T-cell interferon- release assays: can we do better?

Abstract

Accurate diagnosis of latent tuberculosis (TB) infection (LTBI) is scientifically challenging because of the low burden of dormant tubercle bacilli, which are not directly detectable or quantifiable. However, the strong cellular immune response triggered by LTBI serves as an amplified signal for the presence of these dormant bacilli. The first measure of the cellular immune response to be exploited as a marker of Mycobacterium tuberculosis infection, developed at the end of the 19th century, was the tuberculin skin test (TST), which measures a delayed type hypersensitivity response to tuberculin purified protein derivative. The development and validation of T-cell-based interferon (IFN)-γ release assays (IGRAs) over the past decade represents a 100-yr upgrade in the diagnosis of LTBI 1, 2; the amplified signal measured is the ex vivo release of T-cell-derived IFN-γ. IGRAs’ clearest advantage is increased specificity for detection of M. tuberculosis infection thanks to their utilisation of M. tuberculosis -specific antigens encoded in region of difference (RD)1, a genomic segment absent from the Bacille Calmette-Guerin (BCG) vaccine and most environmental mycobacteria. Currently, there are two forms of IGRA: either IFN-γ secretion is measured in whole-blood by ELISA or IFN-γ-secreting T-cells are directly enumerated by enzyme-linked immunospot (ELISpot) assay. The two assays are commercially licensed as QuantiFERON-TB Gold In-Tube (Cellestis Inc, Carnegie, Australia) and T-SPOT. TB (Oxford Immunotec Ltd, Abingdon, UK). Because these tests measure IFN-γ released in response to the RD1-encoded antigens early secretory antigenic target (ESAT)-6 and culture filtrate protein (CFP)-10 (and TB7.7 (Rv2645) in QuantiFERON-TB Gold In-Tube), specificity in populations at very low risk for LTBI is higher than that of TST, approaching 100% 2, 3. Because there is no gold standard test for LTBI, surrogate markers are used to estimate the diagnostic sensitivity of IGRAs. When using active TB as a surrogate for …