Bone marrow repopulation by human marrow stem cells after long-term expansion culture on a porcine endothelial cell line

In vitro exposure of murine hematopoietic stem cells (HSCs) to cell cycle-inducing cytokines has been shown to result in a defect in the ability of these cells to engraft. We used a porcine microvascular endothelial cell (PMVEC) line in conjunction with exogenous interleukin (IL)-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and stem cell factor (SCF) to expand human HSCs that express the CD34 and Thy-1 antigens but lack lineage-associated markers (CD34⁺Thy-1⁺Lin^- cells). Ex vivo expansion of hematopoietic cells was evaluated in comparison to stromal cell- free, cytokine-supplemented cultures. Cells expressing the CD34⁺Thy-1⁺Lin^- phenotype were detectable in both culture systems for up to 3 weeks. These cells were reisolated from the cultures and their ability to engraft human fetal bones implanted into SCID mice (SCID-hu bone) was tested. HSCs expanded in PMVEC coculture were consistently capable of competitive marrow repopulation with multilineage (CD19⁺ B lymphoid, CD33⁺ myeloid, and CD34⁺ cells) progeny present 8 weeks postengraftment. In contrast, grafts composed of cells expanded in stroma-free cultures did not lead to multilineage SCID- hu bone repopulation. Proliferation analysis revealed that by 1 week of culture more than 80% of the cells in the PMVEC cocultures expressing the primitive CD34⁺CD38^- phenotype had undergone cell division. Fewer than 1% of the cells that proliferated in the absence of stromal cells remained CD34⁺CD38^-. These data suggest that the proliferation of HSCs in the presence of IL-3, IL-6, GM-CSF, and SCF without stromal cell support may result in impairment of engraftment capacity, which may be overcome by coculture with PMVECs.