Immunogenicity of peptides having pre-determined alpha-helical and alpha-alpha fold topologies

A panel of three synthetic peptides based on the 310-327 region of mouse LDH-C4 was used to examine the effect of peptide conformation on immunogenicity. The peptides, without prior conjugation to carrier molecules, were injected into outbred mice and the antisera were assayed for peptide- and LDH-C4-reactive antibodies by ELISA. An 18-residue random coil peptide (alpha N) and an 18-residue amphipathic alpha-helix peptide (alpha 1) were weakly immunogenic. A conformationally stable 40-residue alpha-alpha fold peptide (alpha 3) was highly immunogenic. The antibodies elicited by alpha 3 reacted strongly with the native molecule by ELISA. Solution-phase binding assays were used to further characterize the specificity of the sera from two mice immunized with alpha 3. Antibodies from one of the mice appeared to recognize the helical portion of the peptides, while antibodies from the other mouse reacted only with the immunogen and may be specific for the non-natural beta bend residues or possibly a topographic determinant peculiar to the anti-parallel helices. Serum from neither mouse was able to recognize the native molecule in solution. Peptides intended to mimic topographic determinants for the purpose of synthetic vaccine development may have to be more complex than those used in this study in order to induce high-affinity antibodies capable of exerting a significant biological effect.