Experience and lessons from the use of lepromin and Mycobacterium leprae-specific serology

Skin testing with lepromin, which produces a delayed-type hypersensitivity reaction, has been used in the classification of leprosy, and a good correlation has been found between immunological status and the reaction to lepromin. In addition, the prognostic value of the lepromin test has been demonstrated. More recently, skin testing with two soluble antigens of Mycobacterium leprae showed no difference of the mean size of the reaction between household contacts and non-contacts, indicating that these antigens are not useful for the diagnosis of leprosy. This and other evidence points to the need for a better skin test antigen capable of detecting infection of individuals by M. leprae. Whereas serological assays for antibodies against both PGL-1 and the 35 kDa antigen of M. leprae have been found to yield positive results in 90-100% of patients with lepromatous (BL/LL) leprosy, these assays fail to identify 40-60% of patients with tuberculoid (BT/TT) leprosy, because of the presence of only an insignificant level of antibody against components of M. leprae in these patients' serum, although, in many BT patients, antibody signal could be detected in the local lesions. These data indicate that there remains a need for a specific diagnostic test for leprosy.