Anaerobic Mycobacterium tuberculosis Cell Death Stems from Intracellular Acidification Mitigated by the DosR Regulon.

Mycobacterium tuberculosis is a strict aerobe capable of prolonged survival in the absence of oxygen. We investigated the ability of anaerobic M. tuberculosis to counter challenges to internal pH homeostasis in the absence of aerobic respiration, the primary mechanism of proton efflux for aerobic bacilli. Anaerobic M. tuberculosis populations were markedly impaired for survival under a mildly acidic pH relative to standard culture conditions. An acidic environmental pH greatly increased the susceptibilities of anaerobic bacilli to the collapse of the proton motive force by protonophores, to antimicrobial compounds that target entry into the electron transport system, and to small organic acids with uncoupling activity. However, anaerobic bacilli exhibited high tolerance against these challenges at a near-neutral pH. At a slightly alkaline pH, which was near the optimum intracellular pH, the addition of protonophores even improved the long-term survival of bacilli. Although anaerobic M. tuberculosis bacilli under acidic conditions maintained 40% lower ATP levels than those of bacilli under standard culture conditions, ATP loss alone could not explain the drop in viability. Protonophores decreased ATP levels by more than 90% regardless of the extracellular pH but were bactericidal only under acidic conditions, indicating that anaerobic bacilli could survive an extreme ATP loss provided that the external pH was within viable intracellular parameters. Acidic conditions drastically decreased the anaerobic survival of a DosR mutant, while an alkaline environment improved the survival of the DosR mutant. Together, these findings indicate that intracellular acidification is a primary challenge for the survival of anaerobic M. tuberculosis and that the DosR regulon plays a critical role in sustaining internal pH homeostasis.IMPORTANCE During infection, M. tuberculosis bacilli are prevalent in environments largely devoid of oxygen, yet the factors that influence the survival of these severely growth-limited and metabolically limited bacilli remain poorly understood. We determined how anaerobic bacilli respond to fluctuations in environmental pH and observed that these bacilli were highly susceptible to stresses that promoted internal acidic stress, whereas conditions that promoted an alkaline internal pH promoted long-term survival even during severe ATP depletion. The DosR regulon, a major regulator of general hypoxic stress, played an important role in maintaining internal pH homeostasis under anaerobic conditions. Together, these findings indicate that in the absence of aerobic respiration, protection from internal acidification is crucial for long-term M. tuberculosis survival.

Mycobacterium tuberculosis Phosphate Uptake System Component PstA2 Is Not Required for Gene Regulation or Virulence.

The Mycobacterium tuberculosis genome encodes two complete high-affinity Pst phosphate-specific transporters. We previously demonstrated that a membrane-spanning component of one Pst system, PstA1, was essential both for M. tuberculosis virulence and for regulation of gene expression in response to external phosphate availability. To determine if the alternative Pst system is similarly required for virulence or gene regulation, we constructed a deletion of pstA2. Transcriptome analysis revealed that PstA2 is not required for regulation of gene expression in phosphate-replete growth conditions. PstA2 was also dispensable for replication and virulence of M. tuberculosis in a mouse aerosol infection model. However, a ΔpstA1ΔpstA2 double mutant was attenuated in mice lacking the cytokine interferon-gamma, suggesting that M. tuberculosis requires high-affinity phosphate transport to survive phosphate limitation encountered in the host. Surprisingly, ΔpstA2 bacteria were more resistant to acid stress in vitro. This phenotype is intrinsic to the alternative Pst transporter since a ΔpstS1 mutant exhibited similar acid resistance. Our data indicate that the two M. tuberculosis Pst transporters have distinct physiological functions, with the PstA1 transporter being specifically involved in phosphate sensing and gene regulation while the PstA2 transporter influences survival in acidic conditions.

Mycobacterium tuberculosis requires phosphate-responsive gene regulation to resist host immunity.

Mycobacterium tuberculosis persists in the tissues of mammalian hosts despite inducing a robust immune response dominated by the macrophage-activating cytokine gamma interferon (IFN-γ). We identified the M. tuberculosis phosphate-specific transport (Pst) system component PstA1 as a factor required to resist IFN-γ-dependent immunity. A ΔpstA1 mutant was fully virulent in IFN-γ(-/-) mice but attenuated in wild-type (WT) mice and mice lacking specific IFN-γ-inducible immune mechanisms: nitric oxide synthase (NOS2), phagosome-associated p47 GTPase (Irgm1), or phagocyte oxidase (phox). These phenotypes suggest that ΔpstA1 bacteria are sensitized to an IFN-γ-dependent immune mechanism(s) other than NOS2, Irgm1, or phox. In other species, the Pst system has a secondary role as a negative regulator of phosphate starvation-responsive gene expression through an interaction with a two-component signal transduction system. In M. tuberculosis, we found that ΔpstA1 bacteria exhibited dysregulated gene expression during growth in phosphate-rich medium that was mediated by the two-component sensor kinase/response regulator system SenX3-RegX3. Remarkably, deletion of the regX3 gene suppressed the replication and virulence defects of ΔpstA1 bacteria in NOS2(-/-) mice, suggesting that M. tuberculosis requires the Pst system to negatively regulate activity of RegX3 in response to available phosphate in vivo. We therefore speculate that inorganic phosphate is readily available during replication in the lung and is an important signal controlling M. tuberculosis gene expression via the Pst-SenX3-RegX3 signal transduction system. Inability to sense this environmental signal, due to Pst deficiency, results in dysregulation of gene expression and sensitization of the bacteria to the host immune response.

The Mycobacterium tuberculosis DosR regulon assists in metabolic homeostasis and enables rapid recovery from nonrespiring dormancy.

Mycobacterium tuberculosis survives in latently infected individuals, likely in a nonreplicating or dormancy-like state. The M. tuberculosis DosR regulon is a genetic program induced by conditions that inhibit aerobic respiration and prevent bacillus replication. In this study, we used a mutant incapable of DosR regulon induction to investigate the contribution of this regulon to bacterial metabolism during anaerobic dormancy. Our results confirm that the DosR regulon is essential for M. tuberculosis survival during anaerobic dormancy and demonstrate that it is required for metabolic processes that occur upon entry into and throughout the dormant state. Specifically, we showed that regulon mechanisms shift metabolism away from aerobic respiration in the face of dwindling oxygen availability and are required for maintaining energy levels and redox balance as the culture becomes anaerobic. We also demonstrated that the DosR regulon is crucial for rapid resumption of growth once M. tuberculosis exits an anaerobic or nitric oxide-induced nonrespiring state. In summary, the DosR regulon encodes novel metabolic mechanisms essential for M. tuberculosis to survive in the absence of respiration and to successfully transition rapidly between respiring and nonrespiring conditions without loss of viability.

Unique roles of DosT and DosS in DosR regulon induction and Mycobacterium tuberculosis dormancy.

In Mycobacterium tuberculosis, the sensor kinases DosT and DosS activate the transcriptional regulator DosR, resulting in the induction of the DosR regulon, which is important for anaerobic survival and perhaps latent infection. The individual and collective roles of these sensors have been postulated biochemically, but their roles in vivo have remained unclear. This work demonstrates distinct and additive roles for each sensor during anaerobic dormancy. Both sensors are necessary for wild-type levels of DosR regulon induction, and concomitantly, full induction of the regulon is required for wild-type anaerobic survival. In the anaerobic model, DosT plays an early role, responding to hypoxia. DosT then induces the regulon and with it DosS, which sustains and further induces the regulon. DosT then loses its functionality as oxygen becomes limited, and DosS alone maintains induction of the genes from that point forward. Thus, M. tuberculosis has evolved a system whereby it responds to hypoxic conditions in a stepwise fashion as it enters an anaerobic state.

Reconstitution of hepatitis C virus-specific T-cellmediated immunity after liver transplantation.

Hepatitis C virus (HCV)-related liver failure is the leading indication for liver transplantation worldwide. After transplantation, virological recurrence is the rule, but the spectrum of histological injury is wide, ranging from the development of allograft cirrhosis within a few years to minimal hepatitis despite long-term follow-up. The immunological correlates of this variable natural history are poorly understood. Here, we studied the kinetics of the cellular immune responses, viral replication, and allograft histology in 24 patients who had undergone liver transplantation for HCV-related liver failure. Using direct ex vivo methodologies (i.e., interferon-gamma ELISPOT and major histocompatibility complex class I-peptide tetrameric complexes), we found that patients who experienced viral eradication after antiviral therapy showed restoration of HCV-specific T-cell responses, whereas patients with progressive HCV recurrence that failed to respond to therapy showed declining frequencies of these viral-specific effector cells. The cytotoxic T lymphocytes that peripherally reconstituted after transplantation were clonotypically identical to those present within the recipient explant liver, defined at the level of the T-cell receptor beta chain (one epitope/one clone). Moreover, the subset of patients who spontaneously demonstrated minimal histologic recurrence had more vigorous CD4+ T-cell responses in the first 3 months, particularly targeting nonstructural proteins. We provide evidence that T-cell responses emerge after liver transplantation, and their presence correlates with improved histological and clinical outcomes. In conclusion, these results may help identify patients more likely to develop severe HCV recurrence and therefore benefit from current antiviral therapy, as well as provide a rationale for the future use of novel immunotherapeutic approaches. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience. wiley.com/jpages/0270-9139/suppmat/index.html).

Cutting edge: identification of hepatitis C virus-specific CD8+ T cells restricted by donor HLA alleles following liver transplantation.

By necessity, human liver transplantation is performed across HLA barriers. As a result, intracellular infection of the allograft presents a unique immunologic challenge for the recipient's immune system. In this study, we describe the presence of HLA-A2-restricted, hepatitis C virus (HCV)-specific CD8+ T cells in liver transplant recipients in whom the allograft is HLA-A2 positive and the recipient is HLA-A2 negative. These memory-effector T cells are recipient derived and recognize HCV peptide uniquely in the context of HLA-A2. Furthermore, these cells were absent before the transplant, suggesting that the allograft is capable of selectively expanding naive CD8+ T cells. The in vitro specificity to donor HLA allele-restricted CD8+ T cells suggests that these cells may function to control HCV spread in the allograft.