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β2-Glycoprotein I Autoantibodies
CHAPTER
β2-Glycoprotein I Autoantibodies
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Eiji Matsuura1,2 and Luis R. Lopez3 1Collaborative
Research Center,
2Department
of Cell Chemistry, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, Okayama, Japan, 3Corgenix, Inc., Broomfield, CO
Historical notes In 1990, three groups of investigators independently reported that anticardiolipin (aCL) antibodies were directed against β2-glycoprotein I (β2GPI)/cardiolipin (CL) complexes rather than CL alone [1–3]. It is now widely agreed that β2GPI is the major antigenic target for antiphospholipid antibodies derived from patients with antiphospholipid syndrome (APS). Pathogenic antiphospholipid antibodies are directed to β2GPI/phospholipid complexes, likely to cryptic epitopes generated on domain I of the β2GPI molecule upon its interaction with phospholipids or when bound to high density phospholipid (or artificial) surfaces. These represent two possible antibody-binding mechanisms, each supported by experimental data. These antibody-binding mechanisms may not be mutually exclusive; perhaps they all play different roles depending on the type of surface, the interacting molecules, or particular experimental settings. The β2GPI region responsible for phospholipid binding activity has been identified in domain V and is fairly well characterized. However, the precise nature and pathophysiologic consequences triggered by this interaction are now been unraveled. It seems clear that β2GPI needs to interact with phospholipids exposed by activated cell membranes to trigger prothrombotic changes. In contrast, the location and nature of the cryptic epitope(s) responsible for antibody binding remain less clear. Experimental evidence points to the location of cryptic epitopes at the opposite end of the β2GPI molecule (domain I). The exact manner in which antiphospholipid antibodies trigger prothrombotic activity in the venous or arterial vasculature when binding β2GPI/phospholipid complexes is a subject of intense research. More recently, it has been recognized that β2GPI may have more diverse functions than previously thought. β2GPI binds other lipoproteins of pathological significance (i.e., oxidized low-density lipoprotein (oxLDL)), suggesting a role of these complexes in autoimmune-mediated atherogenesis.
The β2GPI autoantigen: definition
β2GPI is a 50-kDa single-chain polypeptide present in plasma at a concentration of approximately 200 μg/mL. The complete amino acid sequence of human β2GPI has been established by complementary deoxyribonucleic acid (cDNA) cloning and sequencing from a human hepatoma cell line (HepG2). It is composed of 326 amino acid residues and consists of five homologous domains. Each of the first four Autoantibodies. http://dx.doi.org/10.1016/B978-0-444-56378-1.00081-2 Copyright © 2014 Elsevier B.V. All rights reserved.
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domains (domains I–IV) is composed of 60 amino acids with highly conserved prolines, cystines, and tryptophans. Each motif is characterized by conserved half-cysteine residues with two internal disulfide bridges. These repeating motifs have been designated short consensus repeats or shushi domains. The fifth domain (domain V) toward the carboxyl terminal is different, with 82 amino acids residues and three disulfide bridges. This domain is particularly important because it contains the sequence 282KNKEKK287, which is responsible for phospholipid binding, as alterations of this sequence produced a complete elimination of β2GPI binding to phospholipids. Elucidation of the crystal structure of β2GPI has confirmed these observations.
β2GPI biologic functions
The physiologic functions of β2GPI are becoming more evident and diverse. β2GPI binds in vitro to negatively charged molecules such as phospholipids (CL, phosphatidylserine), heparin, and certain lipoproteins, as well as to the cell membranes of activated platelets and endothelial cells. It has been widely reported that β2GPI also influences coagulation and platelet function by inhibiting contact activation of the intrinsic coagulation pathway, platelet prothrombinase activity, and adenosine diphosphate (ADP)-mediated platelet aggregation. More recently, β2GPI has been involved in the removal of apoptotic cells through a phosphatidylserine-binding mechanism and shown to interfere with the protein C and protein S anticoagulant pathways. β2GPI appears to influence lipoprotein metabolism by promoting clearance of oxidation products such as oxLDL and even angiogenesis [4]. However, there is less information available about the precise functions and mechanisms of action of β2GPI in vivo. Free or monomeric β2GPI has a relatively low affinity for negatively charged phospholipids. Anti-β2GPI antibodies may cross-link two β2GPI/phospholipid complexes, increasing its binding affinity over 100-fold. How does cross-linking on cellular surfaces activate prothrombotic mechanisms? A model to explain how anti-β2GPI antibodies activate platelets proposes that anti-β2GPI antibodies bind two β2GPI molecules, inducing conformational changes. This interaction increases the affinity of dimeric β2GPI to cell surface phospholipids and protein-binding sites. β2GPI may interact with a cell surface receptor of the low-density lipoprotein (LDL) receptor family, apolipoprotein E receptor 2 (apoER2). This may result in phosphorylation of apoER2, followed by phosphorylation of MAPp38 kinase and synthesis of thromboxane A2. These events may shift the hemostatic balance toward a prothrombotic state, increasing the risk of developing thrombosis in patients with circulating anti-β2GPI antibodies.
Source and methods of β2GPI purification
β2GPI is purified from normal plasma by sequential perchloric acid precipitation or CL affinity and ion-exchange chromatography. β2GPI activates lipoprotein lipase in vitro and appears in the lipoprotein ultracentrifugation fraction. For these reason, it has also been referred to as apolipoprotein H. Human and bovine β2GPI show a high degree of homology (∼85%), and both have been widely used in diagnostic systems. Recombinant human β2GPI has been produced using baculovirus and insect cell systems and has contributed significantly in studies of the biological functions of β2GPI.
β2-GPI in atherogenesis
β2-GPI in atherogenesis
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Once ascertained that β2GPI was the main antigenic target for antiphospholipid antibodies, other functions of β2GPI started to become apparent; initially described as a natural anticoagulant, β2GPI has more pleiotropic functions affecting fibrinolysis, angiogenesis, and apoptosis as well as atherogenesis due to its interaction with oxLDL [5]. In fact, unlike native LDL, β2GPI binds oxLDL via specific oxidative-derived ligands to form stable and proatherogenic oxLDL/β2GPI complexes. The interaction between oxLDL and β2GPI suggests an antioxidant role of β2GPI by quenching the proinflammatory and proatherogenic effects of the oxLDL molecule. But in doing so, oxLDL/β2GPI complexes may also become immunogenic, triggering the production of autoantibodies and immune complexes. Serum levels of oxLDL/β2GPI complexes are elevated in patients with systemic autoimmune diseases characterized by cardiovascular complications, implicating oxLDL/β2GPI complexes as atherogenic autoantigens. Indeed, autoantibodies to oxLDL/β2GPI (and to β2GPI) frequently coexist in the same autoimmune disorders [6]. Current evidence points toward the atherosclerotic lesions as the primary site of oxLDL/β2GPI complex formation with subsequent release into the circulation. oxLDL/β2GPI complexes and their immune complexes upregulate the macrophage expression of scavenger and FcγRI receptors that further stimulate the oxLDL/β2GPI uptake and rapid intracellular accumulation. It is possible that oxidative stress and β2GPI-containing complexes are capable of activating chronic inflammation via innate immune responses that include the interleukin (IL)-1β inflammasome pathway. Published experimental evidence for a role of β2GPI in the regulation of oxidative stress and adaptive and innate immunity is summarized in Table 81.1. Recent reports on the role of oxidized β2GPI in vascular atherothrombosis of autoimmune patients require further discussion. β2GPI is susceptible to reduction by the thiol oxidoreductase thioredoxin-1. The reduction may take place on the surface of platelets where the Cys288-Cys326 disulfide bond in domain V of β2GPI is reduced by thioredoxin-1, favoring the binding of von Willebrand factor (vWF) to platelets and endothelial cells [7]. Thus, the presence of the reduced form of β2GPI in circulation may be relevant to atherothrombosis. The main function(s) of β2GPI remains unclear, but the high plasma concentration makes β2GPI readily available to the effect and regulation by thioredoxin-1. β2GPI with free thiols has been detected in both human and murine sera, which suggests that free thiol-containing β2GPI exerts a powerful protection of vascular elements such as platelets and endothelial cells from oxidative stress-injury and cell death. Further, the oxidative post-translational modification of β2GPI may trigger a Th1 immune response [8]. Direct electron microscopy visualization of β2GPI purified from plasma in the absence and presence of antibodies shows that β2GPI is a flexible molecule that can exist in two different conformations. In plasma it is mostly present as a circular protein in which domain I binds to domain V. When β2GPI interacts with anionic surfaces, it opens and adopts the J conformation. The closed (circular) conformation would hide domain V from binding to cellular receptors or phospholipids and even protect domain I from interacting with the antibody, while the open conformation makes both β2GPI sites available for biological/immunological interactions [9]. The in vivo mechanisms that control the conformation of β2GPI and their pathophysiologic significance remain to be elucidated. Whether the reduced or oxidized forms of β2GPI correspond to the open or closed conformation has yet to be determined.
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Table 81.1 Evidence for a Role of β2GPI in the Regulation of Oxidative Stress and Immune (Innate and Adaptive) Responses Author/year
Study Description
Findings
Kajiwara et al. (2007) [23]
Intracellular trafficking of β2GPI complexes with oxLDL. In vitro murine macrophages J774 Redox (thioredoxin-1) control of β2GPI. In vitro vWF platelet adhesion β2GPI novel component of innate immunity. In vitro monocytes and endothelial cells Reduced β2GPI protect in vitro endothelial cells from oxidative stress-induced cell injury Detection of oxidized β2GPI in autoimmune and vascular diseases
Only complexed β2GPI localized in lysosomes and upregulated expression of CD36 and FcγRI Thioredoxin-1 reduced β2GPI (Cys288-Cys326 Dom V), had increased binding to vWF and platelet adhesion β2GPI bound LPS and inhibited LPS-induced TF and IL-6 expression
Passam et al. (2010) [7]
Agar et al. (2011) [9]
Ioannou et al. (2010) [24]
Ioannou et al. (2011) [25]
Passan et al. (2011) [8]
Review paper and unpublished recent data
Thioredoxin-1 reduced β2GPI (Cys288–Cys326 Dom V), had antioxidant and platelet adhesion Post-translational oxidative modification of β2GPI (lacking free thiols) may participate in thrombosis Reduced β2GPI with abundant free thiols is antioxidant reservoir. Oxidation of β2GPI (lacking free thiols) increases immunogenicity via Th1 response
β2GPI: β2-glycoprotein I; IL: interleukin; LPS: lipopolysaccharide; oxLDL: oxidized low-density lipoprotein; TF: tissue factor; vWF: von Willebrand factor.
The β2GPI autoantibody: definition
Antiphospholipid antibodies (aCL and lupus anticoagulants (LA)) are a heterogeneous group of autoantibodies with a pathogenic role in the development of the vascular complications (venous and arterial thrombosis) of APS. These antibodies were initially thought to be specific for negatively charged phospholipids; however, it is now widely accepted that protein/phospholipid complexes are the more relevant antigenic targets. Several plasma proteins that participate in coagulation have been described as antiphospholipid antigenic targets (i.e., β2GPI, prothrombin, annexin V, etc.). β2GPI interacts with negatively charged phospholipids and is considered the most clinically important antiphospholipid antigen. Antiphospholipid antibodies also recognize β2GPI immobilized on oxygenated polystyrene surfaces in the absence of phospholipids [10]. These findings suggest that antiphospholipid antibodies recognize cryptic epitopes formed on β2GPI upon its interaction with an oxygenated surface.
Pathogenic role NZW x BXSB (W/B) F1 mice present multiple autoantibodies and a systemic lupus-like disease [11]. Myocardial infarction is common in the W/B F1 male mice, and its incidence increases with age to over 80%. The titer of antiphospholipid antibodies (including anti-β2GPI antibodies) also
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increases with age in these mice. Two monoclonal anti-β2GPI antibodies have been raised from W/B F1 mice that induce thrombosis when injected into normal mice. One of these monoclonal antibodies (WB-CAL-1) binds to the complexed form but not to free or monomeric β2GPI. This finding supports the role of β2GPI cryptic epitopes in antibody-mediated thrombosis. Monoclonal anti-β2GPI antibodies have been derived from APS patients and even proposed to be useful for assay standardization. Antiphospholipid antibodies may interfere with β2GPI’s natural anticoagulant properties, thus promoting thrombosis. Anti-β2GPI antibodies have been reported to be more specific for thrombosis and APS than aCL antibodies. Recent prospective studies have shown that aCL antibodies, particularly those β2GPI-dependent or anti-β2GPI antibodies, are important predictors for arterial thrombosis (myocardial infarction and stroke) in men. These studies suggested that β2GPI and anti-β2GPI antibodies play a central role in the pathogenesis of thrombosis, particularly in arterial thrombosis and atherosclerosis seen in systemic lupus erythematosus (SLE) and APS patients. Dysregulation of both the adaptive and inflammatory immune responses are important pathologic mechanisms underlying the clinical manifestations characteristic of autoimmune diseases such as SLE and APS. The cause–effect relationship between antiphospholipid antibodies and vascular thrombotic events is now widely accepted and better understood. In addition to venous thromboembolism, patients with SLE and APS may develop premature atherosclerotic cardiovascular disease associated with significant morbidity and mortality not fully explained by the classic risk factors. Further, the contributory role of antibodies to β2GPI and oxLDL in autoimmune-mediated atherothrombosis is now being recognized [12].
Newer pathogenic roles β2GPI may interact in vitro with other lipid molecules to form potentially pathogenic complexes. oxLDL plays a central role in atherogenesis. We have demonstrated that β2GPI binds oxLDL, not native LDL, initially forming dissociable electrostatic complexes, followed by more stable complexes bound by covalent interactions [13]. In addition, lipid peroxidation (oxidative stress) is common in SLE and APS, and circulating “stable” β2GPI/oxLDL complexes were demonstrated in these patients. Because oxLDL is highly proinflammatory, unstable, and short lived in circulation, we hypothesized that β2GPI binding neutralizes the deleterious effect of oxLDL while promoting its clearance. However, chronic “oxidative stress” may overcome this physiologic mechanism and over time become rather proatherogenic. oxLDL and β2GPI/oxLDL complexes are also highly immunogenic. We have demonstrated the presence of anti-β2GPI/oxLDL antibodies in patients with systemic autoimmune diseases (SLE, systemic sclerosis (SSc), and APS) [14]. These antibodies were significantly higher and more prevalent (75%) in secondary APS patients compared to 37% in SLE controls without history of thrombosis, with 93.7% specific for APS with a predictive value (PV) of 90% (P < 0.001). IgG aCL antibodies were 80% specific with a PV of 71.4% (P < 0.001). In addition, APS patients with a history of arterial thrombosis had a higher mean level and prevalence (38%) of IgG anti-β2GPI/oxLDL antibodies compared with APS patients with history of venous thrombosis (19%). The PV of IgG anti-β2GPI/oxLDL antibodies for total (arterial + venous) thrombosis was 92% (P = 0.018): 89% (P = 0.04) for arterial thrombosis and 77% (P = 0.168) for venous thrombosis [15].
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Patients with autoimmune diseases present chronic dyslipidemia characterized by decreased high-density lipoprotein (HDL), changes in the HDL subpopulations, raised triglycerides, and unchanged or only slightly elevated LDL levels [16]. On this background, the systemic generation of free radicals by endothelial and circulating mononuclear cells may induce oxidative modifications of LDL (oxLDL). The uptake of oxLDL by arterial mononuclear cells results in the release of inflammatory and chemotactic cytokines in the early stages of atherosclerosis, leading to an excessive i ntracellular accumulation of oxLDL. This uptake is mediated by scavenger receptors [17]. Immunostaining of human atherosclerotic lesions colocalized β2GPI with oxLDL and suggested a close relationship of these molecules [18,19]. When IgG anti-β2GPI antibodies were coincubated with oxLDL and β2GPI, the monocyte/ macrophage uptake and intracellular accumulation of oxLDL was accelerated, likely via FcγRI receptors, also leading to upregulation and enhanced surface expression of scavenger (CD36) and FcγRI receptors [20]. It therefore seems that while antibodies against β2GPI are strongly implicated in the development of autoimmune arterial and venous thromboembolism, antibodies against oxLDL/β2GPI are involved in atherogenesis, lending further support to the general concept that autoimmunity plays a role in development of premature cardiovascular disease. Outside the autoimmune setting, antiphospholipid antibodies (anti-β2GPI and anti-oxLDL/β2GPI) as well as oxLDL/β2GPI complexes have been demonstrated in patients with type 2 diabetes and cardiovascular disease, including acute coronary syndromes (ACS). The presence of these antibodies in ACS was associated with a 2.9-fold risk of adverse outcomes [21]. Serum levels of oxLDL/β2GPI complexes in higher quartiles positively correlated with the severity of the disease assessed by angiography and with a 3.5-fold increased risk [22]. When both antibodies and complexes were present, the risk increased 14-fold, suggesting a synergistic effect between oxLDL/β2GPI complexes and their antibodies. An interesting but unexplained observation is that the most common aPL isotype in ACS patients was IgM. Previous reports suggested a protective (antiatherogenic) role of natural IgM antibodies.
Method of detection The 1998 Sapporo and Sydney serologic criteria for the classification of definite APS requires the demonstration of persistent LA and/or high titers of IgG aCL and anti-β2GPI antibodies. LAs have been reported as more specific for thrombosis and APS than aCL antibodies. β2GPI role in LA activity has also been demonstrated. Detection of anti-β2GPI antibodies in patient sera is important for the diagnosis of APS and it has been recently incorporated as a major serologic criteria. P urified human β2GPI can be adsorbed on two general types of surfaces: CL-coated polystyrene to assess β2GPI-dependency or oxygenated polystyrene plates. β2GPI adsorbed on oxygenated polystyrene plates would create the increasing density and the cryptic epitopes recognized by clinically relevant anti-β2GPI antibodies. This system eliminates most of the false-positive or irrelevant reactivity frequently seen in CL-coated surfaces. The nature of the surface seems to be important, and anti-β2GPI enzyme-linked immunosorbent assay (ELISA) using such surfaces have been shown to be more s pecific for thrombosis and APS than ELISAs using CL-coated surfaces.
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In addition, ELISAs for anti-β2GPI antibodies allow not only the quantification but also the d etermination of antibody isotypes. IgG anti-β2GPI antibodies are considered to be more specific than IgM, and more recently, several reports indicate that IgA anti-β2GPI antibodies may also have a pathogenic role in thrombosis and APS.
Clinical utility: disease association APS is the most common cause of acquired thrombophilia (hypercoagulability) in the general populations. In a recent study of the relationship between antiphospholipid antibodies and the type of thrombotic manifestation, the PV and association of APS with aCL, antiphosphatidylserine antibodies, anti-β2GPI antibodies, and antiprothrombin antibodies were evaluated in 100 APS and 90 SLE patients [15]. APS patients were classified according to the clinical history of arterial thrombosis, venous thrombosis, or pregnancy morbidity. Three isotypes (IgG, IgM, and IgA) of each antibody were measured by ELISA. Individually, IgG, IgM, and IgA anti-β2GPI and antiphosphatidylserine a ntibodies had the strongest specificity (> 94%) and PV for APS (86.4–94.1%; P < 0.001) in patients with SLE. In comparison, the IgG specificity of aCL antibodies was only 78% with a PV of 60.7% (P = 0.063). The PV for APS reached 100% when two or more antiphospholipid antibodies were present. Similarly, anti-β2GPI antibodies and antiphosphatidylserine antibodies had a stronger PV and association for arterial thrombosis (87– 95%; P < 0.001) compared to venous thrombosis (80– 92%; P = 0.01) in APS patients. These results suggested an important pathogenic role of anti-β2GPI in arterial thrombosis and provided the best diagnostic value for the laboratory assessment of APS.
Diagnostic value aCL ELISAs are the most commonly used tests by clinical laboratories to assess the risk of thrombosis and diagnose APS. Low specificity (high false-positive rates) and standardization problems remain major controversial issues. Accurate serologic diagnosis for APS is of paramount importance as anticoagulation is the main treatment, frequently long-term if not life-long. Complications from recommended anticoagulation programs may potentially be serious, underscoring the need for better understanding of the role of antiphospholipid antibodies in thrombosis and the availability of highly specific assays. The discovery of β2GPI as a clinically relevant antigenic target stimulated the development and use of more specific anti-β2GPI ELISA tests. In addition to the participation of β2GPI and anti-β2GPI antibodies in venous thrombosis, there is newer experimental evidence that β2GPI participates in atherosclerosis and arterial thrombosis by interacting with oxLDL, forming β2GPI/oxLDL complexes in the arterial wall. β2GPI/oxLDL complexes are taken up by macrophages and stimulate the production of anti-β2GPI/oxLDL antibodies, which will further enhance macrophage uptake via Fcγ receptors, promoting foam cell and atherosclerotic plaque formation. Additional functions or interactions of β2GPI that may trigger anti-β2GPI antibody production will likely be discovered and provide new insights into the pathogenesis of autoimmune-mediated thrombosis.
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FIGURE 81.1 Schematic representation of mechanisms involved in anti-β2GPI antibody-dependent atherothrombosis. β2GPI: β2-glycoprotein I; Ab: antibody; LDL: low-density lipoprotein; oxLDL: oxidized low-density lipoprotein.
Take-home messages • β 2GPI is the most clinically relevant antigenic target for antiphospholipid antibodies, suggesting a central role in the pathogenesis of autoimmune-mediated thrombosis. • The interaction of β2GPI with activated phospholipid cell membranes (platelets or endothelial cells) may produce a prothrombotic surface. This interaction generates cryptic epitopes on the β2GPI that promote anti-β2GPI autoantibody production. • Cryptic epitopes and/or increased β2GPI density on suitable surfaces favor anti-β2GPI antibody binding, triggering prothrombotic mechanisms. • Measuring anti-β2GPI antibodies in the clinical laboratory provides additional and relevant serologic information to assess the risk of thrombosis or APS diagnosis properly. • β2GPI binding to oxLDL to form β2GPI/oxLDL complexes and autoantibodies to these complexes represents another important mechanism that leads to atherosclerosis and arterial thrombosis in autoimmune patients.
References [1] Galli M, Comfurius P, Maassen C, Hemker HC, de Baets MH, van Breda-Vriesman PJC, et al. Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor. Lancet 1990;335:1544–7. [2] McNeil HP, Simpson RJ, Chesterman CN, Krilis SA. Antiphospholipid antibodies are directed against a complex antigen that includes a lipid-binding inhibitor of coagulation: β2-glycoprotein I (apolipoprotein H). Proc Natl Acad Sci U S A 1990;87:4120–4. [3] Matsuura E, Igarashi Y, Fujimoto M, Ichikawa K, Koike T. Anticardiolipin cofactor(s) and differential diagnosis of autoimmune disease. Lancet 1990;336:177–8. [4] Kobayashi K, Kishi M, Atsumi T, Bertolaccini ML, Makino H, Sakairi N, et al. Circulating oxidized LDL forms complexes with β2-glycoprotein I: implication as an atherogenic autoantigen. J Lipid Res 2003;44:716–26.
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[5] De Groot PG, Meijers JCM. β2-glycoprotein I: evolution, structure and function. J Thomb Haemost 2011;9:1275–84. [6] Lopez LR, Salazar-Paramo M, Palafox-Sanchez C, Hurley BL, Matsuura E, Garcia-De La Torre I. Oxidized low-density lipoprotein and β2-glycoprotein I in patients with systemic lupus erythematosus and increased carotid intima-media thickness: implications in autoimmune-mediated atherosclerosis. Lupus 2006; 15:80–6. [7] Passam FH, Rahgozar S, Qi M, Raftery MJ, Wong JW, Tanaka K, et al. Redox control of β2GPI-von Willebrand factor interaction by thioredoxin-1. J Thromb Haemost 2010;8:1754–62. [8] Passam FH, Giannakopoulos B, Mirarabshahi P, Krillis SA. Molecular pathophysiology of the antiphospholipid syndrome: the role of oxidative post-translational modification of β2-glycoprotein I. J Thromb Haemost 2011;9(Suppl. 1):275–82. [9] Agar C, van Os GM, Mörgelin M, Sprenger RR, Marquart JA, Urbanus RT, et al. β2-glycoprotein I can exist in 2 conformations: implications for our understanding of the antiphospholipid syndrome. Blood 2011;116:1336–43. [10] Matsuura E, Igarashi Y, Yasuda T, Triplett DA, Koike T. Anticardiolipin antibodies recognize β2-glycoprotein I structure altered by interacting with an oxygen-modified solid-phase surface. J Exp Med 1994;179:457–62. [11] Hashimoto Y, Kawamura M, Ichikawa K, Suzuki T, Sumida T, Yoshida S, et al. Anticardiolipin antibodies in NZW x BXSB F1 mice: a model of antiphospholipd syndrome. J Immunol 1992;149:1063–8. [12] Kobayashi K, Lopez LR, Shoenfeld Y, Matsuura E. The role of innate and adaptive immunity to oxidized low-density lipoprotein in the development of atherosclerosis. Ann NY Acad Sci 2005;1051:442–54. [13] Matsuura E, Lopez LR. Are oxidized LDL/β2-glycoprotein I complexes pathogenic antigens in autoimmunemediated atherosclerosis? Clin Dev Immunol 2004;11:103–11. [14] Lopez LR, Simpson DF, Hurley BL, Matsuura E. OxLDL/β2GPI complexes and autoantibodies in systemic lupus erythematosus, systemic sclerosis and antiphospholipid syndrome. Pathogenic implications for vascular involvement. Ann NY Acad Sci 2005;1051:313–22. [15] Lopez LR, Dier KJ, Lopez D, Merrill JT, Fink CA. Anti-β2-glycoprotein I and antiphosphatidylserine antibodies are predictors of arterial thrombosis in patients with antiphospholipid syndrome. Am J Clin Pathol 2004;121:142–9. [16] Frostegard J. Atherosclerosis in patients with autoimmune disorders. Arterioscler Thromb Vasc Biol 2005;25:1776–85. [17] Hasunuma Y, Matsuura E, Makita Z, Katahira T, Nishi S, Koike T. Involvement of β2-glycoprotein I and anticardiolipin antibodies in oxidatively modified low-density lipoprotein uptake by macrophages. Clin Exp Immunol 1997;107:569–73. [18] Ylä-Herttuala S, Palinski W, Rosenfeld ME, Parthasarathy S, Carew TE, Butler S, et al. Evidence for presence of oxidatively modified low-density lipoprotein in atherosclerosis lesions of rabbit and man. J Clin Invest 1989;84:1086–95. [19] George J, Harats D, Gilburd B, Afek A, Levy Y, Schneiderman J, et al. Immunolocalization of β2-glycoprotein I (apolipoprotein H) to human atherosclerotic plaques: potential implications for lesion progression. Circulation 1999;99:2227–30. [20] Matsuura E, Kobayashi K, Matsunami Y, Lopez LR. The immunology of atherothrombosis in the antiphospholipid syndrome: antigen presentation and lipid intracellular accumulation. Autoimmunity Rev 2009;8:500–5. [21] Greco TP, Conti-Kelly AM, Greco Jr T, Doyle R, Matsuura E, Anthony JR, et al. Newer antiphospholipid antibodies predict adverse outcomes in patients with acute coronary syndrome. Am J Clin Pathol 2009;132:613–20. [22] Greco TP, Conti-Kelly AM, Anthony JR, Greco Jr T, Doyle R, Boisen M, et al. Oxidized-LDL/ β2-glycoprotein I complexes are associated with disease severity and increased risk of adverse outcomes in patients with acute coronary syndromes. Am J Clin Pathol 2010;133:737–43.
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[23] Kajiwara T, Yasuda T, Matsuura E. Intracellular trafficking of β2-glycoprotein I complexes with lipid vesicles in macrophages: implications on the development of antiphospholipid syndrome. J Autoimmun 2007;29(2-3):164–73. [24] Ioannou Y, Zhang JY, Passam FH, Rahgozar S, Qi JC, Giannakopoulos B, et al. Naturally occurring free thiols within β2-glycoprotein I in vivo: nitrosylation, redox modification by endothelial cells, and regulation of oxidative stress-induced cell injury. Blood 2010;116:1961–70. [25] Ioannou Y, Zhang JY, Qi M, Gao L, Qi JC, Yu DM, et al. Novel assays of thrombogenic pathogenicity in the antiphospholipid syndrome based on the detection of molecular oxidative modification of the major autoantigen β2-glycoprotein I. Arthritis Rheum 2011;63:2774–82.
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Eiji Matsuura1,2 and Luis R. Lopez3 1Collaborative
Research Center,
2Department
of Cell Chemistry, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, Okayama, Japan, 3Corgenix, Inc., Broomfield, CO
Historical notes In 1990, three groups of investigators independently reported that anticardiolipin (aCL) antibodies were directed against β2-glycoprotein I (β2GPI)/cardiolipin (CL) complexes rather than CL alone [1–3]. It is now widely agreed that β2GPI is the major antigenic target for antiphospholipid antibodies derived from patients with antiphospholipid syndrome (APS). Pathogenic antiphospholipid antibodies are directed to β2GPI/phospholipid complexes, likely to cryptic epitopes generated on domain I of the β2GPI molecule upon its interaction with phospholipids or when bound to high density phospholipid (or artificial) surfaces. These represent two possible antibody-binding mechanisms, each supported by experimental data. These antibody-binding mechanisms may not be mutually exclusive; perhaps they all play different roles depending on the type of surface, the interacting molecules, or particular experimental settings. The β2GPI region responsible for phospholipid binding activity has been identified in domain V and is fairly well characterized. However, the precise nature and pathophysiologic consequences triggered by this interaction are now been unraveled. It seems clear that β2GPI needs to interact with phospholipids exposed by activated cell membranes to trigger prothrombotic changes. In contrast, the location and nature of the cryptic epitope(s) responsible for antibody binding remain less clear. Experimental evidence points to the location of cryptic epitopes at the opposite end of the β2GPI molecule (domain I). The exact manner in which antiphospholipid antibodies trigger prothrombotic activity in the venous or arterial vasculature when binding β2GPI/phospholipid complexes is a subject of intense research. More recently, it has been recognized that β2GPI may have more diverse functions than previously thought. β2GPI binds other lipoproteins of pathological significance (i.e., oxidized low-density lipoprotein (oxLDL)), suggesting a role of these complexes in autoimmune-mediated atherogenesis.
The β2GPI autoantigen: definition
β2GPI is a 50-kDa single-chain polypeptide present in plasma at a concentration of approximately 200 μg/mL. The complete amino acid sequence of human β2GPI has been established by complementary deoxyribonucleic acid (cDNA) cloning and sequencing from a human hepatoma cell line (HepG2). It is composed of 326 amino acid residues and consists of five homologous domains. Each of the first four Autoantibodies. http://dx.doi.org/10.1016/B978-0-444-56378-1.00081-2 Copyright © 2014 Elsevier B.V. All rights reserved.
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domains (domains I–IV) is composed of 60 amino acids with highly conserved prolines, cystines, and tryptophans. Each motif is characterized by conserved half-cysteine residues with two internal disulfide bridges. These repeating motifs have been designated short consensus repeats or shushi domains. The fifth domain (domain V) toward the carboxyl terminal is different, with 82 amino acids residues and three disulfide bridges. This domain is particularly important because it contains the sequence 282KNKEKK287, which is responsible for phospholipid binding, as alterations of this sequence produced a complete elimination of β2GPI binding to phospholipids. Elucidation of the crystal structure of β2GPI has confirmed these observations.
β2GPI biologic functions
The physiologic functions of β2GPI are becoming more evident and diverse. β2GPI binds in vitro to negatively charged molecules such as phospholipids (CL, phosphatidylserine), heparin, and certain lipoproteins, as well as to the cell membranes of activated platelets and endothelial cells. It has been widely reported that β2GPI also influences coagulation and platelet function by inhibiting contact activation of the intrinsic coagulation pathway, platelet prothrombinase activity, and adenosine diphosphate (ADP)-mediated platelet aggregation. More recently, β2GPI has been involved in the removal of apoptotic cells through a phosphatidylserine-binding mechanism and shown to interfere with the protein C and protein S anticoagulant pathways. β2GPI appears to influence lipoprotein metabolism by promoting clearance of oxidation products such as oxLDL and even angiogenesis [4]. However, there is less information available about the precise functions and mechanisms of action of β2GPI in vivo. Free or monomeric β2GPI has a relatively low affinity for negatively charged phospholipids. Anti-β2GPI antibodies may cross-link two β2GPI/phospholipid complexes, increasing its binding affinity over 100-fold. How does cross-linking on cellular surfaces activate prothrombotic mechanisms? A model to explain how anti-β2GPI antibodies activate platelets proposes that anti-β2GPI antibodies bind two β2GPI molecules, inducing conformational changes. This interaction increases the affinity of dimeric β2GPI to cell surface phospholipids and protein-binding sites. β2GPI may interact with a cell surface receptor of the low-density lipoprotein (LDL) receptor family, apolipoprotein E receptor 2 (apoER2). This may result in phosphorylation of apoER2, followed by phosphorylation of MAPp38 kinase and synthesis of thromboxane A2. These events may shift the hemostatic balance toward a prothrombotic state, increasing the risk of developing thrombosis in patients with circulating anti-β2GPI antibodies.
Source and methods of β2GPI purification
β2GPI is purified from normal plasma by sequential perchloric acid precipitation or CL affinity and ion-exchange chromatography. β2GPI activates lipoprotein lipase in vitro and appears in the lipoprotein ultracentrifugation fraction. For these reason, it has also been referred to as apolipoprotein H. Human and bovine β2GPI show a high degree of homology (∼85%), and both have been widely used in diagnostic systems. Recombinant human β2GPI has been produced using baculovirus and insect cell systems and has contributed significantly in studies of the biological functions of β2GPI.
β2-GPI in atherogenesis
β2-GPI in atherogenesis
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Once ascertained that β2GPI was the main antigenic target for antiphospholipid antibodies, other functions of β2GPI started to become apparent; initially described as a natural anticoagulant, β2GPI has more pleiotropic functions affecting fibrinolysis, angiogenesis, and apoptosis as well as atherogenesis due to its interaction with oxLDL [5]. In fact, unlike native LDL, β2GPI binds oxLDL via specific oxidative-derived ligands to form stable and proatherogenic oxLDL/β2GPI complexes. The interaction between oxLDL and β2GPI suggests an antioxidant role of β2GPI by quenching the proinflammatory and proatherogenic effects of the oxLDL molecule. But in doing so, oxLDL/β2GPI complexes may also become immunogenic, triggering the production of autoantibodies and immune complexes. Serum levels of oxLDL/β2GPI complexes are elevated in patients with systemic autoimmune diseases characterized by cardiovascular complications, implicating oxLDL/β2GPI complexes as atherogenic autoantigens. Indeed, autoantibodies to oxLDL/β2GPI (and to β2GPI) frequently coexist in the same autoimmune disorders [6]. Current evidence points toward the atherosclerotic lesions as the primary site of oxLDL/β2GPI complex formation with subsequent release into the circulation. oxLDL/β2GPI complexes and their immune complexes upregulate the macrophage expression of scavenger and FcγRI receptors that further stimulate the oxLDL/β2GPI uptake and rapid intracellular accumulation. It is possible that oxidative stress and β2GPI-containing complexes are capable of activating chronic inflammation via innate immune responses that include the interleukin (IL)-1β inflammasome pathway. Published experimental evidence for a role of β2GPI in the regulation of oxidative stress and adaptive and innate immunity is summarized in Table 81.1. Recent reports on the role of oxidized β2GPI in vascular atherothrombosis of autoimmune patients require further discussion. β2GPI is susceptible to reduction by the thiol oxidoreductase thioredoxin-1. The reduction may take place on the surface of platelets where the Cys288-Cys326 disulfide bond in domain V of β2GPI is reduced by thioredoxin-1, favoring the binding of von Willebrand factor (vWF) to platelets and endothelial cells [7]. Thus, the presence of the reduced form of β2GPI in circulation may be relevant to atherothrombosis. The main function(s) of β2GPI remains unclear, but the high plasma concentration makes β2GPI readily available to the effect and regulation by thioredoxin-1. β2GPI with free thiols has been detected in both human and murine sera, which suggests that free thiol-containing β2GPI exerts a powerful protection of vascular elements such as platelets and endothelial cells from oxidative stress-injury and cell death. Further, the oxidative post-translational modification of β2GPI may trigger a Th1 immune response [8]. Direct electron microscopy visualization of β2GPI purified from plasma in the absence and presence of antibodies shows that β2GPI is a flexible molecule that can exist in two different conformations. In plasma it is mostly present as a circular protein in which domain I binds to domain V. When β2GPI interacts with anionic surfaces, it opens and adopts the J conformation. The closed (circular) conformation would hide domain V from binding to cellular receptors or phospholipids and even protect domain I from interacting with the antibody, while the open conformation makes both β2GPI sites available for biological/immunological interactions [9]. The in vivo mechanisms that control the conformation of β2GPI and their pathophysiologic significance remain to be elucidated. Whether the reduced or oxidized forms of β2GPI correspond to the open or closed conformation has yet to be determined.
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Table 81.1 Evidence for a Role of β2GPI in the Regulation of Oxidative Stress and Immune (Innate and Adaptive) Responses Author/year
Study Description
Findings
Kajiwara et al. (2007) [23]
Intracellular trafficking of β2GPI complexes with oxLDL. In vitro murine macrophages J774 Redox (thioredoxin-1) control of β2GPI. In vitro vWF platelet adhesion β2GPI novel component of innate immunity. In vitro monocytes and endothelial cells Reduced β2GPI protect in vitro endothelial cells from oxidative stress-induced cell injury Detection of oxidized β2GPI in autoimmune and vascular diseases
Only complexed β2GPI localized in lysosomes and upregulated expression of CD36 and FcγRI Thioredoxin-1 reduced β2GPI (Cys288-Cys326 Dom V), had increased binding to vWF and platelet adhesion β2GPI bound LPS and inhibited LPS-induced TF and IL-6 expression
Passam et al. (2010) [7]
Agar et al. (2011) [9]
Ioannou et al. (2010) [24]
Ioannou et al. (2011) [25]
Passan et al. (2011) [8]
Review paper and unpublished recent data
Thioredoxin-1 reduced β2GPI (Cys288–Cys326 Dom V), had antioxidant and platelet adhesion Post-translational oxidative modification of β2GPI (lacking free thiols) may participate in thrombosis Reduced β2GPI with abundant free thiols is antioxidant reservoir. Oxidation of β2GPI (lacking free thiols) increases immunogenicity via Th1 response
β2GPI: β2-glycoprotein I; IL: interleukin; LPS: lipopolysaccharide; oxLDL: oxidized low-density lipoprotein; TF: tissue factor; vWF: von Willebrand factor.
The β2GPI autoantibody: definition
Antiphospholipid antibodies (aCL and lupus anticoagulants (LA)) are a heterogeneous group of autoantibodies with a pathogenic role in the development of the vascular complications (venous and arterial thrombosis) of APS. These antibodies were initially thought to be specific for negatively charged phospholipids; however, it is now widely accepted that protein/phospholipid complexes are the more relevant antigenic targets. Several plasma proteins that participate in coagulation have been described as antiphospholipid antigenic targets (i.e., β2GPI, prothrombin, annexin V, etc.). β2GPI interacts with negatively charged phospholipids and is considered the most clinically important antiphospholipid antigen. Antiphospholipid antibodies also recognize β2GPI immobilized on oxygenated polystyrene surfaces in the absence of phospholipids [10]. These findings suggest that antiphospholipid antibodies recognize cryptic epitopes formed on β2GPI upon its interaction with an oxygenated surface.
Pathogenic role NZW x BXSB (W/B) F1 mice present multiple autoantibodies and a systemic lupus-like disease [11]. Myocardial infarction is common in the W/B F1 male mice, and its incidence increases with age to over 80%. The titer of antiphospholipid antibodies (including anti-β2GPI antibodies) also
Newer pathogenic roles
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increases with age in these mice. Two monoclonal anti-β2GPI antibodies have been raised from W/B F1 mice that induce thrombosis when injected into normal mice. One of these monoclonal antibodies (WB-CAL-1) binds to the complexed form but not to free or monomeric β2GPI. This finding supports the role of β2GPI cryptic epitopes in antibody-mediated thrombosis. Monoclonal anti-β2GPI antibodies have been derived from APS patients and even proposed to be useful for assay standardization. Antiphospholipid antibodies may interfere with β2GPI’s natural anticoagulant properties, thus promoting thrombosis. Anti-β2GPI antibodies have been reported to be more specific for thrombosis and APS than aCL antibodies. Recent prospective studies have shown that aCL antibodies, particularly those β2GPI-dependent or anti-β2GPI antibodies, are important predictors for arterial thrombosis (myocardial infarction and stroke) in men. These studies suggested that β2GPI and anti-β2GPI antibodies play a central role in the pathogenesis of thrombosis, particularly in arterial thrombosis and atherosclerosis seen in systemic lupus erythematosus (SLE) and APS patients. Dysregulation of both the adaptive and inflammatory immune responses are important pathologic mechanisms underlying the clinical manifestations characteristic of autoimmune diseases such as SLE and APS. The cause–effect relationship between antiphospholipid antibodies and vascular thrombotic events is now widely accepted and better understood. In addition to venous thromboembolism, patients with SLE and APS may develop premature atherosclerotic cardiovascular disease associated with significant morbidity and mortality not fully explained by the classic risk factors. Further, the contributory role of antibodies to β2GPI and oxLDL in autoimmune-mediated atherothrombosis is now being recognized [12].
Newer pathogenic roles β2GPI may interact in vitro with other lipid molecules to form potentially pathogenic complexes. oxLDL plays a central role in atherogenesis. We have demonstrated that β2GPI binds oxLDL, not native LDL, initially forming dissociable electrostatic complexes, followed by more stable complexes bound by covalent interactions [13]. In addition, lipid peroxidation (oxidative stress) is common in SLE and APS, and circulating “stable” β2GPI/oxLDL complexes were demonstrated in these patients. Because oxLDL is highly proinflammatory, unstable, and short lived in circulation, we hypothesized that β2GPI binding neutralizes the deleterious effect of oxLDL while promoting its clearance. However, chronic “oxidative stress” may overcome this physiologic mechanism and over time become rather proatherogenic. oxLDL and β2GPI/oxLDL complexes are also highly immunogenic. We have demonstrated the presence of anti-β2GPI/oxLDL antibodies in patients with systemic autoimmune diseases (SLE, systemic sclerosis (SSc), and APS) [14]. These antibodies were significantly higher and more prevalent (75%) in secondary APS patients compared to 37% in SLE controls without history of thrombosis, with 93.7% specific for APS with a predictive value (PV) of 90% (P < 0.001). IgG aCL antibodies were 80% specific with a PV of 71.4% (P < 0.001). In addition, APS patients with a history of arterial thrombosis had a higher mean level and prevalence (38%) of IgG anti-β2GPI/oxLDL antibodies compared with APS patients with history of venous thrombosis (19%). The PV of IgG anti-β2GPI/oxLDL antibodies for total (arterial + venous) thrombosis was 92% (P = 0.018): 89% (P = 0.04) for arterial thrombosis and 77% (P = 0.168) for venous thrombosis [15].
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Patients with autoimmune diseases present chronic dyslipidemia characterized by decreased high-density lipoprotein (HDL), changes in the HDL subpopulations, raised triglycerides, and unchanged or only slightly elevated LDL levels [16]. On this background, the systemic generation of free radicals by endothelial and circulating mononuclear cells may induce oxidative modifications of LDL (oxLDL). The uptake of oxLDL by arterial mononuclear cells results in the release of inflammatory and chemotactic cytokines in the early stages of atherosclerosis, leading to an excessive i ntracellular accumulation of oxLDL. This uptake is mediated by scavenger receptors [17]. Immunostaining of human atherosclerotic lesions colocalized β2GPI with oxLDL and suggested a close relationship of these molecules [18,19]. When IgG anti-β2GPI antibodies were coincubated with oxLDL and β2GPI, the monocyte/ macrophage uptake and intracellular accumulation of oxLDL was accelerated, likely via FcγRI receptors, also leading to upregulation and enhanced surface expression of scavenger (CD36) and FcγRI receptors [20]. It therefore seems that while antibodies against β2GPI are strongly implicated in the development of autoimmune arterial and venous thromboembolism, antibodies against oxLDL/β2GPI are involved in atherogenesis, lending further support to the general concept that autoimmunity plays a role in development of premature cardiovascular disease. Outside the autoimmune setting, antiphospholipid antibodies (anti-β2GPI and anti-oxLDL/β2GPI) as well as oxLDL/β2GPI complexes have been demonstrated in patients with type 2 diabetes and cardiovascular disease, including acute coronary syndromes (ACS). The presence of these antibodies in ACS was associated with a 2.9-fold risk of adverse outcomes [21]. Serum levels of oxLDL/β2GPI complexes in higher quartiles positively correlated with the severity of the disease assessed by angiography and with a 3.5-fold increased risk [22]. When both antibodies and complexes were present, the risk increased 14-fold, suggesting a synergistic effect between oxLDL/β2GPI complexes and their antibodies. An interesting but unexplained observation is that the most common aPL isotype in ACS patients was IgM. Previous reports suggested a protective (antiatherogenic) role of natural IgM antibodies.
Method of detection The 1998 Sapporo and Sydney serologic criteria for the classification of definite APS requires the demonstration of persistent LA and/or high titers of IgG aCL and anti-β2GPI antibodies. LAs have been reported as more specific for thrombosis and APS than aCL antibodies. β2GPI role in LA activity has also been demonstrated. Detection of anti-β2GPI antibodies in patient sera is important for the diagnosis of APS and it has been recently incorporated as a major serologic criteria. P urified human β2GPI can be adsorbed on two general types of surfaces: CL-coated polystyrene to assess β2GPI-dependency or oxygenated polystyrene plates. β2GPI adsorbed on oxygenated polystyrene plates would create the increasing density and the cryptic epitopes recognized by clinically relevant anti-β2GPI antibodies. This system eliminates most of the false-positive or irrelevant reactivity frequently seen in CL-coated surfaces. The nature of the surface seems to be important, and anti-β2GPI enzyme-linked immunosorbent assay (ELISA) using such surfaces have been shown to be more s pecific for thrombosis and APS than ELISAs using CL-coated surfaces.
Diagnostic value
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In addition, ELISAs for anti-β2GPI antibodies allow not only the quantification but also the d etermination of antibody isotypes. IgG anti-β2GPI antibodies are considered to be more specific than IgM, and more recently, several reports indicate that IgA anti-β2GPI antibodies may also have a pathogenic role in thrombosis and APS.
Clinical utility: disease association APS is the most common cause of acquired thrombophilia (hypercoagulability) in the general populations. In a recent study of the relationship between antiphospholipid antibodies and the type of thrombotic manifestation, the PV and association of APS with aCL, antiphosphatidylserine antibodies, anti-β2GPI antibodies, and antiprothrombin antibodies were evaluated in 100 APS and 90 SLE patients [15]. APS patients were classified according to the clinical history of arterial thrombosis, venous thrombosis, or pregnancy morbidity. Three isotypes (IgG, IgM, and IgA) of each antibody were measured by ELISA. Individually, IgG, IgM, and IgA anti-β2GPI and antiphosphatidylserine a ntibodies had the strongest specificity (> 94%) and PV for APS (86.4–94.1%; P < 0.001) in patients with SLE. In comparison, the IgG specificity of aCL antibodies was only 78% with a PV of 60.7% (P = 0.063). The PV for APS reached 100% when two or more antiphospholipid antibodies were present. Similarly, anti-β2GPI antibodies and antiphosphatidylserine antibodies had a stronger PV and association for arterial thrombosis (87– 95%; P < 0.001) compared to venous thrombosis (80– 92%; P = 0.01) in APS patients. These results suggested an important pathogenic role of anti-β2GPI in arterial thrombosis and provided the best diagnostic value for the laboratory assessment of APS.
Diagnostic value aCL ELISAs are the most commonly used tests by clinical laboratories to assess the risk of thrombosis and diagnose APS. Low specificity (high false-positive rates) and standardization problems remain major controversial issues. Accurate serologic diagnosis for APS is of paramount importance as anticoagulation is the main treatment, frequently long-term if not life-long. Complications from recommended anticoagulation programs may potentially be serious, underscoring the need for better understanding of the role of antiphospholipid antibodies in thrombosis and the availability of highly specific assays. The discovery of β2GPI as a clinically relevant antigenic target stimulated the development and use of more specific anti-β2GPI ELISA tests. In addition to the participation of β2GPI and anti-β2GPI antibodies in venous thrombosis, there is newer experimental evidence that β2GPI participates in atherosclerosis and arterial thrombosis by interacting with oxLDL, forming β2GPI/oxLDL complexes in the arterial wall. β2GPI/oxLDL complexes are taken up by macrophages and stimulate the production of anti-β2GPI/oxLDL antibodies, which will further enhance macrophage uptake via Fcγ receptors, promoting foam cell and atherosclerotic plaque formation. Additional functions or interactions of β2GPI that may trigger anti-β2GPI antibody production will likely be discovered and provide new insights into the pathogenesis of autoimmune-mediated thrombosis.
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FIGURE 81.1 Schematic representation of mechanisms involved in anti-β2GPI antibody-dependent atherothrombosis. β2GPI: β2-glycoprotein I; Ab: antibody; LDL: low-density lipoprotein; oxLDL: oxidized low-density lipoprotein.
Take-home messages • β 2GPI is the most clinically relevant antigenic target for antiphospholipid antibodies, suggesting a central role in the pathogenesis of autoimmune-mediated thrombosis. • The interaction of β2GPI with activated phospholipid cell membranes (platelets or endothelial cells) may produce a prothrombotic surface. This interaction generates cryptic epitopes on the β2GPI that promote anti-β2GPI autoantibody production. • Cryptic epitopes and/or increased β2GPI density on suitable surfaces favor anti-β2GPI antibody binding, triggering prothrombotic mechanisms. • Measuring anti-β2GPI antibodies in the clinical laboratory provides additional and relevant serologic information to assess the risk of thrombosis or APS diagnosis properly. • β2GPI binding to oxLDL to form β2GPI/oxLDL complexes and autoantibodies to these complexes represents another important mechanism that leads to atherosclerosis and arterial thrombosis in autoimmune patients.
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