Helix stabilization in the C-terminal peptide of chicken riboflavin carrier protein enhances immunogenicity and prolongs contraceptive potential as an epitope-based vaccine in female rats

Earlier investigations have shown that (a) antibodies against a carrier-cou pled 20-residue synthetic peptide (C-20), (200)HACQKKLLKFEALQQEEGEE(219), c orresponding to the C-terminal partially helical sequence of chicken ribofl avin carrier protein (RCP; 219 AA) curtail pregnancy in mammals and (b) hel ix stabilization by introducing appropriately spaced salt bridges in the fl anking sequences of its B-cell epitopic structure enhances RCP antigenicity to peptide antibodies. Among such engineered C-20 analogs, HE-20 (HAEQKKLL KFEALEQEKGKE) exhibited maximum helical propensity. Since C-20 per se, i.e. , without carrier conjugation, elicits RCP-reactive neutralizing antibodies in rodents, we mapped its T-cell epitope which overlaps its B-cell epitope , both of which remain unmodified in HE-20. Comparative evaluation of immun ogenicity of the two epitope-based peptide vaccines showed that HE-20 was f ar superior to C-20 in generating RCP-reactive antibodies in terms of both affinity and titer. With regard to bioefficacy, passive immunoneutralizatio n of RCP in pregnant rats by administering purified IgG from either of the antipeptide sera terminated pregnancy. Similarly, active immunization of fe rtile female rats with the individual peptide analogs curtailed pregnancy. However, HE-20 was more efficient in eliciting higher affinity, longer-last ing, RCP-crossreactive antibodies with consequently more prolonged immunoco ntraceptive efficacy. (C) 2001 Academic Press.