TGF-β1 inhibits 4-1BB induction and the TGF-β1 effect can be reversed by anti-4-1BB costimulation. IL-15-stimulated CD8⁺ T cells were activated with plate-coated anti-CD3 plus either control IgG, anti-CD28, or anti-4-1BB in the absence (upper panels) or presence (lower panels) of TGF-β1 for 3 days. Cells were analyzed for expression of 4-1BB on CD8⁺ T cells. (A) Means ± SD of the percentage of 4-1BB⁺ cell populations in CD8⁺ T cells from 4 experiments are shown in the histograms. (B) P values between cells treated with and without TGF-β1 in the presence of control IgG, anti-CD28, or anti-4-1BB are shown.