"Stem cell candidates" purified by liquid culture in the presence of Steel factor, IL-3, and 5FU are strictly stroma-dependent and have myeloid, lymphoid, and megakaryocytic potential.

Berardi et al. (Science 1995; 267:105) reported recently that combined cytokine stimulation and antimetabolite treatment were able to isolate cells with characteristics of hemopoietic stem cells. Bone marrow (BM) low-density cells were cultured for 1 week in the presence of Steel factor (SF), IL-3, and the antimetabolite 5-FU. Following this approach one in 10(5) BM cells were purified. These cells showed no clonogenic potential in soft gel assays but presented a striking myeloid-lymphoid potential as long-term culture-initiating cells (LTC-ICs). We investigated this new "stem cell candidate." following a similar approach we purified one in 55,000/130,000 cells from cord blood and peripheral blood in mobilized cancer patients. These cells displayed no clonogenic potential in methylcellulose assays in the presence of different cytokine combinations and generated very few or no clonogenic progenitors in liquid cultures in the presence of cytokines. When seeded on layers derived from the murine BM stromal cell line M2-10B4, 38-86% of the cells purified using this approach generated multilineage progenitors acting as LTC-lCs. In a different series of studies, after 5-week culture on human BM stromal cell line L87/4 layers, cells were forced to selected lineage differentiation by culture in the presence of low concentrations of SF and high concentrations of lineage-specific cytokines such as Flt3-ligand (myeloid and pre-B cell differentiation), Tpo (megakaryocytic differentiation). IL-7, and IL-2 (pre-B and NK differentiation). After 12-day culture under these conditions, generation of myeloid, pre-B, megakaryocytic, and NK progenitors was assessed by immunohistochemistry, flow cytometry, and mRNA expression of CD7, 14, 19, 41b, S6, and 61. We conclude that this procedure for multilineage progenitor cell purification is simple and effective and could have major implications for gene transfer and stem cell transplantation.