Progress in serodiagnosis of Mycobacterium tuberculosis infection

Mycobacterium tuberculosis pili (MTP), a putative biomarker for a tuberculosis diagnostic test

Novel biomarkers are urgently needed for point of care TB diagnostics. In this study, we investigated the potential of the pilin subunit protein encoded by the mtp gene as a diagnostic biomarker. BLAST analysis of the mtp gene on published genome databases, and amplicon sequencing were performed in Mycobacterium tuberculosis Complex (MTBC) strains and other organisms. The protein secondary structure of the amino acid sequences of non-tuberculous Mycobacteria that partially aligned with the mtp sequence was analysed with PredictProtein software. The mtp gene and corresponding amino acid sequence of MTBC were 100% homologous with H37Rv, in contrast to the partial alignment of the non-tuberculous Mycobacteria. The mtp gene was present in all 91 clinical isolates of MTBC. Except for 2 strains with point mutations, the sequence was 100% conserved among the clinical strains. The mtp gene could not be amplified in all non-tuberculous Mycobacteria and respiratory organisms. The predicted MTP protein structure of Mycobacterium avium, Mycobacterium ulcerans and Mycobacterium abscessus differed significantly from that of the M. tuberculosis, which was similar to Mycobacterium marinum. The absence of the mtp gene in non-tuberculous Mycobacteria and other respiratory bacteria suggests that its encoded product, the pilin subunit protein of M. tuberculosis may be a suitable marker for a point of care TB test.

Plasma antibody profiles in non-human primate tuberculosis

BACKGROUND

Tuberculosis (TB) in non-human primates (NHPs) is highly contagious, requiring efficient identification of animals infected with Mycobacterium tuberculosis. Tuberculin skin test is usually used but lacks desirable sensitivity/specificity and efficiency.

METHODS

We aimed to develop an immunoassay for plasma antibodies against M. tuberculosis. A key challenge is that not all infected animals contain antibodies against the same M. tuberculosis antigen. Therefore, a multiplex panel of 28 antigens (Luminex(®) -Platform) was developed.

RESULTS

Data revealed antibodies against eight antigens (Rv3875, Rv3875-Rv3874 fusion, Rv3874, Rv0934, Rv3881, Rv1886c, Rv2031, Rv3841) in experimentally infected (M. tuberculosis strains: Erdman and H37Rv) NHPs (rhesus and cynomolgus macaques). In a naturally acquired M. tuberculosis infection, rhesus macaques (n = 15) with lung TB pathology (n = 10) contained antibodies to five additional antigens (Rv0831, Rv2220, Rv0054, Rv1099, and Rv0129c).

CONCLUSIONS

Results suggest that this user-friendly and easily implementable multiplex panel, containing 13 M. tuberculosis antigens, may provide a high-throughput alternative for NHP TB screening.

Exploration of novel cellular and serological antigen biomarkers in the ORFeome of Mycobacterium tuberculosis

Increasing evidence demonstrates that antigen-specific cellular and humoral immunity plays an indispensable role in protection against Mycobacterium tuberculosis infection. Antigen is a key element in the development of a successful diagnostic method and vaccine. However, few antigens are available, and a systemic study on M. tuberculosis ORFeome-based antigen screening is still lacking. In the current study, a genome-wide examination was conducted on high-throughput M. tuberculosis encoding proteins and novel antigens were identified via a comprehensive investigation of serological and antigen-specific cellular responses. The serological immunoglobulin G level of each protein was detected in pooled sera from 200 pulmonary tuberculosis patients by means of semi-quantitative Western blot. Of the 1,250 detected proteins, 29 were present at a higher level relative to the commercialized 38-kDa protein. Furthermore, the top 12 of the 29 proteins had not been previously reported, and their antigenicity was validated in serum from each individual patient. Results confirmed that the 12 proteins displayed nearly identical immunoglobulin G antibody levels in patients with pulmonary and extrapulmonary tuberculosis. Antigen-specific cellular interferon-γ secretion was also evaluated using a cell-based ELISPOT assay. Thirty-four of the proteins were able to induce positive interferon-γ production by peripheral blood mononuclear cells from pulmonary tuberculosis patients as judged by positive (commercial ESAT-6 antigen) and negative controls. The top 4 candidates out of the 34 proteins displayed good accuracy ranging from 50% to 80% compared with the commercial ESAT-6 antigen. Subsequent epitope examination confirmed that a pool of peptides, including a 25aa peptide from Rv1198, demonstrated significant tuberculosis-specific cellular interferon-γ production. Overall, the current study draws significant attention to novel M. tuberculosis antigens, many of which have not been previously reported. This discovery provides a large amount of useful information for the diagnosis of tuberculosis and the development of vaccines to provide protection against tuberculosis.

Immunologic evaluation and validation of methods using synthetic peptides derived from Mycobacterium tuberculosis for the diagnosis of tuberculosis infection

The goal of this study was to demonstrate the usefulness of an enzyme-linked immunosorbent assay (ELISA) for the serodiagnosis of pulmonary tuberculosis (PTB) and extrapulmonary TB (EPTB). This assay used 20 amino acid-long, non-overlapped synthetic peptides that spanned the complete Mycobacterium tuberculosis ESAT-6 and Ag85A sequences. The validation cohort consisted of 1,102 individuals who were grouped into the following five diagnostic groups: 455 patients with PTB, 60 patients with EPTB, 40 individuals with non-EPTB, 33 individuals with leprosy and 514 healthy controls. For the PTB group, two ESAT-6 peptides (12033 and 12034) had the highest sensitivity levels of 96.9% and 96.2%, respectively, and an Ag85A-peptide (29878) was the most specific (97.4%) in the PTB groups. For the EPTB group, two Ag85A peptides (11005 and 11006) were observed to have a sensitivity of 98.3% and an Ag85A-peptide (29878) was also the most specific (96.4%). When combinations of peptides were used, such as 12033 and 12034 or 11005 and 11006, 99.5% and 100% sensitivities in the PTB and EPTB groups were observed, respectively. In conclusion, for a cohort that consists entirely of individuals from Venezuela, a multi-antigen immunoassay using highly sensitive ESAT-6 and Ag85A peptides alone and in combination could be used to more rapidly diagnose PTB and EPTB infection.

Antigen 85A and mycobacterial DNA-binding protein 1 are targets of immunoglobulin G in individuals with past tuberculosis

Development of accurate methods for predicting progression of tuberculosis (TB) from the latent state is recognized as vitally important in controlling TB, because a majority of cases develop from latent infections. Past TB that has never been treated has a higher risk of progressing than does latent Mycobacterium tuberculosis infection in patients who have previously received treatment. Antibody responses against 23 kinds of M. tuberculosis proteins in individuals with past TB who had not been medicated were evaluated. These individuals had significantly higher concentrations of antibodies against Antigen 85A and mycobacterial DNA-binding protein 1 (MDP1) than did those with active TB and uninfected controls. In addition, immunohistochemistry revealed colocalization of tubercle bacilli, antigen 85 and MDP1 inside tuberculous granuloma lesions in an asymptomatic subject, showing that M. tuberculosis in lesions expresses both antigen 85 and MDP1. Our study suggests the potential usefulness of measuring antibody responses to antigen 85A and MDP1 for assessing the risk of TB progression.

Antibody responses to mycobacterial antigens in children with tuberculosis: challenges and potential diagnostic value

The identification of easily detectable biomarkers for active tuberculosis (TB) is a global health priority. Such biomarkers would be of particular value in childhood TB, which poses greater diagnostic challenges than adult TB. Serum antibodies can be detected by simple formats that provide extremely rapid results. However, attempts to develop accurate serodiagnostic tests for TB have been unsuccessful. Whereas antibody responses to mycobacterial antigens in adult TB have been studied extensively and reviewed, the same cannot be said for serologic data in pediatric populations. Here we appraise studies on serological responses in childhood TB and discuss findings and limitations in the context of the developing immune system, the age range, and the spectrum of TB manifestations. We found that the antibody responses to mycobacterial antigens in childhood TB can vary widely, with sensitivities and specificities ranging from 14% to 85% and from 86% to 100%, respectively. We conclude that the limitations in serodiagnostic studies of childhood TB are manifold, thereby restricting the interpretation of currently available data. Concerns about the methodology used in published studies suggest that conclusions about the eventual value of serodiagnosis cannot be made at this time. However, the available data suggest a potential adjunctive value for serology in the diagnosis of childhood TB. Despite the difficulties noted in this field, there is optimism that the application of novel antigens and the integration of those factors which contribute to the serological responses in childhood TB can lead to useful future diagnostics.

Immunodiagnosis of tuberculosis: a dynamic view of biomarker discovery

Infection with Mycobacterium tuberculosis causes a variety of clinical conditions ranging from life-long asymptomatic infection to overt disease with increasingly severe tissue damage and a heavy bacillary burden. Immune biomarkers should follow the evolution of infection and disease because the host immune response is at the core of protection against disease and tissue damage in M. tuberculosis infection. Moreover, levels of immune markers are often affected by the antigen load. We review how the clinical spectrum of M. tuberculosis infection correlates with the evolution of granulomatous lesions and how granuloma structural changes are reflected in the peripheral circulation. We also discuss how antigen-specific, peripheral immune responses change during infection and how these changes are associated with the physiology of the tubercle bacillus. We propose that a dynamic approach to immune biomarker research should overcome the challenges of identifying those asymptomatic and symptomatic stages of infection that require antituberculosis treatment. Implementation of such a view requires longitudinal studies and a systems immunology approach leading to multianalyte assays.

Identification of putative biomarkers for the serodiagnosis of drug-resistant Mycobacterium tuberculosis

Background

Early diagnosis and treatment of Mycobacterium tuberculosis infection can prevent most deaths resulting from this pathogen; however, multidrug-resistant strains present serious threats to global tuberculosis control and prevention efforts. In this study, we identified antigens that could be used for the serodiagnosis of drug-resistant M. tuberculosis strains, using a proteomics-based analysis.

Results

Serum from patients infected with drug-resistant or drug-susceptible M. tuberculosis strains and healthy controls was subjected to two-dimensional gel electrophoresis using a western blot approach. This procedure identified nine immunoreactive proteins, which were subjected to MALDI-TOF-MS analysis. Six recombinant proteins, namely rRv2031c, rRv0444c, rRv2145c, rRv3692, rRv0859c, and rRv3040, were expressed and used to determine the immuno-reactivity of 100 serum samples. Antibody reactivity against rRv2031c, rRv3692, and rRv0444c was consistently observed. Among them, the best sensitivity and specificity of rRv3692 were 37% and 95% respectively. Furthermore, when rRv2031c and rRv3692 or rRv2031c, rRv3692, and rRv0444c were combined in 2:1 or equal amounts, the assay sensitivity and specificity were improved to 56.7% and 100% respectively.

Conclusions

These results suggest that Rv2031c, Rv3692, and Rv0444c are possible candidate biomarkers for effective use in the serodiagnosis of drug-resistant tuberculosis infections, and a combined formula of these antigens should be considered when designing a subunit assay kit.

A bispecific antibody based assay shows potential for detecting tuberculosis in resource constrained laboratory settings

The re-emergence of tuberculosis (TB) as a global public health threat highlights the necessity of rapid, simple and inexpensive point-of-care detection of the disease. Early diagnosis of TB is vital not only for preventing the spread of the disease but also for timely initiation of treatment. The later in turn will reduce the possible emergence of multi-drug resistant strains of Mycobacterium tuberculosis. Lipoarabinomannan (LAM) is an important non-protein antigen of the bacterial cell wall, which is found to be present in different body fluids of infected patients including blood, urine and sputum. We have developed a bispecific monoclonal antibody with predetermined specificities towards the LAM antigen and a reporter molecule horseradish peroxidase (HRPO). The developed antibody was subsequently used to design a simple low cost immunoswab based assay to detect LAM antigen. The limit of detection for spiked synthetic LAM was found to be 5.0 ng/ml (bovine urine), 0.5 ng/ml (rabbit serum) and 0.005 ng/ml (saline) and that for bacterial LAM from M. tuberculosis H37Rv was found to be 0.5 ng/ml (rabbit serum). The assay was evaluated with 21 stored clinical serum samples (14 were positive and 7 were negative in terms of anti-LAM titer). In addition, all 14 positive samples were culture positive. The assay showed 100% specificity and 64% sensitivity (95% confidence interval). In addition to good specificity, the end point could be read visually within two hours of sample collection. The reported assay might be used as a rapid tool for detecting TB in resource constrained laboratory settings.

The pathology of Mycobacterium tuberculosis infection

Mycobacterium tuberculosis is an old enemy of the human race, with evidence of infection observed as early as 5000 years ago. Although more host-restricted than Mycobacterium bovis, which can infect all warm-blooded vertebrates, M. tuberculosis can infect, and cause morbidity and mortality in, several veterinary species as well. As M. tuberculosis is one of the earliest described bacterial pathogens, the literature describing this organism is vast and overwhelming. This review strives to distill what is currently known about this bacterium and the disease it causes for the veterinary pathologist.

Poor performance of serological tests in the diagnosis of pulmonary tuberculosis: evidence from a contact tracing field study

BACKGROUND

Delayed or missed diagnosis of TB continues to fuel the global TB epidemic, especially in resource limited settings. Use of serology for the diagnosis of tuberculosis, commonly used in India, is another factor. In the present study a commercially available serodiagnostic assay was assessed for its diagnostic value in combination with smear, culture and clinical manifestations.

METHODOLOGY/PRINCIPAL FINDINGS

A total of 2300 subjects were recruited for the study, but 1041 subjects were excluded for various reasons. Thus 1259 subjects were included in the study of which 470 were pulmonary tuberculosis cases (440 of 470 were culture-positive) and 789 were their asymptomatic contacts. A house-to-house survey method was used. Blood samples were tested for IgM, IgA, and IgG antibodies using the Pathozyme Myco M (IgM), Myco A (IgA) and Myco G (IgG) enzyme immunoassay (EIA). Out of 470 PTB cases, BCG scar was positive in 82.34%. The Mantoux test and smear positivity rates in PTB cases were 94.3% (430/456), and 65.32% (307/470), respectively. Among the asymptomatic contacts, BCG scar was positive in 95.3% and Mantoux test was positive in 80.66% (442/548) contacts. No contact was found falsely smear positive. The sensitivity of IgM, IgA, and IgG EIA tests was 48.7%, 25.7% and 24.4%, respectively, while the specificity was 71.5%, 80.5%, 76.6%, respectively. Performance of EIAs was not affected by the previous BCG vaccination. However, prior BCG vaccination was statistically significantly (p = 0.005) associated with Mantoux test positivity in PTB cases but not in contacts (p = 0.127). The agreement between serology and Mantoux test was not significant.

CONCLUSION

The commercial serological test evaluated showed poor sensitivity and specificity and suggests no utility for detection of pulmonary tuberculosis.

Mycobacterial antibody levels and immune restoration disease in HIV patients treated in South East Asia

AIM

Immune restoration disease (IRD) associated with Mycobacterium tuberculosis parallels the reconstitution of a pathogen-specific Th1 response. However, it is not clear whether humoral responses to M. tuberculosis antigens also rise, or whether antibody levels predict IRD. Here, humoral immunity to M. tuberculosis antigens was investigated in four Asian cohorts.

METHODS

Plasma samples were obtained from longitudinal prospective studies of HIV patients beginning antiretroviral therapy (ART) in New Delhi (India), Kuala Lumpur (Malaysia), Jakarta (Indonesia) and Phnom Penh (Cambodia). IgG antibodies to purified protein derivative, lipoarabinomannan and 38-kDa antigens of M. tuberculosis were quantitated using in-house ELISAs. IRD was defined as exacerbated symptoms of tuberculosis in patients on anti-tuberculosis therapy or a novel presentation of tuberculosis on ART.

RESULTS

Pre-ART IgG levels to purified protein derivative, lipoarabinomannan and 38-kDa antigen were similar in the IRD and control groups from each site. Compared with non-IRD controls, a higher proportion of IRD patients had elevated IgG levels to lipoarabinomannan (defined as a greater than twofold increase) over 12 weeks of ART. However, this trend was not significant for the other antigens and longitudinal analyses did not reveal clear rises in antibody levels at the time of IRD.

CONCLUSION

Levels of antibody to mycobacterial antigens do not predict IRD, but levels of antibody reactive with lipoarabinomannan rise during an IRD in some patients.

Adjunctive tests for diagnosis of tuberculosis: serology, ELISPOT for site-specific lymphocytes, urinary lipoarabinomannan, string test, and fine needle aspiration

The diagnostic gold standard for active tuberculosis (TB) is the detection of Mycobacterium tuberculosis (MTB) by culture or molecular methods. However, despite its limited sensitivity, sputum smear microscopy is still the mainstay of TB diagnosis in resource-limited settings. Consequently, diagnosis of smear-negative pulmonary and extrapulmonary TB remains challenging in such settings. A number of novel or alternative techniques could provide adjunctive diagnostic use in the context of difficult-to-diagnose TB. These may be especially useful in certain patient groups such as persons infected with human immunodeficiency virus (HIV) and children, who are disproportionably affected by smear-negative and extrapulmonary disease and who are also most adversely affected by delays in TB diagnosis and treatment. We review a selection of these methods that are independent of nucleic acid amplification techniques and could largely be implemented in resource-limited settings in current or adapted versions. Specifically, we discuss the diagnostic use and potential of serologic tests based on detection of antibodies to MTB antigens; interferon gamma release assays using site-specific lymphocytes; detection of lipoarabinomannan, a glycolipid of MTB, in urine; the string test, a novel technique to retrieve lower respiratory tract samples; and fine needle aspiration biopsy of lymph nodes.

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.

Comparison of membrane proteins of Mycobacterium tuberculosis H37Rv and H37Ra strains

Background

The potential causes for variation in virulence between distinct M. tuberculosis strains are still not fully known. However, differences in protein expression are probably an important factor. In this study we used a label-free quantitative proteomic approach to estimate differences in protein abundance between two closely related M. tuberculosis strains; the virulent H37Rv strain and its attenuated counterpart H37Ra.

Results

We were able to identify more than 1700 proteins from both strains. As expected, the majority of the identified proteins had similar relative abundance in the two strains. However, 29 membrane-associated proteins were observed with a 5 or more fold difference in their relative abundance in one strain compared to the other. Of note, 19 membrane- and lipo-proteins had higher abundance in H37Rv, while another 10 proteins had a higher abundance in H37Ra. Interestingly, the possible protein-export membrane protein SecF (Rv2586c), and three ABC-transporter proteins (Rv0933, Rv1273c and Rv1819c) were among the more abundant proteins in M. tuberculosis H37Rv.

Conclusion

Our data suggests that the bacterial secretion system and the transmembrane transport system may be important determinants of the ability of distinct M. tuberculosis strains to cause disease.

Tuberculosis in the global aging population

Despite the decline in the overall incidence of tuberculosis (TB) in many developed countries, it remains an important problem among the older population. The control of TB in the elderly remains a major challenge because of the limitations of the existing tools for the diagnosis and treatment of latent TB infection and clinically active disease. This article examines the current and possible future status of TB in the elderly, focusing on epidemiology, risk factors, preventive treatment strategies, and clinical disease.

Recombinant Mycobacterium smegmatis expressing an ESAT6-CFP10 fusion protein induces anti-mycobacterial immune responses and protects against Mycobacterium tuberculosis challenge in mice

The currently used vaccine against tuberculosis, Bacille Calmette-Guérin (BCG), has variable efficacy, so new vaccine development is crucial. In this study, we evaluated a recombinant vaccine prepared from non-pathogenic Mycobacterium smegmatis (rMS) that expresses a fusion of early secreted antigenic target 6-kDa antigen (ESAT6) and culture filtrate protein 10 (CFP10). C57BL/6 mice were immunized with the rMS expressing the ESAT6-CFP10 fusion protein (rM.S-e6c10) or with BCG. The mice in the rM.S-e6c10 group had a significantly higher titre of anti-ESAT6-CFP10 antibodies than did animals in the BCG or saline groups. Spleen cells from rM.S-e6c10-immunized mice exhibited a cytotoxic response to ESAT6 and CFP10-expressed target cells, but spleen cells from animals in the other groups did not. Levels of IFN-γ and IL-2 production by purified T cells from spleens were significantly higher in rM.S-e6c10 group than in BCG group. Finally, after M. tuberculosis (MTB)-challenged mice, dramatic reduction in the numbers of MTB colony-forming units (CFUs) in the lungs was observed for the mice immunized with the rMS. The protective efficacy of rM.S-e6c10 and BCG vaccination was similar based on measures of MTB burden and lung pathology. Our data indicate that the recombinant M. smegmatis vaccine expressing the ESAT6-CFP10 fusion protein has potential in clinic application.

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.

Definition of novel cell envelope associated proteins in Triton X-114 extracts of Mycobacterium tuberculosis H37Rv

Background

Membrane- and membrane-associated proteins are important for the pathogenicity of bacteria. We have analysed the content of these proteins in virulent Mycobacterium tuberculosis H37Rv using Triton X-114 detergent-phase separation for extraction of lipophilic proteins, followed by their identification with high resolution mass spectrometry.

Results

In total, 1417 different proteins were identified. In silico analysis of the identified proteins revealed that 248 proteins had at least one predicted trans-membrane region. Also, 64 of the identified proteins were predicted lipoproteins, and 54 proteins were predicted as outer membrane proteins. Three-hundred-and-ninety-five of the observed proteins, including 91 integral membrane proteins were described for the first time. Comparison of abundance levels of the identified proteins was performed using the exponentially modified protein abundance index (emPAI) which takes into account the number of the observable peptides to the number of experimentally observed peptide ions for a given protein. The outcome showed that among the membrane-and membrane-associated proteins several proteins are present with high relative abundance. Further, a close examination of the lipoprotein LpqG (Rv3623) which is only detected in the membrane fractions of M. tuberculosis but not in M. bovis, revealed that the homologous gene in M. bovis lack the signal peptide and lipobox motif, suggesting impaired export to the membrane.

Conclusions

Altogether, we have identified a substantial proportion of membrane- and membrane-associated proteins of M. tuberculosis H37Rv, compared the relative abundance of the identified proteins and also revealed subtle differences between the different members of the M. tuberculosis complex.

CysA2: A candidate serodiagnostic marker for Mycobacterium tuberculosis infection

BACKGROUND AND OBJECTIVE

It is important to identify and test serologically active antigens in order to devise a mixture of antigens or peptides that is most useful for serodiagnosis. This study evaluated the serodiagnostic potential of CysA2, which has not previously been described as a serological antigen, together with those of PstS1, HspX, antigen 85 complex and CFP-10 proteins.

METHODS

Serum IgG antibody titres against each antigen and a mixture of the antigens were measured by ELISA, in subjects with pulmonary tuberculosis and in healthy control subjects.

RESULTS

CysA2 showed diagnostic value comparable to that of PstS1 and HspX. Mixtures of these three proteins provided the highest diagnostic sensitivity. CysA2 was useful for identifying patients who did not react to HspX or PstS1, and was most valuable in increasing the sensitivity of testing. Furthermore, CysA2 efficiently overcame the limitation associated with use of PstS1, that is, significantly lower sensitivity for subjects who are negative for acid-fast bacilli.

CONCLUSIONS

These findings suggest that CysA2 can be used in combination with HspX and/or PstS1 to increase the accuracy of tuberculosis diagnoses.

New insights into protein O-mannosylation in actinomycetes

Glycosylation is a common post-translational modification of surface exposed proteins and lipids present in all kingdoms of life. Information derived from bacterial genome sequencing, together with proteomic and genomic analysis has allowed the identification of the enzymatic glycosylation machinery. Among prokaryotes, O-mannosylation of proteins has been found in the actinomycetes and resembles protein O-mannosylation in fungi and higher eukaryotes. In this review we summarize the main features of the biosynthetic pathway of O-mannosylation in prokaryotes with special emphasis on the actinomycetes, as well as the biological role of the glycosylated target proteins.

Serodiagnostic efficacy of Mycobacterium tuberculosis 30/32-kDa mycolyl transferase complex, ESAT-6, and CFP-10 in patients with active tuberculosis

Elimination of tuberculosis (TB) largely depends upon definitive rapid diagnosis and treatment. Widely used diagnostic tests do not qualify for use in a developing country due to lack of either desired accuracy or their cost. In the present study an enzyme-linked immunosorbent assay was used to evaluate the diagnostic potential of an immuno-dominant 30/32-kDa mycolyl transferase complex (Ag85 complex) and Mycobacterium tuberculosis-specific proteins (ESAT-6 and CFP-10) of the RD1 region. Higher sensitivity (84.1%) with Ag85 complex was observed compared with ESAT-6 (64.9%) and CFP-10 (66%), with almost similar specificity (Ag85: 85.2%, ESAT-6: 88.9%, CFP-10: 85.2%), whereas the individual components of Ag85 complex, i.e. Ag85A, Ag85B, and Ag85C, showed sensitivities of 44.6, 34, and 80.9% and specificities of 55.6, 74.1, and 40.7% respectively. A cocktail of Ag85 complex, ESAT-6, CFP-10, Ag85A, Ag85B, and Ag85C antigens also could not help in increasing either sensitivity (51.1%) or specificity (85.2%). Furthermore, immunoblot analysis using clinical isolates as well as a standard strain (H37Rv) of M. tuberculosis also showed strong reactivity of sera from TB patients to Ag85 complex and, to a lesser extent, also to ESAT-6. To conclude, use of Ag85 complex along with ESAT-6 and CFP-10 seems to be promising in minimizing the heterogeneous sero-responses of adult TB cases.

Identification of Rv2041c, a novel immunogenic antigen from Mycobacterium tuberculosis with serodiagnostic potential

Novel immunogenic antigens are continually required for the improvement of diagnostic techniques for Mycobacterium tuberculosis infection. Some proteins with serodiagnostic value are not expressed under normal culture conditions, but may be induced under specific conditions such as gradual oxygen depletion and low pH, and from inside macrophages. Using a customized amplification library, we previously found that Rv2041c from M. tuberculosis H37Rv was highly expressed in vitro under conditions of low pH and hypoxia. In this study, recombinant (r)Rv2041c was produced in Escherichia coli to examine its role in immune responses. Increased Rv2041c expression in vitro during dormancy and during infection in human macrophages was confirmed by Western blotting and reverse transcription polymerase chain reaction, respectively. Interestingly, positive antibody responses to rRv2041c were detected only in those patients with active tuberculosis (TB) and in mice infected with M. tuberculosis H37Rv. Finally, Rv2041c was used successfully in the serodiagnosis of active M. tuberculosis infection in Korean patients in conjunction with other M. tuberculosis proteins, including Ag85 complex, 38 kDa, rESAT-6, rHSP-X and rCFP-10. Our Rv2041c-ELISA had comparable diagnostic sensitivity and equivalent specificity to the use of an M. tuberculosis H37Rv cellular extract. In addition, seven of 46 serum samples collected from TB patients (15.28%) showed positive antibody responses to Rv2041c, but not to the other proteins. These results suggest that Rv2041c can be used to increase assay sensitivity alongside well-known antigens for the serodiagnosis of M. tuberculosis infection.

Exploring the immune response against Mycobacterium tuberculosis for a better diagnosis of the infection

Tuberculosis (TB) still represents a monumental problem, with more than two million deaths every year worldwide. The current diagnostics for TB offer sub-optimal accuracy both for the active and the latent form of infection and are often based on technologies unaffordable in low-income settings. The tuberculin skin test was the first diagnostic based on an acquired immune response towards Mycobacterium tuberculosis (MTB). Advances in molecular and cellular biology and the elucidation of the mechanisms governing the relation between MTB and the human immune system form the basis for new and more accurate assays, potentially able to fill the gaps and limits of classical diagnostics. However, the process of validating new tests is still complex and hampered by specific questions regarding TB immunology and natural history. We present here a summary of the current approaches to validate new diagnostics based on the detection of immunological biomarkers of TB infection.

Utility of a combination of RD1 and RD2 antigens as a diagnostic marker for tuberculosis

We evaluated the diagnostic potential of a cocktail of 4 antigens encoded by regions of difference (RD) 1 and 2 of Mycobacterium tuberculosis, that is, early secretory antigenic target-6, culture filtrate protein-10 (CFP-10), CFP-21, and mycobacterial protein from species tuberculosis-64 (MPT-64) on the basis of antigen and antibody detection by enzyme-linked immunosorbent assay. Parallel detection of antigens and antibodies in the serum samples of pulmonary tuberculosis (PTB) patients resulted in higher sensitivity as compared to either of the single tests in both smear-positive (90%) and smear-negative (60%) PTB patients. In addition, combined detection of antigens and antibodies in the fluids of extrapulmonary tuberculosis (EPTB) patients could detect >90% of the patients with high specificity. These results demonstrate the ability of the combination of antigen and antibody detection assays based on the cocktail of RD antigens to diagnose a substantial number of PTB and EPTB cases with high specificity.

The protective role of antibody responses during Mycobacterium tuberculosis infection

Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) is one of the most important infectious diseases globally. Immune effector mechanisms that lead to protection or development of clinical disease are not fully known. It is generally accepted that cell-mediated immunity (CMI) plays a pivotal role in controlling Mtb infection, whereas antibody responses are believed to have no protective role. This generalization is based mainly on early classical experiments that lacked standard protocols, and the T helper type 1 (Th1)/Th2 paradigm. According to the Th1/Th2 paradigm Th1 cells protect the host from intracellular pathogens, whereas Th2 cells protect form extracellular pathogens. During the last two decades, the Th1/Th2 paradigm has dominated not only our understanding of immunity to infectious pathogens but also our approach to vaccine design. However, the last few years have seen major discrepancies in this model. Convincing evidence for the protective role of antibodies against several intracellular pathogens has been established. Studies of B cell-deficient mice, severe combined immunodeficiency (SCID) mice, passive immunization using monoclonal (mAb) and polyclonal antibodies and immune responses against specific mycobacterial antigens in experimental animals reveal that, in addition to a significant immunomodulatory effect on CMI, antibodies play an essential protective role against mycobacterial infections. In this review, our current understanding of the essential role of antibodies during Mtb infections, limitations of the Th1/Th2 model and the unfolding interdependence and mutual regulatory relationships between the humoral and CMI will be presented and discussed.

Peptides of a novel Mycobacterium tuberculosis-specific cell wall protein for immunodiagnosis of tuberculosis

The sequencing of the Mycobacterium tuberculosis genome revealed the existence of several genes encoding novel proteins with unknown functions, one of which is the proline-threonine repetitive protein (PTRP; Rv0538). Genomic studies of various mycobacterial species and M. tuberculosis clinical isolates demonstrate that ptrp is specific to the M. tuberculosis complex and ubiquitous in clinical isolates. Enzyme-linked immunosorbent assay, Western blot analysis, and electron microscopic evaluation of M. tuberculosis subcellular fractions and intact bacteria confirm that PTRP is a cell wall protein. Antibodies to PTRP are present in serum specimens from human immunodeficiency virus (HIV)-negative, tuberculosis (TB)-positive and HIV-positive, TB-positive patients but not purified protein derivative (PPD)-negative or PPD-positive healthy control subjects, demonstrating its diagnostic potential. Epitope mapping of PTRP delineated 4 peptides that can identify >80% of sputum smear-positive and >50% of smear-negative, HIV-negative, TB-positive patients and >80% of HIV-positive, TB-positive patients. These results demonstrate that immunodominant epitopes of carefully selected M. tuberculosis-specific proteins can be used to devise a simple peptide-based serodiagnostic test for TB.

Increased levels of immunological markers in the respiratory tract but not in serum correlate with active pulmonary mycobacterial infection in mice

Immunological tests for the diagnosis of tuberculosis (TB) have relied mostly on detection of immune markers in serum or release of cytokines by mononuclear cells in vitro. These tests, although useful, sometimes fail to discriminate between active infection and contact with mycobacteria or vaccination. TB is primarily a disease of the lung, and therefore identification of immunological markers in the respiratory tract will be more likely to reflect the infection status or disease activity. In this study, it is demonstrated that active infection of mice with Mycobacterium bovis bacille Calmette-Guérin (BCG), but not exposure to heat-killed BCG, induced production of interleukin-12 (IL-12), interferon-gamma (IFN-gamma) or soluble tumour necrosis factor receptors (sTNFRs) locally in the lungs, as detected in bronchoalveolar lavage (BAL) fluid. There was a strong correlation between bacterial growth in the lung and levels of sTNFRs, and to some extent IL-12 and IFN-gamma, in BAL fluid. Furthermore, sTNFR levels increased significantly in BAL fluid after reactivation of controlled infection with dexamethasone, and this correlated with increased bacterial growth in the lungs. Finally, infection, but not exposure to non-replicating mycobacteria, induced specific IgG and IgA in BAL fluid. Elevated levels of all biomarkers measured were also detected in the serum, but correlation with infection was not as clear as in the case of BAL fluid. Taken together, the detection of sTNFRs and mycobacterium-specific antibodies, especially IgA, locally in the lungs could be used as immunological markers for the diagnosis of TB.

Mycobacterium tuberculosis 38-kDa lipoprotein is apoptogenic for human monocyte-derived macrophages

Mycobacterium tuberculosis is the main aetiologic agent of tuberculosis, a disease of great concern in less-developed regions. Apoptosis is a conspicuous event in macrophages infected in vitro with mycobacteria, a phenomenon also observed in vivo in granulomas of patients with tuberculosis. To determine its significance, it is important to define the mycobacterial moieties involved and how they cause apoptosis. Here we show that the 38-kDa lipoprotein induces macrophage caspase-dependent apoptosis involving TNF-alpha and FasL and, interestingly, with the upregulation of cell-death receptors TNFR1, TNFR2 and Fas. A role for the Toll-like receptor 2 was also demonstrated. In conclusion, the ability to induce apoptosis of host cells is another property of the 38-kDa lipoprotein, a molecule that has focused attention for being an immunodominant antigen that participates in phosphate transport.

Molecular findings and approaches spotlighting Mycobacterium bovis persistence in cattle

Mycobacterium tuberculosis (Mtb) and Mycobacterium bovis (M. bovis) are the etiological agents of human and bovine tuberculosis (TB, bTB) respectively, and share genetic identity over 99% at the whole genome level. Progress has been made towards explaining how mycobacteria and their infected hosts remain in balance without producing clinical symptoms of disease, a phenomenon referred to as latency or persistence, which can be mimicked by certain in vitro conditions. Latency/persistence has mainly been studied using Mtb, where the two-component signalling system, dosRS, has been assigned an instrumental role, and even constitutes the current basis for development of new diagnostic methods and treatment addressing this particular stage of TB. M. bovis conserves homolog genes that in Mtb play a role in human latent TB infection and that, by analogy, would allow it to enter a persistent state in infected cattle; nevertheless, little attention has been paid to this stage in bovine hosts. We suggest that many of the advances acquired through the study of Mtb can and should be taken into consideration by research groups and veterinary professionals dealing with bTB. The study of the infection in bovines, paying particular attention to defining the molecular and cellular markers of a M. bovis persistent infection in cattle, presents great opportunities for the development and trial of new diagnostic tests and vaccines, tools that will surely help in promoting eradication of bTB in high-burden settings.

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.

Peptide-based antibody detection for tuberculosis diagnosis

Tuberculosis (TB) is a major cause of morbidity and mortality, especially in developing countries. Despite significant limitations, microscopy remains the cornerstone of the global TB control strategy. As the TB epidemic escalates, new diagnostic methods that are accurate and also economical and simple to manufacture and deploy are urgently needed. Although several promising antigens have been identified and evaluated in recent years, the reproducible production of high-quality recombinant mycobacterial proteins with minimal batch-to-batch variation is difficult, laborious, and expensive. To determine the feasibility of devising a synthetic peptide-based diagnostic test for TB, we have delineated the immunodominant epitopes of three candidate antigens, Ag85B, BfrB, and TrxC, that were previously identified to be immunogenic in TB patients. The results demonstrate that combinations of carefully selected synthetic peptides derived from highly immunogenic proteins can be the basis for devising an immunodiagnostic test for TB.

Assessment of three commercially available serologic assays for detection of antibodies to Mycobacterium tuberculosis and identification of active tuberculosis

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a major world disease, with approximately 9 million new cases each year. Identification and treatment of active disease are essential for TB control. Serology may offer increased detection of active disease in patients with a positive tuberculin skin test (TST) or QuantiFERON-TB (QFT-G). The InBios Active TbDetect immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA), IBL M. tuberculosis IgG ELISA, and Anda Biologics TB ELISAs were evaluated for the ability to detect M. tuberculosis antibodies in patients with active disease. Agreement, sensitivity, and specificity for each ELISA were determined and compared to those for culture or amplified direct detection and M. tuberculosis low-risk control patients. The InBios Active TbDetect ELISA had an agreement of 96.2%, a sensitivity of 83.3%, and a specificity of 98.9%. The IBL M. tuberculosis ELISA had an agreement of 84.0%, a sensitivity of 5.6%, and a specificity of 100.0%. The agreement, sensitivity, and specificity of the Anda Biologics TB ELISA were 74.2%, 83.3%, and 72.0%, respectively. The sensitivity for detecting M. tuberculosis antibodies in human immunodeficiency virus-associated TB was 50% for both the InBios Active TbDetect ELISA and the Anda Biologics TB ELISA and 0% for the IBL M. tuberculosis ELISA. The positivity rates for InBios Active TbDetect ELISA, IBL M. tuberculosis ELISA, and Anda Biologics TB ELISA in latently infected individuals positive by TST and/or QFT-G were 5.1%, 0.0%, and 30.8%, respectively. It can be concluded that the InBios Active TbDetect IgG ELISA is superior to the other ELISAs in accurately detecting active TB.

Diagnosing latent tuberculosis infection in the HIV era

Tuberculin skin testing (TST) and Interferon-gamma (IFNγ)release assays (IGRAs) are presently the only available assays for the detection of Mycobacterium tuberculosis infected individuals. IGRAs might progressively replace TST, as numerous published reports establish their higher specificity and similar sensitivity when tested in BCG vaccinated, immunocompetent individuals or in populations who may have been in contact with atypical mycobacteria. However, few published reports have commented on their role in TB diagnosis in immunocompromised individuals (HIV, immunosuppressive therapy, cancer…). It is the purpose of this report to review IGRAs published studies in HIV individuals in endemic and non endemic area for tuberculosis (TB). IGRAs were tested in the presence or absence of active TB but correlated to duration of exposure. In newly diagnosed active TB, IGRAs demonstrated a similar sensitivity to TST. In TB non infected individuals, TST and IGRAs also gave similar values when categorization of individuals was correlated to the risk of infection. A higher number of positive IGRAs was observed in individuals from TB endemic areas, in similar proportions to immunocompetent individuals. Comparison between the two IGRAs: QuantiFERON-TB Gold^® (QF-TB, Cellestis, Australia) and T-SPOT-TB^® (Oxford Immunotec, UK), and against TST, in the same HIV population demonstrates a higher sensitivity of T-SPOT-TB and TST than QF-TB. Indeterminate results, which correspond to the absence of a positive T-cell IFNγ response towards phytohemaglutinin (PHA), is a key point when comparing both IGRAs. This PHA control is indicative of the level of immunosuppression observed in the tested individual. QF-TB seems to present, in HIV populations, more indeterminate results than T-SPOT-TB. The calibration and/or concentration of PBMC on nitrocellulose membrane for the T-SPOT-TB, as compared to a whole blood assay, might explain this difference, with less indeterminate results with the T-SPOT-TB assay. Neither assay is able to differentiate active TB from latent TB infection (LTBI). Several laboratories have tried new antigenic epitopes to solve this issue. It is of importance that these studies need to be repeated on a larger scale by others to validate their results. Two blood assays might add information characterising the evolution from LTBI to active TB: either by losing protective immunity, as demonstrated by the whole blood killing assay, or by evaluating the kinetics of the antibodies synthesized against M. tuberculosis specific antigens. In conclusion, longitudinal studies are still needed to validate IGRAs and other assays and to define their respective predictive values.