Identification of an HLA-A24-restricted cytotoxic T lymphocyte epitope from human papillomavirus type-16 E6: The combined effects of bortezomib and interferon-γ on the presentation of a cryptic epitope

About 50% of cervical cancers are associated with human papillomavirus type 16 (HPV-16), and since the HPV-16 E6 and E7 oncoproteins are constitutively expressed in the tumor cells, they are attractive targets for cytotoxic T lymphocyte (CTL)-mediated immunotherapy. Nevertheless, only a limited number of HPV-16 E6 epitopes have been identified to date. Using reverse immunological methods, we have generated a CTL clone against the HPV-16 E6_49-57 epitope restricted by HLA-A*2402, which is the most commun allele in Japan and relatively frequent worldwide, capable of lysing 293T cells transduced with HLA-A*2402 and HPV-16 E6. Although it was unable to recognize the SiHa cervical cancer cell line positive for HPV-16 and HLA-A*2402, the cells became susceptible to lysis when transduced with E6-E7 genes, which was unexpectedly offset by pretreatment with interferon (IFN)-γ alone. Interestingly, however, combined pretreatment with a proteasome inhibitor, bortezomib and IFN-γ fully restored CTL-mediated lysis of the original SiHa cells. Furthermore, such intervention of 2 of 4 other cervical cancer cell lines expressing HPV-16 E6 and HLA-A*2402 was found to induce IFN-γ production by specific CTLs. Tetramer analysis further revealed that induction of E6_49-57-specific T cells was possible in 5 of 7 patients with HPV-16-positive high grade cervical intraepithelial neoplasia or cervical cancer by in vitro stimulation with E6_49-57 peptide. Thus, these findings together indicate that E6_49-57 is a candidate epitope for immunotherapy and immunological monitoring of such patients.