27 Ex Vivo Expansion of Hematopoietic Stem Cells

A number of studies have demonstrated the feasibility of amplifying the short-term repopulating progenitors in vitro and have shown the relevance of ex vivo expansion approaches in hematopoietic transplantation (1 -3 ) and gene therapy protocols (4 -5 ). In spite of the beneficial effects generally associated with ex vivo expansion strategies, a number of observations suggest that further studies on the biology of ex vivo expansion are required to facilitate the optimal implantation of this strategy in the clinics (6 -7 ). In this respect, although in some instances data showing an impairment in the long-term repopulating capacity of ex vivo expanded grafts t(6 ), ex vivo amplifications in other cases of very primitive progenitors have been observed (9 ). To prevent the prompt differentiation of the hematopoietic stem cells (HSCs) during the ex vivo expansion process, new combinations of early-acting cytokines have been used to facilitate the self-renewal divisions in the HSC compartment (1 ,10 -12 ).