Effects of AMG on the NK cells cytotoxic activity. NK cells were purified by negative selection as described in Materials and Methods and stimulated for 3 days with 5 U/mL IL-12 or/and 20 ng/mL TNF in the regular medium RPMI 1640 or in the same medium adjusted to a L-arginine concentration of 2 mmol/L. AMG (4 mmol/L) was added during the afferent step of sensitization (a.s.) or during the effector step (e.s). Cytotoxicity against K562 target cells (A) and Daudi target cells (B) was tested in a standard 4-hour ⁵¹Cr- release assay at an E:T ratio of 12.5:1 for K562 cells and 25:1 for Daudi cells. Bars represent the means ± SD for three independent experiments. *P < .05, **P < .01 by the Student’st-test comparing IL-12/IL-12 + TNF in the presence or absence of AMG (a.s.)