Repression of Inflammasome by Francisella tularensis during Early Stages of Infection

Rachel J Dotson,Seham M Rabadi,Elizabeth L Westcott,Stephen Bradley,Sally V Catlett,Sukalyani Banik,Jonathan A Harton,Chandra Shekhar Bakshi,Meenakshi Malik

doi:10.1074/jbc.m113.490086

Abstract

Francisella tularensis is an important human pathogen responsible for causing tularemia. F. tularensis has long been developed as a biological weapon and is now classified as a category A agent by the Centers for Disease Control because of its possible use as a bioterror agent. F. tularensis represses inflammasome; a cytosolic multi-protein complex that activates caspase-1 to produce proinflammatory cytokines IL-1β and IL-18. However, the Francisella factors and the mechanisms through which F. tularensis mediates these suppressive effects remain relatively unknown. Utilizing a mutant of F. tularensis in FTL_0325 gene, this study investigated the mechanisms of inflammasome repression by F. tularensis. We demonstrate that muted IL-1β and IL-18 responses generated in macrophages infected with F. tularensis live vaccine strain (LVS) or the virulent SchuS4 strain are due to a predominant suppressive effect on TLR2-dependent signal 1. Our results also demonstrate that FTL_0325 of F. tularensis impacts proIL-1β expression as early as 2 h post-infection and delays activation of AIM2 and NLRP3-inflammasomes in a TLR2-dependent fashion. An enhanced activation of caspase-1 and IL-1β observed in FTL_0325 mutant-infected macrophages at 24 h post-infection was independent of both AIM2 and NLRP3. Furthermore, F. tularensis LVS delayed pyroptotic cell death of the infected macrophages in an FTL_0325-dependent manner during the early stages of infection. In vivo studies in mice revealed that suppression of IL-1β by FTL_0325 early during infection facilitates the establishment of a fulminate infection by F. tularensis. Collectively, this study provides evidence that F. tularensis LVS represses inflammasome activation and that F. tularensis-encoded FTL_0325 mediates this effect.

Highlights

The mechanism of repression of inflammasome caused by Francisella tularensis is not known
We demonstrate that muted IL-1␤ and IL-18 responses generated in macrophages infected with F. tularensis live vaccine strain (LVS) or the virulent SchuS4 strain are due to a predominant suppressive effect on TLR2-dependent signal 1
Muted IL-1␤ and IL-18 Responses Are Observed in Naïve F. tularensis-infected Macrophages—We first examined the amounts of IL-1␤ and IL-18 produced by macrophages in response to infection with F. tularensis LVS and SchuS4 and compared with those obtained for F. novicida

Summary

Background

The mechanism of repression of inflammasome caused by Francisella tularensis is not known. Our results demonstrate that FTL_0325 of F. tularensis impacts proIL-1␤ expression as early as 2 h post-infection and delays activation of AIM2 and NLRP3inflammasomes in a TLR2-dependent fashion. Mice deficient for inflammasome components apoptosis-associated speck-like protein containing a CARD domain (ASC) or caspase-1 exhibit a susceptible phenotype accompanied by increased morbidity and mortality, higher bacterial burden, and pyroptotic cell death, demonstrating a key role of these molecules in innate immunity to F. novicida [24]. There are reports demonstrating that F. tularensis LVS and F. novicida are sensed by NLRP3 and a non-NLR protein, absent in melanoma 2 (AIM2) in human and mouse macrophages, respectively [25, 26]. Several bacterial pathogens have evolved sophisticated mechanisms to avoid their detection by cytosolic NLR sensors to prevent inflammasome activation and subvert innate immune responses. We report that F. tularensis represses both the AIM2 and NLRP3 inflammasomes and that FTL_0325 is required for this repression

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION