Naive monocytes can trigger transendothelial migration of peripheral bloodcells through the induction of endothelial tumour necrosis factor-alpha

In this manuscript we describe a potentially new mechanism by which unstimu lated human monocytes activate endothelial cells (EC) through the secondary induction of endothelial tumour necrosis factor alpha (TNF-alpha). Serum f ree supernatants (SN) of peripheral blood mononuclear cells (PBMC) strongly induce the expression of intercellular adhesion molecule 1 (ICAM-1, CD54), vascular cell adhesion molecule 1 (VCAM-1, CD106), and endothelial-leukocy te adhesion molecule 1 (ELAM-1, CD62E) on human EC 24 and 4 h post treatmen t, respectively. Further characterization of the responsible subpopulation revealed the CD14(+) monocytes and a monocytic cell line (MM6) to produce a n endothelial activating factor (EAF). The EAF also triggers an adhesion an d a transendothelial migration (TEM) of peripheral blood cells. Using neutr alization with an anti TNF-alpha MoAb MAK195, EAF is not identical with TNF -alpha, but induces the expression of endothelial TNF-alpha, since MAK195 b locked TEM only when coincubated with EC, not with monocytes. Furthermore, intracellular TNF-alpha was significantly upregulated in EC after treatment with SN-MM6. Another evidence for a secondary autocrine mechanism was prov ided by culturing the EC with a conditioned medium of SN-MM6 treated EC. Th is conditioned medium induces an adhesion molecule expression and TEM in a similar way to SN-MM6 and can completely be inactivated by anti TNF-alpha. Taken together, these data may have an impact for, e.g. transplantational s ettings that donor monocytes may trigger an inflammatory response in the ab sence of further activation signals by eliciting an endogenous TNF-alpha re sponse in the host.