Abstract
IL-1β is produced from inactive pro-IL-1β by activation of caspase-1 brought about by a multi-subunit protein platform called the inflammasome. Many bacteria can trigger inflammasome activity through flagellin activation of the host protein NLRC4. However, strains of the common human pathogen Pseudomonas aeruginosa lacking flagellin can still activate the inflammasome. We set out to identify what non-flagellin components could produce this activation. Using mass spectroscopy, we identified an inflammasome-activating factor from P. aeruginosa as pilin, the major component of the type IV bacterial pilus. Purified pilin introduced into mouse macrophages by liposomal delivery activated caspase-1 and led to secretion of mature IL-1β, as did recombinant pilin purified from Escherichia coli. This was dependent on caspase-1 but not on the host inflammasome proteins NLRC4, NLRP3 or ASC. Mutants of P. aeruginosa strain PA103 lacking pilin did not activate the inflammasome following infection of macrophages with live bacteria. Type III secretion remained intact in the absence of pili, showing this was not due to a lack of effector delivery. Our observations show pilin is a novel activator of the inflammasome in addition to flagellin and the recently described PrgJ protein family, the basal body rod component of the type III apparatus.
Highlights
IL-1b is a key inflammatory cytokine important in host defence that mediates a diverse range of effects, includ-Received 25 August, 2010; revised 5 October, 2010; accepted 11 October, 2010. *For correspondence
Purified pilin introduced into mouse macrophages by liposomal delivery activated caspase-1 and led to secretion of mature IL-1b, as did recombinant pilin purified from Escherichia coli
This was dependent on caspase-1 but not on the host inflammasome proteins NLRC4, NLRP3 or ASC
Summary
IL-1b is a key inflammatory cytokine important in host defence that mediates a diverse range of effects, includ-Received 25 August, 2010; revised 5 October, 2010; accepted 11 October, 2010. *For correspondence. The NLRC4 inflammasome is activated by intracellular microbial products (Evans, 2009), the best characterized of cellular microbiology which is the bacterial protein flagellin, the main component of flagella (Franchi et al, 2006; Miao et al, 2006). Both NLRP3 and NLRC4 inflammasomes usually contain the adaptor protein ASC, which is required for activation (Mariathasan et al, 2004)
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