Tryptophan metabolism reprogramming regulates Th1/Th2 immune balance and inhibits mast cell activation

Asthma is characterized by chronic airway inflammation and an imbalanced Th1/Th2 response. Although tryptophan metabolism has been implicated in immune regulation, its direct influence on Th1/Th2 differentiation and mast-cell activation remains insufficiently understood. CD4⁺ T cells were cultured under graded tryptophan concentrations (25, 50, 75 and 100 µM) to examine how tryptophan availability alters Th1/Th2 polarization. Flow cytometry, western blotting and RT-qPCR were used to evaluate phenotype markers and related metabolic pathways. In parallel, IgE-activated LAD2 mast cells were exposed to different tryptophan concentrations with or without the IDO1/TDO inhibitor HY-149,411, followed by immunofluorescence staining and ELISA to assess tryptase expression and histamine release. High tryptophan availability markedly enhanced Th1 differentiation, with increased Notch1/Jagged1 levels and elevated IL-2 and IFN-γ, while IL-4 expression was reduced. RT-qPCR showed upregulated T-bet and mTOR and downregulated GATA3, together with increased IDO1 and TDO mRNA levels in CD4⁺ T cells, indicating Th1-biased immunometabolic activation. In mast cells, tryptophan treatment suppressed tryptase expression and lowered histamine secretion, demonstrating reduced activation. HY-149,411 attenuated tryptophan-dependent differences in histamine release, suggesting that tryptophan catabolism contributes to mast-cell regulation. Tryptophan availability reprograms immune metabolism to promote Th1 differentiation and suppress mast-cell activation, offering a dual mechanism through which tryptophan may help correct Th1/Th2 imbalance and ameliorate allergic inflammation. These findings highlight tryptophan metabolism as a potential immunometabolic target for asthma therapy.