ST2–IL-33 signaling in CD4 impacts CD8 antitumor activity. (A) Granzyme B and perforin expression in CD8 T cells cultured with WT or ST2^−/− CD4 cells together or through a Transwell under T9_IL-33 conditions (n = 3, from three independent experiments, unpaired t test; data are shown as mean ± SEM; *, P < 0.05). (B) Splenic CD4 and CD8 cells were either co-cultured together or separated in a Transwell under T9_IL-33 conditions for 5 d. Representative plots and mean ± SEM of IL-9 expression in CD8 T cells (left), IL-9 secretion from total T9_IL-33 (center), and PU.1 expression in CD8 T cells (right) either from co-culture (Co) or through Transwell (TW; n = 3, unpaired t test; data are shown as mean ± SEM; **, P < 0.01). (C) KLRG1 expression on CD8 cells cultured together or through a Transwell with CD4 T cells. (D) Cytolytic assays of purified CD8 cells differentiated into Tc9_IL-33 cells or co-cultured in the presence of CD4 in MLR conditions (n = 4, from two independent experiments, unpaired t test; data are shown as mean ± SEM; *, P < 0.05).