Interleukin-6 synergizes with M-CSF in the formation of macrophage colonies from purified human marrow progenitor cells

We examined the in vitro stimulative effects of recombinant human interleukin-6 (IL-6, or interferon-beta 2) on purified human bone marrow progenitor cells. IL-6 alone or in combination with erythropoietin (Epo), IL-3, GM-CSF, or G-CSF did not induce colony formation. However, IL-6 strongly synergized with M-CSF in stimulating macrophage colony formation (colony numbers and size). The magnitude of IL-6 synergism with M-CSF was dose dependent; maximal potentiation of M- colony formation was evident at approximately 100 to 1,000 U/mL IL-6. When the addition of IL-6 to M-CSF-supplemented cultures was delayed for more than one day after the beginning of culture, enhancement of macrophage colony formation was lost. IL-6 stimulation of M-CSF- responsive colony formation was not apparent when nonpurified marrow cells were plated, most likely due to endogenous IL-6 release. These observations suggest that IL-6, in addition to playing a role in B- lymphocyte proliferation can potentiate the human immune defence mechanism by stimulating monocyte-macrophage development as well.