Analysis of the repopulating potential of cord blood hematopoietic cells post ex-vivo expansion: CD34+CD38- phenotype does not predict function

We have recently demonstrated that a human brain endothelial cell line (HUBEC) supports the ex-vivo expansion of human bone marrow CD34⁺CD38^- cells capable of repopulating NOD/SCID mice at high levels, whereas stroma-free liquid cultures do not maintain bone marrow cells with SCID-repopulating capacity (SRC). In this study, we examined the capacity for HUBEC monolayers vs. stroma-free liquid cultures toward supporting the expansion of cord blood CD34⁺ cells and we compared the engraftment capacity of the expanded populations vs. Fresh CB CD34⁺ cells. HUBEC co-culture supplemented with GMCSF/IL-3/IL-6/SCF/FU-3 lig for 7 days supported a 40-fold increase in total hematopoietic cells, a 20-fold increase in CD34⁺ cells, and a 1022-fold increase in cells bearing the CD34⁺CD38 phenotype. Stroma-free liquid cultures supplemented with the identical cytokines supported a 41-fold increase in total cells and a 10-fold increase in CD344 cells, but less than 1% of the expanded population expressed the CD34⁺CD38 phenotype. HUBEC-expanded hematopoietic cells (culture starting dose: 5 × 10⁵ CD34⁺ cells) engrafted in all recipient NOD/SCID mice at high levels (mean 36.3% huCD45⁺ cells). Despite the lack of cells expressing the CD34⁺CD38 phenotype stroma-free liquid cultured cells also engrafted NOD/SCID mice, but at lower levels (mean 22.6% huCD45⁺ cells) which were comparable to the engraftment of Fresh CB CD34⁺ cells (mean 28.7% huCD45⁺ cells; 5 × 10⁵ cell dose per animal). These data demonstrate that HUBEC co-culture may offer advantages in the ex-vivo expansion of human cord blood by producing large numbers of both CD34⁺ and CD34⁺CD38^- target cells. However, unlike adult bone marrow stem cells, the expression of the CD34⁺CD38 phenotype in expanded cord blood populations does not appear to be a requirement for engraftment and repopulation in NOD/SCID mice. We are currently performing subset analyses to determine which expanded cord blood populations (CD34⁺CD38^-, CD34⁺CD38^-, CD34^-) are critically required to achieve hematopoietic repopulation.