Anti-CD137 therapy requires NK and CD8 T cells and is enhanced by CD4 T-cell depletion. (A-B) BALB/c mice were inoculated subcutaneously with 5 × 10⁶ A20 tumor cells. Depletion was initiated before tumor challenge as described in “Depletion of NK cells, CD4 T cells, CD8 T cells, and Tregs.” Mice received 1 intraperitoneal injection of either rat IgG or anti-CD137 mAb at day 6 after tumor inoculation. Groups include: rat IgG alone, anti-CD137 alone, CD4 depletion + anti-CD137, CD8 depletion + anti-CD137, or asialo GM1 depletion + anti-CD137. Mice were monitored for tumor growth (A, mean ± SEM) and overall survival (B). (C-E) BALB/c mice were inoculated subcutaneously with 5 × 10⁶ A20 tumor cells and received either no treatment (Tx) or 1 intraperitoneal injection of anti-CD137 mAb at day 8 after tumor inoculation. Five days after treatment, mice were killed and tumors and spleens were collected for analysis. (C) Representative data of the percentage of CD25⁺FoxP3⁺ T_regs among CD4 T cells in both spleen and tumor from untreated and treated groups. (D-E) The average percentage of CD25⁺FoxP3⁺ T_regs among CD4 T cells (D) and CD8 T cells (E) among total lymphocytes between the untreated and treated groups (3 mice per group).