Peripheral T Cell Interferon-γ Responses and Latent Tuberculosis

Prevalence of latent tuberculosis infection in Sudan: a case-control study comparing interferon-γ release assay and tuberculin skin test

BACKGROUND

Most people exposed to M. tuberculosis show no evidence of clinical disease. Five to 10% of individuals with latent infection progress to develop overt disease during their life time. Identification of people with latent TB infection will increase case detection rates and may dictate new treatment policies to control tuberculosis. This study aimed to determine LTBI point prevalence in a population from Sudan using two different diagnostic methods: the tuberculin skin test (TST) and the IFN-γ release assay (IGRA).

METHODS

This was a prospective, community-based and case-controlled study. Following informed consent, household contacts (HHCs; n = 98) of smear-positive index cases and Community controls (CCs; 186), were enrolled. Tuberculin skin test (TST), whole blood stimulation with ESAT-6/CFP-10 ± TB7.7 antigens or purified protein derivative (PPD) and IFN-γ levels determination with ELISA were performed. The levels of IFN-γ and TST induration between the CCs and the HHCs were compared using student t-test, Chi-square and Kappa coefficient. Pearson correlation test was used to compare TST and IFN-γ. P levels of <0.05 were considered significant.

RESULTS

TST induration of ≥ 10 mm gave an LTBI point prevalence of 327 cases/1000 individuals among HHCs compared to 126 cases/1000 individuals among CCs (p = 0.000). PPD-induced IFN-γ release assay gave an LTBI point prevalence of 418 cases/1000 individuals among HHCs compared to 301 cases/1000 individuals among CCs (p =0.06). On the other hand ESAT-6/CFP-10 ± TB7.7-induced IFN-γ gave an LTBI point prevalence of 429 cases/1000 individuals among HHCs compared to 268 cases/1000 individuals among CCs (p = 0.01). IFN-γ productions levels induced by ESAT-6/CPF-10 ± TB7.7 antigens in HHCS and CCs were not significantly different from those induced by PPD (p = 0.7).

CONCLUSION

IFN-γ release assay (IGRA) gave higher LTBI point prevalence compared to TST in HHCs and CCs. PPD gave comparable results to ESAT-6/CFP-10 ± TB7.7 antigens in whole blood IFN-γ release, making it a cheap alternative to the recombinant antigens.

Rapid diagnosis of smear-negative tuberculosis by bronchoalveolar lavage enzyme-linked immunospot

RATIONALE

In a large proportion of patients with active pulmonary tuberculosis (pTB), acid-fast bacilli smear results for sputum and bronchial secretions are negative. Detectable growth of Mycobacterium tuberculosis (MTB) in cultures takes several weeks and MTB-specific DNA amplification results on sputum and bronchial secretions are variable in these patients.

OBJECTIVE

We investigated whether a rapid diagnosis of pTB can be established by enumeration of MTB-specific mononuclear cells from bronchoalveolar lavage (BAL) fluid in routine clinical practice.

METHODS

Patients presenting to a tertiary hospital with medical histories and pulmonary infiltrates compatible with tuberculosis, and negative acid-fast bacilli smear results (three) from sputum, were prospectively enrolled in this study. An MTB-specific enzyme-linked immunospot assay (ELISPOT [T-SPOT.TB; Oxford Immunotec, Abingdon, UK]) with early antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) peptides was performed on peripheral blood mononuclear cells (PBMCs) and mononuclear cells from the BAL fluid (BALMCs).

MEASUREMENTS AND MAIN RESULTS

Of 37 patients, 12 were found to have smear-negative pTB and 25 were found to have an alternative diagnosis. Patients with tuberculosis had a median number of 17 ESAT-6-specific cells and 24.5 CFP-10-specific cells per 200,000 PBMCs and 37.5 ESAT-6-specific cells and 49.5 CFP-10-specific cells per 200,000 cells in the BAL fluid. Control patients had a median of 1 ESAT-6-specific cell and 1 CFP-10-specific cell per 200,000 PBMCs and no ESAT-6- and CFP-10-specific cells per 200,000 cells in the BAL fluid (p < 0.0001). All patients with TB but none of the control subjects had more than 5 spot-forming cells per 200,000 BALMCs with either peptide in the BAL fluid ELISPOT.

CONCLUSION

Smear-negative pulmonary tuberculosis can be diagnosed rapidly by identification of MTB-specific cells in the BAL fluid.

Utility of the antigen-specific interferon-gamma assay for the management of tuberculosis

PURPOSE OF REVIEW

The tuberculin skin test, now over a century old, is prone to reader variability, and outcomes are influenced by cross-reactivity with environmental mycobacteria, previous bacillus Calmette-Guerin (BCG) vaccination, and anergy in immunosuppressed individuals. More recently, T-cell-based interferon-gamma responses to Mycobacterium tuberculosis - specific antigens have been investigated for their role in diagnosing latent tuberculosis infection.

RECENT FINDINGS

We review the evidence supporting the utility of the interferon-gamma assay for the diagnosis of latent tuberculosis infection (LTBI) in low-prevalence countries. We discuss the principle of the test, technical factors related to performance, and its utility in active tuberculosis, in specialised subgroups such as immunocompromised patients, and its applicability in developing countries.

SUMMARY

Compared with the tuberculin skin test, the antigen-specific interferon-gamma assay, when used in a standardised protocol (overnight incubation assay using a combination of two antigens) for the diagnosis of LTBI, has greater specificity in BCG-vaccinated individuals, displays a stronger association with exposure, and is less biased by environmental mycobacteria such as Mycobacterium avium. Prospective studies are required, however, to confirm that treating LTBI, as defined by the interferon-gamma assay, will reduce the tuberculosis burden in low-prevalence countries and whether interferon-gamma responses are predictive of those who have a high risk of progression to active tuberculosis. Further studies are required to address the utility of the interferon-gamma assay in specialised subgroups of patients, in developing countries, and as a marker of disease activity.