Inhibition of antibody-dependent, cell-mediated cytotoxicity by serum from lepromatous leprosy patients.

Antibody-dependent cellular cytotoxicity (ADCC) and total and alternative pathway complement (C) activity were found to be normal in lepromatous leprosy (LL) patients in the presence of elevated circulating immune complexes (CIC) measured by the 125I-Clq binding assay. Heat inactivation (56 degrees C, 30 min) uncovered the ADCC inhibitory activity of LL sera. The effect of C was exerted both by interfering with the blocking action of CIC and by recovering ADCC activity of CIC-blocked effector cells. Heat inactivation allowed the expression of total and alternative C pathway fixing ability of the LL sera. Thus, immune complexes potentially able to block Fc receptor-dependent functions or capable of fixing C can be detected in LL sera. We postulate that compensatory mechanisms such as those described in vitro may contribute to maintain intact ADCC activity in vivo.