PRESERVED LONG-TERM REPOPULATION AND DIFFERENTIATION PROPERTIES OF HEMATOPOIETIC GRAFTS SUBJECTED TO EX VIVO EXPANSION WITH STEM CELL FACTOR AND INTERLEUKIN 111

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Abstract
Background. The ex vivo expansion of hematopoietic grafts has been proposed as an efficient procedure for improving the hematological recovery of recipients. The fate of the long-term repopulating cells during the ex vivo manipulation of the graft is, however, a critical issue in ex vivo expansion protocols and ultimately will define the applicability of this new technology in hematopoietic transplants. Methods. The repopulating ability of mouse hematopoietic samples was determined by means of bone marrow (BM*) competition assays, using congenic strains that express the pan-leukocyte Ly-5.1 and Ly-5.2 antigens. The generation of potential changes in the repopulating properties of human hematopoietic samples subjected to ex vivo expansion was determined by comparing the engraftment of fresh and ex vivo-manipulated CD34+ cord blood cells in irradiated nonobese diabetic/severe-combined immunodeficient (NOD/SCID) mice. Results. Under our optimized conditions of mouse BM incubation (stem cell factor plus interleukin-11, either with or without macrophage inflammatory protein-1α or Flt3 ligand), both the short-term and the mid-term repopulating ability of the ex vivo-expanded samples were significantly improved when compared with fresh samples. In the long-term, no changes in the repopulation and differentiation properties of the graft were observed as a result of the ex vivo expansion process. As deduced from the analysis of NOD/SCID mice transplanted with fresh and ex vivo expanded human CD34+ cord blood cells, the in vitro stimulation mediated by SCF/IL-11/FLT3L was capable of preserving the ability of the grafts to repopulate the lympho-hematopoiesis of recipents for at least 3 months. Conclusion. These results indicate that under our optimized conditions of ex vivo expansion, the amplification of the hematopoietic progenitors responsible for the short- and mid-term repopulating properties of the graft can take place without compromising the long-term lympho-hematopoietic repopulating properties.

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