CD137 expression on both OT-1 Tms and host cells is required for the effect of anti-CD137 mAb. (A) CD137 on OT-1 Tms is required for the effect of anti-CD137 mAb. Spleen cells were prepared from wild-type OT-1 (OT-1/Wt) or CD137-deficient OT-1 (OT-1/CD137KO) mice, and activated by anti-CD3/CD28 mAb in vitro for 2 days. In addition, spleen cells from Thy1.1 mice (CD8⁺Thy1.1) were also activated with the same procedure as controls. Activated cells were transferred into naive B6 mice for memory T-cell development. One month later, mice were treated with rat Ig or anti-CD137 mAb as indicated. One week after antibody treatment, B16-OVA was inoculated subcutaneously. Tumor sizes were measured regularly. Each point is the mean (± SD) tumor diameter in a group of 5 mice, and the result is a representative of 3 independent experiments. **P < .01 compared with the OT-1/Wt Tms treated with control rat Ig. Mice treated with OT-1/CD137KO Tms and anti-CD137 mAb had a significantly weaker response against tumor in comparison with those treated with OT-1/Wt Tms and anti-CD137 mAb (P = .026, 2-way ANOVA). (B) CD137 on host cells is required for the effect of anti-CD137 mAb. OT-1/Wt and OT-1/CD137KO were transferred into naive CD137KO mice and subsequently treated with rat Ig or anti-CD137 mAb. Each point indicates the mean tumor diameters in a group of 5 mice, and error bars show SD. Results are 1 representative of 3 independent experiments. *P < .05 compared with the OT-1/CD137KO treated with anti-CD137 mAb.