CD47/SIRPα interaction can strongly reduce macrophage phagocytosis of 6A6-opsonized platelets. (A) Platelets were isolated from CD47^+/+, CD47^+/-, or CD47^-/- C57BL/6 mice, labeled with the cell tracker CMFDA, opsonized with 0.65 μg/mL mAb 6A6, and incubated with CD47^+/+ bone marrow–derived macrophages for 30 minutes at 37°C and 5% CO₂. Macrophages were pretreated with 75 μg/mL anti-SIRPα mAb P84 (▪) or isotype control mAb (□) for 15 minutes before addition of platelets. Macrophages were then treated and scored for phagocytosis as described in “Materials and methods.” Data are means ± SEMs for 3 to 4 experiments in each group. ^***P < .001, using Student t test for paired or unpaired comparisons. (B) Platelets were isolated from CD47^+/+ or CD47^+/- C57BL /6 mice, labeled with the cell tracker CMFDA, and injected into CD47^+/+ recipient mice. Experimental ITP was then induced by injection of mAb 6A6 at 0.3 μg/g body weight, and clearance of CD47^+/- (□) or CD47^+/+ platelets (▪) was followed as described in the legend to Figure 3. Data are means ± SEMs for 5 mice in each group. ^*P < .05 and ^***P < .001, using Student t test for paired comparisons.