Transmembrane TNF-α Reverse Signaling Inhibits Lipopolysaccharide-Induced Proinflammatory Cytokine Formation in Macrophages by Inducing TGF-β: Therapeutic Implications

TNF-alpha, a potent proinflammatory cytokine, is generated in a precursor form called transmembrane (m)TNF-alpha that is expressed as a type II polypeptide on the surface of certain cells. mTNF-alpha was shown to act both as a ligand by binding to TNF-alpha receptors, as well as a receptor that transmits outside-to-inside (reverse) signals back into the mTNF-alpha bearing cells. In this study, we show that nonactivated macrophages express basal levels of mTNF-alpha and respond to anti TNF-alpha Abs by triggering the MAPK kinase 4 signaling pathway. The pathway induces TGF-beta. Based on inhibitory experiments, the production of TGF-beta 1 is regulated via Jun kinases, whereas that of other TGF-beta s is regulated via p38 MAPKs. Exposure to LPS further induced the expression of mTNF-alpha, and triggering of mTNF-alpha strongly suppressed the LPS-induced proinflammatory response. Neutralizing TGF-beta by Abs prevented the mTNF-alpha mediated suppression of LPS-induced proinflammatory cytokine formation, indicating that the immune suppressive effect of mTNF-alpha is mediated via TGF-beta. Although apoptotic cells are also known to suppress LPS-induced proinflammatory cytokine formation in macrophages by upregulating TGF-beta, we show that they do not use the mTNF-alpha signaling pathway. Because TGF-beta possesses a wide range of immune-suppressive effects, our data indicate that upregulation of TGF-beta synthesis by those TNF-alpha targeting molecules, which are able to trigger mTNF-alpha, might contribute to their therapeutic effect in the treatment of certain inflammatory diseases such as Crohn's disease, Wegener's granulomatosis, or sarcoidosis. Additionally, none of the TNF-alpha targeting molecules is expected to interfere with the immune-silencing effects of apoptotic cells.