Altering lipid droplet homeostasis affects Coxiella burnetii intracellular growth

Coxiella burnetiiis an obligate intracellular bacterial pathogen and a causative agent of culture-negative endocarditis. WhileC. burnetiiinitially 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 ofC. burnetii-infected macrophages reported differential regulation of the LD coat protein-encoding gene perilipin 2(plin-2). To further investigate the relationship between LDs andC. burnetii, we compared LD numbers using fluorescence microscopy in mock-infected andC. 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 blockingC. burnetiiprotein synthesis. The observed LD accumulation was dependent on theC. 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 duringC. burnetiiinfection, we manipulated LD homeostasis and assessedC. burnetiiintracellular 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, increasedC. burnetiigrowth 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 forC. burnetii.Together these data suggest that maintenance of LD homeostasis, possibly via theC. burnetiiT4BSS, is critical for bacterial growth.IMPORTANCEHost 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 pathogenCoxiella burnetii, a significant cause of culture-negative endocarditis. WhileC. burnetiiwas found in LD-rich foamy macrophages in endocarditis patients, little is known about the host LD-C. burnetiirelationship. We demonstratedC. burnetiiType 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 duringC. burnetiiinfection. AsC. burnetiiendocarditis has a 19% mortality rate even in treated patients, exploring the LD-C. burnetiiassociation might identify novel therapeutic targets.