Production of citrulline and nitrate/nitrite in cultures of NK cells. Purified NK cells (10⁶ cells/mL) were incubated in Dulbecco’s modified Eagle’s medium (DMEM; arginine content, 0.4 mmol/L) with IL-12 (5 U/mL) or/and TNF (20 ng/mL) without or with AMG (4 mmol/L) for 3 days. Supernatants were then collected and production of citrulline (A) and nitrate/nitrite (B) levels was determined as described in Materials and Methods. The results are expressed as the mean ± SE of three different experiments. The asterisks denote a statistically significant difference (P < .01) between IL-12 + AMG and IL-2 or/and IL-12 + TNF + AMG and IL-12 + TNF, as determined by the Student’s t-test.