Coculture and transplant of purified CD34+Lin− and CD34−Lin− cells reveals functional interaction between repopulating hematopoietic stem cells

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 EMVnull 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.