IDO1 inhibition increases the proliferation and functional activity of CD4⁺ T cells, CD8⁺ T cells, and NK cells. Indoleamine 2,3-dioxygenase-1 (IDO) in DCs differentiated from human monocytes were induced by LPS or PGN. The IDO-positive or -negative DCs were cocultured with primary human CD4⁺ (A) or CD8⁺ (B) T cells (2 × 10⁵ cells/well; 1:1) in the presence of OKT3 (100 ng/mL) with or without INCB024360 (1μM) for 4 days. The cell cultures were then pulsed with ³H-thymidine overnight, and the amount of radioactivity incorporated into the cells were measured. The radioactivity from IDO⁻ DCs and T-cell cocultures were normalized to 100%. (C) Supernatants were harvested from these cultures before ³H-thymidine addition and assayed for IFN-γ levels. mDC indicates mature DCs. (D) NK cells (2 × 10⁵/well) purified from allogeneic PBMCs with anti-CD56 microbeads and magnetic sorting were cocultured with IDO-positive or -negative DCs (2 × 10⁵/well) for 3 days in the presence of IL-2 with or without INCB024360 (1μM). The cell cultures were then pulsed with ³H-thymidine as above. The data represent the average of duplicate wells from 4 independent experiments. Error bars represent SD.