CD137 stimulation induces proliferation of memory but not naive T cells. In vitro activated OT-1 cells were adoptively transferred into naive B6 mice and rested for more than 40 days to generate memory OT-1 cells. B6 mice containing naive (upper panels) or memory OT-1 × RAG1KO TCR transgenic T cells (lower panels) were subsequently treated with rat IgG or 2A. The mice were fed with BrdU-containing drinking water for 5 days. Spleen cells were harvested and stained for CD8, OT-1 tetramer, and anti-BrdU. Data shown were gated on CD8⁺ cells. The results are from one representative of 2 independent experiments with similar results.