Affinity consideration in the design of synthetic vaccines intended to elicit antibodies

As a model for synthetic vaccines BALB/c mice were injected with a large cyanogen bromide cleaved fragment of horse cytochrome c, containing residues 1-80 of the 104 residue long polypeptide chain; then individual B cells specific for the peptide were challenged in vitro in splenic fragment cultures, with either the fragment or intact cytochrome c, both coupled to hemocyanin. The splenic environment in which the B cells were cultured contained hemocyanin-primed T cells, which provided equivalent T cell help for both the peptide and protein immunogens. In two experiments, intact cytochrome c-hemocyanin activated a total of only five peptide-primed B cells, compared to 66 that were activated by the peptide-hemocyanin conjugate. Furthermore, antibodies from the few cells that appeared to be activated by the protein did not bind the native protein with appreciable affinity in competitive ELISA. Of the mAbs elicited by the peptide, 51 were shown to have detectable affinity for native cytochrome c, but their affinity was dramatically less than that previously observed for antibodies elicited by protein-primed B cells and also less than that for the peptide. Thus, although the Ig receptors on many of the peptide-primed B cells did bind the protein to some extent, most such B cells were not activated. These results demonstrate that, in the development of synthetic vaccines, the affinity of a protein for peptide-primed antibodies (Ig receptors) is an important criterion to be considered. Qualitative examination of the binding of an anti-peptide antibody to a protein antigen, especially in denaturing conditions such as Western blots or in potentially denaturing conditions such as ELISA, is not an accurate indication of the efficacy of the peptide to prime B cells that can be activated upon challenge from the native protein.