CHARACTERIZATION OF A POPULATION OF CELLS IN THE BONE-MARROW THAT PHENOTYPICALLY MIMICS HEMATOPOIETIC STEM-CELLS - RESTING STEM-CELLS OR MYSTERY POPULATION

We have identified a population of cells in murine bone marrow that ha s many of the phenotypic characteristics attributed to resting hematop oietic stem cells but does not reconstitute irradiated mice, These cel ls express high levels of Sca-1, H-2K and CD38 and low levels of Thy-1 .1, but do not express CD34 nor any of the lineage markers including C D3, CD4, CD5, CD8 NK1.1, I-A, B220, Ig(MGA), CD40, kappa, Mac-1, Gr-1 or Ter119, In addition, this population can be found at normal frequen cy in nu/nu as well as rag-1(-/-) mice, These cells incorporate only l ow levels of Rh123, are resistant to the cytotoxic effects of 5-fluoro uracil and, consistent with their resting phenotype, less than 2% of t hese cells are in the S/G(2)/M phases of the cell cycle. The only phen otypic characteristic that distinguishes these cells from the lineage( -) Sca-1(+), Thy-1.1(low) long-term reconstituting hematopoietic stem cell population is their lack of c-kit expression, Here we have explor ed the possibility that these cells represent a truly resting populati on of hematopoietic stem cells, We found that the lineage(-), Sca-1(+) , c-kit(-) cells do not respond to hematopoietic growth factors in vit ro, either alone or in combination with stromal layers, Furthermore, t hese cells do not form in vivo spleen colonies nor do they have the ab ility to reconstitute irradiated mice, Thus, this population may repre sent either a population of resting stem cells for which we lack the a ppropriate activating stimulus, or simply represent a ''mystery popula tion'' that phenotypically mimics most of the physical properties of r esting stem cells, Given the close phenotypic similarity of the c-kit- mystery population cells to the c-kite long-term reconstituting stem c ells, investigators must be rigorous to exclude their effects from oth er stem cell assays.