Dormant Mycobacterium tuberculosis Fails To Block Phagosome Maturation and Shows Unexpected Capacity To Stimulate Specific Human T Lymphocytes

Abstract

Dormancy is defined as a stable but reversible nonreplicating state of Mycobacterium tuberculosis. It is currently thought that dormant M. tuberculosis (D-Mtb) is responsible for latent tuberculosis (TB) infection. Recently, D-Mtb was also shown in sputa of patients with active TB, but the capacity of D-Mtb to stimulate specific immune responses was not investigated. We observed that purified protein derivative–specific human CD4+ T lymphocytes recognize mycobacterial Ags more efficiently when macrophages are infected with D-Mtb instead of replicating M. tuberculosis (R-Mtb). The different Ag recognition occurs even when the two forms of mycobacteria equally infect and stimulate macrophages, which secrete the same cytokine pattern and express MHC class I and II molecules at the same levels. However, D-Mtb but not R-Mtb colocalizes with mature phagolysosome marker LAMP-1 and with vacuolar proton ATPase in macrophages. D-Mtb, unlike R-Mtb, is unable to interfere with phagosome pH and does not inhibit the proteolytic efficiency of macrophages. We show that D-Mtb downmodulates the gene Rv3875 encoding for ESAT-6, which is required by R-Mtb to block phagosome maturation together with Rv3310 gene product SapM, previously shown to be downregulated in D-Mtb. Thus, our results indicate that D-Mtb cannot escape MHC class II Ag-processing pathway because it lacks the expression of genes required to block the phagosome maturation. Data suggest that switching to dormancy not only represents a mechanism of survival in latent TB infection, but also a M. tuberculosis strategy to modulate the immune response in different stages of TB.

Innate and Adaptive Cellular Immune Responses toMycobacterium tuberculosisInfection

“…Once engulfed by the macrophage, Mtb potently inhibits macrophage activation and becomes highly resistant to clearance. Virulent Mtb manipulates the response of infected cells to avoid detection and elimination through a variety of immune evasion strategies, including inhibition of phago-lysosome fusion and detoxification of nitrogen and oxygen radicals and dormancy (Flynn and Chan 2003;Pieters 2008;Gengenbacher and Kaufmann 2012;Mariotti et al 2013). When the cell-intrinsic response to Mtb proves inadequate and/or the bacilli replicate to sufficient numbers within AMs, the infected cells burst.…”

Challenging Mycobacterium tuberculosis dormancy mechanisms and their immunodiagnostic potential

“…Actually, as demonstrated by Mariotti et al, 36 dormant mycobacteria lack the capacity to subvert the host immune system due to its inability to block the phagosome maturation. Such findings could be associated with persistent antigen presentation to CD4 + T cells by dendritic cells.…”

“…It is unlikely that dormant mycobacteria could infect tissue macrophages during latency, as they do not replicate and cannot invade host cells unless they reactivate to sense the environment. 36 Another mechanism for host protection against M. tuberculosis infection was recently described by Venkatasubramanian et al 37 A particular subset of regulatory T cells, named CD4 + CD25 + FoxP3 + D4GDI + T cells, was found to contribute in the contention of M. tuberculosis in both human and murine models. These cells do not produce immunomodulatory cytokines, such as TGF-␤ or IL-10, secreting Rho GDP dissociation inhibitor (D4GDI) instead.…”

Mycobacterium tuberculosis and the human macrophage : shifting the balance through inflammasome activation

“…However, DosR is also required for exiting dormancy (181), indicating a constant turnover of the bacterial population in vivo (182). Entering dormancy as a defensive strategy is underlined by the fact that already dormant bacteria infecting human macrophages fail to inhibit phagosomal maturation, but are still not killed by the macrophage (183). In a way, being phagocytosed by a macrophage able to impose stress on the bacterium can be seen as a maturation process of the bacterium itself leading to the induction of dormancy.…”