Pathogenic mycobacteria such as Mycobacteria tuberculosis (Mtb) target host cellular pathways in order to survive and persist long-term within the endocytic system of host macrophages. This represents an important therapeutic target for this disease, for which there are currently limited options. We have been studying a rare human genetic disease, Niemann-Pick disease type C (NPC), which is characterised at the cellular level by the storage of multiple lipid species including cholesterol, reduced acidic store calcium levels, and a block in late endosome: lysosome fusion. We have been struck by the presence of multiple convergent features to the cellular pathology of Mtb infected cells and NPC disease cells. For example, both become lipid laden with cholesterol and have a block in phagosome: lysosome fusion. We speculated that this could indicate mechanistic convergence and that Mtb may shed lipids that inhibit the NPC cell biological pathway. This would prevent fusion with the lysosome by reducing acidic store calcium levels, which would in turn prevent the mycobacteria from being cleared. Here, we present a substantial body of preliminary data demonstrating that Mtb and BCG do shed lipids that inhibit the NPC cellular pathway. In murine tissue culture model small molecule therapies that we and others have developed for treating NPC disease, efficacy in clearing mycobacteria from infected cells is demonstrated by targeting host cell pathways that correct or compensate for dysfunction of the NPC pathway. In this application we will test approved therapeutics in human macrophage culture systems, and mouse infection models of Mtb. In addition, we will identify the biochemical nature of the shed lipids, and determine how they inhibit the function of the NPC pathway.
We have found an unexpected mechanistic link between Mycobacterial tuberculosis (Mtb) infection and the rare genetic disease Niemann-Pick type C disease (NPC). This novel discovery suggests new therapeutic approaches to treat Mtb that target the host cell. NPC disease involves dysfunction of a cell biological pathway that regulates aspects of lipid homeostasis. When dysfunctional in NPC disease, the lysosome stores sphingosine, which causes a defect in acidic store calcium filling. Insufficient calcium is released from the acidic store to mediate late endosome: lysosome fusion. This results in a block of late endosome: lysosome fusion and the secondary storage of glycosphingolipids and cholesterol. We have found that Mtb and BCG shed lipids that inhibit the NPC pathway, thereby blocking phagosome:lysosome fusion. This allows the mycobacteria to avoid lysosomal destruction and to persist long-term in the host cell. We have been developing therapeutics for NPC that target the host cell and either correct or compensate for this defect. These therapeutics have shown efficacy in a murine BCG model. In this application our objective is to extend our novel findings by evaluating drugs that correct NPC cells in primary human macrophage:Mtb infection models and in mouse models of Mtb infection as proof of principle prior to clinical trials.