The Latest in SLE-Accelerated Atherosclerosis: Related Mechanisms Inform Assessment and Therapy

Abstract

Purpose of Review.

Accelerated atherosclerosis is a significant co-morbidity and the leading cause of death for patients with systemic lupus erythematosus (SLE). It is now apparent that SLE-accelerated atherosclerosis is not driven solely by traditional cardiovascular risk factors, adding complexity to disease characterization and mechanistic understanding. In this review, we will summarize new insights into SLE-accelerated atherosclerosis evaluation, treatment, and mechanism.

Recent Findings.

Recent work highlights the need to incorporate inflammatory biomarkers into cardiovascular disease (CVD) risk assessments. This is especially true for SLE patients, where mechanisms of immune dysfunction likely drive CVD progression. There is new evidence that commonly prescribed SLE therapeutics hinder atherosclerosis development. This effect is achieved both by reducing SLE-associated inflammation and by directly improving measures of atherosclerosis, emphasizing the interconnected mechanisms of the two conditions.

Summary.

SLE-accelerated atherosclerosis is most likely the consequence of chronic autoimmune inflammation. Therefore, diligent management of atherosclerosis requires assessment of SLE disease activity as well as traditional cardiovascular risk factors. This supports why many of the therapeutics classically used to control SLE also modulate atherosclerosis development. Greater understanding of the mechanisms underlying this condition will allow for the development of more targeted therapeutics and improved outcomes for SLE patients.

Introduction

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease affecting over 1.5 million Americans and at least 5 million individuals worldwide. Patients are predominantly female, and are two to three times more likely to be women of color. SLE is characterized by autoantibody producing B cells and dysfunctional CD4⁺ T cells, and can result in end organ damage to the kidneys, joints, and cardiovascular system [1]. Life expectancy for SLE patients has improved but remains lower than that of the general population [2], and the leading cause of death among SLE patients is cardiovascular disease (CVD) driven by accelerated atherosclerosis [3–7]. It is estimated that women with SLE between the ages of 35 and 44 have a 50-fold increased risk of myocardial infarction compared with age- and gender-matched controls [8]. Even if we assume that this estimate is inflated due to the fact that healthy, pre-menopausal women are typically protected from CVD development [9], post-menopausal women with SLE are still five times more likely to develop atherosclerosis than healthy, age-matched controls [10]. Statin therapy has been shown to provide less cardiovascular protection in SLE patients compared to the general population [11], suggesting the SLE-accelerated atherosclerosis is not entirely lipid dependent. In this review we highlight recent findings regarding mechanisms driving SLE-accelerated atherosclerosis as well as the best techniques to evaluate and treat this disease.

Disease Risk Assessment

Atherosclerosis is associated with a number of risk factors and the likelihood of a related adverse event (coronary heart disease, stroke, peripheral artery disease, or heart failure) occurring in the long term in the general population is often determined using composite measurements such as the Framingham Risk Score [12] or Systematic Coronary Risk Evaluation (SCORE) [13]. The Framingham Risk Score is a 10-year predictive measurement that uses sex, age, total and high-density lipoprotein (HDL) cholesterol levels, systolic blood pressure, hypertension, smoking, and diabetes status to determine an individual’s CVD risk [12]. SCORE utilizes many of the same metrics, but also incorporates geography as a variable to consider whether an individual’s country as a whole has rising or falling CVD rates [13]. In SLE patients these scores may inaccurately predict an individual’s true risk of CVD [14,15]. This is likely due to several factors; the lack of consideration of inflammatory variables that are increased in SLE, the heterogeneity of the disease, and our incomplete understanding of disease mechanism. Therefore, improved methods for predicting risk SLE are required to ensure appropriate management of CVD and other life-threatening co-morbidities.

Techniques for Improved Risk Assessment.

One strategy for adjusting the Framingham risk score in SLE has been to simply multiply the traditional score by a factor of two [16]. A similar technique was successfully applied to rheumatoid arthritis patients, who also experience increased rates of CVD related death [17,18]. Unfortunately, given the heterogeneous nature of SLE, a blanket correction factor may oversimplify the contribution of autoimmune factors to CVD risk. Therefore, it became necessary to develop an SLE-centered cardiovascular risk score which uses traditional cardiovascular risk factors like age, gender, systolic blood pressure, cholesterol, smoking, and diabetes, but also SLE-specific risk factors. Variables such as time since SLE diagnosis, corticosteroid and hydroxychloroquine use, C3 and C4 levels, anti-double stranded DNA (dsDNA) titer, proteinuria, disease activity score (SELENA-SLEDAI score), estimated glomerular filtration rate, and history of lupus anticoagulant and anticardiolipin were evaluated. In the subjects of this study, only disease activity score, C3 level, and lupus anticoagulant titer were predictive of cardiovascular outcomes [19**]. Using the SLE-specific risk score incorporating these select variables, Petri et al. determined that some SLE patients were in appropriately assessed by the traditional Framingham risk score while others, particularly those with higher SLEDAI scores, had their ten-year risk underestimated by as much as a factor of 10 [19**]. In agreement with these findings, a separate seven-year surveillance study found no difference in the progression of subclinical atherosclerosis between the control group and SLE patients with mild, well controlled disease [20], highlighting the need to use SLE-specific criteria and disease activity level in CVD risk assessment.

Inaccurate CVD risk assessment is especially prevalent in young SLE patients (≤45 years). These patients are not likely to experience adverse cardiovascular events within 10 years (the range in which classical scoring methods are designed to predict), but still have elevated risk compared to their healthy, age-matched counterparts, including a 50-fold increase in the risk of myocardial infarction [8,21]. Clinical studies have shown that coronary artery calcification (CAC), a known predictor of CVD mortality [22], is increased in SLE patients anywhere from 2.8 to 4 times compared to controls and may be an accurate predictor of outcomes [23,24]. These studies were performed using older, mostly white cohorts (72% and 65%, respectively), perhaps not accurately representing the predominately non-white SLE patient population. Gartshteyn et al. studied SLE patients aged 18–65, who were primarily African American and Hispanic, and had no clinical CVD. They found that CAC severity increased with age in patients, and that CAC was significantly more prevalent in young SLE patients compared to age-matched controls, 32% vs 9.6%, respectively [25]. In a separate study, a related, though independent condition, aorta calcification (AC), was found to occur earlier, at a higher incidence, and in a wider range of ages than CAC in SLE patients highlighting its importance in predicting atherosclerosis development [26].

Collectively, studies indicate risk assessment for SLE-accelerated atherosclerosis needs to be distinct from that of standard atherosclerosis. By integrating more inflammatory variables into predictive equations and/or adopting regular monitoring of CAC and AC, SLE patients will experience improved care with more timely therapeutic interventions.

Biomarkers.

In addition to improved techniques to assess atherosclerosis development in SLE patients, there has also been progress in terms of new biomarkers to identify potentially high CVD risk patients. Biomarkers associated with SLE-accelerated atherosclerosis include proinflammatory HDL (piHDL), increased circulating leptin, elevated homocysteine, and the presence of soluble tumor necrosis factor-like weak inducer of apoptosis (sTWEAK) [27].

Soluble CD163.

A biomarker that was previously associated with increased glomerular inflammation in SLE patients is the soluble form of the scavenger receptor CD163 (sCD163). sCD163 was found at high levels in both serum and urine of SLE patients with more CD163⁺ macrophage glomerular infiltrate and severe inflammation (r = 0.635) as part of lupus nephritis [28]. CD163 expression on macrophages has typically been thought of as an indication of alternative activation and an anti-inflammatory phenotype [29]. However, recent findings demonstrated CD163⁺ macrophages actually promote atherosclerotic plaque progression in both humans and mice via a CD163/HIF1α/VEGF-A pathway [30]. The involvement of CD163⁺ macrophages in both SLE and atherosclerosis suggest expression of this scavenger receptor may be an underlying mechanism of SLE-accelerated atherosclerosis and that its soluble form could be a biomarker of the disease. This was tested in one study of 63 SLE patients whose carotid atherosclerotic plaque was measured by ultrasound and serum sCD163 levels monitored both at baseline of the study and during follow-up. Results indicated that serum sCD163 is elevated in SLE patients compared to controls, and is further increased in patients with carotid plaque. Additionally, higher levels of sCD163 correlated with the development of new carotid plaque over time and positively associated with subclinical atherosclerosis in SLE independent of classic cardiovascular risk factors [31*]. These results indicate sCD163 is a promising biomarker of SLE-accelerated atherosclerosis.

FcγRIIA Polymorphism.

Fcγ receptors (FcγRs) are expressed on antigen presenting cells like macrophages, dendritic cells (DCs), and B cells, and bind to the Fc domain of IgG antibodies. Depending on whether the FcγR is activating or inhibitory, it will promote pro- or anti-inflammatory responses [32]. In mouse models, activating FcγRs are proatherogenic [33]. Functional polymorphism in the activating Fcγ receptor IIA (FcγRIIA) have previously been associated with SLE and lupus nephritis in humans [34,35]. In heparin-induced thrombocytopenia, a functional polymorphism (single amino acid substitution H131R) in FcγRIIA caused a hyperactive platelet phenotype characterized by increased expression of P-selectin and CD40 ligand, greater binding to annexin V, and increased formation of platelet-leukocyte complexes [36–40]. A similar phenotype was described in SLE where hyperactive platelets promote vascular pathogenesis by activating endothelial cells [41]. Based on these works, Clancy et al. investigated whether the H131R FcγRIIA polymorphism impacted cardiovascular outcomes in SLE patients. The H131R allelic variant was found in a similar proportion (~40%) of both the SLE patient and healthy control populations. When SLE patients were assessed by carotid ultrasound, 58% of SLE patients with the allelic variant had carotid plaque compared to only 25% of SLE patients with the ancestral variant. Yet, the FcγRIIA allelic variant did not associate with carotid plaque in heathy control subjects [42*]. This suggests the H131R functional polymorphism confers CVD risk specifically in the context of SLE and could potentially acts as a genetic CVD biomarker in patients.

Appropriate CVD risk assessment for the SLE patient population has long been needed. The techniques and biomarkers outlined above have the potential to improve CVD risk detection in patients, hopefully resulting in prompt therapeutic intervention and prevention of devastating cardiovascular outcomes.

Mechanism and Treatment

Treatments for SLE are limited in comparison to other chronic illnesses and therapeutic advancement has been relatively slow, in part because of lack of understanding of disease mechanisms. Unfortunately, the impact of some of the most common SLE therapeutics specifically on SLE-accelerated atherosclerosis has been understudied. The following highlights recent work that has contributed to determining the impact of SLE treatments on atherosclerosis and the mechanism behind SLE-accelerated atherosclerosis. Table 1 summarizes the treatments discussed and their effects.

Table 1.

Review of SLE-Accelerated Atherosclerosis Treatments

Treatment	Effect in SLE-Accelerated Atherosclerosis	References
BAFF Inhibition	Improves SLE responder index in patients with ongoing B cell dysfunction Reduces circulating anti-dsDNA antibodies in mice Inhibits antibody and complement deposition in the kidney in mice Atheroprotective in animals with low (<5mmol/L) cholesterol and atherogenic in animals with high cholesterol	[52,54**]
Hydroxychloroquine	Reduces interferon-α production Lessens aortic stiffness in SLE patients Corrects lipoprotein profile (increases HDL and lowers cholesterol, LDL, VLDL, triglycerides, and cylomicrons) in SLE patients Improves glycemic control in non-diabetic women with SLE Reduces the risk of all thrombovascular events in SLE patients SLE patients with carotid plaque are less likely to be using HCQ than those with no plaque (63% vs. 82.3%) Long term HCQ treatment in ApoE−/− mice with pristane-induced SLE reduces atherosclerotic lesion size, anti-dsDNA antibodies, total leukocyte numbers, macrophages, and DCs, and increases lymohocytes HDL from SLE patients is more functional following 12 weeks of treatment with HCQ.	[58–67*,70*]
Low-dose IL-2	IL-2 treatment reduces renal inflammation and activation of kidney-infiltrating CD4+ T cells in a mouse model of lupus nephritis Targeted IL-2 treatment in ApoE−/− mice reduces the size of pre-established atherosclerotic lesions Currently under study in clinical trials to determine its effectiveness in SLE and atherosclerosis	[96*,97–99]
Mycophenolate	Treatment with MMF for 12 weeks improves HDL function in SLE patients Treatment with MMF for 12 weeks improves CVD biomarkers like sTWEAK in SLE patients Reduces atherosclerosis in Ldlr−/− B6.Sle1.2.3 bone marrow chimeras and limits recruitment of CD4+ T cells to atherosclerotic lesions	[70*,71]

VLDL = very low-density lipoprotein

In SLE, B cells are hyperactive leading to the production of autoantibodies, inappropriate T cell activation and DC recruitment, and inhibition of regulatory T cells (T_regs) [43]. In the context of atherosclerosis, B cells were initially thought to be protective [44,45], but recent work has demonstrated B2 B cells and production of IgG are proatherogenic [46,47]. The cytokine B-cell activating factor (BAFF) is required for the maturation and survival of B2 B cells and plays a major role in SLE [48–51]. Benlysta^®, a B-cell-depleting monoclonal antibody that targets BAFF, was the first new SLE therapy in 50 years [52]. Therefore, inhibition of BAFF signaling seemed an attractive target for treating SLE-accelerated atherosclerosis. To test this, Saidoune et al. crossed atherosclerosis-susceptible ApoE^−/− to Qa-1 knock-in mice (D227K) which have an amino acid mutation that hinders CD8⁺ T_regs and develop an SLE-like phenotype [53]. Interestingly, when ApoE^−/− D227K mice were treated with a BAFF neutralizing antibody, SLE disease activity was impaired but atherosclerosis severity was only improved in animals with low cholesterol levels (<5mmol/L). This unexpected finding was the result of BAFF signaling through transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) on macrophages. Inhibiting BAFF-TACI signaling on macrophages promoted foam cell formation in high lipid environments and was largely atherogenic, overriding the atheroprotective effect of disrupting BAFF-BAFF receptor (BAFFR) on B cells in SLE [54**]. A separate study demonstrated BAFFR is also expressed on circulating endothelial progenitor cells (EPCs) [55*], a population of cells that are meant to maintain vascular homeostasis but that is reduced in SLE patients [56,57]. BAFF signaling on EPCs induced apoptosis suggesting inhibition of BAFF signaling could be beneficial in SLE-accelerated atherosclerosis [55*]. Overall, BAFF signaling seems to be a critical mechanism of SLE-accelerated atherosclerosis, but requires further study due to its complex interaction with multiple cell types.

Hydroxychloroquine (HCQ) is one of the most widely used medications for autoimmune diseases and is regularly prescribed for SLE [58]. It is known to elicit atheroprotective effects like improving lipid profiles [59–61] and glycemic control [62], lessening aortic stiffness [63], and reducing thrombotic events [64,65]. There is a documented negative correlation between HCQ treatment and atherosclerosis in SLE [66], but the mechanism has not been elucidated. By studying pristane-induced SLE in atherosclerosis-susceptible ApoE^−/− mice, Liu et al. showed long term HCQ treatment reduced atherosclerosis largely through reversal of SLE-associated autoinflammation. Specifically, HCQ treatment lowered anti-dsDNA antibodies and reduced total leukocytes, macrophages, and DCs [67*]. Pristane-induced SLE is known to reduce lymphocyte numbers in the spleen by triggering their apoptosis [68]. This phenotype was also reversed by HCQ treatment in both the aorta and the spleen which authors hypothesize increased the numbers of atheroprotective T_regs [67*]. This mechanistic study provides some insight to how HCQ may also be beneficial in treating SLE-accelerated atherosclerosis.

Although typically considered to be atheroprotective due to its anti-inflammatory properties, HDL in SLE patients can become piHDL. SLE patients with carotid artery plaque are more likely to have piHDL than SLE patients without plaque [69], making it an interesting biomarker for SLE-accelerated atherosclerosis [27]. Little is known about the effects of regularly prescribed SLE disease-modifying therapies like mycophenolate (MMF), azathioprine (AZA), and HCQ on piHDL. In a recent observational study, SLE patients starting a new disease-modifying therapy (MMF, AZA, or HCQ) had plasma collected and disease activity measured at baseline, six weeks post initiation of therapy, and 12 weeks post initiation of therapy [70*]. MMF and HCQ significantly improved HDL function over the course of the 12-week treatment, though not to normal levels, while AZA had no effect on HDL function. MMF also significantly improved other biomarkers associated with plaque and intima-media thickness progression in SLE, suggesting MMF could have potential to treat both SLE associated glomerulonephritis and accelerated atherosclerosis [70*].

Findings from our own work demonstrate MMF treatment reduces atherosclerosis in Ldlr^−/− B6.Sle1.2.3 bone marrow chimeric mice (which spontaneously develop SLE and are atherosclerosis prone) and inhibits CD4⁺ T cell activation and recruitment to atherosclerotic plaques [71]. In general, T cells are known to significantly contribute to the development of atherosclerosis [72,73], with different subsets playing unique roles. For instance, Th1 cells are widely accepted to be proatherogenic [74–76] while functional T_regs are atheroprotective [77,78]. T cells are also critical in SLE where they are long-lived, hyperactive, and produce proinflammatory cytokines like IFN-γ and IL-17 [79–82]. Much of the work from our group has shown the importance of T cells in mouse models of SLE-accelerated atherosclerosis [83–85], and our findings are in agreement with results from human studies [86,87].

Previous results from Laurence Morel’s group identified the lupus susceptibility gene, Pbx1d, as overexpressed in CD4⁺ T cells from SLE patients and using mouse models of SLE described its role in expanding follicular helper T cells (Tfh) and impairing T_reg homeostasis [88,89]. Given the importance of CD4⁺ T cells in atherosclerosis, recent work explored the effect of Pbx1d transgenic CD4⁺ T cells in atherosclerosis in a Ldlr^−/− bone marrow chimeras. Pbx1d overexpression in CD4⁺ T cells resulted in thicker arterial walls and larger necrotic cores within lesions, which is thought to be an indication of plaque instability and severity. Recipients of Pbx1d transgenic bone marrow had expanded Tfh cells and impaired T_regs, that was increased with western diet feeding [90**], indicating a lupus susceptibility allele can drive atherosclerosis, while at the same time dyslipidemia can enhance autoimmune phenotypes.

Impaired IL-2 signaling in T cells due to decreased IL-2 production in SLE and its contribution to disease pathogenesis is well known [91–94]. Recent findings identify a regulatory subunit (PPP2R2D) of protein phosphatase 2A (an enzyme which is increased in T cells from SLE patients and leads to decreased IL-2 production) that limits the accessibility of the IL-2 gene and other transcription factors important to IL-2 expression via chromatin remodeling. PPP2R2D is increased in T cells from SLE patients and PPP2R2D deficiency in T cells prevents autoimmunity in an imiquimod-induced lupus-like mouse model [95*]. A separate group demonstrated that treating (NZB x NZW) F1 SLE mice with IL-2 reduced renal inflammation and lessened the activity of kidney-infiltrating CD4⁺ T cells [96*]. Interestingly, IL-2 delivered specifically to atherosclerotic lesions through fusion to fibronectin targeting antibody reduced plaque size by activating and expanding T_regs [97]. This suggests IL-2 supplementation could be beneficial in both SLE and atherosclerosis, a hypothesis which is currently being tested in low-dose IL-2 treatment clinical trials [98,99].

In order to determine how the risk of atherosclerotic vascular events (AVE) in SLE patients has changed over time in response to new therapeutic strategies, Urowitz et al. compared two cohorts of newly identified (enrolled in the study within 12 months of diagnosis) SLE patients. Cohort 1 included patients who entered the University of Toronto Lupus Clinic between 1975 and 1987 and were then followed through 1992, while Cohort 2 included patients entering between 1999 and 2011 and were followed through 2016. Over the course of study SLE disease activity, treatment regimens, blood pressure, cholesterol, blood sugar, smoking status, and AVE were monitored. Patients in Cohort 2 had significantly less AVE than Cohort 1 over the 17-year study period. Additionally, patients in Cohort 2 had lower SLE disease burdens, smoked less and had longer periods of time with normal blood pressure, cholesterol, and glucose than patients in Cohort 1, demonstrating interventions for both SLE and CVD have improved over time [100*].

Understanding how current treatment regimens effect SLE-associated atherosclerosis is an important goal which will provide near immediate benefit to individuals living with the disease. Of equal importance, is the more long-term goal of elucidating the mechanisms which drive SLE-accelerated atherosclerosis. Though findings made in this area may not immediately translate to the clinic, they will eventually lead to more targeted therapeutics for patients. These works suggest a highly interconnected, positive feedback loop between progression of SLE and atherosclerosis, and may provide further insight as to why therapeutics designed to treat one impact the other.

Conclusion.

SLE-accelerated atherosclerosis is a complex disease in which immunological and cardiovascular mechanisms are intertwined to drive pathogenesis. While recent work has begun to elucidate this interconnected relationship, further study is needed to gain a complete mechanistic understanding of the disease and to identify potential therapeutic targets. In the meantime, the field has grown in its understanding of how to best evaluate current SLE patients for their cardiovascular risk, integrating traditional cardiovascular risk factors with information about inflammatory disease activity. This improvement in disease risk assessment will lead to better monitoring of high-risk patients as well as improved treatment regimens.

Key Points.

• Accelerated atherosclerosis is the leading cause of death among SLE patients.

• CVD risk assessment in SLE patients must integrate inflammatory as well as traditional cardiovascular risk factors in order to get an accurate assessment.

• Many of the immune mechanisms that drive SLE also promote atherosclerosis.

• Many of the existing SLE treatments hold promise for improving SLE-accelerated atherosclerosis.