Biologic Significance of Constitutive and Subliminal Growth Factor Production by Bone Marrow Stroma

The "stromal" or adherent cells of long-term murine Dexter explant bone marrow cultures provide the best in vitro model of the bone marrow microenvironment. Colony-stimulating factor-1 (CSF-1) is produced constitutively by these cells and is easily detected, but most investigators have not found constitutive production of the other hemolymphopoietic cytokines. We have previously reported the detection of granulocyte-macrophage-CSF (GM-CSF) in murine stromal cultures and its induction by the lectin Pokeweed mitogen. The present studies analyzing stromal cytokine messenger RNA (mRNA) production by standard Northern blot analysis show constitutive production of mRNAs for CSF-1, GM-CSF, granulocyte-CSF (G-CSF), c-kit ligand (KL), and interleukin-6 (IL-6), but not IL-3, IL-4, or IL-5 by 3-week irradiated or nonirradiated murine Dexter stromal cells. Exposure of stromal cells to Pokeweed mitogen or IL-1 16 hours before RNA harvest induces the messages for GM-CSF, G-CSF, KL, and IL-6, but not IL-3, IL-4, IL-5, or CSF-1. Polymerase chain reaction amplification of cDNA made with reverse transcriptase from stromal RNA using two separate sets of IL-3-specific primers shows the presence of IL-3 message in irradiated stromal cells, which is only detectable with this more sensitive technique. The factor-dependent cell lines FDC-P1 and 32D are supported by the stromal cells without the addition of exogenous growth factors, demonstrating a cytokine activity in these cultures that is inhibited by the addition of anti-IL-3 or anti-GM-CSF antibodies. These data indicate that murine Dexter stromal cells constitutively produce CSF-1, GM-CSF, G-CSF, IL-6, KL, and IL-3. This growth factor production could explain the support of granulocyte, macrophage, and megakaryocyte production and stem cell maintenance in Dexter-type long-term murine bone marrow cultures.