Role of lipid droplets and prostaglandinE2 in Coxiella burnetii intracellular growth

Coxiella burnetii is an obligate intracellular bacterium that typically infects livestock. In humans, inhalation of Coxiella results in Q fever presented as an atypical pneumonia which may relapse after a prolonged latent period leading to potentially fatal endocarditis. Our goal is to determine the mechanisms Coxiella employs to survive long-term in the host. While the bacterium initially infects alveolar macrophages, in endocarditis patients Coxiella is found in foamy macrophages that are rich in 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, blocking LD breakdown almost completely inhibits bacterial growth suggesting that LD-derived lipids are critical for Coxiella’s intracellular survival. LD breakdown releases arachidonic acids, precursors for the lipid immune mediator prostaglandin E2 (PGE2) which promotes an immunosuppressive environment within alveolar macrophages. 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 production during infection with and without LD breakdown inhibitor, Atglistatin. ELISA at 24 and 48 hours post-infection showed a LD breakdown-dependent significant increase in PGE2 levels. Fluorescence microscopy revealed that blocking PGE2 production using FDA-approved COX-2 inhibitors significantly decreased bacterial intracellular growth which was rescued with addition of exogenous PGE2. To directly assess the effect of PGE2, we treated infected macrophages with a PGE2-specific mPGES-1 inhibitor. Decreased Coxiella growth in mPGES-1-treated cells which was rescued with PGE2 suggests the importance of PGE2 during Coxiella infection. Our studies show that PGE2 promotes Coxiella survival in macrophages. Ongoing studies are identifying the correlation between LDs and PGE2 production and the contribution of LDs to immunosuppression during Coxiella infection