Intestinal epithelial cells express immunomodulatory ISG15 during active ulcerative colitis and Crohn’s disease

BACKGROUND AND AIMS: Intestinal epithelial cells (IECs) secrete cytokines that recruit immune cells to the mucosa and regulate immune responses that drive inflammation in inflammatory bowel disease (IBD). However, experiments in patient-derived IEC models are still scarce. Here, we aimed to investigate how innate immunity and IEC-specific pattern recognition receptor (PRR) signaling can be involved in an enhanced type I interferon (IFN) gene signature observed in colon epithelium from patients with active IBD, with a special focus on secreted ubiquitin-like protein ISG15. METHOD: Gene and protein-expression in whole mucosa biopsies and in microdissected human colonic epithelial lining, in HT29 human intestinal epithelial cells and primary 3D colonoids treated with PRR-ligands and cytokines were detected by transcriptomics, in situ hybridisation, immunohistochemistry, western blots and ELISA. Effects of IECs secreted cytokines were examined in human peripheral blood mononuclear cells (PBMCs) by multiplex chemokine profiling and ELISA. RESULTS: The type I IFN gene signature in human mucosal biopsies was mimicked in TLR3 and to some extent TNF treated human IECs. In intestinal biopsies, ISG15 expression correlated with expression of the newly identified receptor for extracellular ISG15, LFA-1 integrin. ISG15 was expressed and secreted from HT29-cells and primary 3D colonoids through both JAK1-pSTAT-IRF9 dependent and independent pathways. In experiments using PBMCs we show that ISG15 releases IBD relevant proinflammatory cytokines such as CXCL1, CXCL5, CXCL8, CCL20, IL1, IL6, TNF and IFNgamma. CONCLUSION: ISG15 is secreted from primary IECs upon extracellular stimulation and mucosal ISG15 emerges as an intriguing candidate for immunotherapy in IBD.