AUTHOR=Yang Jungmin , Hwang Inhwa , Lee Eunju , Shin Sung Jae , Lee Eun-Jin , Rhee Joon Haeng , Yu Je-Wook 

TITLE=Bacterial Outer Membrane Vesicle-Mediated Cytosolic Delivery of Flagellin Triggers Host NLRC4 Canonical Inflammasome Signaling

JOURNAL=Frontiers in Immunology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2020.581165

DOI=10.3389/fimmu.2020.581165

ISSN=1664-3224

ABSTRACT=<p>Bacteria-released components can modulate host innate immune response in the absence of direct host cell–bacteria interaction. In particular, bacteria-derived outer membrane vesicles (OMVs) were recently shown to activate host caspase-11-mediated non-canonical inflammasome pathway <italic>via</italic> deliverance of OMV-bound lipopolysaccharide. However, further precise understanding of innate immune-modulation by bacterial OMVs remains elusive. Here, we present evidence that flagellated bacteria-released OMVs can trigger NLRC4 canonical inflammasome activation <italic>via</italic> flagellin delivery to the cytoplasm of host cells. <italic>Salmonella typhimurium</italic>-derived OMVs caused a robust NLRC4-mediated caspase-1 activation and interleukin-1β secretion in macrophages in an endocytosis-dependent, but guanylate-binding protein-independent manner. Notably, OMV-associated flagellin is crucial for <italic>Salmonella</italic> OMV-induced inflammasome response. Flagellated <italic>Pseudomonas aeruginosa</italic>-released OMVs consistently promoted robust NLRC4 inflammasome activation, while non-flagellated <italic>Escherichia coli</italic>-released OMVs induced NLRC4-independent non-canonical inflammasome activation leading to NLRP3-mediated interleukin-1β secretion. Flagellin-deficient <italic>Salmonella</italic> OMVs caused a weak interleukin-1β production in a NLRP3-dependent manner. These findings indicate that <italic>Salmonella</italic> OMV triggers NLRC4 inflammasome activation <italic>via</italic> OMV-associated flagellin in addition to a mild induction of non-canonical inflammasome signaling <italic>via</italic> OMV-bound lipopolysaccharide. Intriguingly, flagellated <italic>Salmonella</italic>-derived OMVs induced more rapid inflammasome response than flagellin-deficient <italic>Salmonella</italic> OMV and non-flagellated <italic>Escherichia coli</italic>-derived OMVs. Supporting these <italic>in vitro</italic> results, <italic>Nlrc4</italic>-deficient mice showed significantly reduced interleukin-1β production after intraperitoneal challenge with <italic>Salmonella</italic>-released OMVs. Taken together, our results here propose that NLRC4 inflammasome machinery is a rapid sensor of bacterial OMV-bound flagellin as a host defense mechanism against bacterial pathogen infection.</p>