l-kynurenine affects surface phenotype of NK cells cultured in the presence of IL-12. NK cells cultured for 2 days either in the presence of IL-12 alone (2.5 ng/mL; white profiles) or in combination with l-kynurenine (0.5 mM; gray profiles) were analyzed by immunofluorescence and flow cytometry analysis for the expression of the indicated receptors. Identical results were also obtained when NK cells were cultured in the presence of 5 ng/mL IL-12.