Defining Current Strategies for Future Human Trials in Gene Therapy

Current application of gene transfer technology to human blood diseases in clinical protocols has been hampered by inefficient transduction of hematopoietic stem cells. The limitation of efficient transduction appears to relate to multiple aspects of retroviral and stem cell biology, including retroviral receptor expression on stem cells, requirement for division of normally quiescent stem cells for integration of Moloney leukemia-based vectors, and loss of repopulating ability due to growth factor or manipulation-induced changes in stem cell behavior. These limitations suggest specific strategies which are necessary in the design of future human gene therapy trials if significant therapeutic benefit are to be realized. Such strategies are simultaneously being pursued in a number of laboratories, and include: 1. optimal definition of the expression and modulation of viral receptors for a number of different retrovirus envelopes; 2. exploitation of viruses which have the capacity to integrate without the requirement for cell division; 3. definition of optimal ex-vivo conditions for hematopoietic stem cells, allowing maintenance of repopulating capacity in the presence of limited cell division; 4. characterization of specific primitive hematopoietic cell populations which might allow short term (or permanent) correction/change in the phenotype of the manipulated cells, due to differences in proliferative capacity; and 5. development of selection schemes to enrich transduced populations, either in vivo or in vitro. sThese strategies will be reviewed with specific examples of modifications in the infection protocols using physiologic ligands for co-localization of vectors and target cells.