Identification of an immunostimulating protein from Mycobacterium leprae

Despite the recent identification of a number of Mycobacterium leprae proteins, the major immunogenic determinants of this organism remain obscure. We isolated from M. leprae a potent immunostimulatory preparation, designated the MLP fraction, which contains a major protein of 35 kilodaltons (kDa). This protein was precipitated by monoclonal antibody ML03-A1, which recognizes a 35-kDa protein of M. leprae, and by sera obtained from patients with lepromatous leprosy. Neither sera from healthy controls nor sera from patients with pulmonary tuberculosis recognized the 35-kDa protein, and only one of four serum samples from patients with borderline tuberculoid leprosy reacted with this protein. The MLP fraction stimulated T-cell proliferation in patients with leprosy whose T cells proliferate in response to whole M. leprae cells. Apparently, the T-cell epitope associated with MLP is also expressed on M. tuberculosis and M. bovis BCG, since patients with pulmonary tuberculosis and BCG-vaccinated individuals demonstrated significant responses to the MLP fraction. The 35-kDa M. leprae protein, purified to homogeneity in the laboratory of P. J. Brennan, stimulated T-cell proliferative responses in all MLP-responsive subjects. These findings suggest that the 35-kDa protein present in MLP is an immunostimulatory component of M. leprae. In addition to serving as a useful probe for study of the T-cell anergy associated with lepromatous disease, this protein may ultimately be useful as a component of a vaccine designed to provide protection against infection with M. leprae.