Enhancement of the antilymphoma activity of anti-CD20 mAb by anti-CD137 agonistic mAb is dependent on NK cells and macrophages. (A) Peripheral blood cell subsets from lymphoma-bearing C57BL/6 mice 4 days after tumor inoculation treated on day 3 with either IgG control or anti-CD20 antibody were analyzed for CD137 expression on CD3⁻NK1.1⁺ NK cells (NK), F4/80⁺ macrophages (Mϕ), CD3⁺CD8⁺ T cells (CD8), and CD3⁺CD4⁺ T cells (CD4) (n = 3 mice per group, *P = .001). (B) Tumor-infiltrating lymphocytes from lymphoma-bearing C57BL/6 mice 7 days after tumor inoculation treated on day 3 with either IgG control or anti-CD20 antibody were analyzed for CD137 expression on CD3⁻NK1.1⁺ NK cells (NK), F4/80⁺ macrophages (Mϕ), CD3⁺CD8⁺ T cells (CD8), and CD3⁺CD4⁺ T cells (CD4) (n = 3 mice per group, *P = .012; NS, not significant). (C-D) C57BL/6 mice were inoculated with 5 × 10⁶ BL3750 lymphoma tumor cells. After tumor inoculation, mice received rat IgG control on day 3 (●), anti-Asialo-GM1 on days −1, 0, 5, 10, 15, 20, and 25 with anti-CD20 antibody on day 3 and anti-CD137 antibody on day 4 (■), liposomal clodronate on days −2, 0, 4, 8, 12, 16, 20, and 24 with anti-CD20 antibody on day 3 and anti-CD137 antibody on day 4 (▾), anti-CD8 mAb on days −1, 0, 5, 10,15, 20, and 25 with anti-CD20 antibody on day 3 and anti-CD137 antibody on day 4 (♦), or anti-CD20 antibody on day 3 and anti-CD137 antibody on day 4 (▴). Mice (10 per group) were then monitored for tumor growth (C; *P = .002) and overall survival (D; *P < .001).