Drivers of the immunopathogenesis in systemic lupus erythematosus

Abstract

This review summarises a number of current insights into the pathogenesis of SLE. On the basis of the interaction of environmental factors within a predisposed host, a chronic autoimmune process gains function with a positive feed-forward loop between innate and adaptive immunity. A current focus of SLE pathogenesis is on the enhanced production of certain cytokines, such as type I interferons and BLyS/BAFF, suggesting continuous plasmacytoid dendritic and myeloid cell activity together with disturbances of B lineage cells (increased autoantibodies, including anti-dsDNA and plasmablasts, which correlate with SLE activity and memory B-cell abnormalities). Recent studies provided evidence that CD4⁺ and CD8⁺ T cells and B cells are hyporesponsive in SLE, likely reflecting their ‘post-activation status’. Data of enhanced protein tyrosine phosphatase activity of lymphocytes in SLE require consideration if they represent a disease characteristic. Better understanding of the chronic autoimmune phase is needed in addition to those phases during flares and will permit improved treatment of SLE.

Introduction

Systemic lupus erythematosus (SLE) is a prototypic, systemic autoimmune disease with a wide clinical spectrum that ranges from typical organ manifestations, such as joints, skin and kidneys to infrequent manifestations, e.g. shrinking lung syndrome [1]. The clinical heterogeneity of SLE is accompanied by complex disturbances in the immune system, with the hallmark of characteristic autoantibodies and an enhanced type I interferon (IFN) and B-cell activating factor (BAFF)/B lymphocyte stimulator (BlyS) system [2]. A number of alterations in SLE have been targeted in clinical trials in the last decade, but so far, only belimumab obtained FDA and EMA approval [3]. Owing to the various abnormalities in the innate and adaptive immune system, unspecific inhibition with NSAID, glucocorticoids, antimalarials, methotrexate, cyclophosphamide or mycophenolate mofetil remains the first choice to control lupus activity. Prior successes and failures of certain targeted therapies in SLE, however, have permitted deeper insights into the mechanisms of the disease. For example, as IFNs have pleiotropic effects on the immune system, anifrolumab, a monoclonal antibody against the IFN receptor, holds promise as another targeted immune therapy and is in advanced development followed by many other [3].

SLE pathogenesis can only be incompletely explained by a single cause, and it is believed that a complex interplay of environmental factors (e.g. sex hormones, UV light and smoking), genetic and epigenetic factors contributes to SLE pathology. The idea of a complex interplay of various factors led to the concept of an ‘exposome’ affecting healthy but genetically predisposed individuals [4] (Fig. 1). This concept has been addressed in Gene, Environment Association Studies [5] that will provide further insights in the interaction between host and environment.

Environmental factors, such as exposure to silicates, smoking, UV light and certain medications; vitamin D deficiency; viral infections; and oestrogens are considered potential risk factors in the development of lupus. The National Institute of Environmental Health Science Expert Panel reviewed and evaluated environmental influences on autoimmunity and found evidence for silica exposure and a likely association for smoking as contributors to SLE development [6].

In the last decade, vitamin D with its broad effects on the immune system has been studied extensively in autoimmune diseases [7]. Sun exposure is important for vitamin D production. As 80% of patients with SLE show a photosensitivity and consequently protect themself from UV-light exposure, vitamin D deficiency was found in most SLE patients [8]. As such, the role of vitamin D and SLE is complex. Few studies investigated vitamin D deficiency as a risk factor for the development of SLE, and up to now there is no clear answer to this question. In addition, an association between vitamin D receptor single-nucleotide polymorphisms and a higher long-term cumulative damage in SLE has been reported [9]. In this context, 25-OH vitamin D levels were inversely related to lupus disease activity [10], [11], and therefore, a contribution to lupus pathology could not be ruled out. Supplementation of vitamin D is safe in SLE, but the therapeutic effect on disease activity remains to be delineated because studies including randomised controlled trials supplementing vitamin D in SLE arrived at inconclusive results [7], [12], [13]. However, the use of vitamin D to protect against glucocorticoid-related osteoporosis may superimpose its effect on lupus activity and make it difficult to discern its real impact in clinical grounds.

Although frequently taken for granted, it is still a matter of debate whether UV-light exposure is a risk factor for the induction of SLE [14]. On the one hand, UV-light exposure in SLE is known to aggravate pre-existing skin manifestation and can cause severe lupus flares with a plausible pathogenic mechanism: UV-light exposure induces increased apoptosis of keratinocytes, followed by a prolonged exposure of self-antigens because of known clearance defects that subsequently causes precipitation of immune complexes (ICs) in the skin (‘lupus band’) and results in the initiation of an inflammation [2], [14]. Sun exposure, on the other hand, leads to higher serum concentration of vitamin D and thus might be protective [14]. It is also possible that the timing of the UV-light exposure in the context of other environmental or individual predispositions is important, as known from acquired neonatal lupus syndrome, in which skin manifestation develops in the majority of cases six weeks postpartum following UV-light exposure [15].

Bacterial and viral infections, such as EBV, CMV, parvovirus B19, human endogenous retroviruses and others, have been implicated in the development of SLE where these agents apparently ‘kick-start’ immune activation and result in chronic inflammation [16]. One of the best understood mechanisms of ongoing autoimmunity is molecular mimicry [17], [18]: viral infection can trigger B cells to produce antibodies that recognise the viral antigen, but can also cross-react with autoantigens, causing a lasting autoimmune response [2]. It has been shown that the Epstein Barr Virus Antigen 1 (EBNA-1) contains regions that are homologous to sequences of self-proteins such as Ro60 kDa and snRNP [17]. In SLE, McClain and colleagues could detect anti-EBNA-1 prior to the detection of anti-Ro. This suggests that EBV infection started with anti-EBNA-1 and due to similarities to Ro antigen, ‘switched’ to anti-Ro antibody production (probably after Ro60 kDa exposure due to apoptosis induced by UV light). This study demonstrated that anti-Ro antibodies cross-react with EBNA-1. Similar findings were also reported for other autoantigens (e.g. Sm and La) and support the idea that certain infections can induce the development of autoimmunity in predisposed individuals.

Most SLE patients, however, cannot be traced back to a single infection, and thus, disturbed general immune response mechanisms, such as the activation of the IFN system through toll-like receptors (TLRs) [19] and cytosolic sensors, are considered to be involved in the induction process. Fever in early SLE has gained recent awareness [20], reflecting systemic immune activation (not essentially an infection).

Cells are equipped with membrane-bound receptors and cytosolic sensors that allow the immune system to detect and fight infections. Membrane-bound pattern recognition receptors such as TLRs detect pathogen-associated molecular patterns such as viral, bacterial and fungal proteins in the extracellular department and endosome [21], [19].

The early recognition by certain individuals is crucial for the subsequent dendritic cell (DC) response and type of T-cell response (TH1, 2, 17, Treg) and involvement of B cells [2]. In SLE, the activation of TLR7 and 9 in plasmacytoid dendritic cells (pDC) through RNA- and DNA-containing antigens, antibodies and ICs lead to an inflammatory cascade, mainly with a pronounced IFN type I response [22], [19]. Bacterial infections are discussed as contributors of SLE pathology through similar mechanisms. Endotoxins, such as lipopolysaccharides (LPS) of gram-negative bacteria, can lead to the secretion of polyreactive antibodies in mice and various cytokines because of TLR4 activation [22], [23], [24]. In SLE, elevated LPS serum levels were consistent with a significant overlap in genes induced by LPS/endotoxin together with the known IFN type I signature [25]. Interestingly, TLR4 was recently described to be upregulated in skin and renal biopsies of SLE patients, and the level of TLR4 expression was correlated with lupus disease activity [26], [27]. Thus, TLR4 activation has been found in affected organs of SLE patients [25].

In the last decade, other antiviral mechanisms next to cell-surface-dependent TLR signalling were discovered, which can also elicit an IFN type I response. After virus invasion into the cell, cytosolic sensors detect foreign RNA molecules and respond mainly with IFN type I or III production. Different cytosolic sensors, such as RIG-I-like receptors, are known, which detect viral RNA in the cytosol, cyclic GMP-AMP synthase, IFN gamma-inducible protein 16 and DNA-dependent activator of IFN-regulatory factors [21]. However, these sensors can detect other ligands independently of infections and lead to a comprehensive immune reaction mainly through the stimulation of IFN genes (STING) [21]. Very recently, a gain-of-function mutation in STING, causing familial chilblain lupus with an activation of the IFN type I system, has been reported [28]. Interestingly, STING-deficient autoimmune-prone mice showed an exacerbation of disease by lack of expression of negative regulators controlling immune activation and hyperresponsiveness to TLR agonists [29]. It has been concluded that TLR and STING pathways might be cross-regulated [30]. The role of STING in SLE is studied less, but data implicate STING as a regulator of immune responses [31]; however, how it is involved in lupus warrants further investigations.

Family studies could show that monozygotic twins have a ten-fold higher risk to develop SLE than dizygotic twins [32] and a clear aggregation of SLE in families [33]. This together with the findings in large-scale genome-wide association studies (GWAS) [34], [35], [36], [37] implicate a strong genetic background for SLE. More than 80 genetic risk loci involved in different immunological pathways affecting the innate and adaptive immune system such as IC processing, clearance of cellular debris including ICs, type I IFN and TLR signalling as well as lymphocyte activation are associated with SLE risk (some examples are shown in Fig. 1) [38]. Noteworthy, more than 50% of the lupus susceptibility genes are linked to the IFN system [39], [40]. In addition to this pathway, gene loci involved in NFkB signalling, DNA degradation, apoptosis, phagocytosis, neutrophil, and monocyte/macrophage function and signalling have been identified (reviewed [41]). The most pronounced association, however, was found for HLA-DR2/DR3, followed by BLK and PTPN22 involved in T- and especially B-cell activation and therefore adaptive immunity. Current concepts suggest that these genetic abnormalities provide a susceptibility basis for the induction and chronic phase of SLE related to a positive feed-forward loop with a connecting point of antigen presentation between innate and adaptive immunity [2].

As a substantial proportion of heritability in SLE cannot be explained by GWAS data alone [38], epigenetic mechanisms, such as DNA methylation, post-translational histone modification and microRNAs (miRNAs), regulating the gene expression appear to play a role in lupus pathogenesis. Each cell type might have a specific and dynamic epigenetic profile [42]. The interaction between host and the presumed environmental factors resurfaces and suggests that they may directly interact on this level.

Recent data provided evidence that most methylation sites in lupus patients are hypomethylated and subsequently permit enhanced transcription. In this context, striking hypomethylation of loci associated with type I IFN signalling was found in monocytes and B- and T-cells from lupus patients, suggesting that these cells might be hypersensitive to IFN [43]. Apart from IFN activation, the methylation status of CD4⁺ T cells in SLE has been found to undergo an epigenetic shift towards an inflammatory status [44]. Recently, Chen and colleagues also reported alterations in DNA methylation in lupus [45]. However, the nature of epigenetic modulations during the disease course, especially related to disease activity, remains to be delineated [43], [44].

Environmental, dietary and lifestyle factors influence epigenetic modulation [46], [47], [48] and may explain why monozygotic twins develop different epigenetic profiles when they become older [49]. The importance of environmental factors and their impact on the epigenome became apparent in a transgenic lupus model with a transmethylation micronutrients diet that altered DNA methylation status [50], indicating the capacity of diet on epigenetic modulation in SLE.

Smoking is considered a key environmental factor in SLE pathogenesis [2], [6] and has also been shown to affect DNA methylation with long-lasting imprints (for years) that continue even after smoking cessation [48]. Procainamide and hydralazine are medications that can result in drug-induced SLE. Both are able to inhibit DNA methylation in T cells and induce autoimmunity [51]. Notably, a previous study showed that mycophenolic acid used in treating lupus nephritis patients is capable of modulating the epigenetic status through histone modification [52]. Currently, clinical studies in SLE are evaluating the impact of histone deacetylase inhibitors (vorinostat and panobinostat target class I, II and IV HDACs; romidepsin is a selective class I HDAC) [3].

Another possibility of epigenetic modulation is interference with miRNA. These are noncoding RNAs involved in the regulation of about 90% of protein-coding genes [53]. The complex mode of action of miRNA is not fully understood, but one potential mechanism is translational repression of miRNA, leading to a downregulation of gene expression [54]. In autoimmune diseases, many different miRNAs have been reported to be up- or downregulated [53]. Although we are just beginning to understand the importance of certain miRNAs in SLE, miRNA(miR)-30a in B cells has been shown to decrease the expression of Lyn as central protein tyrosine kinase of B-cell receptor signalling. The study arrived at the hypothesis that low levels of Lyn expression in SLE are due to increased expression of miR-30a, which may facilitate B-cell proliferation and antibody production [55]. Further studies unravelling the complexity of miRNAs involved in regulating normal immunity and SLE are warranted.