Searching for Hematopoietic Stem Cells : Evidence

Hematopoietic stem cells (HSCs) are defined in mice by three activities: they must rescue lethally irradiated mice (radioprotection), they must self-renew, and they must restore all blood cell lineages permanently. We initially demonstrated that HSCs were contained in a rare ('~0.05%) subset of bone marrow cells with the following surface marker profile: Thy-l.1 l~ LinSca-l+. These cells were capable of long-term, multi-lineage reconstitution and radioprotection of lethally irradiated mice with an enrichment that mirrors their representation in bone marrow, namely, 1,000-2,000fold. However, the experiments reported did not exclude the possibility that stem cell activity may also reside in populations that are Thy-l . l , Sca-l-, or Lin +. In this article stem cell activity was determined by measuring: (a) radioprotection provided by sorted cells; (b) longterm, multi-lineage reconstitution of these surviving mice; and (c) long-term, multi-lineage reconstitution by donor cells when radioprotection is provided by coinjection of congenic host bone marrow cells. Here we demonstrate that HSC activity was detected in Thy-l.l+, Sca-l+, and Linfractions, but not Thy-l . l , Sca-l-, or Lin + bone marrow cells. We conclude that Thy-l.1 l~ LinSca-1 + cells comprise the only adult C57BL/Ka-Thy-l.1 mouse bone marrow subset that contains pluripotent HSCs.