Date Published: December 20, 2018
Publisher: Public Library of Science
Author(s): Claudia Sala, Nina T. Odermatt, Paloma Soler-Arnedo, Muhammet F. Gülen, Sofia von Schultz, Andrej Benjak, Stewart T. Cole, Christopher M. Sassetti.
The ESX-1, type VII, secretion system represents the major virulence determinant of Mycobacterium tuberculosis, one of the most successful intracellular pathogens. Here, by combining genetic and high-throughput approaches, we show that EspL, a protein of 115 amino acids, is essential for mediating ESX-1-dependent virulence and for stabilization of EspE, EspF and EspH protein levels. Indeed, an espL knock-out mutant was unable to replicate intracellularly, secrete ESX-1 substrates or stimulate innate cytokine production. Moreover, proteomic studies detected greatly reduced amounts of EspE, EspF and EspH in the espL mutant as compared to the wild type strain, suggesting a role for EspL as a chaperone. The latter conclusion was further supported by discovering that EspL interacts with EspD, which was previously demonstrated to stabilize the ESX-1 substrates and effector proteins, EspA and EspC. Loss of EspL also leads to downregulation in M. tuberculosis of WhiB6, a redox-sensitive transcriptional activator of ESX-1 genes. Overall, our data highlight the importance of a so-far overlooked, though conserved, component of the ESX-1 secretion system and begin to delineate the role played by EspE, EspF and EspH in virulence and host-pathogen interaction.
Mycobacterium tuberculosis, the etiological agent of human tuberculosis, is arguably the world’s most successful human pathogen. It is estimated that one third of the world’s population is latently infected by the bacterium , which can survive in a dormant state inside specialized cellular structures in the lung parenchyma called granulomas [2,3]. As a consequence of immunodeficiency or co-morbidities, like HIV or diabetes [4,5], latent M. tuberculosis can reactivate and establish an acute infectious process which leads to the disease. Host-pathogen interaction and disease progression are mediated by various virulence factors encoded by the bacterial genome, the most important of them being the ESX-1 or type VII secretion system .
The data presented here demonstrate the essentiality of EspL for ESX-1-dependent virulence and for stabilizing the intracellular levels of EspE, EspF and EspH in M. tuberculosis. ESX-1-dependent secretion in ΔespL was severely compromised, with undetectable levels of EsxA, EsxB, EspA and EspD in the culture filtrates. Conversely, secretion of EspB was not affected, confirming previous data generated by our group . In line with the secretion profile, virulence and innate cytokine production were compromised when THP-1 cells were infected by the espL knock-out strain. In this regard, ΔespL behaves like the ESX-1-null mutants ΔRD1  and ΔΔRD1 , which fail to stimulate the innate immune response . These phenotypes tally with those previously reported for an espL transposon mutant in a clinical isolate of the W/Beijing family, which was shown to have lost the ability to arrest phagosomal maturation , and with the need for espL in order to fully complement an espB mutant in M. marinum . While these phenotypic traits are most likely attributable to lack of secretion of the main ESX-1 substrates, whether all of them are directly caused by lack of EspL or are mediated by EspE, EspF or EspH is currently unknown and additional research is required.