CD4⁺CD25⁺ T cells stimulated by allogeneic IDO-competent DCs suppress the proliferation of third-party T cells. DCs were activated with LPS/IFN-γ for 48 hours and used as stimulators of an MLR, and the CD4⁺ T cells retrieved from the MLR were subjected to subsequent suppressor assays as in Figure 4. (Left) CFSE-stained fresh T cells and CD4⁺ T cells were autologous. (Right) CFSE-stained fresh T cells and CD4⁺ T cells were allogeneic (third party). Proliferative responses of fresh T cells are shown by CFSE dilution (open histograms with thick black line) on coculture with sorted CD4⁺CD25⁻ (top) and CD4⁺CD25⁺ T cells (bottom). Filled gray histograms indicate CFSE dilution of anti-CD3–stimulated fresh T cell only; open histograms with thin line, CFSE dilution of unstimulated fresh T cells only. The numbers indicate the percentage of CFSE-negative cells in suppressor assays. Results of 1 experiment, representative of 3 experiments, are shown.