Epigenetic regulation of normal hematopoietic development

Mammalian blood cell production ultimately originates from a pool of pluripotent hematopoietic stem cells (PHSC) which are capable of undergoing self-renewal or commitment to a variety of lineage-committed hematopoietic progenitor cells (HPC) which undergo progressive terminal differentiation, resulting in the production of mature blood cells. Allogeneic cord blood (CB) cells are an important alternative source of marrow-repopulating cells (MRC) which can serve as a stem cell graft for patients with hematological malignancies and blood-related genetic disorders. CB grafts frequently contain less HSC than bone marrow or mobilized peripheral blood grafts which leads to a higher incidence of graft failure and a more prolonged time for hematological reconstitution to occur. The fixed number of HSC within a single CB unit has limited the use of this alternative source of stem cells in adults. Numerous investigators have tried to increase the CB stem cell dose by culturing CB CD34+ cells ex vivo under a variety of conditions in order to shorten the time to engraftment and to make these grafts accessible to adult recipients. We have reported that the expansion of CB CD34+ cells in the presence of cytokines alone alters HSC function leading to the loss of functional HSC after prolonged culture and that the ex vivo sequential addition of chromatin-modifying agents (CMA) to specific cytokine combinations can affect HSC fate decisions resulting in increased numbers of CB MRC. In addition there is presently a need for blood products with rare antigen profiles or for use in alloimmunized individuals. We have attempted to generate such products ex vivo using histone deacetylase inhibitors (HDACI). These data indicate that the HDACI-treated CD34+ cell product was capable of producing human erythroid cells which persisted for least 3 weeks in a xenogeneic transfusion model providing the rationale for further exploring the utility of such ex vivo-generated EPCs as a red cell transfusion product in man. These studies provide insight into the potential utility of CMA as a means of altering the HSC/HPC fate decisions for graft engineering.