Altering lipid droplet homeostasis affects Coxiella burnetii intracellular growth

Abstract

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 using fluorescence microscopy in mock-infected and C. burnetii-infected alveolar macrophages. On average, C. burnetii-infected macrophages contained twice as many LDs as mock-infected macrophages. LD numbers increased as early as 24 hours post-infection, an effect reversed by blocking C. burnetii protein synthesis. The observed LD accumulation was dependent on the C. burnetii Type 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 manipulated LD homeostasis and assessed C. burnetii intracellular growth. Surprisingly, blocking LD formation with the pharmacological inhibitors triacsin 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 maintenance of LD homeostasis, possibly via the C. burnetii T4BSS, is critical for bacterial growth.\n\nIMPORTANCEHost neutral lipid storage organelles known as lipid droplets (LDs) serve as a source of energy, nutrients, and signaling lipids. LDs are associated with infection of the intracellular bacterial pathogen Coxiella burnetii, a significant cause of culture-negative endocarditis. While C. burnetii was found in LD-rich foamy macrophages in endocarditis patients, little is known about the host LD-C. burnetii relationship. We demonstrated C. burnetii Type 4B Secretion System (T4BSS)-dependent LD accumulation in macrophages, suggesting a T4BSS-mediated regulation of host LD homeostasis. Further, manipulating LD homeostasis significantly affected bacterial growth, indicating LDs play an important role during C. burnetii infection. As C. burnetii endocarditis has a 19% mortality rate even in treated patients, exploring the LD-C. burnetii association might identify novel therapeutic targets.