Characterization of murine hematopoietic progenitor subsets involved in interleukin-3-induced interleukin-6 production

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.