• William J Janssen, Lea Barthel, Alaina Muldrow, Rebecca E Oberley-Deegan, Mark T Kearns, Claudia Jakubzick, and Peter M Henson.
• Division of Pulmonary Medicine, Department of Medicine, National Jewish Health, Denver, Colorado 80206, USA. janssenw@NJHealth.org
• Am. J. Respir. Crit. Care Med.. 2011 Sep 1;184(5):547-60.

RationaleDuring acute lung injury (ALI) the macrophage pool expands markedly as inflammatory monocytes migrate from the circulation to the airspaces. As inflammation resolves, macrophage numbers return to preinjury levels and normal tissue structure and function are restored.ObjectivesTo determine the fate of resident and recruited macrophages during the resolution of ALI in mice and to elucidate the mechanisms responsible for macrophage removal.MethodsALI was induced in mice using influenza A (H1N1; PR8) infection and LPS instillation. Dye labeling techniques, bone marrow transplantation, and surface immunophenotyping were used to distinguish resident and recruited macrophages during inflammation and to study the role of Fas in determining macrophage fate during resolving ALI.Measurements And Main ResultsDuring acute and resolving lung injury from influenza A and LPS, a high proportion of the original resident alveolar macrophages persisted. In contrast, recruited macrophages exhibited robust accumulation in early inflammation, followed by a progressive decline in their number. This decline was mediated by apoptosis with local phagocytic clearance. Recruited macrophages expressed high levels of the death receptor Fas and were rapidly depleted from the airspaces by Fas-activating antibodies. In contrast, macrophage depletion was inhibited in mice treated with Fas-blocking antibodies and in chimeras with Fas-deficient bone marrow. Caspase-8 inhibition prevented macrophage apoptosis and delayed the resolution of ALI.ConclusionsThese findings indicate that Fas-induced apoptosis of recruited macrophages is essential for complete resolution of ALI.

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