Indole-acetaldehyde from Rothia mucilaginosa activates the PXR/NRF2 axis to enhance alveolar macrophage phagocytosis and protect against ARDS

Despite advances in therapeutic strategies, acute respiratory distress syndrome (ARDS) mortality remains high. Growing evidence links respiratory microbiome composition to ARDS outcomes. This investigation sought to elucidate how colonizing bacteria and their metabolites influence ARDS pathogenesis. Bronchoalveolar lavage fluid (BALF) from patients with pulmonary infections was analyzed by metagenomic next-generation sequencing (mNGS) to identify characteristic bacteria. Bacterial culture supernatants were analyzed by untargeted metabolomics (LC-MS) to identify metabolites. A murine ARDS model was established through intratracheal LPS instillation. Single-cell sequencing datasets from the GEO database were analyzed to reveal differential cell populations and functional alterations in murine ARDS. Potential molecular mechanisms were explored through molecular docking, RNA-seq analysis, Western boltting, and targeted gene knockdown in murine and cellular model. R. mucilaginosa demonstrated enrichment in patients without ARDS (nARDS). The bacterial culture supernatant conferred substantial protection in murine models, whereas viable bacteria showed minimal efficacy. LC-MS analysis identified indole-3-acetaldehyde (IAAld) as the predominant metabolite in the supernatant. Single-cell sequencing suggested that resident alveolar macrophages (RAMs) were pivotal cells in murine ARDS model. IAAld enhanced RAMs phagocytosis, facilitating neutrophil and LPS clearance. Mechanistic studies revealed that IAAld likely activated PXR signaling, promoted NRF2 nuclear translocation, and upregulated the phagocytosis-related gene CD36. Targeted PXR knockdown eliminated these protective effects. The respiratory commensal R. mucilaginosa synthesizes IAAld, which—independent of bacterial colonization per se—ameliorates ARDS through PXR/NRF2/CD36 axis activation, thereby enhancing macrophage phagocytic function. These findings suggest that therapeutic targeting of microbial metabolites represents a novel ARDS treatment paradigm.