Ma-Xing-Shi-Gan decoction alleviates allergic asthma by modulating the gut microbiota-tryptophan metabolism-ILC2 axis

Ma-Xing-Shi-Gan decoction (MXSG) shows clinical efficacy in asthma, yet how it shapes gut-lung immunity-particularly type 2 innate lymphoid responses-remains poorly defined.

To investigate whether MXSG mitigates asthma by restraining group 2 innate lymphoid cells (ILC2s) via a gut microbiota-tryptophan metabolic pathway, and to identify microbiota-dependent active compounds.

An asthma mouse model was used. ILC2 in the lung and intestinal lamina propria were assessed by flow cytometry. Rag1^⁻/⁻ mice were used to assess T and B cell-independent effects. Untargeted fecal metabolomics and antibiotic-mediated microbiota depletion were conducted to evaluate metabolic and microbial contributions. Microbiota-dependent MXSG constituents were traced using anaerobic fecal fermentation coupled with LC-MS/MS profiling, followed by in vivo validation.

MXSG significantly alleviated pulmonary inflammation, reduced bronchoalveolar lavage eosinophils and improved histopathology. It decreased ILC2s populations in lung and gut. These effects were preserved in Rag1^⁻/⁻ mice but abolished with antibiotics pretreatment, indicating microbiota dependence. Metabolomics revealed that MXSG reprogrammed tryptophan metabolism, restoring tryptamine and rebalancing kynurenine, indole, and serotonin-related branches. Anaerobic fermentation and LC-MS/MS profiling identified microbiota-dependent flavonoids, and isorhamnetin partially reproduced the anti-inflammatory and ILC2-modulating effects in vivo.

MXSG exerts its anti-asthmatic effects via the gut microbiota-tryptophan metabolism-ILC2 axis. These findings reveal a novel gut-lung mechanism centered on type 2 innate immunity and microbiota-derived indole metabolism.