IL-17A amplifies TNF-alpha-induced inflammation in intestinal epithelial cells via up-regulation of pro-inflammatory chemokines and cytokines including IL-17C and inflammatory bowel disease risk gene transcripts

Background and aims: IL-17-secreting Th17 cells play a major role in the pathogenesis of inflammatory bowel diseases (IBD). However, clinical trials in IBD patients using anti-IL-17 antibodies lacked clinical efficacy. We therefore analyzed how IL-17 modulates the proinflammatory response elicited by TNF-alpha in intestinal epithelial cells (IEC).

Methods: Target gene activation by TNF-alpha, IL-17A and by combined TNF/IL-17A treatment were investigated by microarray and qPCR. Signaling pathways were identified by receptor neutralization, RNA silencing and kinase inhibitor experiments. Selected target genes were analyzed in the serum and in intestinal biopsies of IBD patients.

Results: Microarray analysis demonstrated that IL-17A alone is only a weak inducer of gene expression in IEC (29 regulated transcripts, P<0.05). However, the combined stimulation with IL-17A and TNF significantly (P<0.05) affected the expression of 823 genes, with a strong up-regulation of proinflammatory chemokines and cytokines. Particularly the gene expression of the chemokines CCL20, CXCL1 and CXCL8 was strongly enhanced when IEC were stimulated with both IL-17A and TNF-alpha (>200-fold increase compared to non-stimulated controls, P<0.01). Furthermore, we demonstrate for the first time that IL-17A and TNF-alpha increase the expression of IBD susceptibility genes (CCL2, FUT2, ICAM1, ICOSLG, JAK2, LTB, SMAD3, TNFSF15) and related genes (IL10RA, IL23A). We found that the enhanced gene transcription resulting from the combined action of IL-17A and TNF-alpha is dependent on IL-17RA, ACT1, NF-κB and MAPK signaling. In addition, IL-17C, which has been previously shown to elicit a pro-inflammatory response, is strongly enhanced when IEC are stimulated with both IL-17A and TNF-alpha. IL-17C in turn induces CCL20 and CXCL8 expression in IEC. Furthermore, intestinal IL17C mRNA expression and serum IL-17C are increased in patients with active IBD.

Conclusions: This study shows that IL-17A alone is only a weak transcriptional activator in IEC but amplifies the TNF-induced gene expression of proinflammatory chemokines and cytokines including IL-17C as well as IBD susceptibility gene transcripts, suggesting a beneficial effect of the combined inhibition of TNF-alpha and IL-17A in human IBD.