recombinant domain V significantly inhibited platelet and fibrin thrombus size in both ß2GPI -/- mice and WT mice. These findings reveal a major role for ß2GPI as a natural anticoagulant and implicate domain V of ß2GPI as a potential antithrombotic therapy.

Oxidative stress in the pathogenesis of atherothrombosis associated with anti-phospholipid syndrome and systemic lupus erythematosus: new therapeutic approaches.

Atherothrombosis is a recurrent complication in APS and SLE patients. Oxidative stress has been suggested as a key player underlying this process. Autoantibodies have been pointed to as the main contributors to abnormality in the oxidative status observed in APS and SLE patients, promoting the increased production of oxidant species and the reduction of antioxidant molecules. This imbalance causes vascular damage through the activation of immune cells, including monocytes, lymphocytes and neutrophils, causing the expression of pro-inflammatory and procoagulant molecules, the formation of neutrophil extracellular traps and the adhesion of these cells to the endothelium; the induction of cellular apoptosis and impaired cell clearance, which in turn enhances autoantibody neogeneration; and cytotoxicity of endothelial cells. This review describes the mechanisms underlying the role of oxidative stress in the pathogenesis of atherothrombosis associated with APS and SLE, focused on the effect of autoantibodies, the different cell types involved and the diverse effectors, including cytokines, procoagulant proteins and their main modulators, such as oxidant/antioxidant species and intracellular pathways in each pathology. We further discuss new therapies aimed at restoring the oxidative stress balance and subsequently to tackle atherothrombosis in APS and SLE.

Anti-ß2 glycoprotein I antibodies in complex with ß2 glycoprotein I induce platelet activation via two receptors: apolipoprotein E receptor 2' and glycoprotein I bα.

Anti-ß2 glycoprotein I (anti-ß2GPI ) antibodies are important contributors to thrombosis, especially in patients with antiphospholipid syndrome (APS). However, the mechanism by which anti-ß2GPI antibodies are involved in the pathogenesis of thrombosis is not fully understood. In this report, we investigated the role of anti- ß2GPI antibodies in complexes with ß2GPI as mediators of platelet activation, which can serve as a potential source contributing to thrombosis. We examined the involvement of the apolipoprotein E receptor 2' (apoER2') and glycoprotein I ba (GP I ba) in platelet activation induced by the anti-ß2GPI /ß2GPI complex. The interaction between the anti-ß2GPI /ß2GPI complex and platelets was examined using in vitro methods, in which the Fc portion of the antibody was immobilized using protein A coated onto a microtiter plate. Platelet activation was assessed by measuring GPII b/III a activation and P-selectin expression and thromboxane B2 production as well as p38 mitogen-activated protein kinase phosphorylation. Our results revealed that the anti-ß2GPI /ß2GPI complex was able to activate platelets, and this activation was inhibited by either the anti-GP I bα antibody or the apoER2' inhibitor. Results showed that the anti-ß2GPI /ß2GPI complex induced platelet activation via GPI ba and apoER2', which may then contribute to the prothrombotic tendency in APS patients.

Antiphospholipid Syndrome--Not a Noninflammatory Disease.

The autoimmune disease antiphospholipid syndrome (APS) is characterized by thrombosis or pregnancy morbidity in patients with persistent antiphospholipid antibodies (aPLs). Although inflammation is not a key feature of the clinical presentation of the syndrome, there are indications that the inflammatory response plays an important role in APS. The major antigen of aPLs, the plasma protein ß2-glycoprotein I, is involved in clearance of microparticles and in the innate immune response. In light of these physiological functions, the formation of antibodies against the protein is easily understood, as antibodies might augment the clearance reaction. In addition, inflammatory mediators are thought to play a role in the activation of leukocytes and the induction of endothelial dysfunction in APS. Moreover, evidence for a role of complement activation in the pathogenesis of the syndrome is accumulating. This review will provide an overview of current knowledge on the physiological function of ß2-glycoprotein I, the formation of autoantibodies against ß2-glycoprotein I and will explore the contribution of inflammation to the clinical manifestations of APS.

Attenuation of early atherosclerotic lesions by immunotolerance with ß2 glycoprotein I and the immunomodulatory effectors interleukin 2 and 10 in a murine model.

OBJECTIVE: This study assessed the effect of cellular and humoral autoimmune response inhibition after immunization with ß2-glycoprotein I (ß2-GPI) and the effect of immunomodulation with interleukin (IL)-2 and IL-10 in the development of early atherosclerotic vascular lesion in a murine model. Atherosclerosis is increasingly considered a chronic inflammatory disease with pathogenic autoimmune processes. Regulatory T cells, and their cytokines, have been implicated in the inhibition of the development of atherosclerotic lesions and involved in the immunologic tolerance induction. METHODS: Eight-week-old male C57BL6 LDL-receptor deficient (LDLR(-/-)) mice were fed a cholesterol-rich (2.8%), high-saturated-fat (82%) diet for a week and divided in five groups. The groups received the following intravenous immunizations: group I (control group): one dose of 5 µg ß2-GPI; group II: 5 µg ß2-GPI I and 1 µg IL-2; group III: 5 µg ß2-GPI and 0.75 µg of IL-10; and group IV: 5 µg ß2-GPI, 1 µg IL-2, and 0.75 µg IL-10. The aortas of the mice were assessed 8 weeks after inoculation to determine the aortic lesion size and composition in all groups. RESULTS: ß2-GPI immunization attenuated the early atherosclerotic lesions development compared with the control group (P = .001). Macroscopic and histologic aortic atherosclerotic lesions were significantly decreased in the IL-2 and IL-10-treated groups in ß2-GPI-tolerant mice compared with the ß2-GPI-tolerant group without cytokine injection (P = .001). The association of both cytokines did not provoke a major inhibition in the atherosclerosis development when compared with groups injected with the two cytokines separately. CONCLUSIONS: The immunotolerance induction against ß2-GPI attenuates the development of atherosclerosis lesions in an animal model, enhanced by downregulation of the cellular and humoral autoimmune response provoked by IL-2 and IL-10.

Cardiovascular disease in the antiphospholipid syndrome.

The prevalence of cardiovascular disease (CVD) in antiphospholipid syndrome (APS) varies from one series to another depending on the definition of CVD and tools used for its detection. Atherosclerosis, the usual cause of CVD, starts when the endothelium becomes damaged and is considered to be an autoimmune-inflammatory disease. The excessive cardiovascular events observed in patients with APS are not fully explained by traditional risk factors. Therefore, several novel risk factors contribute to the development of premature CVD and accelerated vascular damage in those patients. Herein, the significance and outcomes of CVD in APS are reviewed.

Toll-like receptor 4 and ß2 glycoprotein I interaction on endothelial cells.

Antiphospholipid syndrome (APS) is a systemic autoimmune disease characterized by thrombotic events and/or pregnancy morbidity in the persistent presence of antiphospholipid antibodies (aPL). aPL targeting ß2 glycoprotein I (anti-ß2GPI Abs) provide the main pathogenic autoantibody subset. In monocytes, platelets and endothelial cells (ECs), the interaction of circulating aPL with membrane-bound ß2GPI results in cell activation, and EC perturbation provides an important player in clotting. Several receptors have been suggested to mediate ß2GPI/EC binding. AnnexinA2 provides a high-affinity binding site for ß2GPI but, since it does not span the cell membrane, an adaptor protein is required to trigger signal. Consistent evidence supports the role of Toll-like receptor (TLR) 4 as co-receptor for ß2GPI on ECs. ß2GPI was recently reported to behave as lipopolysaccharide (LPS) scavenger. In monocytes, TLR4 activation was shown to be apparent, due to LPS/ß2GPI complexes. Conversely, our findings in ECs demonstrate that ß2GPI interacts directly with TLR4, and that such interaction may contribute to ß2GPI-dependent aPL-mediated EC activation. LPS enhanced anti-ß2GPI Ab binding to EC only at cell-activating concentrations, able to up-regulate TLR4. This in vitro model may explain why LPS behaves as a second hit increasing the expression of ß2GPI in vascular tissues and triggering aPL-mediated thrombosis in vivo.

Phospholipid-binding proteins differ in their capacity to induce autoantibodies and murine systemic lupus erythematosus.

We have previously shown that immunization of nonautoimmune mice with the phospholipid-binding protein ß2-glycoprotein I (ß2GPI), in combination with lipopolysaccharide (LPS), induces a murine model of systemic lupus erythematosus (SLE), with sequential emergence of autoantibodies and glomerulonephritis. Here, we determine whether the paradigm for induction of murine SLE extends to other phospholipid-binding proteins. Mice were immunized with a phospholipid-binding protein (prothrombin (PT), protein S, or ß2GPI), or a nonphospholipid-binding protein (glu-plasminogen), in the presence of LPS. The breadth and degree of the autoantibody response, and the frequency of glomerulonephritis, varied among the three proteins, with ß2GPI being the most effective in inducing SLE-like disease. The phospholipid-binding proteins also differed in the pattern of serum cytokines they elicited. The most apparent difference between ß2GPI and the other phospholipid-binding proteins was in their ability to bind to LPS: ß2GPI bound to LPS, while PT and protein S did not. Our data suggest that binding to phospholipid(s) is a necessary, but not sufficient, condition for full induction of murine SLE. We propose that other properties, such as physiologic function, avidity for anionic phospholipids, and degree of interaction with other cell surface and/or circulating molecules (particularly LPS) may determine the range and severity of disease.

ß2-glycoprotein I inhibits VEGF-induced endothelial cell growth and migration via suppressing phosphorylation of VEGFR2, ERK1/2, and Akt.

ß(2)-glycoprotein I (ß(2)-GPI) is a plasma glycoprotein with diverse functions, but the impact and molecular effects of ß(2)-GPI on vascular biology are as yet unclear. Based on the limited information available on the contribution of ß(2)-GPI to endothelial cells, we investigated the effect of ß(2)-GPI on cell growth and migration in human aortic endothelial cells (HAECs). The regulation of ß(2)-GPI as part of intracellular signaling in HAECs was also examined. Vascular endothelial growth factor (VEGF) is a pro-angiogenic factor that may regulate endothelial functions. We found that ß(2)-GPI dose-dependently inhibited VEGF-induced endothelial cell growth using the 3-(4,5-dimethylthiazol-2-yl)-2,5-dipenyl tetrazolium bromide assay and cell counts. Using wound healing and Boyden chamber assays, ß(2)-GPI remarkably reduced VEGF-increased cell migration at the physiological concentration. Furthermore, ß(2)-GPI suppressed VEGF-induced phosphorylation of VEGF receptor 2 (VEGFR2), extracellular signal-regulated kinase 1/2 (ERK1/2), and Akt. These results suggest that ß(2)-GPI plays an essential role in the down-regulation of VEGF-induced endothelial responses and may be a useful component for anti-angiogenic therapy.

Val247Leu ß2-glycoprotein-I allelic variant is associated with antiphospholipid syndrome: systematic review and meta-analysis.

INTRODUCTION: Previous studies have suggested that the possession of the Val/Val genotype of the Val247Leu polymorphism of the ß(2)-glycoproteinI (ß(2)-GPI) gene may be associated with antiphospholipid syndrome (APS), and, among patients with APS, with the production of anti-ß(2)-GPI antibodies or the development of thrombosis. Given the controversial results reported, the aim of this work is to combine previous findings by means of a systematic review and a meta-analysis. METHODS: We retrieved studies analyzing the genotype of the above-mentioned polymorphism among patients with APS by means of electronic database search. A meta-analysis was conducted in a random effects model and calculations of odds ratio (OR) and confidence intervals (CI) were done. Sensitivity analysis and tests for heterogeneity of the results were performed. RESULTS: Eight previous studies analyzed the association of APS, anti-ß(2)-GPI antibodies and/or thrombosis with the Val247Leu polymorphism. After meta-analysis, patients with APS had a significantly higher prevalence of the Val/Val genotype of this genetic variant when compared with controls (OR=2.04; 95% CI: 1.12, 3.73; P=0.02). Among patients with APS, those with anti-ß(2)-GPI antibodies had a higher prevalence of this genotype (OR=1.73; 95% CI: 1.04, 2.87; P=0.03). No significant results were found for the presence of arterial or venous thrombosis. CONCLUSIONS: Val/Val genotype of ß(2)-GPI gene is associated with a significant excess risk to suffer from APS and, among patients with APS, to have anti-ß(2)-GPI antibodies. No definite conclusions can be made regarding the association of this polymorphism with thrombosis among APS patients.

ß2-glycoprotein I and protection from anti-SSA/Ro60-associated cardiac manifestations of neonatal lupus.

One mechanism to molecularly explain the strong association of maternal anti-Ro60 Abs with cardiac disease in neonatal lupus (NL) is that these Abs initiate injury by binding to apoptotic cardiomyocytes in the fetal heart. Previous studies have demonstrated that ß(2)-glycoprotein I (ß(2)GPI) interacts with Ro60 on the surface of apoptotic Jurkat cells and prevents binding of anti-Ro60 IgG. Accordingly, the current study was initiated to test two complementary hypotheses, as follows: 1) competition between ß(2)GPI and maternal anti-Ro60 Abs for binding apoptotic induced surface-translocated Ro60 occurs on human fetal cardiomyocytes; and 2) circulating levels of ß(2)GPI influence injury in anti-Ro60-exposed fetuses. Initial flow cytometry experiments conducted on apoptotic human fetal cardiomyocytes demonstrated dose-dependent binding of ß(2)GPI. In competitive inhibition experiments, ß(2)GPI prevented opsonization of apoptotic cardiomyocytes by maternal anti-Ro60 IgG. ELISA was used to quantify ß(2)GPI in umbilical cord blood from 97 neonates exposed to anti-Ro60 Abs, 53 with cardiac NL and 44 with no cardiac disease. ß(2)GPI levels were significantly lower in neonates with cardiac NL. Plasmin-mediated cleavage of ß(2)GPI prevented binding to Ro60 and promoted the formation of pathogenic anti-Ro60 IgG-apoptotic cardiomyocyte complexes. In aggregate these data suggest that intact ß(2)GPI in the fetal circulation may be a novel cardioprotective factor in anti-Ro60-exposed pregnancies.

ß(2) -Glycoprotein I: evolution, structure and function.

ß(2) -Glycoprotein I (ß(2) -GPI) is a protein that circulates in blood at high concentrations. The function of ß(2) -GPI has long been an enigma. More than 20 years ago, it was discovered that ß(2) -GPI is the major antigen for the circulating antibodies in the antiphospholipid syndrome. However, this knowledge has not advanced our understanding of the physiologic role of the protein. In recent years, new insights have suggested an important function of this protein in innate immunity. ß(2) -GPI was found to scavenge lipopolysaccharide and was able to clear unwanted anionic cellular remnants such as microparticles from the circulation. The function of ß(2) -GPI seems to depend on the structural conformation of the protein, and it has been established that ß(2) -GPI can exist in at least two conformations. In this review, we will highlight and summarize the current knowledge on this protein.

Effects of anti-ß2-glycoprotein I antibody on PlGF, VEGF and sVEGFR1 production from cultured choriocarcinoma cell line.

AIM: Anti-ß2-glycoprotein I (anti-ß2-GPI) antibody has been detected in cases of recurrent abortion and intrauterine death. Placenta growth factor (PlGF) and vascular endothelial growth factor (VEGF) are growth factors that accelerate villous proliferation. Soluble VEGF receptor-1 (sVEGFR1) is a soluble receptor for PlGF and suppresses the function of PlGF. In order to study the pathological mechanism of anti-ß2-GPI antibody, we evaluated the effects of anti-ß2-GPI antibody on PlGF, VEGF and sVEGFR1 production from cultured choriocarcinoma cells. METHODS: The sera were taken from six anti-ß2-GPI antibody-positive and six anti-ß2-GPI antibody-negative patients with recurrent miscarriages. Choriocarcinoma cells (JEG-3) were cultured in 24-well plates. After each serum and isolated immunoglobulin G (IgG) were added to the culture medium, the PlGF, VEGF and sVEGFR1 in the culture medium were measured by ELISA 24 h later. When the isolated IgG was added to culture medium, only the levels of PlGF were measured. RESULTS: The anti-ß2-GPI antibody-positive sera and isolated IgG significantly suppressed the production of PlGF from JEG-3 cells more strongly than the anti-ß2-GPI antibody-negative sera. The suppressive effects were not changed by serum inactivation. There was no significant difference in the values of the XTT assay and the production of VEGF and sVEGFR1 between the antibody-positive and antibody-negative sera. CONCLUSIONS: Anti-ß2-GPI antibody-positive sera and IgG suppress the production of PlGF from JEG-3 cells. We suggest that the anti-ß2-GPI antibodies may suppress PlGF production from trophoblasts and cause the failure of placenta formation and function.

Antiphospholipid syndrome: long-time research on pathogenic mechanisms has finally lead to new therapeutic strategies.

The antiphospholipid syndrome is characterized by the presence of arterial or venous thrombosis or recurrent miscarriages in a patient with positive laboratory tests for antiphospholipid antibodies (anticardiolipin antibodies and/or lupus anticoagulant and/or anti-beta2-glycoprotein I). Despite the strong association between antiphospholipid antibodies and thrombosis and obstetric morbidity, their pathogenic role in the development of these clinical features has not been fully elucidated. However, the knowledge of new pathogenic mechanisms might identify novel therapeutic targets and therefore may improve the management of these patients.

Plasmin-clipped beta(2)-glycoprotein-I inhibits endothelial cell growth by down-regulating cyclin A, B and D1 and up-regulating p21 and p27.

beta(2)-Glycoprotein-I (beta(2)gpI), an abundant plasma glycoprotein, functions as a regulator of thrombosis. Previously, we demonstrated that plasmin-clipped beta(2)gpI (cbeta(2)gpI) exerts an anti-angiogenic effect on human umbilical vein endothelial cells (HUVEC). The present study was focused on the molecular background responsible for this phenomenon. cbeta(2)gpI strongly reduced HUVEC growth and proliferation as evidenced by the MTT and BrdU assay and delayed cell cycle progression arresting HUVEC in the S-and G2/M-phase. Western blot analysis indicated that cbeta(2)gpI inhibited cyclin A, B and D1, and enhanced p21 and p27 expression. Activity of p38 was down-regulated independently from the cbeta(2)gpI incubation time. Phosphorylation of ERK1/2 was not changed early (30 and 60 min) but became enhanced later (90 min, 4h). JNK activity was reduced rapidly after cbeta(2)gpI treatment but compared to controls, increased thereafter. Annexin II blockade prevented growth inhibition and cell cycle delay evoked by cbeta(2)gpI. We assume that cbeta(2)gpI's effects on HUVEC growth is mediated via cyclin A, B and D1 suppression, up-regulation of p21 and p27 and coupled to modifications of the mitogen-activated protein (MAP) kinase signalling pathway. cbeta(2)gpI may represent a potential endogenous angiogenesis-targeted compound, opening the possibility of a novel tool to treat cancer.

Impaired fibrinolysis in the antiphospholipid syndrome.

The antiphospholipid syndrome (APS) is characterized by venous and/or arterial thrombosis, or recurrent fetal loss, in the presence of antiphospholipid antibodies (APL). The pathogenesis of APS is multifaceted and involves numerous mechanisms including activation of endothelial cells, monocytes, and/or platelets; inhibition of natural anticoagulant pathways such as protein C, tissue factor inhibitor, and annexin A5; activation of the complement system; and impairment of the fibrinolytic system. Fibrinolysis--the process by which fibrin thrombi are remodeled and degraded--involves the conversion of plasminogen to plasmin by tissue plasminogen activator (tPA) or urokinase-type plasminogen activator, and is tightly regulated. Although the role of altered fibrinolysis in patients with APS is relatively understudied, several reports suggest that deficient fibrinolytic activity may contribute to the pathogenesis of disease in these patients. This article discusses the function of the fibrinolytic system and reviews studies that have reported alterations in fibrinolytic pathways that may contribute to thrombosis in patients with APL. Some of these mechanisms include elevations in plasminogen activator inhibitor-1 levels, inhibitory antibodies against tPA or other components of the fibrinolytic system, antibodies against annexin A2, and finally, antibodies to beta(2)-glycoprotein-I (beta(2)GPI) that block the ability of beta(2)GPI to stimulate tPA-mediated plasminogen activation.

Autoimmune-mediated atherothrombosis.

Autoimmune vascular inflammation and oxidative stress (lipid peroxidation) are common in systemic autoimmune diseases and contribute to the oxidative modification of low-density lipoprotein (oxLDL) and oxLDL/beta2GPI complex formation. Circulating oxLDL/beta2GPI complexes have been detected in patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). The presence of antibodies to oxLDL/beta2GPI complexes indicates that these complexes are immunogenic, and the coexistence of complexes and antibodies has pointed to an active proatherogenic role in the development of autoimmune vascular complications. Immunohistochemical staining of atherosclerotic lesions suggest that these complexes are formed in the arterial wall and released into circulation. The in vitro macrophage uptake of oxLDL/beta2GPI complexes was significantly increased in the presence of antiphospholipid antibodies, either beta2GPI-dependent anticardiolipin or anti-beta2GPI antibodies, suggesting that macrophage Fcgamma receptors are involved in lipid intracellular influx and foam cell formation. These findings provide an explanation for the accelerated development of atherosclerosis seen in SLE and APS. The presence of circulating oxLDL/beta2GPI complexes and IgG antibodies to these complexes indicate significant vascular injury and oxidative stress as well as an active role in autoimmune-mediated atherothrombosis.

Prothrombotic mechanisms based on the impairment of fibrinolysis in the antiphospholipid syndrome.

Antiphospholipid syndrome (APS) is a clinical autoimmune disorder characterised by thrombosis, venous or arterial, and recurrent pregnancy morbidity in association with the persistence of antiphospholipid antibodies (aPL). The clinical variety of aPL ranges from asymptomatic individuals to those with multiple organ thromboses and failure developing over a short period, also known as catastrophic APS. An increasing number of phospholipid-binding proteins with crucial functions in the regulation of blood coagulation and fibrinolysis are targeted by APS-related autoantibodies. Disruption of fibrinolysis is one of the proposed pathophysiological mechanisms for the APS. There are some experimental data for an association between impaired overall fibrinolytic activity and autoimmune aPL; however, evidence is still inconclusive and more studies are needed in this area. In this article, we review the evidence by which aPL may disturb fibrinolysis.