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Keywords:

  • Cross-reactivity;
  • epitope;
  • human chorionic gonadotropin (hCG);
  • luteinizing hormone (LH);
  • vaccine

PROBLEM: Human chorionic gonadotropin (hCG) is a placental glycoprotein hormone, a heterodimeric molecule, consisting of α and β chains. It induces the synthesis of progesterone, which is essential for the maintenance of the fertilized egg. Antibodies directed against hCG can, therefore, prevent pregnancy and serve as a vaccine. hCG belongs to the glycoprotein hormone family and shares the α chain with the other members. The β chain is a hormone-specific subunit that is unique to hCG, but still possesses 85% amino acid homology with the β chain of luteinizing hormone (LH), which means that prolonged immunization with hCG produces antibodies that cross-react with LH.

METHOD OF STUDY: We have taken an approach involving the mutation of βhCG to eliminate cross-reactive epitopes without affecting the natural folding of the polypeptide chain and thus the unique βhCG-specific epitopes.

RESULTS: Several mutants have been constructed that have maintained the binding to hCG-specific monoclonal antibodies (mAbs) but have lost the ability to bind to a panel of LH cross-reactive mAbs. To investigate the immunogenicity of selected mutants, mice were immunized with expression plasmid DNA, containing the gene for wild-type βhCG and two mutants: mutant 3, with four amino acid substitutions (68 Arg[RIGHTWARDS ARROW]Glu; 74 Arg[RIGHTWARDS ARROW]Ser; 75 Gly[RIGHTWARDS ARROW]His; 79 Val[RIGHTWARDS ARROW]His), and mutant 7, with a single amino acid substitution (68 Arg[RIGHTWARDS ARROW]Glu).

CONCLUSIONS: Athough both mutants were able to elicit antibody responses in at least some animals, the levels were less than those seen with the wild-type βhCG DNA, and there seems still to be a residual cross-reacitivity with LH. Attempts to improve the immunogenicity of the mutants and to further modify the sequence to remove the cross-reacitivty are currently underway.