Refocusing of B-cell responses following a single amino acid substitution in an antigen

Intranasal immunization of BALB/c strain mice was carried out using baculov irus-derived human chorionic gonadotrophin (hCG) beta -chain, together with Escherichia coli heat-labile enterotoxin. Gonadotrophin-reactive immunoglo bulin A (IgA) was induced in a remote mucosal site, the lung, in addition t o a systemic IgG response. The extensive sequence homology with luteinizing hormone (LH) results in the production of LH cross-reactive antibodies whe n holo-hCG is used as an immunogen. In contrast to wild-type hCG beta, a mu tated hCG beta -chain containing an arginine to glutamic acid substitution at position 68 did not induce the production of antibodies which cross-reac t with LH. Furthermore, the epitopes utilized in the B-cell response to the mutated hCG beta shifted away from the immunodominant region of the parent wild-type molecule towards epitopes within the normally weakly immunogenic C terminus. This shift in epitope usage was also seen following intramuscu lar immunization of rabbits. Thus, a single amino acid change, which does n ot disrupt the overall structure of the molecule, refocuses the immune resp onse away from a disadvantageous cross-reactive epitope region and towards a normally weakly immunogenic but antigen-unique area. Similar mutational s trategies for epitope-refocusing may be applicable to other vaccine candida te molecules.