Abstract

Background Type 1 diabetes (T1D) is an autoimmune disease characterized by profound dysregulation of CD4+ T cell subsets, particularly impaired regulatory T cell (Treg) function accompanied by excessive Th17 and T follicular helper (Tfh) cell responses. Interleukin-21 (IL-21) has been implicated in T cell–mediated autoimmunity; however, the immunoregulatory mechanisms linking IL-21 signaling to T cell imbalance in pediatric T1D remain incompletely understood. Methods Single-cell RNA sequencing data from children with T1D were analyzed to characterize IL-21/STAT3 pathway activity across immune cell subsets. Functional assays were performed using primary human CD4+ T cells and Tregs treated with IL-21 and the STAT3 inhibitor Stattic. T cell differentiation, suppressive function, cytokine production, and STAT3 activation were assessed in vitro. The immunological and pathological effects of STAT3 inhibition were further evaluated in a non-obese diabetic (NOD) mouse model. Results Single-cell transcriptomic analysis revealed enhanced IL-21/STAT3 signaling activity in CD4+ T cell populations from children with T1D. IL-21 stimulation induced STAT3 phosphorylation and nuclear translocation, leading to reduced FoxP3 expression, impaired Treg-associated suppressive function, and a shift in CD4+ T cell differentiation toward Th17 and Tfh phenotypes. Pharmacological inhibition of STAT3 effectively reversed IL-21–mediated Treg dysfunction and restored CD4+ T cell balance in vitro. In NOD mice, STAT3 inhibition ameliorated hyperglycemia, reduced pancreatic inflammation, preserved insulin-positive islets, and corrected systemic T cell subset imbalance. Conclusion Aberrant activation of the IL-21/STAT3 signaling axis may contribute to impaired Treg function and CD4+ T cell imbalance in pediatric T1D.