CD137 on T cells is essential and sufficient for the anti-CD137 mAb effect. (A) Purified T cells from WT or CD137KO mice were adoptively transferred into WT B6/Thy1.1 mice. Mice treated with rat IgG or 2A (shown) were fed with BrdU as described. The data are gated on spleen CD8⁺ CD44^hi cells and show the BrdU incorporation from host versus donor origin distinguished by Thy1.2 staining. The results are from one representative of 2 independent experiments with 3 mice each. (B) WT B6 or CD137KO mice injected with PBS, poly I:C or 2A on day 0 were fed with BrdU. Spleen cells were harvested and stained for CD8, CD44, and anti-BrdU. Data shown were gated on CD8⁺ cells. The results are from one representative of 2 independent experiments with 3 mice. (C) Purified T cells from WT B6/Thy1.1 mice were transferred into WT or CD137KO mice. Mice were treated as described in panel A. The data are gated on spleen CD8⁺ CD44^hi cells and show the BrdU incorporation from host versus donor origin distinguished by Thy1.1 staining.