IDO activity of human DCs treated with IDO1-siRNA or 1MT stereoisomers. (A) Tryptophan degradation and kynurenine accumulation were measured in cell culture supernatants¹²  (n = 7). (B) iDCs were stimulated with a maturation cocktail and IFN-γ to induce IDO. D-1MT and L-1MT (Sigma) were tested at 200 μM and 1 mM for their potential to block the enzymatic activity (n = 15, mean value ± SD).