Serological expression cloning and immunological evaluation of MTB48, a novel Mycobacterium tuberculosis antigen

Antibodies as key mediators of protection against Mycobacterium tuberculosis

Tuberculosis (TB) is caused by infection with the bacterial pathogen Mycobacterium tuberculosis (M.tb) in the respiratory tract. There was an estimated 10.6 million people newly diagnosed with TB, and there were approximately 1.3 million deaths caused by TB in 2022. Although the global prevalence of TB has remained high for decades and is an annual leading cause of death attributed to infectious diseases, only one vaccine, Bacillus Calmette-Guérin (BCG), has been approved so far to prevent/attenuate TB disease. Correlates of protection or immunological mechanisms that are needed to control M.tb remain unknown. The protective role of antibodies after BCG vaccination has also remained largely unclear; however, recent studies have provided evidence for their involvement in protection against disease, as biomarkers for the state of infection, and as potential predictors of outcomes. Interestingly, the antibodies generated post-vaccination with BCG are linked to the activation of innate immune cascades, providing further evidence that antibody effector functions are critical for protection against respiratory pathogens such as M.tb. In this review, we aim to provide current knowledge of antibody application in TB diagnosis, prevention, and treatment. Particularly, this review will focus on 1) The role of antibodies in preventing M.tb infections through preventing Mtb adherence to epithelium, antibody-mediated phagocytosis, and antibody-mediated cellular cytotoxicity; 2) The M.tb-directed antibody response generated after vaccination and how humoral profiles with different glycosylation patterns of these antibodies are linked with protection against the disease state; and 3) How antibody-mediated immunity against M.tb can be further explored as early diagnosis biomarkers and different detection methods to combat the global M.tb burden. Broadening the paradigm of differentiated antibody profiling and antibody-based detection during TB disease progression offers new directions for diagnosis, treatment, and preventative strategies. This approach involves linking the aforementioned humoral responses with the disease state, progression, and clearance.

A novel Mycobacterium Tuberculosis antigen, MTB48 enhances inflammatory response in LPS-induced RAW264.7 macrophage immune cells

Mtb (Mycobacterium tuberculosis) is a pathogenic bacterium that causes tuberculosis infection (TB). Mtb-secreted proteins have recently been investigated as virulence factors, as well as therapeutic and vaccine possibilities. The early-secreted antigen target MTB48 is one of these proteins that has been explored as a cocktail antigen in the serodiagnosis of active tuberculosis. However, there exists no information about the function or control of MTB48's inflammatory activity in macrophages at the site of inflammation. As a result, the goal of this research was to figure out what processes are involved in MTB48's function. MTB48 stimulated inflammation in LPS induced macrophages at both the protein and mRNA levels, which was interesting. MTB48 aided LPS induced IB phosphorylation and NF-κB translocation. MTB48 also led to the phosphorylation of MAPK signaling protein. These findings imply that MTB48 can enhance inflammatory activity via NF-κB and MAPK signaling by upregulating COX-2, iNOS, NO and PGE2. Many tuberculosis antigens have been tested for the development of rapid serological diagnosis. The results of this study suggest that MTB48 is a very high conservative antigen and is a major factor causing inflammatory reactions, suggesting that it can help control and diagnose tuberculosis.

Antibodies as clinical tools for tuberculosis

Tuberculosis (TB) is a leading cause of morbidity and mortality worldwide. Global research efforts to improve TB control are hindered by insufficient understanding of the role that antibodies play in protective immunity and pathogenesis. This impacts knowledge of rational and optimal vaccine design, appropriate diagnostic biomarkers, and development of therapeutics. Traditional approaches for the prevention and diagnosis of TB may be less efficacious in high prevalence, remote, and resource-poor settings. An improved understanding of the immune response to the causative agent of TB, Mycobacterium tuberculosis (Mtb), will be crucial for developing better vaccines, therapeutics, and diagnostics. While memory CD4+ T cells and cells and cytokine interferon gamma (IFN-g) have been the main identified correlates of protection in TB, mounting evidence suggests that other types of immunity may also have important roles. TB serology has identified antibodies and functional characteristics that may help diagnose Mtb infection and distinguish between different TB disease states. To date, no serological tests meet the World Health Organization (WHO) requirements for TB diagnosis, but multiplex assays show promise for improving the sensitivity and specificity of TB serodiagnosis. Monoclonal antibody (mAb) therapies and serum passive infusion studies in murine models of TB have also demonstrated some protective outcomes. However, animal models that better reflect the human immune response to Mtb are necessary to fully assess the clinical utility of antibody-based TB prophylactics and therapeutics. Candidate TB vaccines are not designed to elicit an Mtb-specific antibody response, but evidence suggests BCG and novel TB vaccines may induce protective Mtb antibodies. The potential of the humoral immune response in TB monitoring and control is being investigated and these studies provide important insight into the functional role of antibody-mediated immunity against TB. In this review, we describe the current state of development of antibody-based clinical tools for TB, with a focus on diagnostic, therapeutic, and vaccine-based applications.

Priming mycobacterial ESX-secreted protein B to form a channel-like structure

ESX-1 is a major virulence factor of Mycobacterium tuberculosis, a secretion machinery directly involved in the survival of the microorganism from the immune system defence. It disrupts the phagosome membrane of the host cell through a contact-dependent mechanism. Recently, the structure of the inner-membrane core complex of the homologous ESX-3 and ESX-5 was resolved; however, the elements involved in the secretion through the outer membrane or those acting on the host cell membrane are unknown. Protein substrates might form this missing element. Here, we describe the oligomerisation process of the ESX-1 substrate EspB, which occurs upon cleavage of its C-terminal region and is favoured by an acidic environment. Cryo-electron microscopy data shows that quaternary structure of EspB is conserved across slow growing species, but not in the fast growing M. smegmatis. EspB assembles into a channel with dimensions and characteristics suitable for the transit of ESX-1 substrates, as shown by the presence of another EspB trapped within. Our results provide insight into the structure and assembly of EspB, and suggests a possible function as a structural element of ESX-1.

Performance of a Tuberculosis Serologic Assay in Various Patient Populations

OBJECTIVES

Detection of the humoral response to diagnose active tuberculosis has had varied success. We sought to further characterize the performance of a commercial serologic assay (Active TBDetect IgG ELISA; InBios International, Seattle, WA), which had demonstrated promising results in prior studies.

METHODS

Blood specimens from patients with mycobacterial infections, autoimmune disorders, and documented nonmycobacterial infections were prospectively collected for testing by the Active TBDetect IgG ELISA. Pertinent medical records were reviewed.

RESULTS

The sensitivity of the InBios IgG ELISA for active tuberculosis cases was 54.1% (20/37). Reactivity occurred in 24.1% (14/58) of nontuberculous mycobacterium cases, 10.4% (7/67) of nonmycobacterial infections, 10.5% (11/105) of autoimmune disorder cases, 8.7% (8/92) of noninfected patients, 14.3% (1/7) of patients with latent tuberculosis, and 10.7% (3/28) of control pediatric cases. Overall specificity was 87.5% (288/329). Receiver operator curve analysis demonstrated an area under the curve of 0.74. Reactivity with nontuberculous mycobacterium infection occurred with Mycobacterium avium-intracellulare complex, Mycobacterium chelonae/abscessus complex, Mycobacterium simiae, and Mycobacterium gordonae and was positively associated with having a positive acid-fast bacilli smear.

CONCLUSIONS

This study confirmed the limitations of serodiagnosis for active tuberculosis, including poor sensitivity and increased reactivity with nontuberculous mycobacterium-positive patients.

Mycobacterium tuberculosis Secreted Proteins As Potential Biomarkers for the Diagnosis of Active Tuberculosis and Latent Tuberculosis Infection

Background

The detection of Mycobacterium tuberculosis (Mtb) specific human antibodies has been an important diagnostic aid in the diagnosis of tuberculosis (TB) cases with smear‐negative sputum samples, especially for the screening of high‐risk population. This study focused on the analysis and comparison of the four potential Mtb‐secreted proteins (ESAT6, CFP10, Ag85B, Hsp16.3) and the fusion protein Ag85B‐Hsp16.3 as new markers in the serodiagnosis between active TB and latent TB infection (LTBI).

Methods

These five recombinant proteins were produced and used in optimized ELISA to detect IgG serum antibodies against the four secreted proteins. The capacity of identifying infection was evaluated either in active TB patients or LTBI individuals, which was compared with the control groups consisting of hospitalized non‐TB individuals.

Results

The results showed that Ag85B‐Hsp16.3/ESAT6 and Hsp16.3/ESAT6 were the best‐associated antigens for serology diagnosis of the active TB and LTBI individuals because of their specificity, sensitivity, YI values, and positive rates, respectively. ELISA test demonstrated that 41.67% (25/60) of blood donors respond to Ag85B‐Hsp16.3/ESAT6. The consistency of this positive respond with clinical diagnosis almost reached 84% (21/25).

Conclusion

Thus, a combined test of multiple Mtb‐secreted proteins Ag85B, Hsp16.3, and ESAT6 may be the ascendant preliminary screening antigens for active TB or LTBI patients.

High-level expression of Mycobacterium tuberculosis protein EspB in E. coli and preparation of novel anti-EspB monoclonal antibodies

The ESX-1 secretion system plays a critical role in the virulence of Mycobacterium tuberculosis. The ESX-1 secreted protein EspB is cleaved close to its C-terminus during secretion and is necessary for inhibiting phagosome maturation. In this study, the EspB gene of M. tuberculosis H37Rv was cloned into the expression vector of pET21a(+) and was effectively expressed in Escherichia coli BL21(DE3). The expressed fusion protein existed as a soluble form in cell lysate and was first purified by a column packed with Ni-NTA resin and then a column packed with DEAE-Sepharose Fast Flow matrix. Using the purified protein to immunize BALB/c mice, five monoclonal antibodies were produced. As shown by ELISA and immunoblotting, the five respective antibodies could recognize the EspB protein. These monoclonal antibodies will provide powerful reagents for further investigation of EspB protein functions.

Serum antibody responses to 10 Mycobacterium tuberculosis proteins, purified protein derivative, and old tuberculin in natural and experimental tuberculosis in rhesus monkeys

Old tuberculin (OT) and purified protein derivative (PPD) are widely used for tuberculin skin testing (TST) in diagnosis of tuberculosis (TB) but often yield poor specificity and anergy in reaction. Therefore, it is necessary to develop new serological methods as a possible auxiliary diagnostic method for TB. In this study, we characterized the dynamic antibody responses of 10 purified recombinant antigens, PPD, and OT in rhesus monkeys experimentally infected with Mycobacterium tuberculosis and analyzed the time to antibody detection, antibody levels, and their association with the infectious doses. The antibodies were detected as early as 4 weeks after infection in response to 5 antigens (CFP10, CFP10-ESAT-6, U1, MPT64, and Ag85b). Antibodies against most of the other antigens were detected between 4 and 12 weeks after infection. The levels of antibodies were dose dependant. We further evaluated the serodiagnostic potential of these antigens by using indirect enzyme-linked immunosorbent assay in 71 TST-positive and 90 TST-negative serum samples from monkeys. For all 12 antigens, the median optical density values of TST-positive monkeys were statistically significantly higher than those of TST-negative monkeys (P < 0.001). Among those antigens, Ag85b and CFP10 showed higher diagnostic potential than others. A combination of results from Ag85b, the 38-kDa antigen (Ag38kDa), and Ag14kDa reaches a sensitivity of 95.77%, indicating that these antigens may be ideal cocktails in TB diagnosis.

Serodiagnosis of tuberculosis: due to shift track

Development of novel diagnostics for tuberculosis has so far been governed by the clinical requirement of improving the detection of patients with paucibacillary forms of the disease. For this aim, serological assays have been evaluated using several antigens, but were found insufficiently sensitive, because antibody production associates with the bacterial load of the disease. Consequently, detection of antibodies against a relatively small number of selected well-defined antigens has a much higher sensitivity for sputum smear-positive pulmonary disease in adult HIV-negative patients. They are the most active in generating and spreading aerosols containing live tubercle bacilli, but their detection is often delayed, thus perpetuating the transmission of the infection and disease in the population. High volume throughput serological screening of clinical suspects with mild clinical symptoms may help to achieve diagnosis earlier, than currently used procedures. Such expanded testing could be done more efficiently in laboratories, than at 'points-of-care' and at a lower cost than other tests. The feasibility of this approach towards reducing the delayed diagnosis of the most infectious cases of pulmonary tuberculosis needs to be ascertained in prospective diagnostic trials, in populations at a high risk. Reducing the transmission of tuberculosis is of key importance for achieving its continued decline and therefore it is proposed, that the aims of serological screening should shift from clinical to public health priorities.

Identification of Mycobacterium tuberculosis antigens of high serodiagnostic value

Tuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis, with several million new cases detected each year. Current methods of diagnosis are time-consuming and/or expensive or have a low level of accuracy. Therefore, new diagnostics are urgently needed to address the global tuberculosis burden and to improve control programs. Serological assays remain attractive for use in resource-limited settings because they are simple, rapid, and inexpensive and offer the possibility of detecting cases often missed by routine sputum smear microscopy. The aim of this study was to identify M. tuberculosis seroreactive antigens from a panel of 103 recombinant proteins selected as diagnostic candidates. Initial library screening by protein array analysis and enzyme-linked immunosorbent assay (ELISA) identified 42 antigens with serodiagnostic potential. Among these, 25 were novel proteins. The reactive antigens demonstrated various individual sensitivities, ranging from 12% to 78% (specificities, 76 to 100%). When the antigens were analyzed in combinations, up to 93% of antibody responders could be identified among the TB patients. Selected seroreactive proteins were used to design 3 new polyepitope fusion proteins. Characterization of these antigens by multiantigen print immunoassay (MAPIA) revealed that the vast majority of the TB patients (90%) produced antibody responses. The results confirmed that due to the remarkable variation in immune recognition patterns, an optimal multiantigen cocktail should be designed to cover the heterogeneity of antibody responses and thus achieve the highest possible test sensitivity.

Comparison of antibody responses to seventeen antigens from Mycobacterium tuberculosis

BACKGROUND

The detection of Mycobacteriumtuberculosis (MTB)-specific human antibodies has been an important diagnostic aid in the diagnosis of TB, especially for the bacterium-negative TB. The humoral antibody responses to different antigens of M.tuberculosis (MTB) are heterogeneous in active TB patients. Hence, detection of antibody responses to several MTB antigens may improve the sensitivity and specificity of serological diagnosis of active TB.

METHODS

Seventeen MTB antigens (38kD, 16kD, Ag85A, Ag85B, MPT32, MPT63, MPT64, Mtb39, MTB48, Mtb81, MTC28, Rv1009, ESAT6, CFP10, CFP10-ESAT6, katG, and LAM) were prepared by cloning, expression, and purification from E. coli, and their antigenicities were evaluated in the antibody responses of 210 active TB patients (103 sera from smear- or culture-positive patients, and 107 from smear- or culture-negative patients) and 192 healthy control (95 sera from purified protein derivative-negative healthy donors, and 97 sera from BCG-vaccinated individuals) by an enzyme-linked immunosorbent assay (ELISA).

RESULTS

The levels of antibodies against these antigens in bacterium-negative TB patients were significantly higher than that in healthy controls (p<0.001). The sensitivity with individual antigens to detect antibody responses ranged from 55.7 to 82.9%, with the specificity from 62.0 to 92.2%. Importantly, the sensitivity with five antigens (LAM, 38kD, katG, 16kD, and MPT63 or Mtb39) to detect antibody responses reached 69.5% (146/210), with a specificity of 91.1% (17/192), and the sensitivity with another five antigens (LAM, katG, 16kD, Mtb39 and Mtb81) to detect antibody responses reached 67.1% (141/210), with a specificity of 92.7% (14/192).

CONCLUSIONS

The combination of optimal multiple antigens to detect anti-MTB antibody responses increased the sensitivity and specificity. Therefore, detection of anti-MTB antibody responses with multiple antigens may be valuable in the clinical diagnosis of TB patients.

Humoral immune responses against the Mycobacterium tuberculosis 38-kilodalton, MTB48, and CFP-10/ESAT-6 antigens in tuberculosis

The diagnosis of smear-negative and culture-negative patients with active tuberculosis (TB) is challenging. The detection of Mycobacterium tuberculosis-specific antibodies in human sera has been an important diagnostic aid. However, detection of antibody responses to a single antigen usually has a low sensitivity for diagnosis of TB. In this study, humoral immune responses against recombinant M. tuberculosis 38-kDa, MTB48, and CFP-10/ESAT-6 (culture filtrate protein 10/6-kDa early secreted antigen target of M. tuberculosis) antigens in 250 Chinese TB patients and 260 healthy subjects were evaluated by an enzyme-linked immunosorbent assay (ELISA). The levels of antibodies against those antigens in TB patients, even in bacterium-negative ones, were significantly higher than those in healthy subjects (P < 0.001). The serodiagnostic sensitivities to detect antibodies against individual antigens, i.e., recombinant M. tuberculosis 38-kDa, MTB48, and CFP-10/ESAT-6 antigens, in TB patients were 73.6%, 73.2%, and 60.4%, respectively, with specificities of 85.4%, 77.7%, and 73.8%, respectively. Importantly, the sensitivity to positively detect humoral responses to one of the antigens increased further. Our data suggest that the humoral immune responses to M. tuberculosis antigens in TB patients are heterogeneous. The 38-kDa, MTB48, and CFP-10/ESAT-6 antigens can be used as the cocktail antigens in the serodiagnosis of active TB, especially for smear- or culture-negative TB cases.

Screening and assessing 11 Mycobacterium tuberculosis proteins as potential serodiagnostical markers for discriminating TB patients from BCG vaccinees

Purified protein derivative (PPD) skin tests often yield poor specificity, so that to develop new serological antigens for distinguishing between Mycobacterium tuberculosis infection and Bacille Calmette-Guerin (BCG) vaccination is a priority, especially for developing countries like China. We predicted the antigenicity for selected open reading frames (ORFs) based on the genome sequences of M. tuberculosis H37Rv and M. bovis BCG, as well as their functions and differences of expression under different stimulus. The candidate ORFs were cloned from H37Rv sequences and expressed as recombinant proteins in Escherichia coli. We studied the serodiagnostic potential of 11 purified recombinants by using enzyme-linked immunosorbent assay (ELISA) and involving a cohort composed of 58 TB patients (34 males and 24 females), 8 healthy volunteers and 50 PPD-negative individuals before and after BCG vaccination. For all the 11 antigens, the median OD values for the sera from TB patients were statistically significantly higher than those for PPD-negative individuals before or after BCG vaccination (P<0.01). They had at least 92% specificity in healthy controls and six seroantigens (Rv0251c, Rv1973, Rv2376c, Rv2537c, Rv2785c and Rv3873A) were never reported with seroantigenicities previously. Thus the approach combining comparative genomics, bioinformatics and ELISA techniques can be employed to identify new seroantigens distinguishing M. tuberculosis infection from BCG vaccination.

Performance of purified antigens for serodiagnosis of pulmonary tuberculosis: a meta-analysis

Serological antibody detection tests for tuberculosis may offer the potential to improve diagnosis. Recent meta-analyses have shown that commercially available tests have variable accuracies and a limited clinical role. We reviewed the immunodiagnostic potential of antigens evaluated in research laboratories (in-house) for the serodiagnosis of pulmonary tuberculosis and conducted a meta-analysis to evaluate the performance of comparable antigens. Selection criteria included the participation of at least 25 pulmonary tuberculosis patients and the use of purified antigens. Studies evaluating 38 kDa, MPT51, malate synthase, culture filtrate protein 10, TbF6, antigen 85B, alpha-crystallin, 2,3-diacyltrehalose, 2,3,6-triacyltrehalose, 2,3,6,6'-tetraacyltrehalose 2'-sulfate, cord factor, and TbF6 plus DPEP (multiple antigen) were included in the meta-analysis. The results demonstrated that (i) in sputum smear-positive patients, sensitivities significantly >or=50% were provided for recombinant malate synthase (73%; 95% confidence interval [CI], 58 to 85) and TbF6 plus DPEP (75%; 95% CI, 50 to 91); (ii) protein antigens achieved high specificities; (iii) among the lipid antigens, cord factor had the best overall performance (sensitivity, 69% [95% CI, 28 to 94]; specificity, 91% [95% CI, 78 to 97]); (iv) compared with the sensitivities achieved with single antigens (median sensitivity, 53%; range, 2% to 100%), multiple antigens yielded higher sensitivities (median sensitivity, 76%; range, 16% to 96%); (v) in human immunodeficiency virus (HIV)-infected patients who are sputum smear positive, antibodies to several single and multiple antigens were detected; and (vi) data on seroreactivity to antigens in sputum smear-negative or pediatric patients were insufficient. Potential candidate antigens for an antibody detection test for pulmonary tuberculosis in HIV-infected and -uninfected patients have been identified, although no antigen achieves sufficient sensitivity to replace sputum smear microscopy. Combinations of select antigens provide higher sensitivities than single antigens. The use of a case-control design with healthy controls for the majority of studies was a limitation of the review. Efforts are needed to improve the methodological quality of tuberculosis diagnostic studies.

Improved diagnosis of tuberculosis in HIV-positive patients using RD1-encoded antigen CFP-10

OBJECTIVE

The present study was aimed at determining the serodiagnostic potential of 38-kDa (Rv0934, Mycobacterium tuberculosis complex-specific antigen) and CFP-10 (Rv3874, RD1 antigen) antigens among HIV-positive and HIV-negative patients with pulmonary TB.

METHODS

The diagnostic potential of native 38-kDa (n38-kDa) and recombinant CFP-10 (rCFP-10) antigens was ascertained in terms of sensitivity and specificity using an indirect ELISA. The study included 508 HIV-seronegative TB patients (TB), 54 HIV-seropositive TB patients (HIV-TB), 30 HIV-positive patients without TB (HIV), and 256 controls.

RESULTS

In HIV-TB, the sensitivities for individual antigens ranged from 14.8% to 31.5% and the specificity was >98% for IgG. When IgA results were added to IgG, the sensitivity increased to 25.9% for 38-kDa and 57.4% for CFP-10; specificity changed to 97.5% for 38-kDa and 98.1% for CFP-10. The combined results of both the antigens gave 59.3% sensitivity and 95.6% specificity. In TB, the sensitivity was 82.8% when the antigen results were combined. None of the HIV-infected controls showed positivity for IgG to either of the two antigens.

CONCLUSION

Use of CFP-10 enhances the sensitivity of 38-kDa, and therefore the 38-kDa and CFP-10 antigen combination can be a diagnostic marker in HIV-TB.

Improved diagnosis of pulmonary tuberculosis by detection of antibodies against multiple Mycobacterium tuberculosis antigens

Two secreted antigens (38 and 30 kDa) and 1 cytosolic antigen (16 kDa) were purified in our laboratory from Mycobacterium tuberculosis culture filtrate and cytosol using chromatographic/electrophoretic methods. One recombinant antigen (27 kDa, MPT51) expressed in Escherichia coli was also isolated. All the 4 antigens were tested individually for detection of serum IgG, IgA, and IgM (a total of 476 sera from 5 groups) by indirect enzyme-linked immunosorbent assay. Keeping the well-reported 38 kDa as the main candidate, the usefulness of the other antigens, which may add to the test positivity in cases not diagnosed by 38 kDa, was analyzed. The individual antigens ranged in their sensitivity from 57% to 67% (IgG). Addition of other antigen results, with that of 38 kDa, offered a sensitivity of 91% in smear- and culture-positive tuberculosis (TB), 78% in smear-negative culture-confirmed TB, and 97% specificity in normal healthy subjects. IgG antibody to multiple antigens (38, 30, and 16 kDa) may be a sensitive, specific, rapid, and cost-effective test to rule-in clinical suspicion of pulmonary TB.

A unique Mycobacterium ESX-1 protein co-secretes with CFP-10/ESAT-6 and is necessary for inhibiting phagosome maturation

The ESX-1 secretion system plays a critical role in the virulence of Mycobacterium tuberculosis and M. marinum. To date, three proteins are known to be secreted by ESX-1 and necessary for virulence, two of which are CFP-10 and ESAT-6. The ESX-1 secretion and the virulence mechanisms are not well understood. In this study, we have examined the M. marinum secretomes and identified four proteins specific to ESX-1. Two of those are CFP-10 and ESAT-6, and the other two are novel: MM1553 (homologous to Rv3483c) and Mh3881c (homologous to Rv3881c). We have shown that Mh3881c, CFP-10 and ESAT-6 are co-dependent for secretion. Mh3881c is being cleaved at close to the C-terminus during secretion, and the C-terminal portion is critical to the co-dependent secretion, the ESAT-6 cellular levels, and interaction with ESAT-6. The co-dependent secretion is required for M. marinum intracellular growth in macrophages, where the Mh3881c C-terminal portion plays a critical role. The role of the co-dependent secretion in intracellular growth correlates with its role in inhibiting phagosome maturation. Both the secretion and the virulence defects of the Mh3881c mutant are complemented by Mh3881c or its M. tuberculosis homologue Rv3881c, suggesting that in M. tuberculosis, Rv3881c has similar functions.

Progress in serodiagnosis of Mycobacterium tuberculosis infection

One-third of the world population is estimated to have Mycobacterium tuberculosis infection. Accurate and timely identification of infected individuals is critical for treatment and control. The current diagnostic methods lack the desired sensitivity and specificity, require sophisticated equipment and skilled workforce or take weeks to yield results. Diagnosis of extrapulmonary TB, TB-HIV co-infection, childhood TB and sputum smear-negative pulmonary TB pose serious challenges. Interest in developing serodiagnostic methods is increasing because detection of antibody is rapid, simple and relatively inexpensive, and does not require a living cell for detection. Three types of tests, namely screening tests to overcome diagnostic delay, specific tests for diagnosis of extrapulmonary TB and other bacteriologically negative cases, and tests for vaccine-induced immunity need critical consideration. Several factors must be considered to develop serodiagnostic methods for TB. Antigen recognition by infected individuals is highly heterogeneous due to stage of disease, differences in HLA types, strain of the bacilli, health of the patient and bacillary load. With advances in molecular biological techniques, a number of novel antigens have been identified. Some of these antigens have proven valuable in detecting specific antibodies in some of the most challenging TB patients. The best example is a fusion protein containing several M. tuberculosis proteins (e.g. CFP-10, MTB8, MTB48, MTB81 and the 38-kDa protein) which showed encouraging results in detecting antibodies in sera of patients, including TB-HIV co-infection. This review presents progress made in the serodiagnosis of TB during the last decade.

[Innovative strategies to diagnose and monitor tuberculosis patients]

Mycobacterium tuberculosis is the single most deadly microorganism worldwide. A third of the world population is thought to have latent tuberculosis and approximately 2 million people die of the disease each year. Short and closely supervised treatment regimens are needed, but it is also essential to develop new strategies to ensure prompt diagnosis of the disease. In particular, cheap methods are needed to tackle tuberculosis from a population perspective. The present article reviews the advances in immunology and molecular strategies for epidemiological diagnosis and monitoring of tuberculosis patients.

PPE protein (Rv3425) from DNA segment RD11 of Mycobacterium tuberculosis: a potential B-cell antigen used for serological diagnosis to distinguish vaccinated controls from tuberculosis patients

Proteins encoded by a 9.5-kb DNA segment, termed the region of difference (RD), of Mycobacterium tuberculosis have been demonstrated to be important in bacterial virulence, vaccine development and the design of diagnostic reagents. This study evaluated the immunogenic properties of Rv3425, a member of the PPE family of proteins, encoded by an open reading frame found in RD11 of M. tuberculosis, in comparison with two other well-known antigens, the early secreted antigen target 6 (ESAT-6) and the 10-kDa culture filtrate protein (CFP-10). RT-PCR demonstrated that Rv3425 mRNA is expressed in liquid culture by M. tuberculosis H37Rv. When tested in a conventional ELISA in the form of a His-tagged recombinant protein, Rv3425 revealed a statistically significant antigenic distinction between healthy bacille Calmette-Guérin (BCG)-vaccinated controls and tuberculosis (TB) patients (p <0.0001). The anti-IgG response to recombinant Rv3425 was almost equal to that for CFP-10, and was higher than that for ESAT-6. The results highlight the immunosensitive and immunospecific nature of Rv3425, which shows promise for use in the serodiagnosis of TB.

ML0405 and ML2331 are antigens of Mycobacterium leprae with potential for diagnosis of leprosy

Despite the success of multidrug therapy in reducing the number of registered leprosy cases worldwide, evidence suggests that Mycobacterium leprae continues to be transmitted. A serological diagnostic test capable of identifying and allowing treatment of early-stage disease could reduce transmission and prevent the onset of the disability, a common complication of the disease in later stages. Serological diagnosis based on antibody recognition of phenolic glycolipid I (PGL-I) cannot reliably identify individuals with lower bacterial indices (BI). One strategy that might improve this situation is the provision of highly specific serological antigens that may be combined with PGL-I to improve the sensitivity of diagnosis. Using serological expression cloning with a serum pool of untreated lepromatous leprosy (LL) patients, we identified 14 strongly reactive M. leprae proteins, 5 of which were previously unstudied. We present results suggesting that two of these proteins, ML0405 and ML2331, demonstrate the ability to specifically identify LL/borderline lepromatous (BL) patients on the basis of immunoglobulin G (IgG) reactivity. In a household contact study, LL index cases were identified on the basis of this reactivity, while household contacts of these patients demonstrated undetectable reactivity. At a serum dilution of 1:800, suitable to reduce background PGL-I IgM reactivity, two BL patients with a BI of <4 showed anti-human polyvalent immunoglobulin G, A, and M reactivity measured with a combination of ML0405, ML2331, and natural disaccharide O-linked human serum albumin (NDOHSA) (synthetic PGL-I) that was markedly higher than IgM reactivity to NDOHSA alone. We suggest that ML0405 and ML2331 may have utility in serological leprosy diagnosis.

Evaluation of the Mtb72F polyprotein vaccine in a rabbit model of tuberculous meningitis

Using a rabbit model of tuberculous meningitis, we evaluated the protective efficacy of vaccination with the recombinant polyprotein Mtb72F, which is formulated in two alternative adjuvants, AS02A and AS01B, and compared this to vaccination with Mycobacterium bovis bacillus Calmette-Guérin (BCG) alone or as a BCG prime/Mtb72F-boost regimen. Vaccination with Mtb72F formulated in AS02A (Mtb72F+AS02A) or Mtb72F formulated in AS01B (Mtb72F+AS01B) was protective against central nervous system (CNS) challenge with Mycobacterium tuberculosis H37Rv to an extent comparable to that of vaccination with BCG. Similar accelerated clearances of bacilli from the cerebrospinal fluid, reduced leukocytosis, and less pathology of the brain and lungs were noted. Weight loss of infected rabbits was less extensive for Mtb72F+AS02A-vaccinated rabbits. In addition, protection against M. tuberculosis H37Rv CNS infection afforded by BCG/Mtb72F in a prime-boost strategy was similar to that by BCG alone. Interestingly, Mtb72F+AS01B induced better protection against leukocytosis and weight loss, suggesting that the polyprotein in this adjuvant may boost immunity without exacerbating inflammation in previously BCG-vaccinated individuals.

Expression and purification of recombinant antigens of Mycobacterium tuberculosis for application in serodiagnosis

Accurate diagnosis is essential for the treatment, prevention, and control of tuberculosis. Poor specificity of the tuberculin skin test in BCG-vaccinated populations and constraints to implementation of PCR and CMI-based diagnostic assays in developing countries warrant development of easy-to perform robust serological tests. Due to great heterogeneity in humoral response in TB patients, it will be necessary to include several antigens in any diagnostic assay to achieve useful levels of sensitivity and specificity. This needs production of recombinants, soluble versions of mycobacterial antigens in high yields. We have cloned, expressed, and purified a number of mycobacterial proteins in Escherichia coli. This paper describes the expression and purification of four promising sero-reactive proteins namely, ESAT6, CFP10, MTC28, and 14-kDa antigen of Mycobacterium tuberculosis. The protocol involves regulated and slow expression of proteins by using a T7 promoter-based expression vector for obtaining soluble protein followed by a three-step column chromatography procedure employing media with high binding capacity and flow characteristics. The yields of these proteins obtained were several folds higher than previously reported. The purified proteins were useful in detecting antibodies in sera of TB patients (smear positive, smear negative, and extra-pulmonary categories) and in combination with other immunodominant antigens will be useful in increasing the sensitivity to detect M. tuberculosis specific antibodies.

Epitope-specific antibody levels demonstrate recognition of new epitopes and changes in titer but not affinity during treatment of tuberculosis

Antibody levels rise during treatment of tuberculosis. This study examined when this rise occurred, whether there was recognition of new antigen binding sites (epitopes) on the same or different antigens, and how long specific antibody persisted. Forty patients with smear-positive pulmonary tuberculosis provided serum before and during treatment. Antibody levels were measured using a monoclonal antibody competition assay to epitopes restricted to the Mycobacterium tuberculosis complex and an enzyme-linked immunosorbent assay for lipoarabinomannan. Significant increases in antibody levels were apparent after 7 days of treatment. Five samples (12.5%) had positive titers to all epitopes at the start of treatment, and this increased to 23 (58%) during treatment. Antibody to epitopes with the poorest sensitivity (the TB23 epitope of the 19-kDa antigen and the TB78 epitope of hsp65) showed the greatest increases after treatment. Antibody to these two epitopes was also absent in some patients with relapsed tuberculosis until after treatment. Antibody titers showed a biphasic response, with a fall at 2 to 3 months of treatment. Sera from two patients showed changes in the affinity of epitope-specific antibody during treatment, whereas the majority did not. Those infected with isoniazid-resistant strains of M. tuberculosis showed a late rise in antibody. Antibody to the TB68 epitope of the 16-kDa alpha-crystallin homolog was short-lived, but it recurred with bacteriological relapse during treatment. Positive antibody titers persisted for at least 3 to 18 months after treatment. Diagnostic tests for tuberculosis should be evaluated using only pretreatment sera. Delayed antigenic recognition could be due to active suppression and/or failure to engage internal antigens of M. tuberculosis.

Assessing the serodiagnostic potential of 35 Mycobacterium tuberculosis proteins and identification of four novel serological antigens

Improved diagnostic reagents are needed for the detection of Mycobacterium tuberculosis infections, and the development of a serodiagnostic test would complement presently available diagnostic methods. The aim of the present study was to identify novel serological targets for use for the future serodiagnosis of tuberculosis (TB). We cloned and expressed 35 M. tuberculosis proteins as recombinant proteins in Escherichia coli and analyzed their serodiagnostic potentials. By a two-step selection process, four superior seroantigens, TB9.7, TB15.3, TB16.3, and TB51, were identified, none of which has been described before. The four novel antigens were tested with panels of sera from smear-positive and smear-negative TB patients from areas both where TB is endemic and where TB is not endemic, with recognition frequencies ranging from 31 to 93% and with a specificity of at least 97%. The single most potent antigen was TB16.3, which had a sensitivity of 48 to 55% with samples from Danish resident TB patients and a sensitivity of 88 to 98% with samples from African TB patients. Importantly, the TB16.3 and the TB9.7 antigens were recognized by more than 85% of the samples from TB patients coinfected with human immunodeficiency virus, a patient group for which it is in general difficult to detect M. tuberculosis-specific antibodies.

Differential Immune Responses and Protective Efficacy Induced by Components of a Tuberculosis Polyprotein Vaccine, Mtb72F, Delivered as Naked DNA or Recombinant Protein

Key Ags of Mycobacterium tuberculosis initially identified in the context of host responses in healthy purified protein derivative-positive donors and infected C57BL/6 mice were prioritized for the development of a subunit vaccine against tuberculosis. Our lead construct, Mtb72F, codes for a 72-kDa polyprotein genetically linked in tandem in the linear order Mtb32(C)-Mtb39-Mtb32(N). Immunization of C57BL/6 mice with Mtb72F DNA resulted in the generation of IFN-gamma responses directed against the first two components of the polyprotein and a strong CD8(+) T cell response directed exclusively against Mtb32(C). In contrast, immunization of mice with Mtb72F protein formulated in the adjuvant AS02A resulted in the elicitation of a moderate IFN-gamma response and a weak CD8(+) T cell response to Mtb32c. However, immunization with a formulation of Mtb72F protein in AS01B adjuvant generated a comprehensive and robust immune response, resulting in the elicitation of strong IFN-gamma and Ab responses encompassing all three components of the polyprotein vaccine and a strong CD8(+) response directed against the same Mtb32(C) epitope identified by DNA immunization. All three forms of Mtb72F immunization resulted in the protection of C57BL/6 mice against aerosol challenge with a virulent strain of M. tuberculosis. Most importantly, immunization of guinea pigs with Mtb72F, delivered either as DNA or as a rAg-based vaccine, resulted in prolonged survival (>1 year) after aerosol challenge with virulent M. tuberculosis comparable to bacillus Calmette-Guérin immunization. Mtb72F in AS02A formulation is currently in phase I clinical trial, making it the first recombinant tuberculosis vaccine to be tested in humans.

Comparison of antibody responses to a potential combination of specific glycolipids and proteins for test sensitivity improvement in tuberculosis serodiagnosis

The humoral response to different proteinaceous antigens of Mycobacterium tuberculosis is heterogeneous among patients with active disease, and this has originated in the proposal to use a combination of several specific antigens to find an efficient serodiagnostic test for tuberculosis (TB). However, to date, comparisons of antibody responses to several antigens in the same population have been carried out without consideration of antigenic cell wall glycolipids. In the present study the presence of immunoglobulin G (IgG), IgM, and IgA antibodies to M. tuberculosis glycolipids (sulfolipid I, diacyltrehaloses, triacyltrehaloses, and cord factor) was compared with the response to four commercially available tests based on the 38-kDa protein mixed with the 16-kDa protein or lipoarabinomannan. Fifty-two serum samples from TB patients and 83 serum samples from control individuals (48 healthy individuals and 35 non-TB pneumonia patients) were studied. Three relevant results were obtained. (i) Smear-negative TB patients presented low humoral responses, but the sera which did react principally showed IgA antibodies to some glycolipidic antigens. (ii) TB patients exhibit heterogeneous humoral responses against glycolipidic antigens. (iii) Finally, test sensitivity is improved (from 23 to 62%) when IgG and IgA antibodies are detected together in tests based on different antigens (proteins and glycolipids). We conclude that it is possible to include glycolipidic antigens in a cocktail of specific antigens from M. tuberculosis to develop a serodiagnostic test.

Potential serological use of a recombinant protein that is a replica of a Mycobacterium tuberculosis protein found in the urine of infected mice

The recent availability of numerous well-characterized Mycobacterium tuberculosis recombinant proteins has revived interest in the serological diagnosis of tuberculosis. Several promising results have been reported, particularly when more than one antigen is used in the test. However, thus far these antigens have not been used in routine diagnostic tests because they lack sufficient sensitivity. In addition, with the exception of one antigen, most recombinant M. tuberculosis proteins do not identify the majority of tuberculosis patients coinfected with human immunodeficiency virus (HIV). Here, we report a newer M. tuberculosis protein that is a promising candidate for increasing the sensitivity of the serological tests, in particular for patients coinfected with HIV. The protein was found in the urine of mice during the early stages of infection with M. tuberculosis (10 to 14 days), thus suggesting that the antigen is abundantly released during the in vivo growth of the mycobacterium. Reverse genetics was used to produce the recombinant protein, which we named U1 (for urine protein 1). Using a conventional enzyme-linked immunosorbent assay (ELISA), antibody to U1 could be detected in 60% of patients with pulmonary tuberculosis with no signs of coinfection with HIV (n = 83). Conversely, anti-U1 antibody was detected in 87% of the sera from tuberculosis patients coinfected with HIV (n = 47). Out of 12 HIV-infected nontuberculosis patients' sera, 9 did not react with U1 and three sera gave borderline ELISA signals (signal/cutoff of < or =1.75). These results suggest that the high efficiency of U1 in identifying tuberculosis patients coinfected with HIV may be related to abundant release of this protein during the initial phase of the HIV coinfection. The immediate availability of the antigen at a time point in which the patient's immune system is still competent would lead to a secondary immune response to U1 that persists for months in the patient's serum.

Human T-cell responses to the RD1-encoded protein TB27.4 (Rv3878) from Mycobacterium tuberculosis

In recent years, there has been considerable focus on the discovery and characterization of proteins derived from Mycobacterium tuberculosis leading to the identification of a number of candidate antigens for use in vaccine development or for diagnostic purposes. Previous experiments have demonstrated an important immunological role for proteins encoded by the RD1 region, which is absent from all strains of bacillus Calmette-Guérin (BCG) but present in the genomes of virulent M. bovis and M. tuberculosis. Herein, we have studied human T-cell responses to the antigen encoded by the putative open reading frame (rv3878) of the RD1 region. Immunoblot analysis revealed that rv3878 was expressed and the native protein was designated TB27.4. Immunological evaluations demonstrate that TB27.4 elicits a prominent immune response in human tuberculosis patients with a dominant region in the C-terminal part of the molecule. In contrast, very limited responses were seen in M. bovis BCG-vaccinated donors. This study therefore emphasizes the diagnostic potential of proteins encoded by the RD1 region.

PPE antigen Rv2430c of Mycobacterium tuberculosis induces a strong B-cell response

The variation in sequence and length in the C-terminal region among members of the unique PE (Pro-Glu) and PPE (Pro-Pro-Glu) protein families of Mycobacterium tuberculosis is a likely source of antigenic variation, giving rise to the speculation that these protein families could be immunologically important. Based on in silico analysis, we selected a hypothetical open reading frame (ORF) encoding a protein belonging to the PPE family and having epitopes with predictably higher antigenic indexes. Reverse transcriptase PCR using total RNA extracted from in vitro-cultured M. tuberculosis H37Rv generated an mRNA product corresponding to this gene, indicating the expression of this ORF (Rv2430c) at the mRNA level. Recombinant protein expressed in Escherichia coli was used to screen the sera of M. tuberculosis-infected patients, as well as those of clinically healthy controls (n = 10), by enzyme-linked immunosorbent assay. The panel of patient sera comprised sera from fresh infection cases (category 1; n = 32), patients with relapsed tuberculosis (category 2; n = 30), and extrapulmonary cases (category 3; n = 30). Category 2 and 3 sera had strong antibody responses to the PPE antigen, equal to or higher than those to other well-known antigens, such as Hsp10 or purified protein derivative (PPD). However, a higher percentage of patients belonging to category 1, as opposed to clinically healthy controls, showed stronger antibody response against the PPE protein when probed with anti-immunoglobulin M (IgM) (71 versus 37.5%) or anti-IgG (62.5 versus 28.12%). Our results reveal that this PPE ORF induces a strong B-cell response compared to that generated by M. tuberculosis Hsp10 or PPD, pointing to the immunodominant nature of the protein.

Use of multiepitope polyproteins in serodiagnosis of active tuberculosis

Screening of genomic expression libraries from Mycobacterium tuberculosis with sera from tuberculosis (TB) patients or rabbit antiserum to M. tuberculosis led to the identification of novel antigens capable of detecting specific antibodies to M. tuberculosis. Three antigens, Mtb11 (also known as CFP-10), Mtb8, and Mtb48, were tested together with the previously reported 38-kDa protein, in an enzyme-linked immunosorbent assay (ELISA) to detect antibodies in TB patients. These four proteins were also produced as a genetically fused polyprotein, which was tested with two additional antigens, DPEP (also known as MPT32) and Mtb81. Sera from individuals with pulmonary and extrapulmonary TB, human immunodeficiency virus (HIV)-TB coinfections, and purified protein derivative (PPD)-positive and PPD-negative status with no evidence of disease were tested. In samples from HIV-negative individuals, the ELISA detected antibodies in >80% of smear-positive individuals and >60% smear-negative individuals, with a specificity of approximately 98%. For this group, smears detected 81.6% but a combination of smear and ELISA had a sensitivity of approximately 93%. The antigen combination detected a significant number of HIV-TB coinfections as well as antibodies in patients with extrapulmonary infections. Improved reactivity in the HIV-TB group was observed by including the antigen Mtb81 that was identified by proteomics. The data indicate that the use of multiple antigens, some of which are in a single polyprotein, can be used to facilitate the development of a highly sensitive test for M. tuberculosis antibody detection.