Mycobacterium tuberculosis cords within lymphatic endothelial cells to evade host immunity.

Thomas R Lerner,Lucy Collinson,Robert J Wilkinson,Maximiliano G Gutierrez,Rachel P Lai,Matthew R.G Russell,Daniel J Greenwood,Antony Fearns,Christophe J Queval

doi:10.1172/jci.insight.136937

Abstract

The ability of Mycobacterium tuberculosis to form serpentine cords is intrinsically related to its virulence, but specifically how M. tuberculosis cording contributes to pathogenesis remains obscure. Here, we show that several M. tuberculosis clinical isolates form intracellular cords in primary human lymphatic endothelial cells (hLECs) in vitro and in the lymph nodes of patients with tuberculosis. We identified via RNA-Seq a transcriptional program that activated, in infected-hLECs, cell survival and cytosolic surveillance of pathogens pathways. Consistent with this, cytosolic access was required for intracellular M. tuberculosis cording. Mycobacteria lacking ESX-1 type VII secretion system or phthiocerol dimycocerosates expression, which failed to access the cytosol, were indeed unable to form cords within hLECs. Finally, we show that M. tuberculosis cording is a size-dependent mechanism used by the pathogen to avoid its recognition by cytosolic sensors and evade either resting or IFN-γ-induced hLEC immunity. These results explain the long-standing association between M. tuberculosis cording and virulence and how virulent mycobacteria use intracellular cording as strategy to successfully adapt and persist in the lymphatic tracts.

Highlights

Mycobacterium tuberculosis is one of the deadliest bacterial pathogens of humankind and still constitutes a global health challenge [1]
We have previously shown in extrapulmonary TB that a subpopulation of M. tuberculosis is found in human lymphatic endothelial cells in lymph node biopsies and these cells could represent a reservoir for M. tuberculosis in infected patients [19]
By monitoring GFPexpressing M. tuberculosis H37Rv (GFP-M. tuberculosis) replication in human lymphatic endothelial cells (hLECs) at different time points after infection, we observed a striking ability of M. tuberculosis to form distinctive intracellular cords over the time (Figure 1A). 3D confocal imaging of M. tuberculosis–infected hLECs for 72 hours confirmed that these cords were intracellular rather than on the cell surface (Figure 1B)

Summary

Introduction

Mycobacterium tuberculosis is one of the deadliest bacterial pathogens of humankind and still constitutes a global health challenge [1]. A striking phenotype of M. tuberculosis growing in nutrient broth is the ability of this pathogen to form serpentine cords, a morphological observation originally described by Robert Koch [2]. This cording phenotype is intimately associated with virulence and immune evasion [3]. Similar cording has been reported in other pathogenic mycobacteria, primarily in liquid media or extracellularly in various cell and organism models of infection. M. abscessus released from apoptotic macrophages grows extracellularly, forming cords [8]. There are, a few reports showing that cording can occur intracellularly. Ferrer and coworkers [11] showed that an attenuated mutant of M. tuberculosis formed cords in fibroblasts

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Conclusion