Cytosolic flagellin requires Ipaf for activation of caspase-1 and interleukin 1 in salmonella-infected macrophages

Luigi Franchi^1, 5, Amal Amer^1, 5, Mathilde Body-Malapel¹, Thirumala-Devi Kanneganti¹, Nesrin Özören¹, Rajesh Jagirdar¹, Naohiro Inohara¹, Peter Vandenabeele², John Bertin^3, 4, Anthony Coyle^3, 4, Ethan P Grant^3, 4 & Gabriel Núñez¹

¹  Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA.

²  Molecular Signalling and Cell Death Unit, Department of Molecular Biomedical Research, VIB, Ghent 9052, University, Ghent, Belgium.

³  Millennium Pharmaceuticals, Cambridge, Massachusetts 02139, USA.

⁴  Present addresses: Synta Pharmaceuticals, Bedford, Massachusetts 01730, USA (J.B. and E.P.G.) and MedImmune, Gaithersburg, Maryland 20878, USA (A.C.).

⁵  These authors contributed equally to this work.

Gram-negative bacteria that replicate in the cytosol of mammalian macrophages can activate a signaling pathway leading to caspase-1 cleavage and secretion of interleukin 1, a powerful host response factor. Ipaf, a cytosolic pattern-recognition receptor in the family of nucleotide-binding oligomerization domain–leucine-rich repeat proteins, is critical in such a response to salmonella infection, but the mechanism of how Ipaf is activated by the bacterium remains poorly understood. Here we demonstrate that salmonella strains either lacking flagellin or expressing mutant flagellin were deficient in activation of caspase-1 and in interleukin 1 secretion, although transcription factor NF-B–dependent production of interleukin 6 or the chemokine MCP-1 was unimpaired. Delivery of flagellin to the macrophage cytosol induced Ipaf-dependent activation of caspase-1 that was independent of Toll-like receptor 5, required for recognition of extracellular flagellin. In macrophages made tolerant by previous exposure to lipopolysaccharide, which abrogates activation of NF-B and mitogen-activated protein kinases, salmonella infection still activated caspase-1. Thus, detection of flagellin through Ipaf induces caspase-1 activation independently of Toll-like receptor 5 in salmonella-infected and lipopolysaccharide-tolerized macrophages.

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