l-kynurenine induces modulation of certain NK-cell–triggering surface receptors. (A) (Left) Freshly isolated NK cells were cultured in the presence of IL-2 either alone or in combination with increasing concentrations of l-kynurenine (indicated on the x-axis). After 48 hours of culture, cells were evaluated by cytofluorimetric analysis for the surface expression of the main NK-cell–triggering receptors. (Right) Freshly isolated NK cells were cultured in the presence of IL-2. At day 2, l-kynurenine was added at the indicated concentrations. After a further 48 hours, cells were analyzed as in the left panel. For each determination, the mean fluorescence intensity (mfi) was calculated and reported: NKp46 (triangles), NKG2D (squares), or NKp30 (circles). (B) Freshly drawn NK cells were cultured for 2 days in medium alone or in the presence of IL-2. At the onset of the cultures, NK cells were given either l-kynurenine (at the final concentration of 0.5 mM) or dilution buffer alone. Treated (gray profiles) and untreated (white profiles) NK cells were analyzed by flow cytometry for the expression of the indicated triggering receptors. (C) Freshly drawn NK cells were cultured in the presence of IL-2 at the indicated concentrations. Either at the onset (0 U/mL) or after 2 days of culture (12, 50, 100 U/mL), NK cells were analyzed for the expression of the indicated triggering receptors (gray profiles). For comparison, in each panel the profile (white) indicating the maximum expression level for each receptor obtained by culturing cells for the same time period in the presence of 300 U/mL IL-2 is also reported. Results shown in panels A-C are representative of those obtained in 10 independent experiments.