IL-4 allows TGF-β1 to inhibit granzyme B expression by abrogating effect of anti-4-1BB. (A) IL-15-stimulated CD8⁺ T cells were activated with anti-CD3 and anti-CD28 (open bars) or anti-4-1BB (filled bars) for 3 days in the absence or presence of IL-4 (10 ng/mL) or IL-12 (2 ng/mL). All activation cultures were done with (right graph) or without (left graph) TGF-β1 (10 ng/mL). Cells were analyzed for intracellular levels of granzyme B. Mean ± SD of the percentage of granzyme B⁺ cells from 4 experiments are shown in the histograms. P values between effects of anti-CD28 and anti-4-1BB in the presence and absence of IL-4 or IL-12 were compared for the cells treated with or without TGF-β1. (B) IL-15-stimulated CD8⁺ T cells were activated with anti-CD3 and anti-4-1BB for 3 days as in panel A in the presence of 0, 2, 5, and 10 ng/mL IL-4 with or without 10 ng/mL IL-12. All the activations were undertaken with TGF-β1 (10 ng//mL). Cells were analyzed for the expression of granzyme B, and percentages of granzyme B⁺ cells were plotted against the concentrations of IL-4 and IL-12. Data are shown as mean ± SD of 4 independent experiments, each performed in triplicate. P values between cells treated with and without IL-4 in the presence (^**) and absence (^*) of IL-12 are shown.