Memory T-cell proliferation induced by CD137 stimulation is antigen and MHC independent. (A) Memory OT-1 cells generated as described Figure 4 were labeled with CFSE and adoptively transferred into H-2K^b KO mice and subsequently treated with CD137 mAb or control antibody on day 1 and 3. On day 7, spleen cells were harvested and stained for CD8, OT-1 tetramer. CFSE intensity of transferred memory OT-1 cells was shown by flow cytometry gated on CD8 and OT-1 tetramer-positive cells. (B) B6 mice transferred with naive OT-1 T cells were preimmunized with OVA plus poly I:C to generate memory OT-1 cells. Fifty days later, mice were injected with anti–H-2K^b blocking mAb on day −1 and 2. On day 0 and day 2, mice were treated with CD137 mAb or control mAb, respectively, and fed with PBS containing BrdU as shown previously. Spleen cells were prepared on day 7 after treatment and analyzed by flow cytometry. Histogram plots of CFSE intensity of transferred OT-1 cells (gated on CD8⁺ OT-1 tetramer-positive cells) in the spleen are shown. The results are from one representative of 2 independent experiments with 3 mice in each group.