We have developed an in vitro clonal assay of murine hematopoietic precursor cells that form spleen colonies (CFU-S day 12) or produce in vitro clonable progenitors in the marrow (MRA cells) of lethally irradiated mice. The assay is essentially a long-term bone marrow culture in microtiter wells containing marrow-derived stromal “feeders” depleted for hematopoietic activity by irradiation. To test the validity of the assay as a quantitative in vitro stem cell assay, a series of unsorted and physically sorted bone marrow cells were simultaneously assayed in vivo and overlaid on the feeders in a range of concentrations, while frequencies of cells forming hematopoietic clones (cobblestone area forming cells, CAFC) were calculated by means of Poisson statistics. Linear regression analysis of the data showed high correlations between the frequency of CFU-S day 12 and CAFC day 10, and between MRA cells and CAFC day 28. A majority of MRA activity and CAFC day 28 was separable from CFU-S day 12 and CAFC day 10. This correlation study validates the CAFC system as a clonal assay facilitation both the quantitative assessment of a series of subsets in the hematopoietic stem cell hierarchy and the study of single long-term repopulating cells in vitro.

We have developed an in vitro clonal assay of murine hematopoietic precursor cells that form spleen colonies (CFU-S day 12) or produce in vitro clonable progenitors in the marrow (MRA cells) of lethally irradiated mice. The assay is essentially a long-term bone marrow culture in microtiter wells containing marrow-derived stromal “feeders” depleted for hematopoietic activity by irradiation. To test the validity of the assay as a quantitative in vitro stem cell assay, a series of unsorted and physically sorted bone marrow cells were simultaneously assayed in vivo and overlaid on the feeders in a range of concentrations, while frequencies of cells forming hematopoietic clones (cobblestone area forming cells, CAFC) were calculated by means of Poisson statistics. Linear regression analysis of the data showed high correlations between the frequency of CFU-S day 12 and CAFC day 10, and between MRA cells and CAFC day 28. A majority of MRA activity and CAFC day 28 was separable from CFU-S day 12 and CAFC day 10. This correlation study validates the CAFC system as a clonal assay facilitation both the quantitative assessment of a series of subsets in the hematopoietic stem cell hierarchy and the study of single long-term repopulating cells in vitro.

Various murine cell populations were tested for their ability to generate interleukin-6 (IL-6) in response to IL-3. Among these, bone marrow cells exhibit the most prominent IL-6 production. The responder cells in this organ have been further characterized by cell fractionation on a discontinuous Ficoll gradient, fluorescence- activated cell sorting, and in situ hybridization. These procedures have allowed us to ascribe the following features to the cells mainly responsible for IL-3-induced IL-6 production: (1) they possess a low density and a relatively high forward and perpendicular light scatter (FLS/PLS); (2) they are characterized by a high rhodamine (Rh) retention; and (3) their enrichment in various subpopulations is similar to that obtained for progenitors forming colonies in the methylcellulose assay colony-forming units (CFU-C). In contrast, IL-3 target cells in terms of IL-6 production are absent both in the mature and in the most immature bone marrow compartment. Indeed, the Rh-dull population that is enriched for cells with marrow repopulating activity does not respond to the growth factor and mature cells cannot be induced to express IL-6 as assessed by (1) FLS/PLS characteristics, (2) the monoclonal antibody ER-MP 20 recognizing monocytes and granulocytic cells, and (3) in situ hybridization. Taken together, our data support the conclusion that the bone marrow cells generating IL-6 in response to IL-3 belong to a progenitor population with enhanced mitochondrial activity, comprising probably several types of immature cells of the myeloid lineage including macrophage/granulocyte precursors.

Various murine cell populations were tested for their ability to generate interleukin-6 (IL-6) in response to IL-3. Among these, bone marrow cells exhibit the most prominent IL-6 production. The responder cells in this organ have been further characterized by cell fractionation on a discontinuous Ficoll gradient, fluorescence- activated cell sorting, and in situ hybridization. These procedures have allowed us to ascribe the following features to the cells mainly responsible for IL-3-induced IL-6 production: (1) they possess a low density and a relatively high forward and perpendicular light scatter (FLS/PLS); (2) they are characterized by a high rhodamine (Rh) retention; and (3) their enrichment in various subpopulations is similar to that obtained for progenitors forming colonies in the methylcellulose assay colony-forming units (CFU-C). In contrast, IL-3 target cells in terms of IL-6 production are absent both in the mature and in the most immature bone marrow compartment. Indeed, the Rh-dull population that is enriched for cells with marrow repopulating activity does not respond to the growth factor and mature cells cannot be induced to express IL-6 as assessed by (1) FLS/PLS characteristics, (2) the monoclonal antibody ER-MP 20 recognizing monocytes and granulocytic cells, and (3) in situ hybridization. Taken together, our data support the conclusion that the bone marrow cells generating IL-6 in response to IL-3 belong to a progenitor population with enhanced mitochondrial activity, comprising probably several types of immature cells of the myeloid lineage including macrophage/granulocyte precursors.