Treatment by anti-CD137 mAb promotes functions of memory OT-1 T cells in vivo. (A) In vivo CTL assay. Naive B6 spleen cells were labeled with CFSE at a high (5μM, CFSE^hi, right peaks) or low dose (0.5μM, CFSE^lo, left peaks) and pulsed with OVA (SIINFEKL) or the control peptide, respectively, as target cells. Target cells at 1 × 10⁷ were injected intravenously into the recipient mice 1 month after transfer of activated OT-1 cells and treatment with anti-CD137 mAb or control Ig. Splenocytes were collected from the recipient mice for detection of CFSE-labeled cells by flow cytometry at 6 and 24 hours. Percentage lysis of peptide-pulsed target cells was calculated from the ratio of CFSE^hi/CFSE^lo target cells. (B) ELISPOT assays for IFN-γ–producing cells. ELISPOT assays were performed in triplicate, using spleen cells with OVA peptide or mitomycin C–treated B16-OVA tumor cells. The spots that represent IFN-γ–producing cells were counted, and the mean values ± SD are shown. Results represent 3 independent experiments. *P < .05; **P < .01.