Cellular immune responses to ESAT-6 discriminate between patients with pulmonary disease due to Mycobacterium avium complex and those with pulmonary disease due to Mycobacterium tuberculosis.

ESAT-6 (for 6-kDa early secreted antigenic target) is a secreted antigen found almost exclusively in organisms of the Mycobacterium tuberculosis complex. We compared in vitro gamma interferon (IFN-gamma) responses by peripheral blood mononuclear cells to this antigen in patients with pulmonary disease due to either Mycobacterium avium complex (MAC) or Mycobacterium tuberculosis with those in healthy, skin test-negative, control subjects. Significant IFN-gamma responses to ESAT-6 were detected in 16 (59%) of 27 M. tuberculosis pulmonary disease patients, 0 (0%) of 8 MAC disease patients, and 0 (0%) of 8 controls. Significant IFN-gamma responses to M. tuberculosis purified protein derivative were detected in 23 (85%) of 27 M. tuberculosis disease patients, 2 (25%) of 8 MAC disease patients, and 5 (63%) of 8 healthy controls. M. avium sensitin was recognized in 24 (89%) of 27 M. tuberculosis disease patients, 4 (50%) of 8 MAC disease patients, and 1 (13%) of 8 controls. IFN-gamma responses to ESAT-6 are specific for disease due to M. tuberculosis and are not observed in patients with MAC disease or in healthy controls.