Leprolomin

Before the discovery of armadillo as a susceptible animal the source of M.leprae was limited and hence the use of lepromin was not common in the field. In recent times, the soluble antigens of armadillo-derived M.leprae have been used extensively in the field. Although the results of the study show that these antigens do not differentiate always a susceptible form from the resistant form, they are able to segregate the polar forms of leprosy. In a given field situation the criteria for diagnosis is so stressed that leprosy is overdiagnosed and within one year of follow up nearly half the number of cases are noted as not leprosy. Hence, in such situations lepromin reaction would be definitely a poor correlate with the type of leprosy. However, in hospital based studies the lepromin reaction has always been and would remain useful in confirming the classification (Sengupta et al 1984). Lepromins and M.leprae soluble antigens have gone through extensive standardization procedures. As these antigens contain mostly common mycobacterial antigens along with the M.leprae-specific antigens, these antigens are unable to specifically diagnose M.leprae infection. After purification of M.leprae from infected armadillo tissue, it was expected that the soluble antigen of M.leprae would probably be as useful as tuberculin. However, this was not found to be true in case of lepromin. Specificity for M.leprae has been noted in the epitopes (antigenic sites) on cross reacting molecules (12 kd, 18 kd, 28 kd, 35 kd, 36 kd) of mycobacteria (Ivanyi et al 1983; Watson 1989). These specific epitopes, if synthesized, could be of use as skin test antigens for determining M.leprae infection.

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.