Target For Antiphospholipid Antibodies Research Articles

The J-elongated conformation of β2-glycoprotein I predominates in solution: implications for our understanding of antiphospholipid syndrome

β2-Glycoprotein I (β2GPI) is an abundant plasma protein displaying phospholipid-binding properties. Because it binds phospholipids, it is a target of antiphospholipid antibodies (aPLs) in antiphospholipid syndrome (APS), a life-threatening autoimmune thrombotic disease. Indeed, aPLs prefer membrane-bound β2GPI to that in solution. β2GPI exists in two almost equally populated redox states: oxidized, in which all the disulfide bonds are formed, and reduced, in which one or more disulfide bonds are broken. Furthermore, β2GPI can adopt multiple conformations (i.e. J-elongated, S-twisted, and O-circular). While strong evidence indicates that the J-form is the structure bound to aPLs, which conformation exists and predominates in solution remains controversial, and so is the conformational pathway leading to the bound state. Here, we report that human recombinant β2GPI purified under native conditions is oxidized. Moreover, under physiological pH and salt concentrations, this oxidized form adopts a J-elongated, flexible conformation, not circular or twisted, in which the N-terminal domain I (DI) and the C-terminal domain V (DV) are exposed to the solvent. Consistent with this model, binding kinetics and mutagenesis experiments revealed that in solution the J-form interacts with negatively charged liposomes and with MBB2, a monoclonal anti-DI antibody that recapitulates most of the features of pathogenic aPLs. We conclude that the preferential binding of aPLs to phospholipid-bound β2GPI arises from the ability of its preexisting J-form to accumulate on the membranes, thereby offering an ideal environment for aPL binding. We propose that targeting the J-form of β2GPI provides a strategy to block pathogenic aPLs in APS.

Anti-domain 1 \u03b22 glycoprotein antibodies increase expression of tissue factor on monocytes and activate NK Cells and CD8+ cells in vitro

Backgroundβ2-Glycoprotein I (β2GPI) represents the major antigenic target for antiphospholipid antibodies (aPL), with domain 1 (D1) being identified as a risk factor for thrombosis and pregnancy complications in APS. We aimed to analyse the ability of aPL, and particularly anti-D1 β2GPI, to stimulate prothrombotic and proinflammatory activity of immune cells in vitro.MethodsPeripheral blood mononuclear cells (PBMCs) from 11 healthy individuals were incubated with: (1) “anti-D1(+)”—pooled plasma derived from patients suspected of having APS contained anticardiolipin antibodies (aCL), lupus anticoagulant (LA), anti-β2GPI and anti-D1 β2GPI; (2) “anti-D1(−)”—pooled plasma from patients suspected of having APS contained aCL, LA, anti-β2GPI, and negative for anti-D1 β2GPI; (3) “seronegative”—negative for aPL.ResultsThe presence of anti-D1(+) and anti-D1(−) plasma resulted in increased HLA-DR and CD11b on monocytes. While only anti-D1(+) plasma markedly increased the percentage and median fluorescence intensity (MFI) of CD142 (tissue factor, TF) on monocytes in comparison with those cultured with anti-D1(−) and seronegative plasma. Anti-D1(+) plasma resulted in increased percentage and MFI of activation marker CD69 on NK and T cytotoxic cells. Expression of IgG receptor FcγRIII(CD16) on monocytes and NK cells was down-regulated by the anti-D1(+) plasma.ConclusionsTaking together, our study shows the ability of patient-derived aPL to induce immune cell activation and TF expression on monocytes. For the first time, we demonstrated the influence of anti-D1 β2GPI on the activation status of monocytes, NK and cytotoxic T cells. Our findings further support a crucial role of D1 epitope in the promotion of thrombosis and obstetrical complications in APS.

Reactivity against the Five Domains of β2 Glycoprotein I: A Focus on Systemic Lupus Erythematosus

Beta-2 glycoprotein I (β2GPI) is the main antigenic target for antiphospholipid antibodies (aPL), the serological markers of antiphospholipid syndrome (APS), a systemic autoimmune disease characterized by vascular thrombosis and/or pregnancy morbidity. aPL are detected in 20 to 50% of patients with systemic lupus erythematosus (SLE), representing a poor prognostic factor. Indeed, thrombotic events heralds high morbidity and mortality, being regarded as the strongest predictors of death in the first five years after SLE onset. It would be thus very important to identify a reliable laboratory tool such as anti-domain antibodies to better risk-stratify lupus patients according to the aPL profile, in order to tailor treatment strategies. Domain I (DI) of β2GPI has lately been identified as the main epitope targeted by antibodies reacting against β2GPI isolated from APS patients, well correlating with thrombotic as well as obstetric manifestations. Interestingly, anti-DI antibodies have been shown to well correlate with lupus anticoagulant and to predict the clinical manifestations of the syndrome, thrombotic events as well as pregnancy complications. Further, anti-DI antibodies allow to identify patients at highest clinical risk, presenting a good specificity for APS. On the other hand, anti- β2GPI antibodies from aPL asymptomatic carriers or subjects with infectious diseases preferentially display reactivity towards DIV or DV of the molecule.Few studies have to date evaluated the domain profile of anti-β2GPI antibodies in subjects with SLE, even though it would be very relevant from a clinical point of view to understand whether among patients with SLE the domain specificities of anti-β2GPI antibodies display a clinical meaning comparable to the one they exert in APS. It is therefore rather appropriate to review the available evidence about anti-domain specificities with a particular focus on SLE.

Atherosclerosis and Autoimmunity

Atherosclerosis is a chronic pathological condition in which arteries undergo gradual thickness of the intima causing decreased elasticity, narrowing, and reduced blood supply to target organs. The blood vessels most commonly affected and of clinically relevance include the aorta, coronary, carotid, and cerebral arteries, collectively classified under cardiovascular disease. Several inherited and environmental risk factors have been identified as contributors to the initiation and progression of atherosclerosis. These risk factors are thought to disrupt a number of lipid regulatory and inflammatory mechanisms within the arterial wall. The appearance of lipid-laden foam cells in the arterial intima is a characteristic histological finding of early atherosclerosis. High plasma levels of low-density lipoprotein (LDL) in combination with arterial shear stress may contribute to vascular inflammation that promotes the oxidative modification of LDL. The causal relationship between oxidized LDL (oxLDL) and atherosclerosis is almost universally accepted. OxLDL transported into the subendothelial space is ingested by macrophages in the arterial intima via scavenger receptors, resulting in excessive intracellular accumulation of oxLDL and foam cell (plaque) formation. OxLDL is the principal plasma lipoprotein found in atherosclerotic lesions and it colocalizes with β2-glycoprotein I (β2GPI), the major antigenic target for antiphospholipid antibodies. Furthermore, there is wide consensus that anti-β2GPI antibodies play a central role in the development of the clinical complications that characterize the antiphospholipid syndrome (APS). It has been recently demonstrated that oxLDL forms complexes with β2GPI in atherosclerotic lesions and that such oxLDL/β2GPI complexes are present in the bloodstream of patients with

Research around β2-glycoprotein I: A major target for antiphospholipid antibodies

β2-Glycoprotein I (β2GPI), a phospholipid-binding protein, is one of the major target antigens for antiphospholipid antibodies (aPL) found in patients with antiphospholipid syndrome (APS). Thrombophilic disorders in APS patients are strongly associated with aPL, and their pathogenic properties depend on the presence of β2GPI. Procoagulant cell stimulation by aPL, via β2GPI, is one of the most plausible mechanisms of thrombosis in APS, and p38 mitogen activated protein kinase (MAPK) pathway plays a crucial role in such activation. β2GPI is proteolytically cleaved in domain V by activated factor X or plasmin, leading to the generation of the nicked form of β2GPI. Recently, increasing attention is focused on the role of nicked-β2GPI as a regulator of extrinsic fibrinolysis pathway.

Sulfatides: targets for anti-phospholipid antibodies.

Sulfatides are sulfated glycosphingolipids expressed on the surface of erythrocytes, leukocytes, and platelets. Sulfatides interact with several cell adhesion molecules involved in hemostasis. Beta2-glycoprotein I is an anionic phospholipid-binding plasma protein, and the phospholipid-bound form is the target for most anti-phospholipid antibodies that are associated with recurrent thrombosis, miscarriages, and neurological symptoms. In this study, we examined whether beta2-glycoprotein I forms a complex with sulfatides and thereby becomes a target for anti-phospholipid antibodies. Beta2-glycoprotein I binds to surface-bound sulfatides but not to other glycolipids, such as ceramide, cerebrosides, sphingomyelin, or ganglioside. At a sulfatide coating density of 1 microg/well, beta2-glycoprotein I reaches half-maximal binding at 2.5 microg/mL, and the binding is saturated at 10 microg/mL. The binding of beta2-glycoprotein I also depends on the coating density of sulfatides in the well. At a constant beta2-glycoprotein I concentration of 5 microg/mL, maximal binding of beta2-glycoprotein I is observed at a coating density of 1 mug/well. The serum from 14 patients with anti-cardiolipin antibodies, a subset of anti-phospholipid antibodies, bound to sulfatide-bound beta2-glycoprotein I and previous absorption on cardiolipin-coated surfaces decreased the immunoreactivity toward sulfatide-beta2-glycoprotein I complex by >50% in 12 of 14 patients. Furthermore, immunoaffinity-purified anti-cardiolipin antibodies from 4 of 5 patients reacted with sulfatide-bound beta2-glycoprotein I. These results show that not only anionic phospholipids, as commonly known, but also sulfatides are targets for most anti-phospholipid antibodies. We therefore postulate that interactions of these antibodies with sulfatides may contribute to some of the clinical symptoms of the anti-phospholipid antibody syndrome.

Endothelium as a target for antiphospholipid antibodies

Several, and not necessarily alternative, pathogenic mechanisms have been suggested to sustain the thrombophilic diathesis of the anti-phospholipid syndrome. Among them, interference of anti-phospholipid antibodies with cell acting in the coagulation cascade likely plays a major role. Anti-phospholipid antibodies have been shown to react with endothelial cells mainly by reacting with beta 2 glycoprotein I expressed on the cell membrane surface. Beta 2 glycoprotein I can adhere to endothelial cell surface through the Annexin II receptor and through negatively charged structures (heparin-like molecules) that are bound by the phospholipid-binding site of the molecule. The autoantibody binding involves a yet unknown receptor that activates a signalling pathway able to translocate NFkappaB from the cytoplasm to the nucleus and to activate genes for adhesion molecule, pro-inflammatory cytokine and Tissue Factor up-regulation. The ultimate effect is the induction of a pro-inflammatory and a pro-coagulant endothelial phenotype that has been reproduced both in vitro and in vivo experimental models. Additional effects of anti-phospholipid-mediated endothelial cell activation are the interference with the protein C/S system, with the Annexin V binding, the up-regulation of endothelin I synthesis and the induction of apoptosis. Altogether these effects cooperate in switching endothelium from an anti-coagulant to a pro-coagulant surface.

Regulated Expression of Signal Transducer and Activator of Transcription, Stat5, and its Enhancement of PRL Expression in Human Endometrial Stromal Cells in Vitro

Differentiation of human endometrium during the secretory phase of the menstrual cycle is characterized by expression of a variety of genes implicated in the establishment and maintenance of pregnancy. An increased abundance of signal transducers and activators of transcription (Stats) in the secretory phase suggests Stat5 as a component of the differentiation of endometrium in response to ovarian hormone stimulation in vivo. Decidualization is initiated in a subset of endometrial stromal cells (ESC) in vivo during the secretory phase, but it is unclear whether regulated expression of Stat5 is a feature of these cells. Here, therefore, the abundance and subcellular distribution of Stat5 in ESC after a decidualization stimulus of cAMP plus medroxyprogesterone acetate (MPA) has been investigated in vitro. Western blotting revealed an increase in the apparent abundance of Stat5a and Stat5b, in the cytosolic and nuclear fractions, at 2, 3, and 4 d after stimulation. The potential functional relevance of this increase in Stat5 is suggested by the ability of transiently transfected Stat5a or Stat5b to significantly enhance the response of the decidual PRL promoter to cAMP/MPA and attenuation of the response to cAMP/MPA by dominant negative Stat5. Recent evidence suggests endometrial differentiation, including PRL production, as a possible target of antiphospholipid antibodies (aPL) prevalent in recurrent miscarriage. Monoclonal antibody, ID2, which has similar reactivity as human aPL, significantly decreased the apparent abundance of nuclear Stat5b in response to cAMP/MPA and was associated with decreased decidual PRL promoter activation and PRL secretion. Regulated expression of Stat5 is therefore a component of decidual differentiation of human ESC and contributes significantly to activation of the decidual PRL promoter. Alteration of this process by an aPL component suggests decidual differentiation as a potential clinical target in recurrent early miscarriages.

Faible prévalence des anticorps anti-annexine V dans le syndrome des antiphospholipides avec pertes fœtales

Annexin V, a protein with potent anticoagulant activity has a calcium-dependent binding affinity for phospholipids. Annexin V is distributed in many organs, especially in the placenta and endothelium. Various studies have shown that placental annexin V is decreased in women with anti-phospholipid syndrome. It has been suggested that annexin V might be a target of anti-phospholipid antibodies and that the subsequent decrease in annexin V might be associated with obstetrical complications. We investigated the presence of anti-annexin V antibodies in the plasma of women with anti-phospholipid syndrome and obstetrical complications. Twenty-three patients with at least one spontaneous abortion were included in the study. Anti-cardiolipin antibodies and lupus anticoagulant were present in 87% and 30% of the patients, respectively. A group of 40 healthy women were included in the control group. Anti-annexin V IgG and IgM antibodies were measured by ELISA. The IgG mean OD was 0.07 +/- 0.013 in patients and 0.042 +/- 0.06 in the control group. There was no significant difference between the two groups (P = NS). Only two out of the 23 patients and two out of the 40 healthy women were positive for IgG (OD > 0.25). The sensitivity of the assay was poor (8.7%). Even when the threshold was adjusted according to the mean OD in control subjects +2 SD, the sensitivity was still poor, reaching only 13%. The prevalence of anti-annexin V was low in patients with anti-phospholipid syndrome and repetitive spontaneous abortions. Anti-annexin V assay does not appear to be sensitive enough for the identification of anti-phospholipid antibodies that might be involved in the decrease in annexin V leading subsequently to thrombosis risk.

Endothelial Cells as a Target for Antiphospholipid Antibodies: Role of Anti‐Beta 2 Glycoprotein I Antibodies

To investigate the role of the plasma cofactor for antiphospholipid antibodies in antibody binding to endothelial cells. a) Evaluation of endothelial cell binding of polyclonal and monoclonal anti-beta 2 glycoprotein I antibodies; and b) study of the effects of antibody binding: adhesion molecule expression, leukocyte adhesion, and interleukin-1 beta secretion. Anti-beta 2 glycoprotein I antibodies bind endothelial cell monolayers in vitro by reacting with the cofactor adhered to the cell membranes. Antibody binding induces an up-regulation of adhesion molecules, favours leukocyte adherence, and increases interleukin-1 beta secretion. Interleukin-1 beta plays an active role to mediate adhesion molecule expression through an autocrine loop, as shown by the inhibitory effect of interleukin 1 receptor antagonist. Antiphospholipid antibodies do react with endothelium through the co-factor adhered to their cell membranes and induces a pro-adhesive cell phenotype.

Endothelial cells as target for antiphospholipid antibodies. Human polyclonal and monoclonal anti-beta 2-glycoprotein I antibodies react in vitro with endothelial cells through adherent beta 2-glycoprotein I and induce endothelial activation.

To investigate the ability of human anti-beta 2-glycoprotein I (anti-beta 2 GPI) antibodies to recognize the cofactor adherent on endothelial cells (EC) and to modulate endothelial functions. Six human affinity-purified polyclonal anti-beta 2 GPI IgG and 2 IgM monoclonal antibodies (MAb) were obtained from patients with the antiphospholipid syndrome. The antibodies were tested for their ability to 1) bind to endothelial monolayers through the adherent beta 2 GPI and 2) modulate endothelial adhesion molecule expression and interleukin-6 (IL-6) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) secretion. The affinity-purified IgG and the MAb with anti-beta 2 GPI activity, but not the respective controls, displayed EC binding, which declined on cells incubated in serum-free medium and was restored in a dose-dependent manner by exogenous human beta 2 GPI. After EC binding, both polyclonal and monoclonal antibodies up-regulated adhesion molecule expression. Anti-beta 2 GPI MAb also significantly increased IL-6 and 6-keto-PGF1 alpha secretion. These findings support the hypothesis that anti-beta 2 GPI antibodies bind and activate EC through the adherent cofactor beta 2 GPI, likely leading to a procoagulant state.

Crossreaction between antibodies to oxidised low-density lipoprotein and to cardiolipin in systemic lupus erythematosus

Serum lipoproteins contain phospholipids and modified low-density lipoprotein (LDL) may thus act as a target for antiphospholipid antibodies. Raised concentrations of IgG antibodies against oxidised LDL were found in 47 of 61 (80%) patients with systemic lupus erythematosus (SLE). 46% of patients also had raised concentrations of IgG anticardiolipin antibodies. Binding of anticardiolipin antibodies to solid-phase cardiolipin was inhibited by oxidised LDL but not by native LDL in 16 of 21 sera from SLE patients. These observations suggest crossreactivity between antiphospholipid antibodies, which are closely associated with thrombosis in SLE, and antibodies to oxidised LDL, thus providing a possible link between thrombotic and atherosclerotic complications in SLE.