Coxiella burnetii causes Q fever endocarditis, a disease that requires 18-24 months of antibiotic treatment and lacks an FDA-approved vaccine. The goal of our research is to understand how Coxiella survives in the host cell, and identify new therapeutic targets. During infection, Coxiella infects macrophages and survives in a unique, lysosomal-like compartment called the Coxiella Containing Vacuole (CCV). CCV formation is essential for bacterial survival, yet we know little about CCV biogenesis and maintenance. We are especially interested in how cholesterol and other lipids contribute to CCV formation and the unique properties of the CCV. (Image to right: fibroblast cells infected with Coxiella (red) and stained with filipin, a fluorescent sterol-binding compound.
Cholesterol - Coxiella's Achilles Heel?
We developed a novel cholesterol-free cell culture model system which not only revealed that Coxiella grows best in the absence of cholesterol, but Coxiella is very sensitive to elevated host cholesterol. Using live cell imaging to directly measure the pH of individual CCVs, we discovered the CCV cholesterol levels are directly linked to CCV pH. When cholesterol is trapped in the Coxiella CCV membrane through a variety of approaches, the CCV becomes more acidic and kills the bacteria. We are currently trying to elucidate how cholesterol influences the CCV microenvironment, and how Coxiella regulates CCV pH.
Inter-organelle membrane contact sites
We discovered that Coxiella induces inter-organelle membrane contact sites to facilitate cholesterol transfer away from the CCV. First, Coxiella recruits the host cell sterol binding protein ORP1L to mediate contact sites between the CCV and host endoplasmic reticulum. Second, Coxiella secretes a unique effector protein, CbEPF1, into the host cytoplasm; CbEPF1 not only induces formation of lipid droplets, but increases lipid droplet size by forming membrane contact sites between the ER and lipid droplets. Lipid droplets store cholesterol, and serve as a source of fatty acids and lipid immune mediators. Current efforts focus on elucidating structure and function of these unique inter-organelle contact sties.
IL-17 signaling during Coxiella infection
During natural infection, Coxiella first infects alveolar macrophages. Despite the macrophage’s innate ability to kill intracellular pathogens and induce a pro-inflammatory response, Coxiella survives, spreads, and causes disease. Understanding how Coxiella evades the host innate immune system is critical for developing new methods to control disease progression. We discovered that Coxiella inhibits the pro-inflammatory IL-17 signaling pathway in macrophages, resulting in decreased reactive oxygen and chemokine secretion. We are currently identifying the underlying molecular mechanism as well as the role during animal infection.