Exposure to bacterial lipopolysaccharide aggravates H1N1-mediated lung immunopathology via neutrophil extracellular trap formation.

Influenza A virus (IAV) is a prevalent pathogen that causes severe respiratory infections worldwide. Currently, the incidence of coinfections between IAV and Gram-negative bacteria has been steadily increasing, and the synergistic interaction between IAV infection and bacterial exposure represents a pivotal risk factor for disease exacerbation. Lipopolysaccharide (LPS), a major Gram-negative bacterial virulence factor, is hypothesized to regulate IAV-induced immune response intensity and disease severity by reshaping the pulmonary immune microenvironment. Here, we established an in vivo coadministration model via intratracheal instillation of low-dose LPS and H1N1 to dissect their synergistic pathogenesis. LPS exposure potentiated H1N1-induced severe lung injury, manifested as persistent neutrophil and monocyte infiltration, elevated levels of cytokines (e.g., CXCL1, CCL2, and IL-6), and alveolar damage without affecting viral replication. Mechanistically, LPS triggered immune hyperactivation through early neutrophil recruitment, neutrophil extracellular trap (NET) formation, and a self-sustaining positive feedback loop. Intervention experiments revealed that the degradation of NETs by DNase I or neutrophil inhibition significantly alleviated lung damage and systemic inflammatory responses. This study establishes NET-driven innate immune amplification as a key mechanism underlying LPS-enhanced H1N1 pathogenicity, offers an immunomodulatory approach for severe bacterial-viral coinfections, and underscores the parity of immunomodulatory therapy with anti-infective treatment.