Francisella tularensis (Ft), the causative agent of fatal human disease tularemia has long been developed as a biological weapon due to its ability to cause severe illness. Ft is now classified as a category A select agent by the CDC based on its possible use as a bioterror agent. Ft is an intracellular pathogen. During its replication cycle, Ft escapes from the phagosome and replicates within the cytosol of several cell types; thus, the role of cytosolic pattern recognition receptors, especially the NLRs and inflammasome in innate immunity against Ft assumes great importance. The inflammasome is a cytosolic multi-protein complex that activates caspase1 to produce pro-inflammatory cytokines IL-1β and IL-18 required for recruitment and activation of other immune cells to promote bacterial clearance. Very little is known regarding the role of the inflammasome in host defense against virulent Ft strains. Our overall hypothesis is that active repression of the inflammasome is essentially required for intracellular survival of Ft. Results from the studies conducted during our two previous R15 funding cycles have demonstrated that Ft encoded factors suppress Aim2 and Nlrp3 inflammasomes; both the Aim2 and Nlrp3 are dispensable for vaccine-induced protective immune responses; and Nlrp3 plays an inflammasome-independent detrimental role in Ft infected macrophages and mice. The scientific premise of the current proposal is built upon two unanswered questions emanating from these studies: (1) what are the consequences of suppression of the Aim2 inflammasome by Ft on the overall development of innate immune responses? (2) How does Nlrp3 exert its detrimental effects on the host following Ft infection? In specific aim 1, we will investigate the mechanisms through which Aim2 regulates the host’s immunity to Ft infection. We will investigate the consequences of suppression of the Aim2 inflammasome on the overall development of innate immune responses during Francisella infection. The inflammasome-independent roles of NLRs, especially as negative regulators of inflammation, have come to the forefront in recent years. Our preliminary studies demonstrate that Nlrp3 increases the susceptibility of mice and suppresses the production of pro-inflammatory cytokines in mice and macrophages infected with Ft. In specific aim 2, we will investigate the mechanisms through which Nlrp3 regulates the host’s immunity and plays a detrimental role by dampening the host’s innate immune responses against Francisella. These proposed studies are novel and will enhance our understanding of the unique pathogenic mechanisms of Ft, which are essential for the development of effective therapeutics and vaccines against tularemia.