Peripheral blood and pleural fluid mononuclear cell responses to low-molecular-mass secretory polypeptides of Mycobacterium tuberculosis in human models of immunity to tuberculosis

Modulating mycobacterial envelope integrity for antibiotic synergy with benzothiazoles

Developing effective tuberculosis drugs is hindered by mycobacteria's intrinsic antibiotic resistance because of their impermeable cell envelope. Using benzothiazole compounds, we aimed to increase mycobacterial cell envelope permeability and weaken the defenses of Mycobacterium marinum, serving as a model for Mycobacterium tuberculosis Initial hit, BT-08, significantly boosted ethidium bromide uptake, indicating enhanced membrane permeability. It also demonstrated efficacy in the M. marinum-zebrafish embryo infection model and M. tuberculosis-infected macrophages. Notably, BT-08 synergized with established antibiotics, including vancomycin and rifampicin. Subsequent medicinal chemistry optimization led to BT-37, a non-toxic and more potent derivative, also enhancing ethidium bromide uptake and maintaining synergy with rifampicin in infected zebrafish embryos. Mutants of M. marinum resistant to BT-37 revealed that MMAR_0407 (Rv0164) is the molecular target and that this target plays a role in the observed synergy and permeability. This study introduces novel compounds targeting a new mycobacterial vulnerability and highlights their cooperative and synergistic interactions with existing antibiotics.

HigA2 (Rv2021c) Is a Transcriptional Regulator with Multiple Regulatory Targets in Mycobacterium tuberculosis

Toxin-antitoxin (TA) systems are the major mechanism for persister formation in Mycobacterium tuberculosis (Mtb). Previous studies found that HigBA2 (Rv2022c-Rv2021c), a predicted type II TA system of Mtb, could be activated for transcription in response to multiple stresses such as anti-tuberculosis drugs, nutrient starvation, endure hypoxia, acidic pH, etc. In this study, we determined the binding site of HigA2 (Rv2021c), which is located in the coding region of the upstream gene higB2 (Rv2022c), and the conserved recognition motif of HigA2 was characterized via oligonucleotide mutation. Eight binding sites of HigA2 were further found in the Mtb genome according to the conserved motif. RT-PCR showed that HigA2 can regulate the transcription level of all eight of these genes and three adjacent downstream genes. DNA pull-down experiments showed that twelve functional regulators sense external regulatory signals and may regulate the transcription of the HigBA2 system. Of these, Rv0903c, Rv0744c, Rv0474, Rv3124, Rv2603c, and Rv3583c may be involved in the regulation of external stress signals. In general, we identified the downstream target genes and possible upstream regulatory genes of HigA2, which paved the way for the illustration of the persistence establishment mechanism in Mtb.

Immunological Characterization of Proteins Expressed by Genes Located in Mycobacterium tuberculosis-Specific Genomic Regions Encoding the ESAT6-like Proteins

The 6 kDa early secreted antigen target (ESAT6) is a low molecular weight and highly immunogenic protein of Mycobacterium tuberculosis with relevance in the diagnosis of tuberculosis and subunit vaccine development. The gene encoding the ESAT6 protein is located in the M. tuberculosis-specific genomic region known as the region of difference (RD)1. There are 11 M. tuberculosis-specific RDs absent in all of the vaccine strains of BCG, and three of them (RD1, RD7, and RD9) encode immunodominant proteins. Each of these RDs has genes for a pair of ESAT6-like proteins. The immunological characterizations of all the possible proteins encoded by genes in RD1, RD7 and RD9 have shown that, besides ESAT-6 like proteins, several other proteins are major antigens useful for the development of subunit vaccines to substitute or supplement BCG. Furthermore, some of these proteins may replace the purified protein derivative of M. tuberculosis in the specific diagnosis of tuberculosis by using interferon-gamma release assays and/or tuberculin-type skin tests. At least three subunit vaccine candidates containing ESAT6-like proteins as antigen components of multimeric proteins have shown efficacy in phase 1 and phase II clinical trials in humans.

B cells response directed against Cut4 and CFP21 lipolytic enzymes in active and latent tuberculosis infections

BACKGROUND

Better understanding of the immune response directed against Mycobacterium tuberculosis (Mtb) is critical for development of vaccine strategies and diagnosis tests. Previous studies suggested that Mtb enzymes involved in lipid metabolism, are associated with persistence and/or reactivation of dormant bacilli.

METHODS

Circulating antibodies secreting cells (ASCs), memory B cells, and antibodies directed against Cut4 (Rv3452) and CFP21 (Rv1984c) antigens were explored in subjects with either active- or latent-tuberculosis (LTB), and in Mtb-uninfected individuals.

RESULTS

Circulating anti-Cut4 ASCs were detected in 11/14 (78.6%) subjects from the active TB group vs. 4/17 (23.5%) from the LTB group (p = 0.001). Anti-CFP21 ASCs were found in 11/14 (78.6%) active TB vs. in 5/17 (29.4%) LTB cases (p = 0.01). Circulating anti-Cut4 and anti-CFP21 ASCs were not detected in 38 Mtb uninfected controls. Memory B cells directed against either Cut4 or CFP21 were identified in 8/11 (72.7%) and in 9/11 (81.8%) subjects with LTB infection, respectively, and in 2/6 Mtb uninfected individuals (33.3%). High level of anti-Cut4 and anti-CFP21 IgG were observed in active TB cases.

CONCLUSION

Circulating IgG SCs directed against Cut4 or CFP21 were mostly detected in patients presenting an active form of the disease, suggesting that TB reactivation triggers an immune response against these two antigens.

Structural and genetic analysis of START superfamily protein MSMEG_0129 from Mycobacterium smegmatis

Mycobacterium tuberculosis is a notorious pathogen that continues to threaten human health. Rv0164, an antigen of both T- and B cells conserved across mycobacteria, and MSMEG_0129, its close homolog in Mycobacterium smegmatis, are predicted members of the START domain superfamily, but their molecular function is unknown. Here, gene knockout studies demonstrate MSMEG_0129 is essential for bacterial growth, suggesting Rv0164 may be a potential drug target. The MSMEG_0129 crystal structure determined at 1.95 Å reveals a fold similar to that in polyketide aromatase/cyclases ZhuI and TcmN from Streptomyces sp. Structural comparisons and docking simulations, however, infer that MSMEG_0129 and Rv0164 are unlikely to catalyze polyketide aromatization/cyclization, but probably play an irreplaceable role during mycobacterial growth, for example, in lipid transfer during cell envelope synthesis.

The putative polyketide cyclase MSMEG_0129 from Mycobacterium smegmatis: purification, crystallization and X-ray crystallographic analysis

Mycobacterium tuberculosis Rv0164 has previously been identified as a human T-cell antigen that induces significant production of IFN-γ in human peripheral blood mononuclear cells. M. smegmatis MSMEG_0129 shares 59% sequence identity with Rv0164. Based on sequence alignment, both proteins are predicted to be members of the cyclase/dehydrase family, which is part of a large group of enzymes referred to as type II polyketide synthases (PKSs). In biosynthetic pathways mediated by type II PKSs, cyclases catalyze the conversion of linear poly-β-ketones to cyclized intermediates. To date, no mycobacterial type II PKSs have been reported. Here, the goal is to determine whether these proteins adopt similar folds to reported cyclase structures, and to this end MSMEG_0129 was cloned, expressed, purified and crystallized. An X-ray diffraction data set was collected to 1.95 Å resolution from a crystal belonging to space group P6₂, with unit-cell parameters a = 109.76, b = 109.76, c = 56.5 Å, α = 90, β = 90, γ = 120°. Further crystallographic analysis should establish a basis for investigating the structure and function of this putative mycobacterial type II PKS enzyme.

Glycosylation of Recombinant Antigenic Proteins from Mycobacterium tuberculosis: In Silico Prediction of Protein Epitopes and Ex Vivo Biological Evaluation of New Semi-Synthetic Glycoconjugates

Tuberculosis is still one of the most deadly infectious diseases worldwide, and the use of conjugated antigens, obtained by combining antigenic oligosaccharides, such as the lipoarabinomannane (LAM), with antigenic proteins from Mycobacterium tuberculosis (MTB), has been proposed as a new strategy for developing efficient vaccines. In this work, we investigated the effect of the chemical glycosylation on two recombinant MTB proteins produced in E. coli with an additional seven-amino acid tag (recombinant Ag85B and TB10.4). Different semi-synthetic glycoconjugated derivatives were prepared, starting from mannose and two disaccharide analogs. The glycans were activated at the anomeric position with a thiocyanomethyl group, as required for protein glycosylation by selective reaction with lysines. The glycosylation sites and the ex vivo evaluation of the immunogenic activity of the different neo-glycoproteins were investigated. Glycosylation does not modify the immunological activity of the TB10.4 protein. Similarly, Ag85B maintains its B-cell activity after glycosylation while showing a significant reduction in the T-cell response. The results were correlated with the putative B- and T-cell epitopes, predicted using a combination of in silico systems. In the recombinant TB10.4, the unique lysine is not included in any T-cell epitope. Lys30 of Ag85B, identified as the main glycosylation site, proved to be the most important site involved in the formation of T-cell epitopes, reasonably explaining why its glycosylation strongly influenced the T-cell activity. Furthermore, additional lysines included in different epitopes (Lys103, -123 and -282) are also glycosylated. In contrast, B-cell epitopic lysines of Ag85B were found to be poorly glycosylated and, thus, the antibody interaction of Ag85B was only marginally affected after coupling with mono- or disaccharides.

O-mannosylation of the Mycobacterium tuberculosis adhesin Apa is crucial for T cell antigenicity during infection but is expendable for protection

Glycosylation is the most abundant post-translational polypeptide chain modification in nature. Although carbohydrate modification of protein antigens from many microbial pathogens constitutes important components of B cell epitopes, the role in T cell immunity is not completely understood. Here, using ELISPOT and polychromatic flow cytometry, we show that O-mannosylation of the adhesin, Apa, of Mycobacterium tuberculosis (Mtb) is crucial for its T cell antigenicity in humans and mice after infection. However, subunit vaccination with both mannosylated and non-mannosylated Apa induced a comparable magnitude and quality of T cell response and imparted similar levels of protection against Mtb challenge in mice. Both forms equally improved waning BCG vaccine-induced protection in elderly mice after subunit boosting. Thus, O-mannosylation of Apa is required for antigenicity but appears to be dispensable for its immunogenicity and protective efficacy in mice. These results have implications for the development of subunit vaccines using post-translationally modified proteins such as glycoproteins against infectious diseases like tuberculosis.

Expression of the ARPC4 subunit of human Arp2/3 severely affects mycobacterium tuberculosis growth and suppresses immunogenic response in murine macrophages

BACKGROUND

The search for molecules against Mycobacterium tuberculosis is urgent. The mechanisms facilitating the intra-macrophage survival of Mycobacterium tuberculosis are as yet not entirely understood. However, there is evidence showing the involvement of host cell cytoskeleton in every step of establishment and persistence of mycobacterial infection.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that expression of ARPC4, a subunit of the Actin related protein 2/3 (Arp2/3) protein complex, severely affects the pathogen's growth. TEM studies display shedding of the mycobacterial outer-coat. Furthermore, in infected macrophages, mycobacteria expressing ARPC4 were cleared off at a much faster rate, and were unable to mount a pro-inflammatory cytokine response. The translocation of ARPC4-expressing mycobacteria to the lysosome of the infected macrophage was also impaired. Additionally, the ARPC4 subunit was shown to interact with Rv1626, an essential secretory mycobacterial protein. Real-time PCR analysis showed that upon expression of ARPC4 in mycobacteria, Rv1626 expression is downregulated as much as six-fold. Rv1626 was found to also interact with mammalian cytoskeleton protein, Arp2/3, and enhance the rate of actin polymerization.

CONCLUSIONS/SIGNIFICANCE

With crystal structures for Rv1626 and ARPC4 subunit already known, our finding lays out the effect of a novel molecule on mycobacteria, and represents a viable starting point for developing potent peptidomimetics.

Deciphering the proteome of the in vivo diagnostic reagent "purified protein derivative" from Mycobacterium tuberculosis

Purified protein derivative (PPD) has served as a safe and effective diagnostic reagent for 60 years and is the only broadly available material to diagnose latent tuberculosis infections. This reagent is also used as a standard control for a number of in vitro immunological assays. Nevertheless, the molecular composition and specific products that contribute to the extraordinary immunological reactivity of PPD are poorly defined. Here, a proteomic approach was applied to elucidate the gene products in the U.S. Food and Drug Administration (FDA) standard PPD-S2. Many known Mycobacterium tuberculosis T-cell antigens were detected. Of significance, four heat shock proteins (HSPs) (GroES, GroEL2, HspX, and DnaK) dominated the composition of PPD. The chaperone activities and capacity of these proteins to influence immunological responses may explain the exquisite solubility and immunological potency of PPD. Spectral counting analysis of three separate PPD reagents revealed significant quantitative variances. Gross delayed-type hypersensitivity (DTH) responses in M. tuberculosis infected guinea pigs were comparable among these PPD preparations; however, detailed histopathology of the DTH lesions exposed unique differences, which may be explained by the variability observed in the presence and abundance of early secretory system (Esx) proteins. Variability in PPD reagents may explain differences in DTH responses reported among populations.

T-cell recognition of iron-regulated culture filtrate proteins of Mycobacterium tuberculosis in tuberculosis patients and endemic normal controls

BACKGROUND

Culture filtrate proteins (CFPs) of Mycobacterium tuberculosis are potential vaccine candidates.

OBJECTIVE

The aim was to study the influence of iron levels on CFPs and assess the immuno-protective potential of defined antigenic fractions from high (8 μg Fe/mL) and low iron (0.02 μg Fe / mL) cultures of M. tuberculosis.

MATERIALS AND METHODS

The CFPs of M. tuberculosis from high (CFP-high) and low (CFP-low) iron conditions were first compared to identify iron-regulated proteins and then fractionated to obtain ten antigen pools (CF-Ags H1- H5 and L1-L5) that were used to assess the immune response of TB patients and normal healthy controls.

RESULTS

Iron limitation resulted in the up-regulation of two novel iron-regulated low-molecular-weight proteins Irp-1 (in CF-Ag L4) and Irp-2 (in CF-Ag L5) and repression of two ESAT proteins (identified with monoclonal antibody HYB 76.8). The median stimulation indices (SIs) against most of the CF-Ags were high in pulmonary TB patients. The CF-Ags L1 and L2 showed statistically significant SI (P values of 0.0027 and 0.0029 respectively); the % case recognition was high with these antigens as well as with L4 ( P = 0.0275). IFN-γ in response to these CF-Ags was significantly high in the endemic normals; maximal expression was seen with CF-Ag L5 (median value of 233 pg mL -1 ) that was higher than the corresponding H5 (140 pg mL -1 ) and H3 and L3 (205 and 206 pg mL -1 respectively).

CONCLUSIONS

CF-Ags L5, H3 and L3 showed immuno-protective potential in this geographical location.

The two PPX-GppA homologues from Mycobacterium tuberculosis have distinct biochemical activities

Inorganic polyphosphate (poly-P), guanosine pentaphosphate (pppGpp) and guanosine tetraphosphate (ppGpp) are ubiquitous in bacteria. These molecules play a variety of important physiological roles associated with stress resistance, persistence, and virulence. In the bacterial pathogen Mycobacterium tuberculosis, the identities of the proteins responsible for the metabolism of polyphosphate and (p)ppGpp remain to be fully established. M. tuberculosis encodes two PPX-GppA homologues, Rv0496 (MTB-PPX1) and Rv1026, which share significant sequence similarity with bacterial exopolyphosphatase (PPX) and guanosine pentaphosphate 5′-phosphohydrolase (GPP) proteins. Here we delineate the respective biochemical activities of the Rv0496 and Rv1026 proteins and benchmark these against the activities of the PPX and GPP proteins from Escherichia coli. We demonstrate that Rv0496 functions as an exopolyphosphatase, showing a distinct preference for relatively short-chain poly-P substrates. In contrast, Rv1026 has no detectable exopolyphosphatase activities. Analogous to the E. coli PPX and GPP enzymes, the exopolyphosphatase activities of Rv0496 are inhibited by pppGpp and, to a lesser extent, by ppGpp alarmones, which are produced during the bacterial stringent response. However, neither Rv0496 nor Rv1026 have the ability to hydrolyze pppGpp to ppGpp; a reaction catalyzed by E. coli PPX and GPP. Both the Rv0496 and Rv1026 proteins have modest ATPase and to a lesser extent ADPase activities. pppGpp alarmones inhibit the ATPase activities of Rv1026 and, to a lesser extent, the ATPase activities of Rv0496. We conclude that PPX-GppA family proteins may not possess all the catalytic activities implied by their name and may play distinct biochemical roles involved in polyphosphate and (p)ppGpp metabolic pathways.

The role of the novel exopolyphosphatase MT0516 in Mycobacterium tuberculosis drug tolerance and persistence

Inorganic polyphosphate (poly P) has been postulated to play a regulatory role in the transition to bacterial persistence. In bacteria, poly P balance in the cell is maintained by the hydrolysis activity of the exopolyphosphatase PPX. However, the Mycobacterium tuberculosis PPX has not been characterized previously. Here we show that recombinant MT0516 hydrolyzes poly P, and an MT0516-deficient M. tuberculosis mutant exhibits elevated intracellular levels of poly P and increased expression of the genes mprB, sigE, and rel relative to the isogenic wild-type strain, indicating poly P-mediated signaling. Deficiency of MT0516 resulted in decelerated growth during logarithmic-phase in axenic cultures, and tolerance to the cell wall-active drug isoniazid. The MT0516-deficient mutant showed a significant survival defect in activated human macrophages and reduced persistence in the lungs of guinea pigs. We conclude that exopolyphosphatase is required for long-term survival of M. tuberculosis in necrotic lung lesions.

Identification of murine T-cell epitopes on low-molecular-mass secretory proteins (CFP11, CFP17, and TB18.5) of Mycobacterium tuberculosis

The low-molecular-mass secretory proteins of Mycobacterium tuberculosis have been shown to be major T-cell antigens during infection with the pathogenic bacterium. In this study, we determined murine T-cell epitopes on three low-molecular-mass proteins, CFP11 (Rv2433c), CFP17 (Rv1827), and TB18.5 (Rv0164) using DNA immunization of inbred mice. We analyzed interferon-gamma production from immune splenocytes in response to overlapping peptides covering these proteins. We identified two CD8+ T-cell epitopes on CFP11 and CFP17, one in BALB/c mice and the other in C57BL/6 mice, respectively. On TB18.5, we identified a CD8+ T-cell epitope in BALB/c mice and a CD4+ T-cell epitope in C57BL/6 mice. With the aid of computer algorithms, we could identify the minimal CD8+ T-cell epitopes. These T-cell epitopes are feasible for analysis of the role of antigen-specific T cells during M. tuberculosis infection.

Antituberculous vaccine development: a perspective for the endemic world

Several new antituberculous vaccine candidates that are effective against primary infection in preclinical animal models have now entered the early phases of clinical trials. Many of these clinical trials involve subunit vaccines, recombinant bacillus Calmette-Guérin (BCG), or improvement of BCG immunity by boosting with subunit vaccines or recombinant viral vectors expressing immunodominant TB antigens. The burning question at this stage is: will the current vaccines be effective in the endemic world where the diverse and complex challenges of TB exist? These challenges include protection of those individuals who are already vaccinated with BCG, those already exposed to environmental mycobacteria and those infected with latent TB or HIV. This review focuses on the available BCG vaccine, new TB vaccines in the pipeline and what type of vaccines are actually needed in high-burden endemic countries.

Immunoproteomic identification of human T cell antigens of Mycobacterium tuberculosis that differentiate healthy contacts from tuberculosis patients

Identification of Mycobacterium tuberculosis antigens inducing cellular immune responses is required to improve the diagnosis of and vaccine development against tuberculosis. To identify the antigens of M. tuberculosis that differentiated between tuberculosis (TB) patients and healthy contacts based on T cell reactivity, the culture filtrate of in vitro grown M. tuberculosis was fractionated by two-dimensional liquid phase electrophoresis and tested for the ability to stimulate T cells in a whole blood assay. This approach separated the culture filtrate into 350 fractions with sufficient protein quantity (at least 200 microg of protein) for mass spectrometry and immunological analyses. High levels of interferon-gamma (IFN-gamma) secretion were induced by 105 fractions in healthy contacts compared with TB patients (p < 0.05). Most interesting was the identification of 10 fractions that specifically induced strong IFN-gamma production in the healthy contact population but not in TB patients. Other immunological measurements showed 42 fractions that induced significant lymphocyte proliferative responses in the healthy contact group compared with the TB patients. The tumor necrosis factor-alpha response for most of the fractions did not significantly differ in the tested groups, and the interleukin-4 response was below the detectable range for all fractions and both study groups. Proteomic characterization of the 105 fractions that induced a significant IFN-gamma response in the healthy contacts compared with the TB patients led to the identification of 59 proteins of which 24 represented potentially novel T cell antigens. Likewise, the protein identification in the 10 healthy "contact-specific fractions" revealed 16 proteins that are key candidates as vaccine or diagnostic targets.

Cutinase-like protein-6 of Mycobacterium tuberculosis is recognised in tuberculosis patients and protects mice against pulmonary infection as a single and fusion protein vaccine

Infection with Mycobacterium tuberculosis continues to be a leading cause of death in many regions of the world, and control of this disease is hampered by the lack of a safe and effective vaccine. Secreted proteins of M. tuberculosis are an important group of antigens for subunit vaccines which target this infection. We have tested three secreted members of the cutinase-like protein (CULP) family of M. tuberculosis for their potential as protein vaccine antigens. Culp6 elicited a strong T lymphocyte response in M. tuberculosis infected mice, and importantly, in tuberculosis (TB) patients tested. Culp1, Culp2 and Culp6 when delivered as protein vaccines to mice, induced potent IFN-gamma responses which in turn translated into a significant level of protection against aerosol M. tuberculosis infection. A Culp1-6 fusion protein provided an increased level of protection against infection compared to Culp1 or Culp6 alone. The data presented here may indicate that the cell wall-associated, putatively essential protein Culp6, shown here for the first time to be recognised in TB patients, is an attractive candidate for inclusion in future subunit vaccines.

Exopolyphosphatases PPX1 and PPX2 from Corynebacterium glutamicum

Corynebacterium glutamicum accumulates up to 300 mM of inorganic polyphosphate (PolyP) in the cytosol or in granules. The gene products of cg0488 (ppx1) and cg1115 (ppx2) were shown to be active as exopolyphosphatases (PPX), as overexpression of either gene resulted in higher exopolyphosphatase activities in crude extracts and deletion of either gene with lower activities than those of the wild-type strain. PPX1 and PPX2 from C. glutamicum share only 25% identical amino acids and belong to different protein groups, which are distinct from enterobacterial, archaeal, and yeast exopolyphosphatases. In comparison to that in the wild type, more intracellular PolyP accumulated in the Deltappx1 and Deltappx2 deletion mutations but less when either ppx1 or ppx2 was overexpressed. When C. glutamicum was shifted from phosphate-rich to phosphate-limiting conditions, a growth advantage of the deletion mutants and a growth disadvantage of the overexpression strains compared to the wild type were observed. Growth experiments, exopolyphosphatase activities, and intracellular PolyP concentrations revealed PPX2 as being a major exopolyphosphatase from C. glutamicum. PPX2(His) was purified to homogeneity and shown to be active as a monomer. The enzyme required Mg2+ or Mn2+ cations but was inhibited by millimolar concentrations of Mg2+, Mn2+, and Ca2+. PPX2 from C. glutamicum was active with short-chain polyphosphates, even accepting pyrophosphate, and was inhibited by nucleoside triphosphates.

Cutinase-like proteins of Mycobacterium tuberculosis: characterization of their variable enzymatic functions and active site identification

Discovery and characterization of novel secreted enzymes of Mycobacterium tuberculosis are important for understanding the pathogenesis of one of the most important human bacterial pathogens. The proteome of M. tuberculosis contains over 400 potentially secreted proteins, the majority of which are uncharacterized. A family of seven cutinase-like proteins (CULPs) was identified by bioinformatic analysis, expressed and purified from Escherichia coli, and characterized in terms of their enzymatic activities. These studies revealed a functional diversity of enzyme classes based on differential preferences for substrate chain length. One member, Culp1, exhibited strong esterase activity, 40-fold higher than that of Culp6, which had strong activity as a lipase. Another, Culp4, performed moderately as an esterase and weakly as a lipase. Culp6 lipase activity was optimal above pH 7.0, and fully maintained to pH 8.5. None of the CULP members exhibited cutinase activity. Site-directed mutagenesis of each residue of the putative catalytic triad in Culp6 confirmed that each was essential for activity toward all fatty acid chain lengths of nitrophenyl esters and lipolytic function. Culp1 and Culp2 were present only in culture supernatants of M. tuberculosis, while Culp6, which is putatively essential for mycobacterial growth, was retained in the cell wall, suggesting the proteins play distinct roles in mycobacterial biology.

Programmed death (PD)-1:PD-ligand 1/PD-ligand 2 pathway inhibits T cell effector functions during human tuberculosis

Protective immunity against Mycobacterium tuberculosis requires the generation of cell-mediated immunity. We investigated the expression and role of programmed death 1 (PD-1) and its ligands, molecules known to modulate T cell activation, in the regulation of IFN-gamma production and lytic degranulation during human tuberculosis. We demonstrated that specific Ag-stimulation increased CD3+PD-1+ lymphocytes in peripheral blood and pleural fluid from tuberculosis patients in direct correlation with IFN-gamma production from these individuals. Moreover, M. tuberculosis-induced IFN-gamma participated in the up-regulation of PD-1 expression. Blockage of PD-1 or PD-1 and its ligands (PD-Ls: PD-L1, PD-L2) enhanced the specific degranulation of CD8+ T cells and the percentage of specific IFN-gamma-producing lymphocytes against the pathogen, demonstrating that the PD-1:PD-Ls pathway inhibits T cell effector functions during active M. tuberculosis infection. Furthermore, the simultaneous blockage of the inhibitory receptor PD-1 together with the activation of the costimulatory protein signaling lymphocytic activation molecule led to the promotion of protective IFN-gamma responses to M. tuberculosis, even in patients with weak cell-mediated immunity against the bacteria. Together, we demonstrated that PD-1 interferes with T cell effector functions against M. tuberculosis, suggesting that PD-1 has a key regulatory role during the immune response of the host to the pathogen.

Supplementation with RD antigens enhances the protective efficacy of BCG in tuberculous mice

Different combinations of ESAT-6, CFP-10, CFP-21, MPT-64, encoded by RD1 and RD2 of Mycobacterium tuberculosis were evaluated on the basis of antigenicity in PPD positive TB contacts and immunogenicity in C57BL/6J mice immunized with the combination of all four RD antigens. The peripheral blood mononuclear cells of TB contacts showed maximum recognition in response to the combination of ESAT-6+MPT-64 in terms of predominant lymphoproliferation, IFN-gamma levels and the number of responders. On the contrary, the combination of ESAT-6+CFP-21+MPT-64 was found to be most immunogenic based on both T-cell and antibody responses in immunized mice. Prophylactic potential of the selected combinations was assessed as supplementation vaccines to BCG against intravenous challenge with M. tuberculosis in mice. BCG supplementation with the selected combinations resulted in significantly greater protection as compared to BCG alone against experimental tuberculosis and thus appears to be a promising approach to enhance the protective efficacy of the existing vaccine.

Differential cytokine levels and immunoreactivities against Mycobacterium tuberculosis antigens between tuberculous and malignant effusions

Much effort has been devoted to the identification of immunologically important factors in tuberculous pleurisy (TBP) and malignant pleurisy (MP) to improve the differential diagnosis of the two major causes of lymphocyte-dominant pleurisy. This study evaluated the immunoreactivity and potential diagnostic utility of both host (cytokines and chemokines) and pathogen (mycobacterial proteins) factors in pleural effusions. Effusion samples were collected from 41 patients with MP caused by lung cancer and from 81 patients with TBP. The concentrations of nine cytokines and chemokines (interleukin (IL)-12 p40, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, IL-6, IL-10, CXCL8/IL-8, CXCL10/IP-10, CCL3/MIP-1alpha, and CCL4/MIP-1beta) and antibody responses (IgG, IgM, and IgA) against five Mycobacterium tuberculosis antigens (early secreted antigenic target (ESAT)-6, 30-kDa, MTB12, 38-kDa, and a heparin-binding hemagglutinin (HBHA)) were determined in pleural fluids using enzyme-linked immunosorbent assays (ELISA). In the logistic regression, IFN-gamma (odds ratio, 7.178; 95% confidence interval (CI), 2.258-22.817; p=0.001), IL-12 p40 (odds ratio, 11.037; 95% CI, 3.38-36.037; p<0.001), and IL-6 (odds ratio, 3.295; 95% CI, 1.147-9.463; p=0.027) were found to be statistically significant cytokines predicting tuberculous from malignant effusions. Although IgG responses to all of the M. tuberculosis antigens tested were significantly higher in effusions from TBP (p<0.001) compared with those from MP, the logistic regression showed IgG levels for ESAT-6 and MTB12 to be statistically significant for differentiation of TBP from MP. HBHA showed the highest sensitivity of IgM antibody responses in TBP in comparison with other antigens. These data indicate that selected mycobacterial antigens (ESAT-6 and MTB12) and cytokine markers (IFN-gamma, IL-12p40, and IL-6) provide useful information for differentiating tuberculous and malignant effusions in clinical practice.

Mycobacterium tuberculosis-induced gamma interferon production by natural killer cells requires cross talk with antigen-presenting cells involving Toll-like receptors 2 and 4 and the mannose receptor in tuberculous pleurisy

Tuberculous pleurisy allows the study of human cells at the site of active Mycobacterium tuberculosis infection. In this study, we found that among pleural fluid (PF) lymphocytes, natural killer (NK) cells are a major source of early gamma interferon (IFN-gamma) upon M. tuberculosis stimulation, leading us to investigate the mechanisms and molecules involved in this process. We show that the whole bacterium is the best inducer of IFN-gamma, although a high-molecular-weight fraction of culture filtrate proteins from M. tuberculosis H37Rv and the whole-cell lysate also induce its expression. The mannose receptor seems to mediate the inhibitory effect of mannosylated lipoarabinomannan, and Toll-like receptor 2 and 4 agonists activate NK cells but do not induce IFN-gamma like M. tuberculosis does. Antigen-presenting cells (APC) and NK cells bind M. tuberculosis, and although interleukin-12 is required, it is not sufficient to induce IFN-gamma expression, indicating that NK cell-APC contact takes place. Indeed, major histocompatibility complex class I, adhesion, and costimulatory molecules as well as NK receptors regulate IFN-gamma induction. The signaling pathway is partially inhibited by dexamethasone and sensitive to Ca2+ flux and cyclosporine. Inhibition of p38 and extracellular-regulated kinase mitogen-activated protein kinase pathways reduces the number of IFN-gamma+ NK cells. Phosphorylated p38 (p-p38) is detected in ex vivo PF-NK cells, and M. tuberculosis triggers p-p38 in PF-NK cells at the same time that binding between NK and M. tuberculosis reaches its maximum value. Thus, interplay between M. tuberculosis and NK cells/APC triggering IFN-gamma would be expected to play a beneficial role in tuberculous pleurisy by helping to maintain a type 1 profile.

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.

Tuberculosis subunit vaccine design: the conflict of antigenicity and immunogenicity

The attempts to find an effective antituberculous subunit vaccine are based on the assumption that it must drive a Th1 response. In the absence of effective correlates of protection, a vast array of mycobacterial components are being evaluated worldwide either on the basis of their ability to be recognized by T lymphocytes in in vitro assays during early stage of animal or human infection (antigenicity) or their capacity to induce T cell response following immunization in animal models (immunogenicity). The putative vaccine candidates selected using either of these strategies are then subjected to challenge studies in different animal models to evaluate the protective efficacy. Here we review the outcome of this current scheme of selection of vaccine candidates using an 'antigenicity' or 'immunogenicity' criterion on the actual protective efficacy observed in experimental animal models. The possible implications for the success of some of the leading vaccine candidates in clinical trials will also be discussed.