TNF Antagonists in IBD: Novel Antiinflammatory Mechanisms Beyond Cytokine Inhibition.

Tumor necrosis factor (TNF) antagonists, such as infliximab, adalimumab, or etanercept, belong to the top 10 best-selling drugs worldwide and have markedly influenced the treatment of inflammatory bowel disease. Despite their efficacy, the mechanism of action of TNF antagonists is not fully understood. Anti-TNF therapy interferes with pathogenic processes, presumably at multiple levels, by lowering cytokine production, inhibiting inflammatory cell recruitment, and by inducing cell death in inflammatory cells. A recent report by Werner et al reports that anti-TNF therapy involves activation of Notch-1 signaling, a pathway that has not so far been implicated in the antiinflammatory effects of TNF inhibition. The authors show that TNF inhibition significantly induces apoptosis and prevents activation of activated peripheral blood and lamina propria mononuclear cells. They further demonstrate that TNF antagonists interrupt cell cycling of activated T cells, associated with an upregulation of the cyclin-dependent kinase inhibitor p21, downregulation of CD25, and inhibition of activation-induced cytokine secretion. TNF is expressed as a transmembrane form (tmTNF) that can be processed by TNF alpha-converting enzyme (TACE) into a soluble form. Both forms of TNF are bound by adalinumab and infliximab. Werner et al demonstrate that apoptosis in response to TNF antagonists was blocked by either silencing TNF expression, by inhibition of tmTNF cleavage on the cell surface, or by neutralizing the TNF receptors, TNF-R1 or TNF-R2. T-cell activation and cycling, in contrast, remained unaffected under these conditions. Interestingly, however, the authors identify Notch-1 as the responsible mediator for the cycle arrest induced by TNF inhibitors in T cells. The Notch pathway, which has been mainly linked to developmental and differentiation processes, is triggered upon proteolytic cleavage of Notch by ADAM and c-secretase proteases. Werner et al found that Notch-1 is activated by TNF inhibitors and binds to TNF. Conversely, Notch-1 inhibition prevented T-cell cycle arrest but not apoptosis in response to TNF antagonists. The molecular mechanism of how TNF antagonists trigger Notch1 activation remains unclear. Nevertheless, by connecting Notch and TNF signaling, this study provides novel mechanistic insights into the pleiotropic effects of TNF inhibitors. The mechanisms by which TNF antagonists ameliorate inflammation in Crohn’s disease are likely complex and multifactorial. In addition to neutralizing soluble TNF, TNF inhibitors bind to tmTNF. It is known that tmTNF functions not only as an activating ligand to trigger forward signaling into target cells, but also as a receptor that can transmit reverse signals via its cytoplasmic domain in tmTNFbearing cells. By their ability to induce crosslinking of TNF to an active trimer, infliximab and adalimumab have been found to trigger apoptosis of monocytes and lymphocytes in such a retrograde pathway. Moreover, it was demonstrated that anti-TNFinduced reverse signaling depends on the activation of c-Jun NH2-terminal kinase and the proapoptotic Bcl-2 protein Bax, but is independent of TNF receptors. The reverse signaling by tmTNF remains poorly understood in terms of biological function. Of special interest is the finding that reverse signaling by infliximab also induces the expression of antiinflammatory cytokines, such as interleukin (IL)-10. Therefore, reverse signaling seems to be an additional mechanism explaining some therapeutic effects of TNF antagonists. TNF receptor-mediated apoptosis is further complicated by the increasing evidence of crosstalk between TNF-R1 and TNF-R2. In most cases TNF-R1 signaling induces apoptosis, whereas TNF-R2 prevents cell death. Unlike TNF-R2, the ubiquitously expressed TNF-R1 consists of an intracellular death domain and accounts for the majority of proinflammatory, toxic, and apoptotic effects of TNF. In contrast, TNF-R2 is often inducibly expressed and more restricted to lymphocytes, where it mediates signals promoting lymphocyte activation and proliferation. The interaction between TNF receptors is orchestrated by many factors, including the intraand extracellular environment, activation of inflammatory pathways, and the cell type, which might determine the final outcome of cell survival or death. In this context, Notch-1 that is activated by TNF inhibitors might represent an interesting candidate. As shown by Werner et al, Notch-1 activation in response to TNF antagonists inhibits cell cycle progression and activation of T cells, thereby contributing to the antiinflammatory activity of TNF-blocking agents. Whether Notch-1 can directly affect apoptosis of immune cells remains a matter of debate. Notch activation can promote cell survival, but can also induce cell death. Unfortunately, Werner et al did not differentiate between various T-cell populations, e.g., effector and regulatory (Treg) T cells, when analyzing the effect of anti-TNF treatment on Notch-1 activation. In Treg cells, we have recently shown that anti-TNF treatment results in decreased rather than elevated apoptosis, which might explain the increase of Treg cells induced by antiCopyright VC 2012 Crohn’s & Colitis Foundation of America, Inc. DOI 10.1002/ibd.22988 Published online inWiley Online Library (wileyonlinelibrary.com).