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Increased expression of interleukin (IL)-7 and its receptor is suggested to play a critical role in immunopathology of primary Sjögren's syndrome (pSS).1–3 Data from humans and mice demonstrate that IL-7 drives a range of processes involved in pSS immunopathology, including epithelial cell apoptosis, lymphocyte infiltration and reduction of salivary output.3 The IL-7/IL-7R axis is involved in the formation of (ectopic) lymphoid structures in salivary glands (SGs),3 ,4 which is a predictor for lymphoma development in pSS.5 ,6 IL-7 activity is potentiated by the soluble form of its receptor (sIL-7R), which is produced in inflamed tissues by activated stromal cells.7 As sIL-7R is a possible biomarker for IL-7-driven immune activation and lymphoid neogenesis, we studied the expression of sIL-7R in pSS in relation to markers of inflammation and saliva production.
Ninety-five patients with pSS were diagnosed according to the 2002 criteria (table 1).8 sIL-7R was measured in serum of 68 patients with pSS using ELISA as previously described9 and compared with 51 healthy individuals (HC). Labial SG biopsy tissues were taken from 27 patients and, after thorough rinsing, were incubated in 200 µL of sterile saline for 1 hour at room temperature. In these tissue supernatants, sIL-7R was measured using multicytokine analysis and compared with 24 patients with sicca not meeting criteria for pSS or any other connective tissue disease and defined as non-Sjögren's sicca (nSS). Weights of biopsy tissues analysed did not differ between the groups. Patients with sIL-7Rhigh were defined as those patients with pSS with sIL-7R concentrations in serum or SG tissue supernatant above the mean plus twice the SD of the concentrations measured in the respective control group. We used Mann–Whitney U-test to compare groups, Spearman's rank correlation coefficient to assess correlations and Fisher's exact test to compare proportions. Values of p<0.05 were considered statistically significant, and p values were not corrected for multiple hypothesis testing.
Serum levels of sIL-7R were significantly higher in patients with pSS compared with healthy controls (figure 1A). In the patients with pSS, serum sIL-7R concentrations correlated with serum IgG (r=0.30, p=0.015), lymphocytic focus score (LFS) (r=0.36, p=0.008), stimulated whole saliva (SWS) (r=−0.48, p=0.011) and unstimulated whole saliva (UWS) (r=−0.54, p=0.004). A group of patients with pSS with sIL-7R levels above those of HC could be discerned and was defined as sIL-7Rhigh (n=23/34%). These patients showed increased serum IgG and LFS compared with the other patients with pSS (sIL-7Rlow) (figure 1B). Furthermore, the sIL-7Rhigh group had significantly decreased salivary output, as measured by SWS (median (range): 0.2 (0.0–0.9) vs 1.2 (0.0–4.6) mL/min, p=0.015) and UWS (figure 1C).
In addition, we observed increased sIL-7R production in SG supernatants from patients with pSS compared with nSS (figure 1D). In the patients with pSS, supernatant sIL-7R concentrations correlated with serum IgG (r=0.52, p=0.007). Similar to the serum data, a sIL-7Rhigh group could be discerned (n=13/46%), with higher levels of serum IgG (figure 1E), increased EULAR Sjögren's syndrome disease activity index (ESSDAI) scores (median (range): 3.0 (2.0–11) vs 1.0 (0.0–7.0), p=0.017) and increased prevalence of anti-Ro/Sjogren's syndrome A antigen (SSA) and anti-La/Sjogren's syndrome B antigen (SSB) autoantibodies (figure 1F) compared with the sIL-7Rlow group.
Thus, sIL-7R is increased in serum and SG supernatant of patients with pSS, in which high sIL-7R expression identifies patients with increased markers of inflammation and decreased salivary output. The sIL-7R increase in pSS SG supernatants indicates that the increased systemic sIL-7R levels are at least partially mediated by local production. High local production of sIL-7R was related to increased B-cell activity and autoimmunity, in line with the described role of IL-7 in lymphoid neogenesis.10
Our data indicate that sIL-7R may be a biomarker for patients with more severe disease, which is in line with data from systemic lupus erythematosus where increased serum sIL-7R is a marker for increased disease activity and lupus nephritis.9 As the IL-7/IL-7R axis is critical for lymphoid neogenesis,10 increased sIL-7R may also contribute to lymphoma development. However, larger patient cohorts are necessary to validate the potential of sIL-7R as a disease marker and study its possible contribution to lymphoid neogenesis. Dissecting the functional role of sIL-7R in immunopathology may have important implications with regard to therapeutic targeting of the IL-7/IL-7R axis in pSS.
Acknowledgments
We thank Dr Marjolein de Hair for statistical support and the Multiplex Core Facility of the Laboratory for Translational Immunology (UMC Utrecht, the Netherlands) for performing the in-house developed and validated multiplex immunoassay.
References
Footnotes
MRH and SLMB shared first authorship.
Contributors MRH, SLMB, JAGvR and TRDJR were involved in study design. MRH, SLMB, APR, AB, BRL and AAK were involved in data collection. MRH, SLMB and APR performed the analysis. All authors were involved in interpretation of the data, drafting of the work, revising it critically and gave final approval of the version published.
Competing interests None declared.
Ethics approval Sample collection was performed according to ethical regulations of the UMC Utrecht and patients gave their written informed consent.
Provenance and peer review Not commissioned; externally peer reviewed.