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THE PRESENT AND FUTURE

STATE-OF-THE-ART REVIEW

Antiphospholipid SyndromeRole of Vascular Endothelial Cells and Implications forRisk Stratification and Targeted Therapeutics

Michel T. Corban, MD,a Ali Duarte-Garcia, MD,b Robert D. McBane, MD,a Eric L. Matteson, MD, MPH,b,c

Lilach O. Lerman, MD, PHD,a,d Amir Lerman, MDa

ABSTRACT

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Antiphospholipid syndrome (APS) is an autoimmune disease characterized by venous thromboembolism, arterial thrombosis,

and obstetric morbidities in the setting of persistently positive levels of antiphospholipid antibodies measured on 2 different

occasions 12 weeks apart. Patients with APS are at increased risk for accelerated atherosclerosis, myocardial infarction,

stroke, and valvular heart disease. Vascular endothelial cell dysfunction mediated by antiphospholipid antibodies and sub-

sequent complement systemactivation play a cardinal role in APS pathogenesis. Improved understanding of their pathogenic

function could help in the risk stratification of patients with APS and provide new molecular therapeutic targets.

(J Am Coll Cardiol 2017;69:2317–30) © 2017 by the American College of Cardiology Foundation.

T he term antiphospholipid syndrome (APS) wascoined in the 1980s to describe a conditionof autoantibody-induced thrombophilia (1).

This autoimmune prothrombotic condition is charac-terized by venous thromboembolism, arterial throm-bosis, and pregnancy morbidity in the setting oflaboratory evidence of elevated levels of antiphos-pholipid antibodies (aPLs). aPLs can be identified by1 of 3 assay platforms, namely, clot-based assaysto identify lupus anticoagulant, or enzyme-linkedimmunosorbent assays to identify anticardiolipin(aCL) or anti–b2-glycoprotein 1 (b2-GP1) antibodies(immunoglobulin G or immunoglobulin M). Regard-less of the assay, this class of antibodies targetsantiphospholipid-bound proteins. The prevalence ofaPLs in a random sample of 552 healthy blooddonors was found to be 6.5% and 9.4% for aCL immu-noglobulin G and immunoglobulin M antibodies,respectively. None of those normal subjects with

m the aDepartment of Cardiovascular Diseases, Mayo Clinic College of Me

Rheumatology, Department of Medicine, Mayo Clinic College of Medici

idemiology, Department of Health Sciences Research, Mayo Clinic CollegedDivision of Nephrology and Hypertension, Department of Medicine, May

nnesota. Dr. McBane has a research grant from Bristol-Myers Squibb. Dr.

thors have reported that they have no relationships relevant to the conte

nuscript received January 2, 2017; revised manuscript received February

positive aPLs developed thrombotic events at 1-yearfollow-up (2).

A definite diagnosis of APS requires the presence ofat least 1 clinical and 1 laboratory criterion. Clinicalcriteria may include objectively confirmed venous,arterial, or small-vessel thrombosis or pregnancymorbidity attributable to placental insufficiency,including pregnancy loss or premature birth. Labo-ratory criteria encompass persistently positive testresults for at least 1 of these 3 aPLs measures on 2 ormore occasions 12 weeks apart. The 12-week testinginterval is particularly important, given that someinfections and medications can cause transientaPL-positive testing (3,4).

APS typically presents in the fourth decade of lifeand is classified as either a primary disease or sec-ondary to another underlying autoimmune disease,solid tumor, or hematologic disorder. Approximately10% to 40% of patients with systemic lupus

dicine and Science, Rochester, Minnesota; bDivision

ne and Science, Rochester, Minnesota; cDivision of

of Medicine and Science, Rochester, Minnesota; and

o Clinic College of Medicine and Science, Rochester,

Leman is a consultant for Itamar Medical. All other

nts of this paper to disclose.

21, 2017, accepted February 28, 2017.

ABBR EV I A T I ON S

AND ACRONYMS

aCL = anticardiolipin antibody

aPL = antiphospholipid

antibody

apoER2 = apolipoprotein E

receptor 2

APS = antiphospholipid

syndrome

b2-GP1 = b2-glycoprotein 1

CAPS = catastrophic

antiphospholipid syndrome

eNOS = endothelial nitric oxide

synthase

HCQ = hydroxychloroquine

INR = international normalized

ratio

mTORC = mammalian target of

rapamycin complex

NO = nitric oxide

NOAC = new oral

anticoagulant agents

SLE = systemic lupus

erythematosus

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erythematosus (SLE) (5) and up to 20% ofpatients with rheumatoid arthritis (6) havepositive aPL serologies. Moreover, patientswith SLE who have positive lupus anticoagu-lant have a 50% chance of developing venousor arterial thrombotic events within a 20-yearfollow-up period (5). APS can affect any bodyorgan, and patients with APS have increasedrisk for thrombosis, accelerated atheroscle-rosis, myocardial infarction, and stroke (7,8).In a large prospective cohort study of 1,000patients with APS followed over a 10-yearperiod, deep vein thrombosis was the mostcommon presenting clinical manifestation(39%), followed by thrombocytopenia (30%),livedo reticularis (24%), stroke (20%), pul-monary embolism (14%), and myocardialinfarction (6%). In that study, thromboticevents occurred in 16.6% of patients duringthe first 5-year period and in 14.4% during thesecond 5-year period; 90.7% of patients werestill alive at 10 years (9). Despite high survivalrates, these data reflect a substantial diseaseburden and morbidity in patients with APS oncurrent treatment.

Obstetric morbidities in APS are thought to besecondary to placental vascular insufficiencies,with early (<10 weeks) and late ($10 weeks) mis-carriages being the most common obstetric manifes-tations (35% vs. 17%, respectively) of APS, followedby premature labor (11%), pre-eclampsia and/oreclampsia (5%), and intrauterine growth restriction(2%) (9).

A recent meta-analysis demonstrated that APS isassociated not only with clinical adverse cardiovas-cular events but also with subclinical cardiovascularrisk factors associated with endothelial cell dysfunc-tion, such as increased carotid artery intima-mediathickness and lower flow-mediated dilation, higherfrequency of carotid plaques, and increased preva-lence of pathological ankle-brachial indexes (10). Therenal vasculature may also be affected in APS, leadingto nephropathy, renal vein thrombosis, renal arterystenosis, thrombotic microangiopathy, hypertension,kidney infarction, and ultimately end-stage kidneydisease (11,12). The most severe, yet fortunatelyinfrequent form of APS is catastrophic APS (CAPS).CAPS, with an incidence of 0.9% (9) and mortality>50% (9,13), is defined as small-vessel thrombosisinvolving $3 organs, organ systems, and/or tissues,occurring simultaneously or in <1 week, along withpositive laboratory evidence of aPLs and no alterna-tive diagnosis (13).

PATHOGENESIS

THE CARDINAL ROLE OF ENDOTHELIAL CELLS. Thefact that normal healthy subjects with circulating aCLantibodies remained free of thrombotic events at1-year follow-up on one hand and the important directrole of aPLs in the pathogenesis of APS on the otherhand imply that additional underlying host suscepti-bility characteristics are necessary for the develop-ment of the disease. More than a decade ago, it wassuggested that endothelial cells play a central role inAPS pathogenesis and may represent the commonpathway through which autoimmunity and inflam-mation participate in APS (14). Although circulatingaPLs and underlying endothelial dysfunction are anecessary “first hit” for thrombosis in APS, an inflam-matory “second hit” is needed to precipitate a throm-botic event (15–17). Although elevated levels of aPLshave been associated with both vascular thrombosisand pregnancy morbidity in patients with APS (18,19),human aPLs injected into mice did not promotethrombosis in the absence of endothelial injury (20,21).Growing evidence demonstrates that endothelial celldysfunction, mediated by aPLs binding to endothelialcell b2-GP1 receptors, results in increased risk forthrombosis, accelerated atherosclerosis, myocardialinfarction, and stroke in patients with APS (7). Simi-larly, a recent in vitro study associated obstetricmorbidity in APS to placental inflammatory responsemediated by binding of aPLs to b2-GP1 receptors on thesurface of placental trophoblasts (22). Moreover, aninflammatory insult secondary to surgery, trauma, orinfection has been demonstrated to up-regulateexpression of b2-GP1 receptors on the endothelial cellsurface (Figure 1A) (15,17).Endothe l ia l n i t r i c ox ide synthase inh ib i t ion andthrombos is . Endothelium-derived nitric oxide (NO),produced by endothelial NO synthase (eNOS), isimportant for normal endothelial function andvascular health (23–25). Patients with APS have lowerplasma nitrite levels and an impaired endothelium-dependent vascular response, suggestive ofimpaired eNOS activity and reduced NO productionand endothelial function (26,27). The antithromboticeffects of eNOS and NO secreted from endothelialcells and platelets have been demonstrated in multi-ple animal and human studies. Indeed, eNOS-knockout mice demonstrated enhanced plateletaggregation (28) and increased predisposition tothrombosis, stroke, and atherosclerosis (29–32), anddecreased endogenous NO production in humanshas also been associated with increased risk forthrombosis (33–35).

FIGURE 1 Antiphospholipid Syndrome Pathogenesis

(A) Up-regulation of beta-2 glycoprotein 1 (b2-GP1) receptors on endothelial cells following a “second hit” inflammatory insult. (B) Antiphospholipid antibody

(aPL)–mediated endothelial nitric oxide synthase (eNOS) inhibition, impaired nitric oxide (NO) production and release, and endothelial dysfunction. “First hit.”

(C) aPL-mediated endothelial cell proliferation, intimal hyperplasia, and nonatherosclerotic vascular stenosis. (D) Antiphospholipid syndrome (APS)–associated

accelerated atherosclerosis. (E) aPL-mediated platelet cell activation, aggregation, and thrombosis. (F) Complement system activation and thrombosis.

apoER2 ¼ apolipoprotein E receptor 2; C5a ¼ complement component 5a fragment; C5aR ¼ complement component 5a fragment receptor; C5b ¼ complement

component 5b fragment; C5bR ¼ complement component 5b fragment receptor; DI ¼ domain I of b2-GP1 receptor; DV ¼ domain V of b2-GP1 receptor;

LDL ¼ low-density lipoprotein; MAC ¼ membrane attack complex; mTORC ¼ mammalian target of rapamycin complex; PI3K-AKT ¼ phosphatidylinositol 3-kinase–AKT

pathway; PP2A ¼ protein phosphatase 2 A; VCAM ¼ vascular cell adhesion molecule.

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Recent elegant in vitro and animal studies havedemonstrated that aPL-mediated eNOS inhibition isthe molecular basis of endothelial dysfunction,increased leukocyte–endothelial cell adhesion, andthrombus formation in patients with APS (36,37).Binding of circulating aPLs to domain I of the b2-GP1receptor on endothelial cells induces b2-GP1 dimer-ization (38,39). Subsequent interaction betweendomain V of dimerized b2-GP1 and low-densitylipoprotein–binding domain 1 of apolipoprotein Ereceptor 2 (apoER2) mediates aPL-induced eNOSinhibition, up-regulation of adhesion molecule

expression, and increased production of endothelin-1and tissue factor, ultimately leading to thrombusformation (37,38,40–44). It was recently shown thatthe intracellular pathway of aPL-induced eNOS inhi-bition following aPL, b2-GP1, and apoER2 interactionis mediated by protein phosphatase 2 A dephosphor-ylation of a critical serine residue (Ser1177) ofeNOS (Figure 1B) (37,45,46). Thus, circulatingaPL-associated NO-dependent endothelial dysfunc-tion, mediated by inhibitory dephosphorylation ofeNOS, plays an important role in the pathogenesis ofAPS-related thrombosis.

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Endothel ia l ce l l dysfunct ion and vascu lopathy .Circulating aPL-induced endothelial cell dysfunctionmay also lead to vasculopathy. Steps to aPL-mediatedvasculopathy include diffuse intimal hyperplasia,accelerated atherosclerosis, and microvasculardysfunction.Endothelial cell proliferation and intimal

hyperplasia. Patients with APS have significantlyincreased carotid intima-media thickness andsmaller carotid luminal diameter in the absenceof atherosclerotic disease (47,48). Moreover,pathological specimens of patients with APSnephropathy show evidence of vascular cellularinfiltrates, severe intimal hyperplasia, and fibrosisof the intima and media (49–51).

Interestingly, the pathogenesis of acute peripheralarterial occlusion (49) and ischemic cerebrovascularevents (52,53) in patients with APS has been associ-ated primarily with severe concentric intimal andmedial hyperplasia and fibrous occlusions with vari-able evidence of thrombosis and thrombus stages.Two important recent studies by Canaud et al. (54,55)have demonstrated that aPLs, specifically aCL andanti-b2-GP1 antibodies, induce endothelial cell pro-liferation and intimal hyperplasia through activationof the phosphatidylinositol 3-kinase–AKT pathwayand, subsequently, the mammalian target of rapa-mycin complex (mTORC) (Figure 1C). Moreover,mTORC pathway activation in endothelial cellscorrelated with aPL titers, and an mTORC pathwayinhibitor (sirolimus) prevented recurrent renal allo-graft vasculopathy in patients with APS (54,55).Endothelial cell dysfunction and accelerated

atherosclerosis. Endothelial dysfunction is theearliest detectable stage of atherosclerosis (56–59).Circulating aPL-mediated endothelial dysfunction isassociated with accelerated peripheral and coronaryatherosclerosis (60–66) relative to the generalpopulation with comparable risk factors (67).Circulating aCL and anti-b2-GP1 antibodies mediateinternalization of oxidized low-density lipoprotein–b2-GP1 complexes into macrophages to formfoam cells, which leads to the development ofatherosclerotic lesions (Figure 1D) (68–70).

Multiple clinical studies have demonstrated astrong correlation between circulating aPLs andsystemic atherosclerosis in humans, including pe-ripheral arterial disease, coronary artery disease,acute myocardial infarction, and ischemic stroke(8,66,71–76). Moreover, aPL levels have been associ-ated with increased mortality rates due to coronaryartery disease (73). Premature atherosclerosis inthese patients has been linked to localized proin-flammatory nontraditional immunopathological risk

factors mediated by circulating aPLs. Aggressivecontrol of traditional cardiovascular risk factors istherefore recommended to prevent further endothe-lial injury.

Interestingly, recurrent thrombosis was recentlysuggested to be a potential mechanism for coronaryplaque progression in cardiac allograft transplantvasculopathy (77). Taking into account a similaritybetween these 2 features of accelerated atheroscle-rosis, the association between recurrent clinical orsubclinical thrombosis and accelerated atheroscle-rosis in patients with APS warrants furtherinvestigation.Endothelial cell dysfunction and coronary microvascular

obstruction. Coronary microvascular endothelialdysfunction in APS has also been associated withischemic heart disease. Cardiac magnetic resonancestudies of these patients with otherwise low pre-testprobability for coronary artery disease revealed ahigher than anticipated incidence of late myocardialgadolinium enhancement consistent with subclinicalcoronary microvascular dysfunction (78). Similarly,60% of asymptomatic patients with APS and noknown systemic risk factors had subclinicalmyocardial perfusion abnormalities (23% exerciseinduced and 42% persistent) on single-positronemission computed tomography (technetium-99msestamibi) imaging, suggestive of endothelialdysfunction and associated thrombosis in coronarymicrovasculature (79). In both studies, the level ofcirculating aPLs positively correlated with detectedmyocardial perfusion defects.

THROMBOSIS: ACTIVATION OF PLATELETS AND

COMPLEMENT SYSTEM. Binding of circulating aPLsto domain I of b2-GP1 receptors on platelets inducesvenous and/or arterial thrombosis through increasedproduction of thromboxane B2. Thromboxane A2, apotent platelet activator, results in enhanced adhe-sion of platelets to collagen and increased plateletaggregation before being degraded to thromboxane B2

(Figure 1E) (80). Indeed, a strong correlation has beenreported between the level of anti-b2-GP1 domain 1aPLs and adverse clinical outcomes in patients withAPS (81,82).

In addition to eNOS inhibition and platelet aggre-gation, the thrombogenic effects of aPLs also involveactivation of the classical complement system(21,83,84). Complement activation and depositionhave been associated with placental injury in femalepatients with APS experiencing obstetric complica-tions (85).

Girardi et al. (86) suggested that heparin preventedobstetric complications by blocking complement

FIGURE 2 Libman-Sacks Endocarditis

Transthoracic echocardiographic (TTE) and transesophageal echocardiographic (TEE) images of a 33-year-old woman who presented with

acute-onset shortness of breath secondary to flash pulmonary edema in the setting of new diagnosis of antiphospholipid syndrome (strongly

positive anticardiolipin immunoglobulin G antibody, >100 GPL), Libman-Sacks endocarditis, and moderate-to-severe mitral regurgitation.

(A) TTE parasternal long-axis view of the mitral valve with thickened anterior and posterior leaflets and adherent deposits on the leaflets

coaptation point. (B) TEE images demonstrating mitral valve vegetations on the atrial aspect of both leaflets tips consistent with

Libman-Sacks endocarditis. (C) Moderate-to-severe mitral valve regurgitation associated with incomplete coaptation of mitral valve leaflets

secondary to Libman-Sacks endocarditis lesions.

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activation, rather than by directly preventingplacental thrombosis. In mice, C3 and C5 activationwas required for aPL-induced thrombosis andincreased leukocyte adhesion to endothelium (84,87).In addition, interaction between C5a and C5aR wasrequired for aPL-induced pregnancy loss (88).Furthermore, C5a binding to endothelial cells resul-ted in increased neutrophil adhesion, tissue factorexpression, and release of other procoagulant sub-stances (89,90). C5b-initiated assembly of the mem-brane attack complex subsequently triggersproinflammatory signaling pathways and promotesthrombus formation (Figure 1F) (91). In contrast, C3convertase inhibitor and anti-C5 mouse antibodiesblocked aPL-induced thrombus formation in thismodel (84,87). Similarly, blockade of C5aR preventedthrombus formation in a rat model of antibody-mediated thrombotic glomerulonephritis (92). Eculi-zumab, a C5 inhibitor, prevented APS-associatedthrombotic microangiopathy in a post–renal trans-plantation patient with CAPS and in other patientswith recurrent CAPS (93,94).

VALVULAR HEART DISEASE IN APS

Between 15% and 30% of patients with APS havevalvular heart disease, known as Libman-Sacksendocarditis or nonbacterial thrombotic endocarditis(9,95). The reported incidence rate is 5% over a10-year period (9). Although the revised classification

criteria for definite APS diagnosis do not includevalvular heart disease, the international consensusstatement defines aPL-associated cardiac valve dis-ease as the coexistence of aPLs and echocardio-graphic evidence of mitral or aortic valve lesions.These criteria include valve thickness >3 mm, local-ized thickening of the proximal or middle portion ofthe valve leaflet, irregular nodules on the atrial aspectof the mitral valve or the vascular surface of the aorticvalve, and/or moderate-to-severe valvular regurgita-tion or stenosis (Figure 2). Infective endocarditisand rheumatic heart disease history are importantexclusions (4).

Histologically, Libman-Sacks endocarditis lesionsare small (up to 3 to 4 mm), sterile, fibrinous vege-tations with varying fibroblastic organization, neo-revascularization, and mononuclear inflammatorycell infiltration. Fibrous plaque formation, scarring,and focal calcifications may be seen in advancedstages (96).

Although the risk for valvular heart disease is 3-foldhigher in patients with circulating aPLs (97), its precisepathophysiology remains unclear. Ziporen et al. (98)demonstrated deposits of aPLs and complementon deformed heart valves of patients with APS.Blank et al. (99) identified target epitopes of anti-b2-GP1 antibodies on valvular endothelial cell b2-GP1receptors. These findings suggest that the pathogen-esis could include aPL-mediated valvular endothelialcell activation with complement fixation, similar to

TABLE 1 Primary Thromboprophylaxis in

Antiphospholipid Syndrome

General measures for allantiphospholipid-positive patients

� Assessment and management oftraditional cardiovascular risk factors

� In high-risk situations (puerperium,surgery, prolonged immobilization),use usual doses of LMWH forthromboprophylaxis

Antiphospholipid-positivenon-SLE patients(obstetric APS andasymptomatic carriers)

� Low-dose aspirin (75–100 mg/day) inthose with a high-risk aPL profile,especially in the presence of otherthrombotic risk factors

Patients with SLE andpositive aPLs

� Hydroxychloroquine (200–400 mg/day)þ low-dose aspirin (75–100 mg/day)

aPL ¼ antiphospholipid antibody; APS ¼ antiphospholipid syndrome;LMWH ¼ low–molecular weight heparin; SLE ¼ systemic lupus erythematosus.

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endothelial cell injury in other vascular beds. It isunclear whether antithrombotic therapy limits thedevelopment of valvular injury in this setting.

RISK STRATIFICATION: INCREMENTAL

VALUE OF ENDOTHELIAL FUNCTION TESTING

AND CLINICAL IMPLICATIONS

Tools capable of early and reliable identification ofthe high-risk APS patient population would be clini-cally useful. Known thrombotic risk variables includea “triple-positive” aPL profile (positive for lupusanticoagulant, aCL, and anti-b2-GP1 antibodies)(100,101), high mean platelet volumes as a surrogatefor increased platelet activation (102), and decreasedplasminogen and elevated plasminogen activator in-hibitor-1 levels (103). The Global APS Score, a novel,promising risk assessment tool, may be useful for thispurpose. The tool, on the basis of prospective studiesin primary and secondary APS (104,105), uses the aPLprofile, hypertension, dyslipidemia, diabetes melli-tus, and the presence of other autoimmune anti-bodies, namely, antinuclear antibodies, extractablenuclear antibodies (anti-Ro, La, Smith, ribonucleo-protein, scleroderma-70, Jo-1 antibodies), anddouble-stranded deoxyribonucleic acid antibodies topredict thrombosis risk (106).

Measures of endothelial cell dysfunction are notincluded in this risk assessment tool. Other importantrisk factors, including obesity and cigarette smoking,are also not used. Given the central role of endothelialdysfunction in APS, future risk stratification toolsmight benefit from incorporating either invasive ornoninvasive measures of endothelial function, suchas intracoronary acetylcholine challenge testing, orbrachial reactivity and plethysmography finger arte-rial pulsatile volume changes (EndoPAT, ItamarMedical, Caesarea, Israel). On the basis of the growingbody of evidence that endothelial dysfunction is apredictor of adverse cardiovascular events indepen-dently of traditional risk factors (107), the role ofendothelial function assessment for risk stratificationand potential therapeutic implications in APS needsfurther investigation. Moreover, the benefit of theassessment of APS score in individuals with unex-plained endothelial dysfunction and acceleratedatherosclerosis may be explored.

PHARMACOLOGICAL THERAPY: CURRENT

RECOMMENDATIONS, EMERGING THERAPIES,

AND POTENTIALLY PROMISING

TARGETED THERAPIES

PRIMARY THROMBOPROPHYLAXIS. Thrombophiliaremains the hallmark of APS, and thrombosis

prevention is a major goal of therapy. Asymptomaticpatients with positive APS-related antibodies maydevelop thrombotic events at a rate of 0% to 4% peryear, especially in the setting of SLE (108). Althoughthromboembolic risk stratification tools for patientswith APS are limited, contemporary recommenda-tions for primary thromboprophylaxis in high-riskSLE and non-SLE patients with positive aPL serologyhave been proposed (Table 1).

The 13th International Congress on Anti-phospholipid Antibodies Prevention and Manage-ment Task Force recommended evaluation and tightcontrol of traditional cardiovascular risk factors aspart of primary prophylaxis in APS (108). Althoughhypertension has been associated with thrombosis inaPL-positive patients, hypertriglyceridemia, lowhigh-density lipoprotein levels, and central obesityare the most common cardiovascular risk factorsassociated with primary APS (109). Control of tradi-tional cardiovascular risk factors is indeed importantto help prevent additional insult to the vascularendothelium in patients with APS. The task force alsorecommended that all aPL-positive patients receiveappropriate thromboprophylaxis with heparin inhigh-risk situations, such as surgery, postpartum, andprolonged immobilization (108).

Two recent meta-analyses have suggested that therisk for first thrombotic event was significantlydecreased by low-dose aspirin among asymptomaticaPL-positive subjects, patients with SLE, and patientswith obstetric APS (110,111). A recent open-labelrandomized trial showed no incremental primarythromboprophylaxis benefit in adding anticoagu-lation with warfarin to low-dose aspirin alone (112).As such, primary prophylaxis with anticoagulantagents is not recommended in otherwise healthyambulatory patients with no prior thrombotic history.Currently, guidelines recommend the use of low-dose

TABLE 2 Secondary Thromboprophylaxis in

Antiphospholipid Syndrome

Definite APS and a firstvenous event

� Indefinite oral anticoagulant therapyto a target INR of 2.0–3.0

Definite APS and arterialthrombosis

� Indefinite oral anticoagulant therapyto a target INR >3.0 or combinedantiaggregant-anticoagulant (INR2.0–3.0) therapy

Patients with venous orarterial thrombosiswho do not fulfillcriteria for APS

� Treatment as per usual recommen-dations for arterial or venousthrombosis

APS ¼ antiphospholipid syndrome; INR ¼ international normalized ratio.

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aspirin in aPL-positive patients, with or without SLE,with a high-risk aPL profile and/or additional throm-botic risk factors (108).

Hydroxychloroquine (HCQ) is an antimalarial usedextensively in autoimmune disease that has immu-nomodulatory and possibly antithrombotic proper-ties. HCQ has been shown to reduce thrombus sizeand platelet aggregation in mouse models (113,114).The antithrombotic efficacy of HCQ in humans hasbeen shown during its use for deep vein thrombosisprophylaxis after hip surgery (115). HCQ is widelyused in SLE for its disease-modifying effects (116),and its use in asymptomatic patients with positiveaPL serologies resulted in a reduction of thrombo-embolic events (117). In patients with obstetric APS,HCQ has been considered after failure of standardtreatment with heparin and aspirin (118). Currentguidelines recommend the use of HCQ in addition tolow-dose aspirin only in aPL-positive patients withSLE (108). There are no recommendations regardingthe use of HCQ for primary thrombosis prevention inother aPL-positive populations.

SECONDARY THROMBOPROPHYLAXIS. Anticoagulationis the mainstay therapy for secondary thrombopro-phylaxis in patients with APS. Current guidelines forsecondary thromboprophylaxis, summarized inTable 2, recommend consideration of whether thefirst thrombotic event was venous or arterial (108). Ifthe initial event was venous thromboembolism, it isrecommended that moderate-intensity warfarintherapy targeting international normalized ratio (INR)values of 2.0 to 3.0 be initiated, with heparinbridging. Two systematic reviews have confirmedboth the efficacy and safety of this approach in pre-venting recurrent venous thrombosis (119,120).

In comparison, management of arterial thromboticevents has been less well studied and more contro-versial, with 2 approaches proposed: moderate in-tensity (INR goal 2 to 3) versus high-intensity (INR >3)warfarin anticoagulation. A systematic review ofobservational studies concluded that the rate ofrecurrent arterial thrombosis was significantly lowerwith INR>3 (3.8%) versus INR<3 (23%) (120). AlthoughAPASS (Antiphospholipid Antibodies and StrokeStudy), a large prospective cohort study of 1,770 pa-tients, found no difference in recurrent stroke anddeath in patients treated with moderate-intensitywarfarin (INR 1.4 to 2.8) versus those treated withhigh-dose aspirin (325 mg/day) (121), a small random-ized controlled trial of 20 patients with APS andischemic stroke showed that patients treated with acombination of low-dose aspirin (100 mg/day) andmoderate-intensity warfarin (INR 2.0 to 3.0) had a

significantly lower incidence of recurrent strokecompared with those treated with low-dose aspirinalone (122). However, it is important to note that noneof the patients included in the APASS study met thecurrently accepted diagnostic criteria for APS. Thetask force of the 13th International Congress onAntiphospholipid Antibodies recommended high-intensity warfarin (INR >3.0) or combined moderate-intensity warfarin (INR 2.0 to 3.0) therapy and anantiplatelet agent for secondary thromboprophylaxisin patients with APS with arterial thrombotic events(108); however, this recommendation did not reachpanel consensus. Importantly, patients with venous orarterial thrombosis who do not fulfill criteria for aconclusive APS diagnosis should be treated the sameas aPL-negative patients with similar venous or arte-rial thrombotic event presentation (108).

Secondary thromboprophylaxis, whether forvenous or arterial thrombosis, should be continuedindefinitely (108). A retrospective study revealed asignificant increase (relative risk: 4.55 [95% confi-dence interval: 2.67 to 7.43] vs. 0.36 [95% confidenceinterval: 0.24 to 0.53] on any treatment, p < 0.001 forboth) in recurrent thrombosis soon after stopping oralanticoagulant agents (123). Elevated titers of aCL an-tibodies predict recurrent thrombosis and death inthe 6-month period following the initial episode ofvenous thromboembolism (124).

NEW ORAL ANTICOAGULANT AGENTS. The efficacyof the new oral anticoagulants (NOACs), the directthrombin inhibitor dabigatran, and direct anti–factorXa inhibitors rivaroxaban, apixaban, and edoxaban,in APS remains unclear, and is largely on the basis ofcase reports and case series. A recent systematic re-view showed that approximately 20% of patients withAPS on NOACs developed vascular events during amean follow-up period of 12 months (125), whereasone-third of patients developed recurrent eventsduring 2 years of follow-up (126).

TABLE 3 Alternative and Adjunctive Therapeutic Options for Specific Clinical

Scenarios in Antiphospholipid Syndrome

Known warfarin allergy,warfarin intolerance, orpoor anticoagulant controlon warfarin despitetherapeutic target (as perTable 2)

� Treatment with, or addition of, NOAC: directthrombin inhibitor dabigatran or direct anti–factor Xa inhibitors rivaroxaban, apixaban, oredoxaban

Heparin-inducedthrombocytopenia

� Treatment with fondaparinux or argatroban

Refractory APS despiteadequate anticoagulation(as per Table 2)

� Consider starting statin therapy� Consider adding rituximab therapy� Consider adding glucocorticosteroids and

IVIG þ plasma exchange

CAPS � Heparin anticoagulation þ glucocorticosteroids þIVIG and/or plasma exchange reduces mortality

� Eculizumab reduces mortality

Renal transplantation patientswith APS

� Sirolimus decreases recurrent vascular lesionsand vascular proliferation

� Eculizumab for treatment and prevention ofthrombotic microangiopathy in patients withhistory of CAPS

Obstetric APS � Heparin (unfractionated or LMWH) þ lowdose aspirin (75–100 mg/day)

� Patients on warfarin should be switched toheparin (unfractionated or LMWH) immedi-ately upon pregnancy confirmation to avoidteratogenicity

� Extended thromboprophylaxis (up to 6 weeksafter delivery) for high-risk patients

� Indefinite anticoagulation for APS patientswith prior thrombotic events

CAPS ¼ catastrophic antiphospholipid syndrome; IVIG ¼ intravenous immunoglobulin;NOAC ¼ new oral anticoagulant; other abbreviations as in Table 1.

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The RAPS (Rivaroxaban Versus Warfarin to TreatPatients With Thrombotic Antiphospholipid Syn-drome, With or without Systemic Lupus Erythema-tosus) trial enrolled only patients with venousthrombotic events, and a laboratory surrogateoutcome measure (endogenous thrombin potential)was the primary outcome. Rivaroxaban did not reachthe noninferiority threshold from the laboratory sur-rogate standpoint, but there was no increase inthrombotic risk compared with standard-intensitywarfarin (127).

Two currently ongoing randomized controlledclinical trials, ASTRO-APS (Apixaban for the Second-ary Prevention of Thromboembolism Among PatientsWith the Antiphospholipid Syndrome; NCT02295475)and TRAPS (Rivaroxaban in Thrombotic Anti-phospholipid Syndrome; NCT02157272), will helpclarify the role of NOACs in the management of APS.Currently, the 14th International Congress on Anti-phospholipid Antibodies task force recommends theuse of NOACs only when there is a known warfarinallergy, warfarin intolerance, or poor anticoagulationcontrol (Table 3) (128).

OTHER ANTICOAGULANT AGENTS. The evidencebase for the use of other anticoagulant agents, such as

fondaparinux or argatroban, in APS is also limited tocase reports and case series and is predominantly inthe setting of heparin-induced thrombocytopenia(129,130). The 14th International Congress on Anti-phospholipid Antibodies task force recommends theuse of these anticoagulant agents only in the settingof heparin-induced thrombocytopenia in APS(Table 3) (128).

STATINS. The immunomodulatory and anti-inflammatory properties of statins can be beneficialin APS (131). Simvastatin and fluvastatin suppressanti-b2GPI antibody–mediated endothelial activation(132), and fluvastatin also reduced up-regulation oftissue factor expression by aPLs on endothelial cells(133). Moreover, patients with APS treated with flu-vastatin showed decreases in multiple inflammatoryand prothrombotic biomarkers, including interleukin1b, vascular endothelial growth factor, tumor necrosisfactor–a, inducible protein–10, soluble CD40 ligand,and soluble tissue factor (134). To date, there are noformal recommendations for the use of statins inpatients with APS who have normal lipid profiles;however, statins can be considered in patients withrefractory APS despite adequate anticoagulation(Table 3) (128). Furthermore, we submit that endo-thelial function assessment may play a role in iden-tifying patients with APS and normal lipid profileswho might benefit from statin therapy.

mTOR INHIBITORS. Patients with APS nephropathywho required transplantation and were receivingsirolimus had no recurrence of vascular lesions and adecrease in vascular proliferation on biopsycompared with patients with aPLs who were not onsirolimus (Table 3) (54). Although the mTOR pathwaymay represent potential future therapeutic targets fortreatment of APS vasculopathy, the prothromboticeffects of mTOR inhibitors might limit their efficacyin this syndrome (135–138).

CELL DEPLETION THERAPY. Rituximab, a chimericmonoclonal antibody targeting CD20 cells, has beeneffectively used in cases of CAPS, anticoagulationfailure, recurrent thrombosis, and thrombocytopenia(139). In a retrospective single-center study, ritux-imab decreased thrombotic events (average follow-upperiod 40 months) in patients with SLE-associatedAPS with recurrent thrombosis despite adequatewarfarin therapy or life-threatening active diseaserefractory to conventional therapy (140). It is impor-tant to note that immunosuppression was added toongoing anticoagulation therapy (141).

Moreover, rituximab therapy was associated withclinical response in non-thrombosis-related APSmanifestations, including thrombocytopenia, cardiac

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valve disease, skin ulcers, nephropathy, and cogni-tive dysfunction, in a small pilot open-label phase2 trial of 20 patients with APS. Interestingly, therewas no significant variation in the aPL titers atfollow-up (142).

Although the role of rituximab in APS is not yetclearly defined, anti-CD20 antibodies may be animportant future tool in the management of hema-tologic and microangiopathic manifestations, aswell as thrombotic cases of APS refractory to anti-coagulation therapy (Table 3) (128).

COMPLEMENT INHIBITION. Complement systemactivation plays an important role in pathogenesisand thrombus formation in APS. Eculizumab is amonoclonal antibody that binds the C5 protein,inhibiting its cleavage into C5a and C5b. By blockingthe complement cascade, the formation of themembrane attack complex is inhibited. A smallnumber of case reports and case series have reportedsuccessful use of eculizumab in acute CAPS, aswell as in treatment and prevention of thromboticmicroangiopathy following renal transplantation inpatients with CAPS (Table 3) (93,94,143). Currently,there are no recommendations for the global useof eculizumab in APS; however, selective treatmentin refractory cases remains an attractive tool forthe expanding tool box for these complicatedpatients (128).

GLUCOCORTICOSTEROIDS, PLASMA EXCHANGE,

AND INTRAVENOUS IMMUNOGLOBULINS. The use ofglucocorticosteroids, plasma exchange, and intrave-nous immunoglobulins in APS is limited to refractorycases of recurrent thrombosis despite adequateanticoagulation (144,145) and CAPS (Table 3). Thecombination of heparin anticoagulation, glucocorti-costeroids, and plasma exchange, intravenous im-munoglobulins, or both reduced mortality comparedwith other treatment strategies in patients withCAPS (146).

MANAGEMENT OF OBSTETRIC APS. The ninthedition of the American College of Chest Physiciansevidence-based clinical practice guidelines and the14th International Congress on AntiphospholipidAntibodies task force report on obstetric APS recom-mend treatment of women with obstetric APS with acombination of heparin, either unfractionated orlow–molecular weight heparin, and low-dose aspirin(75 to 100 mg/day) (Table 3) (147,148). Although2 randomized controlled trials reported a higher rateof live births with the addition of unfractionatedheparin to low-dose aspirin (149,150), these resultswere not confirmed in 2 additional randomizedcontrolled trials (151,152). Pooled data of all available

randomized controlled trials showed a beneficialeffect on live births with unfractionated heparin andlow-dose aspirin, whereas data on the efficacy of low–

molecular weight heparin and low-dose aspirin wereinconclusive (148).

For patients already receiving warfarin, therapyshould be transitioned to either low–molecularweight heparin or unfractionated heparin andlow-dose aspirin. This transition should occurimmediately upon pregnancy confirmation to avoidteratogenicity. The risk for warfarin-induced fetaleffects is highest during the first 6 weeks, especiallyfor patients requiring more than 5 mg of warfarindaily. For selected high-risk patients in whom sig-nificant risk factors persist following delivery, inter-national guidelines suggest extended prophylaxis(up to 6 weeks after delivery) following dischargefrom the hospital (108,147). For patients with APSwith prior thrombotic events, anticoagulant agentsshould be continued indefinitely. Importantly,maternal warfarin therapy is safe for nursing infants.

b2-GP1 DOMAINS I AND V ANTAGONISTS. Thebinding of aPLs to domain I of b2-GP1 receptors onendothelial cells induces a conformational change inthe receptor and promotes binding of domain V toapoER2. This binding results in eNOS inhibition-mediated endothelial cell dysfunction and ulti-mately thrombosis (Figure 1). Similarly, binding ofaPLs to domain I of the b2-GP1 receptor on plateletsinduces thrombus formation because of increasedthromboxane B2 secretion, enhanced adhesionof platelets to collagen, and increased plateletaggregation.

To date, only a handful of in vitro and mousestudies investigated potential b2-GP1 antagonists.Ioannou et al. (153) demonstrated that the mutateddomain I of b2-GP1 receptors was associated withreduced binding to aPLs in vitro, and that therecombinant antigenic target peptide domain I ofb2-GP1 inhibited venous thrombotic events anddecreased vascular cell adhesion molecule–1 expres-sion on aortic endothelial cells in mice injected withhuman aPLs (154). Moreover, Ostertag et al. (155)demonstrated that TIFI, a 20–amino acid syntheticpeptide with conformational similarities to domain Vof b2-GP1, inhibited APS-induced thrombosis in mice.Similarly, Kolyada et al. (156) demonstrated that theA1-A1 molecule, which inhibits binding of apoER2 todomain V of b2-GP1, reduced arterial thrombus size inAPS mice. These studies support the pursuit ofunique targets, possibly within the b2-GP1 receptor,for future drug development beyond simple antico-agulant therapy.

CENTRAL ILLUSTRATION Antiphospholipid Syndrome Pathogenesis

Corban, M.T. et al. J Am Coll Cardiol. 2017;69(18):2317–30.

Pathogenesis of antiphospholipid syndrome and recommended areas for research: “first hit” circulating antiphospholipid antibodies (aPLs) and underlying endothelial

dysfunction and “second hit” inflammatory insult result in impaired nitric oxide (NO)–dependent endothelial function, accelerated atherosclerosis, nonatherosclerotic

vasculopathy, platelet activation and aggregation, and complement system activation. Research in aPL and beta-2 glycoprotein 1 receptor (b2-GP1) interaction,

b2-GP1 and apolipoprotein E receptor 2 (apoER2) interaction, and complement system activation is recommended. C5a ¼ complement component 5a fragment;

C5b ¼ complement component 5b fragment.

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CONCLUSIONS

APS is an autoimmune disease characterized bythrombophilia, adverse obstetric events and recur-rent miscarriages, accelerated atherosclerosis,increased risk for myocardial infarction and stroke,and valvular heart disease. Morbidity and mortalityin APS is strongly associated with aPL-mediatedvascular endothelial cell dysfunction and comple-ment system activation. Although thrombophilia isthe hallmark of APS, accurate identification of pa-tients at increased risk for thrombosis remains achallenge.

To date, therapeutic efforts have focused mostlyon preventing recurrent thrombotic events in

patients with APS, with only limited research directedtoward new therapies for primary prevention.Although anticoagulation is the mainstay therapy forsecondary thromboprophylaxis in patients with APS,a significant number of those patients developrecurrent thrombosis despite conventional thera-peutic anticoagulation targets. Simply increasing thedoses of anticoagulant agents comes at the expense ofincreased risk for bleeding. Indeed, hemorrhageremains the most common cause of death (23%) inpatients with primary APS (9).

The cardinal role of vascular endothelial dysfunc-tion in APS pathogenesis (summarized in the CentralIllustration) calls for more focused researchregarding aPLs and b2-GP1 interaction, b2-GP1 and

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apoER2 interaction, and associated intracellularsignaling pathways. Large prospective cohort studiesand randomized clinical trials are warranted to studythe role of endothelial function testing in therisk stratification of patients with APS, investigatethe association between recurrent thrombosis andaccelerated atherosclerosis in patients with APS, andevaluate targeted therapies against endothelialdysfunction-mediated APS thrombophilia and vas-culopathy. A paradigm shift aimed at investigatingnew therapeutic targets is of the utmost importanceand is potentially highly rewarding and timely

for reducing morbidity and mortality in patientswith APS.

ACKNOWLEDGMENTS The authors thank Mr. KevinYouel for his artistic contribution to the Figure 1 andthe Mayo Clinic Echocardiography Laboratory forproviding the echocardiographic images in Figure 2.

ADDRESS FOR CORRESPONDENCE: Dr. AmirLerman, Mayo Clinic, Department of CardiovascularDiseases, 200 First Street SW, Rochester,Minnesota 55905. E-mail: lerman.amir@mayo.edu.

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KEY WORDS antiphospholipid antibodies,atherosclerosis, epidemiology,thrombophilia, vascular endothelium,warfarin