Interferon-gamma release assays for tuberculosis: current and future applications

Abstract

Interferon-gamma release assays (IGRAs) represent the first new tool to diagnose latent tuberculosis infection for more than 100 years. They have advantages over the traditional tuberculin skin test which has a poor specificity due to false-positive responses in people who are BCG vaccinated as there is a cross-reactivity of proteins present in both BCG and the tuberculin skin test. IGRAs rely on the concept of detecting the ex vivo release of the cytokine IFN-γ, a key anti-Mycobacterium tuberculosis cytokine, from T cells which react specifically to antigens from M. tuberculosis. T cells are sensitized to the antigens in vivo, and then react when they encounter the same proteins ex vivo. The readouts are used to determine presence of sensitized cells, acting as a surrogate for latent tuberculosis infection. IGRAs are now being incorporated into national guidance for diagnosis and research is ongoing into next-generation versions of the test.

Keywords::

• immunodiagnosis
• interferon-gamma release assays
• latent infection
• mycobacteria
• national guidelines
• tuberculin skin test
• tuberculosis

Financial & competing interests disclosure

A Lalvani is a NIH Research Senior Investigator. A Lalvani is the inventor of several patents underpinning T cell-based diagnosis, including the ESAT-6/CFP-10 ELISpot-based IGRA, T-Spot.TB, which was commercialised by a University of Oxford spin-off company, Oxford Immunotec Ltd., in which the University of Oxford and A Lalvani have minority shareholdings and royalty entitlements. A Lalvani’s research is independent of Oxford Immunotec Ltd., which has no access to his research results and has never funded his research. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• The tuberculin skin test (TST) is a crude mix of >200 proteins from Mycobacterium tuberculosis (MTB). It is injected into the skin causing a delayed-type hypersensitivity reaction to diagnose infection. It may be falsely positive in patients who are BCG vaccinated or exposed to environmental mycobacteria and may be falsely negative in immunosuppressed individuals.

• Interferon-gamma release assays (IGRAs) represent an ex vivo T cell response toward proteins specific to MTB and not found in BCG. They need expertise in laboratory processing of blood samples (with the ELISpot) as well as antigens and assays. They have been shown to be more specific and sensitive than TST for diagnosis of latent tuberculosis infection (LTBI) but must be used with caution in patients who are immunosuppressed. They are more expensive than the TST but cost-effective in terms of reducing unnecessary chemoprophylaxis in persons who have false-positive TST responses due to prior BCG vaccination.

• There are two commercially available IGRAs, the ELISpot and ELISA, both of which have government approval in a number of countries where they are either used alone (USA, Japan) or in conjunction with the TST for diagnosis of LTBI (Canada, Europe). They are not recognized for use in countries with a high burden of disease, nor are they recommended in active TB but here they may be able to be used as a rule-out test to exclude TB when combined with the TST if both tests are negative.

• IGRAs have been shown to be positive in bronchoalveolar lavage fluid, pleural effusions and cerebrospinal fluid where those areas are the predominant site of TB infection. Some patients have been shown to have a positive transient response which becomes negative, perhaps representing an acute resolving infection in contacts not given chemoprophylaxis. There are cases of IGRAs which are negative and then become positive with an increasing level of response prior to conversion from latent to active TB.

• Future areas of research include next-generation IGRAs incorporating additional antigens and cytokines, transcriptomics and proteomics.

Notes