Enzyme-linked immunospot assay responses to early secretory antigenic target 6, culture filtrate protein 10, and purified protein derivative among children with tuberculosis: implications for diagnosis and monitoring of therapy

Response to M. tuberculosis selected RD1 peptides in Ugandan HIV-infected patients with smear positive pulmonary tuberculosis: a pilot study

Background

Tuberculosis (TB) is the most frequent co-infection in HIV-infected individuals still presenting diagnostic difficulties particularly in developing countries. Recently an assay based on IFN-gamma response to M. tuberculosis RD1 peptides selected by computational analysis was developed whose presence is detected during active TB disease. Objective of this study was to investigate the response to selected RD1 peptides in HIV-1-infected subjects with or without active TB in a country endemic for TB and to evaluate the change of this response over time.

Methods

30 HIV-infected individuals were prospectively enrolled, 20 with active TB and 10 without. Among those with TB, 12 were followed over time. IFN-gamma response to selected RD1 peptides was evaluated by enzyme-linked immunospot (ELISPOT) assay. As control, response to RD1 proteins was included. Results were correlated with immune, microbiological and virological data.

Results

Among patients with active TB, 2/20 were excluded from the analysis, one due to cell artifacts and the other to unresponsiveness to M. tuberculosis antigens. Among those analyzable, response to selected RD1 peptides evaluated as spot-forming cells was significantly higher in subjects with active TB compared to those without (p = 0.02). Among the 12 TB patients studied over time a significant decrease (p =< 0.007) of IFN-gamma response was found at completion of therapy when all the sputum cultures for M. tuberculosis were negative. A ratio of RD1 peptides ELISPOT counts over CD4⁺ T-cell counts greater than 0.21 yielded 100% sensitivity and 80% specificity for active TB. Conversely, response to RD1 intact proteins was not statistically different between subjects with or without TB at the time of recruitment; however a ratio of RD1 proteins ELISPOT counts over CD4⁺ T-cell counts greater than 0.22 yielded 89% sensitivity and 70% specificity for active TB.

Conclusion

In this pilot study the response to selected RD1 peptides is associated with TB disease in HIV-infected individuals in a high TB endemic country. This response decreases after successful therapy. The potential of the novel approach of relating ELISPOT spot-forming cell number and CD4⁺ T-cell count may improve the possibility of diagnosing active TB and deserves further evaluation.

Long-term follow-up of the QuantiFERON TB-2G test for active tuberculosis disease

OBJECTIVE

To evaluate transitional changes in QuantiFERON TB-2G (QFT-2G) test results in the serial testing on the same patients and to reevaluate the optimal threshold of positive response of QFT-2G test as a cure of TB infection.

METHODS

We prospectively investigated transitional changes of QFT-2G test results in 22 patients with active tuberculosis (TB) over three years after the initiation of treatment with antituberculosis drugs. Treatment using antituberculosis drugs was performed for six months in all patients.

RESULTS

The positive rate of QFT-2G test results decreased 50% at the treatment completion. Thereafter, although the positive rate of QFT-2G test results has been decreased 45% six months later even if treatment was finished, it decreased slightly to 41% two years later and 36% three years later. If the cut-off value was situated below 50% (IFN-gamma level three years later/ IFN-gamma level of baseline peak value), we could judge the conversion of QFT-2G test in most cases except for two cases three years after the initiation of antituberculosis treatment through this study.

CONCLUSION

It may be difficult to monitor markers in the cure of TB infection using QFT-2G tests. The cut-off level for a positive response on QFT-2G test may need to be reconsidered when the test is used to monitor the response of active TB to therapy.

Accuracy of an immune diagnostic assay based on RD1 selected epitopes for active tuberculosis in a clinical setting: a pilot study

A previous case-control study reported that an in-vitro interferon (IFN)-gamma response to early secreted antigenic target (ESAT)-6 selected peptides was associated with active tuberculosis (A-TB). The objective of the present pilot study was to evaluate the diagnostic accuracy of this assay for TB disease in a clinical setting. An IFN-gamma ELISPOT assay was performed on samples from patients with suspected A-TB using two peptides selected from ESAT-6 protein and three peptides selected from culture filtrate 10 (CFP-10) proteins. The results were compared with those obtained by two commercially available assays approved for diagnosis of TB infection (T SPOT-TB and QuantiFERON-TB Gold) which use ESAT-6/CFP-10 (RD1) overlapping peptides. Sensitivity to the RD1 selected peptides was 70% (positive for 16 of 23 patients with microbiologically diagnosed A-TB) and specificity was 91% (positive for three of 32 controls). In contrast, the sensitivity and specificity were 91% and 59%, respectively, for T SPOT-TB, and were 83% and 59%, respectively, for QuantiFERON-TB Gold. The RD1 selected peptides assay had the highest diagnostic odds ratio for A-TB. Thus, the results suggest that an assay based on RD1 selected peptides has a higher diagnostic accuracy for A-TB in a clinical setting compared with commercially available assays based on RD1 overlapping peptides.

Microbiological diagnostic procedures in respiratory infections: mycobacterial infections

This article will review traditional and newer microbiological techniques for the diagnosis of mycobacterial respiratory infections. It will concentrate on the diagnosis of infections due to Mycobacterium tuberculosis, the main mycobacterium causing respiratory infections of clinical and public health importance. The diagnosis of respiratory disease associated with non-tuberculous mycobacteria (NTM), particularly in children with underlying airway pathology such as cystic fibrosis (CF) or bronchiectasis, will be briefly discussed. With respect to the diagnosis of tuberculosis (TB), the review will concentrate on the diagnosis of patients with symptoms and/or signs of clinical disease, rather than the detection of exposure or asymptomatic infection. It will not specifically address the assessment of pre-test probability based on clinical or epidemiological factors, the use of radiological investigations or the investigation of extrathoracic lymph node disease or chest wall disease. The role of newer diagnostic modalities including nucleic acid detection (NAD) and gamma-interferon assays in paediatric practice will be reviewed, and suggestions made as to how they may fit into contemporary diagnostic algorithms.

Interferon- Release Assays Improve the Diagnosis of Tuberculosis and Nontuberculous Mycobacterial Disease in Children in a Country with a Low Incidence of Tuberculosis

BACKGROUND

Diagnosis of childhood tuberculosis (TB) is challenging. The widely used tuberculin skin test (TST) may produce -positive results because of cross-reactivity with nontuberculous mycobacteria or bacille Calmette-Guerin vaccination, resulting in unnecessary treatment. Two recently developed interferon- gamma release assays (IGRAs) show good diagnostic accuracy for active TB in adults; pediatric data are limited, particularly in areas with a low incidence of TB. We assessed the diagnostic accuracy of IGRAs for TB in children in an area with a low incidence of TB.

METHODS

In a hospital-based study, the diagnostic accuracy of the TST and 2 IGRAs (T SPOT-TB [T-SPOT; Oxford Immunotec] and QuantiFERON-TB Gold In-Tube [QFT-IT; Cellestis]) were assessed in a cohort of 73 children (median age, 39 months); 28 children with bacteriologically confirmed TB were compared with children without TB (23 with bacteriologically confirmed nontuberculous mycobacterial lymphadenitis and 22 with other nonmycobacterial respiratory tract infections).

RESULTS

The specificity for TB of QFT-IT was 100% (95% confidence interval [CI], 91%-100%), and the specificity of T-SPOT was 98% (95% CI, 87%-100%), both of which were considerably higher than the specificity of TST (58%; 95% CI, 42%-73%). The specificity of the TST was 10.5% (95% CI, 1%-33%) in children with nontuberculous mycobacterial lymphadenitis and was 100% (95% CI, 83%-100%) in children with other nonmycobacterial respiratory tract infections. The sensitivity of both QFT-IT and T-SPOT was 93% (95% CI, 77%-99%), and the sensitivity of the TST was 100% (95% CI, 88%-100%). Agreement between the IGRAs was 95.6% ( kappa =0.91); 6.8% of the IGRAs showed indeterminate results.

CONCLUSIONS

Both IGRAs showed high diagnostic value in bacteriologically confirmed childhood TB. Their advantage in this study, when performed in addition to the TST, was the ability to distinguish -positive TST results caused by nontuberculous mycobacterial disease, thereby reducing overdiagnosis of TB and guiding clinical management.

Diagnosis of tuberculosis by an enzyme-linked immunospot assay for interferon-gamma

*National Taiwan University Hospital, Taipei, Taiwan,

This assay for interferon-γ can rapidly and accurately diagnose active tuberculosis in a disease-endemic area.

We evaluated an enzyme-linked immunospot assay for interferon-γ (T SPOT-TB) for rapid diagnosis of active tuberculosis (TB) in a disease-endemic area. From January to June 2005, patients whose clinical symptoms and radiographic findings were compatible with TB were recruited, and a blood sample was obtained for T SPOT-TB assay within 7 days of microbiologic studies. Sixty-five patients were studied, including 39 (60%) with active TB. Thirty-five (53.8%) patients had underlying medical conditions. Thirty-seven patients had positive cultures for Mycobacterium tuberculosis, and 11 patients had positive cultures for nontuberculous mycobacteria. The sensitivity, specificity, positive predictive value, and negative predictive value of the T SPOT-TB assay were 87.2%, 88.5%, 91.9%, and 82.1%, respectively. The accuracy of this test in diagnosing active TB is >80%, even in an area with a high incidence of nontuberculous mycobacteria disease.

T-cell assays for the diagnosis of latent tuberculosis infection: moving the research agenda forward

For nearly a century, the tuberculin skin test was the only tool available for the detection of latent tuberculosis infection. A recent breakthrough has been the development of T-cell-based interferon-gamma release assays. Current evidence suggests interferon-gamma release assays have higher specificity than the tuberculin skin test, better correlation with surrogate markers of exposure to Mycobacterium tuberculosis in low-incidence settings, and less cross-reactivity as a result of BCG vaccination compared with the tuberculin skin test. The body of literature supporting the use of interferon-gamma release assays has rapidly expanded. However, several unresolved and unexplained issues remain. To address these issues, a group of experts met in Geneva, Switzerland, in March, 2006, to discuss the research evidence on T-cell-based assays, their clinical usefulness, limitations, and directions for future research, with a specific focus on resource-limited and high HIV prevalence settings. On the basis of 2 days of discussions, a comprehensive research agenda was generated, which will propel the field forward by stimulating focused high-impact research and encourage the investment of resources needed to tackle priority research questions, especially in resource-limited settings. Ultimately, if adequately financed, the research findings will inform appropriate use of novel latent tuberculosis infection diagnostics in global tuberculosis control.

Longitudinal assessment of an ELISPOT test for Mycobacterium tuberculosis infection

BACKGROUND

Very little longitudinal information is available regarding the performance of T cell-based tests for Mycobacterium tuberculosis infection. To address this deficiency, we conducted a longitudinal assessment of the enzyme-linked immunosorbent spot test (ELISPOT) test in comparison to the standard tuberculin skin test (TST).

METHODS AND FINDINGS

In tuberculosis (TB) contacts we repeated ELISPOT tests 3 mo (n = 341) and 18 mo (n = 210) after recruitment and TSTs at 18 mo (n = 130). We evaluated factors for association with conversion and reversion and investigated suspected cases of TB. Of 207 ELISPOT-negative contacts, 51 (24.6%) had 3-mo ELISPOT conversion, which was associated with a positive recruitment TST (odds ratio [OR] 2.2, 95% confidence interval [CI] 1.0-5.0, p = 0.048) and negatively associated with bacillus Calmette-Guérin (BCG) vaccination (OR 0.5, 95% CI 0.2-1.0, p = 0.06). Of 134 contacts, 54 (40.2%) underwent 3-mo ELISPOT reversion, which was less likely in those with a positive recruitment TST (OR 0.3, 95% CI 0.1-0.8, p = 0.014). Between 3 and 18 mo, 35/132 (26.5%) contacts underwent ELISPOT conversion and 28/78 (35.9%) underwent ELISPOT reversion. Of the 210 contacts with complete results, 73 (34.8%) were ELISPOT negative at all three time points; 36 (17.1%) were positive at all three time points. Between recruitment and 18 mo, 20 (27%) contacts had ELISPOT conversion; 37 (50%) had TST conversion, which was associated with a positive recruitment ELISPOT (OR 7.2, 95% CI 1.4-37.1, p = 0.019); 18 (32.7%) underwent ELISPOT reversion; and five (8.9%) underwent TST reversion. Results in 13 contacts diagnosed as having TB were mixed, but suggested higher TST sensitivity.

CONCLUSIONS

Both ELISPOT conversion and reversion occur after M. tuberculosis exposure. Rapid ELISPOT reversion may reflect M. tuberculosis clearance or transition into dormancy and may contribute to the relatively low reported ELISPOT conversion rate. Therefore, a negative ELISPOT test for M. tuberculosis infection should be interpreted with caution.

New approaches and emerging technologies in the diagnosis of childhood tuberculosis

Childhood tuberculosis (TB) has long been neglected by TB control programmes, as children tend to develop sputum smear-negative disease and rarely contribute to disease transmission. However, children suffer severe TB-related morbidity and mortality in areas with endemic TB and carry a significant proportion of the global disease burden. Apart from improved control of the global TB epidemic, access to accurate diagnosis and effective treatment is essential to reduce the disease burden associated with childhood TB. Access to child friendly anti-TB treatment is improving, but establishing an accurate diagnosis remains a challenge. This review provides an overview of recent advances in the diagnosis of childhood TB, focusing on bacteriological, immunological, radiological and symptom-based approaches. It is possible to establish a fairly accurate diagnosis of either latent infection or active TB in immunocompetent children, even in resource-limited settings, but establishing an accurate diagnosis of TB in HIV-infected (immunocompromised) children remains a major challenge.

T cell-based diagnosis of childhood tuberculosis infection

PURPOSE OF REVIEW

T-cell interferon-gamma release assays (TIGRAs), available as enzyme-linked immunospot (ELISpot) and enzyme-linked immunoassay (ELISA), potentially significantly advance on the tuberculin skin test (TST) for diagnosis of tuberculosis infection. We review all publications using TIGRAs in children to appraise paediatricians of the advantages and limitations of these new blood tests.

RECENT FINDINGS

Unlike TST, both tests are independent of Bacille Calmette-Guérin vaccination status, providing higher diagnostic specificity. In children with active tuberculosis ELISpot is more sensitive than TST and is unaffected by HIV infection, age under 3 years or malnutrition; ELISA data are currently limited. In the absence of a gold-standard test for latent tuberculosis infection, tuberculosis exposure was used as a surrogate marker; ELISpot generally correlates better with tuberculosis exposure than TST, while ELISA correlates broadly similarly. Indeterminate test results in young children are rare with ELISpot and are more common with ELISA.

SUMMARY

Although longitudinal studies quantifying risk of progression to tuberculosis in tuberculosis-exposed children with positive TIGRA results are required urgently, the small but rapidly expanding evidence-base since the first application of TIGRAs to childhood tuberculosis in 2003 combined with recent national guidelines makes a strong case for judicious use of TIGRAs in clinical management of paediatric tuberculosis.

Clinical, immunological, and epidemiological importance of antituberculosis T cell responses in HIV-infected Africans

BACKGROUND

Human immunodeficiency virus (HIV)-associated tuberculosis is a major cause of mortality in Africa. The assay of T cell interferon- gamma released in response to antigens of greater specificity than purified protein derivative is a useful improvement over the Mantoux tuberculin skin test, but few studies have evaluated interferon-gamma secretion in HIV-infected individuals.

METHODS

Mycobacterium tuberculosis antigen-specific interferon-gamma secretion was assessed by whole blood assay and enzyme-linked immunospot, which were compared with the Mantoux tuberculin skin test in HIV-infected and HIV-uninfected individuals without active tuberculosis and HIV-infected patients with pulmonary tuberculosis in Khayelitsha, South Africa.

RESULTS

The skin test and whole blood assay responses to purified protein derivative in HIV-positive subjects were decreased, compared with responses in HIV-negative subjects (P < .001). By contrast, the responses to M. tuberculosis antigens (early secreted antigenic target 6, culture filtrate protein 10, TB10.3, and alpha-crystallin 2) were less affected, indicating a high prevalence of latent tuberculosis (approximately 80%) in both HIV-negative and HIV-positive subject groups. Whole blood assay responses did not differ between the HIV-positive subjects without tuberculosis and HIV-positive subjects with tuberculosis, but the enzyme-linked immunospot method response to early secreted antigenic target 6 and culture filtrate protein 10 was higher in the group of HIV-infected subjects with tuberculosis (P < or = .04), although this group had lower CD4+ cell counts. A ratio of the combined enzyme-linked immunospot method response divided by the CD4+ cell count of > 1.0 had 88% sensitivity and 80% specificity for active pulmonary tuberculosis in HIV-infected individuals.

CONCLUSIONS

Interferon-gamma release appears to be less impaired than skin testing by HIV coinfection. The novel potential to relate the enzyme-linked immunospot method and CD4+ cell count to assist diagnosis of active tuberculosis in patients with HIV infection is important and deserves further evaluation.

Recent advances in the diagnosis of childhood tuberculosis

Children account for a major proportion of the global tuberculosis disease burden, especially in endemic areas. However, the accurate diagnosis of childhood tuberculosis remains a major challenge. This review provides an overview of the most important recent advances in the diagnosis of intrathoracic childhood tuberculosis: (1) symptom-based approaches, including symptom-based screening of exposed children and symptom-based diagnosis of active disease; (2) novel immune-based approaches, including T cell assays and novel antigen-based tests; and (3) bacteriological and molecular methods that are more rapid and/or less expensive than conventional culture techniques for tuberculosis diagnosis and/or drug-resistance testing. Recent advances have improved our ability to diagnose latent infection and active tuberculosis in children, but establishing a diagnosis of either latent infection or active disease in HIV-infected children remains a major challenge, particularly in high-burden settings. Although improved access to diagnosis and treatment is essential, ultimately the burden of childhood tuberculosis is determined by the level of epidemic control achieved in a particular community. Several recent initiatives, in particular the United Nations Millennium Developmental Goals, deal with the problem of poverty and disease in a holistic fashion, but global political commitment is required to support these key initiatives.

Enhanced anti-mycobacterial immunity in children with erythema nodosum and a positive tuberculin skin test

Erythema nodosum (EN) may follow a variety of infections, but in regions with a high prevalence of tuberculosis, is frequently associated with a positive tuberculin skin test (TST) and tuberculosis infection. We aimed to investigate the immunological differences between patients with EN as a manifestation of primary tuberculosis, and those with progressive pulmonary tuberculosis (PTB) or asymptomatic infection. We studied the inflammatory response to both mycobacterial and non-mycobacterial antigens in 11 children with EN associated with a positive TST, 22 children with culture-confirmed tuberculosis, and 53 healthy skin test-positive children. In addition, we evaluated functional anti-mycobacterial immunity using an ex vivo assay of mycobacterial growth restriction in five children with EN and 15 with PTB. Patients with EN were distinguished by enhanced mycobacterial growth restriction on the functional assay, which was associated with a markedly increased production of IFNgamma in response to stimulation with purified protein derivative of Mycobacterium tuberculosis. Children presenting with EN and a positive TST show evidence of responses associated with enhanced anti-mycobacterial immunity.

Dynamic relationship between IFN-gamma and IL-2 profile of Mycobacterium tuberculosis-specific T cells and antigen load

Distinct IFN-gamma and IL-2 profiles of Ag-specific CD4(+) T cells have recently been associated with different clinical disease states and Ag loads in viral infections. We assessed the kinetics and functional profile of Mycobacterium tuberculosis Ag-specific T cells secreting IFN-gamma and IL-2 in 23 patients with untreated active tuberculosis when bacterial and Ag loads are high and after curative treatment, when Ag load is reduced. The frequencies of M. tuberculosis Ag-specific IFN-gamma-secreting T cells declined during 28 mo of follow-up with an average percentage decline of 5.8% per year (p = 0.005), while the frequencies of Ag-specific IL-2-secreting T cells increased during treatment (p = 0.02). These contrasting dynamics for the two cytokines led to a progressive convergence of the frequencies of IFN-gamma- and IL-2-secreting cells over 28 mo. Simultaneous measurement of IFN-gamma and IL-2 secretion at the single-cell level revealed a codominance of IFN-gamma-only secreting and IFN-gamma/IL-2 dual secreting CD4(+) T cells in active disease that shifted to dominance of IFN-gamma/IL-2-secreting CD4(+) T cells and newly detectable IL-2-only secreting CD4(+) T cells during and after treatment. These distinct T cell functional signatures before and after treatment suggest a novel immunological marker of mycobacterial load and clinical status in tuberculosis that now requires validation in larger prospective studies.

Les nouveaux tests diagnostiques de la tuberculose

INTRODUCTION

A major challenge in tuberculosis (TB) control is the diagnosis and the treatment of latent tuberculosis infection.

STATE OF THE ART

At the time, the diagnosis is based on tuberculin skin test (TST). TST is not specific, has poor sensitivity and is not easy to perform.

PERSPECTIVES

Two interferon-based tests for the diagnosis of tuberculosis have just been licensed. These tests have some advantages on TST. They require only a blood sample and their results are not dependent on the examinator. Their specificity is higher than TST because they don't cross-react with BCG vaccination and with most of the environmental Mycobacterium species. Their sensitivity is higher for the diagnosis of active tuberculosis too. For latent tuberculosis, the interferon-gamma assays show a better correlation with the exposure to Mycobacterium tuberculosis than TST. The ability to detect TB of the two tests seem to be reduced in immunocompromised patients, specially in medically ones.

CONCLUSIONS

Interferon-gamma assays seems to be useful tools in TB detection, but these good results have to be confirmed in larger studies with unselected patients.

Avantages et limites des tests sanguins in vitro lymphocytes T/interféron gamma comparativement au test intradermique à la tuberculine pour le diagnostic de tuberculose☆

The development of in vitro blood tests that measure the delayed hypersensitivity reaction developed after contact with Mycobacterium tuberculosis will change progressively the diagnosis of M. tuberculosis infection. These blood assays (Quantiferon TB Gold, Cellestis, Australia; T-SPOT.TB, Oxford Immunotec, United Kingdom) use specific, complex M. tuberculosis antigens (ESAT-6 and CFP-10), whereas the intra-dermal Mantoux test is done with tuberculin, a complex mixture of more than 200 antigens. ESAT-6 and CFP-10 are absent from all the BCG vaccine strains used throughout the world. Significant improvement in the specificity with equivalent or increased sensitivity of the in vitro tests compared to the Mantoux test will lead eventually to replacement of the latter.

Latent Tuberculosis Infection Treatment and T-Cell Responses toMycobacterium tuberculosis–specific Antigens

RATIONALE

There is currently no available test for monitoring the effect of treatment of latent tuberculosis infection (LTBI) to indicate cure or predict risk of subsequent progression to disease.

OBJECTIVE

We used the T-SPOT.TB assay, which measures T-cell interferon-gamma responses to the Mycobacterium tuberculosis-specific peptides early secretory antigenic target 6-kD protein (ESAT-6) and culture filtrate protein 10 (CFP-10), to determine the effect of LTBI treatment on these responses.

METHODS

A total of 226 tuberculosis contacts with positive T-SPOT.TB results underwent repeat testing on LTBI treatment completion. The majority (96%) received 6 months of isoniazid. The pre- and post-treatment T-SPOT.TB results were analyzed according to the combined and separate responses to ESAT-6 and CFP-10 antigens.

RESULTS

The T-SPOT.TB reverted to negative in 85 (37.6%) contacts at treatment completion. Treatment had a significant effect on the response to CFP-10 (p < 0.001; reversion rate, 48.6%), but not on the response to ESAT-6 (p = 0.081; reversion rate, 21.6%). The median number of spot-forming cells (SFCs)/2.5 x 10(5) peripheral blood mononuclear cells (PBMCs) pre- and post-treatment was 6 versus 4.5 for ESAT-6 (p = 0.116) and 11 versus 4 for CFP-10 (p < 0.001). There was a significant difference between the change (fall) in the pre- and post-treatment responses to CFP-10 (6 SFCs/2.5 x 10(5) PBMCs) and ESAT-6 (0 SFCs/2.5 x 10(5) PBMCs; p < 0.001). Significantly different age-related T-cell responses to the two antigens were found.

CONCLUSION

LTBI treatment had a differential effect on T-cell responses to ESAT-6 and CFP-10 as measured by the T-SPOT.TB. The quantitative response to CFP-10 may be a useful LTBI treatment-monitoring tool.

Isoniazid prophylaxis differently modulates T-cell responses to RD1-epitopes in contacts recently exposed to Mycobacterium tuberculosis: a pilot study

RATIONALE

Existing data on the effect of treatment of latent tuberculosis infection (LTBI) on T-cell responses to Mycobacterium tuberculosis (MTB)-specific antigens are contradictory. Differences in technical aspects of the assays used to detect this response and populations studied might explain some of these discrepancies. In an attempt to find surrogate markers of the effect of LTBI treatment, it would be important to determine whether, among contacts of patients with contagious tuberculosis, therapy for LTBI could cause changes in MTB-specific immune responses to a variety of RD1-antigens.

METHODS AND RESULTS

In a longitudinal study, 44 tuberculin skin test+ recent contacts were followed over a 6-month period and divided according to previous exposure to MTB and LTBI treatment. The following tests which evaluate IFN-gamma responses to RD1 antigens were performed: QuantiFERON TB Gold, RD1 intact protein- and selected peptide-based assays. Among the 24 contacts without previous exposure that completed therapy, we showed a significant decrease of IFN-gamma response in all tests employed. The response to RD1 selected peptides was found to be more markedly decreased compared to that to other RD1 antigens. Conversely, no significant changes in the response to RD1 reagents were found in 9 treated subjects with a known previous exposure to MTB and in 11 untreated controls.

CONCLUSION

These data suggest that the effect of INH prophylaxis on RD1-specific T-cell responses may be different based on the population of subjects enrolled (recent infection versus re-infection) and, to a minor extent, on the reagents used.

Clinical Reevaluation of the QuantiFERON TB-2G Test as a Diagnostic Method for Differentiating Active Tuberculosis from Nontuberculous Mycobacteriosis

INTRODUCTION

We reevaluated the usefulness of a whole-blood interferon-gamma enzyme-linked immunosorbent assay (QuantiFERON TB-2G [QFT-TB]; Cellestis) in obtaining a differential diagnosis between active tuberculosis (TB) and nontuberculous mycobacteriosis (NTM).

METHODS

The subjects were 50 healthy volunteers, 50 patients with active TB, and 100 patients with NTM who satisfied the diagnostic guidelines of the American Thoracic Society from April 2005 through June 2006. The tuberculin skin test (TST) and the QFT-TB test were performed for all subjects. The QFT-TB test was performed every 2 months.

RESULTS

Of the healthy volunteers, 64% had a negative TST result and 94% had a negative QFT-TB test result. Of the patients with active TB, 64% had a positive TST result and 4% had a negative QFT-TB test result. Of the patients with pulmonary Mycobacterium avium complex disease, 60% had a positive TST result and 7% had a positive QFT-TB test result. The QFT-TB test had a mean sensitivity of 86% and a mean specificity of 94%. The QFT-TB test results for patients with active TB transiently decreased during treatment involving antituberculous drugs. The rate of positive QFT-TB test results was 86% at the initiation of treatment, 48% 6 months later, and 33% 12 months later.

CONCLUSIONS

We confirmed that the QFT-TB test is a useful diagnostic method for differentiating active pulmonary TB from NTM, compared with the TST. However, because it is possible that the effect of the QFT-TB test may be long lasting after treatment and may not be resolved over time, even with treatment, as in this study, it may not provide any level of certainty regarding cure of infection.

Dynamic Antigen-specific T-Cell Responses after Point-Source Exposure toMycobacterium tuberculosis

RATIONALE

The kinetics of Mycobacterium tuberculosis-specific Th1-type T-cell responses after M. tuberculosis infection are likely to be important in determining clinical outcome.

OBJECTIVE

To investigate the kinetics of T-cell responses, in the context of a point-source school tuberculosis outbreak, in three groups of contacts who differed by preventive treatment status and tuberculin skin test (TST) results: 38 treated TST-positive students, 11 untreated TST-positive staff, and 14 untreated students with negative or borderline TST results.

METHODS

We used the ex vivo IFN-gamma enzyme-linked immunospot assay (ELISpot) to track T cells specific for two region of difference 1 (RD1) antigens, early secretory antigenic target 6 and culture filtrate protein 10, for 18 mo after cessation of tuberculosis exposure.

MAIN RESULTS

The treated TST-positive students had an average 68% decline in frequencies of RD1-specific IFN-gamma-secreting T cells per year (p < 0.0001) and 6 of 38 students had no detectable RD1-specific T cells by 18 mo. No change in frequencies of these cells was observed in the untreated TST-positive staff (p = 0.38) and none were ELISpot-negative at 18 mo. Of the 14 untreated students, 7 were persistently ELISpot-positive (all of whom had borderline TST results), and 7 became ELISpot-negative (all but one had negative TST results) during follow-up.

CONCLUSIONS

The decrease in M. tuberculosis-specific T cells and their disappearance in a proportion of treated students likely reflect declining antigenic and bacterial load in vivo induced by antibiotic treatment. The observed disappearance of M. tuberculosis-specific T cells in the untreated TST-negative contacts suggests that an acute resolving infection may occur in some contacts.

Risk for tuberculosis among children

Contacts of adults with tuberculosis (TB) are at risk for infection. Tests based on interferon-gamma (IFN-gamma) expression in response to Mycobacterium tuberculosis antigens may be more sensitive than the tuberculin skin test (TST). Risk for infection was assessed by using TST and an IFN-y-based assay (QuantiFERON Gold in Tube [QFT-IT] test) for 207 children in Nigeria in 1 of 3 groups: contact with adults with smear-positive TB, contact with adults with smear-negative TB, and controls. For these 3 groups, respectively, TST results were >10 mm for 38 (49%) of 78, 13 (16%) of 83, and 6 (13%) of 46 and QFT-IT positive for 53 (74%) of 72, 8 (10%) of 81, and 4 (10.3%) of 39 (p < 0.01). Most test discrepancies were TST negative; QFT-IT positive if in contact with TB-positive persons; and TST positive, QFT-IT negative if in contact with TB-negative persons or controls. TST may underestimate risk for infection with TB in children.

Selected RD1 peptides for active tuberculosis diagnosis: comparison of a gamma interferon whole-blood enzyme-linked immunosorbent assay and an enzyme-linked immunospot assay

We recently set up a gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT), using selected early secreted antigenic target 6 (ESAT-6) peptides, that appears specific for active tuberculosis (A-TB). However, ELISPOT is difficult to automate. Thus, the objective of this study was to determine if the same selected peptides may be used in a technique more suitable for routine work in clinical laboratories, such as whole-blood enzyme-linked immunosorbent assay (WBE). For this purpose, 27 patients with A-TB and 41 control patients were enrolled. Our WBE, using the already described selected peptides from ESAT-6 plus three new ones from culture filtrate protein 10, was performed, and data were compared with those obtained by ELISPOT. Using our selected peptides, IFN-gamma production, evaluated by both WBE and ELISPOT, was significantly higher in patients with A-TB than in controls (P < 0.0001). Statistical analysis showed a good correlation between the results obtained by WBE and ELISPOT (r = 0.80, P < 0.001). To substantiate our data, we compared our WBE results with those obtained by QuantiFERON-TB Gold, a whole-blood assay based on region of difference 1 (RD1) overlapping peptides approved for TB infection diagnosis. We observed a slightly higher sensitivity with QuantiFERON-TB Gold than with our WBE (89% versus 81%); however, our test provided a better specificity result (90% versus 68%). In conclusion, results obtained by WBE based on selected RD1 peptides significantly correlate with those generated by ELISPOT. Moreover, our assay appears more specific for A-TB diagnosis than QuantiFERON-TB Gold, and thus it may represent a complementary tool for A-TB diagnosis for routine use in clinical laboratories.

Comparison of a whole blood interferon-gamma assay with tuberculin skin testing for the detection of tuberculosis infection in hospitalized children in rural India

OBJECTIVE

In vitro interferon-gamma (IFN-gamma) assays have emerged as novel alternatives to the tuberculin skin test (TST) for the diagnosis of latent tuberculosis (TB) infection. These assays have been evaluated in low incidence countries, mainly in adults, and have been shown to be more specific than TST. Because few studies have been done in high incidence countries, and because paediatric data are limited, we compared a whole-blood IFN-gamma assay with TST among hospitalized Indian children.

METHODS

Between July 2004 and June 2005, a total of 105 consecutively admitted children (median age 6 years; 82% had BCG scars) in whom TB was suspected or had history of contact with an index case were recruited at a rural hospital in India. All children underwent TST, and the QuantiFERON-TB-Gold In Tube (QFT) assay.

RESULTS

The overall prevalence of TB infection was similar with both tests. With a TST cut-off point of > or =10mm, 10 of 105 (9.5%; 95% CI 3.8, 15.2) children were TST positive. With a cut-off point of IFN-gamma> or =0.35IU/ml, 11 of 105 (10.5%; 95% CI 4.5, 16.4) were QFT positive. The concordance between TST and QFT was substantial (agreement 95.2%; kappa [kappa] 0.73; 95% CI for kappa 0.53, 0.92). Agreement between TST and QFT results was 100% (kappa 1.0) in BCG scar-negative children as compared to 94% (kappa 0.63) in scar-positive children. BCG was not associated with the results of either TST or QFT (P>0.05 for both tests). The number of children with bacteriologically confirmed active TB was too small to permit the estimation of sensitivity of the tests.

CONCLUSIONS

In a rural, predominantly BCG-vaccinated paediatric population in India, the TST and QFT assay produced comparable results. BCG vaccination did not significantly affect either TST or QFT results. Larger studies are needed to compare the sensitivity of the IFN-gamma assay with that of the TST in children with bacteriologically and/or clinically confirmed TB.

Early detection of perinatal tuberculosis using a whole blood interferon-gamma release assay

BACKGROUND

The diagnosis of perinatal tuberculosis (TB) is problematic because of its nonspecific presentation, the difficulty of obtaining microbiological confirmation, and the unreliability of the tuberculin skin test. Immunodiagnosis of TB has received new attention with the discovery of Mycobacterium tuberculosis-specific immunodominant antigens (early secreted antigenic target 6 [ESAT-6] and culture filtrate protein 10 [CFP-10]) that are encoded by the RD1 region of the pathogen. A whole blood assay has recently been developed to quantitatively measure interferon- gamma production by lymphocytes specific to these antigens, but its evaluation in the diagnosis of TB in infants and children has been limited to date.

METHODS

In addition to routine diagnostic evaluation (tuberculin skin tests, culture of early-morning gastric aspirate samples, and chest radiographs), 2 infants with suspected perinatal TB were investigated with a whole blood interferon-gamma release assay.

RESULTS

The results of the tuberculin skin tests were negative for both patients. The findings of the chest radiographs were abnormal with features suggestive of miliary TB. A whole blood interferon- gamma release assay was performed and yielded positive results within 48 h after admission to the hospital for both patients, prompting early antituberculous treatment. M. tuberculosis was cultured after 6 weeks from gastric aspirate samples collected on admission to the hospital from both infants. At 6 months of age, both infants were thriving and had acheived normal developmental milestones.

CONCLUSIONS

The advent of interferon- gamma release assays may prove to be useful in the evaluation of infants with suspected perinatal TB.

Performance of a whole blood interferon gamma assay for detecting latent infection with Mycobacterium tuberculosis in children

BACKGROUND

The diagnosis of latent Mycobacterium tuberculosis (MTB) infection with a tuberculin skin test (TST) in children is complicated by the potential influence of prior exposure to Bacille Calmette Geurin (BCG) vaccination or environmental mycobacteria. A whole blood assay has recently been developed to quantitatively measure interferon gamma (IFN-gamma) production by lymphocytes specific to the MTB antigens ESAT-6 and CFP-10, but its use and assessment in children has been limited. A study was undertaken to compare the performance of the whole blood IFN-gamma assay with the TST in diagnosing latent tuberculosis (TB) infection or TB disease in children in routine clinical practice.

METHODS

One hundred and six children with a high risk of latent TB infection or TB disease were enrolled in the study. High risk was defined as contact with TB disease, clinical suspicion of TB disease, or recent arrival from an area of high TB prevalence. The whole blood IFN-gamma assay was undertaken in 101 children.

RESULTS

Seventeen (17%) of the 101 assays yielded inconclusive results due to failure of positive or negative control assays. There was poor correlation between the whole blood IFN-gamma assay and the TST (kappa statistic 0.3) with 26 (70%) of the 37 children defined as latent TB infection by TST having a negative whole blood IFN-gamma assay. There were no instances of a positive whole blood IFN-gamma assay with a negative TST. Mitogen (positive) control IFN-gamma responses were significantly correlated with age (Spearman's coefficient = 0.53, p<0.001) and, in children with latent TB infection identified by TST, those with a positive IFN-gamma assay were older (median 12.9 v 6.92 years, respectively, p = 0.007). The whole blood IFN-gamma assay was positive in all nine children with TB disease.

CONCLUSION

There was poor agreement between the whole blood IFN-gamma assay and TST for the diagnosis of latent TB. The whole blood IFN-gamma assay may have lower sensitivity than the TST in diagnosing TB infection in children. A significant proportion of whole blood IFN-gamma assays fail when used as a screening assay in routine practice.

Latent tuberculosis in HIV positive, diagnosed by the M. tuberculosis specific interferon-gamma test

Background

Although tuberculosis (TB) is a minor problem in Denmark, severe and complicated cases occur in HIV positive. Since the new M. tuberculosis specific test for latent TB, the QuantiFERON-TB In-Tube test (QFT-IT) became available the patients in our clinic have been screened for the presence of latent TB using the QFT-IT test. We here report the results from the first patients screened.

Methods

On a routine basis the QFT-IT test was performed and the results from 590 HIV positive individuals consecutively tested are presented here. CD4 cell count and TB risk-factors were recorded from patient files.

Main findings

27/590(4.6%) of the individuals were QFT-IT test positive, indicating the presence of latent TB infection. Among QFT-IT positive patients, 78% had risk factors such as long-term residency in a TB high endemic area (OR:5.7), known TB exposure (OR:4.9) or previous TB disease (OR:4.9). The prevalence of latent TB in these groups were 13%, 16% and 19% respectively. There was a strong correlation between low CD4 T-cell count and a low mitogen response (P < 0.001;Spearman) and more patients with low CD4 cell count had indeterminate results.

Conclusion

We found an overall prevalence of latent TB infection of 4.6% among the HIV positive individuals and a much higher prevalence of latent infection among those with a history of exposure (16%) and long term residency in a high endemic country (13%). The QFT-IT test may indeed be a useful test for HIV positive individuals, but in severely immunocompromised, the test may be impaired by T-cell anergy.

Reversion of the ELISPOT test after treatment in Gambian tuberculosis cases

Background

New tools are required to improve tuberculosis (TB) diagnosis and treatment, including enhanced ability to compare new treatment strategies. The ELISPOT assay uses Mycobacterium tuberculosis-specific antigens to produce a precise quantitative readout of the immune response to pathogen. We hypothesized that TB patients in The Gambia would have reduced ELISPOT counts after successful treatment.

Methods

We recruited Gambian adults with sputum smear and culture positive tuberculosis for ELISPOT assay and HIV test, and followed them up one year later to repeat testing and document treatment outcome. We used ESAT-6, CFP-10 and Purified Protein Derivative (PPD) as stimulatory antigens. We confirmed the reliability of our assay in 23 volunteers through 2 tests one week apart, comparing within and between subject variation.

Results

We performed an ELISPOT test at diagnosis and 12 months later in 89 patients. At recruitment, 70/85 HIV-negative patients (82%) were ESAT-6 or CFP-10 (EC) ELISPOT positive, 77 (90%) were PPD ELISPOT positive. Eighty-two cases (96%) successfully completed treatment: 44 (55%; p < 0.001) were EC ELISPOT negative at 12 months, 17 (21%; p = 0.051) were PPD ELISPOT negative. Sixty (73%) cured cases had a CFP-10 ELISPOT count decrease, 64 (78%) had an ESAT-6 ELISPOT count decrease, 58 (70%) had a PPD ELISPOT count decrease. There was a mean decline of 25, 44 and 47 SFU/2 × 10⁵ cells for CFP-10, ESAT-6 and PPD respectively (p < 0.001 for all). Three of 4 HIV positive patients were cured, all 3 underwent ELISPOT reversion; all 4 not cured subjects (3 HIV-negative, 1 HIV positive) were ESAT-6, CFP-10 and PPD ELISPOT positive at 12 months.

Conclusion

Successful tuberculosis treatment is accompanied by a significant reduction in the M. tuberculosis-specific antigen ELISPOT count. The ELISPOT has potential as a proxy measure of TB treatment outcome. Further investigation into the decay kinetics of T-cells with treatment is warranted.