Kuijieling decoction regulates the Treg/Th17 cell balance in ulcerative colitis through the RA/RAR signaling pathway

Ethnopharmacological relevance: Ulcerative colitis (UC) is an idiopathic intestinal disease characterized by chronic inflammation with unknown etiology. Kuijieling decoction is a traditional Chinese Medicine with unique therapeutic efficacy for UC. Aim of the study: To validate the effects of Kuijieling decoction on vitamin A metabolism and retinoic acid (RA) production, along with evaluation of its immunomodulatory activity, and further clarify the upstream mechanisms underlying Treg/Th17 regulation that contribute to therapeutic effects against UC. Materials and methods: Network pharmacology and molecular docking analyses were employed to predict the potential anti-UC targets of Kuijieling and associated pathways. A rat model of UC was generated by treatment with trinitrobenzene sulfonic acid (TNBS) to induce inflammation. T lymphocytes were induced to differentiate into Th17 via combined stimulation with the cytokines TGF-beta 1, IL-6 and IL-23. Expression levels of RA/RAR alpha-related factors (RARa, CRABPII, Smad3, IL-6R and IL-23R) and Treg/Th17 cell-related factors (Foxp3, ROR gamma t) were measured via western blot (WB), quantitative real-time PCR (RT-qPCR), and immunohistochemistry analyses. Components of the vitamin A metabolic pathway (vitamin A, retinol, retinoic acid) and Treg/Th17 cell-related cytokines (IL-10, IL-17, IL-21) were evaluated using ELISA. Flow cytometry was performed to determine the percentages of Treg and Th17 cells. Results: The action targets of Kuijieling were significantly associated with T cell activation, Th17 cell differentiation and the immune response. IL-2, IL-6, STAT3 and RARa displayed strong binding affinities with the main components of Kuijieling, suggesting an important role in its therapeutic efficacy. Kuijieling promoted vitamin A metabolism and RA/RAR alpha signaling in UC rats and T lymphocytes. Moreover, Kuijieling effectively regulated the Treg/Th17 cell balance in UC rats and T lymphocytes and relieved inflammation. The protective effect of Kuijieling was weakened after inhibition of the RA/RAR alpha signaling pathway. Conclusions: Kuijieling promotes vitamin A metabolism and RA synthesis, enhances interactions between RA, intracellular binding protein CRABPII and nuclear receptor RAR alpha, and upregulates Smad3 and Foxp3, thus promoting Treg differentiation. Simultaneously, Kuijieling inhibited expression of IL-6R and IL-23R genes and production of ROR gamma t, leading to suppression of Th17 differentiation, and ultimately, reduction of the Th17/Treg cell ratio.