Epithelial-specific deletion of XBP1 in mice resulted in spontaneous ileitis and increased susceptibility to chemically induced colitis, linking cell-specific read this ER stress to organ-specific inflammation [27]. In addition, the absence of IRE1 in mice led to an increased susceptibility to experimental colitis [28]. In this study, we performed an extensive analysis of transcript and protein levels of genes involved in the three UPR pathways in colonic and ileal biopsies of healthy controls and patients with CD and UC. Significant increase in transcript levels of HSPA5, PDIA4 and splicing of XBP1 was observed along with increased protein concentrations of HSPA5, PDIA4 and pEIF2A/EIF2A in colonic IBD-associated inflammation. Notably, GADD34, which is involved in blocking the PERK pathway, was not observed to be modulated.

Whereas no significant induction of any of the three pathways was seen in ileal inflammation, the majority of UPR-related genes revealed a significantly increased expression in healthy ileal tissue as compared to healthy colonic tissue. These findings suggest a higher basal UPR engagement in ileal tissue, most likely reflecting the presence of highly secretory cell types, high concentrations of exogenous antigens and a higher metabolic activity. Stimulation of colonic and ileal samples of healthy controls with tunicamycin, a well-known inducer of ER stress, revealed a higher response of the ileal mucosa compared to the colonic mucosa, confirming that this higher basal level does not prevent further UPR engagement.

The increased response of the ileum to alterations in the UPR is consistent with the spontaneous development of ileitis in mouse models with deleted XBP1, where the absence of XBP1 prevents its beneficial effect on ER homeostasis and consequently increase the burden on the ER [27]. Collectively, these results point to a role for the UPR in the pathogenesis of IBD with different implications for colonic and ileal disease. Results Correlation of endoscopic inflammation and transcriptional expression of IL8 IL8 is used as a reliable marker of intestinal inflammation [29], [30], [31]. An extensive set of colonic (Fig. 1A) and ileal samples (Fig. 1B) of healthy controls, UC and CD patients was evaluated for IL8 mRNA to confirm and define inflammation in endoscopic pinch biopsies.

For biopsy samples taken in endoscopically affected areas, IL8 was increased by as much as 38-fold in UC (p<0.0001) (Fig. 1A), 35-fold in colonic CD (p<0.0001) (Fig. 1A) and 35-fold in ileal CD (p=0.001) (Fig. 1B) when Entinostat compared to healthy controls. Comparison of endoscopically non-inflamed samples of active UC patients to healthy controls revealed no difference. In contrast, IL8 was induced by as much as 17-fold (p<0.0001) (Fig. 1A) in colonic samples and 8-fold (p=0.040) (Fig. 1B) in ileal samples taken in endoscopically non-inflamed areas of active CD patients, demonstrating a clear residual inflammation.