Thrombophilia in Systemic Lupus Erythematosus: A Review of Multiple Mechanisms and Resultant Clinical Outcomes

2.1. Vascular thrombosis

The epidemiology of vascular thrombosis is well described in SLE and with several studies showing an increased risk for arterial and venous thrombosis. Arterial and venous thrombosis occurs in approximately 10% of SLE patients [3-7] with thrombosis being a major cause of death in patients with lupus [8]. Gender differences have been reported with male SLE patients having a higher prevalence of thrombosis and antiphospholipid antibody syndrome compared to women with lupus [9]. Thrombovascular events occur throughout the course of lupus disease with an increase risk over time [10-11]. Arterial vascular events occur more frequently in post- menopausal women with lupus and are associated with age, disease duration, smoking and mean dose of glucocorticoids [12]. Venous thrombosis is also associated with smoking in lupus [12]. Other risk factors for thrombosis in lupus include high disease activity, lupus nephritis /nephrotic syndrome, elevated homocysteine, and the presence of antiphospholipid antibodies [11, 13-16].

2.2. Pregnancy loss

The factors which increase thrombosis risk also promote pregnancy loss in lupus. Patients with SLE have increased frequencies of intrauterine growth restriction and fetal demise (miscarriage and stillbirth) which can predate the diagnosis of lupus [17-18]. Pregnancy complications in SLE are fairly common with maternal hypertensive complications occurring in 10-20%, preterm births in 20% and fetal growth restriction occurring in about 28%, with an average drop in fetal growth weight to be 16% [18-19]. Fetal wastage is markedly increased in SLE with stillbirths occurring in 4-22% and miscarriage rates reported to range from approximately 10-46% [18-19]. The increased stillbirth rate in SLE is 4 fold greater than the general population [18]. Intrauterine demise and adverse fetal outcome in SLE are related in great part to lupus specific thrombophilic factors, which can cause placental infarctions, decidual vasculopathy, and lower placental weight [20-22]. Placental histopathological findings in SLE patients with lupus anticoagulant or anti-cardiolipin antibodies include extensive infarctions due to decidual vasculopathy (related to fibrinoid necrosis in the wall of decidual arterioles and thrombosis), syncytial knots, and perivillous fibrinoid change [20-22]. In addition, there is a decrease in vasculo-syncytial membranes,an increase in fibrosis and an increase in hypovascular villi all which can lead to fetal growth restriction or demise [20-22]. Pregnancy itself is a hypercoagulable state with fetal demise, thrombosis and pre-eclampsia being related to Factor V Leiden mutation, prothrombin gene mutation 20210A, and deficiencies of anti-thrombin III, protein C and protein S [23]. Thus, SLE specific thrombophilic factors are additive to the background of pregnancy related hypercoagulability (multiple hits), and this increases the occurrence of adverse fetal outcomes in lupus.

2.3. Premature cardiovascular disease

Lupus-specific thrombophilic factors contribute to premature cardiac disease and atherosclerosis in this population. These lupus specific factors can affect the endothelium resulting in premature arterial vascular disease contributing to the accelerated/premature atherosclerosis observed in SLE [24]. Atheroma formation is initiated when oxidized low density lipoprotein (LDL) is taken up by foam cells in vascular endothelium [24-25]. High density lipoprotein (HDL) and Apo-A1 are protective factors against atherosclerosis. β-2 glycoprotein-1 binding to oxidized LDL facilitates uptake by foam cells [26]. Patients with SLE have antibodies to LDL/ β-2 glycoprotein-1 complexes, HDL, and Apo –A1 [26-27]. Antibodies to HDL and Apo A-1 cross react with cardiolipin and prevent HDL and Apo-1 protection against atherosclerosis. Antibodies to oxidized LDL may cross react with β-2 glycoprotein-1 and may enhance uptake, thus promoting plaque formation [25-28]. Anticardiolipin antibody binding exposes immunogenic and normally hidden (or cryptic) epitopes on β-2 glycoprotein-1, which can bind to anti- β-2 glycoprotein-1 antibodies. These antibodies bind to adhered β-2 glycoprotein-1 on endothelial cells which causes endothelial activation and subsequent up regulation of inflammatory and procoagulant factors [24-26]. In addition, adhered β-2 glycoprotein-1 on oxidized LDL promotes LDL uptake by macrophages, thus facilitating plaque formation [24-26, 28]. During the acute phase response, HDL can be converted to pro-inflammatory molecules which promote LDL oxidation. Chronic inflammation can cause HDL dysfunction in SLE. HDL has been found to be pro-inflammatory in women with SLE and is termed pro-inflammatory HDL or piHDL [25-26, 28]. More than 85% patients with SLE and carotid plaques had piHDL vs. 40% in those without plaques [27]. Pro- inflammatory HDL is an independent risk factor for atherosclerosis in rheumatoid arthritis and antiphospholipid antibody syndrome [26, 28]. The prevalence of moderate to severe atherosclerosis in SLE at autopsy is 52% [29]. There is a higher prevalence of coronary artery disease in SLE than general population which is not predicted by traditional risk factors or increase in lupus activity alone [30-39]. Mortality studies show coronary artery disease (CAD) /myocardial infarction (MI) is a frequent cause of death in SLE and occurs in 11-48% of SLE patients [3, 30-35]. Death from CAD in SLE is disproportionately larger in late disease, with CAD/MI being the leading cause of death in SLE survivors. This accounts for the late peak in mortality in SLE [36-38]. Although there are a greater number of CAD risk factors (hypertension, diabetes mellitus, dyslipidemia, sedentary life style) in SLE patients than matched controls, the increased atherosclerosis in SLE is not fully attributed to these traditional risk factors [39-41]. Several other studies have reported an increased risk of cardiovascular disease in SLE. After controlling for age, sex, cholesterol, hypertension, diabetes mellitus, tobacco use the relative risk was 10.1 for non-fatal myocardial infarction [95%, CI 5.8-15.6] and 17.0 for death due to coronary artery disease (CI 8.1-29.7) [42]. Esdaile also reported a 7.5X increased risk for developing coronary artery disease in SLE [42]. Progression of coronary artery calcification has been associated with age, cholesterol and smoking [43]. In particular, premenopausal aged women with lupus seem to have premature CAD with >50X increase in MI in SLE women 35-44 years, and the risk of cardiovascular events being 8-9 X increased in middle age SLE women [44-45]. We have also reported an increased risk of myocardial infarction in premenopausal women with SLE [46]. Thus, myocardial infarctions occur in patients with SLE at younger age than the general population, and using traditional risk factors alone is inadequate for developing prevention and treatment programs in asymptomatic SLE patients [42, 44].

2.4. Cerebrovascular disease

Cerebrovascular disease is increased in SLE with an increased risk of stroke reported to be 1.67, 3.2 and 7.9 X the general population [44, 47-48]. Age and hypertension are associated with progression of carotid intima media thickness and carotid artery plaque in SLE [43]. Male lupus patients tend to have a higher prevalence of strokes than females [49]. Antiphospholipid antibodies are established risk factors for ischemic stroke in lupus [50] and can increase cerebrovascular atherosclerotic disease [51].

3.1. Chronic inflammation

Chronic Inflammation, which occurs in SLE, contributes to the development of thrombosis and accelerated atherosclerosis [52]. Inflammation and infections are known epidemiologic risk factors for venous thrombosis [53]. Inflammation can cause an acquired thrombophilia and subsequent thrombosis [54]. Inflammation activates the procoagulant arm of the coagulation system and inhibits anticoagulation and fibrinolysis [55].There is an association of lupus disease activity (i.e., inflammation) and elevated erythrocyte sedimentation rate and/or C-reactive protein levels with vascular thrombosis. [5, 6, 56-57]. In fact, complement activation which occurs in lupus can promote thrombosis by activating the coagulation cascade at multiple levels [58]. It is well established that venous thrombosis can occur with elevated levels of procoagulant factors 2, 8, 9 and 10, as well as with decreased levels of anticoagulant factors (antithrombin 3, protein S and protein S [54]. Coagulation factors that increase with inflammation include Von Willebrand factor, fibrinogen, Factor VII, and Factor VIII. Increases in high sensitivity C –reactive protein (HSCRP), fibrinogen and factor VIII are in seen in lupus anticoagulant-related thrombosis [59]. Additionally, fibrinogen increases with time in SLE patients which may partially explain the increased risk of thrombosis with increasing years of lupus disease [11, 60]. Lupus disease activity is associated with elevated procoagulant markers thrombin-anti thrombin complexes, prothrombin fragment 1+2, and soluble thrombomodulin, suggesting inflammatory mediated hypercoagulability [61]. Inflammatory cytokines promote endothelial damage, plaque formation and vascular smooth muscle hypertrophy [62]. Pro-inflammatory cytokines which activate endothelial and vascular smooth muscle cells include interleukin 1 (IL-1), interleukin 6 (IL-6), tumor necrosis factor (TNF) and vascular endothelial growth factor (VEGF), all of which are increased in active lupus, particularly lupus nephritis [62]. CD 40 ligand is increased in lymphocytes in SLE patients and CD40/CD40 ligand interactions can cause plaque rupture [62]. Other pro-inflammatory effects which promote atherosclerosis include chemokines and adhesion molecules. Inflammation induces thrombosis via endothelial cell dysfunction, tissue factor mediated activation of coagulation, platelet activation, impaired function of anticoagulants and suppressed fibrinolytic activity [55]. These effects all play a role in venous thrombosis in lupus.

3.2. Hypercoagulability

Multiple mechanisms contribute to hypercoagulability in SLE and thrombosis is due to multiple hits to the clotting system (the multiple-hit theory). It has been shown that elevations of procoagulant factors in combination have an additive effect on thrombosis [63]. In lupus, this includes lupus-specific and non- specific thrombogenic factors. As discussed in the previous paragraph, inflammation can cause elevations of non- lupus related procoagulant factors. Other factors contributing to hypercoagulability in SLE includes hyperhomocysteinemia [16, 64-66], and elevated plasminogen activator inhibitor-1 ( PAI-1) which decreases fibrinolytic activity [16, 55], as well as deficiencies of anticoagulant factors such as protein C, protein S and tissue plasminogen activator (tPA)which activates the fibrinolytic system [17, 55]. Lupus-specific procoagulant factors include antiphospholipid antibodies. Other lupus- specific factors include antibodies to factor XII [67], prothrombin [68], and annexin V [69-71]. Antiphospholipid antibodies encompass anticardiolipin antibodies, lupus anticoagulant (LAC), anti- β-2 glycoprotein-1 antibodies and false positive rapid plasma reagin (RPR) tests. These antibodies are prevalent in SLE and are pathogenically involved in the thromboses, accelerated atherosclerosis and fetal demise seen in this population. The prevalence of antiphospholipid antibodies are common in SLE with anticardiolipin antibodies occurring in 17-86% (vs.1-6% in the general population), and LAC noted in 15-30% (vs. 1-4% in the general population) [72]. Antiphospholipid antibodies cause vascular thrombosis and fetal loss in SLE [6, 72-76]. The LAC has a much stronger association with venous thrombosis than anticardiolipin antibodies where the thrombosis risk is directly related to increasing titers of these antibodies, particularly IGG anticardiolipin antibody titers of medium to high titer [5, 72, 77-78]. Low titers or transient presence of these antibodies generally do not cause thrombosis [72, 77]. These antibodies bind to phospholipids and protein epitopes found in cardiolipin, annexin V, prothrombin and β-2 glycoprotein-1 [24]. The antiphospholipid antibodies that recognize epitopes on β-2 glycoprotein-1 also bind serine proteases involved in hemostasis and fibrinolysis and hence promote thrombosis [79].

The prothrombotic effect of these antibodies occur via multiple mechanisms including platelet activation, endothelial cell activation with resultant up regulation of adhesion molecules and production of thromboxane A2, and stimulation of monocytes to make tissue factor, all of which promote clotting and vasoconstriction [80-81]. Tissue factor activates the extrinsic coagulation system while tissue plasminogen activator (tPA) activates fibrinolysis. Tissue factor pathway inhibitor activity is reduced in SLE and this is associated with increased levels of tissue factor and subsequent hypercoagulability [82]. Antiphospholipid antibodies bind to components of the coagulation cascade and activate the coagulation system leading to a procoagulant state along with decreased fibrinolysis via reducing activity of tPA [79-81].

Thrombosis and fetal loss in SLE share the same pathogenic pathways, and the mechanisms by which antiphospholipid antibodies cause fetal loss and thrombosis are complex and varied. They include target antigens in the coagulation, endothelial and immune systems summarized by Tripodi, et al, as follows [83]: 1.) inhibiting the protein C axis (targets- protein C, protein S, thrombomodulin, activated protein C ((APC) resistance, endothelial protein C receptor, 2.) increasing thrombin generation (targets- prothrombin, microparticles, tissue factor pathway inhibitor, protein Z, Factor XI, Factor XII, heparin cofactor II), 3.), disruption of the protective shield (target-annexin A5), 4.), decreasing fibrinolysis (targets-tPA, Annexin A2, β-2 glycoprotein-1 cleavage), 5.), altering complement levels (targets-C3, C5a, membrane attack complex, 6.) increasing platelet adhesion (target- Von Willebrand factor) and activation (targets-low density lipoprotein receptor 8, glycoprotein 1b, platelet factor 4, thromboxane A2, 7.) activation of endothelial cells (targets-Toll like receptor 4, tissue factor, prostacyclin, nitric oxide), 8.) activation of monocytes (target-tissue factor), 9.) activation of neutrophils (target-tissue factor), 10.) trophoblast activation (target-growth factor binding), 11.) angiogenesis (target- vascular endothelial growth factor, basic fibroblast growth factor), and 12.) increased atherosclerosis ( target-oxidized LDL) [83-84]. In fact, β-2 glycoprotein-1 binds to oxidized LDL to form atherogenic complexes as discussed above. These complexes have been detected in patients with autoimmune disease and promote macrophage uptake and subsequent atherosclerosis [85].

6.1. High risk clinical scenarios

Selecting SLE patients for a coagulation assessment is well established for those with a thrombosis or fetal loss but is not well defined for those who are at risk but have not yet had an event. Thus, patients who have had an event should clearly be selected for a coagulation work up. The guidelines for this are the Sydney Clinical Criteria for antiphospholipid antibody syndrome [118]. This includes any 1.) vascular ( arterial, venous or small vessel) thrombosis except for superficial thrombosis, 2.) Pregnancy morbidity (one or more unexplained deaths of a morphologically normal fetus at or beyond 10 weeks of gestation, one or more premature births of a morphologically normal neonate at or before 34 weeks gestation due to severe pre-eclampsia, eclampsia or severe placental insufficiency, or three or more unexplained consecutive spontaneous abortions before the 10^th week of gestation excluding anatomic or hormonal abnormalities or maternal/paternal chromosomal causes [118]. However, medical management should advance towards prevention of thrombosis and adverse fetal outcomes and one should evaluate high risk clinical settings as reason enough for a thrombophilic risk assessment. These would include pregnancy, pre-estrogen hormone therapy, pre-tamoxifen therapy, pre-organ transplant, pre- vascular procedure (such as coronary artery stenting), pulmonary hypertension, nephrotic syndrome, and chronic inflammatory setting, etc. In other words, one should assess the risk of thrombosis in the clinical situation and assess the multiple hits on that background. If the clinical setting is high risk for thrombophilia, one should do the coagulation profile discussed above along with genetics and platelet function studies. Then once this information is obtained, one should assess the number and degree of procoagulant hits and determines a treatment plan. This allows fine tuning of treatment while minimizing bleeding risk. Treatment strategies should be tailored to minimize bleeding complications, reduce recurrence of thrombosis, reduce intrauterine fetal demise, and simplify monitoring. Another complicating issue for thrombophilia management is that SLE patients frequently have mixed disorders with both prothrombotic and bleeding tendencies. Thus, when ordering a laboratory work up, an extensive battery of tests is needed to most accurately define the coagulation status. Unfortunately, since these factor abnormalities are independent of each other for the most part, there is no way to truncate the testing to an algorithm. However, as much information as possible should be obtained to determine the procoagulant and anticoagulant factors that would increase the risk for thrombosis and decide on optimal treatment.

6.2. Thromboprophylaxis for hypercoagulable states

For those SLE patients with hypercoagulability who have not had a thrombosis, treatment options for hypercoagulability in lupus consist of thromboprophylaxis for acute high risk situations, chronic prophylaxis for thrombosis prevention, and full dose anticoagulation therapy. Thromboprophylaxis for patients without any history of thrombosis and presence of antiphospholipid antibodies is controversial. However, evaluating thrombosis risk by assessing the multiple hits with a full thrombophilia profile would provide more support for deciding on intensity and type of thromboprophylactic treatment. High risk settings for which acute/short term thromboprophylaxis is indicated for antiphospholipid antibody positive patients would include surgery, ovarian stimulation or other short term hormonal therapy, pregnancy, vascular procedures, lupus flares, infections, and prolonged immobilization [119]. In pregnant SLE patients positive for lupus anticoagulant, it is recommended that low dose molecular weight or unfractionated heparin be used during pregnancy since neiher cross the placenta [19]. It is not recommended to treat thrombophilia in pregnant SLE patients who are positive for anti cardiolipin antibodies or lupus anticoagulant with corticosteroids since only anticoagulation has been shown to be of proven benefit in preventing thrombosis and fetal loss [19]. Corticosteroids have no benefit in preventing thrombotic complications in this setting and should only be used if any active lupus disease is present [19]. In addition, there is no role for prophylactic corticosteroids in patients who have no active lupus disease. Corticosteroids are relatively safe to use during pregnancy from a fetal standpoint, since the placenta metabolizes 90 % of non-flourinated corticosteroids. However, corticosteroids increase maternal complications such as hypertension and gestational diabetes [19]. Appropriate settings for chronic thromboprophylaxis would include those patients with persistent medium to high titers of anticardiolipin antibodies, those with triple antiphospholipid antibody positivity (+LAC, +anticardiolipin antibody, and +anti β-2 glycoprotein-1 antibody), and those with multiple hits on a background of high risk clinical settings such as, long term hormonal therapy, nephrotic syndrome, cardiovascular disease, and history of obstetric antiphospholipid antibody related events [119]. Both platelet function and the balance of procoagulant and anticoagulant factors should be assessed. Low dose aspirin therapy for prophylaxis is recommended by the Task Force at the 13^th International congress on Antiphospholipid Antibodies for these aforementioned situations along with lifestyle changes and is commonly accepted as standard treatment by many [120-121]. In SLE patients with antiphospholipid antibodies, the task force recommends both low dose aspirin and hydroxychloroquine [120]. In pregnant SLE patients with antiphospholipid antibodies only and no previous history of pregnancy loss, low dose aspirin is recommended [122]. If platelet hyperfunction is present, low dose aspirin is indicated. If hyperhomocysteinemia is present, folic acid and B complex should be used to lower homocysteine levels to below 10 µmoles/liter. However, if other high risk situations are identified in which thrombosis is likely, such as very low protein S activity, then anticoagulation with warfarin or low molecular weight heparin is indicated.

6.3. Treatment of hypercoagulable states with prior vascular thrombosis

For those SLE patients with antiphospholipid antibodies and hypercoagulablity who have had an arterial or venous thrombosis full dose anticoagulation is recommended with unfractionated or low molecular weight heparin (e.g., enoxaparin, dalteparin), fondaparinux (a synthetic of the minimal anti thrombin binding sequence of heparin), vitamin K antagonists (e.g., warfarin), direct thrombin inhibitors (e.g., dabigatran), or direct factor Xa inhibitors (e.g., rivaroxaban) [83, 123]. For acute arterial or venous thrombosis, treatment consists of an initial course of unfractionated or low molecular weight heparin followed by indefinite long term treatment with warfarin to keep the international normalized ration or INR between 2.0-3.0, heparin- type drugs, or more recently, one of the newer thrombin or factor Xa inhibitors. For arterial thrombosis (stroke, myocardial infarction), addition of antiplatelet agents (low dose aspirin, clopidogrel 75 mg) may be helpful, particularly if platelet hyperfunction is present [16, 83]. The disadvantage of using warfarin is the difficulty of maintaining the correct therapeutic INR range and frequency of INR testing that must be done, which is inconvenient to the patient. The disadvantage of the heparin- type drugs is that the patient must administer self-injections daily. The heparin- type drugs do have one advantage in that coagulation lab monitoring is unnecessary. The newer thrombin or factor Xa inhibitor drugs have the dual advantage of being an oral medication and not requiring laboratory anticoagulation monitoring. For catastrophic antiphospholipid antibody syndrome, which has a high mortality rate, treatment with plasma exchange, high dose corticosteroids, intavenous immunoglobulins and anticoagulation is recommended [124].

6.4. Pregnancy

SLE pregnancies should be considered high risk and must be managed in a multidisciplinary setting to address three problem areas: hypertensive pregnancy complications, lupus disease activity and thrombophilia [19]. Pregnancy is a hypercoagulable state which is worsened by the inflammation of active lupus disease. Pregnancy outcomes are worse with active lupus, particularly nephritis and active central nervous system (CNS) disease. Thus, it is recommended that patients with SLE have planned pregnancies and not attempt conception until the disease has been in remission for the preceeding 6 months [19]. Pregnancies that occur during active lupus or in patients with a history of severe major organ involvement such as nephritis or CNS disease are higher risk for poor fetal outcomes and maternal complication [19].Patients with mild disease generally have good pregnancy outcomes. For those pregnant SLE patients with no prior pregnancy loss or previous vascular thrombosis who have anti phospholipid antibodies, it is recommended that prophylaxis with low dose aspirin be used [125]. However if there are multiple hits such as triple antiphospholipid antibody positivity, low dose aspirin along with prophylactic doses of unfractionated or low molecular weight heparin be used [125-126]. Pregnancy- induced protein S deficiency can occur in these patients and when present should be treated with full dose anticoagulation with of unfractionated or low molecular weight heparin to prevent pregnancy loss. For pregnant SLE patients with antiphospholipid antibodies, other hypercoagulability factors, and a previous pregnancy loss or vascular thrombosis, treatment with full dose of unfractionated or low molecular weight heparin along with dose aspirin is recommended [127-128]. Generally anticoagulation is interrupted briefly during the delivery period and resumed and continued post- partum until the protein S levels return to normal and the other coagulation parameters correct. For antiphospholipid antibody persistence post-partum, continued treatment with daily aspirin is often used. Although these treatments for pregnant SLE patients are generally accepted, there is a lack of definitive data from clinical trials to support these accepted regimens [83].

6.5. Adjuvant treatments

Other adjuvant treatments summarized by Mehudi [81] for antiphosphoipid antibodies in SLE include: 1.) statins (which decrease antiphospholipid antibody mediated thrombosis and inflammation, 2.)Ritxuamib (which depletes CD 20 B lymphocyte cells involved in antiphospholipid antibodyl mediated disease), 3.) Hydroxychloroquine (which inhibit platelet aggregation of antiphospholipid- activated platelets by binding to GPIIbIIIa and by binding β-2 glycoprotein-1 or to target cells), 4.) Specific GPIIbIIIa inhibitors (e.g,abciximb) which bind and inactivate GPiia IIIb which is upregulated on antiphospholipid antibody activated platelets, 5.) inhibitors of tissue factor up regulation seen in antiphopholipid antibody activated endothelial cells (e.g., ACE inhibitors), 6.) anti TNF therapy to block high TNF levels seen in antiphospholipid antibody positive patients, 7.) blockage of receptors for β-2 glycoprotein-1 or antiphospholipid antibodies on target cells [81].