Abstract
Macrophages respond to Salmonella typhimurium infection via Ipaf, a NACHT–leucine-rich repeat family member that activates caspase-1 and secretion of interleukin 1β. However, the specific microbial salmonella-derived agonist responsible for activating Ipaf is unknown. We show here that cytosolic bacterial flagellin activated caspase-1 through Ipaf but was independent of Toll-like receptor 5, a known flagellin sensor. Stimulation of the Ipaf pathway in macrophages after infection required a functional salmonella pathogenicity island 1 type III secretion system but not the flagellar type III secretion system; furthermore, Ipaf activation could be recapitulated by the introduction of purified flagellin directly into the cytoplasm. These observations raise the possibility that the salmonella pathogenicity island 1 type III secretion system cannot completely exclude 'promiscuous' secretion of flagellin and that the host capitalizes on this 'error' by activating a potent host-defense pathway.
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Acknowledgements
We thank V. Dixit and Genentech for Ipaf- and ASC-deficient mice; S. Akira for TLR5- and MyD88-deficient mice; G. Chilcott, H. Bonnifield, O. Nanassy, J. Karlinsey and K. Hughes for flagellar mutant strains and advice regarding flagellar biology; C. Carlson, M. Ohl and K. Smith for contributions and discussions; H. Bonnifield, C. Carlson and members of the Aderem lab for critical review of this manuscript; and E. Andersen-Nissen for providing flagellin and ovalbumin protein.
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Supplementary information
Supplementary Fig. 1
Schematic of the structures synthesized by flagellar mutants. (PDF 41 kb)
Supplementary Fig. 2
Mutant strains over expressing flagellin display increased cytotoxicity. (PDF 28 kb)
Supplementary Fig. 3
S. typhimurium repress flagellin transcription through PhoP/PhoQ. (PDF 23 kb)
Supplementary Fig. 4
Ipaf sensitizes macrophages to salmonella infection. (PDF 40 kb)
Supplementary Table 1
Strain list: Chromosomal insertion elements and plasmids. (PDF 27 kb)
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Miao, E., Alpuche-Aranda, C., Dors, M. et al. Cytoplasmic flagellin activates caspase-1 and secretion of interleukin 1β via Ipaf. Nat Immunol 7, 569–575 (2006). https://doi.org/10.1038/ni1344
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DOI: https://doi.org/10.1038/ni1344
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