Toll-like Receptor 4-Myeloid Differentiation Factor 88 Signaling Contributes to Ventilator-induced Lung Injury in Mice

Abstract Background: The mechanisms of ventilator-induced lung injury, an iatrogenic inflammatory condition induced by mechanical ventilation, are not completely understood. Toll-like receptor 4 (TLR4) signaling via the adaptor protein myeloid differentiation factor 88 (MyD88) is proinflammatory and plays a critical role in host immune response to invading pathogen and noninfectious tissue injury. The role of TLR4-MyD88 signaling in ventilator-induced lung injury remains incompletely understood. Methods: Mice were ventilated with low or high tidal volume (HTV), 7 or 20 ml/kg, after tracheotomy for 4 h. Control mice were tracheotomized without ventilation. Lung injury was assessed by: alveolar capillary permeability to Evans blue albumin, wet/dry ratio, bronchoalveolar lavage analysis for cell counts, total proteins and cytokines, results of histopathological examination of the lung, and plasma cytokine levels. Results: Wild-type mice subjected to HTV had increased pulmonary permeability, inflammatory cell infiltration/lung edema, and interleukin-6/macrophage-inflammatory protein-2 in the lavage compared with control mice. In HTV, levels of inhibitor of κB α decreased, whereas phosphorylated extracellular signal-regulated kinases increased. TLR4 mutant and MyD88−/− mice showed markedly attenuated response to HTV, including less lung inflammation, pulmonary edema, cell number, protein content, and the cytokines in the lavage. Furthermore, compared with wild-type mice, both TLR4 mutant and MyD88−/− mice had significantly higher levels of inhibitor of κB α and reduced extracellular signal-regulated kinase phosphorylation after HTV. Conclusions: TLR4-MyD88 signaling plays an important role in the development of ventilator-induced lung injury in mice, possibly through mechanisms involving nuclear factor-κB and mitogen-activated protein kinase pathways.