Interferon-gamma receptor-1 (IFN-γR1) expression and indoleamine 2,3-dioxygenase (IDO) activity in bone marrow-derived mesenchymal progenitor cells (BM-MPCs) and tonsil-derived mesenchymal progenitor cells (T-MPCs). (a) Basal expression level of IFN-γR1. BM-MPCs (n = 4) and T-MPCs (n = 5) were analyzed by flow cytometry. (b) IDO activity. BM-MPCs (n = 3) and T-MPCs (n = 4) were cultured in the absence or presence of IFN-γ (100 ng/mL) for 72 hours. Neutralizing anti-IFN-γR1 antibody (1.5 μg/mL) was also tested on cells cultured with IFN-γ. IDO activity was assayed as described in Materials and methods. No IDO activity was detected in the absence of IFN-γ. The differential induction of IDO activity by T-MPCs and BM-MPCs stimulated by IFN-γ is reversed in the presence of neutralizing anti-IFN-γR1 antibody. OD, optical density; PE, phycoerythrin. *, P < 0.05 versus control.