The role of IL-37 on cytokine responses downstream of TLR4 and TLR5 signaling in the colon epithelial cells.

Microscopic colitis (MC), comprising collagenous colitis (CC) and lymphocytic colitis (LC), is characterized clinically by chronic watery diarrhea, abdominal pain, and weight loss. IL-37 (IL-1F7) is a newly described anti-inflammatory cytokine of the IL-1 family. Previously, reduced gene expression of IL-37 has been observed by our group in the colonic mucosa of MC patients, which was suggested as one of the reasons for the chronicity of the colonic inflammation. We here investigated the role of IL-37 on pro-inflammatory cytokine responses mediated by TLR2, TLR4, or TLR5 signaling pathways in the Caco-2 colon epithelial cell line upon stimulation with peptidoglycan, LPS or Flagellin, which was followed by silencing of IL-37 with siRNA. Gene and protein expressions of IL-37, CXCL8, IL-1β, TNF, CXCL11, and CCL20 were detected using qRT-PCR and ELISA. IL-37 gene and protein levels were significantly increased upon LPS and Flagellin stimulations in Caco-2 cells. We didn’t observe any changes upon peptidoglycan stimulation. IL-37 silenced Caco-2 cells had significantly increased CXCL8, TNF and CCL20 gene expressions, but not IL-1β upon 100 ng/ml Flagellin stimulation, whereas 500 ng/ml LPS stimulation led to up regulation of CXCL8, TNF and IL-1β but not CXCL11 gene expressions compared to cells without siRNA transfections. Flagellin and LPS stimulations of IL-37 silenced Caco-2 cells resulted in significantly increased CXCL8 protein levels. Altogether, this study show the novel regulatory role of IL-37 on pro-inflammatory cytokine production following TLR4 or TLR5 stimulation, corroborating the importance of reduced IL-37 in MC patients as one of the important immunopathological factors.