Macrophages are responsible for clearing transfused stored RBCs. All transfusion recipients and donors were syngeneic male C57BL/6 mice (8-12 weeks of age). (A) Mice were infused intraperitoneally with 2 mg of liposomal clodronate (n = 9) or control PBS-liposomes (n = 10) 48 hours before transfusion with stored RBCs. The 2-hour RBC survival was then measured. The 2-hour RBC survival (■) is indicated for each mouse and the horizontal bar indicates the mean. The results are representative of 2 separate experiments; ***P < .001 compared with treatment with PBS-liposomes. (B) Representative images of histologic sections of liver and spleen from mice treated with liposomal clodronate or control PBS-liposomes 48 hours before transfusion with stored RBCs, and stained with an anti–mouse F4/80 monoclonal antibody, as labeled. Note the absence of tissue macrophages in the liposomal clodronate–treated mice, as evidenced by the absence of brown staining cells. (C) Representative images of histologic sections from the liver of mice transfused with fresh or stored RBCs. Sections were stained with hematoxylin & eosin or with an anti–mouse F4/80 monoclonal antibody, as labeled. Arrows denote tissue macrophages that ingested RBCs. Brown staining is a result of F4/80 immunoreactivity of macrophages; the cytoplasmic staining is displaced to the periphery of the cells in mice transfused with stored RBCs because of the accumulation of ingested RBCs. Original magnification was ×400. Typical representative examples derived from 5 necropsies are shown.