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Induction of surface tumor necrosis factor (TNF) expression and possible facilitation of surface TNF release from human monocytic cells by granulocyte-macrophage colony-stimulating factor or gamma interferon in combination with 1,25-dihydroxyvitamin D3.

1,25-dihydroxyvitamin D3 (1,25(OH)2D3), gamma interferon (IFN-gamma) and granulocyte-macrophage colony-stimulating factor (GM-CSF) can regulate monocyte maturation and activation. Using the human monocytoid cell line U937, we have shown that these agents increase surface tumor necrosis factor (TNF) expression without directly affecting TNF release. GM-CSF and IFN-gamma combined with 1,25(OH)2D3 increased cellular TNF secretion to levels not seen with these agents alone. Ability to express and secrete TNF in part depended on degree of monocytic maturation. The combination of 1,25(OH)2D3 and GM-CSF, however, facilitated lipopolysaccharide (LPS)-mediated release of surface TNF from U937 cells, an effect that was temporally independent of maximal maturation. 1,25(OH)2D3 plus IFN-gamma was less effective than 1,25(OH)2D3 plus GM-CSF at facilitating TNF secretion. We postulate that 1,25(OH)2D3 and GM-CSF are required together to prime a specific mechanism, probably a protease, which cleaves TNF from the surface of monocytic cells. This protease, once primed, can be activated by a secondary stimulus such as LPS.