ATP inhibits CX₃CL1-induced NK-cell cytotoxicity. (A) Effect of 100μM ATP on 10 ng/mL CX₃CL1-induced NK cell–mediated killing of K562 cells *P < .05, CX₃CL1 versus CX₃CL1 + ATP. (B) CX₃CL1 ability to increase HUVEC killing by NK cells is blocked by ATP and ATP-γS and restored by NF157. NK cells were treated with 10 ng/mL CX₃CL1 in the presence of the indicated stimuli (100μM ATP, 10μM ATP-γS, 100μM NF157) for 2 hours before the coculture performed at 80:1 (NK/target cells) ratio. *P < .05 versus cells treated with CX₃CL1 only. (C) ATP (100μM) inhibits NK cell–mediated cytolytic activity against TNF-α–activated ECs. HUVECs were left untreated or stimulated for 16 hours with 50 ng/mL rhTNF-α and used in cytotoxicity assay. *P < .05 versus cells treated with TNF-α only. (D) Effect of 100μM ATP and 10μM ATP-γS on CX₃CL1-induced killing of HCAECs by NK cells. *P < .05 versus cells treated with CX₃CL1 only. (E) Hydrolysis of endogenous extracellular ATP by apyrase (1 U/mL) enhances basal and CX₃CL1-induced NK cell–mediated killing of HUVECs. *P < .05. Data are mean ± SD of percentages of unstimulated NK cells from 3 independent experiments. All P values were calculated by Student t test for paired samples.