Human T-cell responses to 25 novel antigens encoded by genes of the dormancy regulon of Mycobacterium tuberculosis

Not to wake a sleeping giant: new insights into host-pathogen interactions identify new targets for vaccination against latent Mycobacterium tuberculosis infection

Mycobacterium tuberculosisis one of the worlds' most successful and sophisticated pathogens. It is estimated that over 2 billion people today harbour latentM. tuberculosisinfection without any clinical symptoms. As most new cases of active tuberculosis (TB) arise from this (growing) number of latently infected individuals, urgent measures to control TB reactivation are required, including post-exposure/therapeutic vaccines. The current bacille Calmette-Guérin (BCG) vaccine and all new generation TB vaccines being developed and tested are essentially designed as prophylactic vaccines. Unfortunately, these vaccines are unlikely to be effective in individuals already latently infected withM. tuberculosis. Here, we argue that detailed analysis ofM. tuberculosisgenes that are switched on predominantly during latent stage infection may lead to the identification of new antigenic targets for anti-TB strategies. We will describe essential host-pathogen interactions in TB with particular emphasis on TB latency and persistent infection. Subsequently, we will focus on novel groups of late-stage specific genes, encoded amongst others by theM. tuberculosisdormancy (dosR) regulon, and summarise recent studies describing human T-cell recognition of these dormancy antigens in relation to (latent)M. tuberculosisinfection. We will discuss the possible relevance of these new classes of antigens for vaccine development against TB.

New drugs and vaccines for drug-resistant Mycobacterium tuberculosis infections

Tuberculosis remains the most common cause of death due to a single infective organism. Despite the availability of a vaccine and chemotherapeutic options, the global disease burden remains relatively unaffected. The ability of the mycobacterial etiological agents to adopt a semidormant, phenotypically drug-resistant state requires that chemotherapy is both complex and lengthy. The emergence of drug resistance has raised the possibility of virtually untreatable tuberculosis. Furthermore, the currently used bacillus Calmette-Guerin vaccine has had mixed success in protecting susceptible populations. Given this backdrop, the need for novel anti-infectives and more effective vaccines is clearly evident. Recent progress, described herein, has seen the development and entry into clinical trials of several new drugs and vaccine candidates.

Identification of Mycobacterium tuberculosis ornithine carboamyltransferase in urine as a possible molecular marker of active pulmonary tuberculosis

Although the antigen detection assay has the potential to discriminate active tuberculosis from latent infection, development of such a test for the accurate diagnosis of this serious disease has only recently become a matter of interest. Here we present evidence that a Mycobacterium tuberculosis protein (ornithine carboamyltransferase, coded for by MT_1694; Rv1656 [argF]) is an interesting candidate molecule for this test development. The protein was initially discovered by mass spectroscopy in urine of patients with pulmonary tuberculosis and shown by Western blot analysis to be present in M. tuberculosis crude cell extract as well as in the culture supernatant ("secreted" protein). In addition, a recombinant ornithine carboamyltransferase (rMT1694) produced in Escherichia coli was recognized by immunoglobulin G (IgG) antibodies from patients with active tuberculosis but not by IgG from uninfected healthy subjects. Moreover, rMT1694 was strongly recognized by peripheral blood mononuclear cells from both healthy tuberculin purified protein derivative (PPD)-positive individuals and patients with pulmonary tuberculosis. More importantly, a capture enzyme-linked immunosorbent assay formatted with rabbit IgG antibodies specific to rMT1694 was able to identify the presence of this antigen in urine samples from 6 of 16 patients with pulmonary tuberculosis and in none of 16 urine samples collected from healthy PPD(+) controls. These results indicate that an improved antigen detection assay based on M. tuberculosis ornithine carboamyltransferase may represent an important new strategy for the development of a specific and accurate diagnostic test for tuberculosis.

Mycobacterium bovis BCG vaccine strains lack narK2 and narX induction and exhibit altered phenotypes during dormancy

Mycobacterium tuberculosis is the causative agent of tuberculosis, a disease that affects one-third of the world's population. The sole extant vaccine for tuberculosis is the live attenuated Mycobacterium bovis bacillus Calmette-Guerin (BCG). We examined 13 representative BCG strains from around the world to ascertain their ability to express DosR-regulated dormancy antigens. These are known to be recognized by T cells of M. tuberculosis-infected individuals, especially those harboring latent infections. Differences in the expression of these antigens could be valuable for use as diagnostic markers to distinguish BCG vaccination from latent tuberculosis. We determined that all BCG strains were defective for the induction of two dormancy genes: narK2 (Rv1737c) and narX (Rv1736c). NarK2 is known to be necessary for nitrate respiration during anaerobic dormancy. Analysis of the narK2/X promoter region revealed a base substitution mutation in all tested BCG strains and M. bovis in comparison to the M. tuberculosis sequence. We also show that nitrate reduction by BCG strains during dormancy was greatly reduced compared to M. tuberculosis and varied between tested strains. Several dormancy regulon transcriptional differences were also identified among the strains, as well as variation in their growth and survival. These findings demonstrate defects in DosR regulon expression during dormancy and phenotypic variation between commonly used BCG vaccine strains.

Cooperative binding of phosphorylated DevR to upstream sites is necessary and sufficient for activation of the Rv3134c-devRS operon in Mycobacterium tuberculosis: implication in the induction of DevR target genes

The DevR-DevS two-component system of Mycobacterium tuberculosis mediates bacterial adaptation to hypoxia, a condition believed to be associated with the initiation and maintenance of dormant bacilli during latent tuberculosis. The activity of the Rv3134c-devRS operon was studied in M. tuberculosis using several transcriptional fusions comprised of promoter regions and the gfp reporter gene under inducing and aerobic conditions. Aerobic transcription was DevR independent, while hypoxic induction was completely DevR dependent. The hypoxia transcriptional start point, T(H), was mapped at -40 bp upstream of Rv3134c. In contrast, the divergently transcribed Rv3135 gene was not induced under hypoxic conditions. DNase I footprinting and mutational analyses demonstrated that induction required the interaction of DevR-P with binding sites centered at bp -42.5 and -63.5 relative to T(H). Binding to the distal site (D) was necessary to recruit another molecule of DevR-P to the proximal site (P), and interaction with both sequences was essential for promoter activation. These sites did not bind to either unphosphorylated or phosphorylation-defective DevR protein, which was consistent with an essential role for DevR-P in activation. Phosphorylated DevR also bound to three copies of the motif at the hspX promoter. The Rv3134c and hspX promoters have a similar architecture, wherein the proximal DevR-P binding site overlaps with the promoter -35 element. A model for the likely mode of action of DevR at these promoters is discussed.

How a different look at latency can help to develop novel diagnostics and vaccines against tuberculosis

Mycobacterium tuberculosis is one of the most successful human pathogens. It kills every year approximately 1.5 - 2 million people, and at present a third of the human population is estimated to be infected. Fortunately, only a relatively small proportion of the infected individuals will progress to active disease, and most will maintain a latent infection. Although a latent infection is clinically silent and not contagious, it can reactivate to cause highly contagious pulmonary tuberculosis, the most prevalent form of the disease in adults. Therefore, a thorough understanding of latency and reactivation may help to develop novel control strategies against tuberculosis. The most widely held view is that the mycobacteria are imprisoned in granulomatous structures during latency, where they can survive in a non-replicating, dormant form until reactivation occurs. However, there is no hard data to sustain that the reactivating mycobacteria are indeed those that laid dormant within the granulomas. In this review an alternative model, based on evidence from early studies, as well as recent reports is presented, in which the latent mycobacteria reside outside granulomas, within non-macrophage cell types throughout the infected body. Potential implications for new diagnostic and vaccine design are discussed.

T-cell assays for the diagnosis of latent tuberculosis infection: moving the research agenda forward

For nearly a century, the tuberculin skin test was the only tool available for the detection of latent tuberculosis infection. A recent breakthrough has been the development of T-cell-based interferon-gamma release assays. Current evidence suggests interferon-gamma release assays have higher specificity than the tuberculin skin test, better correlation with surrogate markers of exposure to Mycobacterium tuberculosis in low-incidence settings, and less cross-reactivity as a result of BCG vaccination compared with the tuberculin skin test. The body of literature supporting the use of interferon-gamma release assays has rapidly expanded. However, several unresolved and unexplained issues remain. To address these issues, a group of experts met in Geneva, Switzerland, in March, 2006, to discuss the research evidence on T-cell-based assays, their clinical usefulness, limitations, and directions for future research, with a specific focus on resource-limited and high HIV prevalence settings. On the basis of 2 days of discussions, a comprehensive research agenda was generated, which will propel the field forward by stimulating focused high-impact research and encourage the investment of resources needed to tackle priority research questions, especially in resource-limited settings. Ultimately, if adequately financed, the research findings will inform appropriate use of novel latent tuberculosis infection diagnostics in global tuberculosis control.

Lack of immune responses to Mycobacterium tuberculosis DosR regulon proteins following Mycobacterium bovis BCG vaccination

Mycobacterium bovis BCG is widely used as a vaccine against tuberculosis (TB), despite its variable protective efficacy. Relatively little is known about the immune response profiles following BCG vaccination in relation to protection against TB. Here we tested whether BCG vaccination results in immune responses to DosR (Rv3133c) regulon-encoded proteins. These so-called TB latency antigens are targeted by the immune system during persistent Mycobacterium tuberculosis infection and have been associated with immunity against latent M. tuberculosis infection. In silico analysis of the DosR regulon in BCG and M. tuberculosis showed at least 97% amino acid sequence homology, with 41 out of 48 genes being identical. Transcriptional profiling of 14 different BCG strains, under hypoxia and nitric oxide exposure in vitro, revealed a functional DosR regulon similar to that observed in M. tuberculosis. Next, we assessed human immune responses to a series of immunodominant TB latency antigens and found that BCG vaccination fails to induce significant responses to latency antigens. Similar results were obtained with BCG-vaccinated BALB/c mice. In contrast, responses to latency antigens were observed in individuals with suspected exposure to TB (as indicated by positive gamma interferon responses to TB-specific antigens ESAT-6 and CFP-10) and in mice vaccinated with plasmid DNA encoding selected latency antigens. Since immune responses to TB latency antigens have been associated with control of latent M. tuberculosis infection, our findings support the development of vaccination strategies incorporating DosR regulon antigens to complement and improve the current BCG vaccine.

The prognosis of latent tuberculosis: can disease be predicted?

In humans, Mycobacterium tuberculosis persists for long periods in a clinically latent state, creating a huge reservoir of 'silent' tuberculosis (TB) (roughly one-third of the global population) from which new cases continually arise. A prognostic marker for active TB would enable targeted treatment of the small fraction of infected individuals who are most at risk of developing contagious TB, contributing greatly to TB control efforts. Here, we propose that TB-specific interferon-gamma release assays might be useful for identifying individuals with progressive infections who are likely to develop the disease. This might provide an unprecedented advantage for TB control, namely targeted preventive therapy for individuals who are most at risk of developing active contagious TB.

T-cell recognition of the HspX protein of Mycobacterium tuberculosis correlates with latent M. tuberculosis infection but not with M. bovis BCG vaccination

During stationary growth or in vitro conditions mimicking relevant aspects of latency, the HspX protein (Rv2031c) is specifically upregulated by Mycobacterium tuberculosis. In this study we compared T-cell responses against HspX and the secreted M. tuberculosis protein Ag85B (Rv1886c) in tuberculosis (TB) patients, tuberculin skin test-positive individuals, M. bovis BCG-vaccinated individuals, and healthy negative controls. Gamma interferon responses to HspX were significantly higher in M. tuberculosis-exposed individuals than in M. tuberculosis-unexposed BCG vaccinees. In contrast, no such differences were found with respect to T-cell responses against Ag85B. Therefore, BCG-based vaccines containing relevant fragments of HspX may induce improved responses against this TB latency antigen. To identify relevant major histocompatibility complex class I- and class II-restricted HspX-specific T-cell epitopes, we immunized HLA-A2/K(b) and HLA-DR3.Ab(0) transgenic (tg) mice with HspX. Two new T-cell epitopes were identified, p91-105 and p31-50, restricted via HLA-A*0201 and HLA-DRB1*0301, respectively. These epitopes were recognized by human T cells as well, underlining the relevance of HspX T-cell recognition both in vivo and in vitro. In line with the data in humans, BCG immunization of both tg strains did not lead to T-cell responses against HspX-derived epitopes, whereas nonlatency antigens were efficiently recognized. These data support the notion that BCG vaccination per se does not induce T-cell responses against the latency antigen, HspX. Thus, we suggest that subunit vaccines incorporating HspX and/or other latency antigens, as well as recombinant BCG strains expressing latency antigens need to be considered as new vaccines against TB.

Tuberculosis subunit vaccine development: Impact of physicochemical properties of mycobacterial test antigens

Tuberculosis caused by Mycobacterium tuberculosis continues to be one of the major public health problems in the world. The eventual control of this disease will require the development of a safe and effective vaccine. One of the approaches receiving a great deal of attention recently is subunit vaccination. An efficacious antituberculous subunit vaccine requires the identification and isolation of key components of the pathogen that are capable of inducing a protective immune response. Clues to identify promising subunit vaccine candidates may be found in their physicochemical and immunobiological properties. In this article, we review the evidence that the physicochemical properties of mycobacterial components can greatly impact the induction of either protective or deleterious immune response and consequently influence the potential utility as an antituberculous subunit vaccine.

Measurement of antigen specific immune responses: 2006 update

Measuring antigen-specific immune responses (MASIR) is essential for basic immunological research and in the clinical setting. Numerous techniques have been used and the recent years have witnessed a flourishing of flow cytometry based methods for the identification of antigen specific T cells, in addition to other methodologies. The second MASIR conference held in Santorini, Greece, from 14 to 18 June 2006 has been a forum for the discussion of methodological issues and for research or clinical applications of these techniques, as reviewed here. In addition to flow cytometry based techniques, other emerging techniques with different degrees of complexity can be applied. These novel methods are highly promising in numerous conditions to look for correlates of protection, to test responses to natural infections or to vaccination trials, to evaluate the immune status of immunocompromised patients and to monitor persistence and function of specific T cells administered as adoptive therapy.

Immunogenicity of eight dormancy regulon-encoded proteins of Mycobacterium tuberculosis in DNA-vaccinated and tuberculosis-infected mice

Hypoxia and low concentrations of nitric oxide have been reported to upregulate in vitro gene expression of 48 proteins of the dormancy (DosR) regulon of Mycobacterium tuberculosis. These proteins are thought to be essential for the survival of bacteria during persistence in vivo and are targeted by the immune system during latent infection in humans. Here we have analyzed the immunogenicity of eight DosR regulon-encoded antigens by plasmid DNA vaccination of BALB/c and C57BL/6 mice, i.e., Rv1733c, Rv1738, Rv2029c (pfkB), Rv2031c/hspX (acr), Rv2032 (acg), Rv2626c, Rv2627c, and Rv2628. Strong humoral and/or cellular Th1-type (interleukin-2 and gamma interferon) immune responses could be induced against all but one (Rv1738) of these antigens. The strongest Th1 responses were measured following vaccination with DNA encoding Rv2031c and Rv2626c. Using synthetic 20-mer overlapping peptides, 11 immunodominant, predicted major histocompatibility complex class II-restricted epitopes and one K(d)-restricted T-cell epitope could be identified. BALB/c and (B6D2)F(1) mice persistently infected with M. tuberculosis developed immune responses against Rv1733c, Rv2031c, and Rv2626c. These findings have implications for proof-of-concept studies in mice mimicking tuberculosis (TB) latency models and their extrapolation to humans for potential new vaccination strategies against TB.

Vaccine strategies against latent tuberculosis infection

The leading tuberculosis (TB) vaccines currently in clinical trials are all designed as prophylactic vaccines. Although these vaccines are highly active, they will most probably not result in sterilizing immunity and, therefore, will not solve the global problem of latent TB. An attractive strategy is to target the remaining dormant bacteria with vaccines based upon antigens induced as the bacteria change from active multiplication to non-replicating dormancy (latency antigens) or during reactivation as dormant bacteria resume active metabolism (resuscitation antigens). These late-stage antigens might have potential as post-exposure vaccines or could form the basis for a multi-stage vaccine strategy, in which they are combined with prophylactic vaccines based on early antigens from replicating bacteria.