title: Host-dependent trigger of caspases and apoptosis by Legionella pneumophila.
abstract: The Dot/Icm system of Legionella pneumophila triggers activation of caspase-3 during early stages of infection of human macrophages, but apoptosis is delayed until late stages of infection. During early stages of infection of mouse macrophages, the organism triggers rapid caspase-1-mediated cytotoxicity, which is mediated by bacterial flagellin. However, it is not known whether caspase-1 is triggered by L. pneumophila in human macrophages or whether caspase-3 is activated in permissive or nonpermissive mouse macrophages. Using single-cell analyses, we show that the wild-type strain of L. pneumophila does not trigger caspase-1 activation throughout the intracellular infection of human monocyte-derived macrophages (hMDMs), even when the flagellated bacteria escape into the cytoplasm during late stages. Using single-cell analyses, we show that the Dot/Icm system of L. pneumophila triggers caspase-3 but not caspase-1 within permissive A/J mouse bone marrow-derived primary macrophages by 2 to 8 h, but apoptosis is delayed until late stages of infection. While L. pneumophila triggers a Dot/Icm-dependent activation of caspase-1 in nonpermissive BALB/c mouse-derived macrophages, caspase-3 is not activated at any stage of infection. We show that robust intrapulmonary replication of the wild-type strain of L. pneumophila in susceptible A/J mice is associated with late-stage Dot/Icm-dependent pulmonary apoptosis and alveolar inflammation. In the lungs of nonpermissive BALB/c mice, L. pneumophila does not replicate and does not trigger pulmonary apoptosis or alveolar inflammation. Thus, similar to hMDMs, L. pneumophila does not trigger caspase-1 but triggers caspase-3 activation during early and exponential replication in permissive A/J mouse-derived macrophages, and apoptosis is delayed until late stages of infection. The Dot/Icm type IV secretion system is essential for pulmonary apoptosis in the genetically susceptible A/J mice.
authors.affiliation: Department of Microbiology and Immunology, University of Louisville College of Medicine, 319 Abraham Flexner Way 55A, Louisville, KY 40202, USA.
authors.forename: Marina
authors.initials: M
authors.lastname: Santic
authors.forename: Rexford
authors.initials: R
authors.lastname: Asare
authors.forename: Miljenko
authors.initials: M
authors.lastname: Doric
authors.forename: Yousef
authors.initials: Y
authors.lastname: Abu Kwaik
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D001426:Bacterial Proteins / Q000378:metabolism; D020169:Caspases / Q000378:metabolism*; D004789:Enzyme Activation / Q000276:immunology; D006801:Humans; D016952:Legionella pneumophila / Q000235:genetics / Q000378:metabolism*; D008168:Lung / Q000473:pathology; D008264:Macrophages / Q000276:immunology / Q000382:microbiology*; D051379:Mice; D008807:Mice, Inbred BALB C
chemical_list: D001426:Bacterial Proteins; C432955:ICMQ protein, Legionella pneumophila; D020169:Caspases