Role of Lipid Droplets in Prostaglandin E2 Production during Coxiella burnetii Infection

Coxiella burnetii, an obligate intracellular bacterium, causes potentially fatal endocarditis several years after intial infection suggesting the bacterium’s ability to persist long-term in the host. Our overall goal is to determine the mechanisms Coxiella employs for its long-term survival. While Coxiella initially infects alveolar macrophages, in endocarditis patients, it is also found in foamy macrophages containing neutral lipid storage organelles called lipid droplets (LDs). Our previous studies show that Coxiella manipulates host LD metabolism via the Type 4 Secretion System (T4SS), a major virulence factor which secretes bacterial effector proteins into the host cell cytoplasm to manipulate cellular processes. Additionally, inhibiting LD breakdown almost completely inhibits bacterial growth suggesting that LD-derived lipids are critical for Coxiella intracellular survival. LD breakdown releases arachidonic acid, a prostaglandin E2 (PGE2) precursor, which promotes immunosuppressive environment in alveolar macrophages. Hence we hypothesize that Coxiella manipulates host cell LD metabolism to promote a PGE2-mediated immunosuppressive environment and survive long-term in the host. To test this, we quantified PGE2 synthesis enzyme cyclooxygenase-2 (cox-2) gene expression in differentially infected alveolar macrophages. Compared to uninfected cells, cox-2 was upregulated in Coxiella-infected macrophages but not T4SS mutant-infected cells. ELISA further demonstrated Coxiella infection-dependent increase in PGE2 levels. These studies indicate that during infection Coxiella T4SS actively manipulates cox-2 expression resulting in increased PGE2. Studies are ongoing to identify the direct correlation between LDs and PGE2 production in Coxiella-infected cells and their contribution to PGE2-mediated immunosuppression. Future studies will determine the potential of blocking PGE2 production as a supplemental therapy for Coxiella endocarditis