Persistent mycobacteria evade an antibacterial program mediated by phagolysosomal TLR7/8/MyD88 in human primary macrophages

by Alexandre Gidon, Signe Elisabeth Åsberg, Claire Louet, Liv Ryan, Markus Haug, Trude Helen Flo

Pathogenic mycobacteria reside in macrophages where they avoid lysosomal targeting and degradation through poorly understood mechanisms proposed to involve arrest of phagosomal maturation at an early endosomal stage. A clear understanding of how this relates to host defenses elicited from various intracellular compartments is also missing and can only be studied using techniques allowing single cell and subcellular analyses. Using confocal imaging of human primary macrophages infected with Mycobacterium avium (Mav) we show evidence that Mav phagosomes are not arrested at an early endosomal stage, but mature to a (LAMP1⁺/LAMP2+/CD63⁺) late endosomal/phagolysosomal stage where inflammatory signaling and Mav growth restriction is initiated through a mechanism involving Toll-like receptors (TLR) 7 and 8, the adaptor MyD88 and transcription factors NF-κB and IRF-1. Furthermore, a fraction of the mycobacteria re-establish in a less hostile compartment (LAMP1-/LAMP2-/CD63-) where they not only evade destruction, but also recognition by TLRs, growth restriction and inflammatory host responses that could be detrimental for intracellular survival and establishment of chronic infections.

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