Immunoradiometric assay and radioimmunodetection using monoclonal human chorionic gonadotropin (hCG) antibodies recognizing only native hCG

Monoclonal antibodies to human chorionic gonadotropin (hCG), which recognize conformational determinants of native hCG molecules, were used for two-site immunoradiometric assay (IRMA) and radioimmunodetection (RID) of hCG-producing tumours in nude mice. Several monoclonal hCG antibodies were produced against hCG, one of which (21-D-6) reacted with only native hCG with a high Ksub(a) value of 8.8x109L/mol; another antibody (3-D-1) had a specificity for the epitopes located on the β-subunit of hCG. The antibodies 3-A-3, 12-A-5 and 24-B-6 cross-reacted substantially with LH. The antibodies 21-D-6 and 3-D-1, reacting with different antigenic sites, were used in a two-site IRMA in which 3-D-1 was immobilized and 21-D-6 was labelled with 125I. This two-site IRMA was highly specific for hCG and there was no cross-reaction with hCG-α and hCG-β; the sensitivity of this IRMA was comparable with that of hCG radioiummunoassay (RIA) using polyclonal hCG antibodies. The hCG values in amniotic fluids were determined by IRMA and routine RIA, using SB-6 as hCG antiserum. Both values were significantly correlated (r=0.85, p<0.01), and the values obtained by IRMA were lower than those obtained by RIA. The IgG and F(ab prime)2 fractions prepared from 21-D-6 were iodinated by the lactoperoxidase method and used to detect hCG-containing tumours in nude mice by whole-body photoscans with a gamma scintillation camera. Positive RID results were obtained for all three tumour sites, at 144 h and 240 h after 131I-IgG injection with a slightly higher background activity, and at 48 h after 131I-F(ab prime)2 administration with essentially no background activity. These results indicate that monoclonal anti-hCG and its radiolabelled fragment, which recognizes the conformation of the native hCG but not that of the subunit, are preferable for two-site IRMA and for RID of hCG-producing tumours. (author)