DNA vaccination offers a strategy to induce immune attack on
cancer cells, but
tumor Ags are often weak. Inclusion of a "foreign"
protein increases immunogenicity, and we found previously that fusion of the fragment C (FrC) of
tetanus toxin to the
tumor Ag sequence promotes Ab and CD4(+) responses against B cell
tumors. For CTL responses, use of the full two-domain FrC may be less helpful, because known immunogenic MHC class I-binding
peptides in the second domain could compete with attached
tumor-derived
epitopes. Therefore, we removed the second domain, retaining the N-terminal domain, which contains a "universal" helper
epitope. We investigated the ability to induce CTL responses of candidate
peptides placed at the C terminus of this domain. As test
peptides, we repositioned the two known CTL motifs from the second domain to this site. Strong CTL responses to each
peptide were induced by the engineered construct, as compared with the native FrC construct. Induced CTLs were able to specifically kill
tumor cells transfected with FrC as a surrogate
tumor Ag both in vitro and in vivo. Further reduction of the domain to a short helper
epitope generated only weak CTL responses against fused
peptides, and synthetic
peptides mixed with the plasmid containing the first domain were ineffective. The single FrC domain-peptide
vaccine design also was able to induce high levels of CTLs against a known
epitope from carcinoembryonic Ag. Response to
peptide was suppressed if two FrC domains were present, consistent with immunodominance. These principles and designs may have relevance for
cancer vaccines delivered via
DNA.