LIMITATIONS OF PREDICTIVE MOTIFS REVEALED BY CYTOTOXIC T-LYMPHOCYTE EPITOPE MAPPING OF THE HUMAN PAPILLOMA-VIRUS E7 PROTEIN

Human papilloma virus (HPV) type 16 is found in the majority of cervic al cancer patients and the transforming protein E7 is consistently exp ressed in cancer cells, making it a potential target for immune attack . In this study we have investigated whether E7 gains access to the MH C class I processing pathway and provides cytotoxic T lymphocyte (CTL) stimulating peptide epitopes. CTL were induced in H-2b MiCe by immuni zation with recombinant vaccinia virus expressing E7 (Vac-E7). To map CTL recognition, natural peptides were purified from cells expressing either intact or truncated E7 protein. Following peptide separation by HPLC one major CTL epitope was detected and truncated constructs loca lized this epitope to the C-terminal region. Mapping with synthetic pe ptides indicated that residues 49 - 57 (RAHYNIVTF) were recognised by anti-E7 CTL. Synthetic 49 - 57 peptide was used to induce CTL, which r ecognized the same HPLC purified natural peptide fractions as anti-E7 CTL. Binding motifs for H-2b class I molecules did not predict residue s 49 - 57 to be a CTL epitope, but instead the sequence 21 - 28 (DLYCY EQL) which contains a K(b) anchor motif. Synthetic 21 - 28 peptide was found to bind to K(b) class I molecules and readily induced CTL, indi cating that the T cell repertoire of H-2b mice can recognize this epit ope. However, these CTL did not recognize peptides isolated from E7 ex pressing cells, showing that natural processing did not produce detect able levels of the 21 -28 epitope. Together, the data demonstrate that an unexpected E7 peptide can function as a major CTL epitope.