No abstract

The purified protein derivative (PPD) skin test has no predictive value for tuberculosis (TB) in Mycobacterium bovis bacillus Calmette-Guérin (BCG)-vaccinated individuals because of cross-reactive responses to nonspecific constituents of PPD. T cell responses to early-secreted antigenic target 6-kDa protein (ESAT-6) and the newly identified culture filtrate protein 10 (CFP-10), 2 proteins specifically expressed by M. tuberculosis (MTB) but not by BCG strains, were evaluated. Most TB patients responded to ESAT-6 (92%) or CFP-10 (89%). A minority of BCG-vaccinated individuals responded to both ESAT-6 and CFP-10, their history being consistent with latent infection with MTB in the presence of protective immunity. No responses were found in PPD-negative controls. The sensitivity and specificity of the assay were 84% and 100%, respectively, at a cutoff of 300 pg of interferon-gamma/mL. These data indicate that ESAT-6 and CFP-10 are promising antigens for highly specific immunodiagnosis of TB, even in BCG-vaccinated individuals.

Australian Government Department of Foreign Affairs and Trade, WHO, and Indonesian Government.

In this paper we describe identification and characterization of Mycobacterium leprae ESAT-6 (L-ESAT-6), the homologue of M. tuberculosis ESAT-6 (T-ESAT-6). T-ESAT-6 is expressed by all pathogenic strains belonging to the M. tuberculosis complex but is absent from virtually all other mycobacterial species, and it is a promising antigen for immunodiagnosis of tuberculosis (TB). Therefore, we analyzed whether L-ESAT-6 is a similarly powerful tool for the study of leprosy by examining T-cell responses against L-ESAT-6 in leprosy patients, TB patients, and exposed or nonexposed healthy controls from areas where leprosy and TB are endemic and areas where they are not endemic. L-ESAT-6 was recognized by T cells from leprosy patients, TB patients, individuals who had contact with TB patients, and healthy individuals from an area where TB and leprosy are endemic but not by T cells from individuals who were not exposed to M. tuberculosis and M. leprae. Moreover, leprosy patients who were not responsive to M. leprae failed to respond to L-ESAT-6. A very similar pattern was obtained with T-ESAT-6. These results show that L-ESAT-6 is a potent M. leprae antigen that stimulates T-celldependent gamma interferon production in a large proportion of individuals exposed to M. leprae. Moreover, our results suggest that there is significant cross-reactivity between T-ESAT-6 and L-ESAT-6, which has implications for the use of ESAT-6 as tool for diagnosis of leprosy and TB in areas where both diseases are endemic.Tuberculosis (TB) and leprosy are major public health problems in the developing world. One-third of the world's population is infected with Mycobacterium tuberculosis, and around 2 million individuals suffer from leprosy (9). Many studies have shown that early culture filtrate proteins of M. tuberculosis can be dominant target antigens for CD4 ϩ Th1 cells both in animal models of TB (1, 2, 5, 12, 13) and in humans. One of these secreted antigens, called T-ESAT-6, is a 10-kDa protein which is present in M. tuberculosis and virulent M. bovis but not in M. bovis BCG, and this protein could not be detected in M. leprae, M. avium, M. scrofulaceum, M. intracellulare, M. fortuitum, and M. xenopi (3, 11). Indeed, when the T-ESAT-6 protein was used, TB-infected cattle could be distinguished from cattle sensitized by environmental mycobacteria (15), and in humans T-ESAT-6 and T-ESAT-6-derived peptides were shown to be very efficiently and specifically recognized by individuals exposed to M. tuberculosis (4,14,16,21). The gene for ESAT-6 (Rv3875) is in a region of the M. tuberculosis genome, designated RD1, that is indeed absent from M. bovis BCG and most nontuberculous mycobacteria (NTM).The existing diagnostic skin test reagents for leprosy, lepromin and leprosin, are prepared from whole autoclaved M. leprae and from the soluble fraction of M. leprae, respectively, and contain many mycobacterial antigens shared with other species (20). A more specific skin test reagent for leprosy would be a highly desirable diagnostic tool. Becaus...