Th9/IL-9 axis mediates airway fibrosis in traumatic tracheal stenosis via TGF-β1/SMAD2/3 signaling

Although Th9 cells and their hallmark cytokine IL-9 are implicated in fibrotic diseases, their role in traumatic tracheal stenosis (TS) remains unclear. To clarify their role, we examined Th9 and IL-9 in human and rat models both in vitro and in vivo. First, we observed significantly elevated PU.1, IRF4, and IL-9 expression in patients with TS (n = 10), suggesting enhanced Th9-related immune responses. To assess the functional relevance of this finding, we then employed a rat TS model induced by nylon brush injury and a CD4⁺ T cell-fibroblast co-culture system. We found that IL-9 activated fibroblasts and robustly promoted collagen synthesis via the TGF-β1/SMAD2/3 pathway. Notably, neutralization of IL-9 or inhibition of TGF-β1 signaling markedly reduced collagen deposition and restored Th1/Th2 cytokine balance in both in vitro (n = 5) and in vivo (n = 6) models. Proteomic analysis further confirmed IL-9-driven activation of immune-fibrotic networks. These findings establish the Th9/IL-9 axis as a key mediator of airway fibrosis in TS and support its potential as a novel immunotherapeutic target.