Extracellular ATP attenuates phagocytosis of nonopsonized beads via the P2X₇ receptor. (A) Typical time course of uptake of 1.0-μm YG beads by fresh human monocytes with wild-type P2X₇. (B) The composite values for YG bead uptake by monocytes from 28 randomly selected human subjects. Cells were labeled with allophycocyanin (APC)–conjugated anti-CD14 mAb and pretreated with 1mM ATP for 15 minutes or 20μM CytD for 30 minutes before the addition of YG beads. Values for uptake are in arbitrary units of area under uptake curve over 6.5 minutes. ***P < .001. (C) Preincubation of iTHP-1 cells with ATP (1mM) or BzATP (100μM) causes time-dependent inhibition of phagocytosis of YG beads. (D) Preincubation of iTHP-1 cells with ATP or BzATP for 10 minutes causes dose-dependent inhibition of phagocytosis of YG beads. Different levels of bead uptake were standardized so that values with CytD were 40% of basal. (E) Typical uptake curves of YG beads by human monocytes from a subject with genetic loss of P2X₇ pore function (R307Q). (F) Phagocytosis of YG beads by HEK-293 cells transfected with P2X₇-DsRed and P2X₇-R307Q-DsRed. Phagocytosis index of basal cells transfected with wild-type P2X₇ in each separate experiments (n = 5) was normalized to 100%. *P < .05; **P < .02; ***P < .01. n = 4 or 5.