AML-induced CD4⁺CD25⁺ T cells are T_reg cells. (A-B) Secondary allogeneic MLRs consisting of APCs and total or purified T cells previously cultured with IDO-expressing AML cells in the presence and absence of 1-MT (1000 μM). Naive T cells were used as control sample. APC/T-cell ratio equals 1:10. Proliferation results are expressed as stimulation index. IL-2 concentration was measured in 24-hour coculture supernatants. Data are the mean ± SD of 7 independent experiments. (C-D) Naive CD3⁺ T cells were stimulated by allogeneic APCs in the presence of total or purified T cells previously cultured with IDO-expressing AML cells, and tested for proliferation (C). Increasing numbers of purified CD4⁺CD25⁺ T cells obtained after culture with IDO-expressing AML cells were added to MLR cultures (D). Responder and suppressor cells were obtained from the same donor. Experimental T-cell proliferation was compared with that observed in the presence of control T cells and expressed as a percentage of inhibition. Results are the mean ± SD of 6 independent experiments.