T cell responses to DosR and Rpf proteins in actively and latently infected individuals from Colombia

Role of MHC class I pathways in Mycobacterium tuberculosis antigen presentation

MHC class I antigen processing is an underappreciated area of nonviral host–pathogen interactions, bridging both immunology and cell biology, where the pathogen’s natural life cycle involves little presence in the cytoplasm. The effective response to MHC-I foreign antigen presentation is not only cell death but also phenotypic changes in other cells and stimulation of the memory cells ready for the next antigen reoccurrence. This review looks at the MHC-I antigen processing pathway and potential alternative sources of the antigens, focusing on Mycobacterium tuberculosis (Mtb) as an intracellular pathogen that co-evolved with humans and developed an array of decoy strategies to survive in a hostile environment by manipulating host immunity to its own advantage. As that happens via the selective antigen presentation process, reinforcement of the effective antigen recognition on MHC-I molecules may stimulate subsets of effector cells that act earlier and more locally. Vaccines against tuberculosis (TB) could potentially eliminate this disease, yet their development has been slow, and success is limited in the context of this global disease’s spread. This review’s conclusions set out potential directions for MHC-I-focused approaches for the next generation of vaccines.

Preventive effects of Mycobacterium tuberculosis DNA vaccines on the mouse model with latent tuberculosis infection

Background

About a quarter of the world's population with latent tuberculosis infection (LTBI) are the main source of active tuberculosis. Bacillus Calmette Guerin (BCG) cannot effectively control LTBI individuals from developing diseases. Latency-related antigens can induce T lymphocytes of LTBI individuals to produce higher IFN-γ levels than tuberculosis patients and normal subjects. Herein, we firstly compared the effects of M. tuberculosis (MTB) ag85ab and 7 latent DNA vaccines on clearing latent MTB and preventing its activation in the mouse LTBI model.

Methods

A mouse LTBI model was established, and then immunized respectively with PBS, pVAX1 vector, Vaccae vaccine, ag85ab DNA and 7 kinds of latent DNAs (including rv1733c, rv2660c, rv1813c, rv2029c, rv2628, rv2659c and rv3407) for three times. The mice with LTBI were injected with hydroprednisone to activate the latent MTB. Then, the mice were sacrificed for the bacterial count, histopathological examination, and immunological evaluation.

Results

Using chemotherapy made the MTB latent in the infected mice, and then using hormone treatment reactivated the latent MTB, indicating that the mouse LTBI model was successfully established. After the mouse LTBI model was immunized with the vaccines, the lung colony-forming units (CFUs) and lesion degree of mice in all vaccines group were significantly decreased than those in the PBS group and vector group (P<0.0001, P<0.05). These vaccines could induce antigen-specific cellular immune responses. The number of IFN-γ effector T cells spots secreted by spleen lymphocytes in the ag85ab DNA group was significantly increased than those in the control groups (P<0.05). In the splenocyte culture supernatant, IFN-γ and IL-2 levels in the ag85ab, rv2029c, and rv2659c DNA groups significantly increased (P<0.05), and IL-17A levels in ag85ab and rv2659c DNA groups also significantly increased (P<0.05). Compared with the PBS and vector groups, the proportion of CD4⁺CD25⁺FOXP3⁺ regulatory T cells in spleen lymphocytes of ag85ab, rv2660c, rv2029c, and rv3407 DNA groups were significantly reduced (P<0.05).

Conclusions

MTB ag85ab and 7 kinds of latent DNA vaccines showed immune preventive efficacies on a mouse model of LTBI, especially the rv2659c, and rv1733c DNA. Our findings will provide candidates for the development of new multi-stage vaccines against TB.

B21 DNA vaccine expressing ag85b, rv2029c, and rv1738 confers a robust therapeutic effect against latent Mycobacterium tuberculosis infection

Latent tuberculosis infection (LTBI) treatment is known to accelerate the decline in TB incidence, especially in high-risk populations. Mycobacterium tuberculosis (M. tb) expression profiles differ at different growth periods, and vaccines protective and therapeutic effects may increase when they include antigenic compositions from different periods. To develop a post-exposure vaccine that targets LTBI, we constructed four therapeutic DNA vaccines (A39, B37, B31, and B21) using different combinations of antigens from the proliferation phase (Ag85A, Ag85B), PE/PPE family (Rv3425), and latent phase (Rv2029c, Rv1813c, Rv1738). We compared the immunogenicity of the four DNA vaccines in C57BL/6j mice. The B21 vaccine stimulated the strongest cellular immune responses, namely Th1/Th17 and CD8⁺ cytotoxic T lymphocyte responses. It also induced the generation of strengthened effector memory and central memory T cells. In latently infected mice, the B21 vaccine significantly reduced bacterial loads in the spleens and lungs and decreased lung pathology. In conclusion, the B21 DNA vaccine can enhance T cell responses and control the reactivation of LTBI.

Prediction of Th1 and Cytotoxic T Lymphocyte Epitopes of Mycobacterium tuberculosis and Evaluation of Their Potential in the Diagnosis of Tuberculosis in a Mouse Model and in Humans

Latent tuberculosis infection (LTBI) is the primary source of tuberculosis (TB) but there is no suitable detection method to distinguish LTBI from active tuberculosis (ATB). In this study, five antigens of Mycobacterium tuberculosis belonging to LTBI and regions of difference (RDs) were selected to predict Th1 and cytotoxic T lymphocyte (CTL) epitopes. The immunodominant Th1 and CTL peptides were identified in mouse models, and their performance in distinguishing LTBI from ATB was determined in mice and humans. Ten Th1 and ten CTL immunodominant peptides were predicted and synthesized in vitro. The enzyme-linked immunosorbent spot assay results showed that the combination of five Th1 peptides (area under the curve [AUC] = 1, P < 0.0001; sensitivity = 100% and specificity = 93.33%), the combination of seven CTL peptides (AUC = 1, P < 0.0001; 100 and 95.24%), and the combination of four peptide pools (AUC = 1, P < 0.0001; sensitivity = 100% and specificity = 91.67%) could significantly discriminate mice with LTBI from mice with ATB or uninfected controls (UCs). The combined peptides or peptide pools induced significantly different cytokine levels between the three groups, improving their ability to differentiate ATB from LTBI. Furthermore, it was found that pool 2 could distinguish patients with ATB from UCs (AUC = 0.6728, P = 0.0041; sensitivity = 72.58% and specificity = 59.46%). The combination of Th1 and CTL immunodominant peptides derived from LTBI-RD antigens might be a promising strategy for diagnosing ATB and LTBI in mice and patients with ATB and uninfected controls. IMPORTANCE Latent tuberculosis infection (LTBI) is a challenging problem in preventing, diagnosing, and treating tuberculosis (TB). The innate and adaptive immune responses are essential for eliminating or killing the mycobacteria. Antigen-presenting cells (APCs) present and display mycobacterium peptides on their surfaces, and recognition between T cells and APCs is based on some essential peptides rather than the full-length protein. Therefore, the selection of candidate antigens and the prediction and screening of potential immunodominant peptides have become a key to designing a new generation of TB diagnostic biomarkers. This study is the first to report that the combination of Th1 and CTL immunodominant peptides derived from LTBI-RD antigens can distinguish LTBI from active TB (ATB) in animals and ATB patients from uninfected individuals. These findings provide a novel insight for discovering potential biomarkers for the differential diagnosis of ATB and LTBI in the future.

Use of DosR and Rpf antigens from Mycobacterium tuberculosis to screen for latent and relapse tuberculosis infection in a tuberculosis endemic community of Huainan City

The dormancy survival regulator (DosR) antigens upgraded during latency and resuscitation-promoting factors (Rpfs) expressed over the reactivation from dormant Mycobacterium tuberculosis (M. tuberculosis) could be used to diagnose tuberculosis (TB) at different stages. We performed a retrospective cohort study based on four groups, including healthy controls (HCs), active tuberculosis infections (ATBs), latent tuberculosis infections (LTBIs), and relapse tuberculosis infections (RTBs) enrolled between November 2020 and June 2021. Compared to the fusion protein E6-C10, combined with early secreted antigenic target 6 kDa (ESAT-6) and culture filtrate of 10 kDa (CFP-10), the DosR- or Rpf-encoded antigens could not elicit significant IFN-γ concentration for the diagnosis of ATB. Of note, the DosR antigens produce significantly more antigen-specific IFN-γ in LTBIs than Rpfs, and the levels of antigen-specific IFN-γ elicited in RTBs stimulated by Rpfs were higher than the DosR antigens. Among the DosR antigens, Rv2003c was the most immunogenic in diagnosing LTBIs, followed by Rv2007c and Rv2005c. As far as Rpfs are concerned, Rv0867c was the best antigen to identify RTBs, followed by Rv2389c and Rv1009. Both Rv2450c and Rv1884c showed relatively limited IFN-γ concentration in RTBs. Besides, the selected DosR antigens and Rpfs showed ideal specificity and inadequate sensitivity, which could have been enhanced by the fusion antigens prepared by the DosR antigens or Rpfs, respectively. The results of this study can provide more accurate detection methods for LTBIs and RTBs and could be used for screening the dormant M. tuberculosis throughout reactivation.

Immunogenic profiling of Mycobacterium tuberculosis DosR protein Rv0569 reveals its ability to switch on Th1 based immunity

Mycobacterium tuberculosis (M.tb) is a multifaceted bacterial pathogen known to infect more than 2 billion people globally. However, a majority of the individuals (>90%) show no overt clinical symptoms of active Tuberculosis (TB) and, it is reported that M.tb in these individuals resides in the latent form. Therefore, huge burden of latently infected population poses serious threat to human health. Inconsistent efficacy of BCG vaccine and poor understanding of latency-associated determinants contribute to the failure of combating M.tb. The discovery of DosR as the master regulator of dormancy, opened new avenues to understand the pathophysiology of the bacterium. Though the specific functions of various DosR genes are yet to be discovered, they have been reported as potent T-cell activators and could elicit strong protective immune responses. Rv0569 is a DosR-encoded conserved hypothetical protein overexpressed during dormancy. However, it is not clearly understood how this protein modulates the host immune response. In the present study, we have demonstrated that Rv0569 has a high antigenic index and induces enhanced secretion of Th1 cytokines IL-12p40 and TNF-α as compared to Th2 cytokine IL-10 in macrophages. Mechanistically, Rv0569 induced the transcription of these pro-inflammatory signatures through the activation of NF-κB pathway. Further, immunization of mice with DosR protein Rv0569 switched the immune response towards Th1-biased cytokine pattern, characterized by the enhanced production of IFN-γ, IL-12p40, and TNF-α. Rv0569 augmented the expansion of antigen-specific IFN-γ and IL-2 producing effector CD4⁺ and CD8⁺ T-cells which are hallmarks of Th1 biased protective immunity. Additionally, IgG2a/IgG1 and IgG2b/IgG1 ratio in the serum of immunized mice further confirmed the ability of Rv0569 to skew Th1 biased immune response. In conclusion, we emphasize that Rv0569 has the ability to generate signals to switch on Th1-dominated responses and further suggest that it could be a potential vaccine candidate against latent M.tb infection.

Mycobacterium tuberculosis latency-associated antigen Rv1733c SLP improves the accuracy of differential diagnosis of active tuberculosis and latent tuberculosis infection

BACKGROUND

Differential diagnosis of active tuberculosis (ATB) and latent tuberculosis infection (LTBI) has been a challenge for clinicians in high TB burden countries. The purpose of this study was to improve the accuracy of differential diagnosis of ATB and LTBI by using fluorescent immunospot (FluoroSpot) assay to detect specific Th1 cell immune responses. The novel mycobacterium tuberculosis (MTB) latency-associated antigens Rv1733c and synthetic long peptides derived from Rv1733c (Rv1733c SLP) were used based on virulence factors early secreting antigen target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10).

METHODS

Fifty-seven ATB cases, including 20 pathogen-confirmed ATB and 37 clinically diagnosed ATB, and 36 LTBI cases, were enrolled between January and December 2017. FluoroSpot assay was used to detect the interferon γ (IFN-γ) and interleukin 2 (IL-2) secreted by the specific T cells after being stimulated with MTB virulence factors ESAT-6 and CFP-10, MTB latency-associated antigens Rv1733c and Rv1733c SLP. The receiver operating characteristic (ROC) curve was used to define the best cutoff value of latency-associated antigens in the use of differentiating ATB and LTBI. The sensitivity, specificity, predictive value, and likelihood ratio of ESAT-6 and CFP-10-FluoroSpot combined with latency-associated antigen in the differential diagnosis of ATB and LTBI were also calculated.

RESULTS

Following the stimulation with Rv1733c and Rv1733c SLP, the frequency of single IL-2-secreting T cells stimulated by Rv1733c SLP had the largest area under the ROC curve, which was 0.766. With a cutoff value of 1 (spot-forming cells [SFCs]/2.5 × 105 peripheral blood mononuclear cells) for frequency, the sensitivity and specificity of distinguishing ATB from LTBI were 72.2% and 73.7%, respectively. ESAT-6 and CFP-10-FluoroSpot detected the frequency and proportion of single IFN-γ-secreting T cells; the sensitivity and specificity of distinguishing ATB from LTBI were 82.5% and 66.7%, respectively. Combined with the frequency of single IL-2-secreting T cells stimulated by Rv1733c SLP on the basis of ESAT-6 and CFP-10-FluoroSpot, the sensitivity and specificity increased to 84.2% and 83.3%, respectively.

CONCLUSION

Rv1733c SLP, combined with ESAT-6 and CFP-10, might be used as a candidate antigen for T cell-based tuberculosis diagnostic tests to differentiate ATB from LTBI.

Predictive value of TNF-α, IFN-γ, and IL-10 for tuberculosis among recently exposed contacts in the United States and Canada

BACKGROUND

We examined cytokine immune response profiles among contacts to tuberculosis patients to identify immunologic and epidemiologic correlates of tuberculosis.

METHODS

We prospectively enrolled 1272 contacts of culture-confirmed pulmonary tuberculosis patients at 9 United States and Canadian sites. Epidemiologic characteristics were recorded. Blood was collected and stimulated with Mycobacterium tuberculosis culture filtrate protein, and tumor necrosis factor (TNF-α), interferon gamma (IFN-γ), and interleukin 10 (IL-10) concentrations were determined using immunoassays.

RESULTS

Of 1272 contacts, 41 (3.2%) were diagnosed with tuberculosis before or < 30 days after blood collection (co-prevalent tuberculosis) and 19 (1.5%) during subsequent four-year follow-up (incident tuberculosis). Compared with contacts without tuberculosis, those with co-prevalent tuberculosis had higher median baseline TNF-α and IFN-γ concentrations (in pg/mL, TNF-α 129 versus 71, P < .01; IFN-γ 231 versus 27, P < .001), and those who subsequently developed incident tuberculosis had higher median baseline TNF-α concentrations (in pg/mL, 257 vs. 71, P < .05). In multivariate analysis, contact age < 15 years, US/Canadian birth, and IFN or TNF concentrations > the median were associated with co-prevalent tuberculosis (P < .01 for each); female sex (P = .03) and smoking (P < .01) were associated with incident tuberculosis. In algorithms combining young age, positive skin test results, and elevated CFPS TNF-α, IFN-γ, and IL-10 responses, the positive predictive values for co-prevalent and incident tuberculosis were 40 and 25%, respectively.

CONCLUSIONS

Cytokine concentrations and epidemiologic factors at the time of contact investigation may predict co-prevalent and incident tuberculosis.

Cell wall peptidoglycan in Mycobacterium tuberculosis: An Achilles' heel for the TB-causing pathogen

Tuberculosis (TB), caused by the intracellular pathogen Mycobacterium tuberculosis, remains one of the leading causes of mortality across the world. There is an urgent requirement to build a robust arsenal of effective antimicrobials, targeting novel molecular mechanisms to overcome the challenges posed by the increase of antibiotic resistance in TB. Mycobacterium tuberculosis has a unique cell envelope structure and composition, containing a peptidoglycan layer that is essential for maintaining cellular integrity and for virulence. The enzymes involved in the biosynthesis, degradation, remodelling and recycling of peptidoglycan have resurfaced as attractive targets for anti-infective drug discovery. Here, we review the importance of peptidoglycan, including the structure, function and regulation of key enzymes involved in its metabolism. We also discuss known inhibitors of ATP-dependent Mur ligases, and discuss the potential for the development of pan-enzyme inhibitors targeting multiple Mur ligases.

Use of resuscitation promoting factors to screen for tuberculosis infection in household-exposed children in The Gambia

BACKGROUND

Interferon-γ release assays (IGRA) with Resuscitation promoting factor (Rpf) proteins enhanced tuberculosis (TB) screening and diagnosis in adults but have not been evaluated in children. Children often develop paucibacillary TB and their immune response differs from that of adults, which together affect TB disease diagnostics and immunodiagnostics. We assessed the ability of Rpf to identify infection among household TB-exposed children in The Gambia and investigated their ability to discriminate Mycobacterium tuberculosis complex (MTBC) infection from active TB disease in children.

METHODS

Detailed clinical investigations were done on 93 household TB-exposed Gambian children and a tuberculin skin test (TST) was administered to asymptomatic children. Venous blood was collected for overnight stimulation with ESAT-6/CFP-10-fusion protein (EC), purified protein derivative and RpfA, B, C, D and E. Interferon gamma (IFN-γ) production was measured by ELISA in supernatants and corrected for the background level. Infection status was defined by IGRA with EC and TB disease by mycobacterial confirmation and/or clinical diagnosis. We compared IFN-γ levels between infected and uninfected children and between infected and TB diseased children using a binomial logistic regression model while correcting for age and sex. A Receiver Operating Characteristics analysis was done to find the best cut-off for IFN-γ level and calculate sensitivity and specificity.

RESULTS

Interferon gamma production was significantly higher in infected (IGRA+, n = 45) than in uninfected (IGRA-, n = 20) children after stimulation with RpfA, B, C, and D (P = 0.03; 0.007; 0.03 and 0.003, respectively). Using RpfB and D-specific IFN-γ cut-offs (33.9 pg/mL and 67.0 pg/mL), infection was classified with a sensitivity-specificity combination of 73-92% and 77-72% respectively, which was similar to and better than 65-75% for TST. Moreover, IFN-γ production was higher in infected than in TB diseased children (n = 28, 5 bacteriologically confirmed, 23 clinically diagnosed), following RpfB and D stimulation (P = 0.02 and 0.03, respectively).

CONCLUSION

RpfB and RpfD show promising results for childhood MTBC infection screening, and both performed similar to and better than the TST in our study population. Additionally, both antigens appear to discriminate between infection and disease in children and thus warrant further investigation as screening and diagnostic antigens for childhood TB.

Differential T cell responses against DosR-associated antigen Rv2028c in BCG-vaccinated populations with tuberculosis infection

The IFN-γ release assays (IGRAs) based on region of difference 1 (RD1) antigens have improved diagnosis of Mycobacterium tuberculosis (Mtb) infection. However, IGRAs with these antigens could not distinguish latent tuberculosis infection (LTBI) from active tuberculosis (ATB). DosR regulon genes are thought to be important for Mtb dormancy, and their products have higher immunogenicity in LTBI than ATB individuals, suggesting protective immunity mediated by DosR regulon-encoded antigens and potential utility of them for differential diagnostics of Mtb-infected populations or development of therapeutic vaccines against tuberculosis (TB). Among them, Rv2028c is a dormancy-related antigen that has demonstrated potential use in TB control, but its immunological characteristics in the BCG-vaccinated Chinese population are unknown. In this study, a total of 148 individuals, including 98 patients with ATB, 20 cases with LTBI and 30 healthy controls, were tested for Rv2028c-specific T cell responses by using an IFN-γ ELISA assay. The results showed that the T-cell responses in LTBI individuals were almost always higher than those in ATB patients, regardless of the site of infection or the results of bacteriological examination in the patients. This allowed for good differentiation between these two groups of Mtb-infected individuals even in the BCG-vaccinated high TB-incidence setting that pertains in China. In addition, the diagnostic efficacy for ATB was enhanced by combining the results from Rv2028c and RD1 antigen-based IFN-γ ELISA assays. In conclusion, Rv2028c-specific T-cell responses might contribute to natural protection against dormant Mtb infection, and the determination of these responses can aid discrimination between healthy LTBI individuals and ATB patients in the Mtb-infected populations.

Recent Trends in System-Scale Integrative Approaches for Discovering Protective Antigens Against Mycobacterial Pathogens

Mycobacterial infections are one of the deadliest infectious diseases still posing a major health burden worldwide. The battle against these pathogens needs to focus on novel approaches and key interventions. In recent times, availability of genome scale data has revolutionized the fields of computational biology and immunoproteomics. Here, we summarize the cutting-edge ‘omics’ technologies and innovative system scale strategies exploited to mine the available data. These may be targeted using high-throughput technologies to expedite the identification of novel antigenic candidates for the rational next generation vaccines and serodiagnostic development against mycobacterial pathogens for which traditional methods have been failing.

The DosR antigen Rv1737c from Mycobacterium tuberculosis confers inflammation regulation in tuberculosis infection

There is an urgent need to identify the potential risk factors for activating latent Mycobacterium tuberculosis infection. In this study, we evaluated the immune function of Rv1737c, which is a latency-associated antigen of dormancy survival regulator (DosR) of M. tuberculosis in a mouse model. Our data showed that mice pretreated with recombinant Rv1737c (rRv1737c) exhibited higher levels of antigen-specific antibodies (IgG, IgM and IgA) than sham-treated mice. Following Bacilli Calmette-Guerin (BCG) challenge, rRv1737c adjuvanted with cholera toxin subunit B (CTB) induced diffuse lung inflammation and fibrosis compared to the control mice. The inflammatory pathogenesis due to rRv1737c pre-exposure was associated with a switch in the macrophage phenotype from M1 to activated M2 and was characterized by IL-10 production. Intracellular cytokine analysis further showed that the rRv1737c-pretreated mice exhibited an increased frequency of Th2 cells in the lungs, lymph nodes and spleen after BCG challenge. Furthermore, IFN-γ expression increased in the lungs after rRv1737c pretreatment compared to that in the sham mice. Accordingly, lung cells from rRv1737c-immunized mice stimulated with killed BCG produced higher levels of multiple cytokines, such as IFN-γ, IL-10 and IL-6. The results confirmed that the pathological features of rRv1737c promoted inflammation. Overall, our findings provide direct evidence of the pro-inflammatory function of rRv1737c in a murine model of BCG infection, indicating that Rv1737c is a pathogenic antigen of M. tuberculosis and may be key to the recurrence of latent infection.

DosR proteins of Mycobacterium tuberculosis upregulate effector T cells and down regulate T regulatory cells in TB patients and their healthy contacts

It is well established that the current problem of tuberculosis (TB) can be combated by overcoming the drawbacks of the currently available BCG vaccine. This would involve incorporation of antigens that can control TB at all stages including the dormant phase which is generally ignored. Hence, DosR regulon proteins, which are expressed in latent infection, could prove to be very good vaccine candidates as they can possibly target the silent but most predominant form of TB infection. In the present study, the immune response to two DosR proteins Rv2627 and Rv2628 has been studied in PBMCs derived from normal individuals, TB patients and healthy contacts of TB patients. It was found that these antigens were capable of stimulating a strong IFN-γ+ T cell response along with accentuation of memory T cells and other protective cytokines such as IL-2 and IL-17. At the same time these proteins decreased the frequencies of immune-suppressor regulatory T cells in in vitro stimulation of PBMC from both patients and their contacts. Considering all these facts together, we suggest Rv2627 and Rv2628 to be one of the extremely promising candidates for incorporation into a post exposure subunit vaccine against TB.

A Systematic Review on Novel Mycobacterium tuberculosis Antigens and Their Discriminatory Potential for the Diagnosis of Latent and Active Tuberculosis

Background: Current immunodiagnostic tests for tuberculosis (TB) are based on the detection of an immune response toward mycobacterial antigens injected into the skin or following an in-vitro simulation in interferon gamma-release assays. Both tests have limited sensitivity and are unable to differentiate between tuberculosis infection (LTBI) and active tuberculosis disease (aTB). To overcome this, the use of novel Mycobacterium tuberculosis (M. tuberculosis) stage-specific antigens for the diagnosis of LTBI and aTB has gained interest in recent years. This review summarizes current evidence on novel antigens used for the immunodiagnosis of tuberculosis and discrimination of LTBI and aTB. In addition, results on measured biomarkers after stimulation with novel M. tuberculosis antigens were also reviewed.

Methods: A systematic literature review was performed in Pubmed, EMBASE and web of science searching articles from 2000 up until December 2017. Only articles reporting studies in humans using novel antigens were included.

Results: Of 1,533 articles screened 34 were included in the final analysis. A wide range of novel antigens expressed during different stages and types of LTBI and aTB have been assessed. M. tuberculosis antigens Rv0081, Rv1733c, Rv1737c, Rv2029c, Rv2031 and Rv2628, all encoded by the dormancy of survival regulon, were among the most widely studied antigens and showed the most promising results. These antigens have been shown to have best potential for differentiating LTBI from aTB. In addition, several studies have shown that the inclusion of cytokines other than IFN-γ can improve sensitivity.

Conclusion: There is limited evidence that the inclusion of novel antigens as well as the measurement of other biomarkers than IFN-γ may improve sensitivity and may lead to a discrimination of LTBI from aTB.

Novel epitopes identified from Mycobacterium tuberculosis antigen Rv2629induces cytotoxic T lymphocyte response

There is an urgent need for a more effective vaccine against tuberculosis (TB). Cytotoxic T lymphocytes (CTLs) play a critical role in combating Mycobacterium tuberculosis (M.tb). The identification of novel CTL epitopes is essential for the design of peptide-based vaccines. In this study, we predicted CTL epitope peptides of M.tb antigen Rv2629 restricted by HLA-A2, using bioinformatics methods. The affinity and stability of binding of these peptides with HLA-A2 molecules were detected by flow cytometry. Their ability to induce CTLs generation was determined in peripheral blood mononuclear cells (PBMCs) from healthy uninfected subjects, Latent tuberculosis infection (LTBI) subjects, and TB patients ex vivo. The cytotoxic activity induced by the epitope peptides was tested by lactate dehydrogenase (LDH) release assay. Finally, we found four novel CTL epitope peptides, Rv2629-p190-2L, Rv2629-p190-1Y2L, Rv2629-p274, and Rv2629-p315, which had high-affinity and stability of binding with T2 cells. Their ability of inducing CTLs was highest in PBMCs from TB patients (P < 0.05). In addition, these peptides could induce the CTLs to generate specific cytotoxic activity. They showed higher immunogenicity in TB patients and had the potential to become candidate vaccines for TB therapy.

Early clearance versus control: what is the meaning of a negative tuberculin skin test or interferon-gamma release assay following exposure to Mycobacterium tuberculosis?

The elimination of tuberculosis (TB) cannot reasonably be achieved by treatment of individual cases and will require an improved vaccine or immunotherapy. A challenge in developing an improved TB vaccine has been the lack of understanding what is needed to generate sterilizing immunity against Mycobacterium tuberculosis (Mtb) infection. Several epidemiological observations support the hypothesis that humans can eradicate Mtb following exposure. This has been termed early clearance and is defined as elimination of Mtb infection prior to the development of an adaptive immune response, as measured by a tuberculin skin test or interferon-gamma release assay. Here, we examine research into the likelihood of and possible mechanisms responsible for early clearance in household contacts of patients with active TB. We explore both innate and adaptive immune responses in the lung. Enhanced understanding of these mechanisms could be harnessed for the development of a preventative vaccine or immunotherapy.

Potential of DosR and Rpf antigens from Mycobacterium tuberculosis to discriminate between latent and active tuberculosis in a tuberculosis endemic population of Medellin Colombia

Background

Tuberculosis (TB) remains one of the most deadly infectious diseases. One-third to one-fourth of the human population is estimated to be infected with Mycobacterium tuberculosis (Mtb) without showing clinical symptoms, a condition called latent TB infection (LTBI). Diagnosis of Mtb infection is based on the immune response to a mixture of mycobacterial antigens (PPD) or to Mtb specific ESAT-6/CFP10 antigens (IGRA), highly expressed during the initial phase of infection. However, the immune response to PPD and IGRA antigens has a low power to discriminate between LTBI and PTB. The T-cell response to a group of so-called latency (DosR-regulon-encoded) and Resuscitation Promoting (Rpf) antigens of Mtb has been proved to be significantly higher in LTBI compared to active TB across many populations, suggesting their potential use as biomarkers to differentiate latent from active TB.

Methods

PBMCs from a group LTBI (n = 20) and pulmonary TB patients (PTB, n = 21) from an endemic community for TB of the city of Medellín, Colombia, were in vitro stimulated for 7 days with DosR- (Rv1737c, Rv2029c, and Rv2628), Rpf- (Rv0867c and Rv2389c), the recombinant fusion protein ESAT-6-CFP10 (E6-C10)-, or PPD-antigen. The induced IFNγ levels detectable in the supernatants of the antigen-stimulated cells were then used to calculate specificity and sensitivity in discriminating LTBI from PTB, using different statistical approaches.

Results

IFNγ production in response to DosR and Rpf antigens was significantly higher in LTBI compared to PTB. ROC curve analyses of IFNγ production allowed differentiation of LTBI from PTB with areas under the curve higher than 0.70. Furthermore, Multiple Correspondence Analysis (MCA) revealed that LTBI is associated with higher levels of IFNγ in response to the different antigens compared to PTB. Analysis based on decision trees showed that the IFNγ levels produced in response to Rv2029c was the leading variable that best-classified disease status. Finally, logistic regression analysis predicted that IFNγ produced by PBMCs in response to E6-C10, Rv2029c, Rv0867c (RpfA) and Rv2389c (RpfA) antigens correlates best with the probability of being latently infected.

Conclusions

The Mtb antigens E6-C10, Rv2029c (PfkB), Rv0867c (RpfA) and Rv2389c (RpfA), may be potential candidates to discriminate LTBI from PTB.

Electronic supplementary material

The online version of this article (doi: 10.1186/s12879-017-2929-0) contains supplementary material, which is available to authorized users.

Altered expression of antigen-specific memory and regulatory T-cell subsets differentiate latent and active tuberculosis

Although one-third of the world population is infected with Mycobacterium tuberculosis, only 5-10% of the infected individuals will develop active tuberculosis (TB) disease and the rest will remain infected with no symptoms, known as latent TB infection (LTBI). Identifying biomarkers that differentiate latent and active TB disease enables effective TB control, as early detection, treatment of active TB and preventive treatment of individuals with LTBI are crucial steps involved in TB control. Here, we have evaluated the frequency of antigen-specific memory and regulatory T (Treg) cells in 15 healthy household contacts (HHC) and 15 pulmonary TB patients (PTB) to identify biomarkers for differential diagnosis of LTBI and active TB. Among all the antigens tested in the present study, early secretory antigenic target-6 (ESAT-6) -specific CD4⁺ and CD8⁺ central memory (Tcm) cells showed 93% positivity in HHC and 20% positivity in PTB. The novel test antigens Rv0753c and Rv0009 both displayed 80% and 20% positivity in HHC and PTB, respectively. In contrast to Tcm cells, effector memory T (Tem) cells showed a higher response in PTB than HHC; both ESAT-6 and Rv0009 showed similar positivity of 80% in PTB and 33% in HHC. PTB patients have a higher proportion of circulating antigen-reactive Treg cells (CD4⁺  CD25⁺  FoxP3⁺ ) than LTBI. Rv2204c-specific Treg cells showed maximum positivity of 73% in PTB and 20% in HHC. Collectively, our data conclude that ESAT-6-specific Tcm cells and Rv2204c-specific Treg cells might be useful biomarkers to discriminate LTBI from active TB.

Mycobacterium tuberculosis Latent Antigen Rv2029c from the Multistage DNA Vaccine A39 Drives TH1 Responses via TLR-mediated Macrophage Activation

Targeting of Mycobacterium tuberculosis (MTB) latent antigens comprises a crucial strategy for the development of alternative tuberculosis (TB) vaccine(s) that protects against TB reactivation. Here, we generated a multistage DNA vaccine, A39, containing the early antigens Ag85A and Rv3425 as well as the latency-associated protein Rv2029c, which conferred protective immunity in a pre-exposure mouse model. Moreover, administration of the A39 vaccination after MTB exposure inhibited reactivation and resulted in significantly lower bacterial loads in the lungs and spleen of mice, compared to those in the control population. Subsequently, we investigated the effect of Rv2029c on innate immunity and characterized the molecular details of the interaction of this protein with the host via iTRAQ proteomic and biochemical assay analyses. Rv2029c activated macrophages, triggered the production of pro-inflammatory cytokines, and promoted toll-like receptor/mitogen-activated protein kinase (TLR/MAPK)-dependent macrophage apoptosis. Furthermore, Rv2029c treatment enhanced the ability of Mycobacterium bovis Bacillus Calmette-Guérin (BCG)-infected macrophages to present antigens to CD4⁺ T cells in vitro, which correlated with an increase in MHC-II expression. Lastly, Rv2029c-treated macrophages activated T cells, effectively polarized CD4⁺ and CD8⁺ T cells to secrete IFN-γ and IL-2, and specifically expanded a population of CD44^highCD62L^lowCD4⁺/CD8⁺ effector/memory cells, indicating that Rv2029c, as a specific recall antigen, contributes to Th1 polarization in T cell immunity. These results suggest that Rv2029c and A39 comprise promising targets for the development of next-generation clinical TB therapeutic vaccines.

Opening Pandora's Box: Mechanisms of Mycobacterium tuberculosis Resuscitation

Mycobacterium tuberculosis (Mtb) characteristically causes an asymptomatic infection. While this latent tuberculosis infection (LTBI) is not contagious, reactivation to active tuberculosis disease (TB) causes the patient to become infectious. A vaccine has existed for TB for a century, while drug treatments have been available for over 70 years; despite this, TB remains a major global health crisis. Understanding the factors which allow the bacillus to control responses to host stress and mechanisms leading to latency are critical for persistence. Similarly, molecular switches which respond to reactivation are important. Recently, research in the field has sought to focus on reactivation, employing system-wide approaches and animal models. Here, we describe the current work that has been done to elucidate the mechanisms of reactivation and stop reactivation in its tracks.

Identification of novel antigen candidates for a tuberculosis vaccine in the adult zebrafish (Danio rerio)

Tuberculosis (TB) remains a major global health challenge and the development of a better vaccine takes center stage in fighting the disease. For this purpose, animal models that are capable of replicating the course of the disease and are suitable for the early-stage screening of vaccine candidates are needed. A Mycobacterium marinum infection in adult zebrafish resembles human TB. Here, we present a pre-clinical screen for a DNA-based tuberculosis vaccine in the adult zebrafish using an M. marinum infection model. We tested 15 antigens representing different types of mycobacterial proteins, including the Resuscitation Promoting factors (Rpf), PE/PPE protein family members, other membrane proteins and metabolic enzymes. The antigens were expressed as GFP fusion proteins, facilitating the validation of their expression in vivo. The efficiency of the antigens was tested against a low-dose intraperitoneal M. marinum infection (≈ 40 colony forming units), which mimics a primary M. tuberculosis infection. While none of the antigens was able to completely prevent a mycobacterial infection, four of them, namely RpfE, PE5_1, PE31 and cdh, led to significantly reduced bacterial burdens at four weeks post infection. Immunization with RpfE also improved the survival of the fish against a high-dose intraperitoneal injection with M. marinum (≈ 10.000 colony forming units), resembling the disseminated form of the disease. This study shows that the M. marinum infection model in adult zebrafish is suitable for the pre-clinical screening of tuberculosis vaccines and presents RpfE as a potential antigen candidate for further studies.

Resuscitation-promoting factors are important determinants of the pathophysiology in Mycobacterium tuberculosis infection

Resuscitation promoting factors (Rpf) are peptidoglycan-hydrolyzing enzymes that are pivotal in the resuscitation of quiescent actinobacteria including Mycobacterium tuberculosis. From the published data, it is clear that Rpf are required for the resuscitation of non-replicating bacilli and pathogenesis in murine infection model of tuberculosis, although their direct influence on human Mycobacterium tuberculosis infection is ill-defined. In this review, we describe the progress in the understanding of the roles that Rpf play in human tuberculosis pathogenesis and importance of bacilli dependent upon Rpf for growth for the outcome of human tuberculosis. We outline how this research is opening up important opportunities for the diagnosis, treatment and prevention of human disease, progress in which is essential to attain the ultimate goal of tuberculosis eradication.

Multifunctional T Cell Response to DosR and Rpf Antigens Is Associated with Protection in Long-Term Mycobacterium tuberculosis-Infected Individuals in Colombia

Multifunctional T cells have been shown to be protective in chronic viral infections. In mycobacterial infections, however, evidence for a protective role of multifunctional T cells remains inconclusive. Short-term cultures of peripheral blood mononuclear cells stimulated with the Mycobacterium tuberculosis RD1 antigens 6-kDa early secretory antigenic target (ESAT6) and 10-kDa culture filtrate antigen (CFP10), which are induced in the early infection phase, have been mainly used to assess T cell multifunctionality, although long-term culture assays have been proposed to be more sensitive than short-term assays for assessment of memory T cells, which are essential for long-term immunity. Here we used a long-term culture assay system to study the T cell immune responses to the M. tuberculosis latency-associated DosR antigens and reactivation-associated Rpf antigens, compared to ESAT6 and CFP10, in patients with pulmonary tuberculosis (PTB) and household contacts of PTB patients with long-term latent tuberculosis infection (ltLTBI), in a community in which M. tuberculosis is endemic. Our results showed that the DosR antigens Rv1737c (narK2) and Rv2029c (pfkB) and the Rv2389c (rpfD) antigen of M. tuberculosis induced higher frequencies of CD4⁺ or CD8⁺ mono- or bifunctional (but not multifunctional) T cells producing interferon gamma (IFN-γ) and/or tumor necrosis alpha (TNF-α) in ltLTBI, compared to PTB. Moreover, the frequencies of CD4⁺ and/or CD8⁺ T cells with a CD45RO⁺ CD27⁺ phenotype were higher in ltLTBI than in PTB. Thus, the immune responses to selected DosR and Rpf antigens may be associated with long-term latency, correlating with protection from M. tuberculosis reactivation in ltLTBI. Further study of the functional and memory phenotypes may contribute to further discrimination between the different states of M. tuberculosis infections.

A putative nitroreductase from the DosR regulon of Mycobacterium tuberculosis induces pro-inflammatory cytokine expression via TLR2 signaling pathway

Tuberculosis caused by Mycobacterium tuberculosis is a global encumbrance and it is estimated that nearly one third population of the world acts as a reservoir for this pathogen without any symptoms. In this study, we attempted to characterise one of the genes of DosR regulon, Rv3131, a FMN binding nitroreductase domain containing protein, for its ability to alter cytokine profile, an essential feature of M. tuberculosis latency. Recombinant Rv3131 stimulated pro-inflammatory cytokines in THP-1 cells and human peripheral blood mononuclear cells in a time and dose dependent manner. In silico analyses using docking and simulations indicated that Rv3131 could strongly interact with TLR2 via a non-covalent bonding which was further confirmed using cell based colorimetric assay. In THP-1 cells treated with Rv3131 protein, a significant upsurge in the surface expression, overall induction and expression of mRNA of TLR2 was observed when analysed by flow cytometry, western blotting and real time PCR, respectively. Activation of TLR2 by Rv3131 resulted in the phosphorylation of NF- κβ. Results of this study indicate a strong immunogenic capability of Rv3131 elicited via the activation of TLR2 signalling pathway. Therefore, it can be surmised that cytokine secretion induced by Rv3131 might contribute to establishment of M. tuberculosis in the granulomas.

Dynamics of the T cell response to Mycobacterium tuberculosis DosR and Rpf antigens in a Colombian population of household contacts of recently diagnosed pulmonary tuberculosis patients

Immune response to DosR and Rpf antigens from Mycobacterium tuberculosis (Mtb) seems to be important for latency maintenance. Little is known about the dynamics of the immune response to these antigens in an endemic community. Thus, the IFNγ response and cytokine production in response to PPD, Esat6-Cfp10 (E6-C10), DosR and Rpf antigens in healthy HHC of tuberculosis (TB) patients over a 12 (T12) months period (short-term, stLTBI) was investigated. This response was compared with a group of LTBI, who have remained healthy for 5-7 years (long-term, ltLTBI). According to the IFNγ response, two groups of HHCs were identified in stLTBI in response to E6-C10. At T12, E6-C10(+) HHCs displayed a decrease in the IFNγ levels and a generalized decrease in cytokines production. The E6-C10(-) HHC showed an increase in the IFNγ response and cytokine levels. In stLTBI, the responses to E6-C10, DosR, and Rpf may be interpreted as a protective immune response controlling Mtb infection and may be leading to a state of latent infection. Comparing the response of stLTBI and ltLTBI, we observed significant changes in the proportions of CD45RO(+)CD27(+) T cells to specific DosR and Rpf, which may indicate a persistent immune response to Mtb antigens in ltLTBI.

Potential novel markers to discriminate between active and latent tuberculosis infection in Chinese individuals

Latent tuberculosis infection (LTBI) constitutes the main reservoir for reactivation tuberculosis. The finding of potential biomarkers for differentiating between TB and LTBI is very necessary. In this study, the immunological characteristics and potential diagnostic utility of Rv2029c, Rv2628 and Rv1813c proteins were assessed. These three proteins stimulated PBMCs from ELISPOT-positive LTBI subjects produced higher levels of IFN-γ in comparison with TB patients and ELISPOT-negative healthy subjects (p<0.05). BCG vaccination and non-TB respiratory disease had little influence on the immunological responses of Rv2029c and Rv2628 proteins (p>0.05). The LTBI diagnostic performance of Rv2029c was higher than Rv2628 and Rv1813c by ROC evaluation. But Rv2628 had much higher specificity than Rv2029c in active TB patients and uninfected healthy subjects. The IgG level against Rv1813c was higher in the TB group than in LTBI and uninfected healthy subjects (p<0.05). These results suggest that T cell response to Rv2628 and antibody against Rv1813c might be applicable as biomarkers to distinguish TB from LTBI and uninfected individuals.

Immune parameters differentiating active from latent tuberculosis infection in humans

Tuberculosis remains a highly prevalent infectious disease worldwide. Identification of the immune parameters that differentiate active disease from latent infection will facilitate the development of efficient control measures as well as new diagnostic modalities for tuberculosis. Here, we investigated the cytokine production profiles of monocytes and CD4(+) T lymphocytes upon encountering mycobacterial antigens. In addition, cytokines and lipid mediators with immune-modulating activities were examined in plasma samples ex vivo. Comparison of these parameters in active tuberculosis patients and healthy subjects with latent infection revealed that, active tuberculosis was associated with diminished Th1-type cytokine secretion from CD4(+) T cells and less augmented inflammatory cytokine secretion from monocytes induced by IFN-γ than that in latent tuberculosis infection. In addition, a higher plasma concentration of lipoxin A4 and lower ratio of prostaglandin E2 to lipoxin A4 were observed in active cases than in latent infections. These findings have implications for preparing new therapeutic strategies and for differential diagnosis of the two types of tuberculosis infection.

Challenging Mycobacterium tuberculosis dormancy mechanisms and their immunodiagnostic potential

Mycobacterium tuberculosis is the etiologic agent of tuberculosis, one of the world's greatest cause of morbidity and mortality due to infectious disease. Many evolutionary mechanisms have contributed to its high level of adaptation as a host pathogen. Prior to become dormant, a group of about 50 genes related to metabolic changes are transcribed by the DosR regulon, one of the most complex and important systems of host-pathogen interaction. This genetic mechanism allows the mycobacteria to persist during long time periods, establishing the so-called latent infection. Even in the presence of a competent immune response, the host cannot eliminate the pathogen, only managing to keep it surrounded by an unfavorable microenvironment for its growth. However, conditions such as immunosuppression may reestablish optimal conditions for bacterial growth, culminating in the onset of active disease. The interactions between the pathogen and its host are still not completely elucidated. Nonetheless, many studies are being carried out in order to clarify this complex relationship, thus creating new possibilities for patient approach and laboratory screening.

Profile of interferon-gamma response to latency-associated and novel in vivo expressed antigens in a cohort of subjects recently exposed to Mycobacterium tuberculosis

Recently some latency-associated antigens (LAA) of Mycobacterium tuberculosis were described, as Rv2029c, Rv2031c, Rv2034, Rv2628 and Rv3353c. Of which, the Rv2034 and Rv3353c also demonstrated in vivo expression. Therefore evaluating the immune response to these antigens may help to understand their role in latent TB infection. In a 1-year longitudinal study, IFN-γ response by in vitro peripheral blood mononuclear cells stimulation with LAA was investigated in subjects recently exposed to TB, classified by IFN-γ release assay (IGRA) using RD1 antigens (ESAT-6:CFP-10) and tuberculin skin test (TST) response. Except for Rv3353c, all the LAA triggered higher mean IFN-γ response in IGRA-RD1(+) groups (p < 0.05). Combining the IFN-γ-responders to Rv2029c, Rv2031c plus Rv2034 detected 90.3% (28/31) of IGRA-RD1(+) and 66.7% (24/36) of TST(+) contacts, while 95% (19/20) and 11% (2/17) were identified by classifying them according to a TST and IGRA-RD1 double-positive or double-negative response, respectively. In the follow-up, the TST convertors (negative to positive) also demonstrated an IFN-γ conversion to Rv2029c and Rv2031c, whereas the unique TB incident case was exclusively detected via IGRA-Rv2029c and TST before developing TB. A reversion rate to LAA (60%-100%) after prophylactic treatment was observed at TST(+)/IGRA-RD1(+) group. Further studies into the performance of these antigens are thus warranted.

Synthetic Long Peptide Derived from Mycobacterium tuberculosis Latency Antigen Rv1733c Protects against Tuberculosis

Responsible for 9 million new cases of active disease and nearly 2 million deaths each year, tuberculosis (TB) remains a global health threat of overwhelming dimensions. Mycobacterium bovis BCG, the only licensed vaccine available, fails to confer lifelong protection and to prevent reactivation of latent infection. Although 15 new vaccine candidates are now in clinical trials, an effective vaccine against TB remains elusive, and new strategies for vaccination are vital. BCG vaccination fails to induce immunity against Mycobacterium tuberculosis latency antigens. Synthetic long peptides (SLPs) combined with adjuvants have been studied mostly for therapeutic cancer vaccines, yet not for TB, and proved to induce efficient antitumor immunity. This study investigated an SLP derived from Rv1733c, a major M. tuberculosis latency antigen which is highly expressed by "dormant" M. tuberculosis and well recognized by T cells from latently M. tuberculosis-infected individuals. In order to assess its in vivo immunogenicity and protective capacity, Rv1733c SLP in CpG was administered to HLA-DR3 transgenic mice. Immunization with Rv1733c SLP elicited gamma interferon-positive/tumor necrosis factor-positive (IFN-γ(+)/TNF(+)) and IFN-γ(+) CD4(+) T cells and Rv1733c-specific antibodies and led to a significant reduction in the bacterial load in the lungs of M. tuberculosis-challenged mice. This was observed both in a pre- and in a post-M. tuberculosis challenge setting. Moreover, Rv1733c SLP immunization significantly boosted the protective efficacy of BCG, demonstrating the potential of M. tuberculosis latency antigens to improve BCG efficacy. These data suggest a promising role for M. tuberculosis latency antigen Rv1733c-derived SLPs as a novel TB vaccine approach, both in a prophylactic and in a postinfection setting.

Individuality, phenotypic differentiation, dormancy and 'persistence' in culturable bacterial systems: commonalities shared by environmental, laboratory, and clinical microbiology

For bacteria, replication mainly involves growth by binary fission. However, in a very great many natural environments there are examples of phenotypically dormant, non-growing cells that do not replicate immediately and that are phenotypically 'nonculturable' on media that normally admit their growth. They thereby evade detection by conventional culture-based methods. Such dormant cells may also be observed in laboratory cultures and in clinical microbiology. They are usually more tolerant to stresses such as antibiotics, and in clinical microbiology they are typically referred to as 'persisters'. Bacterial cultures necessarily share a great deal of relatedness, and inclusive fitness theory implies that there are conceptual evolutionary advantages in trading a variation in growth rate against its mean, equivalent to hedging one's bets. There is much evidence that bacteria exploit this strategy widely. We here bring together data that show the commonality of these phenomena across environmental, laboratory and clinical microbiology. Considerable evidence, using methods similar to those common in environmental microbiology, now suggests that many supposedly non-communicable, chronic and inflammatory diseases are exacerbated (if not indeed largely caused) by the presence of dormant or persistent bacteria (the ability of whose components to cause inflammation is well known). This dormancy (and resuscitation therefrom) often reflects the extent of the availability of free iron. Together, these phenomena can provide a ready explanation for the continuing inflammation common to such chronic diseases and its correlation with iron dysregulation. This implies that measures designed to assess and to inhibit or remove such organisms (or their access to iron) might be of much therapeutic benefit.

Individuality, phenotypic differentiation, dormancy and 'persistence' in culturable bacterial systems: commonalities shared by environmental, laboratory, and clinical microbiology

For bacteria, replication mainly involves growth by binary fission. However, in a very great many natural environments there are examples of phenotypically dormant, non-growing cells that do not replicate immediately and that are phenotypically 'nonculturable' on media that normally admit their growth. They thereby evade detection by conventional culture-based methods. Such dormant cells may also be observed in laboratory cultures and in clinical microbiology. They are usually more tolerant to stresses such as antibiotics, and in clinical microbiology they are typically referred to as 'persisters'. Bacterial cultures necessarily share a great deal of relatedness, and inclusive fitness theory implies that there are conceptual evolutionary advantages in trading a variation in growth rate against its mean, equivalent to hedging one's bets. There is much evidence that bacteria exploit this strategy widely. We here bring together data that show the commonality of these phenomena across environmental, laboratory and clinical microbiology. Considerable evidence, using methods similar to those common in environmental microbiology, now suggests that many supposedly non-communicable, chronic and inflammatory diseases are exacerbated (if not indeed largely caused) by the presence of dormant or persistent bacteria (the ability of whose components to cause inflammation is well known). This dormancy (and resuscitation therefrom) often reflects the extent of the availability of free iron. Together, these phenomena can provide a ready explanation for the continuing inflammation common to such chronic diseases and its correlation with iron dysregulation. This implies that measures designed to assess and to inhibit or remove such organisms (or their access to iron) might be of much therapeutic benefit.

A Mycobacterium tuberculosis Dormancy Antigen Differentiates Latently Infected Bacillus Calmette-Guérin-vaccinated Individuals

IFN-γ release assays (IGRAs) are better indicators of Mycobacterium tuberculosis infection than the tuberculin skin test (TST) in Bacillus Calmette–Guérin (BCG)-vaccinated populations. However, IGRAs do not discriminate active and latent infections (LTBI) and no gold standard for LTBI diagnosis is available. Thus, since improved tests to diagnose M. tuberculosis infection are required, we assessed the efficacy of several M. tuberculosis latency antigens. BCG-vaccinated healthy donors (HD) and tuberculosis (TB) patients were recruited. QuantiFERON-TB Gold In-Tube, TST and clinical data were used to differentiate LTBI. IFN-γ production against CFP-10, ESAT-6, Rv2624c, Rv2626c and Rv2628 antigens was tested in peripheral blood mononuclear cells. LTBI subjects secreted significantly higher IFN-γ levels against Rv2626c than HD. Additionally, Rv2626c peptide pools to which only LTBI responded were identified, and their cumulative IFN-γ response improved LTBI discrimination. Interestingly, whole blood stimulation with Rv2626c allowed the discrimination between active and latent infections, since TB patients did not secrete IFN-γ against Rv2626c, in contrast to CFP-10 + ESAT-6 stimulation that induced IFN-γ response from both LTBI and TB patients. ROC analysis confirmed that Rv2626c discriminated LTBI from HD and TB patients. Therefore, since only LTBI recognizes specific epitopes from Rv2626c, this antigen could improve LTBI diagnosis, even in BCG-vaccinated people.

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Stimulation with Rv2626c M. tuberculosis antigen induced differential amounts of IFN-γ in LTBI individuals as compared to HD.

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The cumulative response to specific Rv2626c-derived peptide pools improved the discrimination of LTBI individuals from HD.

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PBMC or whole blood stimulation with Rv2626c differentiated latent from active infection (LTBI from TB patients).

Immunogenicity of 60 novel latency-related antigens of Mycobacterium tuberculosis

The aim of our work here was to evaluate the immunogenicity of 60 mycobacterial antigens, some of which have not been previously assessed, notably a novel series of in vivo-expressed Mycobacterium tuberculosis (IVE-TB) antigens. We enrolled 505 subjects and separated them in individuals with and without latent tuberculosis infection (LTBI) vs. patients with active tuberculosis (TB). Following an overnight and 7 days stimulation of whole blood with purified recombinant M. tuberculosis antigens, interferon-γ (IFN-γ) levels were determined by ELISA. Several antigens could statistically significantly differentiate the groups of individuals. We obtained promising antigens from all studied antigen groups [dormancy survival regulon (DosR regulon) encoded antigens; resuscitation-promoting factors (Rpf) antigens; IVE-TB antigens; reactivation associated antigens]. Rv1733, which is a probable conserved transmembrane protein encoded in DosR regulon, turned out to be very immunogenic and able to discriminate between the three defined TB status, thus considered a candidate biomarker. Rv2389 and Rv2435n, belonging to Rpf family and IVE-TB group of antigens, respectively, also stood out as LTBI biomarkers. Although more studies are needed to support our findings, the combined use of these antigens would be an interesting approach to TB immunodiagnosis candidates.

Host cytokine responses induced after overnight stimulation with novel M. tuberculosis infection phase-dependent antigens show promise as diagnostic candidates for TB disease

BACKGROUND

We previously identified Mycobacterium tuberculosis (M.tb) antigen-induced host markers that showed promise as TB diagnostic candidates in 7-day whole blood culture supernatants. The aim of the present study was to evaluate the utility of these markers further, and cross-compare results with short-term antigen stimulated and unstimulated culture supernatants.

METHODS

We recruited 15 culture confirmed TB cases and 15 non-TB cases from a high-TB endemic community in Cape Town, South Africa into a pilot case-control study from an on-going larger study. Blood samples collected from study participants were stimulated with 4 M.tb antigens that were previously identified as promising (ESAT6/CFP10 (early secreted), Rv2029c (latency), Rv2032 (latency) and Rv2389c (rpf)) in a 7-day or overnight culture assay. Supernatants were also collected form the standard QuantiFERON In Tube (QFT-IT) test. The levels of 26 host markers were evaluated in the three culture supernatants using the Luminex platform.

RESULTS

The unstimulated levels of CRP, Serum amyloid P (SAP) and serum amyloid A (SAA) and ESAT-6/CFP-10 specific IP-10 and SAA were amongst the best discriminatory markers in all 3 assays, ascertaining TB with AUC of 72-84%. Four-marker models accurately classified up to 92%, 100% and 100% of study participants in the overnight, 7-day and Quantiferon culture supernatants, respectively, after leave-one-out cross validation.

CONCLUSION

Unstimulated and antigen-specific levels of CRP, SAA, IP-10, MMP-2 and sCD40L hold promise as diagnostic candidates for TB disease in short-term stimulation assays. Larger studies are required to validate these findings but the data suggest that antigen-specific cytokine production and in particular mutimarker biosignatures might contribute to future diagnostic strategies.

Construction and immunogenicity of the DNA vaccine of Mycobacterium Tuberculosis dormancy antigen rv1733c

We aimed to construct the DNA vaccine encoding Mycobacterium Tuberculosis (Mtb) dormancy antigen Rv1733c and investigate its immunogenicity in mice. The recombinant plasmid pcDNA-Rv1733c was transfected into P815 cells and its product was detected by indirect immunofluorescence. The mice were immunized once every 2 weeks by intramuscular injection of pcDNA-Rv1733c plasmid for a total of three times. The specific antibodies in the serum of the immunized mice were detected by enzyme-linked immunosorbent assay at the indicted time. Enzyme-linked immunosorbent spot was applied to determine the levels of IFN-γ, IL-2 and IL-4 secreted by splenic lymphocytes. Total cytotoxicity T lymphocyte (CTL) active of the splenic lymphocytes was detected by lactate dehydrogenase assay. Additionally, we analysed the percentages of CD4⁺ and CD8⁺ T cells in splenic lymphocytes using flow cytometry. The specific antibody was detected at 2 weeks after the first immunization, and the antibody titre was increased with time which was reached to 1:1600 at 8 weeks. The stimulation index of spleen lymphocytes and the levels of IFN-γ, IL-2 and IL-4 of pcDNA-Rv1733c-immunized mice were both higher than those of saline-immunized mice (P < 0.05). However, no difference was found in the percentages of CD4⁺ and CD8⁺ T cells and the activity of CTL between the pcDNA-Rv1733c- and saline-immunized mice (P > 0.05). So we got the conclusion that the plasmid pcDNA-Rv1733c DNA could induce specific humoral and cellular immunity in mice. Improving the immune effect of Rv1733c by several strategies, such as choosing appropriate immunization route and adjuvant, would be significant for Rv1733c as new tuberculosis vaccine.

Comparative genomics for mycobacterial peptidoglycan remodelling enzymes reveals extensive genetic multiplicity

Background

Mycobacteria comprise diverse species including non-pathogenic, environmental organisms, animal disease agents and human pathogens, notably Mycobacterium tuberculosis. Considering that the mycobacterial cell wall constitutes a significant barrier to drug penetration, the aim of this study was to conduct a comparative genomics analysis of the repertoire of enzymes involved in peptidoglycan (PG) remodelling to determine the potential of exploiting this area of bacterial metabolism for the discovery of new drug targets.

Results

We conducted an in silico analysis of 19 mycobacterial species/clinical strains for the presence of genes encoding resuscitation promoting factors (Rpfs), penicillin binding proteins, endopeptidases, L,D-transpeptidases and N-acetylmuramoyl-L-alanine amidases. Our analysis reveals extensive genetic multiplicity, allowing for classification of mycobacterial species into three main categories, primarily based on their rpf gene complement. These include the M. tuberculosis Complex (MTBC), other pathogenic mycobacteria and environmental species. The complement of these genes within the MTBC and other mycobacterial pathogens is highly conserved. In contrast, environmental strains display significant genetic expansion in most of these gene families. Mycobacterium leprae retains more than one functional gene from each enzyme family, underscoring the importance of genetic multiplicity for PG remodelling. Notably, the highest degree of conservation is observed for N-acetylmuramoyl-L-alanine amidases suggesting that these enzymes are essential for growth and survival.

Conclusion

PG remodelling enzymes in a range of mycobacterial species are associated with extensive genetic multiplicity, suggesting functional diversification within these families of enzymes to allow organisms to adapt.

Interferon-γ responses to Mycobacterium tuberculosis Rpf proteins in contact investigation

Mycobacterium tuberculosis Resuscitation-promoting factor proteins (Rpf) induce stronger T-cell responses in latently infected individuals (LTBI) than in pulmonary tuberculosis patients (PTB), but there are scarce data concerning the responses to Rpf among LTBI with different contact levels. We therefore enrolled LTBI individuals infected through household contacts with PTB as well as people with community exposure who were determined to be LTBI through Interferon-γ (IFN-γ) release assays (IGRAs) and TB antibodies test, and we studied interferon-gamma responses to Rv0867c and Rv2389c which demonstrated the highest recognition of all Rpfs. The results demonstrated that LTBI infected through household contacts possessed higher interferon-gamma production and higher frequencies of CD4(+)IFN-γ(+) T-cells to Rv0867c and Rv2389c than did the community exposed individuals. These findings suggest that the interferon-gamma response to Rv0867c and Rv2389c may help to distinguish LTBI caused by different levels of exposure to M. tuberculosis.

Exploring the potential environmental functions of viable but non-culturable bacteria

A conventional plate count is the most commonly employed method to estimate the number of living bacteria in environmental samples. In fact, judging the level of viable culture by plate count is limited, because it is often several orders of magnitude less than the number of living bacteria actually present. Most of the bacteria are in "viable but non-culturable" (VBNC) state, whose cells are intact and alive and can resuscitate when surrounding conditions are more favorable. The most exciting recent development in resuscitating VBNC bacteria is a bacterial cytokine, namely, the resuscitation-promoting factor (Rpf), secreted by Micrococcus luteus, which promotes the resuscitation and growth of high G+C Gram-positive organisms, including some species of the genus Mycobacterium. However, most of studies deal with VBNC bacteria only from the point of view of medicine and epidemiology. It is therefore of great significance to research whether these VBNC state bacteria also possess some useful environmental capabilities, such as degradation, flocculation, etc. Further studies are needed to elucidate the possible environmental role of the VBNC bacteria, rather than only considering their role as potential pathogens from the point view of epidemiology and public health. We have studied the resuscitation of these VBNC bacteria in polluted environments by adding culture supernatant containing Rpf from M. luteus, and it was found that, as a huge microbial resource, VBNC bacteria could provide important answers to dealing with existing problems of environmental pollution. This mini-review will provide new insight for considering the potentially environmental functions of VBNC bacteria.

A multistage-polyepitope vaccine protects against Mycobacterium tuberculosis infection in HLA-DR3 transgenic mice

Mycobacterium tuberculosis (Mtb) is responsible for almost 2 million deaths annually. BCG, currently the only TB vaccine, induces variable protection and does not protect against reactivation of latent TB. Thus, efficient vaccines to supplement BCG are required urgently. Since Mtb's proteome differs qualitatively and quantitatively during bacterial replication stages from that expressed during dormancy, improved TB vaccines should drive immune responses to Mtb antigens expressed during multiple stages of infection. Consequently, such "multistage" vaccines should be composed of (immunodominant) antigens expressed during different phases of Mtb infection. As a concept multistage vaccine, we constructed a polyepitope by fusing five HLA-DR3-restricted T-cell epitopes derived from different Mtb proteins either expressed highly by replicating bacteria (Ag85B, hsp65, 19 kDa lipoprotein), or abundantly expressed by dormant bacilli and recognized preferentially by TST(+) individuals (hsp16, Rv1733c). PBMC of HLA-DR3(+) but not HLA-DR3(-) cured TB patients and TST(+) individuals responded well to the multistage-polyepitope in vitro. The in vivo immunogenicity and protective efficacy of the multistage-polyepitope were analyzed using HLA-DR3 transgenic mice lacking endogenous murine class II as a model. Immunization with the multistage-polyepitope adjuvanted with CpG generated high IgG levels as well as polyfunctional CD4(+) T-cells producing IFN-γ, TNF and IL-2, specific for these HLA-DR3-restricted epitopes. Importantly, multistage-polyepitope immunization reduced the number of bacilli in the lungs after Mtb challenge when administered as prophylactic vaccine. Given the extensive repertoire of potential Mtb antigens available for immune recognition, the data of our model demonstrate the potential of multistage-polyepitope vaccines to protect against TB.

Reduced frequency of memory T cells and increased Th17 responses in patients with active tuberculosis

Phenotypic and functional alterations in Mycobacterium tuberculosis T cell subsets have been reported in patients with active tuberculosis. A better understanding of these alterations will increase the knowledge about immunopathogenesis and also may contribute to the development of new diagnostics and prophylactic strategies. Here, the ex vivo phenotype of CD4(+) and CD8(+) T cells and the frequency and phenotype of gamma interferon (IFN-γ)- and interleukin 17 (IL-17)-producing cells elicited in short-term and long-term cultures following CFP-10 and purified protein derivative (PPD) stimulation were determined in noninfected persons (non-TBi), latently infected persons (LTBi), and patients with active tuberculosis (ATB). Phenotypic characterization of T cells was done based on the expression of CD45RO and CD27. Results show that ATB had a reduced frequency of circulating CD4(+) CD45RO(+) CD27(+) T cells and an increased frequency of CD4(+) CD45RO(-) CD27(+) T cells. ATB also had a higher frequency of circulating IL-17-producing CD4(+) T cells than did LTBi after PPD stimulation, whereas LTBi had more IFN-γ-producing CD4(+) T cells than did non-TBi. The phenotype of IFN-γ-producing cells at 24 h differs from the phenotype of IL-17-producing cells with no differences between LTBi and ATB. At 144 h, IFN-γ- and IL-17-producing cells were mainly CD45RO(+) CD27(+) T cells and they were more frequent in ATB. These results suggest that M. tuberculosis infection induces alterations in T cells which interfere with an adequate specific immune response.