Antibody responses to the 18-kDa protein of Mycobacterium leprae in leprosy and tuberculosis patients.

The 18-kDa protein of Mycobacterium leprae, as recognized by the monoclonal antibody L5, has a restricted species distribution, being confined to M. leprae and M. habana. We have developed a solid-phase ELISA using purified, recombinant M. leprae 18-kDa protein and compared the serological responses of Nepali leprosy and tuberculosis patients and endemic control subjects to the protein and the M. leprae phenolic glycolipid-I (PGL-I). Few control subjects had anti-18-kDa antibodies. A small proportion of paucibacillary (PB) leprosy and 42% of multibacillary (MB) leprosy patients had IgG anti-M. leprae antibodies. A similar proportion (47%) of Nepali tuberculosis (TB) patients were seropositive, and IgG anti-18-kDa antibody levels were significantly higher in MB and TB patients than in control subjects. By comparison, IgM anti-PGL-I antibodies were detected in 88% of MB leprosy patients and only 7% of TB patients. The possible reasons for the 18-kDa protein seroreactivity in TB patients are discussed, and the anti-18-kDa assay is compared with other antibody assays for protein and nonprotein antigens of M. leprae. It is concluded that the sensitivity and specificity of the anti-M. leprae 18-kDa ELISA are insufficient for the assay to be of clinical utility in leprosy patients.