Mild-stretch mechanical ventilation upregulates toll-like receptor 2 and sensitizes the lung to bacterial lipopeptide

Both in vitro and in vivo experiments were conducted. First, TLR2 expression and protein were measured in the A549 pulmonary epithelial cell line submitted to 8-hour cyclic stretch (20% elongation; 20/minute rate). After a 24-hour period of cyclic stretch, the inflammatory response of the A549 cells to the synthetic BLP, Pam3CSK4, was tested after 8 hours of exposure. In a second set of experiments, healthy anesthetized and paralyzed rabbits were submitted to 8-hour MV (tidal volume = 12 ml/kg, zero end-expiratory pressure; FIO2 = 50%; respiratory rate = 20/minute) before being sacrificed for TLR2 lung expression assessment. The lung inflammatory response to BLP was then tested in animals submitted to 24-hour MV before being sacrificed 8 hours after the tracheal instillation of Pam3CSK4.Cyclic stretch of human pulmonary epithelial cell lines increased both TLR2 mRNA and protein expression. Cells submitted to cyclic stretch also increased IL-6 and IL-8 secretion in response to Pam3CSK4, a classical TLR2 ligand. A mild-stretch MV protocol induced a 60-fold increase of TLR2 mRNA expression in lung tissue when compared with spontaneously breathing controls. Moreover, the combination of MV and airway exposure to Pam3CSK4 acted synergistically in causing lung inflammation and injury.Mild-stretch MV increases lung expression of TLR2 and sensitizes the lung to bacterial TLR2 ligands. This may account for the propensity of mechanically ventilated patients to develop acute lung injury in the context of airway bacterial colonization/infection.Bacterial superinfection of the lung is a frequent complication in critically ill patients requiring mechanical ventilation (MV) [1]. An incidence of ventilator-associated pneumonia of up to 30% has been reported in patients requiring prolonged MV. Among the bacteria causing ventilator-associated pneumonia, Staphylococcus aureus is a leading source of infection and mortality [2]. Both in vitro and in vivo experimental studies have demonst