In vivo adaptation of the Wayne model of latent tuberculosis

ESAT-6 undergoes self-association at phagosomal pH and an ESAT-6-specific nanobody restricts M. tuberculosis growth in macrophages

Mycobacterium tuberculosis (Mtb) is known to survive within macrophages by compromising the integrity of the phagosomal compartment in which it resides. This activity primarily relies on the ESX-1 secretion system, predominantly involving the protein duo ESAT-6 and CFP-10. CFP-10 likely acts as a chaperone, while ESAT-6 likely disrupts phagosomal membrane stability via a largely unknown mechanism. we employ a series of biochemical analyses, protein modeling techniques, and a novel ESAT-6-specific nanobody to gain insight into the ESAT-6's mode of action. First, we measure the binding kinetics of the tight 1:1 complex formed by ESAT-6 and CFP-10 at neutral pH. Subsequently, we demonstrate a rapid self-association of ESAT-6 into large complexes under acidic conditions, leading to the identification of a stable tetrameric ESAT-6 species. Using molecular dynamics simulations, we pinpoint the most probable interaction interface. Furthermore, we show that cytoplasmic expression of an anti-ESAT-6 nanobody blocks Mtb replication, thereby underlining the pivotal role of ESAT-6 in intracellular survival. Together, these data suggest that ESAT-6 acts by a pH-dependent mechanism to establish two-way communication between the cytoplasm and the Mtb-containing phagosome.

ESAT-6 undergoes self-association at phagosomal pH and an ESAT-6 specific nanobody restricts M. tuberculosis growth in macrophages

Mycobacterium tuberculosis (Mtb) is known to survive within macrophages by compromising the integrity of the phagosomal compartment in which it resides. This activity primarily relies on the ESX-1 secretion system, predominantly involving the protein duo ESAT-6 and CFP-10. CFP-10 likely acts as a chaperone, while ESAT-6 likely disrupts phagosomal membrane stability via a largely unknown mechanism. we employ a series of biochemical analyses, protein modeling techniques, and a novel ESAT-6-specific nanobody to gain insight into the ESAT-6's mode of action. First, we measure the binding kinetics of the tight 1:1 complex formed by ESAT-6 and CFP-10 at neutral pH. Subsequently, we demonstrate a rapid self-association of ESAT-6 into large complexes under acidic conditions, leading to the identification of a stable tetrameric ESAT-6 species. Using molecular dynamics simulations, we pinpoint the most probable interaction interface. Furthermore, we show that cytoplasmic expression of an anti-ESAT-6 nanobody blocks Mtb replication, thereby underlining the pivotal role of ESAT-6 in intracellular survival. Together, these data suggest that ESAT-6 acts by a pH dependent mechanism to establish two-way communication between the cytoplasm and the Mtb-containing phagosome.

The puzzle of the evolutionary natural history of tuberculosis

Several pieces of the puzzle of the natural history of tuberculosis are assembled in this review to illustrate the potential reservoirs and sources of the Mycobacterium tuberculosis complex (MTBC) mycobacteria, their transmission to animals and humans, and their fate in populations, in a co-evolutionary perspective. Millennia-old companions of mammalian and human populations, MTBC are detected in the soil, in which they infect and survive within vegetative amoebae and cysts, except for Mycobacterium canettii. Never detected in the sphere of plants, they are transmissible by transcutaneous, digestive and respiratory routes and cause an infection of the lymphatic system with secondary dissemination in most tissues, in which they determine a specific and non-pathognomonic granulomatous inflammatory reaction; in which MTBC survives in dormant form irrespective of MTBC species and mammalian species; indicating that the current epidemiology in mammalian populations is essentially governed by the probabilities of contact between mammalian species and MTBC species. Individual variabilities in clinical expression of tuberculosis are related to MTBC species, strain and inoculum; host genetic factors; acquired modulations of the inflammatory response; and probably human microbiota. This review of the literature suggests an evolutionary natural history of telluric environmental mycobacteria, satellites of unicellular eukaryotes, transmissible to mammals via the digestive and then respiratory tracts, in which they determine a fatal contagious infection that is primarily lymphatic and a quiescence-mimicking encysted form. This review opens perspectives for microbiological and translational medical research.

Addressing Latent Tuberculosis: New Advances in Mimicking the Disease, Discovering Key Targets, and Designing Hit Compounds

Despite being discovered and isolated more than one hundred years ago, tuberculosis (TB) remains a global public health concern arch. Our inability to eradicate this bacillus is strongly related with the growing resistance, low compliance to current drugs, and the capacity of the bacteria to coexist in a state of asymptomatic latency. This last state can be sustained for years or even decades, waiting for a breach in the immune system to become active again. Furthermore, most current therapies are not efficacious against this state, failing to completely clear the infection. Over the years, a series of experimental methods have been developed to mimic the latent state, currently used in drug discovery, both in vitro and in vivo. Most of these methods focus in one specific latency inducing factor, with only a few taking into consideration the complexity of the granuloma and the genomic and proteomic consequences of each physiological factor. A series of targets specifically involved in latency have been studied over the years with promising scaffolds being discovered and explored. Taking in account that solving the latency problem is one of the keys to eradicate the disease, herein we compile current therapies and diagnosis techniques, methods to mimic latency and new targets and compounds in the pipeline of drug discovery.

Human and Mouse Hematopoietic Stem Cells Are a Depot for Dormant Mycobacterium tuberculosis

An estimated third of the world's population is latently infected with Mycobacterium tuberculosis (Mtb), with no clinical signs of tuberculosis (TB), but lifelong risk of reactivation to active disease. The niches of persisting bacteria during latent TB infection remain unclear. We detect Mtb DNA in peripheral blood selectively in long-term repopulating pluripotent hematopoietic stem cells (LT-pHSCs) as well as in mesenchymal stem cells from latently infected human donors. In mice infected with low numbers of Mtb, that do not develop active disease we, again, find LT-pHSCs selectively infected with Mtb. In human and mouse LT-pHSCs Mtb are stressed or dormant, non-replicating bacteria. Intratracheal injection of Mtb-infected human and mouse LT-pHSCs into immune-deficient mice resuscitates Mtb to replicating bacteria within the lung, accompanied by signs of active infection. We conclude that LT-pHSCs, together with MSCs of Mtb-infected humans and mice serve as a hitherto unappreciated quiescent cellular depot for Mtb during latent TB infection.

Prophylactic Use of Ganoderma lucidum Extract May Inhibit Mycobacterium tuberculosis Replication in a New Mouse Model of Spontaneous Latent Tuberculosis Infection

A mouse model of spontaneous latent tuberculosis infection (LTBI) that mimics LTBI in humans is valuable for drug/vaccine development and the study of tuberculosis. However, most LTBI mouse models require interventions, and a spontaneous LTBI mouse model with a low bacterial load is difficult to establish. In this study, mice were IV-inoculated with 100 CFU Mycobacterium tuberculosis H37Rv, and a persistent LTBI was established with low bacterial loads (0.5~1.5log₁₀ CFU in the lung; < 4log₁₀ CFU in the spleen). Histopathological changes in the lung and spleen were mild during the first 20 weeks post-inoculation. The model was used to demonstrate the comparative effects of prophylactic and therapeutic administration of Ganoderma lucidum extract (spores and spores lipid) in preventing H37Rv replication in both lung and spleen. H37Rv was inhibited with prophylactic use of G. lucidum extract relative to that of the untreated control and therapy groups, and observed in the spleen and lung as early as post-inoculation week 3 and week 5 respectively. H37Rv infection in the therapy group was comparable to that of the untreated control mice. No significant mitigation of pathological changes was observed in either the prophylactic or therapeutic group. Our results suggest that this new LTBI mouse model is an efficient tool of testing anti-tuberculosis drug, the use of G. lucidum extract prior to M. tuberculosis infection may protect the host against bacterial replication to some extent.

Differential influence of nutrient-starved Mycobacterium tuberculosis on adaptive immunity results in progressive tuberculosis disease and pathology

When infected with Mycobacterium tuberculosis, most individuals will remain clinically healthy but latently infected. Latent infection has been proposed to partially involve M. tuberculosis in a nonreplicating stage, which therefore represents an M. tuberculosis phenotype that the immune system most likely will encounter during latency. It is therefore relevant to examine how this particular nonreplicating form of M. tuberculosis interacts with the host immune system. To study this, we first induced a state of nonreplication through prolonged nutrient starvation of M. tuberculosis in vitro. This resulted in nonreplicating persistence even after prolonged culture in phosphate-buffered saline. Infection with either exponentially growing M. tuberculosis or nutrient-starved M. tuberculosis resulted in similar lung CFU levels in the first phase of the infection. However, between week 3 and 6 postinfection, there was a very pronounced increase in bacterial levels and associated lung pathology in nutrient-starved-M. tuberculosis-infected mice. This was associated with a shift from CD4 T cells that coexpressed gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) or IFN-γ, TNF-α, and interleukin-2 to T cells that only expressed IFN-γ. Thus, nonreplicating M. tuberculosis induced through nutrient starvation promotes a bacterial form that is genetically identical to exponentially growing M. tuberculosis yet characterized by a differential impact on the immune system that may be involved in undermining host antimycobacterial immunity and facilitate increased pathology and transmission.

Mycobacterium tuberculosis metabolism

Metabolism underpins the physiology and pathogenesis of Mycobacterium tuberculosis. However, although experimental mycobacteriology has provided key insights into the metabolic pathways that are essential for survival and pathogenesis, determining the metabolic status of bacilli during different stages of infection and in different cellular compartments remains challenging. Recent advances-in particular, the development of systems biology tools such as metabolomics-have enabled key insights into the biochemical state of M. tuberculosis in experimental models of infection. In addition, their use to elucidate mechanisms of action of new and existing antituberculosis drugs is critical for the development of improved interventions to counter tuberculosis. This review provides a broad summary of mycobacterial metabolism, highlighting the adaptation of M. tuberculosis as specialist human pathogen, and discusses recent insights into the strategies used by the host and infecting bacillus to influence the outcomes of the host-pathogen interaction through modulation of metabolic functions.

Reactivation of dormant "non-culturable" Mycobacterium tuberculosis developed in vitro after injection in mice: both the dormancy depth and host genetics influence the outcome

Three stocks of Mycobacterium tuberculosis H37Rv were cultured in vitro under prolonged hypoxic or acidified conditions until partial or complete loss of the capacity to form colonies on agar medium was achieved. Such dormant "non-culturable" mycobacteria were assessed for the growth resuscitation after intra-tracheal injection into mice of the two inbred strains with different genetic susceptibility to M. tuberculosis-triggered disease: hyper-susceptible I/St and relatively resistant B6. The results indicate that bacteria which are able to resuscitate spontaneously in liquid medium in vitro started to multiply in organs of infected mice, and that the outcome of such infection strongly depended upon the level of genetic TB susceptibility. However, dormant bacteria required inducers for resuscitation in vitro lost the capacity to multiply even in genetically susceptible mice. The established model of dormancy/reactivation is suitable for the studying host-pathogen interactions and testing vaccine and drug candidates specifically targeting latent TB.

Detection of Mycobacterium tuberculosis in latently infected lungs by immunohistochemistry and confocal microscopy

Detection of latent Mycobacterium tuberculosis is a challenge in the diagnosis of asymptomatic, subclinical tuberculosis. We report the development of an immunofluorescence technique to visualize and enumerate M. tuberculosis in latently infected rabbit lungs where no acid-fast-stained organisms were seen and no cultivable bacilli were obtained by the agar-plating method.

Non-replicating Mycobacterium tuberculosis elicits a reduced infectivity profile with corresponding modifications to the cell wall and extracellular matrix

A key feature of Mycobacterium tuberculosis is its ability to become dormant in the host. Little is known of the mechanisms by which these bacilli are able to persist in this state. Therefore, the focus of this study was to emulate environmental conditions encountered by M. tuberculosis in the granuloma, and determine the effect of such conditions on the physiology and infectivity of the organism. Non-replicating persistent (NRP) M. tuberculosis was established by the gradual depletion of nutrients in an oxygen-replete and controlled environment. In contrast to rapidly dividing bacilli, NRP bacteria exhibited a distinct phenotype by accumulating an extracellular matrix rich in free mycolate and lipoglycans, with increased arabinosylation. Microarray studies demonstrated a substantial down-regulation of genes involved in energy metabolism in NRP bacteria. Despite this reduction in metabolic activity, cells were still able to infect guinea pigs, but with a delay in the development of disease when compared to exponential phase bacilli. Using these approaches to investigate the interplay between the changing environment of the host and altered physiology of NRP bacteria, this study sheds new light on the conditions that are pertinent to M. tuberculosis dormancy and how this organism could be establishing latent disease.

Mycobacterium bovis DNA detection in colostrum as a potential indicator of vaccination effectiveness against bovine tuberculosis

Bovine tuberculosis (bTB) remains a problem on many dairy farms in Mexico, as well as a public health risk. We previously found a high frequency of Mycobacterium bovis DNA in colostrum from dairy cows using a nested PCR to detect mpb70. Since there are no reliable in vivo tests to determine the effectiveness of booster Mycobacterium bovis BCG vaccination against bTB, in this work we monitored M. bovis DNA in colostrum by using this nested PCR. In order to decrease the risk of adverse reactions in animals likely containing viable M. bovis, a single application of BCG and a subunit vaccine (EEP-1) formulated with M. bovis culture filtrate proteins (CFP) and a copolymer as the adjuvant was performed in tuberculin skin test-negative cattle (TST(-)), while TST reactor animals (TST(+)) received EEP-1 only. Booster immunization using EEP-1 was applied to both groups, 2 months after primary vaccination to whole herds and 12 months later to lactating cows. Colostrum samples were collected from 6 farms where the cows were vaccinated over a 12-month period postvaccination and, for comparison, from one control farm where the cows were not vaccinated with comparable bTB prevalence. We observed an inverse relationship between the frequency of M. bovis DNA detection and time postvaccination at the first (P < 0.001) and second (P < 0.0001) 6-month periods. Additionally, the concentration of gamma interferon (IFN-γ) was higher in mpb70 PCR-positive colostrum samples (P = 0.0003). These results suggest that M. bovis DNA frequency in colostrum could be a potentially useful biomarker for bTB vaccine efficacy on commercial dairy farms.

Discovery and validation of new antitubercular compounds as potential drug leads and probes

Increasing multidrug resistance in Mycobacterium tuberculosis continues to diminish the number of effective drugs available for treatment of active tuberculosis. Although there are four new products (representing three new chemical classes) in clinical development, an active, robust pipeline of new chemical entities is critical to discovery of medicines to dramatically improve or shorten length of therapy via new mechanisms of action. In the absence of major pharmaceutical industry activity in tuberculosis drug development, the National Institute of Allergy and Infectious Diseases (NIAID) has supported the development of a high throughput screen for growth inhibitors of M. tuberculosis using a chemically diverse commercial library, a compound library available through the NIH Roadmap, Molecular Libraries Screening Center Network, and other compound sources. The rationale for these screens and suggested approaches for follow-up studies to identify compounds for advanced preclinical studies and as chemical probes of critical functions in M. tuberculosis, are discussed.

Interaction of Mycobacterium tuberculosis with the host: consequences for vaccine development

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), remains a major worldwide health problem that causes more than 2 million deaths annually. In addition, an estimated 2 billion people are latently infected with M. tuberculosis. The bacterium is one of the oldest human pathogens and has evolved complex strategies for survival. Therefore, to be successful in the high endemic regions, any future TB vaccine strategy will have to be tailored in accordance with the resulting complexity of the TB infection and anti-mycobacterial immune response. In this review, we will discuss what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge is used in new and more advanced vaccine strategies.

Experimental tuberculosis: the role of comparative pathology in the discovery of improved tuberculosis treatment strategies

The use of laboratory animals is critical to the discovery and in vivo pre-clinical testing of new drugs and drug combinations for use in humans. M. tuberculosis infection of mice, rats, guinea pigs, rabbits and non-human primates are the most commonly used animal models of human tuberculosis. While granulomatous inflammation characterizes the most fundamental host response to M. tuberculosis aerosol infection in humans and animals, there are important species differences in pulmonary and extra-pulmonary lesion morphology which may influence responses to drug therapy. Lesions that progress to necrosis or cavitation are common, unfavorable host responses in naturally occurring tuberculosis of humans, but are not seen consistently in experimental infections in most animal model species. The importance of these unique lesion morphologies is that they represent irreversible tissue damage that can harbor persistent bacilli which are difficult to treat with standard therapies. Understanding the differences in host response to experimental tuberculosis infections may aid in selecting the most appropriate animal models to test drugs that have been rationally designed to have specific mechanisms of action in vivo. A better understanding of lesion pathogenesis across species may also aid in the identification of novel therapeutic targets or strategies that can be used alone or in combination with more conventional tuberculosis treatments in humans.