Mycobacterium tuberculosis Exploits a Molecular Off Switch of the Immune System for Intracellular Survival

Abstract

Mycobacterium tuberculosis (M. tuberculosis) survives and multiplies inside human macrophages by subversion of immune mechanisms. Although these immune evasion strategies are well characterised functionally, the underlying molecular mechanisms are poorly understood. Here we show that during infection of human whole blood with M. tuberculosis, host gene transcriptional suppression, rather than activation, is the predominant response. Spatial, temporal and functional characterisation of repressed genes revealed their involvement in pathogen sensing and phagocytosis, degradation within the phagolysosome and antigen processing and presentation. To identify mechanisms underlying suppression of multiple immune genes we undertook epigenetic analyses. We identified significantly differentially expressed microRNAs with known targets in suppressed genes. In addition, after searching regions upstream of the start of transcription of suppressed genes for common sequence motifs, we discovered novel enriched composite sequence patterns, which corresponded to Alu repeat elements, transposable elements known to have wide ranging influences on gene expression. Our findings suggest that to survive within infected cells, mycobacteria exploit a complex immune “molecular off switch” controlled by both microRNAs and Alu regulatory elements.

Galleria mellonella - a novel infection model for the Mycobacterium tuberculosis complex

“…This organism has been very well-characterized and utilized in a wide range of clinical and non-clinical studies [21,24,27–32,51]. For example, differences in M. bovis BCG lux RLU measurements have been shown to reflect changes in the immune response in human whole blood and cellular models, including differences between tuberculin-positive and -negative individuals [23], changes induced by HIV infection and following antiretroviral treatment [25,26], changes in mycobacterial growth limitation induced by vaccines [24] and Vitamin D supplementation in vitro and in a clinical trial [29–31], and differences in gene expression in immune biological pathways compared to M. tuberculosis [32]. In addition we have also evaluated luminescent mycobacteria in the mouse model [27].…”

“…Due to safety and practical considerations when using M. tuberculosis as an experimental organism, we have carried out this study using the non-pathogenic, luminescent reporter vaccine strain, M. bovis BCG lux [21], which has been well characterized and validated in a wide range of in vitro and in vivo models and clinical studies (e.g. liquid broth, human whole blood and cells, mouse) and has the advantage of faster measurement of mycobacterial viability [22–32]. Over a time-course we have assessed the survival of G. mellonella larvae in response to infection with different inocula of M. bovis BCG lux , and monitored the survival of M. bovis BCG lux by measuring bioluminescence and correlating with CFU.…”

Human genetics of mycobacterial disease

“…A striking effect of M. tuberculosis infection has also been described for whole blood mRNA and miRNA over a 96 h time course in samples from non BCG-vaccinated healthy individuals (von Both et al 2018 ). When considering differentially expressed (DE) genes with an absolute log fold change > 1, 75% were down-regulated in response to M. tuberculosis .…”

Mucosal boosting of H56:CAF01 immunization promotes lung-localized T cells and an accelerated pulmonary response to Mycobacterium tuberculosis infection without enhancing vaccine protection

“…Mtb is known to actively prevent antigen presentation and activation of infected cells, which could have real implications for what a T cell vaccine can effectively achieve in vivo. 39,40 This leads to a model in which infected cells are severely limited in their ability to generate an effective cognate interaction with vaccine-elicited T cells at the site of infection within the first weeks of Mtb infection 41 and suggests limitations of a vaccination approach that depends upon tissue-localized T cells as a first line of defense against infected cells at the site of pulmonary infection. Our combined flow cytometry and histological data confirms that while mucosal H56: CAF01 boosting further enhanced the CD4 T cell response in the lungs within the first 2 weeks post infection, this did not enhance Mtb control.…”