Adoptive transfer of a single naive antigen-specific CD8⁺ T cell can reconstitute protective immunity toward high-dose L.m. infection. (A) Schematic outline of the experimental procedure. RAG^−/− recipient mice received a single naive (CD44^lo) CD45.1-OT-I CD8⁺ T cell by intraperitoneal application. On days 0 and 14 after T-cell transfer, prime/boost IV vaccination was performed with 1 × 10⁸ MVA-Ova. On day 21 after T-cell transfer, mice were challenged IV with an otherwise-lethal dose of L.m.-Ova. Three days later, viable bacteria in tissue homogenates were determined by counting CFUs on brain heart infusion plates. Results are depicted in panel B. Alternatively, RAG^−/− recipient mice received naive CD45.1 OT-I TCR-transgenic CD8⁺ T cells as before, and on the same day recipient mice were challenged IV with L.m.-Ova (infection dose 7500 bacteria). (C) Nine days later, viable bacteria were determined in spleen and liver. Mice receiving 100 CD45.1 OT-I cells served as positive control, whereas mice with undetectable CD45.1 OT-I T cells after single-cell transfer (no cells) served as a negative control (n.d., not detectable). Horizontal bars indicate means. P values were calculated by 1-way analysis of variance.