Complement-fixing CD45 monoclonal antibodies to facilitate stem cell transplantation in mouse and man.

Broadening the applicability of stem cell therapies requires safer preparative regimens for patients. The CD45 antigen is present on all cells of the hematopoietic lineage, and using a murine model, we determined whether a lytic CD45 monoclonal antibody could produce persistent aplasia and whether it could facilitate syngeneic or allogeneic stem cell engraftment. After its systemic administration, we found that all leukocyte subsets in peripheral blood were markedly diminished, but only the effect on the lymphoid compartment was sustained and marrow progenitor cells were spared from destruction. Given the transient effects of the monoclonal antibody on myelopoiesis and the more persistent effects on lymphopoiesis, we asked whether this agent could contribute to donor hemopoietic engraftment after subablative transplantation. Treatment with anti-CD45 alone did not enhance syngeneic engraftment, consistent with its inability to destroy progenitor cells and permit competitive repopulation with syngeneic donor stem cells. By contrast, the combination of anti-CD45 and an otherwise inactive dose of total-body irradiation allowed engraftment of H2 fully allogeneic donor stem cells. We attribute this result to the recipient immunosuppression produced by depletion of CD45-positive lymphocytes. We next assessed a pair of unconjugated rat anti-human CD45 monoclonal antibodies (MAbs), YTH54.12 and YTH25.4, in a clinical trial in patients who were to receive stem cell transplantation for acute leukemia. The maximum tolerated dose of these MAbs, 400 microg/kg/day, produced a pattern of response identical to that seen in the mice, with marked reductions in circulating lymphoid and myeloid cells and sparing of early marrow progenitors. In two of three patients with active leukemia, the MAbs also produced a decrease in the percentage of leukemic blast cells in bone marrow. These pre-clinical and clinical results warrant further evaluation of anti-CD45 MAbs in subablative preparative regimens for stem cell transplantation.

Cells of the hepatic side population contribute to liver regeneration and can be replenished with bone marrow stem cells.

BACKGROUND AND OBJECTIVES: The aim of this study was to determine whether Hoechst effluxing side population cells isolated from murine liver represent hepatic stem cells, and to examine whether hepatic side population cells arise from bone marrow side population cells. DESIGN AND METHODS: Side population cells were isolated from murine liver by flow cytometry after Hoechst staining and injected directly into murine livers of animals pre-treated with the hepatotoxin 3,5 diethoxy carbonyl-1, 4-dihydrocollidine (DDC). Y-chromosome in situ hybridization was used to track donor cells in the livers. In addition, bone marrow side population cells were stably engrafted into the hematopoietic system of sublethally irradiated recipients and CD45 alleleic staining and Y-chromosome in situ hybridization were used to track side population cell progeny in the liver. RESULTS: In vitro, CD45pos and CD45neg hepatic SP cells gave rise to hematopoietic colonies and mixed colonies of hematopoietic and hepatic differentiation. After orthotopic liver cell transplantation, donor hepatic side population cells contributed to the regeneration of mature liver parenchyma and bile duct epithelium. After transplantation of bone marrow side population cells, both CD45pos and CD45neg hepatic side population cells were partially derived from donor stem cells and could be recruited to repair liver damage after treatment with DDC. INTERPRETATION AND CONCLUSIONS: These findings introduce hepatic side population cells as a facultative liver-regenerating population, reveal interchangeability of tissue stem cells at the level of the side population, and suggest that bone marrow-derived side population cells might be exploited for the repair of diseased or damaged liver.

Anti-CD45-mediated cytoreduction to facilitate allogeneic stem cell transplantation.

The CD45 antigen is present on all cells of the hematopoietic lineage. Using a murine model, we have determined whether a lytic CD45 monoclonal antibody can produce persistent aplasia and whether it could facilitate syngeneic or allogeneic stem cell engraftment. After its systemic administration, we found saturating quantities of the antibody on all cells expressing the CD45 antigen, both in marrow and in lymphoid organs. All leukocyte subsets in peripheral blood were markedly diminished during or soon after anti-CD45 treatment, but only the effect on the lymphoid compartment was sustained. In contrast to the prolonged depletion of T and B lymphocytes from the thymus and spleen, peripheral blood neutrophils began to recover within 24 hours after the first anti-CD45 injection and marrow progenitor cells were spared from destruction, despite being coated with saturating quantities of anti-CD45. Given the transient effects of the monoclonal antibody on myelopoiesis and the more persistent effects on lymphopoiesis, we asked whether this agent could contribute to donor hematopoietic engraftment following nonmyeloablative transplantation. Treatment with anti-CD45 alone did not enhance syngeneic engraftment, consistent with its inability to destroy progenitor cells and permit competitive repopulation with syngeneic donor stem cells. By contrast, the combination of anti-CD45 and an otherwise inactive dose of total-body irradiation allowed engraftment of H2 fully allogeneic donor stem cells. We attribute this result to the recipient immunosuppression produced by depletion of CD45(+) lymphocytes. Monoclonal antibodies of this type may therefore have an adjunctive role in nonmyeloablative conditioning regimens for allogeneic stem cell transplantation.