IFN-γ and immune response to alloantigen up-regulate IDO in pDCs. (A) Pre-pDCs from WT B6 BM were exposed to 100 ng/mL IFN-γ for 18 hours, and IDO expression was measured by FACS (heavy line). Pre-DCs from WT mice without IFN-γ treatment and pre-pDCs from IDO1^−/− mice were used as controls. Mean fluorescence intensity (MFI) of intracellular IDO staining for each condition is shown in the table. (B) mRNA levels of pDCs were also assessed by semiquantitative RT-PCR with GAPDH as an internal control. (C) B6→BA.B10 transplant recipients of HSCs, T cells, and 5 × 10⁵ GFP⁺ pre-pDCs were killed at day 10, bone marrow and spleen cells were harvested, and IDO expression in donor GFP⁺ pDCs was detected by intracellular staining (n = 3). (D) FACS-purified pre-pDCs from WT, IFNGR1^−/−, or IDO1^−/− bone marrow were cultured with WT T cells with or without irradiated B10.BR splenocytes (Ag) in 96-well round-bottomed plates, and IDO expression was detected by intracellular staining on day 3. MFI for IDO staining is shown in the table. Experiments for panels A and D were performed twice, with data shown from 1 experiment.