TNF-R and TLR engagement triggers IDO enzymatic activity. DCs were prepared as above, with maturation stimuli consisting of TNFα, LPS, or SAC, either in the presence or absence of PGE₂. (A) IDO protein expression was detected by Western blot, and (B) enzymatic activity was quantified by the colorimetric assay for kynurenine, as described in the “Materials and methods.” Error bars indicate SEM.