TNF-α plus IFN-γ induce connexin43 expression and formation of gap junctions between human monocytes/macrophages that enhance physiological responses

In this work, the effects of bacterial LPS, TNF-α, and IFN-γ on gap junctional communication (dye coupling) and on the expression of connexin43 (immunofluorescence, immunoblotting, and RT-PCR) in monocytes/macrophages were studied. Freshly isolated human monocytes plated at high density and treated either with LPS plus IFN-γ or TNF-α plus IFN-γ became transiently dye coupled (Lucifer yellow) within 24 h. Cells treated with LPS, TNF-α, or IFN-γ alone remained dye uncoupled. In dye-coupled cells, the spread of Lucifer yellow to neighboring cells was reversibly blocked with 18 α-glycyrrhetinic acid, a gap junction blocker, but it was unaffected by oxidized ATP or probenecid, which block ionotropic ATP-activated channels and organic anion transporters, respectively. Abs against TNF-α significantly reduced the LPS plus IFN-γ-induced increase in dye coupling. In dye-coupled monocytes/macrophages, but not in control cells, both connexin43 protein and mRNA were detected, and their levels were higher in cells with an elevated incidence of dye coupling. In dye-coupled cells, the localization of connexin43 immunoreactivity was diffuse at perinuclear regions and thin cell processes. The addition of 18-α-glycyrrhetinic acid induced a profound reduction of monocyte/macrophage transmigration across a blood brain barrier model. It also induced a significant reduction in the secretion of metalloproteinase-2 in cells treated with TNF-α plus IFN-γ. We propose that some monocyte/macrophage responses are coordinated by connexin-formed membrane channels expressed transiently at inflammatory sites in which these cells form aggregates.