IDO expression and activity by CD14⁺ monocyte population. (A) Total PBMCs obtained from normal controls or from HSCT recipients were cultured in medium supplemented with M-CSF (20 ng/mL) for 4 days, and IFN-γ (100 U/mL) that was added for the last 18 hours of culture and examined for IDO expression by flow cytometry as described in “Patients, materials, and methods.” Gray shaded histograms represent the CD14⁺ cell population; dark lines, the CD14^- cell population; and light gray lines, negative control (second antibody only). (B) Total PBMCs were separated into a CD14⁺ (diamonds) and a CD14^- (squares) cell population and were cultured in medium supplemented with M-CSF (20 ng/mL) for 4 days. During the last 18 hours of culture, graded amounts of IFN-γ were added as indicated. At the end of culture, concentrations of neopterin (top row) and of kynurenine (bottom row) were determined in cell culture supernatant by ELISA and HPLC, respectively. Results of one experiment, representative of 3 experiments, are shown.