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-1β 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.

Inflammatory caspases and inflammasomes: master switches of inflammation

“…100,101 Similarly, other studies showed that NAIP5, a mouse parologue of NAIP, possibly in combination with IPAF, recognizes intracellular flagellin from Legionella pneumophila, in order to induce caspase-1 activation. 61,102,103 Moreover, early genetic studies in mice clearly identified NAIP5 as a L. pneumophila susceptibility locus, further delineating the importance of inflammatory caspases in the control of bacterial pathogens. 104 NALPs and the Biology of Reproduction: A Role for the Inflammatory Caspases?…”

Cell death in the host response to infection

“…Several bacterial strains possess delivery systems responsible for the translocation of effector molecules into the host cytosol, which permit bacterial internalization and/or replication. 87,88 Disruption of the type four secretion system in Legionella pneumophila, 53,54,89 of the Listeria monocytogenes listeriolysin O, or of the type three secretion system in Salmonella typhimurium, 47,82,83 Shigella flexneri, 90 Yersinia pseudotuberculosis 55 and Pseudomonas aeruginosa 51 inhibits caspase-1 activation by these pathogens. Although initial reports had proposed that proteins of the type three secretion system, SipB in Salmonella 44 and IpaB in Shigella, 43 could directly bind and activate caspase-1, recent studies indicate a more indirect role for such bacterial delivery systems, presumably by allowing the inflammasome agonist to reach the cytosol.…”

The NLRC4 inflammasome: The pieces of the puzzle are falling into place

“…The activating PAMPs, however, remained unknown. Genetic analyses of Legionella pneumophila and S. Typhimurium revealed that the delivery of flagellin into the cytosol was necessary for robust NLRC4-dependent caspase-1 activation and macrophage cell death [8][9][10][11]. Interestingly, high doses of S. Typhimurium induced flagellin-independent NLRC4 activation, suggesting that which was demonstrated for NLRC4 and also for NLRP1b, where cleavage was sufficient for activation with Bacillus anthracis lethal factor protease [28,29].…”