CD25 depletion promotes strong peripheral antitumor T cell reactivity. Mice were challenged with B16/BL6 melanoma and either left untreated or treated with Gvax/αCTLA-4 on days 8 and 11 after tumor implantation. Some groups also received anti-CD25 mAb 4 d before or after tumor challenge. (A) 14 d after tumor challenge, mice were analyzed for the expression of KI-67 within the CD4⁺Foxp3⁻, CD8⁺, or CD4⁺Foxp3⁺ T cell compartments. (B) Mice were injected with congenically marked pmel Tg CD8⁺ T cells, treated as described in A, and analyzed for KI-67 expression on the pmel compartment 14 d after tumor challenge. Numbers on the top right of the histograms represent the percentage of cells expressing high levels of KI-67. (C–E) CD8⁺ and CD4⁺ T cells were purified from each experimental group described in A and restimulated in vitro with a mix of purified CD11c⁺ dendritic cells and irradiated B16 melanoma or control TRAMP-C2 tumor cell lines for 24 h. Production of IFN-γ by CD8⁺ (C) and CD4⁺ (D), and the production of IL-2 by CD4⁺ T cells (E), was determined as described in the Materials and methods. (A–C) Data are representative of 3 independent experiments (n = 3 mice per group).