Clinical evaluation of the diagnostic value of measuring IgG antibody to 3 mycobacterial antigen preparations in the capillary blood of children with tuberculosis and control subjects

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.

Study of the antibody response against Mycobacterium tuberculosis antigens in Warao Amerindian children in Venezuela

This study was aimed at investigating alternate methods for serodiagnosis of tuberculosis (TB), which are needed because bacteriologic diagnosis of childhood TB is difficult. A selection of 80 serum and saliva samples were tested from Warao indigenous children under 15 years of age; 34 high TB suspects (28 positive and 6 negative for the tuberculin skin test, TST) and 46 healthy contact children (32 positive and 14 negative for the TST). Several enzyme-linked immunosorbent assay (ELISA) serological tests were developed to test for Mycobacterium tuberculosis-specific antibodies, including serum IgA, IgG, IgE, and secretory IgA (sIgA) in saliva against 3 specific antigens (PPD, HSP60, 38 kDa). Of these, 2 antigens, PPD and 38 kDa, showed significantly higher reactivity. The sensitivity and specificity of these tests for diagnosis remained limited, between 26.5% and 38.2%, and 77.4% and 97%, respectively. Of all the samples studied and combinations realized between all isotypes and antigens combined with 3 isotypes (anti-PPD IgG, IgE, and anti-38kDa sIgA) managed to detect the largest number of patients, showing an improved sensitivity level of 64.7%, although specificity levels dropped to 81.8%. These results were compared with the Omega diagnostics commercial kit results. The commercial kits showed significantly lower reactivity (sensitivity of 20% and 13.33% to Myco G and Complex Plus, respectively) and a specificity of 100%. This study shows that in indigenous populations of Venezuela, where invasive procedures cannot be used to select samples but evaluation with a chest X-ray for radiological studies is available, the combination of 3 specific isotypes may be a useful tool to increase diagnostic accuracy with pulmonary TB in this population, when used together with clinical and epidemiological criteria.

Newer diagnostic modalities for tuberculosis

The gold standard for diagnosis of tuberculosis is demonstration of mycobacteria from various body fluids. This is often not possible in children due to pauci-bacillary nature of illness. Significant improvement in understanding of molecular biology of Mycobacterium tuberculosis has led to development of newer diagnostic techniques of tuberculosis. Polymerase chain reaction (PCR) is an emerging diagnostic tool for diagnosis of TB in children. However, its role in day-to-day clinical practice needs to be defined. A negative PCR never eliminates possibility of tuberculosis, and a positive result is not always confirmatory. The PCR may be useful in evaluating children with significant pulmonary disease when diagnosis is not readily established by other means, and in evaluating immunocompromised children (HIV infection) with pulmonary disease. In the absence of good diagnostic methods for tuberculosis, a lot of interest has been generated in serodiagnosis. ELISA has been used to detect antibodies to various purified or complex antigens of M. tuberculosis in children. Despite a large number of studies published over the past several years, serology has found little place in the routine diagnosis of tuberculosis in children, even though it is rapid and does not require specimen from the site of disease. Sensitivity and specificity depend on the antigen used, gold standard for the diagnosis of tuberculosis and the type of tubercular infection. Though most of these tests have high specificity, their sensitivity is poor. In addition, these tests may be influenced by factors such as age, prior BCG vaccination and exposure to environmental mycobacteria. The serological tests, theoretically, may not be able to differentiate between infection and disease. At present, serodiagnosis does not appear to have any role in diagnosis of childhood pulmonary tuberculosis. A new test (QuantiFERON-TB or QFT) that measures the release of interferon-gamma in whole blood in response to stimulation by purified protein derivative is comparable with the tuberculin skin testing to detect latent tubercular infection, and is less affected by BCG vaccination. It can also discriminate responses due to nontuberculous mycobacteria, and avoids variability and subjectivity associated with placing and reading the tuberculin skin test. Polymerase chain reaction based test for identification of katG and rpoB mutation which are associated with isoniazid and rifampicin resistance may help in early identification of drug resistance in mycobacterium.

Evaluation of integral membrane antigens of Mycobacterium habana for serodiagnosis of extrapulmonary tuberculosis: association between levels of antibodies and Mycobacterium tuberculosis antigens

The performance of integral membrane antigens (IMAs) of Mycobacterium habana TMC 5135 in detecting antimycobacterial antibodies in serum and body fluids of patients mainly of extrapulmonary tuberculosis was evaluated. The IMAs were recovered from the detergent phase during Triton X-114 treatment of the plasma membrane of M. habana. Antimycobacterial antibodies were detected by ELISA using IMAs in serum and body fluids of 42 patients and 62 control subjects. As authentic adjunct Mycobacterium tuberculosis antigens were also detected (by ELISA) in body fluids and circulating immune complexes using anti-M. tuberculosis H37Ra antibodies. Anti-M. habana IMA antibody detection increased the positivity rate from 26.% (11/42) and 10% (4/42) obtained by culture and smear microscopy, respectively, to 86% (36/42). M. tuberculosis antigens were also found in 29 out of 36 anti-M. habana IMA antibody-positive cases. Interestingly, all 11 culture-positive cases were also positive for anti-M. habana IMA antibodies. The mean antigen titres in 23 cases, positive for antigens in body fluids, were 2.34 times higher in those who were also positive for anti-IMA antibodies in serum than in those negative for these antibodies. M. habana IMAs may be promising non-tubercular candidate antigens in ELISA-based serodiagnosis of extrapulmonary tuberculosis with substantial sensitivity, specificity and safety.

Serologic response to a secreted and a cytosolic antigen of Mycobacterium tuberculosis in childhood tuberculosis

BACKGROUND AND AIM

Bacteriologic diagnosis of childhood tuberculosis is difficult, and alternate methods are needed. The utility of a serologic test for major secretory antigen (30 kDa) and a cytosolic antigen (16 kDa) of Mycobacterium tuberculosis was evaluated for the diagnosis of tuberculosis in children.

METHODS

Enzyme-linked immunosorbent assay was used. Specific IgG, IgA and IgM antibodies were measured in the sera from 26 clinically and/or bacteriologically diagnosed cases of childhood tuberculosis and 61 normal children.

RESULTS

Anti-IgG antibodies alone, against both 30- and 16-kDa antigens, were detected in 65.4% of patients. However, by combination of all three isotypes, increased sensitivities of 84.6 and 73%, with a specificity of 96.7% each, were obtained for 30- and 16-kDa antigens, respectively.

CONCLUSIONS

We found good specificity and reasonably good sensitivity for detection of antibodies by enzyme-linked immunosorbent assay to 30-kDa antigen alone. The 16-kDa antigen did not perform as well.

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.

Purification and characterisation of isocitrate dehydrogenase and malate dehydrogenase from Mycobacterium tuberculosis and evaluation of their potential as suitable antigens for the serodiagnosis of tuberculosis

SETTING

Enzymes from Mycobacterium tuberculosis are potent antigens and might thus be of interest in the serodiagnosis of tuberculosis.

OBJECTIVE

The purpose of the study was to purify and characterize the two enzymes isocitrate dehydrogenase (IDH) and malate dehydrogenase (MDH) from, M. tuberculosis and to evaluate their potential in the serodiagnosis of tuberculosis.

DESIGN

The two enzymes were analysed for specificity by electrophoresis and then purified by means of affinity chromatography using reactive dyes and ion exchange chromatography. The two isolated enzyme fractions were analysed by ELISA, using antisera against related organisms. They were then tested as antigens in ELISA together with sera from tuberculous patients and controls.

RESULTS

The electrophoretical analyses showed that the two enzymes each differed markedly from the corresponding enzymes of other mycobacteria. The serological analyses, however, could not distinguish between either IDH or MDH from other mycobacteria, but organisms of other genera, such as Nocardia, gave much weaker responses. When IDH and MDH were tested with sera from tuberculous patients and controls the former gave clearly higher optical density values than the latter.

CONCLUSION

The enzymes/antigens IDH and MDH may be of value in developing a serological test for tuberculosis. The latter fraction seemed particularly capable of discriminating patients from controls.