Antibody response against three epitopic domains on human chorionic gonadotropin (hCG) in women and rodents immunized with a beta hCG-based immunocontraceptive vaccine.

The antibody repertoire generated against human chorionic gonadotropin (hCG), following immunization with an immunocontraceptive vaccine based on the beta subunit of the hormone, in humans was compared with that generated in rats. Three epitopic domains represented by the beta hCG loop peptide 38-57, the carboxy-terminal peptide (CTP) 109-145, and a region defined by monoclonal antibody (MAb) 206 were probed. In both species, the titer of antibodies against the MAb 206-defined epitopic domain had a good correlation with the total anti-hCG antibody titers. However, the antibody response against the beta hCG loop peptide (38-57) was not observed in human subjects and there was a weak response against this peptide in rats. Despite the good anti-hCG antibody titers in all animals (n = 8), only two had antibodies against this domain. A good antibody response was observed against CTP in rats, whereas in humans this region was weakly immunogenic. Antibodies against CTP were detected in random samples in only 57% of the subjects and this response had no correlation with the total anti-hCG antibody titers. The high antibody response against CTP in rodents compared to humans may be due to its recognition as a foreign determinant. Our results demonstrate that contraception can be achieved in women despite a poor antibody response against the CTP (109-145) and a receptor binding domain (38-57) of beta hCG.