Rationale: Mechanical ventilation (MV) is life-saving but may evoke ventilator-induced lung injury (VILI). Objectives: To explore how the circadian clock modulates severity of murine VILI via the core clock component BMAL1 (basic helix-loop-helix ARNT like 1) in myeloid cells. Methods: Myeloid cell BMAL1-deficient (LysM (lysozyme 2 promoter/enhancer driving cre recombinase expression)^Bmal1-/-) or wild-type control (LysM^Bmal1+/+) mice were subjected to 4 hours MV (34 ml/kg body weight) to induce lung injury. Ventilation was initiated at dawn or dusk or in complete darkness (circadian time [CT] 0 or CT12) to determine diurnal and circadian effects. Lung injury was quantified by lung function, pulmonary permeability, blood gas analysis, neutrophil recruitment, inflammatory markers, and histology. Neutrophil activation and oxidative burst were analyzed ex vivo. Measurements and Main Results: In diurnal experiments, mice ventilated at dawn exhibited higher permeability and neutrophil recruitment compared with dusk. Experiments at CT showed deterioration of pulmonary function, worsening of oxygenation, and increased mortality at CT0 compared with CT12. Wild-type neutrophils isolated at dawn showed higher activation and reactive oxygen species production compared with dusk, whereas these day-night differences were dampened in LysM^Bmal1-/- neutrophils. In LysM^Bmal1-/- mice, circadian variations in VILI severity were dampened and VILI-induced mortality at CT0 was reduced compared with LysM^Bmal1+/+ mice. Conclusions: Inflammatory response and lung barrier dysfunction upon MV exhibit diurnal variations, regulated by the circadian clock. LysM^Bmal1-/- mice are less susceptible to ventilation-induced pathology and lack circadian variation of severity compared with LysM^Bmal1+/+ mice. Our data suggest that the internal clock in myeloid cells is an important modulator of VILI.

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