Human Papillomavirus 16 (HPV-16) has been identified as the causative agent of 50% of cervical cancers and many other HPV-associated tumors. The transforming potential/tumor maintenance capacity of this high risk HPV is mediated by two viral oncoproteins, E6 and E7, making them attractive targets for therapeutic vaccines. Of 21 E6 and E7 peptides computed to bind HLA-A*0201, 10 were confirmed through TAP-deficient T2 cell HLA stabilization assay. Those scoring positive were investigated to ascertain which were naturally processed and presented by surface HLA molecules for CTL recognition. Because IFN␥ ELISpot frequencies from healthy HPV-exposed blood donors against HLA-A*0201-binding peptides were unable to identify specificities for tumor targeting, their physical presence among peptides eluted from HPV-16-transformed epithelial tumor HLA-A*0201 immunoprecipitates was analyzed by MS 3 Poisson detection mass spectrometry. Only one epitope (E7 11-19 ) highly conserved among HPV-16 strains was detected. This 9-mer serves to direct cytolysis by T cell lines, whereas a related 10-mer (E7 11-20 ), previously used as a vaccine candidate, was neither detected by MS 3 on HPV-transformed tumor cells nor effectively recognized by 9-mer specific CTL. These data underscore the importance of precisely defining CTL epitopes on tumor cells and offer a paradigm for T cellbased vaccine design.The transforming potential of human Papillomavirus (HPV), 4 first suspected in the 1970s, has now been firmly established both biologically and epidemiologically (1-3). The single most important variable linked to malignant transformation is persistent infection with one of the high-risk HPV types. The E6 and E7 proteins encoded by high-risk HPVs have transforming activities and functionally inactivate the p53 and retinoblastoma (Rb) tumor suppressor proteins, respectively (3, 4). HPV-16 is the most abundant high risk HPV and has been detected in Ͼ50% of cervical cancer cases and in most other HPV-induced tumors, such as carcinomas of the vagina, anus, vulva, penis, and oropharynx (3, 5, 6). Worldwide, high risk HPVs are thought to be responsible for Ͼ500,000 malignancies per year, representing more than 5% of human cancers (7).A major breakthrough in combating HPV-induced disease was the development of prophylactic vaccines to prevent HPV infection in previously unexposed individuals. These vaccines are based on virus-like particles consisting of the L1 capsid protein (8, 9). Virus-like particles resemble natural virions and are able to induce high titers of L1-neutralizing antibodies. Two vaccines, one against HPV-16, -18, -6, and -11 and another against HPV-16 and -18, were approved for clinical use in 2006 (10 -12). Although the impact of prophylactic HPV vaccination on the incidence of vaccine type HPV-associated disease and cancer is unquestionable over time, these vaccines have no therapeutic efficacy for established HPV infections. Antibodies neutralize virus particles only before infection. Moreover, as HPV capsid proteins are exc...