TLR4 is required for macrophage efferocytosis during resolution of ventilator-induced lung injury

Abstract

Mechanical ventilation is a life-sustaining therapy for patients with respiratory failure but can cause further lung damage known as ventilator-induced lung injury (VILI). However, the intrinsic molecular mechanisms underlying recovery of VILI remain unknown. Phagocytosis of apoptotic cells (also known as efferocytosis) is a key mechanism orchestrating successful resolution of inflammation. Here we show the positive regulation of macrophage Toll-like receptor (TLR) 4 in efferocytosis and resolution of VILI. Mice were depleted of alveolar macrophages and then subjected to injurious ventilation (tidal volume, 20 mL/kg) for 4 h. On <i>day 1</i> after mechanical ventilation, <i>Tlr4^+/+</i> or <i>Tlr4^-/-</i> bone marrow-derived macrophages (BMDMs) were intratracheally administered to alveolar macrophage-depleted mice. We observed that mice depleted of alveolar macrophages exhibited defective resolution of neutrophilic inflammation, exuded protein, lung edema, and lung tissue injury after ventilation, whereas these delayed responses were reversed by administration of <i>Tlr4^+/+</i> BMDMs. Importantly, these proresolving effects by <i>Tlr4^+/+</i> BMDMs were abolished in mice receiving <i>Tlr4^-/-</i> BMDMs. The number of macrophages containing apoptotic cells or bodies in bronchoalveolar lavage fluid was much less in mice receiving <i>Tlr4^-/-</i> BMDMs than that in those receiving <i>Tlr4^+/+</i> BMDMs. Macrophage TLR4 deletion facilitated a disintegrin and metalloprotease 17 maturation and enhanced Mer cleavage in response to mechanical ventilation. Heat shock protein 70 dramatically increased Mer tyrosine kinase surface expression, phagocytosis of apoptotic neutrophils, and rescued the inflammatory phenotype in alveolar macrophage-depleted mice receiving <i>Tlr4^+/+</i> BMDMs, but not <i>Tlr4^-/-</i> BMDMs. Our results suggest that macrophage TLR4 promotes resolution of VILI via modulation of Mer-mediated efferocytosis.