In this study, we measured simultaneously the in vitro and in vivo T lymphocyte reactivities and the antibody responses of
leprosy patients and healthy family contacts (HFC) toward Mycobacterium leprae
antigens. The in vitro lymphoproliferative response of the HFC to
leprosin A was comparable to that of
tuberculoid leprosy patients. However, their skin-test reactivity to Dharmendra
lepromin was considerably higher compared to the in vitro response to
leprosin A. A significant number of HFC failed to respond to M. leprae
antigens, both in vitro and in vivo, and the unresponsiveness to either test was not related to the type of
leprosy patients in the household. A marginal correlation was observed between the skin-test reactivity of HFC and the age of the individuals. Even though a significant proportion of HFC showed positive anti-PGL-I
IgM levels, none showed a positive titer in the serum antibody competition test toward the M. leprae-specific
epitope My2. A positive anti-PGL-I
IgM response together with a negative
lepromin skin-test reactivity showed a clear downward trend from the lepromatous pole toward the tuberculoid pole. A small number of HFC, all contacts of lepromatous patients, were
lepromin skin-test negative with positive anti-PGL-I
IgM levels, but the majority among them showed T-cell reactivity to mycobacterial
antigens in vitro. These results are discussed in relation to immunological correlates of the susceptibility to M. leprae
infection.