MicroRNA-21 alleviates regulatory T cell transdifferentiation in periodontitis

Periodontitis is one of worldwide prevalent inflammatory diseases, driven by dysregulated host immune responses. MicroRNA‑21 (miR‑21) is a critical modulator of inflammatory responses in various chronic inflammatory conditions. Regulatory T cells (Tregs) and Th17 cells are critical CD4⁺ T cell subsets that govern inflammation, yet their developmental plasticity, transdifferentiation, and functional roles in periodontitis remain poorly defined. In vitro co-culture with heat-killed Porphyromonas gingivalis was conducted to detect T-cell differentiation and cytokine secretion, and Treg suppressive capacity was evaluated using co-culture proliferation assays. We performed adoptive transfer of purified Tregs into ligature-induced periodontitis mice. Flow cytometry was used to detect Treg/Th17 subsets, ELISA to quantify supernatant IL-6, IL-17A and IL-10, and qPCR to detect inflammatory and T lineage-related gene expression in gingiva. Alveolar bone loss was assessed by stereomicroscopic measurement of CEJ–ABC distance, H&E and IHC staining and micro-CT analysis. Using a well‑established ligature‑induced periodontitis mouse model, we observed significantly increased frequencies of Tregs and Th17 cells in the cervical lymph nodes of periodontitis mice compared with healthy controls. Isolated Tregs from periodontitis mice exhibited impaired suppressive capacity and enhanced pro‑inflammatory properties in vitro. Further in vitro experiments demonstrated that P. gingivalis infection promoted Treg proliferation, accompanied by the acquisition of pro‑inflammatory and Th17‑like phenotypes. In vivo adoptive transfer of CD45.1⁺ mouse‑derived Tregs into congenic CD45.2⁺ recipients confirmed that Treg can restore the impaired Treg/Th17 balance during periodontitis. Notably, adoptive Treg transfer significantly attenuated gingival inflammation and alveolar bone loss in periodontitis mice. Mechanistically, we investigated the role of microRNA‑21 (miR‑21) in Treg plasticity. Genetic deletion of miR‑21 exacerbated Treg transdifferentiation and upregulated pro‑inflammatory cytokines associated with alveolar bone resorption. MiR-21 restrains NF-κB signaling to sustain Treg/Th17 balance and alleviate ligature‑induced periodontal inflammation and bone loss, whereas miR-21 deletion aggravates tissue destruction recoverable via Treg adoptive transfer. This study identifies miR-21 as a core modulator regulating the periodontal immune response and alveolar bone turnover and highlights the potential of targeting the Treg/Th17 axis as an immunomodulatory strategy against periodontitis.