Francisella-induced eicosanoid production in the lung: Connection with the inflammasome and consequences on the innate immune responses. – FRANNOÏD

One key feature of the innate immune system is the recognition of pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs), which are localized at the i) cytoplasmic or endosomal membrane or ii) in the cytosol. The innate immune response against intracytosolic pathogens, such as Francisella tularensis (F. tularensis), a facultative intracellular Gram negative pathogen causing tularemia is still poorly understood. Indeed, Absent in Melanoma 2 (AIM2) was recently described as a cytosolic PRR recognising Francisella double-stranded DNA (dsDNA) within the cytosol, and being outstandingly essential for effective pro-inflammatory response against this bacterium. Francisella novicida (F. novicida), a close relative of F. tularensis is emerging as a key model to study in vitro and in vivo in a physiological manner the AIM2-containing inflammasome (AIM2 inflammasome) and the consequences of its activation. The inflammasome output involved during Francisella infection has not been entirely characterized particularly in term of lipid mediators responses. Very interestingly, the activation of the inflammasome containing the nucleotide binding domain-leucine-rich repeat (NLR) protein NLRC4 (NLRC4 inflammasome) has been recently linked to eicosanoid production (von Moltke et al., 2012). Due to the strong stimuli used in this study, inflammasome activation was associated with an eicosanoid storm. The eicosanoid response remains to be studied in a more physiological inflammasome activation model. Indeed, inflammasome dependent eicosanoid production might actually be widespread in organisms during infection and need further investigation. Francisella induced prostaglandin (PG) E2 production has been characterized. To our knowledge, the production and the regulation of other eicosanoids (thromboxane, prostacyclins, leukotrienes…) during Francisella infection have never been described. Therefore, we propose to fill this important gap of knowledge by characterizing i) the profile of Francisella induced eicosanoids in a mouse model of tularemia, and ii) the involvement of AIM2 inflammasome. In a second time, we will characterize in vivo i) the cellular source responsible for this eicosanoid production, considering both the non-phagocytic (epithelial cells) and the phagocytic (macrophages and neutrophils) cells, and ii) the activation of key enzymes involved in eicosanoid production or degradation.
Mice deficient for the enzyme involved in eicosanoid synthesis will be used to assess the contribution of this signaling pathway to resistance or susceptibility to Francisella infection. Once the phenotype of those mice deficient for the enzyme involved in eicosanoid production is confirmed, we will investigate i) the signaling pathways targeted by this eicosanoid production in mice (e.g the AIM2 inflammasome, or the type I interferon (IFN) pathway) and then ii) the mechanisms mediating those effects (e.g. identification of the receptors involved).
This project should help us characterizing AIM2 dependent eicosanoid production induced by Francisella lung infection, and how those lipid mediators affect the host immune response to Francisella. A better understanding of this system may allow the development of new drugs helping the host in its fight against the infection.