Flagellin, a Key Immune Trigger

Abstract

The intracellular pattern recognition receptor Ipaf, which is a member of the NLR (for nucleotide-binding oligomerization domain-leucine-rich repeat or NACHT-leucine-rich repeat receptor) family, is involved in caspase-1 activation, which can stimulate interleukin-1β (IL-1β) secretion or lead to apoptosis, in response to bacterial infection of macrophages. Miao et al . and Franchi et al . both show that caspase-1 activation and IL-1β secretion by macrophages infected with Salmonella typhimurium required the bacterial type III secretion system (TTSS) and the flagellar protein flagellin, of which there are two variants, FliC and FliB. Bacteria lacking FliC and FliB or a functional TTSS did not stimulate caspase-1 activation or IL-1β secretion. Application of flagellin extracellularly did not trigger a macrophage response (because mouse macrophages lack the Toll-like receptor TLR5 that allows recognition of extracellular flagellin). But introduction of flagellin into the cytoplasm of macrophages using agents that mildly disrupt the membrane triggered IL-1β secretion; however, this was not observed in macrophages lacking Ipaf. Thus, the bacteria appear to release flagellin through the TTSS into the host cytoplasm, where it is recognized by Ipaf, which stimulates the inflammatory response. Subramanian and Qadri showed that exposure of Salmonella to intestinal epithelial cells (IECs) stimulated the secretion of monomeric flagellin into the culture medium, which was then able to stimulate IL-8 production in uninfected IECs when the flagellin-containing culture medium was applied to IECs (which have TLR5 and can respond to extracellular flagellin). CaCo-2 cells, an IEC cell line, exposed to various treatments to denature or inactivate proteins, were still capable of stimulating flagellin secretion by Salmonella ; thus, lipid mediators were tested. Exposure of Salmonella to lysophosphatidic acid (LPA) or lysophosphatidylcholine (LPC) stimulated flagellin secretion. Lysophospholipids are released by cells through the activity of phospholipase A 2 (PLA 2 ), and inhibition of two types of PLA 2 suppressed the stimulation of flagellin secretion from the bacteria in response to exposure to IECs or culture medium from IECs. In vivo, inhibition of calcium-independent cell-associated PLA 2 inhibited flagellin release by Salmonella in infected mice. Secretion of flagellin in response to lysophospholipids involved de novo protein synthesis, and LPC and LPA both stimulated a reporter gene driven by the fliC promoter, suggesting that lysophospholipids may trigger the adenosine 3′,5′-monophosphate (cAMP) signaling pathway that activates flagellar biosynthesis. Exposure of Salmonella to glucose, which decreases cAMP concentrations, reduced flagellin secretion in response to LPC. Roy and Zamboni discuss the implications of these three articles and propose that host cells trigger flagellin production and secretion by the bacteria, effectively "'tricking' the bacteria into revealing themselves to host flagellin sensors." E. A. Miao, C. M. Alpuche-Aranda, M. Dors, A. E. Clark, M. W. Bader, S. I. Miller, A. Aderem, Cytoplasmic flagellin activates caspase-1 and secretion of interleukin 1β via Ipaf. Nat. Immunol. 7 , 569-575 (2006). [PubMed] L. Franchi, A. Amer, M. Body-Malapel, T.-D. Kanneganti, N. Özören, R. Jagirdar, N. Inohara, P. Vandenabeele, J. Bertin, A. Coyle, E. P. Grant, G. Núñez, Cytosolic flagellin requires Ipaf for activation of caspase-1 and interleukin 1β in salmonella-infected macrophages. Nat. Immunol. 7 , 576-582 (2006). [PubMed] N. Subramanian, A. Qadri, Lysophospholipid sensing triggers secretion of flagellin from pathogenic salmonella. Nat. Immunol. 7 , 583-589 (2006). [PubMed] C. R. Roy, D. S. Zamboni, Cytosolic detection of flagellin: A deadly twist. Nat. Immunol. 7 , 549-551 (2006). [PubMed]