Altering lipid droplet homeostasis affects Coxiella burnetii intracellular growth

Coxiella burnetii is an obligate intracellular bacterial pathogen and a causative agent of culture-negative endocarditis. While C. burnetii initially infects alveolar macrophages, it has also been found in lipid droplet (LD)-containing foamy macrophages in the cardiac valves of endocarditis patients. In addition, transcriptional studies of C. burnetii-infected macrophages reported differential regulation of the LD coat protein-encoding gene perilipin 2 (plin-2). To further investigate the relationship between LDs and C. burnetii, we compared LD numbers in mock-infected and C. burnetii-infected alveolar macrophages using fluorescence microscopy. Compared to only 10% of mock-infected cells, 50% of C. burnetii-infected cells had more than 50 LDs/cell as early as 24 hours post-infection, indicating a significant increase in LDs in infected cells. Increased LDs required the C. burnetiiType 4B Secretion System (T4BSS), a major virulence factor that manipulates host cellular processes by secreting bacterial effector proteins into the host cell cytoplasm. To determine the importance of LDs during C. burnetii infection, we assessed the effect of manipulating LD homeostasis on C. burnetiiintracellular growth. Surprisingly, blocking LD formation with the pharmacological inhibitors triascin C or T863, or knocking out acyl-CoA transferase-1 (acat-1) in alveolar macrophages, increased C. burnetii growth at least 2-fold. Conversely, preventing LD lipolysis by inhibiting adipose triglyceride lipase (ATGL) with atglistatin almost completely blocked bacterial growth, suggesting LD breakdown is essential for C. burnetii. Together these data suggest that LDs are detrimental to C. burnetiiand maintenance of LD homeostasis, possibly via the T4BSS, is critical for bacterial growth.