Human hematopoietic cells for gene therapy

The bone marrow has been considered a key target tissue from the earliest days of gene therapy research. 1 Experience with bone marrow transplantation demonstrated that within the adult marrow there are cells capable of completely and permanently reconstituting the entire blood and immune systems. Progress in the understanding of human hematologic diseases at the molecular level, development of gene transfer technology, and increasing experience in isolation, ex vivo manipulation, and transplantation of primitive hematopoietic cells all contribute to continued interest in gene transfer into hematopoietic stem cells. 2 – 7 Initial efforts to introduce genes into blood cells were frustrated by the poor efficiency of gene transfer and uncertainties about safety. 8, 9 Two paths of investigation have drastically improved the long-term prospects of stem cell gene therapy. The first is the development of viral vectors capable of integrating DNA stably into target cells. Stable integration appears to be necessary in this system because of the extensive cell proliferation (up to 50 cell doublings) that takes place between the stem cell and the development of terminally differentiated blood cells. The demonstration of retroviral vector transduction of murine hematopoietic progenitors provided the starting point for numerous preclinical models of stem cell gene therapy. 10 A second important contribution has come from studies addressing stem cell phenotype, their enumeration and enrichment, ex vivo maintenance, and developmental capacity after transplantation.