Abstract 834

Transplantation of CD34+ hematopoietic stem cells obtained from either human fetal human liver or umbilical cord blood are considered to be promising therapeutic alternatives for a number of hematologic diseases. These cells express a number of different receptors for soluble mediators that affect cell proliferation and differentiation. Amongst these receptors is FLT3/FLK2, which is a member of the tyrosine kinase receptor family. Additionally, studies have demonstrated that a ligand for this receptor, flt3, may induce the proliferation of CD34+ progenitor cells. We investigated FLT3 receptor expression by CD34+ cells obtained from human cord blood and from human fetal liver using flow cytometry and RT-PCR. We also examined the effects of flt3-ligand when it was added to a combination of stem cell factors including: IL-3, IL-6, GM-CSF, G-CSF and Erythropoietin (Epo) in the long term cultures of CB-derived CD34+ cells. FLT3 receptor expression by CD34+ cells obtained from cord blood was twice as intense as that expressed by cells isolated from human liver. After culturing cord blood cells for 10 or 14 days in the presence of a cocktail of the growth factors identified above, the cultures were analyzed for colony-forming cells (CFC), including erythroid burst-forming unit (BFU-E), granulocyte-macrophage colony-forming unit (CFU-GM) and granulocyte-erythrocyte-megakaryocyte-monocyte colony-forming unit (CFU-GEMM). The number of colonies at both times were counted and used as a control for the effect of flt3 ligand. When flt3 ligand was included in the growth factor cocktail, the number of specific colonies increased. For example, the number of CFU-GM, at day 14, was 1.8-fold more than in the absence of flt3 ligand. Similarly, at day 14, the number of CFU-GEMM colonies was 1.4 greater. However, there was no increase in the number of BFU-E colonies. These results demonstrate that simultaneously committed progenitors as well as the more immature CFU-GEMM and CFU-GM can be substantially amplified from CB-derived CD34+ samples in colony assay within 14 days.

This study was supported, in part, by USPHS grant MH 46815 and the Children's Research Center of Michigan