Role of M. tuberculosis RD-1 region encoded secretory proteins in protective response and virulence

A β-Lactamase based reporter system for ESX dependent protein translocation in mycobacteria

Protein secretion is essential for all bacteria in order to interact with their environment. Mycobacterium tuberculosis depends on protein secretion to subvert host immune response mechanisms. Both the general secretion system (Sec) and the twin-arginine translocation system (Tat) are functional in mycobacteria. Furthermore, a novel type of protein translocation system named ESX has been identified. In the genome of M. tuberculosis five paralogous ESX regions (ESX-1 to ESX-5) have been found. Several components of the ESX translocation apparatus have been identified over the last ten years. The ESX regions are composed of a basic set of genes for the translocation machinery and the main substrate - a heterodimer. The best studied of these heterodimers is EsxA (ESAT-6)/EsxB (CFP-10), which has been shown to be exported by ESX-1. EsxA/B is heavily involved in virulence of M. tuberculosis. EsxG/H is exported by ESX-3 and seems to be involved in an essential iron-uptake mechanism in M. tuberculosis. These findings make ESX-3 components high profile drug targets. Until now, reporter systems for determination of ESX protein translocation have not been developed. In order to create such a reporter system, a truncated β-lactamase (‘bla TEM-1) was fused to the N-terminus of EsxB, EsxG and EsxU, respectively. These constructs have then been tested in a β-lactamase (BlaS) deletion strain of Mycobacterium smegmatis. M. smegmatis ΔblaS is highly susceptible to ampicillin. An ampicillin resistant phenotype was conferred by translocation of Bla TEM-1-Esx fusion proteins into the periplasm. BlaTEM-1-Esx fusion proteins were not found in the culture filtrate suggesting that plasma membrane translocation and outer membrane translocation are two distinct steps in ESX secretion. Thus we have developed a powerful tool to dissect the molecular mechanisms of ESX dependent protein translocation and to screen for novel components of the ESX systems on a large scale.

Early secreted antigen ESAT-6 of Mycobacterium tuberculosis promotes protective T helper 17 cell responses in a toll-like receptor-2-dependent manner

Despite its relatively poor efficacy, Bacillus Calmette-Guérin (BCG) has been used as a tuberculosis (TB) vaccine since its development in 1921. BCG induces robust T helper 1 (Th1) immune responses but, for many individuals, this is not sufficient for host resistance against Mycobacterium tuberculosis (M. tb) infection. Here we provide evidence that early secreted antigenic target protein 6 (ESAT-6), expressed by the virulent M. tb strain H37Rv but not by BCG, promotes vaccine-enhancing Th17 cell responses. These activities of ESAT-6 were dependent on TLR-2/MyD88 signalling and involved IL-6 and TGF-β production by dendritic cells. Thus, animals that were previously infected with H37Rv or recombinant BCG containing the RD1 region (BCG::RD1) exhibited improved protection upon re-challenge with virulent H37Rv compared with mice previously infected with BCG or RD1-deficient H37Rv (H37RvΔRD1). However, TLR-2 knockout (TLR-2⁻/⁻) animals neither showed Th17 responses nor exhibited improved protection in response to immunization with H37Rv. Furthermore, H37Rv and BCG::RD1 infection had little effect on the expression of the anti-inflammatory microRNA-146a (miR146a) in dendritic cells (DCs), whereas BCG and H37RvΔRD1 profoundly induced its expression in DCs. Consistent with these findings, ESAT-6 had no effect on miR146a expression in uninfected DCs, but dramatically inhibited its upregulation in BCG-infected or LPS-treated DCs. Collectively, our findings indicate that, in addition to Th1 immunity induced by BCG, RD1/ESAT-6-induced Th17 immune responses are essential for optimal vaccine efficacy.

Cutting edge: Nicastrin and related components of γ-secretase generate a peptide epitope facilitating immune recognition of intracellular mycobacteria, through MHC class II-dependent priming of T cells

Bacillus Calmette-Guérin (BCG), the antituberculosis vaccine, localizes within immature phagosomes of macrophages and dendritic cells (APCs), and avoids lysosomal degradation. BCG-derived antigenic peptides are thus inefficiently processed by APCs, and we investigated alternate mechanisms of Ag processing. Proteomics identified that BCG phagosomes are enriched for nicastrin, APH, and presenilin components of γ-secretase, a multimeric protease. Using an in vitro Ag presentation assay and BCG-infected APCs, we found γ-secretase components to cleave BCG-derived Ag85B to produce a peptide epitope, which, in turn, primed IL-2 release from Ag85B-specific T cell hybridoma. siRNA knockdown or chemical inhibition of γ-secretase components using L685458 decreased the ability of BCG or Mycobacterium tuberculosis-infected APCs to present Ag85B. In addition, L685485 inhibition of γ-secretase led to a decreased ability of BCG-dendritic cells to immunize mice and induce Ag85B-specific CD4 T cells in vivo. Because BCG and M. tuberculosis sequester within APCs preventing immune recognition, γ-secretase components appear to fortuitously process the immunodominant Ag85B, facilitating immune recognition.

Comparative transcriptional study of the putative mannose donor biosynthesis genes in virulent Mycobacterium tuberculosis and attenuated Mycobacterium bovis BCG strains

Mycobacterium tuberculosis contains mannosylated cell wall components which are important in macrophage recognition and response. The building block for the mannosyl constituents of these components is GDP-mannose, which is synthesized through a series of enzymes involved in the mannose donor biosynthesis pathway. Nothing is known about the expression levels of the genes encoding these enzymes during the course of infection. To generate transcriptional profiles for the mannose donor biosynthesis genes from virulent M. tuberculosis and attenuated Mycobacterium bovis BCG, bacteria were grown in broth culture and within human macrophages. Our results with broth-grown bacteria show that there are differences in expression of the selected genes between M. tuberculosis and BCG, with increased expression of manC in M. tuberculosis and manA in BCG during stationary-phase growth. Results for M. tuberculosis extracted from within macrophages show that whiB2 is highly expressed and manB and manC are moderately expressed during infection. Rv3256c, Rv3258c, and ppm1 have high expression levels early and decreased expression as the infection progresses. Results with BCG show that, as in M. tuberculosis, whiB2 is highly expressed throughout infection, whereas there is either low expression or little change in expression of the remaining genes studied. Overall, our results show that there is differential regulation of expression of several genes in the mannose donor biosynthesis pathway of M. tuberculosis and BCG grown in broth and within macrophages, raising the possibility that the level of mannose donors may vary during the course of infection and thereby impact the biosynthesis of mannose-containing cell wall molecules.

Cytokines in response to proteins predicted in genomic regions of difference of Mycobacterium tuberculosis

Cellular immune responses are responsible for both protection and pathogenesis in tuberculosis, and are mediated/regulated by a complex network of pro-inflammatory, T helper (Th) type 1 and type 2 cytokines. In this study, the secretion of pro-inflammatory cytokines tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, IL-8 and IL-1β; Th1 cytokines interferon-gamma (IFN-γ), IL-2 and tumor necrosis factor-beta (TNF-β); and Th2 cytokines IL-4, IL-5 and IL-10 by the peripheral blood mononuclear cells (PBMCs) of pulmonary tuberculosis patients was studied. PBMCs were cultured in vitro in the absence and presence of complex mycobacterial antigens and peptides corresponding to 11 regions of difference (RD) of Mycobacterium tuberculosis that are deleted/absent in all vaccine strains of Mycobacterium bovis bacillus Calmette-Guérin (BCG). The culture supernatants were tested for secreted cytokines by FlowCytomix assay. PBMCs from the majority of patients (53-100%) spontaneously secreted detectable concentrations of all cytokines tested, except for IL2 (29%) and IL-10 (41%). The profiles of proinflammatory cytokines were largely similar for various complex antigens or RD peptides. However, with respect to Th1 and Th2 cytokines, the antigens could be divided into three groups; the first with Th1-bias (culture filtrate of M. tuberculosis, RD1, RD5, RD7, RD9 and RD10), the second with Th2-bias (whole cells and cell walls of M. tuberculosis, RD12, RD13 and RD15), and the third without Th1/Th2-bias (M. bovis BCG, RD4, RD6 and RD11). Complex mycobacterial antigens and RD proteins with Th1- and Th2-biases may have roles in protection and pathogenesis of tuberculosis, respectively.

Effects of Mycobacterium tuberculosis ESAT-6/CFP-10 fusion protein on the autophagy function of mouse macrophages

Autophagy plays specific roles in host innate and adaptive immune responses to numerous intracellular pathogens, including Mycobacterium tuberculosis. The ESAT-6 and CFP-10 proteins are secreted by M. tuberculosis and play important roles in pathogenesis. We hypothesized that these two proteins may affect the autophagy function of host macrophages during infection with M. tuberculosis, thereby shaping the immune reaction toward the pathogen. Interestingly, we found that rapamycin-induced autophagy of macrophages infected with M. tuberculosis H37Rv enhanced localization of mycobacteria with autophagosomes and lysosomes. Ectopic expression of the ESAT-6/CFP-10 fusion in macrophages dramatically inhibited autophagosome formation, and M. tuberculosis survival inside infected macrophages was significantly affected as well. Further, M. tuberculosis viability was increased by the fusion protein. Expression levels of autophagy-related genes (ATG), especially atg8, also decreased (p<0.05). These results suggested that ESAT-6 and CFP-10 proteins play significant roles in autophagy formation in M. tuberculosis infection and that autophagosome formation is regulated through the expression of ATG.

Tuberculosis immunity: opportunities from studies with cattle

Mycobacterium tuberculosis and M. bovis share >99% genetic identity and induce similar host responses and disease profiles upon infection. There is a rich history of codiscovery in the development of control measures applicable to both human and bovine tuberculosis (TB) including skin-testing procedures, M. bovis BCG vaccination, and interferon-γ release assays. The calf TB infection model offers several opportunities to further our understanding of TB immunopathogenesis. Recent observations include correlation of central memory immune responses with TB vaccine efficacy, association of SIRPα⁺ cells in ESAT-6:CFP10-elicited multinucleate giant cell formation, early γδ T cell responses to TB, antimycobacterial activity of memory CD4⁺ T cells via granulysin production, association of specific antibody with antigen burden, and suppression of innate immune gene expression in infected animals. Partnerships teaming researchers with veterinary and medical perspectives will continue to provide mutual benefit to TB research in man and animals.

Multiple small RNAs identified in Mycobacterium bovis BCG are also expressed in Mycobacterium tuberculosis and Mycobacterium smegmatis

Tuberculosis (TB) is a major global health problem, infecting millions of people each year. The causative agent of TB, Mycobacterium tuberculosis, is one of the world’s most ancient and successful pathogens. However, until recently, no work on small regulatory RNAs had been performed in this organism. Regulatory RNAs are found in all three domains of life, and have already been shown to regulate virulence in well-known pathogens, such as Staphylococcus aureus and Vibrio cholera. Here we report the discovery of 34 novel small RNAs (sRNAs) in the TB-complex M. bovis BCG, using a combination of experimental and computational approaches. Putative homologues of many of these sRNAs were also identified in M. tuberculosis and/or M. smegmatis. Those sRNAs that are also expressed in the non-pathogenic M. smegmatis could be functioning to regulate conserved cellular functions. In contrast, those sRNAs identified specifically in M. tuberculosis could be functioning in mediation of virulence, thus rendering them potential targets for novel antimycobacterials. Various features and regulatory aspects of some of these sRNAs are discussed.

Adjusting to a new home: Mycobacterium tuberculosis gene expression in response to an intracellular lifestyle

Mycobacterium tuberculosis remains the most significant single species of bacteria causing disease in mankind. The ability of M. tuberculosis to survive and replicate within host macrophages is a pivotal step in its pathogenesis. Understanding the microenvironments that M. tuberculosis encounters within the macrophage and the adaptations that the bacterium undergoes to facilitate its survival will lead to insights into possible therapeutic targets for improved treatment of tuberculosis. This is urgently needed with the emergence of multi- and extensively drug resistant strains of M. tuberculosis. Significant advances have been made in understanding the macrophage response on encountering M. tuberculosis. Complementary information is also accumulating regarding the counter responses of M. tuberculosis during the various stages of its interactions with the host. As such, a picture is emerging delineating the gene expression of intracellular M. tuberculosis at different stages of the interaction with macrophages.

Signal regulatory protein alpha (SIRPalpha) cells in the adaptive response to ESAT-6/CFP-10 protein of tuberculous mycobacteria

BACKGROUND

Early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) are co-secreted proteins of Mycobacterium tuberculosis complex mycobacteria (includes M. bovis, the zoonotic agent of bovine tuberculosis) involved in phagolysosome escape of the bacillus and, potentially, in the efficient induction of granulomas. Upon tuberculosis infection, multi-nucleate giant cells are elicited, likely as a response aimed at containing mycobacteria. In tissue culture models, signal regulatory protein (SIRP)alpha (also referred to as macrophage fusion receptor or CD172a) is essential for multi-nucleate giant cell formation.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, ESAT-6/CFP-10 complex and SIRPalpha interactions were evaluated with samples obtained from calves experimentally infected with M. bovis. Peripheral blood CD172a(+) (SIRPalpha-expressing) cells from M. bovis-infected calves proliferated upon in vitro stimulation with ESAT-6/CFP-10 (either as a fusion protein or a peptide cocktail), but not with cells from animals receiving M. bovis strains lacking ESAT-6/CFP-10 (i.e, M. bovis BCG or M. bovis DeltaRD1). Sorted CD172a(+) cells from these cultures had a dendritic cell/macrophage morphology, bound fluorescently-tagged rESAT-6:CFP-10, bound and phagocytosed live M. bovis BCG, and co-expressed CD11c, DEC-205, CD44, MHC II, CD80/86 (a subset also co-expressed CD11b or CD8alpha). Intradermal administration of rESAT-6:CFP-10 into tuberculous calves elicited a delayed type hypersensitive response consisting of CD11c(+), CD172a(+), and CD3(+) cells, including CD172a-expressing multi-nucleated giant cells.

CONCLUSIONS/SIGNIFICANCE

These findings demonstrate the ability of ESAT-6/CFP-10 to specifically expand CD172a(+) cells, bind to CD172a(+) cells, and induce multi-nucleated giant cells expressing CD172a.

A novel DNA vaccine containing multiple TB-specific epitopes casted in a natural structure (ECANS) confers protective immunity against pulmonary mycobacterial challenge

Epitope-based DNA vaccines designed to induce T cell responses specific for Mycobacterium tuberculosis (M. tb) are being developed as a means of addressing vaccine potency. In this study, we predicted 4 T cell epitopes from ESAT-6, Ag85A/B and CFP-10 antigens and constructed an ECANS (epitopes casted in a natural structure) DNA vaccine by inserting the epitope DNA segments separately into the gene backbone of M. tb-derived HSP65 (heat shock protein 65) carrier. The immunogenicity and protective efficacy of pECANS DNA vaccine were assessed in BALB/c mice after intramuscular immunization with 4 doses of 50 microg ECANS DNA and followed by mycobaterial challenge 4 weeks after the last immunization. Compared to plasmid encoding HSP65, pECANS DNA immunization elicited remarkably higher levels of IFN-gamma production by both CD4(+) and CD8(+) T cells, which were coupled with higher frequencies of antigen-specific T cells and higher CTL activity. Significantly enhanced levels of Th1 cytokines (IFN-gamma and IL-12) and increased serum IgG2a/IgG1 ratio were also noted, indicating a predominant Th1 immune response achieved by pECANS DNA immunization. In the consequence, a better protection against Mycobacterium bovis BCG challenge was achieved which was evidenced by reduced bacterial loads in lungs and spleens and profound attenuation of lung inflammation and injury. Our results suggested that multi-T cell-epitope based ECANS gene vaccine induced T cell response to multiple T cell epitopes and led to enhanced protection against mycobacterial challenge. This strategy might be a useful platform to design multi-T cell epitope-based vaccine against M. tb infection.