ATP inhibits chemotaxis of NK cells induced by CX₃CL1. (A) Migratory response of NK cells to CX₃CL1 gradients in the presence (●) or absence (○) of 100μM ATP. *P < .05 versus cells stimulated with the same dose of CX₃CL1 in the absence of ATP as assessed by Student t test for paired samples. (B) Dose dependency of the inhibitory effect of ATP or UTP on the migration of NK cells to CX₃CL1. NK lymphocytes migrated to 10 ng/mL CX₃CL1 added to the bottom chamber along with the indicated concentrations of ATP (●) or UTP (■). *P < .05 versus cells stimulated with UTP assessed by Student t test for paired samples. (C) Flow cytometric analysis of the expression of CX₃CR1 on the membrane of NK cells left untreated or exposed to 100μM ATP for 60 minutes. Numbers represent the net mean fluorescence calculated subtracting the mean fluorescence obtained with isotype control mAb (gray line) to the fluorescence obtained with anti-CX₃CR1 mAb (black line). (A-B) Data are mean ± SD from 4 experiments. In all chemotaxis assays, each concentration used of CX₃CL1 induced statistically significant increase of NK-cell chemotaxis compared with the unstimulated cells. P < .05 assessed by Student t test for unpaired samples. (C) Representative results from 1 of 3 experiments.