Lung microbiota-derived deoxyinosine alleviates TBI-aggravated sepsis-induced lung injury via the S100A9/RAGE pathway

Traumatic brain injury (TBI) frequently leads to severe systemic complications, with pulmonary dysfunction acting as a major determinant of poor prognosis in survivors. While the lung microbiota is increasingly recognized as a critical regulator of pulmonary immune homeostasis, the specific mechanisms by which TBI remotely remodels the lung microenvironment to exacerbate secondary insults, such as sepsis-induced acute lung injury (ALI), remain poorly understood. To investigate this mechanism, we established a murine model combining controlled cortical impact with LPS-induced sepsis and analyzed bronchoalveolar lavage fluid by 16S rRNA sequencing and untargeted metabolomics. We further conducted microbiota depletion and transplantation experiments to establish causality, alongside molecular docking, Co-IP (co-immunoprecipitation), and transgenic mouse models to elucidate molecular pathways. Our results demonstrate that TBI significantly disrupts the lung microbiota, characterized by a reduction in Corynebacterium, and decreases the levels of the metabolite deoxyinosine. Microbiota transplantation from TBI mice worsened sepsis-induced lung injury in recipients, whereas deoxyinosine administration alleviated tissue damage by promoting the polarization of alveolar macrophages from a pro-inflammatory M1 phenotype to an anti-inflammatory M2 phenotype. Mechanistically, deoxyinosine binds directly to S100A9, competitively inhibiting its interaction with the Receptor for Advanced Glycation End Products (RAGE), which subsequently suppresses downstream NF-κB signaling. This study identifies a novel brain-lung axis interaction mediated by microbiota-derived deoxyinosine and highlights the S100A9/RAGE pathway as a promising therapeutic target for preventing post-TBI multi-organ dysfunction. TBI-induced dysbiosis of the mouse lung microbiota reduces deoxyinosine levels, thereby leading to enhanced S100A9/RAGE signaling, M1 polarization of macrophages, and ultimately exacerbating sepsis-induced lung injury TBI induces lung microbiota dysbiosis characterized by a decrease in Corynebacterium and its associated metabolite deoxyinosine. Deoxyinosine administration alleviates TBI-exacerbated sepsis-induced lung injury by shifting alveolar macrophages from M1 to M2 polarization. Deoxyinosine directly binds to S100A9, competitively inhibiting its interaction with RAGE and suppressing downstream NF-κB signaling. S100A9 or RAGE ablation abolishes the protective effects of deoxyinosine, confirming the critical role of the S100A9/RAGE pathway. This study reveals a novel microbiota-mediated brain-lung axis and identifies deoxyinosine as a potential therapeutic agent for post-TBI multi-organ dysfunction.