Preliminary evaluation of a Mycobacterium tuberculosis lipooligosaccharide (LOS) antigen in the serological diagnosis of tuberculosis in HIV seropositive and seronegative patients

A toolbox for tuberculosis (TB) diagnosis: an Indian multi-centric study (2006-2008); evaluation of serological assays based on PGL-Tb1 and ESAT-6/CFP10 antigens for TB diagnosis

BACKGROUND

The aim of this multi-centric prospective study in India was to assess the accuracy of a serological test as an additional tool for diagnosing active tuberculosis (ATB). In particular, an assay based on ELISA using a phenolic glycolipid (PGL-Tb1) or a fusion protein (ESAT-6/CFP10) was compared to the tuberculin skin test (TST) and the microbiological results according to HIV status.

METHODS

Individuals with and without ATB and HIV infection were enrolled. Serology and TST results were analyzed per se and in combination with the microbiological data.

RESULTS

Among the 778 ATB patients, 102 were HIV-infected, 316 HIV-uninfected and 360 had an HIV-unknown status. Of the 945 non-ATB subjects, 559 were at low risk (community adults) and 386 at high risk of M. tuberculosis exposure. Among those with ATB, the sensitivity of ELISA-PGL-Tb1 for ATB was higher than that of ELISA-ESAT-6/CFP10, both in HIV-infected (72.3% versus 63.7%, p = 0.29) and HIV-uninfected/HIV-unknown groups (40.5% versus 28.6%; p<0.0001), whereas the specificity was around 91% for both tests. Sensitivity for ATB increased when the results of the two ELISA were combined, reaching 75.5% in the HIV-infected and 50.9% in the group of HIV-uninfected/HIV-unknown ATB, with a significant decrease of the global specificity (83.9%). Analyzing the ELISA results with the microbiological results, we observed that the sensitivity of both serology tests was independent of the ATB patients' smear microscopy (SM) status and grade. Combining the results of SM with both ELISA, the detection of ATB patients significantly increased (p<0.0001), particularly in those with extrapulmonary TB (up to 45.1%) or HIV infection (up to 83.3%). No significant association was observed between TST and serology results.

CONCLUSIONS

In this prospective multi-centric study, the combination of two rapid tests, such as SM and serology, might be useful in detecting ATB, especially in HIV-infected patients.

Anti-PGL-Tb1 responses as an indicator of the immune restoration syndrome in HIV-TB patients

A prospective and multi-centre study has allowed us to analyse antibody responses and Mycobacterium tuberculosis clinical isolate genotypes on 24 consecutive HIV-TB co-infected patients treated with Highly Active Antiretroviral Therapy (HAART) who either went on to develop a TB Immune Restoration Syndrome (TB-IRS), or not. Circulating free and immune-complexed antibodies against ManLAM, ESAT-6/CFP10 and PGL-Tb1 in HIV-TB co-infected patients were measured by ELISA at the initiation of anti-TB treatment, at the date of HAART initiation and thereafter. Presence of circulating B cells was also monitored by in vitro antibody production (IVAP) against ESAT-6/CFP10 and PGL-Tb1. Finally, 16 out of 24M. tuberculosis clinical isolates from patients with TB-IRS were genotyped using spoligotyping and MIRUs-VNTR typing. Eleven patients (45.8%) experienced TB-IRS (TB-IRS+). Significantly, lower anti-PGL-Tb1 antibody levels were identified in TB-IRS+ compared to TB-IRS-negative patients prior to TB-IRS development. These very low levels were neither related to CD4 counts nor with complexed antibodies. No difference in antibody levels was observed with the other tested antigens. In addition, no specific strain genotype was associated with TB-IRS. The presence of specific anti-PGL-Tb1 antibodies only in TB-IRS-negative patients represents for the first time an indicator of a potential protective response or a diagnostic biomarker for the detection of non-progression to TB-IRS in HIV-TB co-infected patients starting HAART.

Modern approaches to a rapid diagnosis of tuberculosis: promises and challenges ahead

The limitations of the conventional methods for diagnosing tuberculosis have spurred multi-faceted research activities in this field throughout the world. Chromatographic methods appear promising but may not be widely available in the developing countries. Immuno-diagnostic methods using combinations ("cocktails") of antigens have high sensitivity and specificity and can easily be applied in the peripheral laboratories and in the field settings. Though expensive, molecular methods for diagnosis of tuberculosis have advantages of speed, sensitivity, and specificity. Adequate training of the eligible personnels in molecular methods and prevention of laboratory-dependent contamination may help reduce false positive results. Although, there are no clear guidelines, so far on how to make out the best from the gene amplification methods, yet their use may be encouraged with adequate quality controls, because of the inherent ingenuity and promises of these methods. Phage-based molecular methods provide rapid results in susceptibility tests for anti-tubercular drugs. In future, many sophisticated techniques are expected to hit the market for a rapid diagnosis of tuberculosis. In the developing countries, it is necessary to evaluate availability of suitable infrastructure and trained personnels before adopting modern diagnostic methods.

B-cell immune responses in HIV positive and HIV negative patients with tuberculosis evaluated with an ELISA using a glycolipid antigen

The diagnostic value of the PGL-Tb1 enzyme-linked immunosorbent assays (ELISA) was established following a survey study using sera from 220 Tuberculosis patients (including 69 HIV coinfected) and 324 controls. A higher percentage (76.8%) of the HIV-seropositive compared to the HIV-seronegative (58.9%) TB patients were ELISA positive (p=0.02) with a specificity of 94%. In HIV-positive TB patients, ELISA sensitivity was identical for all sites of disease and antibody levels were not affected by the CD4+ counts, PPD results, age or bacterial yield. Combining data for both the smear microscopy and ELISA maximized sensitivity. The kinetics of anti-PGL-Tb1 antibody was evaluated in cohort studies using sera collected before, during and after treatment for clinical TB for 79 TB patients (including 39 HIV coinfected). Statistically significant ELISA signals were observed in 51.3% of HIV-seropositive TB patients prior to the diagnosis of clinical TB and elevated antibody levels persisting 18 months after the end of antituberculous chemotherapy. Asymptomatic development of antibody also occurred in 22.7% of a cohort of 44 HIV-positive patients with a high risk of tuberculosis, but no correlation was found between persisting elevated antibody levels and progression to active disease. This antibody response in absence of disease, might reflect the control of an incipient tuberculosis infection by antituberculous prophylaxis or through an improved protective immune response associated with antiretroviral therapy.

A major cell wall lipopeptide of Mycobacterium avium subspecies paratuberculosis

Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne disease in cattle and other ruminants, is proposed to be at least one of the causes of Crohn disease in humans. MAP and Mycobacterium avium subspecies avium, a closely related opportunistic environmental bacterium, share 95% of their genes and exhibit homologies of more than 99% between these genes. The identification of molecules specific for MAP is essential for understanding its pathogenicity and for development of useful diagnostic tools. The application of gas chromatography, mass spectrometry, and nuclear magnetic resonance led to the structural identification of a major cell wall lipopeptide of MAP, termed Para-LP-01, defined as C20 fatty acyl-D-Phe-N-Me-L-Val-L-Ile-L-Phe-L-Ala methyl ester. Variations of this lipopeptide with different fatty acyl moieties (C16 fatty acyl through C17, C18, C19, C21 to C22) were also identified. Besides the specificity of this lipopeptide for MAP, the presence of an N-Me-L-valine represents the first reported N-methylated amino acid within an immunogenic lipopeptide of mycobacteria. Sera from animals with Johne disease, but not sera from uninfected cattle, reacted with this lipopeptide, indicating potential biological importance.

Analysis of the shotgun expression library of the Mycobacterium tuberculosis genome for immunodominant polypeptides: potential use in serodiagnosis

A recombinant DNA strategy was applied to analyze and screen the shotgun expression library from a clinically confirmed local virulent isolate of Mycobacterium tuberculosis with sera from tuberculosis patients, which led to expression and purification of highly immunoreactive and specific mycobacterial antigens expressed during the course of active disease which could be of diagnostic significance. An enzyme-linked immunoassay for diagnosis of tuberculosis was devised by using a shotgun immunoexpression library in the lambdagt11 vector. DNA from a virulent M. tuberculosis patient isolate (TBW-33) confirmed with the BACTEC 460 system was sheared and expressed to generate shotgun polypeptides. beta-Galactosidase fusion proteins capable of demarcating active tuberculosis infections from Mycobacterium bovis BCG-vaccinated healthy subjects or people harboring environmental mycobacteria were selected by comparative immunoreactivity studies. Promising mycobacterial DNA cassettes were subcloned and expressed into the glutathione S-transferase (GST) fusion vector pGEX-5X-1 with a strong tac promoter and were expressed in Escherichia coli BL21. These fusion proteins were severed at a built-in factor Xa recognition site to separate the GST tags and were utilized in an indirect enzyme-linked immunoassay for serodiagnosis of patients with active tuberculosis. The system offered a clear demarcation between BCG-vaccinated healthy subjects and patients with active tuberculosis and proved to be effective in detecting pulmonary as well as extrapulmonary tuberculosis, with an overall sensitivity of 84.33% and an overall specificity of 93.62%.

Diagnosis of tuberculosis: available technologies, limitations, and possibilities

Rapid diagnosis and treatment are important for preventing transmission of Mycobacterium tuberculosis. However, the diagnosis of tuberculosis continues to pose serious problems, mainly because of difficulties in differentiating between patients with active tuberculosis and those with healed lesions, normal mycobacterium boris BCG (Bacillus Calmette Guerin) vaccinated individuals, and unvaccinated Manteux positives. Physicians still rely on conventional methods such as Ziehl-Neelsen (ZN) staining, fluorochrome staining, sputum culture, gastric lavage, and other non-traditional methods. Although the tuberculin test has aided in the diagnosis of tuberculosis for more than 85 years, its interpretation is difficult because sensitization with nontuberculous mycobacteria leads to false-positive tests. There have been numerous unsuccessful attempts to develop clinically useful serodiagnostic kits for tuberculosis. A number of proteinaceous and nonprotein antigens (such as acyltrehaloses and phenolglycolipids) have been explored from time to time for the development of such assays but they have not proved to be clinically useful. It has been difficult to develop an ELISA utilizing a suitable antigen because M. tuberculosis shares a large number of antigenic proteins with other microorganisms that may or may not be pathogenic. With the advent of molecular biology techniques, there have been significant advances in nucleic acid-based amplification and hybridization, which are helping to rectify existing flaws in the diagnosis of tuberculosis. The detection of mycobacterial DNA in clinical samples by polymerase chain reaction (PCR) is a promising approach for the rapid diagnosis of tuberculous infection. However, the PCR results must be corrected for the presence of inhibitors as well as for DNA contamination. In the modern era of genetics, marked by proteomics and genomics, the day is not far off when DNA chip-based hybridization assays will instantly reveal mycobacterial infections.

Evaluation of a commercial immunodiagnostic kit incorporating lipoarabinomannan in the serodiagnosis of pulmonary tuberculosis in Ghana

We evaluated 'Mycodot', a commercially marketed immunodiagnostic test for tuberculosis which detects antibodies to lipoarabinomannan antigen. Serum was tested from 52 patients with newly diagnosed smear-positive pulmonary tuberculosis, of whom 20 were HIV-positive and 32 HIV-negative. Control sera were taken from 40 patients of whom 20 had acute non-tuberculous lobar pneumonia and 20 patients had no respiratory disease. The test was found to have a very high specificity of 97.5% (95% CI:92.5-100%). However, the sensitivity in HIV-negative patients was 56% (95% CI:39-73%), and was substantially lower at 25% (95% CI:6-44%) in HIV-positive patients.

IN CONCLUSION

'Mycodot' was found to be a highly specific and easily performed assay. However, the poor sensitivity, especially in HIV-infected patients, renders it unlikely to be useful either as a primary or adjunctive diagnostic test for tuberculosis, particularly in countries with a high prevalence of HIV. A larger trial of this assay in Ghana was not deemed necessary.

Mycobacterium tuberculosis Des protein: an immunodominant target for the humoral response of tuberculous patients

The phoA gene fusion methodology permitted the identification of a new Mycobacterium tuberculosis exported protein, Des. This protein has significant sequence similarities to plant acyl-acyl carrier protein desaturases, which are enzymes involved in general fatty acid biosynthesis as well as in mycolic acid biosynthesis in mycobacteria. As shown by Western blot and enzyme-linked immunosorbent assay experiments, the Des protein is a major B-cell antigen that was recognized by all the tuberculous M. tuberculosis- and M. bovis-infected human patients tested.

Clinical utility of serodiagnosis of tuberculosis

The use of TB-ELISA tests as a diagnostic tool offer a lot of scope in early diagnosis of serious forms of childhood tuberculosis. The characteristics of these tests have improved with the availability of purified and recombinant antigens and competition assays using monoclonal antibodies. Lower antibody titres to M. tuberculosis specific antigens in children and the presence of 'natural exposure' antibodies make the interpretation of these tests difficult at times. Caution must be exercised in interpreting their results due to problems inherent on currently available methods of TB-immunodiagnosis. The selection of best combination of antigens for serology, prospective clinical trials comparing success rate of serology with the standard different diagnostic procedures are required.

Comparison of A60 and three glycolipid antigens in an ELISA test for tuberculosis

OBJECTIVES: To compare the diagnostic usefulness in tuberculosis of the serodiagnostic enzyme-linked immunosorbent assay (ELISA) kit A60 (Anda Biologicals, Strasbourg, France) and of our domestic ELISA based on three purified cell wall glycolipid antigens. METHODS: The presence and concentrations of IgG and IgM anti-A60 antibodies and anti-LOS, anti-DAT and anti-PGLTb1 antibodies against the glycolipid antigens were determined by ELISA in 50 HIV-seronegative and 46 HIV-seropositive patients, with documented active tuberculosis. The specificity of these ELISAs was determined with use of sera from 50 healthy blood donors, 29 patients with non-mycobacterial pulmonary diseases and 24 HIV-positive patients with disseminated Mycobacterium avium infection. RESULTS: With a calculated cut-off for each antigen and immunoglobulin that gave a specificity higher than or equal to 98%, the cumulative ELISA results showed that only 36.5% of the patients with tuberculosis had a positive response in the A60 test, as compared with 84.4% who showed a response to the three glycolipid antigens (p<0.001). This striking difference persisted when the cumulative sensitivities were calculated according to the HIV status of the patients and the localization of tuberculosis. The anti-A60 antibody (IgG and IgM) levels and the degree of sensitivity of the ELISA for detection of A60 antigen were always lower in HIV-positive patients with pulmonary and extrapulmonary tuberculosis than in HIV-negative patients with tuberculosis. The sensitivity of A60 ELISA was further decreased in HIV-positive patients with low CD4+ lymphocytes counts, in contrast to the results with the three glycolipid antigens. CONCLUSIONS: These results show the limitations of the A60 ELISA, and confirm the potencies of the glycolipid antigens in serodiagnosis of tuberculosis in HIV-positive and HIV-negative patients.

Analysis of the immunological humoral response to Mycobacterium tuberculosis glycolipid antigens (DAT, PGLTb1) for diagnosis of tuberculosis in HIV-seropositive and -seronegative patients

Using an enzyme immunoassay (EIA) test, the concentrations of IgG antibodies against 2,3 diacyl trehalose (DAT) and phenolic glycolipid Tb1 (PGLTb1) were measured in the sera of 153 patients with active tuberculosis, 50 of whom were coinfected with HIV, and in the sera of 152 healthy blood donors, 149 asymptomatic HIV-seropositive patients, 12 HIV-seronegative patients with conditions simulating tuberculosis, 23 HIV-seropositive patients with disseminated infection caused by mycobacteria other than tuberculosis and 24 HIV-seropositive patients with pulmonary disease from whom mycobacteria was not isolated in culture. A slightly lower percentage (74%) of the HIV-seropositive than the HIV-seronegative (77%) tuberculosis patients were positive for anti-DAT and anti-PGLTb1 IgG antibodies, with a specificity ranging from 91 to 95%. There was no significant difference between EIA sensitivity in smear-positive and smear-negative patients with pulmonary tuberculosis for all HIV immune statuses and sites of disease (pulmonary vs. extrapulmonary). In HIV-seropositive patients, however, sensitivity was always lower for disseminated tuberculosis than for localized tuberculosis. Combining data for both the smear test and the EIA maximized sensitivity. The main value of the EIA test could be to provide early complementary information by antibody detection in patients with tuberculosis, particularly those with a negative smear test.