Novel pathways of human host defence against Mycobacterium tuberculosis

Abstract

A well-functioning immune response is essential for human resistance to Mycobacterium tuberculosis (Mtb), the primary cause of tuberculosis (TB). An improved understanding of host factors that affect Mtb clearance by macrophages, the most important cell for Mtb defence, has the potential to improve treatment regimens for TB. My thesis describes how virulent double auxotroph Mtb H37Rv is distinguished by a greater capacity to cause damage to cell membranes, which resulted in more frequent recognition by the host with greater activation of autophagy and membrane repair mechanisms, than less virulent mycobacterial species. I showed that pre-infection treatment of macrophages with lipopolysaccharide, rapamycin or immunosuppressive treatments chloroquine and bafilomycin A1 worsened outcomes with Mtb infection, illustrating the potential to disrupt macrophage restriction of Mtb. I next characterised the role of the Melanocortin 3 Receptor (MC3R) in macrophages with mycobacterial infection, finding support for a critical role of this receptor in host restriction of Mtb: deletion of MC3R in macrophages resulted in increased Mtb survival; increasing MC3R activity using synthetic agonists, setmelanotide and melanotan II, improved the Mtb killing ability of the macrophage; and a common variant of MC3R in humans was associated with an impairment of mycobacterial immune function. These data implicate MC3R as a host restriction factor for Mtb which may explain variations in host susceptibility and reveal two host-directed therapy candidates for TB. Finally I elucidated the role that Polycomb Repressive Complex 2 (PRC2) plays in the H3K27me3 epigenetic suppression of nitric oxide (NO•) production downstream of Nitric Oxide Synthase 2 (NOS2) transcription in human macrophages. I determined that the inhibition of NO• production could be reversed through pre-infection treatment of the macrophage with GSK126, an inhibitor of the Enhancer of Zeste 2 Polycomb Repressive Complex 2 (EZH2) subunit of PRC2, which resulted in an increase in intracellular killing of Mtb. This was an effect that was dependent on functioning EZH2 and NOS2, supporting a potential therapeutic role for NOS2 de-repression in Mtb infection. Altogether, my experiments provide evidence for the importance of host factors in the outcome of Mtb-infected macrophages, and highlight several potential targets for adjuvant host-directed therapy for tuberculosis disease.