Phagocytosis of viable uPAR^−/− neutrophils by WT macrophages is increased. (A) Increased phagocytosis of uPAR^−/− neutrophils by WT macrophages. A total of 10⁶ viable WT or uPAR^−/− neutrophils were added to WT macrophages. After 60 minutes of incubation, the phagocytic index was determined. ***P < .001, compared with the phagocytosis with viable WT neutrophils. (B) uPAR is expressed on the surface of WT, but not uPAR^−/− neutrophils. Flow cytometry assays were performed with anti-uPAR antibodies as in Figure 1B. (C) Blockade of uPAR enhances the phagocytosis of viable WT neutrophils by WT macrophages. WT neutrophils were preincubated without (con) or with 1 μg/mL anti-uPAR antibodies or rabbit IgG for 30 minutes. The cells were then resuspended in RPMI plus 5% FBS and added to WT macrophages. After 60 minutes of incubation, the phagocytic index was determined. *P < .05, compared with the group treated with IgG. (D) suPAR abrogates the enhanced phagocytosis of viable WT neutrophils by WT macrophages. Viable uPAR^−/− neutrophils were preincubated without (con) or with 1 μg/mL suPAR or BSA for 30 minutes. The cells was then resuspended in RPMI plus 5% FBS and added to WT macrophages. After 60 minutes of incubation, the phagocytic index was determined. ***P < .001, compared with the group treated with BSA. (E) suPAR binds to the surface of viable neutrophils. The cells were incubated with Chromeo 488-labeled mouse suPAR (1 μg/mL) or mouse albumin for 30 minutes and washed 5 times with PBS. Representative confocal images are shown. (F) Integrins participate in the enhanced phagocytosis of viable uPAR^−/− neutrophils by WT macrophages. uPAR^−/− neutrophils were preincubated with 1 μg/mL mouse IgG or antibodies to the integrins α_M, α_V, β₁, β₂, β₃, or β₄ for 30 minutes. The cells were then resuspended in RPMI plus 5% FBS and added to WT macrophages. After 60 minutes of incubation, the phagocytic index was determined. **P < .01, ***P < .001, compared with the control group treated with IgG.