Counting antigen-specific T cells: a new approach for monitoring response to tuberculosis treatment?

Received 18 November 2003; accepted 18 November 2003; electronically published 17 February 2004. Financial support: A.L. ’s research programme is funded by the Wellcome Trust. A.L. is a Wellcome Senior Clinical Research Fellow. A.L. is a named inventor on patents relating to T cell– based diagnosis filed by the University of Oxford. Regulatory approval and commercialization of ELISPOT is being undertaken by a spin-out company of the University of Oxford (Oxford Immunotec Ltd.), in which A.L. has a share of equity and to which he acts as a scientific advisor. Reprints or correspondence: Dr. Ajit Lalvani, Nuffield Dept. of Clinical Medicine, University of Oxford, Level 7, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom (ajit. lalvani@ndm.ox.ac.uk). Clinical Infectious Diseases 2004; 38:757–9 2004 by the Infectious Diseases Society of America. All rights reserved. 1058-4838/2004/3805-0022$15.00 A new approach to diagnosis of Mycobacterium tuberculosis infection based on rapid detection of M. tuberculosis–specific T cells was recently developed [1]. The ex vivo enzyme-linked immunospot (ELISPOT) assay enumerates T cells specific for 2 small, intriguing antigens that are secreted by M. tuberculosis but are absent from all strains of Mycobacterium bovis bacille Calmette-Guérin (BCG): early secretory antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP10). T cells from M. tuberculosis–infected individuals become sensitized to ESAT-6 or CFP10 in vivo; when the T cells reencounter these antigens ex vivo in the overnight ELISPOT assay, they release a cytokine, IFN-g [2]. By the next morning, each such T cell gives rise to a dark spot, which is the “footprint” of an individual M. tuberculosis–specific T cell, and the readout is the number of spots. With use of peptides spanning the length of these antigens, the ELISPOT assay has a sensitivity of 96% for patients with culture-confirmed tuberculosis. This is significantly higher than the sensitivity of the tuberculin skin test (TST) [1]. Unlike the TST, the ELISPOT assay does not seem to be susceptible to false-negative results for patients with disseminated tuberculosis [1], and it maintains its high sensitivity for HIV-infected patients with tuberculosis [3]. In addition, the ELISPOT assay is not confounded by prior BCG vaccination, as evidenced by uniformly negative results for BCG-vaccinated people with no history of tuberculosis exposure [1, 4, 5]; its specificity in these populations is 100%. The ELISPOT assay may thus prove clinically useful in the diagnostic assessment of patients with suspected active tuberculosis in regions where there is a low prevalence of latent tuberculosis infection. The T cells enumerated by the ex vivo ELISPOT assay are effector cells that have recently encountered antigen in vivo and can rapidly release IFN-g when reexposed to antigen [6]. In contrast, long-lived memory T cells, which persist long after clearance of the pathogen, are relatively quiescent and less likely to release IFN-g during the short period of exposure to antigen in the ex vivo ELISPOT assay [6]. In a given individual, the frequency of effector T cells is thought to be largely driven by the antigen load, which is closely related to the bacterial load [6–8]. This is probably why the frequency of ESAT-6 peptide–specific T cells in patients with tuberculosis decreases progressively with successful antituberculous therapy, with an observed rate of decay of 5% (95% CI, 2.4%–8.4%) per week [9]. Thus, unlike the TST and serological tests, the ex vivo ELISPOT assay is dynamic, because it enumerates effector T cells, and the frequency of these cells reflects bacterial burden in vivo. For several chronic viral infections, there are blood tests that directly quantify virus load; in contrast, there is no such quantitative assay for determination of the bacterial load for tuberculosis. Quantitation of M. tuberculosis–specific T cells using the ELISPOT assay might, in theory, serve as an indirect measure of bacterial burden that could be used to monitor the response to tuberculosis treatment. However, individuals naturally differ in the level of T cell response that they mount for a given antigen load in vivo [6–8]. Therefore, although differences in T cell frequency within an individual over time reflect changes in bacterial burden, T cell frequencies cannot be used to compare bacterial load between individuals. In this issue of Clinical Infectious Diseases, a report by Carrara et al. [10] describes the results of an investigation of whether a related ELISPOT assay can be