Understanding host factors controlling intracellular killing of Mycobacterium tuberculosis
Effective stimulation of innate immunity is essential for a successful host response to infection with Mycobacterium tuberculosis (MTB) which causes human tuberculosis (TB). Better understanding of host factors controlling the survival of MTB within the macrophage (the primary site of MTB infection) may present potentially more effective therapeutic strategies for drug-resistant TB. My study describes a novel critical role for Toll-like receptor (TLR) 8 in phagosomal sensing of mycobacterial RNA and subsequent enhancement of intracellular killing of MTB. I showed that TLR8 senses mycobacterial RNA released through mycobacterial extracellular membrane vesicles and stimulates phagolysosomal degradation and autophagic clearance of MTB. I demonstrated that synthetic clinically tested TLR8 agonists can improve intracellular killing of both drug-susceptible and drug-resistant MTB by macrophages, suggesting a potential role for TLR8 activation in host-directed TB therapy. I characterised the mechanism of a polymorphism of TLR8 termed the M1V, which was previously reported to be genetically associated with protection from pulmonary Tuberculosis. I found that the M1V results in altered signal peptide usage leading to altered trafficking of receptors to early rather than late endosomal compartments. Consequently, TLR8 detection of phagosomal MTB is enhanced and results in higher pro-inflammatory cytokine release, greater phagosomal acidification, and improved intracellular killing of MTB. These findings reveal important functions of TLR8 in regulating phagosomal behaviour during MTB infection, and in part explain previous genetic association studies and the basis for enhanced polymorphic receptor function. I have also characterised key interactions between MTB and modulators of the selective autophagy signalling pathway during early infection; and demonstrated how a STAT-cytokine pathway regulates macrophage lipid metabolism to influence host susceptibility to mycobacterial infection. Collectively, my findings provide evidence for the importance of host factors in controlling the survival of MTB within macrophages and highlight opportunities for pharmacological modulation in host-directed TB therapy.
Mycobacterium tuberculosis, Macrophage, Immunity, Host-directed therapy, Phagolysosomal clearance, Autophagy, Toll-like receptors
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