Muramyl dipeptide and mononuclear cell supernatant induce Langhans-type cells from human monocytes

Muramyl dipeptide (MDP) in bacterial cell walls reportedly evokes epithelioid cell granulomas. We examined its effects on multinucleated-giant-cell (MGC) formation from monocytes. Supernatant of concanavalin A-stimulated peripheral blood mononuclear cells (conditioned medium) generated MGCs from monocytes. MDP significantly increased the fusion index of Langhans-type MGCs (LGCs) but did not affect total MGCs. N-Acetylmuramyl-L-alanyl-L-isoglutamine, an MDP analogue, had no effect on MGC formation. MGCs were produced by conditioned medium from CD14(++)/CD16(-) monocytes. MDP enhanced the LGC fusion index from CD14(++)/CD16(-) monocytes. MGCs were not produced from CD14(+)/CD16(+) monocytes or immature dendritic cells induced by granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin (IL) 4 and only weakly produced from macrophage (M)-CSF- or GM-CSF-induced macrophages. Added MDP did not generate MGCs from CD14(+)/CD16(+) monocytes or dendritic cells but enhanced LGC formation from macrophages. Because IFN-gamma, IL-3, and GM-CSF reportedly are important in LGC induction, we added anti-IFN-gamma, anti-IL-3, or anti-GM-CSF monoclonal antibody (mAb) concomitantly to the monocyte culture treated with conditioned medium alone or plus MDP. Anti-IFN-gamma mAb completely abrogated MGC generation, whereas anti-GM-CSF and anti-IL-3 mAbs significantly inhibited LGCs. These findings suggest that CD14(++)/CD16(-) monocytes are fused to form LGCs by MDP derived from granulomatous-disease-causing pathogens with inflammatory mediators such as IFN-gamma, IL-3, and GM-CSF.