Dissociation between stem cell phenotype and NOD/SCID repopulating activity in human peripheral blood CD34(+) cells after ex vivo expansion.

OBJECTIVE: The relationship between phenotype and function in ex vivo-cultured human hematopoietic stem cells (HSC) remains poorly understood. We investigated the effects of a short-term serum-free culture on the relationship between stem cell phenotype, cell division history, and function in human CD34(+) cells.
METHODS: G-CSF-mobilized peripheral CD34(+) cells were cultured for 4 days with stem cell factor, flt-3 ligand, and thrombopoietin. The phenotype (CD34, CD38, HLA-DR, c-kit), cell division history, colony-forming cell (CFC), long-term culture-initiating cell (LTC-IC), and NOD/SCID repopulating activities were evaluated at Day 0 and 4.
RESULTS: We observed a loss of CD38, HLA-DR, and c-kit surface expression resulting in a drastic increase in CD34(+)CD38(-), CD34(+)HLA-DR(-), and CD34(+)c-kit(-/low) cells at Day 4. In contrast, the frequency of Thy-1(+) cells was maintained. We observed a 1.3-fold expansion of CFC, a 4.8-fold increase in LTC-IC, and an overall maintenance of the NOD/SCID repopulating cell activity. CD34(+)CD38(-) and CD34(+)HLA-DR(-) cells detected at Day 4 displayed the most active pattern of division (4 to 5 divisions) whereas 60% of CD34(+)Thy-1(+) cells divided 0 to 2 times during the same period. At Day 4, the NOD/SCID repopulating activity was associated with Thy-1(+) cells with no more than 2 divisions.
CONCLUSIONS: Our results show that the relationship between stem cell phenotype and function is dramatically altered in cultured CD34(+) cells. Thy-1 expression and cell division history appear to be superior to CD38, HLA-DR, and c-kit, or to homing molecules (CXCR4, VLA-4) as predictors of the repopulating activity of cultured peripheral CD34(+) cells.