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Monocyte-conditioned medium and interleukin 1 induce granulocyte-macrophage colony-stimulating factor production in the adherent cell layer of murine bone marrow cultures.

We have studied how production of colony-stimulating factors (CSF) can be induced in murine long-term bone marrow cultures (LTBMC). We found that the adherent cells, but not the nonadherent cells, of LTBMC synthesized and secreted large amounts of CSF upon stimulation with monocyte-conditioned medium (MCM) from the early phase of monocyte culturing. This CSF induced both granulocyte- and macrophage-containing colonies. Interleukin 1 (IL-1) also induced CSF production by the adherent cells, although not to the same extent as MCM. Medium conditioned by E-rosette-positive lymphocytes could not substitute for MCM. CSF production varied in long-term bone marrow cultures less than two weeks old, but thereafter the amount of CSF obtained was relatively independent of the age of the cultures (2-26 weeks). No correlation was found between the content of granulocyte-macrophage colony-forming cells (GM-CFC) in the nonadherent cell fraction of LTBMC and the ability of the adherent cell layer to produce CSF. These results suggest a two-stage process for CSF synthesis. Monocytes produce a factor(s) that, in a second step, leads to bioassayable levels of CSF in the supernatant of adherent cells in LTBMC.