Experimental culture conditions are critical for ex vivo expansion of hematopoietic cells

The ex vivo expansion of hematopoietic stem cells (HSC) for clinical use is now recognized to be a feasible and very promising approach for hematother apy. Expansion of specific HSC subsets is required for different clinical a pplications, for example, to increase the number of mature cells, to produc e specific cells for adoptive therapy, or to increase the number of primiti ve stem cells available for engraftment. Although hematopoietic growth fact ors can play an important role in this setting, in this review we emphasize that other variables affect the outcome of stem and progenitor cell expans ion. These variables include the serum supplement, the purity of CD34(+) ce lls, the initial cell concentration, and the duration of culture. It is als o essential to define standard culture conditions for normal stem cells and to limit or prevent expansion of residual tumor cells. In clinical applica tions, determination of the hematopoietic value of the expanded population is mandatory. Thus, we have to demonstrate the expansion of primitive hemat opoietic progenitor and stem cells, with maintenance of their hematopoietic potential as assessed by in vitro or in vivo assays. We draw attention to the challenges in the clinical application of ex vivo expansion. These incl ude the establishment of well-defined experimental conditions and the deter mination of the hematopoietic value of the expanded grafts, whatever the gr aft source: bone marrow, mobilized peripheral blood, or cord blood. Future studies hopefully will optimize these procedures and allow not only expansi on but engineering of defined cellular functions as HSCs grow under defined conditions.