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Keywords:

  • BLys;
  • IFN;
  • IL-6;
  • IL-17;
  • IL-18;
  • TNF;
  • systemic lupus erythematosus

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

The pathogenesis of systemic lupus erythematosus (SLE) entails a complex interaction between the different arms of the immune system. While autoantibodies production and immune complex deposition are cornered as hallmark features of SLE, there is growing evidence to propose the pathogenic role of cytokines in this disease. Examples of these cytokines include BLys, interleukin-6, interleukin-17, interleukin-18, type I interferons and tumour necrosis factor alpha. These cytokines all assume pivotal functions to orchestrate the differentiation, maturation and activation of various cell types, which would mediate local inflammatory process and tissue injury. The knowledge on these cytokines not only fosters our understanding of the disease, but also provides insights in devising biomarkers and targeted therapies. In this review, we focus on cytokines which have substantial pathogenic significance and also highlight the possible clinical applications of these cytokines.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

Systemic lupus erythematosus (SLE) is an autoimmune disorder which has multi-organ involvements. The pathogenesis of SLE, which involves the various facets of the immune system, is complex and perplexing. The orthodox understanding of this disease encompasses autoantibodies production and immune complex deposition, which will give rise to the subsequent autoimmune phenomenon. However, mounting evidence has emerged to suggest the crucial role of various cytokines in the pathogenesis of SLE. These cytokines are soluble factors which are vibrant mediators for the differentiation, maturation and activation of the various immune cells. The consequence of such would be an immune dysregulation followed by local inflammatory processes and tissue damage. The understanding of these cytokines not only enhances our perception of SLE, but also instills novel ideas for the design of biomarkers and therapeutic agents. In this review, we highlight the cytokines which exert significant effects on the pathogenesis of SLE and their clinical applications.

Interleukin 6 (IL-6)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

IL-6 is one of the first cytokines studied in the pathogenesis of SLE due to its close link with B lymphocytes. This cytokine is primarily secreted by the monocytes, fibroblasts and endothelial cells although the T- and B- lymphocytes also contribute to its production. It has an elaborated interaction with other cytokines as its levels is boosted by IL-1, IL-2 and tumour necrosis factor-α (TNF-α) but diminished by IL-4, IL-10 and IL-13. One of the crucial effects of IL-6 is to promote B lymphocyte maturation into plasma cells and enhance immunoglobulin production.[1, 2] Its pleiotropic actions also include the upregulation of IL-2 and its receptor expression, stimulation of platelet production, promotion of macrophage and osteoclast differentiation and synthesis of acute phase reactants.[2] IL-6 receptors (IL-6R) belong to the type 1 cytokine receptor superfamily and comprise two subunits (IL-6R and the gp130). The coupling of IL-6 and its receptor is followed by gp130 dimerization, Jak1 activation and GP130 tyrosine phosphorylation.[2] Such process is recognized as the classical IL-6 signalling pathway in which membrane-bound IL-6R is required and is largely restricted to hepatocytes, some epithelial cells and leucocytes.[3] Whereas in the alternative pathway, gp130 protein expressing cells – even in the absence of membrane-bound IL-6R can be stimulated by the complex of IL-6 and the soluble IL-6R and this process is known as trans-signalling.[3-5] The pathogenic role of IL-6 in SLE had been elucidated in the following animal and human studies.

Role of IL-6 in lupus mice models

In MRL/lpr mice, investigators have observed an age-related increase of serum IL-6 levels, soluble IL-6 receptors and aberrant expression of the IL-6 receptors.[6, 7] It should be underscored that no other cytokine studies have been demonstrated to possess the capacity of inducing IgG anti-DNA antibodies directly. In the NZB/W mice, exogenous administration of recombinant human IL-6 would lead to an accelerated glomerulonephritis.[8] In IL-6-deficient MRL/lpr mice, investigators have observed a substantial diminution of infiltrating macrophages in the kidney, a decrease in renal IgG and C3 deposition, and a shrunken number of CD4+ and CD8+ lymphocytes.[9] The expression VCAM-1 in the kidneys was also downregulated in MRL-Fas(lpr) IL-6−/− mice compared with IL-6-intact animals.[9] These findings proposed that IL-6 may be a key promoter of lupus nephritis and hence may have a potential role for the treatment of human lupus nephritis. In fact, IL-6 blockade in NZB/W mice could hamper proteinuria, lessen the age-related elevation in anti-dsDNA levels and also significantly improve the survival of these animals.[10, 11] Serum IL-6 levels were raised in B6.Sle1.Yaa mice and such elevation was coupled with the loss of CD19 + B cells and more primitive B-lymphoid progenitors in bone marrow.[12] IL-6 stimulation could trigger transcription factors in these uncommitted progenitor cells, which would deter lymphopoiesis but promote myelopoiesis in SLE. The survival of B lymphocytes can also be attenuated by IL-6 via the recombination-activation gene (Rag) machinery, which are vital for the revision of rearranged immunoglobulin V (D) J genes. IL-6 favours the expression of Rags and hence facilitates the rescue of autoreactive B cells from apoptosis.[13] In MRL/lpr mice, the deficiency in IL-6 led to a delayed onset of lupus nephritis.[9] All these findings suggested that IL-6 antagonism is potential useful for the treatment of lupus nephritis.

Role of IL-6 in human SLE

In human lupus patients, the serum IL-6 levels correlated positively with the disease activity and anti-DNA levels.[14, 15] Lymphoblastoid cells isolated from lupus subjects expressed heightened levels of IL-6 while an blockade of IL-6 will result in diminution of anti-dsDNA in vitro.[16] When compared with healthy individuals, B lymphocytes recovered from SLE patients spontaneously generated increased quantity of circulating immunoglobulins. IL-6 blockade significantly abrogated this spontaneous immunoglobulin secretion, but was restored with exogenous administration of IL-6.[15] It had been shown that B lymphocytes from lupus patients had spontaneous anti-dsDNA production and this autoantibody synthesis ex vivo was predominantly secreted by low density B lymphocytes.[17] One should appreciate that IL-6 can assist these low density B cells from active lupus subjects to differentiate directly into Ig-secreting cells.[17, 18] CD5 expression suppressed BCR signalling in SLE B lymphocytes and IL-6 downregulated CD5 expression via DNA methylation and hence facilitated the activation and expansion of autoreactive B cells in SLE patients.[19]

Genetic polymorphisms of the functional interleukin-6 (IL-6) promoter appear to confer susceptibility of SLE in ethnically different populations. For instance, the IL-6–174 G/C gene polymorphisms would predispose to SLE in Caucasians but such observation is less well established in Asians.[20-22] Apart from its systemic effects, IL-6 was shown to have a tight link with lupus nephritis. Several studies demonstrated elevated urinary IL-6 excretion in patients with active proliferative lupus nephritis who also had high titres of anti-dsDNA antibodies.[23, 24] Moreover, there was enhanced in situ expression of IL-6 along the glomeruli and tubules in lupus nephritis kidneys.[25] In patients with neuropsychiatric manifestation, there was an excessive IL-6 levels in the cerebrospinal fluid.[26] Furthermore, SLE patients with ongoing synovitis (19%) and joint deformities (11%) had raised IL-6 levels and such increase correlated with other serological markers of SLE such as ESR (Erythrocyte Sedimentation Rate) and anti-dsDNA level.[27] While IL-6 is consistently reported to be upregulated in SLE patients, C-reactive protein (which is ordinarily induced by IL-6) and serum amyloid precursor protein (both being pentraxin group) are typically not elevated, and the risk of secondary amyloidosis is uncommon among SLE patients. Recent data have also showed that in SLE patients have specific defect in responding to IL-6 in terms of pentraxin production.[28]

Clinical applications of IL-6 in SLE

IL-6 and its receptors can serve as biomarkers to monitor disease activity and treatment response. IL-6 release from peripheral blood mononuclear cell (PBMC) was associated with disease activity and treatment response in lupus nephritis patients.[29] Other studies have also revealed an upregulation of IL-6 agonistic receptor gp130 on peripheral lymphocytes in SLE patients and its level correlated with the disease activity.[30] In a phase I trial of tocilizumab (antagonist to IL-6 receptor) in patients with SLE, up to 50% of patients had an improvement in the SLEDAI (Systemic Lupus Erythematosus Activity Index) score of ≥4 points.[31] There was also 47% drop in the median anti-dsDNA levels and reduction in circulating plasma cells in patients receiving tocilizumab treatment.[31] Other studies have reported the use of tocilizumab in cases of refractory SLE.[32] Although IL-6 blockade could hamper proteinuria, lessen the age-related elevation in anti-dsDNA levels and also significantly improve the survival in NZB/W mice,[10, 11] IL-6-directed therapies have not been tested in human for the treatment of acute or severe lupus nephritis.

B lymphocytes stimulators (BLys)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

This cytokine belongs to the tumour necrosis factor ligand family and the understanding of this cytokine assumes growing importance due to the recent advancement of SLE treatment related to the manipulation of BLys.[33, 34] BLys is cleaved at the cell surface by furin protease, which leads to the release of a soluble, biologically active molecule.[34] This cytokine is highly expressed on cells of the myeloid lineage and its secretion is promoted by interferon-γ (IFN-γ) and IL-10.[35] It binds to strongly B lymphocytes and is a crucial factor for B lymphocyte proliferation and immunoglobulin secretion.[36] In BLys-deficient mice, there is significant diminution in mature B lymphocytes, depressed baseline serum immunoglobuin levels and a compromised immunoglobulin response to T cell dependent and independent antigens.[37] Three types of BLys receptors have been identified, namely, BAFFR, BCMA and TACI receptors. BLys can engage to these three receptors on B lymphocytes, whereas a proliferation-inducing ligand (APRIL) can only attach to TACI and BCMA.[38] Among these three receptors, the BAFFR receptor assumes the greatest significance as it mediates most of the B cell effects. A deficiency in BCMA and TACI receptors in lupus prone mice display no discernible phenotypic or functional abnormalities.[37, 39] In contrast, A/WySnJ mice (which bear a mutated baffr gene) exhibit diminished mature B cell numbers and antibody levels resembling the BLys-deficient mice.[40] BLys-triggered intracellular events are complex and conducted via the interaction of BLys receptors and several TNF receptor-associated factors. Docking of BLys with its receptors activates phospholipase C-γ2 and subsequently the NF-κB pathways,[41, 42] which is followed by prolonged B lymphocytes survival.

Role of BLys in lupus mice models

In BLys transgenic mice (BLys-Tg mice), excessive production of BLys not only results in polyclonal hypergammaglobulinemia but also raised autoantibodies (including anti-dsDNA) titre, circulating immune complexes and renal immunoglobulin deposition.[43] These mice develop autoimmune disorders resembling SLE and Sjogren syndrome.[43] In SLE-prone mice such as the (NZB/W) F1 mice and MRL-lpr/lpr mice, heightened BLys levels are detected at the onset of disease.[44] Treatment of NZB/W F1 mice with soluble TACI-Ig fusion protein prevented the development of proteinuria and prolonged the survival of the animals.[44] These findings underscored the involvement of BLys and its receptors in the development of SLE and hence the TACI-Ig was proposed as a promising treatment for human autoimmune disease. Furthermore, mice treated with exogenous BLys showed increased numbers of anti-chromatin B cells and augmented anti-dsDNA production.[45] Deletion of either BLys or BR3 critically impaired B cell maturation beyond the transitional developmental stages.[37, 40, 44, 46] T cell-deficient BAFF transgenic (Tg) mice developed SLE similar to T cell-sufficient BAFF Tg mice, and such features were associated with innate B lymphocyte activation and pro-inflammatory autoantibodies release. These data suggest that a dysregulated innate activation of B cells alone can drive disease independently of the T cells.[47]

Role of BLys in human SLE

In human lupus patients, the circulating BLys level was raised in human lupus and is correlated with the anti-dsDNA level.[48] In a survey which measured the serum BLys level and disease activities, healthy subjects universally exhibits a normal longitudinal serum BLys profile, whereas escalated BLys level was observed in SLE patients (persistent rise in 25% and intermittent increase in another 25% of patients). Increased cerebrospinal fluid levels of a proliferation-inducing ligand (APRIL) are also observed SLE patients with neuropsychiatric manifestations.

Clinical applications of BLys in SLE

The antagonism of BLys has been one of the important progresses in the treatment of SLE. Recently, belimumab was approved by the Food and Drug Administration (FDA) for the treatment of SLE. The efficacy and safety of belimumab in active SLE had been evaluated by two large multicentre randomized control trials. Both studies have demonstrated that the use of belimumab is associated with significant improvement in the SLE Responder Index (defined as ≥4 points improvement in SLEDAI) at 52 weeks, reduced SLE activity and severe flares, as well as a comparable tolerability profile to placebo.[33, 34] Analysis of the pooled data from these two large trials showed that belimumab treatment improved overall SLE disease activity mostly in the musculoskeletal and mucocutaneous organ domains and less deterioration occurred in the haematological, immunological and renal domains.[49] In a post-hoc analysis of the BLISS study, the rates of renal flare, renal remission, renal organ disease improvement, proteinuria reduction and serologic activity all favoured belimumab, although the between-group differences in most renal outcomes were not significant. Among the 267 patients with renal involvement at baseline, belimumab resulted in greater renal improvement among patients receiving mycophenolate mofetil or those with active serology at baseline when compared with placebo.[50] While these results suggested that belimumab might offer renal benefits in lupus nephritis patients, this analysis was limited by the small patient numbers and the post-hoc nature of this pooled analysis. Currently, belimumab is only approved for treatment for non-renal SLE. Despite the success of belimumab, the efficacy and safety of antagonism of the TACI receptor needs further evaluation. In this context, the phase III study to examine atacicept (a soluble, fully human, recombinant fusion protein that targets the TACI receptor) in combination with corticosteroids and mycophenolate mofetil was prematurely terminated due to profound drop in serum immunoglobulins and fulminant sepsis among the study subjects.[51]

Interleukin 17 (IL-17)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

IL-17 is a type I transmembrane protein isolated initially from a rodent CD4+ T cell cDNA library.[52] This potent pro-inflammatory cytokine is primarily released by activated T lymphocytes (‘Th17 cells’ being the most vibrant producer). As its name implies, these Th17 cells are a subset of CD4+ T lymphocytes named for its signature cytokine IL-17. The distinctive features of Th17 lymphocyte include their origination from naïve T cells and its characteristic cytokine profile when aptly primed by exclusive transcription factors. Apart from Th17 lymphocytes, recent data showed that neutrophils, gammadelta T cells and mast cells also abundant express IL-17.[53, 54] A total of six family members (IL-17 A to F) and five receptors (IL-17R A to E) were identified in the IL-17 family.[55] IL-17 possesses potent capacity to recruit monocytes and neutrophils, assist T cell infiltration and upregulate adhesion molecule expressions.[56, 57] Several important cytokines such as IL-6, IL-21 and IL-23 are in intimate association with IL-17. IL-6, when combined with transforming growth factor (TGF)β, was capable of inducing murine naïve T cells to differentiate into Th17 cells.[58, 59] On the contrary, mice deficient in IL-6 would experience defective Th17 differentiation.[58] These observations implied that the presence of an inflammatory signal is required to transform the naïve T cells to become pro-inflammatory. IL-21 is another factor which exerts a robust influence for Th-17 differentiation. Unlike IL-6, IL-21 is synthesized by the Th17 cells and T-follicular helper cells but not by antigen presenting cells and, hence, been proposed to act in an auto-amplifier fashion for the Th17 response.[59] Animal studies have also demonstrated that Th17 can be generated from naïve T cells in an IL-23-dependent fashion.[60] In addition, IL-23 elicits IL-17 secretion by memory T cells.[61] Taken together, these findings suggested the IL-23/IL-17 axis may be a novel yet important pathway in the pathogenesis of autoimmune disorders. Although naïve CD4+ T cells can differentiate into Th1, Th2 or Th17 effector subsets, the cytokine milieu characteristic of SLE patients (IL-2 poor but IL-6 and IL-21 rich) favours Th17 expansion. Th17 cells can also act as an independent T helper effector cell subset and perpetuate inflammation via cytokine release. The hallmark cytokines secreted by the Th17 cells include IL-17A, IL-17F, IL-21 and IL-22.[62] This collection of cytokines can excite B lymphocytes, and trigger local inflammation and tissue injury in SLE. The role of IL-17 in SLE pathogenesis has been explored in both human and animal models of lupus.

Role of IL-17 in lupus mice models

In MRL/lpr mice, there was enhanced IL-17 mediated tissue insult after ischemic-reperfusion of the gut.[63] Diminished splenic germinal centre formation as well as suppressed anti-DNA and anti-histone antibodies levels were observed in IL-17R-deficient BXD2 mice.[64] Furthermore, splenocytes from SNF1 mice produced more IL-17 than non-autoimmune B6 mice.[65] CD3+CD4CD8 T cells from MRL/lpr mice secreted abundant IL-17 and the expression of IL-17 and IL-23 receptors in the lymphocytes from these mice were upregulated as the disease progressed.[66] These lymphoid cells from MRL/lpr mice, after treatment with IL-23 in vitro and transferred to non-autoimmune species, can induce nephritis.[66] Mice lacking IL-17 in FcγR2b-deficient lupus mouse model showed better survival and were largely protected from development of glomerulonephritis.[67] In lupus-prone C57BL/6-lpr/lpr mice, IL-23R deficiency was associated with reduced IL-17-producing cells in the lymph nodes, decreased anti-DNA antibodies and abrogation of lupus nephritis.[68] These findings denote that an aberrantly active IL-23/IL-17 axis is responsible for the development of nephritis in lupus-prone mice.

Role of IL-17 in human SLE

Increased circulating IL-17 and IL-23 levels were seen in patients with SLE and such elevation correlates with disease activity.[69] Recent data have suggested that a substantial amount of IL-17 in SLE patients is contributed by the TCR-αβ+CD4CD8 T lymphocytes.[70] These TCR-αβ+CD4CD8 T cells and Th17 cells are also detected in kidney biopsies from SLE patients with renal involvement, hence provide strong evidence for the pathogenic role of IL-17 in lupus nephritis.[70] In addition, IL-17 assumes a crucial role for the survival and proliferation of B lymphocytes and antibody secretion in human SLE.[71] Yang et al. demonstrated the presence of Th17 cells in the PBMC and involved organs of SLE patients and the percentage increased with disease activity.[72] Moreover, the IL-17 from SLE patients can induce adhesion molecule mRNA expression and the adhesion of T cells to endothelial cells.[72]

Clinical applications of IL-17 in SLE

To date, most of the available data of IL-17 and human lupus are derived from observational or correlation studies. Hence, there is limited experience in the manipulation of IL-17 for the treatment of SLE. Therapeutic approaches that limit the cognate interaction between T cells and B cells, prevent inappropriate tissue homing and restore TReg function and the normal cytokine milieu have been explored.[73] Biochemical characterization of SLE T cells has identified distinct early and late signalling aberrations, and has allowed the unveiling of novel molecular targets that can be altered with small molecules, as well as biomarkers that may predict disease activity and organ damage.

Type I Interferons (type I IFN)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

Interferons are proteins, which possess capacity to halt viral replications: the type I IFN being the most essential ones in human lupus. Viral DNA and RNA are classical triggers of type I IFN and the signals are conducted via the Toll-like receptors (TLR) or the retinoic acid-inducible gene I (RIG-I) like receptors.[74] Double-stranded RNA initiates IFN secretion via TLR3 while single stranded RNA provokes IFN via TLR7/8 and the cytosine-phosphate-guanine (CpG) rich DNA via TLR9.[75] Type I IFN are synthesized by all leucocytes with plasmacytoid dendritic cells (PDC) being the most vigorous producer in response to TLR7 or TLR9 activation.[76] Several mechanisms of how IFN may contribute to the pathogenesis of SLE have been postulated. Immune complexes generated from autoantibodies and auto-antigens can activate the dendritic cells, and hence augmented the antigen presentation and boosted IFN secretion.[77] IFN can amplify the expression of auto-antigen such as Ro52 and also the release of auto-antigens by translocation of Ro52 to the nucleus with subsequent induction of apoptosis.[78, 79] Other actions include the promotion of dendritic cell maturation and upregulation of cell surface molecules (MHC classes I and II, co-stimulatory molecules).[80] These concerted effects coordinate the development of Th1 response. In addition, type I IFN also promote antibody production and class switching, reduce B lymphocyte selectivity for CpG-rich DNA and allow stimulation of B lymphocytes even by non-CpG DNA.[81, 82]

Role of type I IFN in lupus mice models

When treated with polyinosinic : polycytidylic acid (a synthetic double-stranded RNA ligand for TLR-3 that strongly induces type I IFN response), autoimmune prone mice would exhibit enhanced anti-dsDNA antibodies levels, increased immune complex deposition, accumulation of activated lymphocytes and macrophages, and augmented metalloproteinase activity. These changes were followed by accelerated lupus nephritis and death of the animals.[83-85] Similar findings were observed in murine models injected with adenovirus expressing IFN-α, which would lead to sustained release of that cytokine, thereby put forward the pathogenic role of Type I IFN in lupus nephritis.[85-89] Additional evidence indicating the pivotal role of type I IFN in lupus nephritis derives from studies in New Zealand Black (NZB), New Zealand mixed 2328 as well as pristane-treated mice deficient of the receptor of type I IFN (IFNAR−/−). The defective signalling through IFNAR in IFNAR−/− mice conferred protection from kidney manifestations and was associated with a reduction in the titres of lupus-specific autoantibodies and disease severity. In these lupus mouse models, the activation and proliferation of dendritic cells as well as B and T lymphocytes was decreased.[90-92]

The role of TLR, especially of TLR-7 (responsive to ssRNA) and TLR-9, (responsive to hypomethylated CpG-rich DNA) in type I IFN production in lupus is well documented. The importance of type I IFN and TLR-7 signalling in aggravating kidney injury was established in mice that overexpress TLR-7 (Y-linked autoimmune accelerating locus mice – Yaa mice) or that were treated with pristane.[93-95] In a pristane-induced mouse model of SLE, it was shown that an intact type I IFN signalling pathway is prerequisite to the upregulation of TLR-7 receptors in B cells and effective activation through TLR-7 and TLR-9 of B cells to produce lupus-specific autoantibodies.[96] These findings suggested that type I IFN is upstream of TLR signalling in the activation of autoreactive B cells in SLE. Furthermore, in lupus-prone mice, severe nephritis can be induced by the activation of TLR-9 signalling pathway through CpG-rich DNA.[97] These observations were supported by a study that tested a dual inhibitor of TLR-7 and TLR-9 (known to inhibit IFN-α production by PDC) in lupus-prone mice. The inhibition of TLR-7 and TLR-9 would lead to a significant improvement of proteinuria, glomerulonephritis, and survival as well as decreased nucleic acid-specific autoantibodies.[98]

Role of type I IFN in human SLE

Elevation of type I IFN in lupus patients was one of the first described cytokine abnormalities in autoimmune diseases. The link between IFN levels and disease activity, anti-dsDNA levels and clinical manifestations backs the role of IFN in SLE pathogenesis.[99] In lupus patients, PDC was detected in the dermal lesions and are responsible for sustained IFN release, although their circulating number is lower in the peripheral blood.[100] Migration of PDC to the glomeruli is observed in patients with lupus nephritis and this movement is thought to be influenced by IL-18.[101] In patients with cerebral lupus, autoantibodies with the capacity to form very potent interferonogenic immune complexes together with RNA-containing auto-antigens were detected in the cerebrospinal fluid.[102] Gene expression profiling showed that SLE patients expressed IFN-inducible genes in PBMC and the expression correlated with disease activities.[78] These findings revealed that raised IFN levels are capable of altering gene expression in active lupus patients and supported the pathogenic role of type I IFN in human lupus. Data derived by the genetic studies had further delineated the causal role of IFN in SLE. Transcription factor IRF5 was the first identified gene directly involved in IFN production and was associated with heightened risk of SLE.[103] Lupus patients with a risk haplotype of IRF5 showed more intense serum IFN activity when compared with patients lacking this risk genotype and the effect was most obvious in patients with autoantibodies against either RNA-binding proteins or double-stranded DNA.[104] Another example is the signal transducer and activator of transcription 4 (STAT4) which interacts with the cytoplasmic part of the IFNAR and variants of STAT4 have been shown to be strongly associated with lupus.[105] A link between the IFN response and SLE had also been established for patients with polymorphism of Jak TYK2.[106] Healthy first degree relatives of lupus patients have more pronounced serum IFN activity and the levels are more abundant in younger individuals.[107, 108] A combination of risk alleles in the type I signalling pathway (e.g. STAT4 and IRF5) may confer an additive predisposition of disease.[109] It can be inferred that the use of genetic mapping may help predicting the development and severity of disease in the future.

Clinical applications of type I IFN in SLE

Interferon-regulated chemokines may be employed to monitor disease activity and organ damage.[110, 111] It has also been proposed that type I IFN-inducible mRNA can be used as pharmacodynamic markers to monitor treatment response of anti-IFN therapy in SLE.[112] The use of anti-IFN-α in the treatment of moderately active SLE was examined in a phase I multicentre double-blind randomized trial. In that study, the use of sifalimumab (an anti-IFN-α monoclonal antibody) led to a dose-dependent inhibition of type I IFN-induced mRNA in whole blood and corresponding changes in related proteins in affected skin. Exploratory analyses showed consistent trends towards improvement in disease activity, less requirement of new or escalation of immunosuppressive treatments and fewer flares in sifalimumab-treated patients.[113] Tolerability profile was acceptable and comparable to patients receiving placebo.

Tumour necrosis factor-α (TNF-α)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

Tumour necrosis factor-α is expressed as a trimer on cell surface and in soluble form after the activation of macrophages and dendritic cells. Being described to have both protective and deleterious effects in SLE, its position in lupus pathogenesis remained controversial.

Role of TNF-α in lupus mice models

In NZB/W mice, there was diminished production of TNF-α.[114] In some mouse model, the deficiency of TNF-α appeared to provoke lupus-like autoimmunity. While TNF-α defective NZB/W mice develop severe disease manifestations, TNF-α intact NZB/W mice only show modest lupus activity.[115] Conversely, TNF-α concentration was elevated in both sera and renal tissue of MRL/lpr lupus mice and the levels of TNF-α correlated with the severity of kidney disease.[116] Moreover, even in NZB/W mice, renal expression of TNF-α is escalated in conjunction with kidney inflammation.[117] In MRL/lpr mice, anti-TNF-α therapy led to improvement of joint and lung manifestations.[118, 119] Whether the controversial role of TNF-α in the pathogenesis of murine SLE could be related to the different animal models used remains unclear.

Role of TNF-α in human SLE

The circulating TNF-α level in active SLE patients closely followed the disease activity and elaborated TNF-α expression was seen in the renal parenchymal tissue in patients with lupus nephritis.[29, 120] Nonetheless, conflicting evidence exists in subjects who had received anti-TNF-α therapy for other autoimmune disorders.[121, 122] These individuals developed lupus-like features coupled with elevated anti-nuclear factors, anti-dsDNA and anti-cardiolipin antibodies. Resolution of these symptoms and autoantibodies followed the cessation of TNF-α blocking therapy. Recent studies on inflammatory bowel disease and ankylosing spondylitis also showed that TNF-α blockade might cause drug-induced lupus.[123-128] However, anti-TNF-induced SLE is a relatively uncommon phenomenon and these patients often only develop multiple autoantibodies but mild clinical manifestations.

Clinical applications of TNF-α in SLE

Given the findings of elevated serum TNF-α in active SLE and overexpression of TNF-α in active lupus nephritis,[29, 129] TNF-α antagonism still appears to be an attractive option for the treatment of active lupus disease. However, evidence for therapeutic efficacy of TNF-α blockade in SLE is still limited.[130, 131] A recent study which reviewed the experience of using inflixmab in SLE patients had raised serious concern of fulminant sepsis and malignancy, and hence the decision to use anti-TNF-α blockade in SLE should not be taken lightly.[132]

Interleukin-18 (IL-18)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

IL-18 belongs to the IL-1 family and is synthesized in an inactive form which requires cleavage by caspase-1 to become biologically active. It exerts a variety of effects on dendritic cells, T lymphocytes and natural killer cells, and is a potent inducer of IFN-α to promote Th1 differentiation. The following discussion focused on the role of IL-18 in the pathogenesis of SLE.

Role of IL-18 in murine SLE

When compared with wild-type MRL/++ mice, MRL/lpr mice demonstrated higher circulating IL-18 levels and daily injections of IL-18 or IL-18 plus IL-12 resulted in accelerated proteinuria, glomerulonephritis, vasculitis and elevated levels of pro-inflammatory cytokines in these animals.[133] Moreover, increased IL-18 expression was observed in the lymph nodes and kidneys of MRL/lpr mice.[134] In MRL/lpr mice, there were renal upregulation of mature IL-18, which was primarily detected in the tubular epithelial cells and such increased expression was in parallel with the severity of nephritis.[135] Recent studies have also further characterized the role of IL-18 in SLE using signal transducers and activators of transcription 4 (Stat4) knockout MRL/lpr mice and found that they did not differ in survival or renal function from Stat4-intact MRL/lpr mice. The circulating IL-18 levels, however, were elevated in Stat4-deficient mice compared with Stat4-intact ones, suggesting the contributory role of IL-18 in the progression of lupus nephritis independent of Stat4.[136] When vaccinated with autologous IL-18, MRL/lpr mice would develop anti-IL18 autoantibodies and these mice displayed a substantial decrease in IFN-α synthesis, alleviated glomerulonephritis and renal damage, and improved survival,[137] indicating an important pathogenic role of this cytokine.

Role of IL-18 in human SLE

Increased serum IL-18 levels had been observed in SLE patients and an association with renal manifestations has been reported.[138-140] Serum IL-18 was higher in lupus patients than in controls and its level was correlated with urinary microalbumin.[141] Moreover, IL-18 expression was also increased within the glomeruli of lupus nephritis patients and such expression was primarily detected within the mesangial matrix and in infiltrating mononuclear cells.[141] Moreover, several studies have described higher circulating IL-18 in SLE patients than in control subjects, and the levels correlates with the anti-dsDNA titres and the SLEDAI score.[138, 140, 142, 143] Apart from the kidneys, IL-18 was also highly relevant in other organ manifestations of lupus. IL-18 was abundantly expressed in biopsy samples of lesional skin from patients with cutaneous lupus.[144] These patients also expressed higher levels of IL-18 receptor on their keratinocyte surface in response to TNF-α and IFN-γ stimulation. Kahlenberg et al. have recently demonstrated that inflammasome activation of IL-18 would result in endothelial progenitor cell (EPC) dysfunction in SLE patients, which might explain premature atherosclerosis in SLE. In these experiments, neutralization of IL-18 in SLE EPC cultures restores their capacity to differentiate into mature endothelial cells, supporting a deleterious effect of IL-18 on vascular repair in vivo.[145]

Clinical applications of IL-18 in SLE

Nold et al. demonstrated that the use of a IL-18 binding protein would significantly inhibit the release of IFN-α and matrix metalloproteinase-9 (MMP-9) from whole blood samples obtained from SLE patients, and anti-IL18 might confer additional inhibitory effect on the pro-inflammatory cytokines when compared with samples incubated with corticosteroids or mycophenolic acid alone.[146] Although IL-18 blockade appeared to a potential therapeutic concept in SLE, the clinical data regarding this approach are still lacking.

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References

In this review, we have highlighted the cytokines which have crucial pathogenic significance in SLE (Fig. 1). The growing knowledge in these cytokines has introduced opportunities for the design of innovative diagnostics and therapeutic approaches (Table 1). Currently, these novel therapies which involve the attenuation of the cytokine system are often used as add-on treatment or for recalcitrant cases. However, one should expand the use of these biologics such as minimization of other immunosuppressive drugs which have more significant toxicities. While some of these agents have proven efficacy and tolerability in the initial studies, the long-term safety remains undefined. Both upcoming randomized trials and long-term follow-up studies are needed to adequately address these concerns. Taken together, data regarding the manipulation of the cytokine systems are encouraging and it is worthwhile to invest resources for the development of therapy in this promising direction.

figure

Figure 1. Schematic diagram to illustrate the complex interaction of different cytokines/immune cells and the rationale of anti-cytokine therapies. IL, interleukin; TNF-α, tumour necrosis factor-α.

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Table 1. Cytokines which have important pathogenic roles in SLE: the major secreting cells and current clinical applications
CytokinesMajor secreting cellsCurrent clinical applications
  1. BLys, B lymphocytes stimulator; IL, interleukin; MMF, mycophenolate mofetil; SLE, systemic lupus erythematosus; SLEDAI, Systemic Lupus Erythematosus Activity Index; TNF-α, tumour necrosis factor-α.

IL-6
  • Monocytes
  •  Fibroblasts
  •  Endothelial cells
  1. Phase I trial of tocilizumab showed improvement in the SLEDAI score of ≥4 points, reduction in anti-dsDNA and circulating plasma cells.[31]
  2. Tocilizumab has been used in cases of refractory SLE.[32]
BLys
  • Monocytes
  •  Macrophages
  •  Dendritic cells
  •  Activated neutrophils
  1. Use of belimumab is associated with significant improvement in the SLE Responder Index (defined as ≥4 points improvement in SLEDAI) at 52 weeks, reduced SLE activity and severe flares, as well as a comparable tolerability profile when compared with placebo.[33, 34]
  2. Belimumab treatment improved overall SLE disease activity in the most common musculoskeletal and mucocutaneous organ domains while less deterioration occurred in the haematological, immunological and renal domains.[49]
IL-17Th-17 LymphocytesStill under investigation
Type 1 interferon (IFN)Plasmacytoid dendritic cells
  1. Use of sifalimumab (anti-IFN-α monoclonal antibody) led to a dose-dependent inhibition of type I IFN-induced mRNA in whole blood and corresponding changes in related proteins in affected skin.[113]
  2. Exploratory analyses showed consistent trends towards improvement in disease activity, less requirement of new or escalation of immunosuppressive treatments and fewer flares in sifalimumab-treated patients.[113]
TNF-α
  • Macrophages
  •  Dendritic cells
  1. Infliximab (anti-TNF-α) improved joint symptoms and proteinuria in SLE patients with moderate activity.[130]
  2. Infliximab (anti-TNF-α) resulted in sustained remission in class IV lupus nephritis patients who failed to achieve remission with steroid/MMF/cyclosporine.[147]

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Interleukin 6 (IL-6)
  5. B lymphocytes stimulators (BLys)
  6. Interleukin 17 (IL-17)
  7. Type I Interferons (type I IFN)
  8. Tumour necrosis factor-α (TNF-α)
  9. Interleukin-18 (IL-18)
  10. Concluding remarks
  11. References