ARTHRITIS & RHEUMATISMVol. 63, No. 9, September 2011, pp 2558–2560DOI 10.1002/art.30382© 2011, American College of Rheumatology

EDITORIAL

Are Posttranslational Modifications of �2-Glycoprotein I Markers for ThromboticRisk? Are They Triggers of Autoimmunity?

Michael D. Lockshin and Jane E. Salmon

For over 30 years, we have known that thrombo-sis and fetal loss are the signature features of theantiphospholipid syndrome (APS). We understand themajor functional components of this syndrome: antibod-ies, primarily of IgG class, bind to an autoantigen, inmany cases �2-glycoprotein I (�2GPI), that itself bindseither to negatively charged phospholipids or, morelikely, to cell surface receptors such as apolipoprotein Ereceptor 2 (Apo ER2). These events initiate a chain ofintracellular or extracellular signals that lead to throm-bosis (1–5). Although the chemistry and cell biology arefamiliar, it has not been possible to predict in whom andat what point in time patients with antiphospholipidantibodies will have clinical events.

Multiple studies in animals and humans haveelucidated elements of the pathophysiology that leads toa diagnosis of clinical APS. The hypotheses tested in themodels focus on specific events from the first appear-ance of antibody to thrombosis, including phenotypicchanges in vulnerable tissue, antibody-triggered path-ways of inflammation and injury, alterations in coagula-tion proteins and platelets, cellular activation by stimu-lation of surface receptors, environmental insults, andcoexistent genetic thrombotic states.

The most robust current theories of pathogenesis(Table 1) incorporate fundamental observations: someform of cell activation occurs, exposing negativelycharged phospholipids on the cell surface or initiatingcell adhesion or thrombosis; �2GPI is involved in thethrombotic process, even though antibody to �2GPI isnot a strong marker of risk; and triggering factors

provoke clinical events (6,7). Early involvement of in-flammation, particularly complement activation, hasbeen established in animal models but is less clear inhumans.

In this issue of Arthritis & Rheumatism, Ioannouand colleagues add to a body of work that secures for�2GPI a central role in the pathophysiology of APS andprovides evidence that alterations in �2GPI are associ-ated with increased risk of thrombosis (8). Beta-2-glycoprotein I (also known as apolipoprotein H), anincompletely understood protein with incompletely de-fined physiologic function, circulates in large quantity inplasma. Subjects deficient in �2GPI appear to behealthy, compounding the mystery of its role.

A part of the complement control protein family,�2GPI consists of 5 “sushi domains” (wrapped likepieces of sushi). The molecule is J-shaped. Its hook end,the fifth domain, is the site of its attachment to the cellsurface. The highly positively charged fifth domain bindsto negatively charged surfaces, such as the phosphatidyl-serine that translocates from the inner cell membrane tothe surface in activated or apoptotic cells. Recent studieshave indicated that the fifth domain of �2GPI also bindsto Apo ER2 receptors and thereby activates plateletsand endothelial cells (1,2,7). Upon binding, �2GPI un-dergoes a conformational change that likely explains itstransformation from a benign circulating protein to anactive participant in thrombosis (4). While the fifthdomain targets �2GPI to cell surfaces, the first domaincontains a neoepitope, exposed after the molecule bindsto a surface, that is recognized by antiphospholipidantibodies. Binding and crosslinking of the first domainby antibodies trigger intracellular and extracellular path-ways that result in thrombosis and may cause pregnancycomplications.

In the current study (8), Ioannou and colleaguesinvestigated whether posttranslational modification of�2GPI, independent of binding-induced conformationalchange, correlates with thrombosis. They posed thequestion through a clinical study, examining total �2GPIand the proportion of oxidized �2GPI (with oxidation

Supported by grants from the NIH, the Barbara VolckerCenter for Women and Rheumatic Diseases, the Mary KirklandCenter for Lupus Research, and Rheuminations, Inc.

Michael D. Lockshin, MD, Jane E. Salmon, MD: Hospital forSpecial Surgery and Weill-Cornell Medical Center, New York, NewYork.

Address correspondence to Michael D. Lockshin, MD, Bar-bara Volcker Center for Women and Rheumatic Diseases, Hospitalfor Special Surgery, 535 East 70th Street, New York, NY 10021.E-mail: LockshinM@hss.edu.

Submitted for publication February 22, 2011; accepted inrevised form March 29, 2011.

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defined as lacking free thiols), in a large multicenterstudy. Patients with nonthrombotic autoimmune disease,patients with thrombosis without autoimmune disease,and healthy individuals served as controls. Pregnancyloss as a manifestation of APS was not considered,although its mechanism may (or may not) be different.Indeed, most women with APS-related fetal loss do notexperience thromboses.

The authors found that APS patients had higherlevels of total �2GPI and proportionally more oxidized�2GPI than any of the control groups. In addition,oxidized �2GPI was associated with lupus anticoagulant,an important observation because lupus anticoagulant isthe dominant predictor of thrombotic clinical events inAPS. They propose that relative levels of oxidized �2GPIcan be used to stratify patients by thrombosis risk.However, they were not able to develop a more com-plete risk profile because the study, which included 139APS patients, did not address the effects of systemiclupus erythematosus, anticoagulation treatment, clinicalmanifestations of APS, or risk associated with a decreasein reduced �2GPI. The authors speculate that clinicalstates characterized by increased oxidative stress, such aspregnancy and infection, lead to elevated levels ofoxidized �2GPI, which is more immunogenic and likelyto break tolerance, and that the relative decrease inreduced �2GPI leads to increased thrombosis risk. Im-portantly, in vitro studies from this group provide evi-dence that reduced �2GPI protects endothelial cellsagainst oxidative injury and maintains their viability (9).These studies identify a new and pivotal role for �2GPIin the induction of thrombosis in APS.

The work of Ioannou et al demonstrates clearbiochemical differences between normal and pathogenic�2GPI, links these differences to clinical phenotype, andsuggests posttranslational mechanisms by which �2GPIcan be modified to become pathogenic. Elucidating thestructural biology of �2GPI is one piece of the puzzlethat we now know has broad implications. To under-stand pathophysiology and use relative levels of the

posttranslationally modified forms of �2GPI for riskstratification, one also must consider antiphospholipidantibody class, antigen localization, activation of effectorcells, initiation of pathways of inflammation (comple-ment and cytokines) and coagulation, and environmen-tal triggers.

A number of questions about antiphospholipidsyndrome cannot be answered by the observations ofIoannou et al. Why do some patients with high-titerantiphospholipid antibodies have no illness? How doesoxidation of �2GPI relate to familial and ethnic skewingin APS? Is �2GPI more antigenic in its oxidized state?Does oxidized �2GPI explain the basis for association ofantiphospholipid antibodies with systemic lupus ery-thematosus? Is there a more effective treatment thatblocks upstream events that will supplant anticoagula-tion?

Larger studies with longitudinal followup mustconfirm and extend the exciting new findings reported byIoannou and colleagues. The onus is on the internationalcommunity of physicians and scientists who care for andstudy APS patients to answer these questions.

AUTHOR CONTRIBUTIONS

Drs. Lockshin and Salmon drafted the article, revised itcritically for important intellectual content, and approved the finalversion to be published.

REFERENCES

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2. Romay-Penabad Z, Aguilar-Valenzuela R, Urbanus RT, DerksenRH, Pennings MT, Papalardo E, et al. Apolipoprotein E receptor2 is involved in the thrombotic complications in a murine model ofthe antiphospholipid syndrome. Blood 2011;117:1408–14.

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Table 1. Theories of how antiphospholipid antibodies cause the antiphospholipid syndrome

Aspects of antiphospholipid antibodies that may contribute to pathogenicityFeatures of the autoantibody: class, subclass, or function that predispose to thrombosisFeatures of the antigen recognized: type of phospholipid or coagulation factorExposure of neoepitope in �2-glycoprotein I associated with conformational changesSignaling via endothelial surface receptors, including apolipoprotein E receptor 2, annexin 2, Toll-like receptorsActivation of endothelial cells or platelets, including expression of tissue factor, or adhesion molecules, and inhibition of endothelial cell nitric

oxide synthaseInitiation of inflammation by targeted antibody, including complement activation and release of proinflammatory cytokinesIncreased vulnerability of endothelium triggered by oxidative stress, oral contraceptives, surgery, infections, smoking, etc.Other prothrombotic abnormalities, including methylenetetrahydrofolate reductase, factor V Leiden, etc.

EDITORIAL 2559

Urbanus RT, et al. �2-glycoprotein I can exist in 2 conformations:implications for our understanding of the antiphospholipid syn-drome. Blood 2010;116:1336–43.

5. Salmon JE, Girardi G, Lockshin MD. The antiphospholipidsyndrome as a disorder initiated by inflammation: implications forthe therapy of pregnant patients. Nat Clin Pract Rheumatol2007;3:140–7.

6. Erkan D, Barbhaiya M, George D, Sammaritano L, Lockshin MD.Moderate versus high-titer persistently anticardiolipin antibodypositive patients: are they clinically different and does high-titeranti-�2-glycoprotein-I antibody positivity offer additional predic-tive information? Lupus 2010;19:613–9.

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9. Ioannou Y, Zhang JY, Passam FH, Rahgozar S, Qi JC, Gianna-kopoulos B, et al. Naturally occurring free thiols within �2-glycoprotein I in vivo: nitrosylation, redox modification by endo-thelial cells, and regulation of oxidative stress-induced cell injury.Blood 2010;116:1961–70.

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