Murine splenic hematopoietic subpopulations: the enlarged undifferentiated subset in New Zealand black mice is multipotent stem cells.

We recently reported that a significant population of the murine splenic non-T, non-B "null" cell compartment consists of non-lineage-specific, undifferentiated cells which are in the G0 and G1 phases of the cell cycle and that their numbers are particularly high in the spleens of New Zealand Black mice. A highly enriched population of these non-lineage-specific cells obtained by successive elimination of differentiated cells was further purified to homogeneity by fluorescence-activated cell sorting. The morphologic, phenotypic, and histochemical characteristics of this purified population suggest that these cells may be primitive hematopoietic stem cells. The germ line configuration of the genomic DNA establishes that these are uncommitted stem cells. In vivo, these cells form day 12 colonies in the spleen and liver of lethally irradiated recipients and confer radioprotection. These cells also differentiate into T- and B-cell lineages and reconstitute the immunodeficiency in mice with severe combined immunodeficiency. In response to a combination of a very few early-acting lymphokines and/or stromal cell-conditioned medium in vitro, these cells differentiate into both myeloid and lymphoid cell types. More of these cells are obtained from the enlarged spleens of New Zealand Black mice than from those of BALB/c mice. The presence of a comparatively higher number of stem cells in the spleen than in the marrow or fetal liver provides an alternative, and possibly superior, source of uncommitted stem cells for a variety of experimental investigations or therapeutic manipulations.