Ventilation-induced airspace acidification via NHE1 increased susceptibility to Pseudomonas aeruginosa pneumonia

Ventilator-associated pneumonia (VAP) is a major cause of nosocomial infections in critically ill patients, associated with increased morbidity and mortality. Local pH fluctuations influence bacterial growth, but whether airway acidification contributes to the pathogenesis of Pseudomonas aeruginosa (PA)-induced VAP is still unknown. C57Bl/6J mice were subjected to mechanical ventilation (MV) with high (HVt:34mL/kg) or low (LVt:9mL/kg) tidal volume for 4h. Subsequently, PA was instilled via the tracheal tube, and animals were allowed to recover from sedation and breathed spontaneously for 24h. Alveolar pH was determined in ex vivo perfused and ventilated (HVt vs LVt) murine lungs via fluorescence microscopy. Bacterial growth and adhesion on cyclically stretched human A549 cells and primary human alveolar epithelial cells were examined. Upon PA infection, HVt mice showed increased alveolo-capillary permeability, elevated lung and blood leukocyte counts, and higher bacterial load in lungs and extrapulmonary organs as compared to LVt controls. HVt-MV induced acidification of airway lining fluid in lungs and decreased pulmonary expression of Na+/H+ exchanger 1 (NHE1), a major regulator of pH. Inhibition of NHE1 increased pulmonary bacterial loads in MV mice, while in vitro, NHE1 inhibition enhanced PA growth and adhesion to alveolar epithelial cells. In our novel murine VAP model, key characteristics of PA-VAP were replicated. HVt-MV induced mild lung injury and acidification of airway lining fluid, increasing susceptibility to PA pneumonia. NHE1 was identified as critical factor for MV-induced airspace acidification, and thus as potential target to conteract PA-VAP.