IGRAs – The gateway to T cell based TB diagnosis

Abstract

Development of Interferon-Gamma Release Assays (IGRAs) and implementation of their use in clinical practice almost 10 years ago has revolutionised diagnosis of latent tuberculosis (TB) infection (LTBI). The commercially available IGRAs, TSPOT.TB (Oxford Immunotech, Oxford, UK) and QuantiFERON Gold In-Tube (Cellestis, Victoria, Australia), allow detection of TB infection with greater specificity and sensitivity than the tuberculin skin test (TST) and are now recommended for diagnosis of LTBI. The TSPOT.TB assay is a simplified enzyme-linked immunospot assay (ELISpot) that enumerates TB-specific T lymphocytes (T cells) secreting interferon-gamma (IFNγ). In comparison, the QuantiFERON Gold In-Tube assay constitutes an enzyme-linked immunosorbent assay (ELISA) to quantify IFNγ released into blood plasma after incubation of whole blood with TB antigens. Release of IFNγ, as a result of antigen stimulation of TB-specific T cells within blood, is indicative of TB infection. Although IGRAs have significant advantages over the TST in diagnosis of latent TB, they have significant limitations. Discovery of new antigens and advances in methodology for measuring cellular immunity have recently paved the way for novel tests that overcome these limitations. By establishing for the first time technological platforms for T cell based diagnosis in diagnostic service laboratories, IGRAs provide a bridgehead to clinical application of T cell based diagnosis in routine practice.

Introduction

With approximately 9 million cases annually, Tuberculosis (TB) contributes significantly to world-wide mortality and morbidity, especially in low-income countries [1]. Despite lower TB mortality rates in high income countries, diagnosis and subsequent treatment of TB remains a health priority in order to prevent spread of disease and reduce the economic cost associated with patient care [1].

Upon initial infection with Mycobacterium tuberculosis (Mtb) a state of dormancy normally occurs, known as latent TB infection (LTBI) [2], [3]. In some patients LTBI may progress to active disease, and it is only in this latter state that TB can be highly infectious and associated with morbidity and mortality [2].

Approximately one third of the world’s population are thought to be latently infected with Mtb [4]. As Mtb infection is a prerequisite for TB disease, latently infected individuals represent a vast and significant reservoir of potential disease. However, if identified, LTBI can be treated by chemoprophylaxis, successfully eradicating the infection in the majority of cases. Diagnosis of LTBI is therefore key to disease control and remains a health policy priority for TB elimination within high income, low TB prevalence countries [5], [6], where a high proportion of TB cases occur in immigrants from countries of high TB incidence [7], [8].

Until a decade previously, the sole available method for diagnosis of LTBI was the tuberculin skin test (TST). This involves intra-dermal injection of Mtb antigens and subsequent observation (at 48–72 h) for the induction of cutaneous induration, resulting from a delayed-type hypersensitivity response to the antigens [9].

Although inexpensive and simple to perform, there are a number of limitations associated with the TST. Logistically, the test requires a second visit from the patient to have the degree of induration assessed. Of particular importance, the test is confounded by prior vaccination with Bacille Calmette-Guérin (BCG) and can produce false positive results in individuals exposed to many non-tuberculosis mycobacteria [10], [11], [12]. Furthermore, the TST lacks sensitivity in patients with immunosuppression [13], for example in young children [12] and individuals with HIV co-infection [14]. These patients represent those most at risk of developing active disease.

Despite the importance of detection and treatment of LTBI in controlling and eliminating TB in high income, low TB prevalence countries, available diagnostic strategies remained bedevilled by poor specificity and logistical drawbacks until the introduction of the T lymphocyte (T cell)-based diagnostic paradigm over the last 10 years. Development and implementation of Interferon-gamma (IFNγ) Release Assays (IGRAs) has revolutionised the diagnosis of LTBI. In this paper we discuss the principles, methodology and application of IGRAs, as well as the future of T cell based diagnosis.