Alveolar macrophage apoptosis-associated bacterial killing helps prevent murine pneumonia

RATIONALE: Antimicrobial resistance challenges therapy of pneumonia. Enhancing macrophage microbicidal responses would combat this problem but is limited by our understanding of how alveolar macrophages (AM) 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 over-expression of the human anti-apoptotic Mcl-1 protein, a factor upregulated in AM 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 (mROS) and nitric oxide (NO), whose peak generation coincided with the late-phase of killing. The CD68.hMcl-1 transgene prevented mROS but not NO generation. Apoptosis-associated killing enhanced pulmonary clearance of Streptococcus pneumoniae and Haemophilus influenzae in wild-type 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 AM to clear ingested bacteria. Engagement of macrophage apoptosis should be investigated as a novel host-based antimicrobial strategy.