Using ELISpot technology to improve the diagnosis of tuberculosis infection: from the bench to the T-SPOT.TB assay

Accurate detection and adequate treatment of latent tuberculosis infection represent a fundamental cornerstone to reduce the incidence of tuberculosis, in particular in low-incidence countries and among high-risk (i.e., immunosuppressed) individuals. Until recently, however, only the century-old tuberculin skin test was available as a diagnostic tool; its poor specificity and low sensitivity among immunosuppressed individuals has been a major limit to the implementation of effective tuberculosis control strategies. In the last years, the achievements of basic research on the genetics and immunology of Mycobacterium tuberculosis infection rapidly translated into clinical practice two elements of the vast amounts of knowledge acquired. First, the identification and use of specific antigens, which are absent in the tuberculosis vaccine and in most nontuberculous mycobacteria; and second, the identification of IFN-gamma as the main fundamental cytokine implicated in the effective immune response against M. tuberculosis. In an incredibly powerful combination, this new knowledge has been applied to enzyme-linked immunospot (ELISpot) technology, the most sensitive technique to quantify an in vitro antigen-specific cellular immune response. In only a few years, a new commercial, regulatory-approved, diagnostic assay has entered clinical practice as a substitute to the tuberculin skin test. The T-SPOT.TB test has already been applied to several hundreds of patients in the context of controlled clinical trials in different countries and prevalence areas, showing improved specificity and sensitivity in the diagnosis of latent tuberculosis infection over the skin test, in particular in those settings where the diagnosis is most needed.