Introduction: Acute lung injury (ALI) due to pneumonia is a major contributor to the development of ARDS, with disease severity linked to excessive inflammatory cell recruitment and compromised lung barrier integrity. Understanding molecular pathways that can be targeted for therapeutic intervention remains crucial.

Objectives: To investigate the role of chemokine CXCL5 during ALI.

Methods: CXCL5 expression was evaluated in patients with severe community-acquired pneumonia, as well as in primary epithelial cells exposed to Streptococcus pneumoniae infection or mechanical stretch. The role of CXCL5 was further examined using Cxcl5-deficient mice in in vivo models of ALI and in vitro human alveolar epithelial barrier assays.

Results: Pneumonia and mechanical ventilation were associated with elevated CXCL5 levels in both human and murine lungs. CXCL5 was produced by bronchial and alveolar epithelial cells and stored in the epithelial glycocalyx. In Cxcl5-deficient mice, alveolar barrier integrity was preserved in ALI models, independent of neutrophil infiltration. Single-cell transcriptomic analysis revealed upregulation of cell junctional genes in epithelial, but not endothelial cells, in the absence of CXCL5. In vitro, CXCL5 exposure impaired the barrier function of TNF-primed human alveolar epithelial cells without affecting pulmonary endothelial cells.

Conclusions: Our findings identify a novel function of CXCL5 in disrupting alveolar epithelial barrier integrity during ALI. Beyond its established role in neutrophil recruitment, CXCL5 contributes directly to lung barrier dysfunction. Targeting CXCL5 may provide a promising adjunctive therapeutic strategy to reduce inflammation and enhance barrier stability in severe bacterial pneumonia.