• Julie A Preston, Martin A Bewley, Helen M Marriott, McGarry Houghton A A 3 Clinical Research Division, Fred Hutchinson Cancer Research Center, and. 4 Divisi, Mohammed Mohasin, Jamil Jubrail, Lucy Morris, Yvonne L Stephenson, Simon Cross, David R Greaves, Ruth W Craig, Nico van Rooijen, Colin D Bingle, Robert C Read, Timothy J Mitchell, Whyte Moira K B MKB 6 MRC Centre for Inflammation Research. 14 Department of Respiratory Medicine, and., Steven D Shapiro, and David H Dockrell.
• 1 The Florey Institute for Host-Pathogen Interactions and.
• Am. J. Respir. Crit. Care Med. 2019 Jul 1; 200 (1): 84-97.

AbstractRationale: Antimicrobial resistance challenges therapy of pneumonia. Enhancing macrophage microbicidal responses would combat this problem but is limited by our understanding of how alveolar macrophages (AMs) kill bacteria. Objectives: To define the role and mechanism of AM apoptosis-associated bacterial killing in the lung. Methods: We generated a unique CD68.hMcl-1 transgenic mouse with macrophage-specific overexpression of the human antiapoptotic Mcl-1 protein, a factor upregulated in AMs from patients at increased risk of community-acquired pneumonia, to address the requirement for apoptosis-associated killing. Measurements and Main Results: Wild-type and transgenic macrophages demonstrated comparable ingestion and initial phagolysosomal killing of bacteria. Continued ingestion (for ≥12 h) overwhelmed initial killing, and a second, late-phase microbicidal response killed viable bacteria in wild-type macrophages, but this response was blunted in CD68.hMcl-1 transgenic macrophages. The late phase of bacterial killing required both caspase-induced generation of mitochondrial reactive oxygen species and nitric oxide, the peak generation of which coincided with the late phase of killing. The CD68.hMcl-1 transgene prevented mitochondrial reactive oxygen species but not nitric oxide generation. Apoptosis-associated killing enhanced pulmonary clearance of Streptococcus pneumoniae and Haemophilus influenzae in wild-type mice but not CD68.hMcl-1 transgenic mice. Bacterial clearance was enhanced in vivo in CD68.hMcl-1 transgenic mice by reconstitution of apoptosis with BH3 mimetics or clodronate-encapsulated liposomes. Apoptosis-associated killing was not activated during Staphylococcus aureus lung infection. Conclusions: Mcl-1 upregulation prevents macrophage apoptosis-associated killing and establishes that apoptosis-associated killing is required to allow AMs to clear ingested bacteria. Engagement of macrophage apoptosis should be investigated as a novel, host-based antimicrobial strategy.

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