title: Cytosolic recognition of flagellin by mouse macrophages restricts Legionella pneumophila infection.
abstract: To restrict infection by Legionella pneumophila, mouse macrophages require Naip5, a member of the nucleotide-binding oligomerization domain leucine-rich repeat family of pattern recognition receptors, which detect cytoplasmic microbial products. We report that mouse macrophages restricted L. pneumophila replication and initiated a proinflammatory program of cell death when flagellin contaminated their cytosol. Nuclear condensation, membrane permeability, and interleukin-1beta secretion were triggered by type IV secretion-competent bacteria that encode flagellin. The macrophage response to L. pneumophila was independent of Toll-like receptor signaling but correlated with Naip5 function and required caspase 1 activity. The L. pneumophila type IV secretion system provided only pore-forming activity because listeriolysin O of Listeria monocytogenes could substitute for its contribution. Flagellin monomers appeared to trigger the macrophage response from perforated phagosomes: once heated to disassemble filaments, flagellin triggered cell death but native flagellar preparations did not. Flagellin made L. pneumophila vulnerable to innate immune mechanisms because Naip5+ macrophages restricted the growth of virulent microbes, but flagellin mutants replicated freely. Likewise, after intratracheal inoculation of Naip5+ mice, the yield of L. pneumophila in the lungs declined, whereas the burden of flagellin mutants increased. Accordingly, macrophages respond to cytosolic flagellin by a mechanism that requires Naip5 and caspase 1 to restrict bacterial replication and release proinflammatory cytokines that control L. pneumophila infection.
authors.affiliation: Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109, USA.
authors.forename: Ari B
authors.initials: AB
authors.lastname: Molofsky
authors.forename: Brenda G
authors.initials: BG
authors.lastname: Byrne
authors.forename: Natalie N
authors.initials: NN
authors.lastname: Whitfield
authors.forename: Cressida A
authors.initials: CA
authors.lastname: Madigan
authors.forename: Etsu T
authors.initials: ET
authors.lastname: Fuse
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Tateda
authors.forename: Michele S
authors.initials: MS
authors.lastname: Swanson
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics; D000818:Animals; D017209:Apoptosis / Q000502:physiology; D002478:Cells, Cultured; D003600:Cytosol / Q000276:immunology*; D005260:Female; D005408:Flagellin / Q000276:immunology*; D007113:Immunity, Innate; D016952:Legionella pneumophila / Q000276:immunology*; D008264:Macrophages / Q000276:immunology* / Q000382:microbiology; D051379:Mice; D008805:Mice, Inbred A; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88; D051616:Neuronal Apoptosis-Inhibitory Protein / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology; D051193:Toll-Like Receptors / Q000502:physiology
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; C496806:Naip5 protein, mouse; D051616:Neuronal Apoptosis-Inhibitory Protein; D051193:Toll-Like Receptors; D005408:Flagellin