ESX‐1‐mediated translocation to the cytosol controls virulence of mycobacteria

Abstract

SummaryMycobacterium species, including Mycobacterium tuberculosis and Mycobacterium leprae, are among the most potent human bacterial pathogens. The discovery of cytosolic mycobacteria challenged the paradigm that these pathogens exclusively localize within the phagosome of host cells. As yet the biological relevance of mycobacterial translocation to the cytosol remained unclear. In this current study we used electron microscopy techniques to establish a clear link between translocation and mycobacterial virulence. Pathogenic, patientderived mycobacteria species were found to translocate to the cytosol, while non-pathogenic species did not. We were further able to link cytosolic translocation with pathogenicity by introducing the ESX-1 (type VII) secretion system into the nonvirulent, exclusively phagolysosomal Mycobacterium bovis BCG. Furthermore, we show that translocation is dependent on the C-terminus of the early-secreted antigen ESAT-6. The C-terminal truncation of ESAT-6 was shown to result in attenuation in mice, again linking translocation to virulence. Together, these data demonstrate the molecular mechanism facilitating translocation of mycobacteria. The ability to translocate from the phagolysosome to the cytosol is with this study proven to be biologically significant as it determines mycobacterial virulence.

Mycobacteria employ two different mechanisms to cross the blood-brain barrier

“…Bacterial levels at 24‐hr postinfection (hpi) were analysed with FACS and compared with uptake of these bacteria by RAW macrophages. RAW macrophages phagocytosed both strains with similar efficiency, both at 2 and 24 hpi (Figure 7a), confirming previous reports that the level of phagocytosis of M. marinum WT and our esx‐1 mutant are similar (Houben et al, 2012; Simeone et al, 2012; van der Wel et al, 2007). …”

“…ESX‐1 secretion mutants are severely attenuated (Davis & Ramakrishnan, 2009; Stoop et al, 2011; Volkman et al, 2004), but most importantly for our purposes, these mutants are unable to complete the macrophage infection cycle and are therefore predominantly located inside phagocytic cells (Houben et al, 2012; Simeone et al, 2012; van der Wel et al, 2007). …”

“…Our data suggest that ESX‐1 substrates mediate an active process in which M. marinum is able to invade endothelial cells. This attributes a novel function to ESX‐1 secretion, in addition to phagosomal escape (Abdallah et al, 2011; Houben et al, 2012; Simeone et al, 2012; van der Wel et al, 2007). Furthermore, the damaging effect of mycobacteria on endothelial cells might be part of the explanation for the extensive vasculopathy, including stroke and vasculitis, found in clinical TBM presentation and autopsy material (Donald & Van Toorn, 2016; van der Flier et al, 2004; Zaharie et al, unpublished).…”

The innate immune response in human tuberculosis

“…In vitro studies using electron microscopy (EM) and a fluorescence resonance energy transfer‐based method showed that Mtb also localizes in the cytosol in THP‐1 macrophages, primary human macrophages and primary human DCs (van der Wel et al ., 2007; Houben et al ., 2012; Simeone et al ., 2012). The ESX‐1 type VII secretion system (T7SS) that is lacking from most of the non‐pathogenic mycobacterial strains (Abdallah et al ., 2007) is required for Mtb localization in the cytosol.…”

The autophagic machinery ensures nonlytic transmission of mycobacteria

“…After uptake by immune phagocytes, the bacteria arrest phagosomal maturation and convert their vacuole into a replication-permissive compartment. Both bacteria can translocate into the host cell cytosol dependent on an intact Region-of-Difference-1-locus (RD1) (8)(9)(10)(11). The genomic RD1-locus encodes a secretion system, ESX-1 (Type-VII secretion system), which has been associated with mycobacterial virulence (ref.…”