Synergistic activation of human monocytes by granulocyte-macrophage colony-stimulating factor and IFN-γ: Increased TNF-α but not IL-1 activity

TNF-α and IL-1 activities and PGE₂ levels were investigated in the supernatants of highly purified human monocytes cultured for 18 h with recombinant human granulocyte-macrophage CSF (GM-CSF). GM-CSF alone did not stimulate IL-1 or TNF-α activities or the production of PGE₂. GM-CSF with IFN-γ, but not with LPS, consistently activated the monocytes for TNF-α activity. In contrast, for increased IL-1 activity, GM-CSF synergized weakly and irregularly with LPS, but not at all with IFN-γ. For the third monocyte product investigated, GM-CSF was a weak and inconsistent inducer of PGE₂ and only in the co-presence of IFN-γ. Thus, GM-CSF can elicit different responses in human monocytes depending both on the co-stimulus as well as the monocyte product being investigated.