CD137 stimulation selectively stimulates proliferation of memory T cells. B6 mice were injected with 100 μg control rat IgG or 2A on day 0 and 2 and fed with drinking water with 0.8 mg/mL BrdU from day 3 to 7. Splenocytes (A) or intrahepatic lymphocytes (B) were harvested on day 7 and stained for CD8, CD4, CD44, and BrdU by flow cytometry. Data were presented by gating on CD8 or CD4. Total numbers of CD44^hi or CD122⁺ cells in CD8⁺ or CD4⁺ T-cell subsets in spleens (C) are shown on days 5 and 8, respectively, after antibody treatment. Error bars indicate SD. The number of total intrahepatic lymphocytes as well as CD4⁺ and CD8⁺ subsets on day 7 are also shown (D). The results are from one representative of 3 independent experiments with similar results. (E) Accumulation of memory phenotype T cells in CD137L transgenic mice. The phenotype of spleen or LN T cells from 1-month-old and 5-month-old CD137L transgenic together with age-matched littermate mice was determined by flow cytometry. Data shown are gated on CD3⁺ (1-month-old), CD8⁺, or CD4⁺ (5-month-old). The percentages of CD44^hi and CD62L^lo cells are indicated. Results are one representative of at least 3 independent experiments. *P < .05, **P < .001.