Abstract
Lipids represent an important source of nutrition for infecting mycobacteria, accumulating within the necrotic core of granulomas and present in foamy macrophages associated with mycobacterial infection. In order to better understand the timing, process and importance of lipid accumulation, we developed methods for direct in vivo visualization and quantification of this process using the zebrafish-M. marinum larval model of infection. We find that neutral lipids accumulate cell-autonomously in mycobacterium-infected macrophages in vivo during early infection, with detectable levels of accumulation by two days post-infection. Treatment with ezetimibe, an FDA-approved drug, resulted in decreased levels of free cholesterol and neutral lipids, and a reduction of bacterial growth in vivo. The effect of ezetimibe in reducing bacterial growth was dependent on the mce4 operon, a key bacterial determinant of lipid utilization. Thus, in vivo, lipid accumulation can occur cell-autonomously at early timepoints of mycobacterial infection, and limitation of this process results in decreased bacterial burden.
Highlights
During the course of Mycobacterium tuberculosis (Mtb) infection, bacilli are taken up by host macrophages
In order to examine the dynamics of lipid accumulation during mycobacterial infection in vivo we developed a Fluorescence-Activated Cell Sorting (FACS) approach and employed high-resolution microscopy using the zebrafish-M. marinum model of infection
In order to assess the presence and timing of cholesterol and neutral lipid accumulation within macrophages during M. marinum infection, we examined the accumulation of both free cholesterol and neutral lipids in infected and uninfected macrophages in vivo
Summary
During the course of Mycobacterium tuberculosis (Mtb) infection, bacilli are taken up by host macrophages. Persistent infection is established, in part, by bacterial manipulation of these macrophages in order to reside and replicate within them [1]. Infected and uninfected macrophages form characteristic aggregated structures called granulomas [2] that, when mature, consist of characteristic layers of epithelialized macrophages organized around a lipid-laden necrotic core [3]. While initially thought to serve as exclusively host protective structures, granulomas contain infection, and represent a protected niche in which bacteria may replicate, shielded in part from the adaptive immune system [2,3,4]. Extensive research has established the importance of lipids and cholesterol as nutritional sources during infection [7].
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