IL-35 enhances IL-10⁺ breg-mediated immunoregulation and attenuates inflammation and fibrosis in systemic sclerosis.

OBJECTIVE: This study aimed to explore the role of interleukin-35 (IL-35)-mediated regulatory B cells (Bregs) in modulating inflammation and fibrosis in systemic sclerosis (SSc). METHODS: Peripheral blood and skin samples from 132 treatment-naïve SSc patients and 58 healthy controls (HCs) were analyzed for IL-35 levels, B cell subsets, and cytokine profiles. CD19⁺ B cells were stimulated with IL-35 or anti-IL-35 monoclonal antibody (mAbIL-35) and co-cultured with autologous CD3⁺ T cells to evaluate immunoregulatory function. A bleomycin-induced SSc mouse model was used to assess in vivo effects. RESULTS: SSc patients exhibited elevated plasma IL-35, IL-10, IL-6, and BAFF levels, along with increased CD19⁺ B cells and IL-6⁺ effector B cells. However, IL-35⁺ and IL-10⁺ Bregs were significantly decreased in both blood and skin. IL-10, IL-6, and TGF-β1 mRNA levels were increased in skin lesions. IL-35 and IL-10 levels correlated with lung function, skin scores, and inflammatory markers. In vitro, IL-35 promoted IL-10⁺ Breg expansion and cytokine secretion, and suppressed Th1, Th17, and CD8⁺IFN-γ⁺ T cell responses via an IL-10-dependent mechanism. In vivo, IL-35 alleviated fibrosis and inflammation in bleomycin-treated mice, whereas mAbIL-35 exacerbated it. CONCLUSION: While IL-35 is compensatorily elevated in SSc, its endogenous levels are insufficient to curb the disease. However, the therapeutic administration of exogenous IL-35 demonstrates significant potential. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13075-026-03735-8.