Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

Figure 1 General outline of ex vivo hematopoietic stem cell gene therapy and pre-selection methods. A: CD34⁺ cells are enriched by CliniMACS after apheresis of peripheral blood of patients following mobilization. These cells are then briefly activated ex vivo and can be modified, commonly by viral transduction, to express a desired therapeutic protein. Cells are then assessed for quality control metrics and engrafted into patients following ablation; B: Pre-selection of transduced cells. Cells can be engineered to express an inert surface marker that can be used to immuno-enrich for the transduced population prior to engraftment. This strategy can increase the chances of hematopoietic reconstitution from the transduced population. Alternatively, cells can be given resistance to cytotoxic drugs. Pre-treatment of the cells ex vivo with drugs can kill off the non-transduced population. Ex vivo treatment allows the use of drugs that would normally not be efficacious in the bone marrow environment at a tolerable dose.