Effect of Ex Vivo Culture of CD34+ Bone Marrow Cells on Immune Reconstitution of XSCID Dogs Following Allogeneic Bone Marrow Transplantation

Successful genetic treatment of most primary immunodeficiencies or hematological disorders will require the transduction of pluripotent, self-renewing hematopoietic stem cells (HSC) rather than their progeny to achieve enduring production of genetically corrected cells and durable immune reconstitution. Current ex vivo transduction protocols require manipulation of HSC by culture in cytokines for various lengths of time depending upon the retroviral vector that may force HSC to enter pathways of proliferation, and possibly differentiation, which could limit their engraftment potential, pluripotentiality and long-term repopulating capacity. We have compared the ability of normal CD34+ cells cultured in a standard cytokine cocktail for 18 hours or 4.5 days to reconstitute XSCID dogs following bone marrow transplantation in the absence of any pretransplant conditioning with that of freshly isolated CD34+ cells. CD34+ cells cultured under standard γ-retroviral transduction conditions (4.5 days) showed decreased engraftment potential and ability to sustain long-term thymopoiesis. In contrast, XSCID dogs transplanted with CD34+ cells cultured for 18 hours showed a robust T cell immune reconstitution similar to dogs transplanted with freshly isolated CD34+ cells, however, the ability to sustain long-term thymopoiesis was impaired. These results emphasize the need to determine ex vivo culture conditions that maintain both the engraftment potential and “stem cell” potential of the cultured cells.