Francisella tularensis (Ft) is an important human pathogen responsible for causing tularemia in the northern hemisphere. Ft has long been developed as a biological weapon due to its ability to cause severe illness and is now classified as a category A agent by the CDC based on its possible use as a bioterror agent. Very little is known regarding the role and composition of the inflammasome, a cytosolic multi-protein complex, that activates caspase-1 to produce pro-inflammatory cytokines, IL-1? and IL-18 in host defense against virulent Ft strains. Our overall hypothesis is that active repression of inflammasome is essentially required for intracellular survival of Ft. Our studies in the previous funding period demonstrated that Ft represses inflammasome by acting at the level of Toll-Like Receptor 2 (TLR2), by blocking the mediators of inflammasome activation, and most importantly, by preventing the release of bacterial ligands into the host cell cytosol. We further demonstrated that Absent In Melanoma 2 (AIM2)-dependent inflammasome is repressed in macrophages infected with Ft. We used a broad genomic approach to identify Ft encoded factors responsible for repression of inflammasome in infected macrophages. In particular, we characterized several Ft encoded factors that mediate the repression of AIM2- dependent inflammasome. These results strongly suggest that repression of inflammasome by Ft is a multifactorial process and involves the targeting of several signaling components required for priming and activation. However, the molecular mechanisms leading to repression caused by these Ft encoded factors still remain undetermined.
In specific aim 1, we will investigate how these Ft encoded factors exert their suppressive effects at the priming and activation steps of inflammasome. We have also demonstrated that vaccination using mutants of Ft in these factors protect vaccinated mice against challenge with highly virulent and category A agent Ft SchuS4. These results indicate that inflammasome-induced pro-inflammatory cytokines provide a link between innate and adaptive responses. The contribution of inflammasome-induced cytokines, IL-1? and IL-18 in the development of protective adaptive immune responses against Ft is not known.
In specific aim 2, we will establish how repression of inflammasome mediated cytokines impacts the development of adaptive immune responses. Dissecting the mechanisms of inflammasome-induced responses in the clearance of primary infection and their impact on the development of adaptive immune responses required for protection against secondary Ft challenge are highly innovative. An understanding of the mechanisms of repression of inflammasome and how it regulates adaptive immunity against tularemia will result in the development of effective therapeutics and vaccines.
This proposal aims to investigate the immune suppressive mechanisms underlying the fatal human disease tularemia caused by Francisella tularensis, a Category A bioterror agent. The potential of F. tularensis to cause severe infection is due to its ability to suppress the host's innate immune system. The proposed studies will explore how Francisella evades the host innate immune responses and blocks the development of adaptive immune responses required for protection from subsequent infections. Defining the molecular mechanism(s) of suppression of innate immunity and how this impacts adaptive immunity will be an essential first step towards development of immunotherapeutic, as well as effective vaccine strategies to combat this fatal disease.
