Hoechst dye efflux reveals a novel CD7+CD34− lymphoid progenitor in human umbilical cord blood

A novel Hoechst 33342 dye efflux assay was recently developed that identifies a population of hematopoietic cells termed side population (SP) cells. In the bone marrow of multiple species, including mice and primates, the SP is composed primarily of CD34(-) cells, yet has many of the functional properties of hematopoietic stem cells (HSCs). This report characterizes SP cells from human umbilical cord blood (UCB). The SP in unfractionated UCB was enriched for CD34(+) cells but also contained a large population of CD34(-) cells, many of which were mature lymphocytes. SP cells isolated from UCB that had been depleted of lineage-committed cells (Lin(-) UCB) contained CD34(+) and CD34(-) cells in approximately equivalent proportions. Similar to previous descriptions of human HSCs, the CD34(+)Lin(-) SP cells were CD38(dim)HLA-DR(dim)Thy-1(dim)CD45RA(-)CD71(-) and were enriched for myelo-erythroid precursors. In contrast, the CD34(-)Lin(-) SP cells were CD38(-)HLA-DR(-)Thy-1(-)CD71(-) and failed to generate myelo-erythroid progeny in vitro. The majority of these cells were CD7(+)CD11b(+)CD45RA(+), as might be expected of early lymphoid cells, but did not express other lymphoid markers. The CD7(+)CD34(-)Lin(-) UCB SP cells did not proliferate in simple suspension cultures but did differentiate into natural killer cells when cultured on stroma with various cytokines. In conclusion, the human Lin(-) UCB SP contains both CD34(+) multipotential stem cells and a novel CD7(+)CD34(-)Lin(-) lymphoid progenitor. This observation adds to the growing body of evidence that CD34(-) progenitors exist in humans.