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Resident Alveolar Macrophages Are Replaced by Recruited Monocytes in Response to Endotoxin-Induced Lung Inflammation

Department of Internal Medicine, Division of Pulmonary and Critical Care Medicine and Infectious Diseases, University Giessen Lung Center (UGLC), Giessen; Hannover School of Medicine, Department of Pulmonary Medicine, Hannover, Germany; Department of Medicine, Division of Allergy, Pulmonary, and Critical Care Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee; and Section of Pulmonary, Critical Care, and Sleep Medicine, University of Illinois at Chicago, Chicago, Ilinois

Correspondence and requests for reprints should be addressed to Ulrich A. Maus, Ph.D., Hannover School of Medicine, Laboratory for Experimental Lung Research, Feodor-Lynen-Strasse 21, Hannover 30625, Germany. E-mail: Maus.Ulrich{at}mh-hannover.de

In the acute respiratory distress syndrome, recruitment of peripheral blood monocytes results in expansion of the total pool of resident alveolar macrophages. The fate of resident macrophages, or whether recruited monocytes are selectively eliminated from the alveolar airspace or differentiate into resident alveolar macrophages during the resolving phase of inflammation, has not been determined. Here, we analyzed the kinetics of resident and recruited macrophage turnover within the alveolar airspace of untreated and LPS-challenged mice. Using bone marrow chimeric CD45.2 mice that were generated by lethal irradiation of CD45.2 alloantigen-expressing recipient mice and bone marrow transplantation from CD45.1 alloantigen-expressing donor mice, we employed a flow cytometric approach to distinguish recipient from donor-type macrophages in bronchoalveolar lavage fluids. Our data show that resident alveolar macrophages of untreated chimeric CD45.2 mice are very slowly replaced by constitutively immigrating CD45.1 positive monocytes, resulting in a replacement rate of 40% by 1 yr. In contrast, more than 85% of the resident CD45.2 positive alveolar and lung homogenate macrophages were exchanged by donor CD45.1-expressing macrophages within 2 mo after treatment with Escherichia coli endotoxin (LPS). Importantly, fluorescence-activated cell sorter analysis of increased annexin V binding to both recipient and donor-type macrophages revealed increased apoptotic events to underlie this endotoxin-driven inflammatory macrophage turnover. Collectively, the data show that under baseline conditions the alveolar macrophage turnover exhibits very slow kinetics, whereas acute lung inflammation in response to treatment with LPS triggers a brisk acceleration of recruitment of monocytes that replace the resident alveolar macrophage population.

Key Words: alveolar macrophage • inflammation • lung • monocyte • turnover

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