Fig 1: Indoleamine 2,3-dioxygenase (IDO) activity is responsible for the immunosuppressive activity of fetal cells. A, Effect of 1-MT on the proliferation of allogeneic PBMC co-cultured with fetal fibroblasts, keratinocytes or both cell types in co-culture. PBMC were initially plated with fetal cells at the ratio 1:4. 1-MT was added at various concentrations to inhibit IDO activity. PBMC were collected from the cultures after 5 days and treated with 3H-thymidine for 16 hours before radioactive counting. Data represent the means (n = 3) ± SD of 3H-thymidine incorporation by proliferative PBMC. Asterisks denote significant differences in PBMC proliferation compared to the positive control (column 2) (*, p<0.05; **, p<0.01). Data are representative of three independent experiments performed with PBMC obtained from different healthy donors. B, IDO-1 and IDO-2 expression in fetal skin. Pieces of the skin sample used to manufacture the fibroblast and keratinocyte cell banks were frozen and embedded for cryosectioning. Skin sections were stained for the immunodetection of IDO-1 (left panel) and IDO-2 (right panel). Isotypic controls for IDO-1 and IDO-2 staining are indicated.