IL-15 and IFN-γ are not required for CD137-induced memory T-cell proliferation. IL-15 KO and IFN-γ KO mice were treated with the indicated mAb and fed with BrdU as shown in Figure 1. Data shown are percentages of BrdU⁺ cells gated on the CD8⁺CD44^hi portion. The results are from one representative of 2 independent experiments with similar results. Error bars indicate SD. No significant difference was found among groups (P > .05).