Atherosclerosis in Autoimmune Diseases Eiji Matsuura, PhD, Kazuko Kobayashi, PhD, and Luis R. Lopez, MD

Corresponding author Eiji Matsuura, PhD Department of Cell Chemistry, Okayama University Graduate School of Medicine Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Okayama 700-8558, Japan. E-mail: [email protected] Current Rheumatology Reports 2009, 11:61–69 Current Medicine Group LLC ISSN 1523-3774 Copyright © 2009 by Current Medicine Group LLC

Lipid peroxidation occurs frequently in patients with systemic autoimmune diseases and contributes to autoimmune vascular inflammation. Oxidized low-density lipoprotein (oxLDL) interacts with β2-glycoprotein I (β2GPI), forming oxLDL/β2GPI complexes. Circulating oxLDL/β2GPI complexes and autoantibodies to these complexes have been demonstrated in patients with systemic lupus erythematosus and antiphospholipid syndrome. These fi ndings suggest an immunogenic nature of the complexes and an active proatherogenic role in autoimmunity. Biochemical characterization of the complexes and immunohistochemical studies of atherosclerotic lesions suggest that most of the complexes originate in the arterial wall and are released into circulation. The in vitro macrophage uptake of oxLDL/β2GPI complexes increased significantly in the presence of antiphospholipid antibodies (anti-β2GPI), suggesting that macrophage Fcγ receptors are involved in the lipid intracellular influx that leads to foam cell formation. These fi ndings provide an immunologic explanation for the accelerated development of atherosclerosis seen in systemic lupus erythematosus and antiphospholipid syndrome.

Introduction Cardiovascular morbidity and mortality caused by the premature or accelerated development of atherosclerosis have been reported in patients with systemic lupus erythematosus (SLE) [1], spurring interest and research into the role of autoimmunity in atherogenesis. The relationship between development of atherosclerosis and plasma cholesterol levels has been well established.

However, newer infl ammatory and immunologic mechanisms have emerged as relevant factors in the initiation and progression of atherosclerotic lesions. The oxidative modification of low-density lipoprotein (oxLDL) and the formation of autoantibodies to oxLDL are thought to be key proatherogenic events that accelerate the formation of macrophage-derived foam cells and atherosclerotic plaques [2,3]. Since the original description of the antiphospholipid syndrome (APS), much consideration has been placed on the basic immunologic mechanisms of vascular injury and thrombosis. Elevated serum levels of antiphospholipid antibodies are associated with venous and arterial thromboembolic events in APS patients, a condition frequently observed in the context of an autoimmune disorder [4]. The exact mechanisms by which antiphospholipid antibodies promote thrombosis are not yet completely understood. However, it is widely accepted that these antibodies play a direct pathogenic role in the development of thrombosis. Venous thrombosis is the most common vascular event; however, one of three APS patients develops arterial thrombosis (eg, myocardial infarction, cerebrovascular accident, angina) during the evolution of the disease [5,6]. Experimental evidence suggests that β2-glycoprotein I (β2GPI) is the major antigenic target for antiphospholipid antibodies, and it is thought to play a central role in the development of the clinical complications of APS [7,8]. Furthermore, anti-β2GPI antibodies have been associated with arterial thrombosis [9,10]. Immunohistochemical analysis of the atherosclerotic lesions shows the lipid core composed mainly of oxLDL, with β2GPI, immunoreactive CD4 and CD8 lymphocytes, and immunoglobulins [11,12]. These fi ndings further support an active role of antiphospholipid antibodies in atherogenesis. OxLDL binds to β2GPI in vitro, forming oxLDL/β2GPI complexes. Circulating oxLDL/β2GPI complexes were demonstrated in patients with various systemic autoimmune and chronic inflammatory diseases, such as SLE, APS, chronic renal disease, diabetes mellitus, and in some patients with acute myocardial infarction [13–15,16••]. IgG antibodies to oxLDL/β2GPI complexes were detected in patients with SLE and APS and were strongly associated with arterial thrombosis [17]. In vitro experiments have shown that oxLDL/β2GPI

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complexes are more rapidly internalized by macrophages when anti-β2GPI antibodies are present, suggesting the participation of Fcγ receptors [14,15,16••]. Thus, circulating IgG immune complexes containing oxLDL and β2GPI may be proatherogenic.

Atherosclerotic Cardiovascular Disease and Autoimmunity Atherosclerosis is an important cause of cardiovascular morbidity and mortality in the world population. In the United States alone, over 61 million Americans have a form of cardiovascular disease; of these, 7.3 million have had a myocardial infarction and 4.5 million have had a cerebrovascular accident [18]. Complications from atherosclerotic disease are chronic, with high recurrence and mortality rates, claiming the lives of approximately 500,000 people each year [18]. The Framingham study identified a series of risk factors for atherosclerosis, including hypertension, hypercholesterolemia, diabetes mellitus, obesity, smoking, family history, and inactive lifestyles. These risk factors are thought to contribute to the initiation and progression of atherosclerosis by disrupting a number of lipid regulatory and inflammatory mechanisms within the arterial wall. The cholesterol in atherosclerotic lesions originates in circulation. LDL cholesterol is proatherogenic, but LDL has to be modified to promote atherosclerosis [19]. OxLDL promotes inflammation by attracting monocytes and T lymphocytes to the lesion, is cytotoxic for endothelial cells (causing a prothrombotic endothelial surface dysfunction), and stimulates the release of various soluble inflammatory and adhesion molecules, such as monocyte chemoattractant protein-1 (MCP-1), macrophage colony-stimulating factor (M-CSF), interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), intercellular adhesion molecule (ICAM), and vascular cell adhesion molecule (VCAM). In addition to being proinflammatory, oxLDL is also highly immunogenic [20]. Anti-oxLDL antibodies have been detected in individuals with atherosclerotic cardiovascular diseases [21]. Although most of the atherosclerotic changes occur inside the arterial wall, there is also an endothelial and platelet prothrombotic dysfunction that leads to increased arterial thrombus formation when plaques become unstable and rupture. Atherothrombosis more appropriately describes the late occurrence of an arterial thrombotic event in complicated atherosclerosis. All these fi ndings point to a complex interrelationship between dyslipoproteinemia, inflammation, and immunologic mechanisms, sharply contrasting with the purely metabolic or passive cause of atherosclerosis that was once proposed. The accelerated or premature development of atherosclerosis has been recently reported in patients with systemic autoimmune diseases [1]. The traditional Framingham risk factors and immunosuppressive therapies (ie,

steroids) failed to account for the accelerated development of atherosclerosis in patients with SLE [22]. With today’s 10-year survival rate over 80%, new causes of SLE morbidity and mortality have emerged. Mortality rates caused by cardiovascular disease have surpassed that from the SLE disease itself or from treatment complications such as infections. The prevalence of symptomatic coronary heart disease in SLE ranges from 6% to 15%, with an overall incidence of myocardial infarction five times higher than the general population [1]. When adjusted for age, the incidence of myocardial infarction can be 50 times as high. In addition, 43% of asymptomatic SLE patients had abnormal myocardial perfusion results by Tc-99m emission tomography, and over 33% had increased carotid intima-media thickness (IMT) with atherosclerotic plaques demonstrated by B-mode ultrasound [1].

Atherothrombosis and APS APS is the most common cause of acquired hypercoagulability [23]. It is characterized by elevated titers of antiphospholipid antibodies associated with thromboembolic complications of venous and arterial blood vessels or with pregnancy morbidity (miscarriages and fetal loss). When APS is present in patients with an underlying systemic autoimmune disorder (ie, SLE), it is referred to as secondary APS. It may be present in the absence of an obvious underlying disease, in which case it is referred to as primary APS [4]. Antiphospholipid antibodies increase the risk of thrombosis by at least twofold when present in the context of an autoimmune disease [24]. In both primary and secondary APS, recurrence rates of up to 30% for thrombosis with a mortality of up to 10% over a 10-year follow-up period have been reported [25]. Venous thromboembolic events are the most common clinical fi nding in APS [5]. In about 25% of APS patients enrolled in a European cohort of 1000 patients, the initial clinical manifestation was an arterial thrombotic event (myocardial infarction, cerebrovascular accident, angina). If all the initial and late arterial thrombotic events were considered, up to 31% of the patients had these complications [26]. These observations support the hypothesis that autoimmune mechanisms mediated by antiphospholipid antibodies are important in promoting atherosclerosis. Antiphospholipid antibodies, such as anticardiolipin antibodies (aCL) and lupus anticoagulants (LA) are a heterogeneous family of autoantibodies characterized by their reactivity to anionic phospholipids, to phospholipid/ protein complexes, and to certain proteins displayed on appropriate surfaces (ie, activated cell membranes, oxygenated polystyrene) [8]. The plasma protein β2GPI is the most relevant antigenic target for antiphospholipid antibodies [7]. Anti-β2GPI antibodies have been reported as more specific for thrombosis and APS than aCL antibodies, and recent prospective studies have shown that aCL antibodies,

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particularly those that are β2GPI-dependent, or the antiβ2GPI antibodies themselves, were important predictors for arterial thrombosis (myocardial infarction and stroke) in men [6,9,10]. β2GPI has been found in human atherosclerotic lesions together with oxLDL, immunoreactive CD4/CD8 lymphocytes, and immunoglobulins [11,12]. These findings provide additional support to the hypothesis that β2GPI and anti-β2GPI antibodies participate in the development of thrombosis, particularly in arterial thrombosis (atherosclerosis). Mice that received syngeneic lymphocytes from β2GPI-immunized LDL receptor–deficient mice developed larger fatty streaks compared with control mice that received lymphocytes from mice immunized with bovine albumin [27]. These findings from an experimental model provided direct evidence that β2GPI-reactive T cells promote atherogenesis.

Autoimmune Mechanisms in Atherosclerosis Atherosclerosis is a multifactorial pathologic process in which arteries undergo thickening of the intima and smooth muscle layer, causing a decrease in their elasticity. The blood vessels commonly affected by this process include the aorta and coronary and cerebral arteries. The appearance of lipid-laden foam cells in the arterial intima is a characteristic histologic finding of early atherosclerotic lesions. Increased cholesterol blood levels are commonly associated with increased LDL. The combination of high LDL with arterial shear stress may produce vascular inflammation that promotes the adherence of circulating monocytes to endothelial cells and the migration of these elements (LDL and monocytes) into the intima [19]. OxLDL from vascular inflammation (and oxidative stress) further activates and attracts inflammatory cells at the site of the arterial lesion. The activation of macrophages would set off a series of proinflammatory events that include the expression of surface receptors, the intracellular influx of LDL, and the foam cell formation (oxLDL-loaded macrophages) [2,3]. Diverse proinflammatory and/or adhesion molecules also participate in atherogenesis. These molecules participate under complicated interrelated conditions and include MCP-1, M-CSF, IFN-γ, TNF-α, interleukin-4 (IL-4), platelet-derived growth factor (PDGF), heparin-binding EGF-like growth factor (HB-EGF), ICAM, VCAM-1, and endothelial selectin (E-selectin) [28,29]. It is widely accepted that LDL must be modified before is taken up by macrophages via their scavenger receptors, and oxidation of LDL represents one such mechanism [19]. Normal blood levels of native (unmodified) LDL and, perhaps, minimally modified LDL, are maintained by an LDL receptor–uptake mechanism on endothelial and monocyte–macrophage cells. LDL receptors are downregulated to prevent excessive intracellular lipid accumulation. OxLDL is removed at a higher rate by macrophage scavenger receptors that are not downregulated, making possible

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an excessive intracellular accumulation of oxLDL and foam cell formation. When the endothelial surface of the atherosclerotic lesion becomes damaged and unstable, it may rupture into the arterial lumen. This event is followed by the activation of blood coagulation mechanisms such as platelet aggregation and thrombi formation, which can then result in a complete occlusion of the blood vessel and tissue or organ necrosis, as seen in acute myocardial and cerebral infarction. Table 1 summarizes recent immunobiological and clinical studies on the role of oxLDL/β2GPI complexes in autoimmune-mediated atherogenesis.

Oxidative Stress and the Modification of LDL OxLDL plays a central pathogenic role in atherosclerosis [2]. The LDL particle contains phospholipids, free cholesterol, cholesteryl esters, triglycerides, and apolipoprotein B (apoB). Both the lipids and apoB are susceptible to oxidation, and apoB may break down into fragments of different sizes (from 14 kDa to over 550 kDa) by this process [30]. An important feature of LDL’s oxidation is the breakdown of polyunsaturated fatty acids into a broad array of smaller fragments (ie, aldehydes and ketones bodies) that may conjugate to amino lipids or to apoB [31]. The polyunsaturated fatty acids in cholesterol esters, phospholipids, and triglycerides are also affected by free radical–initiated oxidation and can participate in a chain of reactions that further amplify the damage. Chemically modified LDLs, such as MDA-LDL, acetylated-LDL and Cu 2+-oxLDL, have been extensively used to study atherogenic mechanisms. Small amounts of Cu 2+ can induce LDL oxidation, resulting in highly reproducible LDL damage [32]. This process leads to an oxLDL structure that shares many functional properties with the LDL oxidized by cells or to oxLDL extracted from arterial atherosclerotic plaques. Incubation of LDL with several different types of cells, or with Cu 2+ even in the absence of cells, results in an oxLDL structure with similar properties [33]. There is general consensus that Cu 2+-oxidized LDL is a relevant autoantigen because the oxLDL found in atheromatous lesions and the oxLDL extracted from atherosclerotic lesions exhibited similar physicochemical and immunologic properties to the Cu 2+-oxLDL [12]. Thus, Cu 2+-mediated oxLDL seems to be a more suitable model for physiologic LDL rather than other chemically modified LDL, such as MDA-LDL. The precise pathways and reactive intermediates involved in in vivo oxidative modification remain to be elucidated; however, the leukocyte protein myeloperoxidase may be one candidate for in vivo lipoprotein and lipid oxidation [34].

Role of Macrophages in Atherosclerosis: Scavenger and Fcγ Receptors Macrophage receptors for LDL were first described by Yamamoto et al. [35]. These receptors are downregulated

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Table 1. Recent immunobiological and clinical publications on oxLDL/β2GPI complexes in autoimmunemediated atherosclerosis Study / year

Observations

Immunohistochemistry of oxLDL/β2GPI complexes Hasunuma et al. [14] / 1997

Interaction of oxLDL with β2GPI; IgG anti-β2GPI dependent uptake of oxLDL/β2GPI complexes by macrophages

Kobayashi et al. [15] / 2001

oxLDL lipid ligand specific for β2GPI

Liu et al. [50] / 2002

Biochemical structure of oxLDL ligands for β2GPI

Kobayashi et al. [13] / 2003

First demonstration of circulating oxLDL/β2GPI complexes in SLE/APS patients

Matsuura et al. [16••] / 2006

Oxidative modification of LDL and immune regulation of atherosclerosis

Tabuchi et al. [51] / 2007

Presence of CRP/oxLDL/β2GPI complexes in diabetic patients

Kobayashi et al. [52] / 2007

Proatherogenic oxLDL/β2GPI complexes and macrophage binding/uptake

Kobayashi et al. [53] / 2007

Characterization of IgG and anti-β2GPI–dependent and independent binding of oxLDL/ β2GPI complexes to macrophages

Intracellular trafficking and antigen presentation Kuwana et al. [54] / 2005

β2GPI/phospholipid complexes facilitate β2GPI presentation to autoreactive CD4+ T cells

Yamaguchi et al. [55] / 2007

IgG immune complexes containing β2GPI/phosphatidylserine or facilitate antigen presentation to T cells

Kajiwara et al. [56] / 2007

Macrophage intracellular trafficking of free and complex forms of β2GPI

Clinical significance of oxLDL complexes Lopez et al. [17] / 2003

oxLDL/β2GPI autoantibodies are associated with arterial thrombosis in APS

Kasahara et al. [57] / 2004

oxLDL/β2GPI complexes in chronic renal disease

Lopez et al. [46] / 2004

Prevalence of oxLDL/β2GPI complexes and autoantibodies in SLE and APS

Lopez et al. [47] / 2005

oxLDL/β2GPI complexes and autoantibodies in SLE, SSc, and APS

Ames et al. [58] / 2005

Preliminary report of atherosclerosis in primary APS

Lopez et al. [47] / 2005

oxLDL/β2GPI complexes in type 2 diabetes mellitus

Matsuura et al. [48] / 2005

Atherogenic role of oxLDL/β2GPI complexes and predictive value of IgG anti-oxLDL/ β2GPI antibodies for arterial thrombosis in SLE and APS

Matsuura et al. [59] / 2006

Accelerated development of atheroma in APS

Margarita et al. [60] / 2007

Subclinical atherosclerosis (increased IMT) in APS patients

Matsuura et al. [61] / 2008

Biological and immunological implications of oxLDL/β2GPI complexes

Ayada et al. [49•] / 2007

Chronic infection may induce atherosclerosis

Ames et al. [62] / 2008

Atherosclerosis in primary APS

Matsuura and Lopez [63] / 2008

Autoimmune-mediated atherothrombosis

Ames et al. [64] / 2008

Primary APS: a low-grade autoinflammatory disease with vascular implications

APS—antiphospholipid syndrome; β2GPI—β2 glycoprotein I; CRP—C-reactive protein; IMT—intima-media thickness; oxLDL—oxidized low-density lipoprotein; SLE—systemic lupus erythematosus; SSc—systemic sclerosis.

to prevent lipid overloading. Another type of chemically modified LDL macrophage receptor was later described and named the scavenger receptor [36]. These scavenger receptors are not downregulated, and thus chronic or excessive exposure to modified LDL may lead to the accumulation of massive amounts of intracellular lipids in macrophages, a process that may result in the formation of macrophagederived lipid-laden foam cells. Initially, acetylated-LDL was used to study scavenger receptors, but acetylation was

not seen under physiologic conditions. In contrast, Cu 2+- or Fe2+-oxLDL was described as a more physiologic ligand for scavenger receptors. Scavenger receptors (ie, SR-A) were first cloned by Kodama et al. [37] and shown to be specific for both acetylated-LDL and Cu2+-oxLDL. This was followed by the description of several different types of scavenger receptors, such as MARCO (a novel macrophage receptor with collagenous structure), SR-B1, Macrosialin, CD36, CD68, LOX-1, SR-PSOX, and SR-EC [38–40].

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The in vitro macrophage uptake of 125I-Cu 2+-oxLDL was significantly enhanced in the presence of β2GPI and IgG anti-β2GPI autoantibodies [14]. Similarly, the macrophage uptake of liposomes containing β2GPI ligands (oxLig-1 and oxLig-2) was also enhanced, confi rming the previous results [13,15,16••]. These fi ndings indicate that IgG anti-β2GPI autoantibodies may be proatherogenic. The in vivo oxLDL uptake is likely mediated by macrophage Fcγ receptors rather than by scavenger receptors. In contrast, Fcγ receptors have poor phagocytic properties, possibly making the IgM class of autoantibodies and/or natural antibodies antiatherogenic (or protective).

OxLDL/β2GPI Complexes in Atherogenesis

β2GPI is a 50-kDa single-chain polypeptide composed of 326 amino acid residues arranged in five homologous repeats known as complement control protein domains. β2GPI’s fifth domain contains a patch of positively charged amino acids that likely represents the binding region for phospholipids [41]. β2GPI binds strongly to negatively charged molecules, such as phospholipids, heparin, and certain lipoproteins, and to activated platelets and apoptotic cell membranes. This binding may aid the clearance of apototic cells from circulation. Furthermore, β2GPI may have anticoagulant properties, as it has been shown to inhibit the intrinsic coagulation pathway, prothrombinase activity, and adenosine diphosphate–dependent platelet aggregation. It has also been reported to interact with several components of the protein C, protein S anticoagulant system [42,43]. We recently demonstrated the specific interaction between Cu 2+-oxLDL and β2GPI by enzyme-linked immunosorbent assay (ELISA) [13–15,16••]. Thus, oxLDL, but not native LDL, binds β2GPI and anti-β2GPI autoantibodies. Two chloroform-extractable lipids (oxLig-1 and oxLig-2) were identified as the LDL-derived ligands for the specific interaction between oxLDL and β2GPI. These oxLDL-derived β2GPI ligands were further purified by reverse-phase high-performance liquid chromatography, and their structures were identified as 7-ketocholesteryl-9carboxynonanoate (9-oxo-9-[7-ketocholest-5-en-3-yloxy] nonanoic acid) and 7-ketocholesteryl-12-carboxy (keto) dodecanoate, respectively. Cholesteryl linoleate present in LDL is a major core lipid and represents the most probable candidate for a precursor of these ligands. The initial in vitro interaction of Cu 2+-oxLDL with β2GPI is due to electrostatic interactions between omegacarboxyl functions and lysine residues of β2GPI, and is reversible by Mg 2+ treatment. This interaction later progresses to a much more stable bond, such as Schiff base formation with an omega-aldehyde. Interestingly, the negative charges generated by Cu 2+-oxLDL were neutralized by interaction with β2GPI. The strength of the bond formed and the neutralization of the charges by the complexes may contribute to their stability in the blood.

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Lipid peroxidation and oxLDL production are common in patients with some systemic autoimmune diseases [44,45]. OxLDL, not native LDL, binds β2GPI in vitro, initially forming dissociable electrostatic complexes, followed by more stable complexes bound by covalent interactions. Circulating oxLDL/β2GPI complexes have been detected in some autoimmune diseases [13,17,46]. High serum levels of stable oxLDL/β2GPI complexes were detected by ELISA in 70% to 80% of patients with SLE and systemic sclerosis (SSc). Patients with rheumatoid arthritis (RA) showed a slight increase of oxLDL/β2GPI complexes compared with healthy controls. Unlike RA, both SLE and SSc are characterized by widespread vascular abnormalities. Serum levels of oxLDL/β2GPI complexes were also signifi cantly elevated in patients with secondary APS and in SLE patients without APS compared with healthy controls. However, these complexes were not associated with SLE disease activity or any major clinical manifestation of APS [46]. Although it can be hypothesized that this interaction might be related to chronic vascular infl ammation that occurs in autoimmune patients, the exact in vivo mechanisms that oxidize LDL and form oxLDL/β2GPI complexes are not fully understood. It is possible that the interaction between β2GPI and oxLDL minimizes the infl ammatory properties of oxLDL while promoting its clearance from circulation. In addition, the binding of β2GPI to oxLDL is likely to occur inside the arterial wall, as the intima microenvironment is conducive to further infl ammation, oxidation, cell activation, and macrophage uptake of oxLDL/β2GPI complexes. Serum levels of oxLDL/β2GPI complexes fluctuated widely when measured in samples obtained at different time intervals over a 12-month follow-up from six SLE patients [46]. This suggests that oxidation and formation of complexes are very active processes under unknown regulatory mechanism(s). Stable oxLDL/β2GPI complexes may be clinically relevant, as they have been implicated as atherogenic autoantigens, and their presence may represent a risk factor or an indirect but significant contributor to thrombosis and atherosclerosis in patients with an autoimmune background.

Autoantibodies to oxLDL/β2GPI Complexes OxLDL/β2GPI complexes are immunogenic. Serum levels of IgG anti-oxLDL/β2GPI antibodies were measured in the same group of SLE, SSc, and RA patients [46]. SLE and SSc patients had significantly higher levels of anti-oxLDL/β2GPI antibodies compared with controls. RA patients showed higher antibody levels than controls, but this difference was not statistically significant. The association of IgG anti-oxLDL/β2GPI antibodies with the major clinical manifestations of APS was evaluated [17,47]. There was a stronger correlation with

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Figure 1. Schematic representation of atherosclerotic plaque formation by oxidized low-density lipoprotein (oxLDL), β2-glycoprotein I (β2GPI), and immunologic activation.

arterial thrombosis compared with venous thrombosis and pregnancy morbidity. Furthermore, the positive predictive value of IgG anti-oxLDL/β2GPI antibodies for total thrombosis (arterial and venous) in patients with secondary APS was 92%, and for arterial thrombosis it was 88.9%. In contrast, the positive predictive value for venous thrombosis was not statistically significant at 77.7%. In addition, anti-oxLDL/β2GPI antibodies were present in three of four SLE patients with active disease followed over a 12-month period, whereas 2 patients with inactive disease and oxLDL/β2GPI complexes did not have these antibodies [46]. The coexistence of oxLDL/β2GPI autoantibodies with oxLDL/β2GPI complexes suggests that these two elements interact, perhaps forming circulating immune complexes (oxLDL/β2GPI antibody). Such immune complexes have been recently detected in patients with SLE and/or APS [13]. These observations, along with the increased in vitro uptake of oxLDL/β2GPI complexes by macrophage in the presence of anti-oxLDL/β2GPI antibodies [14,15,16••] offer an explanation for the accelerated (premature) development of atherosclerosis in autoimmune patients. Although preliminary, IgG anti-oxLDL/β2GPI antibodies represent a distinct subset of antiphospholipid antibodies (ie, anti-β2GPI) that coexist with other antiphospholipid antibodies. Thus, IgG anti-oxLDL/β2GPI antibodies appear to be useful serologic markers for atherosclerotic risk in autoimmune patients with high specificity for APS. The schematic representation of proposed events from oxidative modifi cation of LDL to foam cell and atherosclerotic plaque formation in autoimmune-mediated atherogenesis is presented in Figure 1.

Conclusions

The binding of oxLDL with β2GPI to form circulating complexes strongly suggests that these complexes are atherogenic autoantigens. The nature of the binding has been further characterized and the oxLDL-derived ligand (oxLig-1) specific for β2GPI has been identified and synthesized. SLE and APS patients produce autoantibodies to this complex, and the resulting circulating immune complexes may accelerate the development of atherosclerosis. The physiologic relevance of these fi ndings has been demonstrated in vitro by the enhanced macrophage uptake of oxLDL/β2GPI antibody complexes. The participation of macrophage Fcγ receptors in the uptake of oxLDL-containing complexes seems to be particularly important in the development of foam cells and autoimmune-mediated atherosclerotic plaques [48]. Are there other molecules, antigens, and antibodies or other interactions playing a role in the development of atherosclerosis? OxLDL is highly proinflammatory and immunogenic, making this molecule a prime target for natural defense mechanisms. The resulting complexes may not be irreversibly harmful if the process is self-limiting or self-contained. Chronic and more severe inflammatory processes, especially in autoimmune-prone individuals, may induce immune responses that perpetuate a pathologic process (ie, atherosclerosis). C-reactive protein, fibrinogen, TNF-α, heat-shock protein [49•], homocysteine, and other biomarkers are being use to assess the risk of cardiovascular disease. However, the exact mode of action of these molecules is not completely understood. It has been recently shown that oxLDL can interact with C-reactive protein to form proatherogenic complexes. The product of these and other interactions may not only cause

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5.

vascular inflammation, but also trigger autoantibodies and immune complexes that accelerate the development of atherosclerosis. The development of ELISA test kits to measure oxLDL/β2GPI complexes and anti-oxLDL/β2GPI antibodies has provided additional tools to further study the role of the humoral immune response in the atherosclerotic process. Stable and likely pathogenic oxLDL/β2GPI complexes have been demonstrated in the serum of SLE, SSc, and APS patients. Anti-oxLDL/β2GPI antibodies are detected in SLE and SSc patients, both diseases that are characterized by generalized vascular complications. Further, the association of these antibodies with arterial thrombosis was stronger than with venous thrombosis in APS patients. The role of oxLDL/β2GPI complexes and autoantibodies to these complexes in the vascular complications of SSc remain to be further studied. At this point, these results should be interpreted in the context of an autoimmune disease. However, oxLDL/β2GPI complexes have been demonstrated in patients with syphilis, infectious endocarditis, type 2 diabetes mellitus, and chronic nephritis, indicating that oxidation of LDL and the formation of complexes with β2GPI is not restricted to SLE. In contrast, none of these patients developed significant levels of anti-oxLDL/β2GPI antibodies. These antibodies seem to be restricted to patients with SLE and APS. Thus, it can be hypothesized that these antibodies accelerate the development of atherosclerosis in autoimmune patients.

Acknowledgment Dr. Lopez can be contacted at Corgenix, Inc. in Broomfi eld, CO.

Disclosures No potential confl icts of interest relevant to this article were reported.

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