ABSTRACTMice that express the viral oncoprotein simian virus 40 (SV40) large T antigen (T-Ag) as a transgene provide useful models for the assessment of the state of the host immune response in the face of spontaneous tumor progression. Line SV11 (H2b) mice develop rapidly progressing choroid plexus tumors due to expression of full-length T-Ag from the SV40 promoter. In addition, T-Ag expression in the thymus of SV11 mice results in the deletion of CD8+T cells specific for the three H2b-restricted immunodominant epitopes of T-Ag. Whether CD8+T cells specific for the immunorecessive H2-Db-restricted epitope V of T-Ag survive negative selection in SV11 mice has not been determined. Immunization of SV11 mice with rVV-ES-V, a recombinant vaccinia virus expressing epitope V as a minigene, resulted in the induction of weak, but reproducible, epitope V-specific cytotoxic T-lymphocyte (CTL) responses. This weak lytic response corresponded with a decreased frequency of epitope V-specific CTL that could be recruited in SV11 mice. In addition, CTL lines derived from rVV-ES-V-immunized SV11 mice had reduced avidities compared to that seen with CTL derived from healthy mice. Despite this initial weak response, significant numbers of epitope V-specific CD8+T cells were detected in SV11 mice ex vivo following a priming-boosting approach and these cells demonstrated high avidity for epitope V. The results suggest that low numbers of tumor-reactive CD8+T cells with high avidity for epitope V survive negative selection in SV11 mice but can be expanded by specific boosting approaches in the tumor bearing host.