Characterization of a novel antigen of Mycobacterium tuberculosis K strain and its use in immunodiagnosis of tuberculosis.

Mycobacterium tuberculosis-specific antigens would be of great value in developing immunodiagnostic tests for tuberculosis (TB), but regional differences in molecular types of the organism may result in antigenic variation, which in turn affects the outcome of the tests. For example, the Beijing strains of M. tuberculosis are prevalent in East Asia, and in particular, the K strain and related strains of the Beijing family, are most frequently isolated during school outbreaks of TB in South Korea. From comparison of genome sequences between M. tuberculosis K strain and the H37Rv strain, a non-Beijing type, we identified a K strain-specific gene, InsB, which has substantial homology with the ESAT-6-like proteins. This study was, therefore, initiated to characterize the InsB protein for its immunogenicity in mice and to confirm its expression in TB patients by detecting antibodies to the protein. The InsB gene was cloned from M. tuberculosis K strain and expressed in Escherichia coli. The recombinant InsB protein was used for immunization of mice. All mice showed strong antibody responses to the InsB protein, and splenocytes stimulated with InsB showed strong IFN-γ and IL-17 responses and a weak IL-2 response, all of which have been implicated in disease expression and used for the immunodiagnosis of TB. Serum samples from TB patients also showed significant antibody responses to the InsB protein as compared to healthy control samples. These results indicate that the InsB protein is an M. tuberculosis K-strain-specific antigen that could further improve the current immunodiagnostic methods, especially for the South Korean population.