Effect of l-kynurenine on NKp46, NKG2D, and NKp30 transcript expression in NK cells. Real-time RT-PCR was performed on freshly derived NK cells (□) and on NK cells cultured for 48 hours in IL-2 either untreated (▦) or treated with 0.6 mM l-kynurenine (▪). Bar histograms indicate the expression levels of the different transcripts. The endogenous control β-actin was used to internally standardize the levels of gene expression. Data are expressed as fold difference between freshly derived cells (whose levels of gene expression have been assumed as 1) and cells cultured in the presence of either IL-2 or IL-2 + l-kynurenine. The histograms are representative of results obtained with NK cells derived from 2 different donors. Each experiment was run in triplicate; standard deviation (SD) is indicated.