¹Robarts Research Institute, Stem Cell Biology and Regenerative Medicine, 100 Perth Drive, London, Ontario, Canada

Correspondence: Dr M Bhatia, Robarts Research Institute, 100 Perth Drive, London, Ontario, Canada N6A 5K8. Fax: +1 519 663 2982; E-mail: mbhatia@robarts.ca

Abstract

The human hematopoietic stem cell compartment is comprised of repopulating CD34⁺ and CD34^- cells. The interaction between these subsets with respect to their reconstitution capacity in vivo remains to be characterized. Here, lineage-depleted (Lin^-) human CD34⁺ and CD34^- hematopoietic cells were isolated from human male and female umbilical cord blood (CB) and transplanted into immune-deficient NOD/SCID EMV^null mice, thereby allowing the use of human and Y-chromosome-specific DNA sequences to discriminate human reconstitution contributed by CD34⁺ vs CD34^- repopulating stem cells. Although cultured human CB CD34^-Lin^- cells transplanted alone possessed only minimal repopulating capacity, with 15% of mice achieving low levels of engraftment, transplantation of cocultured male CD34^-Lin^- cells with female CD34⁺Lin^- cells demonstrated human repopulation with a contribution from CD34^-Lin^--derived progeny in 80% of the recipients. After coculture and transplantation, male CD34^-Lin^- cells gave rise to primitive CD34⁺CD38^- cells isolated in vivo, which demonstrated clonogenic progenitor function into multiple lineages. Taken together, our study indicates that the presence of CD34⁺Lin^- cells in coculture enhanced the low repopulating function of human CD34^-Lin^- cells in vivo. We propose that CD34⁺Lin and CD34^-Lin cells represent phenotypically distinct, but related cell types that exhibit unique and previously unappreciated functional interaction.