Immunogenicity of three C-terminal synthetic peptides of the beta subunit of human chorionic gonadotropin and properties of the antibodies raised against 45-amino acid C-terminal peptide.

Immunological studies were carried out in rhesus monkeys and rabbits on three C-terminal synthetic peptides of beta-hCG (115-145; 111-145 and 101-145) after conjugating these to tetanus toxoid (TT). The immunogenicity of the peptide conjugates was comparatively poorer with reference to Pr-beta-hCG-TT conjugates at similar doses and immunization schedule. Amongst the three peptides, the best response was obtained with the 45-amino acid c-terminal peptide (45-CTP; 101-145). The anti-45-CTP recognized native hCG and was devoid of cross-reaction with hLH. hCG-induced testosterone production by mouse Leydig cells was inhibited by anti-45-CTP antiserum, although its neutralization capacity decreased more rapidly upon dilution than anti-beta-hCG sera of comparable titres. Immune complexes formed by the anti-45-CTP with hCG had a lower sedimentation value than those formed by anti-beta-hCG antisera with hCG, suggesting the presence of a limited number of immuno-determinant regions in the 45-amino acid C-terminal synthetic peptide.