Experimental Culture Conditions Are Critical for Ex Vivo Expansion of Hematopoietic Cells

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Abstract
The ex vivo expansion of hematopoietic stem cells (HSC) for clinical use is now recognized to be a feasible and very promising approach for hematotherapy. Expansion of specific HSC subsets is required for different clinical applications, for example, to increase the number of mature cells, to produce specific cells for adoptive therapy, or to increase the number of primitive stem cells available for engraftment. Although hematopoietic growth factors can play an important role in this setting, in this review we emphasize that other variables affect the outcome of stem and progenitor cell expansion. These variables include the serum supplement, the purity of CD34+ cells, the initial cell concentration, and the duration of culture. It is also essential to define standard culture conditions for normal stem cells and to limit or prevent expansion of residual tumor cells. In clinical applications, determination of the hematopoietic value of the expanded population is mandatory. Thus, we have to demonstrate the expansion of primitive hematopoietic 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 include the establishment of well-defined experimental conditions and the determination of the hematopoietic value of the expanded grafts, whatever the graft source: bone marrow, mobilized peripheral blood, or cord blood. Future studies hopefully will optimize these procedures and allow not only expansion but engineering of defined cellular functions as HSCs grow under defined conditions.

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