Summary Francisella tularensis, the causative agent of tularemia, infects host macrophages, which triggers production of the proinflammatory cytokines interleukin 1 β (IL-1 β) and IL-18. We elucidate here how host macrophages recognize Francisella and elicit this pro-inflammatory response. Using mice deficient in the DNA-sensing inflammasome component AIM2, we demonstrate here that AIM2 is required for sensing Francisella. AIM2-deficient mice were extremely susceptible to Francisella infection with higher mortality and bacterial burden compared to wild-type mice. Caspase-1, activation, IL-1β secretion and cell death were absent in Aim2−/− macrophages in response to Francisella infection or presence of cytoplasmic DNA. This study identifies AIM2 as a crucial sensor of F. tularensis infection, and provides genetic proof for its critical role in the host innate immunity to intracellular pathogens.

Background:The NLRP3 inflammasome is a critical mediator of inflammation in response to pathogens, tissue damage, and metabolic diseases. Results: Non-transcriptional priming and activation signals stimulate NLRP3 deubiquitination. Conclusion: The NLRP3 inflammasome is non-transcriptionally primed and activated by a deubiquitination mechanism. Significance: Understanding the mechanism of activation of the NLRP3 inflammasome is crucial for the development of therapeutics to alleviate NLRP3-dependent inflammatory diseases.

Activation of the inflammasome generates the pro-inflammatory cytokines interleukin-1␤ and -18, which are important mediators of inflammation. Abnormal activation of the inflammasome leads to many inflammatory diseases, including gout, silicosis, neurodegeneration, and genetically inherited periodic fever syndromes. Therefore, identification of small molecule inhibitors that target the inflammasome is an important step toward developing effective therapeutics for the treatment of inflammation. Here, we show that the herbal NF-B inhibitory compound parthenolide inhibits the activity of multiple inflammasomes in macrophages by directly inhibiting the protease activity of caspase-1. Additional investigations of other NF-B inhibitors revealed that the synthetic IB kinase-␤ inhibitor Bay 11-7082 and structurally related vinyl sulfone compounds selectively inhibit NLRP3 inflammasome activity in macrophages independent of their inhibitory effect on NF-B activity. In vitro assays of the effect of parthenolide and Bay 11-7082 on the ATPase activity of NLRP3 demonstrated that both compounds inhibit the ATPase activity of NLRP3, suggesting that the inhibitory effect of these compounds on inflammasome activity could be mediated in part through their effect on the ATPase activity of NLRP3. Our results thus elucidate the molecular mechanism for the therapeutic anti-inflammatory activity of parthenolide and identify vinyl sulfones as a new class of potential therapeutics that target the NLRP3 inflammasome.