• Antifertility vaccine;
  • synthetic HCG beta subunit;
  • peptide immunogenicity;
  • carrier-peptide ratio;
  • N- or C-terminal conjugation

ABSTRACT: Peptides representing the amino acid sequence of the carboxy-terminal of the human chorionic gonadotropin (HCG) beta subunit were used in studies to determine whether an immunogen could be prepared that would be suitable for an HCG antifertility vaccine. Peptides of varying length were conjugated to several macro-molecular carriers, in varying peptide-carrier ratios, via an N- or C-terminal group, with and without amino acid spacers. Antibody levels to peptides and intact HCG were used as the criteria for conjugate utility. Immunizations with a peptide of 37 amino acid residues elicited the highest antibody levels to HCG. No differences were found between N- or C-terminal conjugates, but peptides with amino acid spacers elicited higher responses than peptides without spacer. Conjugates with a peptide:carrier ratio greater than 20 peptides/105 daltons carrier were more immunogenic than those with a lower ratio. Diphtheria and tetanus toxoids were consistently the most effective carriers for enhancing antibody responses to peptides. It was concluded that conjugates of β-HCG peptide 109–145 coupled to a toxoid carrier via its N-terminus in a ratio of 20–30 peptides per 105 daltons carrier was a suitable immunogen for further studies for the development of an anti-fertility vaccine.