Human CD34⁺ cell preparations contain over 100-fold greater NOD/SCID mouse engrafting capacity than do CD34^- cell preparations

Objective. The CD34 cell surface marker is used widely for stem/progenitor cell isolation. Since several recent studies reported that CD34^- cells also have in vivo engrafting capacity, we quantitatively compared the engraftment potential of CD34⁺ vs CD34^- cell preparations from normal human placental/umbilical cord blood (CB), bone marrow (BM), and mobilized peripheral blood (PBSC) specimens, using the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model. Methods. CD34⁺ and CD34^- cell preparations were purified by four different approaches in 14 individual experiments involving 293 transplanted NOD/SCID mice. In most experiments, CD34⁺ cells were depleted twice (CD34⁼) in order to obtain efficient depletion of CD34⁺ cells from the CD34^- cell preparations. Results. Dose-dependent levels of human hematopoietic cells were observed after transplantation of CD34⁺ cell preparations. To rigorously assess the complementary CD34^- cell preparations, cell doses 10- to 1000-fold higher than the minimum dose of the CD34⁺ cell preparations necessary for engraftment were transplanted. Nevertheless, of 125 NOD/SCID mice transplanted with CD34^- cell preparations purified from the same starting cells, only six mice had detectable human hematopoiesis, by flow cytometric or PCR assay. Conclusions. CD34^- cells provide only a minor contribution to hematopoietic engraftment in this in vivo model system, as compared to CD34⁺ cells from the same samples of noncultured human cells. Hematopoiesis derived from actual CD34^- cells is difficult to distinguish from that due to CD34⁺ cells potentially contaminating the preparations.