Inflammatory bowel diseases (IBD) are chronic relapsing inflammatory disorders of the gastrointestinal tract with increasing incidence over the past 50 years. Despite a wide variety of possible triggers, inappropriate immune responses to the commensal microbiota are generally considered a central underlying theme in IBD. The intestinal epithelium forms a structural barrier between the commensal bacteria residing in the intestinal lumen and the mucosal immune cells and plays a critical role for the maintenance of gut immune homeostasis.
We showed previously that mice lacking FADD in intestinal epithelial cells (FADD^IEC-KO) develop chronic ileitis and colitis due to RIPK3-mediated death of IECs. Moreover, mice with IEC-specific knockout of caspase-8 (Casp8^IEC‑KO) were reported to develop ileitis but not colitis, suggesting that FADD and caspase-8 may have different functions in the regulation of intestinal epithelial homeostasis and inflammation.
The main aim of thhis project is to dissect the mechanisms by which FADD and caspase-8 regulate intestinal inflammation using genetic mouse models. The recent discovery that human patients with caspase-8 mutations develop severe intestinal inflammation suggests that the FADD/Casp8-dependent pathways regulating intestinal homeostasis and inflammation are conserved between humans and mice.

Therefore, our project is relevant for the pathogenesis of intestinal inflammation in humans and may lead to the identification of new therapeutic targets for patients with IBD.

1. Dissect the upstream pathways that regulate the development of colitis and ileitis in FADD^IEC-KO and CASP8^IEC-KO  mice

2. Unravel the mechanisms by which FADD or Caspase-8 act in intestinal epithelial cells to regulate immune homeostasis and prevent mucosal inflammation

3. Study the interplay of FADD/Casp8 with other pathways that are important for the pathogenesis of inflammatory bowel diseases

Mice with IEC-specific knockout of FADD or caspase-8 were previously shown to develop necroptosis-mediated colitis that depended on TNFR1 signalling and ileitis that was independent of TNFR1. We found that in addition to TNFR1, also Z-DNA Binding Protein 1 (ZBP1) acts as an upstream inducer of IEC death and inflammation in FADD^IEC-KO mice. Our results showed that TNFR1 and ZBP1 exhibit redundant functions inducing necroptosis and ileitis in FADD^IEC-KO mice, while both receptors are important for colitis development (Schwarzer et al, 2020). Moreover, we could show that ZBP1 activation in intestinal epithelial cells of FADD^IEC-KO mice depends on sensing endogenous Z-nucleic acids via its Zα domains (Jiao et al, 2020) . These results revealed a new role of ZBP1, acting as sensor of endogenous Z-nucleic acids to induce necroptosis and intestinal inflammation.

Previous studies showed that RIPK3 knockout prevented both colitis and ileitis in FADD^IEC-KO mice (Welz et al, 2011), suggesting that necroptosis of IECs drives the pathology in both the small and large intestine. Surprisingly, we found that, whereas MLKL deficiency prevented both colitis and ileitis in CASP8^IEC-KO mice, MLKL knockout fully prevented colitis but only partially ameliorated ileitis in FADD^IEC-KO mice, suggesting that necroptosis-independent mechanisms contribute to small intestinal inflammation in mice lacking FADD, but not Caspase-8, in IECs.
Our results showed that MLKL-independent ileitis in FADD^IEC-KO mice is driven by caspase-8, which activates GSDMD-dependent pyroptosis-like death of IECs to induce small intestinal inflammation (Schwarz et al, 2020).

Collectively, our studies revealed an important role of ZBP1 acting as a sensor of endogenous Z-nucleic acids to induce cell death and inflammation in the skin and the intestine. In particular, our work in genetic mouse models provided experimental evidence that ZBP1 cooperates with TNFR1 to drive intestinal inflammation by triggering necroptosis of intestinal epithelial cells when caspase-8 is inhibited. TNF is a key pathogenic factor in inflammatory bowel disease with anti-TNF antibodies constituting a front-line and highly effective therapy in both Crohn’s Disease and Ulcerative Colitis. However, a subset of patients does not respond to anti-TNF therapy whereas a considerable percentage of patients respond initially but show secondary loss of response to anti-TNF, therefore TNF-independent mechanisms drive intestinal inflammation. Based on our results, we hypothesize that ZBP1 may act as a pathogenic factor in IBD to induce TNF-independent inflammation. These results warrant further studies examining whether ZBP1 contributes to IBD and other inflammatory pathologies in humans.