The major hurdle in studies on hematolymphopoietic stem cells (HSCs) has been the lack of a HSC specific marker, comparable to CD34 for hematopoietic progenitor cells (HPCs). Vascular endothelial growth factor receptor 2 (VEGFR2, Flk1 in mouse and KDR in men) is a key functional marker in embyronic hemoangiogenesis. We have investigated the expression of KDR on repopulating HSCs from human post-natal hematopoietic tissues. CD34+ cells comprise 0.1-0.5 percent cells expressing KDR. The KDR marker distinguishes HSCs from HPCs. The KDR+ fraction comprises virtually all HSCs, i.e., repopulating HSCs assayed in NOD-SCID mice and fetal sheep xenografts, and putative HSCs assayed in 12-wk extended Dexter type long term culture (ELTC) as ELTC-initiating cells (ELTC-ICs) or cobblestone area forming cells (CAFSs). Conversely, the KDR- fraction comprises virtually all oligo-unipotent HPCs with no self-renewal capacity. The HSC frequency in bone marrow (BM) KDR+ cells, evaluated by limiting dilution, is 20 percent by NOD-SCID mice assay and 25 percent by 12-wk LTC assay. The latter frequency value rises to 53 percent in LTC supplemented with VEGF, thus suggesting a functional role for KDR in HSCs; it further rises to virtually 100 percent in the KDR+ cell subfraction resistant to prolonged GF starvation in culture. Similar results were obtained in cord blood (CB), normal or mobilized peripheral blood (PB, MPB). On this basis, we hypothesize that in post-natal life KDR is a key functional marker for long-term repopulating CD34+ HSCs. To test this hypothesis, we propose (I) to investigate HSCs activity in CD34+ KDR+ cells. Namely: (Ia) To follow through previous studies on the assay of KDR+ repopulating HSCs, e.g. to test if repopulating HSCs are """"""""pure"""""""" in the KDR+ subset resistant to GF starvation. (Ib) To investigate the self-renewal, differentiation and survival capacity of KDR+ HSCs, including in vivo HSC long-term engaftment in fetal sheep and ex vivo HSC expansion. (II) To transduce the KDR/FLK1 gene into CD34+KDR- cells, to investigate the functional role of Flk1/KDR in HSCs (preliminary studies suggest that Flk1 gene transfer largely suppresses HPC clonogenic activity, but partially rescues HSC activity in ELTC). Since preliminary studies suggest that a discrete number of CD34-lin-KDR+ cells engraft NOD-SCID mice and fetal sheep, while containing ELTC-ICs, we hypothesize that KDR is a key marker for CD34- HSCs. Therefore, we propose (III) to assay HSC activity in CD34-lin-KDR+. We will also attempt to convert in vitro CD34-lin-KDR+ [sic] cells into CD34+lin-KDR+ cells. Other informative studies on CD34+KDR+ and KDR- cells will be comparatively extended to CD34-lin-KDR+ and KDR- cells. We propose that these studies may provide significant contribution to the identification, characterization and utilization of CD34+ and CD34- HSCs, which will reflect at not only biological but also at clinical level, specifically for HSC transplantation, blood cell production in vitro and HSC gene therapy.
