Comparison of IgM, IgG and IgA responses to M.leprae specific antigens in leprosy

Antibodies of IgM, IgG and IgA classes against M.leprae specific antigens (PGL-I, ND-O-BSA, and NT-O-BSA) were determined in the sera of 80 leprosy patients (28 untreated, 34 treated lepromatous and 18 tuberculoid), 25 tuberculosis patients and 33 normal individuals of Northern Thailand. No strong distinction in reactivity could be found between the three antigens. The IgM antibody assay yielded more positive results than assays for IgG and IgA. It was found that the positivity rates of IgM antibodies to all three antigens were highest in untreated lepromatous leprosy (82%). In tuberculoid leprosy, the positivity rates of IgM, IgG and IgA to the antigens were more variable, ranging from 22 to 50 percent. Patients with tuberculosis and normal individuals did not produce IgM antibodies against the antigens. The results suggested that the determination of IgM against the three antigens is a more sensitive and specific test for active leprosy than those of IgG and IgA. The relationship between the duration of treatment and IgM antibody levels in lepromatous leprosy (LL) was studied. Untreated LL patients had significantly higher IgM and IgA antibody levels than treated patients. There was no difference in IgG antibody levels between the two groups, and the levels of both groups were higher than normal controls. Serial determination of IgM antibodies in 7 LL patients revealed that treatment was strongly associated with progressive decrease in IgM antibody levels against all three antigens.