β2-Glycoprotein I can exist in 2 conformations: implications for our understanding of the antiphospholipid syndrome

Blood ◽  
2010 ◽  
Vol 116 (8) ◽  
pp. 1336-1343 ◽  
Author(s):  
Çetin Ağar ◽  
Gwendolyn M. A. van Os ◽  
Matthias Mörgelin ◽  
Richard R. Sprenger ◽  
J. Arnoud Marquart ◽  
...  
Keyword(s):  
Antiphospholipid Syndrome ◽  
Partial Thromboplastin Time ◽  
Antiphospholipid Antibodies ◽  
Fetal Loss ◽  
Activated Partial Thromboplastin Time ◽  
Ample Evidence ◽  
Glycoprotein I ◽  
Open Conformation ◽  
Cryptic Epitope ◽  
Major Antigen

Abstract The antiphospholipid syndrome is defined by the presence of antiphospholipid antibodies in blood of patients with thrombosis or fetal loss. There is ample evidence that β2-glycoprotein I (β2GPI) is the major antigen for antiphospholipid antibodies. The autoantibodies recognize β2GPI when bound to anionic surfaces and not in solution. We showed that β2GPI can exist in at least 2 different conformations: a circular plasma conformation and an “activated” open conformation. We also showed that the closed, circular conformation is maintained by interaction between the first and fifth domain of β2GPI. By changing pH and salt concentration, we were able to convert the conformation of β2GPI from the closed to the open conformation and back. In the activated open conformation, a cryptic epitope in the first domain becomes exposed that enables patient antibodies to bind and form an antibody-β2GPI complex. We also demonstrate that the open conformation of β2GPI prolonged the activated partial thromboplastin time when added to normal plasma, whereas the activated partial thromboplastin time is further prolonged by addition of anti-β2GPI antibodies. The conformational change of β2GPI, and the influence of the autoantibodies may have important consequences for our understanding of the antiphospholipid syndrome.

Autoimmune Antiphospholipid Antibodies Are Directed against a Cryptic Epitope Expressed when β2-Glycoprotein I Is Bound to a Suitable Surface

1995 ◽  
Vol 73 (01) ◽  
pp. 029-034 ◽  
Author(s):  
Vittorio Pengo ◽  
Alessandra Biasiolo ◽  
Maria Grazia Fior
Keyword(s):  
Antiphospholipid Syndrome ◽  
Antiphospholipid Antibodies ◽  
Autoimmune Disorders ◽  
High Antibody ◽  
Anionic Phospholipid ◽  
Glycoprotein I ◽  
Antibody Titres ◽  
Cryptic Epitope ◽  
Antibody Elisa

SummaryThe antiphospholipid antibodies (aPL) present in autoimmune disorders are associated with thromboembolic episodes, and their binding to phospholipids (PL) is mediated by a plasma cofactor, β2-glycoprotein I (β2GPI). Both PL and β2GPI seem necessary for binding, thus indicating that the two components comprise the epitope against which aPL are directed. Using an anti-β2GPI antibody ELISA with the antigen adsorbed onto polyvinylchloride (PVC) plates, we detected high antibody titres in 12 out of 12 plasmas from patients with the antiphospholipid syndrome. No or very low positivity was obtained when the same ELISA was carried out in polystyrene (PST) plates, while an increasing positivity was found when processed (i.e. more hydrophilic) or COOH-surface PST plates were used. When (β2GPI dependent IgG-aPL were purified using agarose-immobilized cardio- lipin, 4 out of 4 preparations were highly positive in anti-β2GPI antibody ELISA using PVC plates, while β2GPI was not fully recognized by aPL-IgG when adsorbed onto PST plates. These findings demonstrate that aPL are, in fact, anti-β2GPI antibodies directed against a cryptic epitope which is expressed when β2GPI is bound to anionic phospholipid, or another suitable surface.

The significance of autoantibodies against β2-glycoprotein I

Blood ◽  
2012 ◽  
Vol 120 (2) ◽  
pp. 266-274 ◽  
Author(s):  
Philip G. de Groot ◽  
Rolf T. Urbanus
Keyword(s):  
Autoimmune Disease ◽  
Infectious Diseases ◽  
Antiphospholipid Syndrome ◽  
Plasma Protein ◽  
Antiphospholipid Antibodies ◽  
Fetal Loss ◽  
Thrombotic Risk ◽  
Pregnancy Morbidity ◽  
Glycoprotein I ◽  
History Of

AbstractThe antiphospholipid syndrome (APS) is defined by the persistent presence of antiphospholipid antibodies in patients with a history of thrombosis and/or pregnancy morbidity, including fetal loss. APS is an autoimmune disease with a confusing name because the pathologic auto-antibodies are shown to be directed against the plasma protein β2-glycoprotein I and not against phospholipids. In fact, auto-antibodies that recognize phospholipids themselves are not associated with thrombosis but with infectious diseases. One of the intriguing questions is why autoantibodies against β2-glycoprotein I are so commonly found in both patients and the healthy. Several potential mechanisms have been suggested to explain the increased thrombotic risk in patients with these autoantibodies. In this overview, we will summarize our knowledge on the etiology of the autoantibodies, and we will discuss the evidence that identify autoantibodies against β2-glycoprotein I as the culprit of APS.

Variability in Exposure of Epitope G40-R43 of Domain I in Commercial Anti-β2-Glycoprotein I IgG Elisas Influences the Diagnosis of the Antiphospholipid Syndrome

Blood ◽  
2012 ◽  
Vol 120 (21) ◽  
pp. 632-632
Author(s):  
Leonie Pelkmans ◽  
Hilde Kelchtermans ◽  
Marisa Ninivaggi ◽  
Theo Lindhout ◽  
Philip G de Groot ◽  
...  
Keyword(s):  
Antiphospholipid Syndrome ◽  
Conflicts Of Interest ◽  
High Variability ◽  
Diagnostic Assays ◽  
Glycoprotein I ◽  
Open Conformation ◽  
Lower Reactivity ◽  
Number Of Patients ◽  
Cryptic Epitope ◽  
Domain I

Abstract Abstract 632 Introduction. Diagnosis of the antiphospholipid syndrome (APS) depends on the detection of autoantibodies against phospholipid-bound β2-glycoprotein I (β2GPI), the most prominent antigen in APS. One of the main problems faced is the high variability observed between different commercially available anti-β2GPI assays. Anti-β2GPI antibodies constitute a heterogeneous population, but predominantly antibodies reacting to a cryptic epitope Glycine40-Arginine43 (G40-R43) in domain I of β2GPI are associated with a strong risk to develop thrombosis. b2GPI is present in blood in a native conformation (either circular or S-shaped). After interaction with anionic surfaces it opens up (J-shaped conformation), resulting in exposure of the epitope G40-R43 in domain I. It is therefore important that in diagnostic assays b2GPI is presented in the open conformation enabling antibodies to react with the G40-R43 epitope. We hypothesize that the high variability between different commercial anti-b2GPI ELISA assays arise from variation in exposure of the G40-R43 epitope of b2GPI. Methods. Two patient-derived monoclonal antibodies P2-6 and P1-117 were tested for their reactivity towards β2GPI in different conformations. Using both antibodies, we compared exposure of epitope G40-R43 on β2GPI in five different commercial anti-β2GPI IgG assays. 10 patient samples selected for their low to high positivity towards epitope G40-R43, were tested in two anti-β2GPI IgG assays. Results. Using neutral versus anionic ELISA plates, we have shown that antibody P1-117 specifically reacts with epitope G40-R43, exposed only in the open conformation, while antibody P2-6 recognizes β2GPI irrespective of its conformation (Fig. 1). In one of the tested anti-β2GPI assays, both antibodies showed equal reactivity towards β2GPI, indicating that all the coated β2GPI exposes epitope G40-R43. In this assay, all ten anti-domain I positive patients tested positive. In other assays P1-117 displayed lower reactivity towards β2GPI than P2-6, demonstrating a reduced exposure of G40-R43. Only 3 out of the 10 patients tested positive in such assay, illustrating that a significant number of patients can be falsely assigned negative in assays characterized by a reduced exposure of epitope G40-R43. Conclusions. The exposure of epitope G40-R43 on β2GPI is highly variable in commercial anti-β2GPI assays, and influences the diagnosis of APS. Our results have major implications for the diagnosis of APS, as it provides suitable controls to ensure sufficient exposure of the G40-R43 epitope or suggests the development of alternative assays coating only domain I of β2GPI. Disclosures: No relevant conflicts of interest to declare.

Current concepts on the pathogenesis of the antiphospholipid syndrome

Blood ◽  
2006 ◽  
Vol 109 (2) ◽  
pp. 422-430 ◽  
Author(s):  
Bill Giannakopoulos ◽  
Freda Passam ◽  
Soheila Rahgozar ◽  
Steven A. Krilis
Keyword(s):  
Experimental Evidence ◽  
Antiphospholipid Syndrome ◽  
Antiphospholipid Antibodies ◽  
Clinical Manifestations ◽  
Fetal Loss ◽  
Peripheral Tolerance ◽  
The Core ◽  
Glycoprotein I ◽  
Antigenic Targets ◽  
Acquired Thrombophilia

Abstract The antiphospholipid syndrome (APS) is an important cause of acquired thrombophilia. It is characterized by the core clinical manifestations of thrombosis, either venous or arterial, and in women it can also be associated with recurrent fetal loss. The detection of persistently elevated levels of antiphospholipid antibodies (aPL Abs) is a requisite laboratory feature for the diagnosis to be made. The dominant antigenic targets in APS are beta 2-glycoprotein I (β2-GPI) and prothrombin. There is an accumulating body of experimental evidence that suggests that specific subgroups of aPL Abs may directly contribute to disease pathogenesis. This review critically examines the experimental evidence underlying the various propositions made to explain how these antibodies may predispose to disease in humans. Furthermore, it also examines the evidence relating to the immunologic mechanisms that may contribute to the breakage of peripheral tolerance in this disorder. Delineating the strengths and limitations of the experimental evidence accumulated thus far will hopefully stimulate further experimentation toward achieving the ultimate goal of precisely defining the dominant pathogenic mechanisms operational in APS. This may pave the way for the development of improved therapies.

β2-glycoprotein I–dependent lupus anticoagulant highly correlates with thrombosis in the antiphospholipid syndrome

Blood ◽  
2004 ◽  
Vol 104 (12) ◽  
pp. 3598-3602 ◽  
Author(s):  
H. Bas de Laat ◽  
Ronald H.W.M. Derksen ◽  
Rolf T. Urbanus ◽  
Mark Roest ◽  
Philip G. de Groot
Keyword(s):  
Confidence Interval ◽  
Antiphospholipid Syndrome ◽  
Partial Thromboplastin Time ◽  
Antiphospholipid Antibodies ◽  
Odds Ratio ◽  
Lupus Anticoagulant ◽  
Thromboembolic Complications ◽  
Glycoprotein I ◽  
Patients At Risk ◽  
History Of

The antiphospholipid syndrome is characterized by the presence of antiphospholipid antibodies in plasma of patients with thromboembolic complications. A major problem in defining the syndrome is that serologic assays to detect antiphospholipid antibodies have a low specificity. We recently published a method that specifically detects lupus anticoagulant (LAC) caused by anti–β2-glycoprotein I antibodies. Here, we studied the clinical relevance of detecting β2-glycoprotein I–dependent LAC. Plasma samples were collected from 198 patients with autoimmune diseases. In those samples with a positive partial thromboplastin time–lupus anticoagulant (PTT-LA), a modified activated partial thromboplastin time (aPTT)–based LAC test was performed with cardiolipin as confirming agent. Twenty-five of 58 patients with an aPTT-based LAC were dependent on the presence of anti–β2-glycoprotein I antibodies. Presence of β2-glycoprotein I–dependent LAC was almost completely associated with a history of thromboembolic complications (odds ratio, 42.3; 95% confidence interval, 194.3-9.9). An increased frequency of thrombosis was not found in 33 patients with LAC independent of anti–β2-glycoprotein I antibodies (odds ratio, 1.6; 95% confidence interval, 3.9-0.8). The use of an LAC assay with cardiolipin as confirming agent strongly improves the detection of patients at risk of thrombosis. Our findings suggest that anti–β2-glycoprotein I antibodies with LAC activity are antibodies that are responsible for the thromboembolic complications in the antiphospholipid syndrome.

scholarly journals Antiphospholipid Antibodies and Acquired Thrombophilia: A Biological Marker for Recurrent Miscarriage

2021 ◽  
pp. 137-143
Author(s):  
Rania Khogli ELsidig Khogli ◽  
Abdel Rahim Mahmoud Muddathir ◽  
Alaa Eltayeb Omer ◽  
Lienda Bashier Eltayeb
Keyword(s):  
Partial Thromboplastin Time ◽  
Antiphospholipid Antibodies ◽  
Tissue Injury ◽  
Recurrent Miscarriage ◽  
Biological Marker ◽  
Activated Partial Thromboplastin Time ◽  
P Value ◽  
Increased Risk ◽  
The One ◽  
Acquired Thrombophilia

Background: Repeated miscarriage can cause tissue injury can lead to the formation of antibodies to the phospholipids. Recurrent miscarriage (RM) is considering the one of the most common cause of sterility. Which has received more attention in recent years as a result of an increase in the number of reproductive-aged women. Materials and Methods: Plasma samples were tested for antiphospholipid antibodies using ELISA, and platelet count using Sysmex (KX21) Heamatology analyzer and Activated Partial Thromboplastin Time using semi-automated machine (STAGO PT31039352 (for coagulation). Results: The prevalence of Anti phospholipid antibodies (APL) was 30.5% in Sudanese patients with recurrent miscarriage, the prevalence of (Anti phospholipid Antibodies-IgM and IgG) was found to be 23.6% in patients with recurrent miscarriage compared to (Anti phospholipid Antibodies-IgG) was found to be 11.1% ((P value≤0.001), low platelets count (<50×109/l) observed in 10 (13.5%), as well as prolongation of activated partial thromboplastin time (APTT) among studied group were detected among 19 (26.1%). Conclusion: Higher prevalence of antiphospolidids antibodies, and acquired thrombophilia was detected among Sudanese women with recurrent abortion; The findings are concerning because they link an increased risk of thrombosis and a hypercoagulable state lead to recurrent miscarriage in pregnant women.

scholarly journals Molecular Mechanisms of “Antiphospholipid Antibodies” and Their Paradoxical Role in the Pathogenesis of “Seronegative APS”

2020 ◽  
Vol 21 (21) ◽  
pp. 8411
Author(s):  
Roberta Misasi ◽  
Agostina Longo ◽  
Serena Recalchi ◽  
Daniela Caissutti ◽  
Gloria Riitano ◽  
...  
Keyword(s):  
Antiphospholipid Antibodies ◽  
Molecular Mechanisms ◽  
Strong Association ◽  
Fetal Loss ◽  
Signal Transduction Pathway ◽  
Therapeutic Strategies ◽  
Transduction Pathway ◽  
Laboratory Methods ◽  
Pregnancy Morbidity ◽  
Glycoprotein I

Antiphospholipid Syndrome (APS) is an autoimmune disease characterized by arterial and/or venous thrombosis and/or pregnancy morbidity, associated with circulating antiphospholipid antibodies (aPL). In some cases, patients with a clinical profile indicative of APS (thrombosis, recurrent miscarriages or fetal loss), who are persistently negative for conventional laboratory diagnostic criteria, are classified as “seronegative” APS patients (SN-APS). Several findings suggest that aPL, which target phospholipids and/or phospholipid binding proteins, mainly β-glycoprotein I (β-GPI), may contribute to thrombotic diathesis by interfering with hemostasis. Despite the strong association between aPL and thrombosis, the exact pathogenic mechanisms underlying thrombotic events and pregnancy morbidity in APS have not yet been fully elucidated and multiple mechanisms may be involved. Furthermore, in many SN-APS patients, it is possible to demonstrate the presence of unconventional aPL (“non-criteria” aPL) or to detect aPL with alternative laboratory methods. These findings allowed the scientists to study the pathogenic mechanism of SN-APS. This review is focused on the evidence showing that these antibodies may play a functional role in the signal transduction pathway(s) leading to thrombosis and pregnancy morbidity in SN-APS. A better comprehension of the molecular mechanisms triggered by aPL may drive development of potential therapeutic strategies in APS patients.

β2-Glycoprotein I: Target antigen for ‘antiphospholipid’ antibodies. Immunological and molecular aspects

Lupus ◽  
1998 ◽  
Vol 7 (2_suppl) ◽  
pp. 5-9 ◽  
Author(s):  
Y Sheng ◽  
DA Kandiah ◽  
SA Krilis
Keyword(s):  
Antiphospholipid Syndrome ◽  
Antiphospholipid Antibodies ◽  
Fluid Phase ◽  
Target Antigen ◽  
Phospholipid Membranes ◽  
Anionic Phospholipid ◽  
Glycoprotein I ◽  
High Concentrations ◽  
Molecular Aspects ◽  
Anionic Phospholipid Membranes

It has become clear that β2-glycoprotein I (β2GPI) is the most common and best-characterised antigenic target for ‘antiphospholipid’ (aPL) autoantibodies. These antibodies preferentially bind β2GPI that has been immobilised on anionic phospholipid membranes or certain synthetic surfaces. These surfaces appear to act by increasing antigen density to allow binding of intrinsically low-affinity anti-β2GPI autoantibodies. Binding of β2GPI in fluid phase is weak and requires high concentrations of β2GPI. Our understanding of the pathophysiology of the ‘Antiphospholipid’ Syndrome (APS) has increased exponentially with the number of studies into the interactions of aPL antibodies and β2GPI.

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