Stroma-dependent maintenance of cytokine responsive hematopoietic progenitor cells derived from long-term bone marrow culture

Hematopoietic cells maintained for long periods on primary cultures of bone marrow stromal cells formed cobblestone colonies (Dexter's long-term bone marrow culture, LTBC). These stably maintained hematopoietic cells (for 4 months) were transferred to a coculture on an established spleen stromal cell line (MSS62), and maintained under stromal cell layer, where they retained their invasive ability in the restricted space between the stromal cell layer and culture substratum (DFC culture). DFC contained lineage-negative (Lin-), c-Kit+, Sca-1- cells and spontaneously produced Mac-1+, Gr-1+ cells. DFC could not grow in the absence of MSS62 stromal cells, although, GM-CSF, IL-3, or IL-7 stimulated its growth. Production of granulocyte and monocytic cells was maintained by GM-CSF or IL-3 while it was decreased by IL-7. RT-PCR analysis showed that the IL-7 responsive cell population expressed early lymphoid markers (Ikaros, Pax-5, Oct-2, Rag-1, TdT, IL-7R and Imu), while lacking expression of receptors for G-CSF (G-CSFR) and for M-CSF (M-CSFR), or myeloperoxidase (MPO). These results suggested that DFC simultaneously contained lymphoid-committed progenitors and myeloid-committed progenitors, and that cytokines may expand their responding progenitor cells under the influence of signals provided by the stromal cells. Such a stromal cell-dependent culture system may be useful to analyze the switching mechanism from constitutive to inducible hematopoiesis in vitro.