Conditioned medium from IDO⁺ AML cells is capable of increasing CD4⁺CD25⁺ T cells. (A,C) HPLC assessment of l-tryptophan and l-kynurenine in the conditioned media obtained from IDO⁺ and IDO⁻ AML cells cultured with and without 1-MT (1000 μM). Starting l-tryptophan concentration was 25 μM (A) and 150 μM (C). *P < .03, IDO⁺ versus IDO⁻ cells. (B,D) CD3⁺ T cells (10 × 10⁵ cells/mL) were cultured with 10⁵/mL IDO⁻ AML cells in the conditioned media containing 25 μM (B) and 150 μM (D) starting l-tryptophan. At the end of culture, T cells were collected and used for flow cytometry analysis of CD25 expression on gated CD4⁺ cells. As control sample (CTR), noncultured CD3⁺ T cells were used. *P < .03, IDO⁺ versus CTR cells. The data report the mean ± SD of independent experiments.