Identification of growth factor conditions that reduce ex vivo cord blood progenitor expansion but do not alter human repopulating cell function in vivo

Robarts Research Institute, Stem Cell Biology and Regenerative Medicine, London, Ontario, Canada.

BACKGROUND AND OBJECTIVES: Ex vivo expansion of primitive hematopoietic cells for transplantation is an important step to realizing the optimal clinical potential of human cord blood (CB). We aimed to characterize minimal growth factor (GF) conditions that allow ex vivo expansion of primitive cells, including candidate hematopoietic stem cells. DESIGN AND METHODS: Here, we directly investigated the effect of thrombopoietin (TPO) on progenitors and repopulating cells using serum-free culture of CB Lin-CD34+CD38- cells in two different minimal GF conditions: stem cell factor (SCF)+FLT-3-L (termed S/F) and SCF+FLT-3-L+TPO (termed S/F/T). RESULTS: While S/F media supported only low levels of total cell and CFU (colony-forming unit) expansion, the addition of TPO (S/F/T) partially restored cell proliferation, and completely restored CFU expansion to levels observed using full GF conditions (SCF+FLT-3-L+interleukin (IL)-3 (IL-3)+ IL-6+ granulocyte colony-stimulating factor (G-CSF). Intravenous transplantation of either S/F- or S/F/T-expanded cells into NOD/SCID mice resulted in similar frequencies and levels of multilineage reconstitution. INTERPRETATION AND CONCLUSIONS: The use of minimal cytokine stimulation and simultaneous assessment of CFU and SRC indicate that hematopoietic progenitors and in vivo-detected repopulating cells are differentially responsive to TPO; CFU expand in response to TPO but SRC do not. In addition, our study suggests that TPO can functionally replace IL-3+IL-6+G-CSF for CFU expansion of ex vivo cultured CB Lin-CD34+CD38- hematopoietic stem cells.