Altered Hematopoiesis in Murine Sickle Cell Disease

We investigated the mechanisms of sickle cell disease (SCD) hematopoietic/erythropoietic defects using bone marrow, spleen, and/or peripheral blood from the transgenic SAD mouse model, which closely reproduces the biochemical and physiological disorders observed in human SCD. First, the erythropoietic lineage late precursors (polychromatophilic normoblasts to the intramedullary reticulocytes) of SAD mouse bone marrow were significantly altered morphologically. These anomalies resulted from high levels of hemoglobin polymers and were associated with increased cell fragmentation occurring during medullary endothelial migration of reticulocytes. Secondly, analysis of bone marrow erythropoiesis in earlier stages showed a marked depletion in SAD erythroid burst-forming units (BFU-E; of approximately 42%) and erythroid colony-forming units (CFU-E; of approximately 23%) progenitors, despite a significant increase in their proliferation, suggesting a compensatory mechanism. In contrast to the bone marrow progenitor depletion, we observed (1) a high mobilization/relocation of BFU-E early progenitors (approximately 4-fold increase) in peripheral blood of SAD mice as well as of colony-forming units-granulocyte-macrophage (CFU-GM) and (2) a 7-fold increase of SAD CFU-E in the spleen. Third, and most importantly, SAD bone marrow multipotent cells (spleen colony-forming units [CFU-S], granulocyte-erythroid-macrophage-megakaryocyte colony-forming units [CFU-GEMM], and Sca(+)Lin(-)) were highly mobilized to the peripheral blood (approximately 4-fold increase), suggesting that peripheral multipotent cells could serve as proliferative and autologous vehicles for gene therapy. Therefore, we conclude the following. (1) The abnormal differentiation and morphology of late nucleated erythroid precursors result in an ineffective sickle erythropoiesis and likely contribute to the pathophysiology of sickle cell disorders; this suggests that transfer of normal or modified SCD bone marrow cells may have a selective advantage in vivo. (2) A hematopoietic compensatory mechanism exists in SAD/SCD pathology and consists of mobilization of multipotent cells from the bone marrow to the peripheral blood and their subsequent uptake into the spleen, an extramedullary hematopoietic site for immediate differentiation. Altogether, these results corroborate the strong potential effectiveness of both autologous and allogeneic bone marrow transplantation for SCD hematopoietic therapy.