Influenza pneumonia mice under different immune conditions: changes in pulmonary microbiota and metabolites.

Pneumonia is the most common complication of influenza virus infection and is associated with a high mortality rate in immunocompromised individuals. Currently, neither vaccination nor antiviral therapy has achieved satisfactory therapeutic outcomes. Unlike other organs, the lungs harbor a distinct microbial community due to their unique exposure patterns to airborne particulates and pathogens. However, the characteristic alterations of the pulmonary microbiota and its metabolic products, as well as their potential association with immune suppression in influenza pneumonia, remain inadequately investigated. Mice were rendered immunocompromised through administration of the immunosuppressant cyclophosphamide. Mouse models of influenza pneumonia were established under both immunocompetent and immunosuppressed conditions. Body weight was routinely monitored, and lung histopathology was assessed via hematoxylin and eosin staining. Viral load in lung tissue, serum inflammatory cytokine levels, pulmonary microbiota composition, and lung metabolites were analyzed using RT-qPCR, enzyme-linked immunosorbent assay, 16S rRNA gene sequencing, and untargeted metabolomics, respectively. Spearman correlation analysis was performed to evaluate significant associations between pulmonary microbial taxa and specific metabolites. Influenza virus infection led to a marked reduction in body weight and significantly increased viral load in lung tissue, as well as elevated serum levels of inflammatory cytokines (IL-1β, IL-6, and TNF-α). These effects were more pronounced in immunosuppressed mice, which also exhibited more severe inflammatory and pathological changes in lung tissues. Characteristic shifts in the pulmonary microbiota were observed in the immunosuppressed influenza pneumonia model, particularly involving increased abundance of