A 35-kilodalton protein is a major target of the human immune response to Mycobacterium leprae.

The control of leprosy will be facilitated by the identification of major Mycobacterium leprae-specific antigens which mirror the immune response to the organism across the leprosy spectrum. We have investigated the host response to a 35-kDa protein of M. leprae. Recombinant 35-kDa protein purified from Mycobacterium smegmatis resembled the native antigen in the formation of multimeric complexes and binding by monoclonal antibodies and sera from leprosy patients. These properties were not shared by two forms of 35-kDa protein purified from Escherichia coli. The M. smegmatis-derived 35-kDa protein stimulated a gamma interferon-secreting T-cell proliferative response in the majority of paucibacillary leprosy patients and healthy contacts of leprosy patients tested. Cellular responses to the protein in patients with multibacillary leprosy were weak or absent, consistent with hyporesponsiveness to M. leprae characteristic of this form of the disease. Almost all leprosy patients and contacts recognized the 35-kDa protein by either a T-cell proliferative or an immunoglobulin G antibody response, whereas few tuberculosis patients recognized the antigen. This specificity was confirmed in guinea pigs, with the 35-kDa protein eliciting strong delayed-type hypersensitivity in M. leprae-sensitized animals but not in those sensitized with Mycobacterium tuberculosis or Mycobacterium bovis BCG. Therefore, the M. leprae 35-kDa protein appears to be a major and relatively specific target of the human immune response to M. leprae and is a potential component of a diagnostic test to detect exposure to leprosy or a vaccine to combat the disease.