ST3Gal-IV^−/− platelets are ingested by ASGPR-expressing liver macrophages and hepatocytes. (A) Human hepatocarcinoma cells (HepG2) cells ingest CM-Orange–labeled ST3Gal-IV^−/− platelets in vitro. The uptake of ST3Gal-IV^−/− platelets by HepG2 cells was 3.8-fold higher (P < .001) than that of WT platelets, as determined by flow cytometry. Platelets were counted as ingested when HepG2 cells contained CM-Orange–labeled platelets that did not label with FITC-labeled anti-β3 IgGs. Asialofetuin, but not fetuin, inhibited the ingestion of ST3Gal-IV^−/− platelets by HepG2 cells (P < .001). WT platelet ingestion was set as 1 (not shown) and the ratio of platelet ingestion compared. Each point is the mean ± SD of 2 independent experiments in duplicates. Significance: *P < .05; **P < .01. (B) Macrophage depletion slows platelet clearance. The circulation of ST3Gal-IV^−/− platelets (□) was significantly increased (P = .007) in macrophage-depleted mice (). ■ shows the survival of WT platelets in WT mice. (Inset) Platelet survival in the first 120 minutes after transfusion. (C) Macrophage depletion was confirmed by injecting FluoSpheres into the clodronate-liposome–treated mice (□). FluoSphere circulation in control (mock liposome-treated) mice was compared (■). The extended recovery and circulation of FluoSpheres demonstrated the effectiveness of the clodronate liposomes in depleting the resident phagocytic cell population. The recovery of FluoSpheres measured 1 minute after transfusion was set as 100%. Each time point is the mean ± SD of 4 mice.