The presence of uPAR on the surface of either macrophages or neutrophils is required for enhanced phagocytosis of viable neutrophils by macrophages. (A) uPAR^−/− macrophages do not demonstrate enhanced phagocytosis when combined with viable uPAR^−/− neutrophils. A total of 10⁶ viable WT or uPAR^−/− neutrophils were added to uPAR^−/− macrophages and phagocytosis assays were performed. ***P < .001, compared with phagocytosis with viable WT neutrophils. (B) Incubation of uPAR^−/− neutrophils or macrophages, but not both, with suPAR restores the ability of uPAR^−/− macrophages to phagocytose viable uPAR^−/− neutrophils. Viable uPAR^−/− neutrophils alone (neutrophil), uPAR^−/− macrophages alone (macrophage), or both (Both) were preincubated with 1 μg/mL suPAR for 30 minutes. The cells were then resuspended in RPMI plus 5% FBS and added to WT macrophages for phagocytosis assays. Phagocytosis of viable uPAR^−/− neutrophils preincubated with 1 μg/mL BSA by uPAR^−/− macrophages preincubated BSA were used as a control. **P < .01, ***P < .001, compared with the control group treated with BSA. (C) WT and uPAR^−/− macrophages demonstrate similar ability in ingesting carboxylate-modified beads. Carboxylate-modified beads (2 μm) were incubated WT or uPAR^−/− macrophages for 1 hour. The phagocytic index was calculated as the percentage of macrophages that ingested beads. (D) suPAR did not enhance the uptake of beads by uPAR^−/− macrophages. uPAR^−/− macrophages were preincubated without or with 1 μg/mL suPAR, and phagocytosis performed as in panel C.