Rejection of human RBCs by recipient macrophages in NOD/SCID mice. (A-C) CFSE-stained human RBCs (1.5 × 10⁸) were intravenously injected into macrophage-depleted (n = 3) or control (n = 4) NOD/SCID mice. Blood was collected at the indicated time points, and the levels (percentages) of injected human RBCs were analyzed by flow cytometry. (A) Representative flow cytometric profiles. (B) Percentages (mean ± SD) of CFSE⁺ human RBCs at the indicated times. (C) Clearance rates of infused human RBCs (ie, percentages of CFSE⁺ human RBCs normalized with the levels at 20 minutes after injection as 100%). (D) CFSE-labeled human or mouse RBCs (1 × 10⁸) were injected into the peritoneal cavity of NOD/SCID mice, and phagocytosis of human RBCs (n = 3) or mouse RBCs (n = 3) by mouse macrophages was measured by staining with anti–mouse F4/80-PE using flow cytometric analysis. Shown are mean (± SD) of phagocytic ratios and representative flow cytometric profiles showing the percentage of CFSE⁺ cells in the gated F4/80⁺ cell population. (E) Human and mouse RBCs were cocultured with NOD/SCID mouse macrophages in media containing 4% NOD/SCID mouse sera (untreated or 56°C-heated), and phagocytosis was measured 4 hours later. Shown are mean ± SD (n = 5 per group) of phagocytic ratios and flow cytometric profiles showing the percentage of CFSE⁺ cells in the gated F4/80⁺ cell population. Data combined from 2 independent experiments are shown. *P < .05; **P < .01; ***P < .001.