Jackson, Alison Moira; (1997) Towards an epitope-specific vaccine: Epitope-loss mutants of the human chorionic gonadotropin beta subunit. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Human chorionic gonadotropin (hCG) is produced by the trophoblastic cells of the early foetus to ensure the continued production of progesterone which is essential for a successful pregnancy. Due to its well studied biological and immunological characteristics, the hormone-specific beta chain (hCGβ) has been the focus of research to develop immunological contraceptive vaccines. However, the beta-chain has extensive homology with the beta chain of human luteinising hormone (hLHβ). When hCGβ is used as an immunogen antibodies are raised which cross-react with hLHβ. Immunopathology which might arise as a result of this cross-reactivity may limit the acceptability of a contraceptive vaccine based on hCGβ. This project has sought to identify amino acid residues involved in the formation of these shared epitopes and replace them with alternative amino acids so the shared epitopes are eliminated. In this way the concept of producing epitope-specific vaccines is demonstrated, where unwanted B-cell epitopes are removed from a globular protein whilst maintaining the overall conformation in order to maintain desired epitopes. Previous attempts to identify the shared epitopes on hCGβ have been unsuccessful due to the discontinuous nature of these epitopes. Site-directed mutagenesis was applied to residues on hCGβ which were identified from structural information on hCG. Analysis of these mutants with a panel of conformational-dependent monoclonal antibodies allowed the allocation of residues to previously determined epitope regions. The immunogenicity of one hCGβ mutant (Mutant 3) was examined in BALB/c mice by nucleic acid immunisation. This showed that the mutations may have knocked out a major T- or B-cell epitope, as it elicited a far lower antibody response than DNA encoding wild-type hCGβ. This may indicate a problem which may be encountered with creating epitope-specific vaccines, where removal of unwanted but immunodominant epitopes leaves a poorly immunogenic molecule.

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