Background: Bone erosion in rheumatoid arthritis (RA) is partly caused by excessive activation of osteoclasts[1]. Osteoclasts can be derived from RA synovium and their differentiation can be inhibited by OPG, a decoy receptor of the osteoclastogenesis promoting cytokine RANKL[2,3]. Fibroblast-like synoviocytes (FLSs) are a main type of stromal cells in the synovium that can secret OPG[4]. The OPG secretion by FLSs can be modulated by various cytokines[5]. Interleukin (IL)-13 is a cytokine rich in early RA synovial fluid and it decreases as RA progresses[6]. IL-13 was reported to alleviate bone erosion in RA mouse models[7]. However, how IL-13 reduces bone destruction remains unclear.

Objectives: To investigate if IL-13 can inhibit osteoclast differentiation by up-regulating OPG in FLSs of RA patients (RA-FLSs), thus reduces bone erosion in RA.

Methods: FLSs were isolated from the synovium tissue of RA patients with informed consent who underwent joint replacement surgery (Ethics No. 2021-544-01). OPG and RANKL expression by RA-FLSs were evaluated by qRT-PCR. OPG secretion was determined by ELISA. Western blot was performed to analyze OPG expression and the activation of STAT6 pathway. IL-13 and OPG siRNA pre-treated RA-FLSs conditioned medium were used in osteoclast induction to test if IL-13 can inhibit osteoclastogenesis by up-regulating OPG in RA-FLSs. Micro-CT scanning and immunofluorescence were done to determine if IL-13 can induce OPG expression and alleviate bone erosion in vivo.

Results: IL-13 can significantly upregulate OPG expression and secretion by RA-FLSs. Meanwhile, RANKL/OPG ratio was downregulated ( Figure 1A ). STAT6 phosphorylation and OPG upregulation of RA-FLSs by IL-13 can be significantly inhibited by a STAT6 inhibitor (inh.) ( Figure 1B ). IL-13 receptor α1 (IL-13Rα1) and IL-13 receptor α2 (IL-13Rα2) knockdown can inhibit OPG upregulation by IL-13 in RA-FLSs, indicating that IL-13 can induce OPG secretion in RA-FLSs through IL-13Rα1 and IL-13Rα2 via STAT6 pathway ( Figure 1C ). 20% conditioned medium of RA-FLSs pretreated by IL-13 can significantly inhibited osteoclast differentiation in vitro. OPG knockdown in RA-FLSs significantly reversed the inhibition, indicating that IL-13 inhibits osteoclastogenesis by upregulating OPG in RA-FLSs ( Figure 1D ). In collagen-induced arthritis mice, IL-13 injection can reduce bone destruction in the ankle joint. Meanwhile, OPG expression in vemintin positive FLSs was increased ( Figure 1E and F).

Figure 1.

Conclusion: IL-13 can inhibit osteoclastogenesis by up-regulating OPG in RA-FLSs through IL-13 receptors via STAT6 pathway, thus ameliorate bone erosion in RA.

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[6]Raza K, Falciani F, Curnow SJ, et al. Early rheumatoid arthritis is characterized by a distinct and transient synovial fluid cytokine profile of T cell and stromal cell origin. Arthritis Res Ther 2005;7(4):R784-95.

[7]Woods JM, Amin MA, Katschke KJ, Jr., et al. Interleukin-13 gene therapy reduces inflammation, vascularization, and bony destruction in rat adjuvant-induced arthritis. Hum Gene Ther 2002;13(3):381-93.

Acknowledgements: This study was supported by the National Natural Science Foundation of China (grant number 81671608). We would like to thank Prof. Zhihong Xu and Dr. Minghui Sun for the assistance in synovial tissue collection, and all the patients and participants for their support for our study.

Disclosure of Interests: None declared.