Introduction

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease, characterized by the production of multiple autoantibodies, complement activation and immune-complex deposition, causing tissue and organ damage. More than one mechanism could contribute to the disease and that different mechanism may be responsible for different disease manifestations [1]. Cytokines produced by abnormal T-helper (TH) cells have been shown to be implicated in the pathogenesis of SLE [2]. Activated T cells have been indicated to differentiate into memory/effector T cells that are classified into T helper 1 (Th1) and Th2 subsets based on their cytokine production profiles [3]. It has been suggested that SLE is a T helper (Th)2-polarized disease because of its production of autoantibodies specific for self-antigens [4]. However, other studies have demonstrated that Th1 response including interferon (IFN)-γ and IL-12 were also significantly increased in patients with SLE [5, 6]. In fact, Th1 dominant immune responses have been commonly considered to be pathological in autoimmune disease via the induction of inflammatory reaction.

In previous studies regarding the correlations between TH1-cell function and autoimmune diseases, however, have contradictions [2]. Mice that lacked components of the IL-12/IFN-γ axis, including IL-12p35, IL-12Rβ2, IFN-γ, IFN-γR and the downstream signaling molecule, Stat1, developed EAE and CIA with a normal or even increased magnitude [7]. By contrast, mice that lacked the p40 subunit shared by IL-12 and IL-23 abstained from contracting EAE and CIA. Moreover, antibodies against the IL-12p40 subunit significantly suppressed the development of autoimmune disease [7, 8].

Recently, these contradictions have been clarified by the studies on the TH17 cells, a novel subset of TH cells that selectively secrete several proinflammatory cytokines, including IL-17, IL-21, IL-22, etc. [9]. IL-17 is a cytokine associated with inflammation, autoimmunity and defense against some bacteria, it has been implicated in allergic rhinitis and many chronic autoimmune diseases including psoriasis, multiple sclerosis and systemic sclerosis [10, 11]. Nevertheless, whether IL-17 plays a role in the pathogenesis of SLE is still unclear. Therefore, in the present study, for better understanding of its interrelationship and immunopathological roles in SLE, we investigated serum IL-17 levels in SLE patients and relations to disease manifestations and activity.

Methods

Patients

A total of 57 patients with SLE (55 females; 2 males; mean age 35.6 ± 13.0 years, range from 16 to 60 years) were recruited from the Department of Rheumatology at Anhui Provincial Hospital. The diagnosis of SLE was established by the presence of four or more American College of Rheumatology (ACR) diagnostic criteria [12]. About 30 healthy volunteers (29 females, 1 male; mean age 43.7 ± 12.27 years, range from 15 to 66 years) were included as normal controls, all of them did not have any rheumatologic conditions. Sera were obtained from 5 ml of whole blood and stored at −80°C until tested. All subjects gave their written consent to participate.

Collection of clinical and laboratory data

Demographic data, clinical data and laboratory data were collected from hospital records or by questionnaire and reviewed by experienced physicians.

Clinical features of SLE patients such as butterfly erythema, oral ulcer, arthritis, nervous system disorder, myocarditis, renal involvement, serositis, alopecia, anemia and photosensitivity were recorded. Renal involvement of SLE was defined according to the ACR criteria, i.e., any one of the following: (1) persistent proteinuria ≥0.5 g/day; (2) the presence of active cellular casts; or (3) biopsy evidence of lupus nephritis [12]. Individual disease activity was quantified using the SLE disease activity index (SLEDAI) score [13]. More active SLE was defined as a SLEDAI score ≥ 10, those patients with SLEDAI < 10 were classed as less active.

Laboratory abnormalities were also recorded, including leukopenia (white blood cell count <4,000/mm3), thrombocytopenia (platelet count <100,000/mm3), the occurrence of blood urine or proteuria, elevated erythrocyte sedimentation rate (Male: >15 mm/h; Female: >20 mm/h), the presence of anti-dsDNA, antinuclear, anti-Sm, anti-SSA, anti-SSB, anti-RNP (by indirect immunofluorescence); IgG, IgA, IgM and serum levels of C3/C4 (by immunoturbidimetry) were also reviewed.

Measurement of serum IL-17 levels

Serum IL-17 concentrations were determined by Specific ELISA kits according to the manufacturer’s recommendation (R&D Systems, Minneapolis, MN, USA), reproducibility of the assay for IL-17 measurements was: Intra-Assay: CV < 10%, Inter-Assay: CV < 15%. The results were expressed as picograms per milliliter.

Statistical analyzes

All quantitative data were expressed as Mean ± SD, statistic analyzes were performed by SPSS 10.01 software. Analysis of covariance was used to compare serum IL-17 levels among different groups. Associations of serum IL-17 levels with clinical and laboratory parameters of SLE patients were analyzed by independent samples t-test. For the correlation analysis between serum IL-17 levels and SLEDAI, Pearson correlation coefficient was used. Differences were considered statistically significant if P < 0.05 in two-tailed tests.

Results

Serum IL-17 levels of SLE patients and controls

In total, 57 patients with SLE were included in the current study, 27 of whom were patients with lupus nephritis (LN). 41 patients were classed as more active SLE. Results showed that serum IL-17 levels were significantly increased in both SLE patients with or without nephritis as compared with normal controls. However, there was no significant difference regarding serum IL-17 level between SLE patients with nephritis and those without nephritis; no significant difference was found between Less active SLE and More active SLE (Table 1).

Table 1 Comparison of serum IL-17 levels between different groups
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Associations of serum IL-17 levels with clinical and laboratory parameters of SLE patients

Associations of serum IL-17 levels with major clinical and laboratory parameters of SLE patients were analyzed, but found no correlation (Tables 2, 3).

Table 2 Associations of serum IL-17 level with clinical parameters of SLE patients
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Table 3 Associations of serum IL-17 level with laboratory parameters of SLE patients
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The correlation of serum IL-17 levels with disease activity

Pearson correlation coefficient was used for correlation analysis between SLEDAI and the level of serum IL-17. Correlation analysis between serum IL-17 levels and SLEDAI showed no association [r s = 0.173, P = 0.197] (Fig. 1).

Fig. 1
figure 1

Correlation between serum IL-17 levels and SLEDAI

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Discussion

IL-17 and other Th17 cytokines have been most commonly described as a proinflammatory cytokine due to its expression in lesions of patients with chronic inflammatory diseases and its induction of proinflammatory cytokines such as IFN-γ, IL-2 and granulocyte–monocyte colony-stimulating factor in SLE [14]. It has been implicated in a variety of autoimmune diseases, such as EAE and CIA are directly linked with IL-17 production, largely through demonstrations that the cytokine is overexpressed in these conditions [2]. Dong et al. [15], found that IL-17 might have an important role in the pathogenesis of lupus nephritis through the induction of IgG, anti-dsDNA antibody overproduction and IL-6 overexpression by peripheral blood mononuclear cells in patients with lupus nephritis. Hsu et al. [16] further confirmed that IL-17 can promote autoimmune disease by inducing the formation of spontaneous germinal centers. A very recent study also demonstrated hyperproduction of IL-17 in patients with SLE [17]. All theses studies suggested a possible pathogenic role of IL-17 in SLE. However, these studies are very limited, and that much of the data regarding IL-17 has been generated using mouse models. Our data show that the level of IL-17 was significantly higher in serum of SLE patients than in normal controls, which also indicated that the elevation of proinflammatory cytokine IL-17 may trigger the inflammatory process in SLE. Nevertheless, no correlation was found between serum IL-17 levels and disease manifestations or SLEDAI. This may be explained by the small sample size of the present study, or the heterogeneity of SLE. Therefore, further studies are needed to confirm this preliminary data.

Komiyama et al. [18] found that IL-17-producing T cells, but not IFN-γ-producing T cells, induced EAE in recipient mice. Consistent with the role of the TH17 lineage in autoimmune inflammation, IL-17-deficient mice either were resistant to EAE and CIA induction or showed reduced disease severity [19]. Moreover, administration of anti-IL-17 mAb in CIA and EAE mouse models significantly reduced disease severity [20], Rohn et al. [21], explored the potential of neutralizing IL-17 by active immunization using virus-like particles conjugated with recombinant IL-17 (IL-17-VLP). Mice immunized with IL-17-VLP induced high levels of anti-IL-17 antibodies, had lower incidence of disease, slower progression to disease and reduced scores of disease severity in both collagen-induced arthritis and experimental autoimmune encephalomyelitis. Therefore, Neutralization of IL-17 by passive and active vaccination maybe a novel therapeutic approach for the treatment of SLE.

Several limitations should be noted in our study. Firstly, we evaluated a limited number of patients. Secondly, the controls were sex matched, but not strictly age matched. Therefore, we adjusted our results by age through analysis of covariance. Thirdly, not only IL-17, but also other Th17 related cytokines like IL-12, IL-21 and IL-23, were also associated with autoimmune rheumatic diseases, but we did not investigate serum IL-12, IL-21 and IL-23 levels in this study, which may weaken the specificity of IL-17 for SLE. Thus, future studies should focus on determine whether IL-17 is a key cytokine in SLE or is simply up-regulated with other pro-inflammatory cytokines such as IFN-gamma, IL-6, IL-12, IL-23, IL-21, etc.

Taken together, the elevation of proinflammatory cytokine IL-17 may trigger the inflammatory process in SLE. Thus, future studies will focus on investigating the role of IL-17 in chronic inflammation and the development and treatment of SLE. IL-17 may prove to be a promising therapeutic target for SLE. However, further studies are awaited to confirm this belief.