The role of TNFR2 in regulating TLR-mediated tissue damage in XIAP-deficiency

• Applicant: Dr. Monica Yabal
• Subject Area: Immunology ; Cell Biology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 504986538

Regulated cell death (RCD) pathways are key regulators of immunity that function to dampen or amplify inflammation in tissues. Necroptosis for instance increases immune cell infiltration, due to the release of intracellular content and chemokines/cytokines, whereas apoptosis avoids induction of inflammation by silently eliminating certain immune cells, such as activated T cells. A central regulator of several RCD pathways is the X-linked inhibitor of apoptosis (XIAP) protein. Depending on the cell type studied, XIAP protects from either apoptosis or necroptosis, thus it has the capacity to either promote or inhibit inflammation. In initial in vitro studies of murine cells, we have shown that XIAP prevents necroptosis of myeloid immune cells after activation by Toll-like receptors (TLRs) ligands. Mechanistically, in the absence of XIAP expression TLR activation leads to cell death, that is activated by TNF-induced signaling and the kinase RIPK3. A key finding was that necroptosis was driven by TNF receptor 2 signaling (TNFR2), which was unexpected as unlike TNFR1, TNFR2 does not contain a death domain that is required to directly activate cell death signaling. We hypothesized that TNFR2-mediated RCD of myeloid cells would be a driver of hyper-inflammation observed in XIAP-deficient patients. Since these patients often present with inflammatory bowel disease (IBD) we focused on the intestinal compartment in an XIAP-deficient mouse model. We identified that in the small intestine of Xiap-/- mice the receptor for bacterial flagellin, TLR5, is involved in inducing cell-death. Intriguingly, TLR5 is uniquely expressed in intestinal type II dendritic cells (cDC2s) and also in specialized intestinal epithelial cells (Paneth cells) in both, mice and humans. While we could detect a direct link between TLR5 and TNFR1-mediated cell death of Paneth cells using small intestinal organoids, the roles of TLR5 and TNFR2 signaling in cDC2s remains to be defined. Thus, the main goal of this application is the dissection of the unique features of TLR5 and TNF-induced signaling in cDC2s, and the impact of RCD for local inflammation in the intestine. We will focus on defining the TLR5 signature of the TLR5+cDC2s and how RCD is controlled in these cells. Furthermore, the impact of RCD of Xiap–/– cDC2s on specialized epithelial and T cell functions will be examined to understand in more detail pathogenesis of local inflammation in the intestine. These findings will finally be expanded to human myeloid cells, where we will characterize how signaling through variant TNFR2 contributes to cell death. Characterizing the regulation of cell death and its consequences will further our understanding of the immune-pathogenesis of inflammatory bowel disease and will hopefully help to identify new molecular targets for future immune interventions.

• DFG Programme: Research Grants