Characterization of cord blood hematopoietic stem cells.

A major problem hampering the development of effective stem cell-based therapies is the absence of a clear understanding of the composition of the hematopoietic stem cell (HSC) pool in humans and how ex vivo manipulation can differentially affect the various HSC classes. This paper will review recent advances in the use of the NOD/SCID xenotransplant assay to characterize the human stem cell compartment and to determine how ex vivo culture affects stem cells. Using lentivector-mediated clonal tracking we found that only 4 days of culture can significantly reduce the number of SCID-repopulating cells (SRCs) contributing to the human graft. Similar results were seen with a competitive assay strategy where non-cultured cells marked with the RFP-lentivector markedly outcompete cultured cells marked with a EGFP-lentivector both transplanted into the same NOD/SCID mouse. A novel intrafemoral (IF) assay was developed to permit the transplantation of human stem cells that might be difficult to detect using the traditional IV injection method. With the IF assay we identified a novel class of human stem cell with the ability to rapidly generate a large graft of human myeloid and erythroid cells within 2 weeks post transplant.