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

The beta-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 alpha-chain being shared with the other members of the glycoprotein hormone family but the beta-chain being unique to hCG. Nevertheless, there is sequence homology between the hCG beta-chain and the beta-chain of human luteinizing hormone (hLH) which results in cross-reactive antibodies being produced following immunization with wild-type hCGbeta. To reduce or eliminate such cross-reactions we generated a number of mutants of the hCGbeta-chain. One mutant (hCGbeta(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 BAChCGbeta(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 BAChCGbeta and with the native hCGalphabeta 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 beta-chain.