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

TNF-a, a potent proinflammatory cytokine, is generated in a precursor form called transmembrane (m)TNF-a that is expressed as a type II polypeptide on the surface of certain cells. mTNF-a was shown to act both as a ligand by binding to TNF-a receptors, as well as a receptor that transmits outside-to-inside (reverse) signals back into the mTNF-a–bearing cells. In this study, we show that nonactivated macrophages express basal levels of mTNF-a and respond to anti–TNF-a Abs by triggering the MAPK kinase 4 signaling pathway. The pathway induces TGF-b. Based on inhibitory experiments, the production of TGF-b1 is regulated via Jun kinases, whereas that of other TGF-bs is regulated via p38 MAPKs. Exposure to LPS further induced the expression of mTNF-a, and triggering of mTNF-a strongly suppressed the LPS-induced proinflammatory response. Neutralizing TGF-b by Abs prevented the mTNF-a–mediated suppression of LPS-induced proinflammatory cytokine formation, indicating that the immunesuppressive effect of mTNF-a is mediated via TGF-b. Although apoptotic cells are also known to suppress LPS-induced proin- flammatory cytokine formation in macrophages by upregulating TGF-b, we show that they do not use the mTNF-a signaling pathway. Because TGF-b possesses a wide range of immune-suppressive effects, our data indicate that upregulation of TGF-b synthesis by those TNF-a–targeting molecules, which are able to trigger mTNF-a, 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-a–targeting molecules is expected to interfere with the immune-silencing effects of apoptotic cells.