Antibody response against epitopes on hCG mapped by monoclonal antibodies in women immunized with an anti-hCG vaccine and its implications for bioneutralization.

The immunological determinants on hCG, to which an antibody response is generated in women by the contraceptive HSD vaccine, were mapped by using a panel of anti-hCG monoclonal antibodies. Two types of inhibition enzyme immunoassays (EIAs) were used to analyze 126 serum samples from 18 subjects immunized with the vaccine and protected from pregnancy. Monoclonal antibody (MAb) 357-2, which recognizes beta-hCG loop peptide 38-57, did not inhibit significantly the binding of immune sera to hCG. Moreover, none of the sera reacted with this loop peptide in a direct binding EIA, suggesting weak immunogenicity of this epitope. All sera competed with MAb 206 and a preponderance of antibodies was seen against this epitopic region on beta-hCG. The antibody titers against the MAb 206 epitope showed a good correlation with the bioneutralization capacity of the sera throughout the course of immunization. These studies indicate the presence of an immunodominant antigenic determinant on hCG as recognized by the human immune system.