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

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.