The effect of l-kynurenine on NK cells is transient. Freshly isolated NK cells were cultured in the presence of IL-2 either alone or in combination with l-kynurenine. At day 2, cells from both culture conditions were harvested, washed, and cultured for a further 3 days in medium containing IL-2 alone. At days 2 and 5, the cells were analyzed for the expression and function of the NKp46-, NKG2D-, and NKp30-triggering receptors. (A) NK cells were assessed by immunofluorescence and flow cytometry analysis for the surface expression of the indicated receptors. Gray profiles indicate cells that have been cultured in the presence of l-kynurenine (0.5 mM) for the first 2 days; white profiles, cells that have never been treated with l-kynurenine. (B) NK cells were analyzed for cytolytic activity against the P815 target cell line either in the absence of mAbs or in the presence of the mAbs to the indicated receptors. ▦ indicate untreated NK cells; ▪, NK cells that have been cultured in the presence of l-kynurenine (0.3 mM) for the first 2 days. (E/T ratio: 2:1.)