Inflammasome Activation by Bacterial Outer Membrane Vesicles Requires Guanylate Binding Proteins.

Thirumala-Devi Kanneganti,Jörn Coers,Eric J Rubin,Eric M Feeley,Arun K Haldar,Nichole Orench-Rivera,Meta J Kuehn,Masahiro Yamamoto,Sarah Luoma,Ryan Finethy,David S Weiss

doi:10.1128/mbio.01188-17

Abstract

ABSTRACTThe Gram-negative bacterial cell wall component lipopolysaccharide (LPS) is recognized by the noncanonical inflammasome protein caspase-11 in the cytosol of infected host cells and thereby prompts an inflammatory immune response linked to sepsis. Host guanylate binding proteins (GBPs) promote infection-induced caspase-11 activation in tissue culture models, and yet their in vivo role in LPS-mediated sepsis has remained unexplored. LPS can be released from lysed bacteria as “free” LPS aggregates or actively secreted by live bacteria as a component of outer membrane vesicles (OMVs). Here, we report that GBPs control inflammation and sepsis in mice injected with either free LPS or purified OMVs derived from Gram-negative Escherichia coli. In agreement with our observations from in vivo experiments, we demonstrate that macrophages lacking GBP2 expression fail to induce pyroptotic cell death and proinflammatory interleukin-1β (IL-1β) and IL-18 secretion when exposed to OMVs. We propose that in order to activate caspase-11 in vivo, GBPs control the processing of bacterium-derived OMVs by macrophages as well as the processing of circulating free LPS by as-yet-undetermined cell types.

Highlights

The Gram-negative bacterial cell wall component lipopolysaccharide (LPS) is recognized by the noncanonical inflammasome protein caspase-11 in the cytosol of infected host cells and thereby prompts an inflammatory immune response linked to sepsis
To determine whether guanylate binding proteins (GBPs) were required for caspase-11 activation by nonpathogenic Gramnegative bacteria, we monitored caspase-11-dependent cellular responses in GBPchr3Ϫ/Ϫ bone marrow-derived macrophages (BMDMs) exposed to the nonpathogenic Escherichia coli strain K-12
We found that the absence of GBPs had no impact on the number of retrievable CFU in IFN-␥-primed BMDMs at 4 h postinfection, indicating that changes in bacterial burden are unlikely to account for any phenotypes associated with the GBPchr3Ϫ/Ϫ genotype at this or later

Summary

Introduction

The Gram-negative bacterial cell wall component lipopolysaccharide (LPS) is recognized by the noncanonical inflammasome protein caspase-11 in the cytosol of infected host cells and thereby prompts an inflammatory immune response linked to sepsis. Mechanistic in vitro studies led to an attractive model, according to which GBPs lyse PVs and thereby release bacteria and their associated LPS into the host cell cytosol for recognition by caspase-11 [10]. This model was based on results obtained from Salmonella infection studies in cultured macrophages [10] but is contradicted by our own studies using Legionella and Chlamydia infection models [11, 12]. While GBPs have been identified as key regulators of caspase-11 function, the mechanism by which they do so remains controversial

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Conclusion