Carrier-induced epitope-specific regulation and its bypass in a protein-protein conjugate.

In the course of clinical trials on a birth control vaccine, it was found that some of the immunized women responded poorly to booster immunizations. This vaccine consists of a dimer of the beta chain of human chorionic gonadotropin (beta hCG) and the alpha chain of ovine luteinizing hormone (alpha oLH), linked to tetanus toxoid (TT) as a carrier. Changing this carrier to diphtheria toxoid resulted in reversion to high anti-hCG antibody titers, indicating the extent to which the carrier influences anti-ligand responses in this system. The suppression of anti-hCG responses after booster immunizations was reminiscent of the phenomenon of carrier-induced, epitope-specific regulation. In a mouse model designed to test the effects of preimmunization with TT on anti-hCG responses, we found that a single preimmunization with TT causes reduced anti-hCG antibody responses in two out of four mouse strains, while anti-alpha oLH antibody responses were not affected by the preimmunization with TT. This is particularly interesting considering that beta hCG and alpha oLH were not presented when linked separately to TT. In an effort to devise a strategy to circumvent this carrier-induced, ligand-specific hyporesponsiveness, we investigated the effectiveness of a synthetic T helper epitope from TT as carrier. We show that preimmunization with TT causes a less profound reduction in anti-hCG titers if the preimmunized mice are subsequently injected with alpha oLH-beta hCG conjugated to a synthetic tetanus toxin peptide recognized by TT-induced and peptide-induced T cells.