Elimination of luteinizing hormone cross-reactive epitopes from human chorionic gonadotropin.

The β-chain of human chorionic gonadotropin (hCG) has been shown to have efficacy in clinical trials when used as a contraceptive vaccine. This hormone is a heterodimer, the α-chain being shared with the other members of the glycoprotein hormone family but the β-chain being unique to hCG. Nevertheless, there is sequence homology between the hCG β-chain and the β-chain of human luteinizing hormone (hLH) which results in cross-reactive antibodies being produced following immunization with wild-type hCGβ. To reduce or eliminate such cross-reactions we generated a number of mutants of the hCGβ-chain. One mutant (hCGβ(R68E)), containing an arginine to glutamic acid replacement at position 68, has been expressed as a recombinant protein in High Five™ insect cells. The recombinant BAChCGβ(R68E) form of this molecule was used to immunize rabbits and the antibody response compared to the response following immunization with the recombinant wild-type protein BAChCGβ and with the native hCGαβ heterodimer isolated from pregnancy urine. The mutant elicited the production of antibodies which avidly recognize native hCG. Compared to immunization with wild-type hCG, the response showed very little cross reactivity with hLH. This is demonstrated to be due to a radically altered epitope usage in the response to the mutant, which now focuses mainly upon the C-terminal region of the β-chain.