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Macrophage Turnover Kinetics in the Lungs of Mice Infected with Streptococcus pneumoniae

¹ Department of Pulmonary Medicine, Laboratory for Experimental Lung Research, and ⁵ Department of Radiotherapy, Hannover School of Medicine, Hannover, Germany; ² Department of Microbiology, University of Alabama at Birmingham, Birmingham, Alabama; ³ School of Molecular and Biomedical Science, University of Adelaide, Adelaide, Australia; and ⁴ Department of Pulmonary, Critical Care, and Sleep Medicine, University of Illinois at Chicago, Chicago, Illinois

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

Streptococcus pneumoniae is the most prevalent cause of community-acquired pneumonia and is known to induce apoptosis and necrosis in macrophages in vivo. We analyzed the kinetics of alveolar and lung parenchymal macrophage replacement by newly recruited exudate macrophages in vehicle-treated and S. pneumoniae–challenged bone marrow chimeric CD45.1 mice. After lethal irradiation, CD45.1 alloantigen-expressing recipient mice were transplanted with bone marrow cells from CD45.2 alloantigen-expressing donor mice. After only 24 hours of low-dose S. pneumoniae infection, approximately 60% of CD45.1^pos recipient-type alveolar macrophages (AM) were replaced by CD45.2^pos donor-type exudate AM in bronchoalveolar lavage fluid, and this increased to more than 80% on Day 7 of infection. In contrast, lung parenchymal macrophages of S. pneumoniae–infected chimeric CD45.1 mice were replaced by only about 10% by 24 hours, although this increased to over 80% by Days 3 to 7 of infection. This dramatic macrophage turnover was accompanied by early induction of apoptosis/necrosis in donor-type exudate AM peaking at 6 hours after infection, whereas peak apoptosis/necrosis induction in recipient-type AM was delayed until Day 7. Collectively, these data for the first time demonstrate that S. pneumoniae infection of the lung triggers a brisk turnover of both resident and recruited mononuclear phagocyte subsets, and suggest an important role of exudate but not resident macrophages in re-establishing alveolar and lung homeostasis.

Key Words: lung • infection • turnover • monocyte • macrophage

CLINICAL RELEVANCE This study demonstrates that Streptococcus pneumoniae infection of the lung triggers a brisk turnover of resident and recruited mononuclear phagocytes, and suggests an important role of exudate but not resident macrophages in re-establishing lung homeostasis.