Reverse signaling through transmembrane TNF confers resistance to lipopolysaccharide in human monocytes and macrophages

We have previously reported that the CD14(+) monocytic subpopulation of hum an PBMC induces programmed cell death (apoptosis) in cocultured endothelial cells (EC) when stimulated by bacterial endotoxin (LPS), Apoptosis is medi ated by two routes, first via transmembrane TNF-alpha (mTNF) expressed on P BMC and, in addition, by TNF-independent soluble factors that trigger apopt osis in EC, Neutralizing anti-TNF mAb completely blocked coculture-mediated apoptosis, despite the fact that there should have been additional soluble cell death factors. This led to the hypothesis that a reverse signal is tr ansmitted from the TNF receptor on EC to monocytes (MO) via mTNF that preve nts the production of soluble apoptotic factors. Here we have tested this h ypothesis. The results support the idea of a bidirectional cross-talk betwe en MO and EC, Peripheral blood MO, MO derived macrophages (M Phi), or the m onocytic cell line Mono Mac 6 were preincubated with human microvascular EC that constitutively express TNF receptor type I (TNF-R1) and subsequently stimulated with LPS. Cell-free supernatants of these preparations no longer induced EC apoptosis, The preincubation of MO/M Phi with TNF-reactive agen ts, such as mAb and soluble receptors, also blocked the production of death factors, providing further evidence for reverse signaling via mTNF, Finall y, we show that reverse signaling through mTNF mediated LPS resistance in M O/M Phi as indicated by the down-regulation of LPS-induced soluble TNF and IL-6 as well as IL-1 and IL-10.