sCR1 prolongs the survival of transfused human RBCs in mice. (A) Fluorescently labeled human group A RBCs equivalent to 10% of total mouse blood volume were injected intravenously into C57Bl/6Ly5.1 mice without (Mock, □, n = 15) or with sCR1 given at a dose of 1.5 mg/kg (▴, n = 5) or 10 mg/kg (▪, n = 9), where n is the number of animals per group. At times indicated, venous blood was sampled and analyzed by flow cytometry for the fraction of fluorescent RBCs. To show the clearance kinetics, injected RBCs at time 0 were taken as 100% and the remaining RBCs were calculated at different time points as the average for each group of mice (error bars depict the SEM). sCR1 at a dose of 10 mg/kg increased the survival of transfused RBCs at all time points (P < .05 as compared with Mock), but at a dose of 1.5 mg/kg, the increased RBC survival was only significant at earlier points (P < .05), and not at 120 minutes after transfusion. (B) Representative immunoblot of plasma from transfused mice coinjected with sCR1 at a dose of 10 mg/kg at the following time points: 1 minute (lane 5), 15 minutes (lane 6), 30 minutes (lane 7), 60 minutes (lane 8), and 120 minutes (lane 9) after injection. To estimate plasma concentrations of sCR1, known concentrations of myc-positope (100 ng in lane1 and 500 ng in lane 2) were used. As a negative control, plasma from noninjected mice (lane 4) was used and as a positive control, injected recombinant sCR1 (lane 3) was used. (C) Comparison of the survival of human group O (triangles) versus group A (squares; identical data as in A) RBCs in mice treated without or with sCR1 administration at a dose of 10 mg/kg. Prolonged RBC survival in mice treated with sCR1 (P < .05 at all time points).