Injection of Flagellin into the Host Cell Cytosol by Salmonella enterica Serotype Typhimurium

Abstract

Bacterial flagella are filamentous appendages on the cell surface that mediate bacterial motility. The filament of the flagellum is composed of ϳ20,000 flagellin subunits (1). Flagellin monomers of many bacterial species function as pathogen-associated molecular patterns and can be detected by host cells using both surface-localized toll-like receptor 5 (TLR5) 2 and cytosolic Nod-like receptors (2-6). The interaction of flagellin with TLR5 at the cell surface has been characterized in detail, identifying the structural basis for the TLR5-flagellin interaction (7,8). TLR5 sensing has been shown to be important in Legionnaires disease, a respiratory tract infection caused by the bacterium Legionella pneumophila, since human TLR5 polymorphisms correlated with disease susceptibility (7,8). In the intestine, TLR5 localizes to the basolateral surface of enterocytes and to CD11c ϩ lamina propria dendritic cells (9, 10). In these locations, TLR5 is thought to be involved in sensing a breach of the intestinal mucosa by enteroinvasive pathogens, triggering an inflammatory response that limits systemic spread of the bacteria (11).In addition to sensing by TLRs, flagellin was recently shown to be sensed by two different Nod-like receptors, Ipaf and Birc1e (also known as Naip5) (2, 4 -6, 12). Ipaf transmits a proinflammatory signal in response to flagellin by activating the inflammasome, resulting in caspase 1-dependent activation of Il-1␤, which is accompanied by cell death (pyroptosis) (13-17). Birc1e/Naip5 signals in response to flagellin, restricting intracellular growth of L. pneumophila by antagonizing the ability of the bacterium to avoid fusion with lysosomes and form a replicative phagosome (18). Several lines of evidence suggest that pathogens residing in a membrane-bound compartment of the cell, such as L. pneumophila and S. Typhimurium (Salmonella enterica serotype Typhimurium), may release flagellins from their vacuole into the host cytosol, which triggers signaling via Ipaf and Birc1e. First, S. Typhimurium and L. pneumophila mutants lacking flagellin elicit reduced caspase activation, IL-1␤ secretion, and cytotoxicity in macrophages (2, 4 -6, 19). Second, mutants of S. Typhimurium lacking the SPI-1 T3SS and L. pneumophila mutants lacking a functional Type IV secretion system (T4SS) fail to trigger Ipaf-dependent signaling (4,5,19). Third, introduction of purified flagellins into the cytosol of macrophages using pore-forming toxins or detergents or by heterologous expression in Escherichia coli expressing listeriolysin O triggers caspase-1 activation and IL-1␤ activation by an Ipaf-dependent mechanism (2, 4 -6, 20).Although injection of effector proteins into host cells by the S. Typhimurium SPI-1 T3SS has been visualized microscopically (21, 22), it has not yet been demonstrated that in infected cells, bacteria are able to translocate flagellin from a membrane-bound compartment into the cytosol. In this report, we show that S. Typhimurium translocates flagellin into the cytosol of infected cells by a process t...

Pseudomonas aeruginosa activates caspase 1 through Ipaf

“…TLR5 signals in response to extracellular flagellin, whereas Ipaf signals in response to flagellin within the mammalian cell cytosol (8)(9)(10)(13)(14)(15). In the case of Salmonella typhimurium, the type III secretion system (T3SS) is required for the delivery of flagellin to the eukaryotic cell cytosol (4,16). We wished to investigate whether the detection of the T3SS and flagellin through the Ipaf-signaling pathway represents a general innate immune strategy.…”

Regulation of phenotypic heterogeneity permits Salmonella evasion of the host caspase-1 inflammatory response

“…Salmonella reside within macrophages in vivo (6) and can translocate flagellin into the cytosol of host cells via the Salmonella pathogenicity island 1 (SPI-1) type III secretion system (8). Macrophages interpret intracytosolic flagellin as a danger signal, and initiate the proinflammatory cell death program pyroptosis in response (9,10).…”

Salmonella: Invasion, Evasion & Persistence

“…For example, the evasiveness of Salmonella is increased via flagellar-based motility, despite the fact several studies have demonstrated that flagellin monomers are potent inducers of innate immunity (Franchi et al, 2006;Miao et al, 2006;Schmitt et al, 2001). Conversely, it has been well demonstrated that flagellin interacts with T3SS-1 from Salmonella-infected macrophages and is translocated into the cytosol resulting in activation of the inflammasome and subsequent cell death via caspase-1 pathway (discussed later in the chapter) (Miao et al, 2007;Ren et al, 2006;Sun et al, 2007).…”