Evidence for a qualitative hierarchy within the Hoechst-33342 'side population' (SP) of murine bone marrow cells

In vitro cobblestone area (CA)-forming cell (CAFC) and in vivo (short-term and competitive repopulation) assays demonstrate that a qualitative hierarchy exists within the Hoechst-33342-defined side population (SP) in murine bone marrow (BM). Consistent with and extending previous studies, we demonstrate that (i) hematopoietic activity found in whole BM (WBM) is concentrated within the SP, rather than the non-SP (NSP); and (ii) within the SP, those cells that more strongly efflux the dye (lower SP, LSP) are qualitatively different from those that less strongly efflux the dye (upper SP, USP). Qualitative differences are highlighted by evidence that (i) CA derived from LSP CAFC persist in culture significantly longer than CA derived from USP CAFC; (ii) short-term, multilineage repopulation of lethally irradiated mice by LSP cells is more rapid than that in mice receiving USP, NSP, whole SP (WSP), or WBM cells and (iii) LSP cells out-compete USP cells in the multilineage hematopoietic repopulation of lethally irradiated recipients. These data suggest that LSP cells are of higher quality than USP cells and potentially provide a means by which qualitative changes in primitive hematopoietic progenitors occurring naturally with aging, or clinically as a consequence of therapeutic manipulation, can be assessed.