Tuberculin skin testing compared with T-cell responses to Mycobacterium tuberculosis-specific and nonspecific antigens for detection of latent infection in persons with recent tuberculosis contact

Abstract

The tuberculin skin test (TST) is used for the identification of latent tuberculosis (TB) infection (LTBI) but lacks specificity in Mycobacterium bovis BCG-vaccinated individuals, who constitute an increasing proportion of TB patients and their contacts from regions where TB is endemic. In previous studies, T-cell responses to ESAT-6 and CFP-10, M. tuberculosis-specific antigens that are absent from BCG, were sensitive and specific for detection of active TB. We studied 44 close contacts of a patient with smear-positive pulmonary TB and compared the standard screening procedure for LTBI by TST or chest radiographs with T-cell responses to M. tuberculosis-specific and nonspecific antigens. Peripheral blood mononuclear cells were cocultured with ESAT-6, CFP-10, TB10.4 (each as recombinant antigen and as a mixture of overlapping synthetic peptides), M. tuberculosis sonicate, purified protein derivative (PPD), and short-term culture filtrate, using gamma interferon production as the response measure. LTBI screening was by TST in 36 participants and by chest radiographs in 8 persons. Nineteen contacts were categorized as TST negative, 12 were categorized as TST positive, and 5 had indeterminate TST results. Recombinant antigens and peptide mixtures gave similar results. Responses to TB10.4 were neither sensitive nor specific for LTBI. T-cell responses to ESAT-6 and CFP-10 were less sensitive for detection of LTBI than those to PPD (67 versus 100%) but considerably more specific (100 versus 72%). The specificity of the TST or in vitro responses to PPD will be even less when the proportion of BCG-vaccinated persons among TB contacts evaluated for LTBI increases.

FIG. 1

T-cell responses to M. tuberculosis sonicate (MTB) were significantly correlated with those to PPD (r = 0.89; 95% CI; 0.82 to 0.93; P < 0.0001) (A) and to ST-CF (r = 0.80; 95% CI,0.70 to 0.87; P < 0.0001) (B) in a group of healthy TB contacts evaluated for LTBI. The dashed lines indicate the lower limit of detection of the IFN-γ ELISA (20 pg/ml).

FIG. 2

T-cell responses to M. tuberculosis sonicate (MTB), PPD, and ST-CF and the individual maximum of the responses to the RD1-encoded antigens ESAT-6 and CFP-10 (RD1-Ag) by category of TST response. The dashed lines indicate the lower limit of detection of the IFN-γ ELISA (20 pg/ml). Statistically significant differences from the group with negative TST results are indicated as follows: #, 0.01 ≤ P < 0.05; ##, 0.001 ≤ P < 0.01; ###, P < 0.001. Comparison of TST-negative persons with other patients with previous TB gave statistically significant differences for all antigens tested (data not shown).