Abstract
Macrophages from C57BL/6J (B6) mice restrict growth of the intracellular bacterial pathogen Legionella pneumophila. Restriction of bacterial growth requires caspase-1 and the leucine-rich repeat-containing protein Naip5 (Birc1e). We identified mutants of L. pneumophila that evade macrophage innate immunity. All mutants were deficient in expression of flagellin, the primary flagellar subunit, and failed to induce caspase-1-mediated macrophage death. Interestingly, a previously isolated flagellar mutant (fliI) that expresses, but does not assemble, flagellin did not replicate in macrophages, and induced macrophage death. Thus, flagellin itself, not flagella or motility, is required to initiate macrophage innate immunity. Immunity to Legionella did not require MyD88, an essential adaptor for toll-like receptor 5 (TLR5) signaling. Moreover, flagellin of Legionella and Salmonella induced cytotoxicity when delivered to the macrophage cytosol using Escherichia coli as a heterologous host. It thus appears that macrophages sense cytosolic flagellin via a TLR5-independent pathway that leads to rapid caspase-1-dependent cell death and provides defense against intracellular bacterial pathogens.
Highlights
Legionella pneumophila is a motile gram-negative bacterium that is the cause of a severe form of pneumonia called Legionnaires’ disease [1]
Since we found that addition of exogenous IL-1 and/or IL-18 did not render macrophages resistant to Legionella, we concluded that caspase-1 is required for host defense via a mechanism that is
We considered two other non-mutually exclusive models: (1) a ‘‘secretion model’’ in which flagellin is secreted into host cells via the type IV apparatus, or leaks into the cytosol via pores in the phagosome generated by the dot/icm apparatus; or (2) a ‘‘two signal’’ model, in which puncture of the cell membrane by the dot/icm apparatus provides a signal that is independent of that provided by flagellin, and both signals are required to initiate cell death
Summary
Legionella pneumophila is a motile gram-negative bacterium that is the cause of a severe form of pneumonia called Legionnaires’ disease [1]. To test whether a dot/icm–dependent, flagellin-independent but non–calcium-mediated signal was required for cell death, we coinfected macrophages with dotA and flaA mutant bacteria. It was interesting that at very high MOIs, the flaA mutant exhibited significant apparent toxicity The basis for this toxicity is unclear, but it appears to depend on the dot apparatus and may represent a ‘‘poreforming’’ cytotoxicity [36] that is revealed at high MOIs. In the above coinfection experiments, the dotA and flaA mutants were not frequently present in the same phagosome (unpublished data), even using specialized infection procedures [39] (Figure 4C) and high MOIs. we could not distinguish whether flagellin is directly transported into the cytosol via the dot/icm secretion apparatus (in cis), or merely leaks into the cytosol via dot/icm–induced pores in the phagosome (in trans).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
