VEGF Production by Ly6C(+high) Monocytes Contributes to Ventilator-Induced Lung Injury

BACKGROUND: Mechanical ventilation is a life-saving procedure for patients with acute respiratory failure, although it may cause pulmonary vascular inflammation and leakage, leading to ventilator-induced lung injury (VILI). Ly6C(+high) monocytes are involved in the pathogenesis of VILI. In this study, we investigated whether pulmonary infiltrated Ly6C(+high) monocytes produce vascular endothelial growth factor (VEGF) and contribute to VILI. METHODS: A clinically relevant two-hit mouse model of VILI, with intravenous lipopolysaccharide (LPS, 20 ng/mouse) immediately before high tidal volume (HTV, 20 mL/kg) ventilation (LPS+HTV), was established. Blood gas and respiratory mechanics were measured to ensure the development of VILI. Flow cytometry and histopathological analyses revealed pulmonary infiltration of leukocytes subsets. Clodronate liposomes were intravenously injected to deplete pulmonary monocytes. In vitro endothelial cell permeability assay with sorted Ly6C(+high) monocytes condition media assessed the role of Ly6C(+high) monocytes in vascular permeability. RESULTS: LPS+HTV significantly increased total proteins, TNF-α, IL-6, vascular endothelial growth factor (VEGF) and mononuclear cells in the bronchoalveolar lavage fluid (BALF). Pulmonary Ly6C(+high) monocytes (SSC(low)CD11b(+)F4/80(+)Ly6C(+high)), but not Ly6C(+low) monocytes (SSC(low)CD11b(+)F4/80(+)Ly6C(+low)), were significantly elevated starting at 4 hr. Clodronate liposomes were able to significantly reduce pulmonary Ly6C(+high) monocytes, and VEGF and total protein in BALF, and restore PaO(2)/FiO(2). There was a strong correlation between pulmonary Ly6C(+high) monocytes and BALF VEGF (R(2) = 0.8791, p<0.001). Moreover, sorted Ly6C(+high) monocytes were able to produce VEGF, resulting in an increased permeability of endothelial cell monolayer in an in vitro endothelial cell permeability assay. CONCLUSION: VEGF produced by pulmonary infiltrated Ly6C(+high) monocytes regulates vasculature permeability in a two-hit model of HTV-induced lung injury. Ly6C(+high) monocytes play an important role in the pathogenesis of VILI.