Immune response to Mycobacterium tuberculosis specific antigen ESAT-6 among south Indians

A new poly(I:C)-decorated PLGA-PEG nanoparticle promotes Mycobacterium tuberculosis fusion protein to induce comprehensive immune responses in mice intranasally

Protein-based subunit vaccine against tuberculosis (TB) is regarded as safer but with lower immunogenicity. To investigate effective adjuvant to improve the immunogenicity of TB subunit vaccine, we modified ploy(I:C) onto PLGA-PEG copolymer nanoparticle with polydopamine to produce a new nanoparticle adjuvant named "PLGA-PEG-poly(I:C)" (NP). M. tuberculosis fusion proteins Mtb10.4-HspX and ESAT-6-Rv2626c (M4) were encapsulated in the nanoparticles to produce the NP/M4 subunit vaccine. The PLGA-PEG/M4 nanoparticle was 200.21 ± 1.07 nm in diameter, and the polydispersity index (PDI) was 0.127 ± 0.02. Following modification with poly(I:C) by polydopamine, the NP/M4 was administered to C57BL/6 female mice intranasally and the immune responses were evaluated. The NP/M4 significantly induced antigen-specific CD4⁺ T cells proliferation, IL-2 and IFN-γ production. In addition, the NP/M4 could promote the production of antigen-specific IgG, IgG1, IgG2c in serum, and sIgA in lung washings. Overall, our results indicated that the NP would be a potential TB subunit vaccine adjuvant with the ability to induce strong Th1-type cell-mediated immunity and humoral immune responses.

Improvement of the immunogenicity of ESAT-6 via fusion with the dodecameric protein dodecin of Mycobacterium tuberculosis

Tuberculosis (TB) is a chronic infectious disease that creates a heavy medical burden worldwide. The only approved vaccine, Bacillus Calmette-Guérin (BCG), cannot fully protect adolescents and adults from TB. Therefore, there is an urgent need to develop an effective new vaccine. Previous studies have found that dodecin, a flavin-binding protein of Mycobacterium tuberculosis (Mtb), can form stable dodecamers and has the potential to improve the immunogenicity of Mtb antigens. In this study, we constructed the fusion protein dodecin-ESAT-6 and evaluated the immunogenicity of dodecin, ESAT-6, and dodecin-ESAT-6 separately. Our results showed that dodecin-ESAT-6 is a dodecameric protein that can withstand heat at 95 °C and under SDS-PAGE conditions. Dodecin-ESAT-6 increased the expression of the costimulatory molecules CD80, CD86, and major histocompatibility complex class II (MHC-II) on the surface of RAW264.7 macrophages. Mice immunized with dodecin-ESAT-6 exhibited higher percentages of antigen-specific CD4⁺ and CD8⁺ T lymphocytes, higher levels of spleen lymphocyte proliferation and IFN-γ and IL-2 secretion, and a lower level of IL-4 secretion than those immunized with ESAT-6. The IgG, IgG1, and IgG2a titers of the dodecin-ESAT-6 group were significantly higher than those of the ESAT-6 group. Dodecin-ESAT-6 elicited a high IgG2a/IgG1 ratio and tended to produce a predominantly Th1-like response. These results support the conclusion that the dodecin-ESAT-6 dodecameric protein induced strong Th1 immune responses and improved the immunogenicity of ESAT-6, which provides a new strategy for TB vaccine development.

Chloroplast-based inducible expression of ESAT-6 antigen for development of a plant-based vaccine against tuberculosis

Mycobacterium tuberculosis causes tuberculosis in humans. The major disease burden of tuberculosis lies in developing countries. Lack of an effective vaccine for adults is one of the major hurdles for controlling this deadly disease. In the present study, 6 kDa early secretory antigenic target (ESAT-6) of M. tuberculosis was inducibly expressed in chloroplasts of Nicotiana tabacum. The expression of ESAT-6 in chloroplasts was controlled by T7 promoter that was activated by nuclear-generated signal peptide. Tobacco plants, containing nuclear component, were transformed via biolistic bombardment with pEXP-T7-ESAT-6 obtained by Gateway® cloning. Transformation and homoplasmic status of transplastomic plants was confirmed by polymerase chain reaction and Southern blotting. Plants were induced for protein expression by spraying with 5% ethanol for 1 day, 3 days, 7 days and 10 days. ESAT-6 protein was detected by immunoblot analysis and maximum protein was obtained for 10 days induced plants that was estimated to accumulate up to 1.2% of total soluble fraction of protein. Transplastomic plants showed completely normal morphology. Transplastomic and untransformed plants became slightly chlorotic upon prolonged exposure to ethanol until 10 days. Taken together, this data could help in the development of an antigen-based subunit vaccine against tuberculosis.

Differential positive TSPOT assay responses to ESAT-6 and CFP-10 in health care workers

BACKGROUND

The TSPOT.TB (TSPOT) diagnostic test for latent tuberculosis infection is based on a cell-mediated response to the Mycobacteria tuberculosis antigens, ESAT-6 and/or CFP-10, producing an "interferon-gamma footprint". We investigated the within-sample and within-subject variability of positive TSPOT assays due to the individual assay antigens' reactivity.

METHODS

Positive TSPOT assay frequencies due to ESAT-6 or CFP-10 among health care workers (HCWs) at 6-month intervals for 18 months were compared. Differences in result interpretation (positive or negative) for ESAT-6 and CFP10 and potential prognostic factors were investigated.

RESULTS

There were 576 positive results in 8805 TSPOT assays representing 2418 participants. A significant difference was detected in positive TSPOT results due to a positive response to either ESAT-6, CFP-10 or both antigens at baseline through 12 M (p < 0.001), but not for the 18 M follow-up. Gender, ethnicity, occupation, previous positive tuberculin skin test (TST) and study site were significantly associated with specific antigen positivity.

CONCLUSIONS

Among our HCW samples with positive TSPOT assays, CFP-10 induced a larger proportion of positive TSPOT results than ESAT-6. Potential causes for this finding include: BCG vaccinated subpopulations, certain jobs, history of positive TST, U.S. birth, and study site. A high proportion of single-positive specimens may reflect false-positives results.

Proteomics Analysis of Three Different Strains of Mycobacterium tuberculosis under In vitro Hypoxia and Evaluation of Hypoxia Associated Antigen's Specific Memory T Cells in Healthy Household Contacts

In vitro mimicking conditions are thought to reflect the environment experienced by Mycobacterium tuberculosis inside the host granuloma. The majority of in vitro dormancy experimental models use laboratory-adapted strains H37Rv or Erdman instead of prevalent clinical strains involved during disease outbreaks. Thus, we included the most prevalent clinical strains (S7 and S10) of M. tuberculosis from south India in addition to H37Rv for our in vitro oxygen depletion (hypoxia) experimental model. Cytosolic proteins were prepared from hypoxic cultures, resolved by two-dimensional electrophoresis and protein spots were characterized by mass spectrometry. In total, 49 spots were characterized as over-expressed or newly emergent between the three strains. Two antigens (ESAT-6, Lpd) out of the 49 characterized spots were readily available in recombinant form in our lab. Hence, these two genes were overexpressed, purified and used for in vitro stimulation of whole blood collected from healthy household contacts (HHC) and active pulmonary tuberculosis patients (PTB). Multicolor flow cytometry analysis showed high levels of antigen specific CD4(+) central memory T cells in the circulation of HHC compared to PTB (p < 0.005 for ESAT-6 and p < 0.0005 for Lpd). This shows proteins that are predicted to be up regulated during in vitro hypoxia in most prevalent clinical strains would indicate possible potential immunogens. In vitro hypoxia experiments with most prevalent clinical strains would also elucidate the probable true representative antigens involved in adaptive mechanisms.

T cell recall response of two hypothetical proteins (Rv2251 and Rv2721c) from Mycobacterium tuberculosis in healthy household contacts of TB - Possible subunit vaccine candidates

The demonstrated variable efficacy of the only licensed TB vaccine Mycobacterium bovis bacillus Calmette-Guérin (M. bovis BCG) encourages the need for new vaccine candidates against TB. Antigen specific cellular immune response is often considered imperative during Mycobacterium tuberculosis (M. tuberculosis) infection and antigens that are strongly associated with the latent phase of infection are drawing increasing attention for anti-TB vaccine development. Here, we investigated the phenotypic and functional profiles of two novel mycobacterial antigens Rv2251 and Rv2721c during T cell recall response via multi-color flow cytometry. Healthy household contacts of TB (latent/HHC) and active pulmonary TB (PTB) patients were recruited to investigate the difference in antigen specific T cell recall response. These two antigens induced expansion of CD45RA^- CCR7⁺ central memory subtypes and CD45RA^- CCR7^- effector memory cells in latent population which suggests their possible association with HHC. Rv2251 and Rv2721c antigen specific IFN-γ, TNF-α and IL-2 response was also significantly high in HHC when compared to the PTB (p < 0.005, p < 0.05 and p < 0.05 respectively). The frequency of multifunctional T cells also was high in HHC compared to the PTB with statistical significance only for the antigen Rv2251. Often, the dominant Th1 immune response in HHC is correlated with the protection against the active TB disease. Collectively, we report the first insights into Rv2251 and Rv2721c antigen specific immune response in human donors of TB and provide the immunologic rationale for selecting them for vaccine development against TB.

PpiA antigen specific immune response is a potential biomarker for latent tuberculosis infection

One third of the world's population is estimated to harbour latent tuberculosis infection (LTBI). Around 10% of them have the life time risk of developing active tuberculosis (PTB). Currently there is no gold standard test for identifying LTBI. Therefore identification of specific markers for LTBI will help as to develop a test specific for LTBI. Earlier, in our immunoproteomic analysis, we found that peptidyl-prolyl cis-trans isomerase A (PpiA) protein-containing fractions induced significantly higher interferon-gamma (IFN-γ) response in LTBI than in PTB. Immunological characterisation of recombinant PpiA protein was carried out in the current study. We have studied 10 cytokines and 2 chemokine responses against PpiA and standard antigens such as early secretory antigenic target-6 (ESAT-6) and culture filtrate antigen-10 (CFP-10). In healthy household contacts (HHC), all the tested antigens induced significantly higher levels of IFN-γ and Interlukin-8 (IL-8) compared with those in PTB. PpiA-specific IL-12p40 response was significantly increased in HHC compared with that in PTB. PpiA antigen-specific IFN-γ and IL-12p40 both showed 86% positivity in HHC, whereas in PTB, they showed 20% and 38% positivity, respectively. In terms of IFN-γ/TNF-α ratio, PpiA displayed 86% (30/35) positivity in HHC and 18% (7/39) positivity in PTB. In summary we found that PpiA-specific IFN-γ and IFN-γ/TNF-α ratio response were specific biomarkers for LTBI identification.

Protective CD4 T cells targeting cryptic epitopes of Mycobacterium tuberculosis resist infection-driven terminal differentiation

CD4 T cells are crucial to the control of Mycobacterium tuberculosis infection and are a key component of current vaccine strategies. Conversely, immune-mediated pathology drives disease, and recent evidence suggests that adaptive and innate responses are evolutionarily beneficial to M. tuberculosis. We compare the functionality of CD4 T cell responses mounted against dominant and cryptic epitopes of the M. tuberculosis 6-kDa early secreted Ag (ESAT-6) before and postinfection. Protective T cells against cryptic epitopes not targeted during natural infection were induced by vaccinating mice with a truncated ESAT-6 protein, lacking the dominant epitope. The ability to generate T cells that recognize multiple cryptic epitopes was MHC-haplotype dependent, including increased potential via heterologous MHC class II dimers. Before infection, cryptic epitope-specific T cells displayed enhanced proliferative capacity and delayed cytokine kinetics. After aerosol M. tuberculosis challenge, vaccine-elicited CD4 T cells expanded and recruited to the lung. In chronic infection, dominant epitope-specific T cells developed a terminal differentiated KLRG1(+)/PD-1(lo) surface phenotype that was significantly reduced in the cryptic epitope-specific T cell populations. Dominant epitope-specific T cells in vaccinated animals developed into IFN-γ- and IFN-γ,TNF-α-coproducing effector cells, characteristic of the endogenous response. In contrast, cryptic epitope-specific CD4 T cells maintained significantly greater IFN-γ(+)TNF-α(+)IL-2(+) and TNF-α(+)IL-2(+) memory-associated polyfunctionality and enhanced proliferative capacity. Vaccine-associated IL-17A production by cryptic CD4 T cells was also enhanced, but without increased neutrophilia/pathology. Direct comparison of dominant/cryptic epitope-specific CD4 T cells within covaccinated mice confirmed the superior ability of protective cryptic epitope-specific T cells to resist M. tuberculosis infection-driven T cell differentiation.

Low cost tuberculosis vaccine antigens in capsules: expression in chloroplasts, bio-encapsulation, stability and functional evaluation in vitro

Tuberculosis (TB) caused by Mycobacterium tuberculosis is one of the leading fatal infectious diseases. The development of TB vaccines has been recognized as a major public health priority by the World Health Organization. In this study, three candidate antigens, ESAT-6 (6 kDa early secretory antigenic target) and Mtb72F (a fusion polyprotein from two TB antigens, Mtb32 and Mtb39) fused with cholera toxin B-subunit (CTB) and LipY (a cell wall protein) were expressed in tobacco and/or lettuce chloroplasts to facilitate bioencapsulation/oral delivery. Site-specific transgene integration into the chloroplast genome was confirmed by Southern blot analysis. In transplastomic leaves, CTB fusion proteins existed in soluble monomeric or multimeric forms of expected sizes and their expression levels varied depending upon the developmental stage and time of leaf harvest, with the highest-level of accumulation in mature leaves harvested at 6PM. The CTB-ESAT6 and CTB-Mtb72F expression levels reached up to 7.5% and 1.2% of total soluble protein respectively in mature tobacco leaves. Transplastomic CTB-ESAT6 lettuce plants accumulated up to 0.75% of total leaf protein. Western blot analysis of lyophilized lettuce leaves stored at room temperature for up to six months showed that the CTB-ESAT6 fusion protein was stable and preserved proper folding, disulfide bonds and assembly into pentamers for prolonged periods. Also, antigen concentration per gram of leaf tissue was increased 22 fold after lyophilization. Hemolysis assay with purified CTB-ESAT6 protein showed partial hemolysis of red blood cells and confirmed functionality of the ESAT-6 antigen. GM1-binding assay demonstrated that the CTB-ESAT6 fusion protein formed pentamers to bind with the GM1-ganglioside receptor. The expression of functional Mycobacterium tuberculosis antigens in transplastomic plants should facilitate development of a cost-effective and orally deliverable TB booster vaccine with potential for long-term storage at room temperature. To our knowledge, this is the first report of expression of TB vaccine antigens in chloroplasts.

Dissecting mechanisms of immunodominance to the common tuberculosis antigens ESAT-6, CFP10, Rv2031c (hspX), Rv2654c (TB7.7), and Rv1038c (EsxJ)

Diagnosis of tuberculosis often relies on the ex vivo IFN-γ release assays QuantiFERON-TB Gold In-Tube and T-SPOT.TB. However, understanding of the immunological mechanisms underlying their diagnostic use is still incomplete. Accordingly, we investigated T cell responses for the TB Ags included in the these assays and other commonly studied Ags: early secreted antigenic target 6 kDa, culture filtrate protein 10 kDa, Rv2031c, Rv2654c, and Rv1038c. PBMC from latently infected individuals were tested in ex vivo ELISPOT assays with overlapping peptides spanning the entirety of these Ags. We found striking variations in prevalence and magnitude of ex vivo reactivity, with culture filtrate protein 10 kDa being most dominant, followed by early secreted antigenic target 6 kDa and Rv2654c being virtually inactive. Rv2031c and Rv1038c were associated with intermediate patterns of reactivity. Further studies showed that low reactivity was not due to lack of HLA binding peptides, and high reactivity was associated with recognition of a few discrete dominant antigenic regions. Different donors recognized the same core sequence in a given epitope. In some cases, the identified epitopes were restricted by a single specific common HLA molecule (selective restriction), whereas in other cases, promiscuous restriction of the same epitope by multiple HLA molecules was apparent. Definition of the specific restricting HLA allowed to produce tetrameric reagents and showed that epitope-specific T cells recognizing either selectively or promiscuously restricted epitopes were predominantly T effector memory. In conclusion, these results highlight the feasibility of more clearly defined TB diagnostic reagent.

In silico identification of potential antigenic proteins and promiscuous CTL epitopes in Mycobacterium tuberculosis

Cell-mediated immunity is critical for the control of Mycobacterium tuberculosis infection. We hypothesized that those proteins of M. tuberculosis (MTB) that do not have homologs in humans as well as human gut flora, would mount a good antigenic response in man, and employed a bioinformatics approach to identify MTB antigens capable of inducing a robust cell-mediated immune response in humans. In the first step we identified 624 MTB proteins that had no homologs in humans. Comparison of this set of proteins with the proteome of 77 different microbes that comprise the human gut flora narrowed down the list to 180 proteins unique to MTB. Twenty nine of the 180 proteins are known to be associated with dormancy. Since dormancy associated proteins are known to harbor CTL epitopes, we selected four representative unique proteins and subjected them to epitope analysis using ProPred1. Nineteen novel promiscuous epitopes were identified in the four proteins. Population coverage for 7 of the 19 shortlisted epitopes including Rv3852 (58-KPAEAPVSL, 112-VPLIVAVTL, 118-VTLSLLALL and 123-LALLLIRQL), Rv2706c (66-RPLSGVSFL) Rv3466 (8- RIVEVFDAL and 38-RSLERLECL) was >74%. These novel promiscuous epitopes are conserved in other virulent MTB strains, and can therefore be further investigated for their immunological relevance and usefulness as vaccine candidates.

Cytotoxicity responses to selected ESAT-6 and CFP-10 peptides in tuberculosis

Cytotoxicity responses were studied for the ESAT-6 peptides Esp1, Esp6, Esp7, Esp8, and CFP-10 peptides, Cfp6, Cfp7, Cfp8, Cfp9 (synthetic 20-mer peptides) and the recombinant ESAT-6, CFP-10 proteins. Cytolytic molecules perforin, granzymes A and B, granulysin responses in healthy household contacts (HHC) and pulmonary tuberculosis patients (PTB), were studied by intracellular flow cytometry. Functional cytotoxicity was studied in both the groups for the peptides Esp6 and Cfp8 by an enzyme (lactate dehydrogenase) based assay. The results revealed that cytolytic molecule positive CD4+ and CD8+ T cells were increased in HHC in response to Esp1, Esp6, Cfp8 and Cfp9 immunogenic peptides compared to PTB. Functional cytotoxicity results showed higher cytotoxicity (not statistically significant) to be exhibited by the peptide Esp6 than Cfp8 in the HHC.