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Role of Lung-marginated Monocytes in an In Vivo Mouse Model of Ventilator-induced Lung Injury

¹ Department of Anaesthetics, Pain Medicine and Intensive Care, Faculty of Medicine, Imperial College London, Chelsea and Westminster Hospital, London, United Kingdom; and ² Vrije Universiteit, Vrije Universiteit Medisch Centrum, Department of Molecular Cell Biology, Faculty of Medicine, Amsterdam, The Netherlands

Correspondence and requests for reprints should be addressed to Masao Takata, M.D., Ph.D., Department of Anaesthetics, Pain Medicine and Intensive Care, Imperial College London, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK. E-mail: m.takata{at}imperial.ac.uk

Rationale: Recruited leukocytes play an important role in ventilator-induced lung injury, although studies have focused predominantly on neutrophils. Inflammatory subset Gr-1^high monocytes are recruited to sites of inflammation and have been implicated in acute lung injury induced by systemic endotoxin.

Objectives: To investigate the recruitment and role of Gr-1^high monocytes in an in vivo mouse model of ventilator-induced lung injury.

Methods: Anesthetized mice were ventilated with low or high stretch. Flow cytometry was used to quantify monocyte subset margination to the lungs, and to assess their in situ cellular activation in response to mechanical stretch. To investigate monocyte involvement in lung injury progression, a two-hit model was used, with a subclinical dose of lipopolysaccharide (intraperitoneal) given 2 hours prior to high-stretch ventilation. In some animals, monocytes were depleted using intravenous clodronate liposomes. Development of lung injury was assessed in ventilated animals by peak inspiratory pressure and respiratory system mechanics.

Measurements and Main Results: High-stretch ventilation induced significant pulmonary margination of Gr-1^high but not Gr-1^low monocytes compared with nonventilated mice. These monocytes displayed increased activation status, with higher CD11b (vs. nonventilated mice) and lower L-selectin expression (vs. low-stretch ventilation). Lipopolysaccharide challenge led to enhanced lung margination of Gr-1^high monocytes and neutrophils, and sensitized the lungs to high stretch–induced pulmonary edema. Clodronate-liposome pretreatment depleted lung monocytes (but not neutrophils) and significantly attenuated lung injury.

Conclusions: High-stretch mechanical ventilation promotes pulmonary margination of activated Gr-1^high monocytes, which play a role in the progression of ventilator-induced lung injury.

Key Words: Gr-1^high monocytes • acute lung injury • stretch • inflammation • mechanical ventilation

AT A GLANCE COMMENTARY Scientific Knowledge on the Subject Recruited leukocytes are known to play an important role in ventilator-induced lung injury (VILI), although studies have focused on neutrophils. Recent studies implicate lung-marginated monocytes in the development of other forms of acute lung injury. What This Study Adds to the Field This study demonstrates that inflammatory subset Gr-1high monocytes are recruited to the lung during high-stretch mechanical ventilation and contribute to the progression of pulmonary edema during VILI.

 

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