Upregulated ST2L-Dependent IL-33 Signaling in Dermal Fibroblasts Contributes to IL-6 Induction and Skin Fibrosis in Systemic Sclerosis.

Abstract

Persistent activation of dermal fibroblasts drives progressive skin fibrosis in systemic sclerosis (SSc), a connective tissue disease. However, the mechanisms sustaining fibroblast activation remain incompletely defined. Interleukin-33 (IL-33), an alarmin-like IL-1 family cytokine, signals via the membrane receptor ST2L along with IL1RAP to activate pathways such as NF-κB, whereas soluble ST2 (sST2) acts as a decoy. The contribution of this axis to skin fibrosis in SSc is unclear. This study was aimed at defining the expression and functional significance of IL-33/ST2L signaling in SSc dermal fibroblasts and experimental dermal fibrosis. IL-33, ST2L, sST2, and IL1RAP expression in cultured dermal fibroblasts and skin samples from patients with diffuse cutaneous SSc and healthy donors was estimated via quantitative real-time PCR and immunoblotting. In vivo relevance of the findings was assessed using a bleomycin-induced skin fibrosis model in IL-33 knockout mice, via histology (hematoxylin and eosin and Masson's trichrome staining), dermal thickness measurements, collagen quantification (Sircol assay), and IL-6 mRNA analysis. At the cellular level, causality was examined by lentiviral overexpression of ST2L in control fibroblasts and its small interfering RNA-mediated knockdown in SSc fibroblasts, with recombinant IL-33 stimulation and readouts, including IL-6 induction and IκBα phosphorylation. IL-33 and ST2L were significantly upregulated in cultured SSc fibroblasts, whereas sST2 and IL1RAP expression was unchanged. Bleomycin increased IL-33 and ST2L expression in mouse skin, and IL-33 deficiency attenuated dermal thickening, collagen accumulation, and IL-6 induction. Mechanistically, ST2L overexpression rendered control fibroblasts IL-33-responsive, inducing IL-6 and transient IκBα phosphorylation, whereas ST2L knockdown reduced basal IL-6 levels and blunted IL-33-induced IL-6 expression in SSc fibroblasts. These results indicate that elevated ST2L expression amplifies IL-33 responsiveness in SSc dermal fibroblasts and promotes IL-6 production via NF-κB signaling, implicating the IL-33/ST2L axis as a contributor to fibrotic skin remodeling and a potential therapeutic target in SSc.