Potentiation of flagellin responses in gut epithelial cells by interferon gamma is associated with STAT-independent regulation of MyD88 expression
Résumé
Flagellin acting via toll-like receptor 5 (TLR5) is a key regulator of the host response to the gut microbial flora in both health and disease. This study has investigated regulation of flagellin /TLR5 signalling in human colonocytes (HT29-19A) by interferon-gamma (IFNγ), a cytokine released early in the inflammatory process which has multiple effects on gut epithelial function that may facilitate abnormal responses to enteric bacteria. Flagellin induced a dose-dependent secretion of chemokines CXCL8 and CCL2 in the human colonocyte line, HT29-19A. Exposure to IFNγ did not induce chemokine secretion but markedly potentiated responses to flagellin increasing CXL8 gene expression and protein secretion by ~4 fold. Potentiation by IFNγ was independent of changes in TLR5 and associated with a rapid, sustained increase in expression of the downstream adaptor molecule MyD88. Knockdown of MyD88 expression using siRNA abolished flagellin-dependent CXCL8 secretion and the potentiating effect of IFNγ. Exposure of non-transformed mouse and human colonocytes to IFNγ also increased MyD88 expression. STAT1 knockdown and use of the broad-spectrum JAK-STAT inhibitor AG490 had no effect on IFNγ mediated upregulation of MyD88. These findings suggest that IFNγ sensitises colonic epithelial cells to bacterial flagellin via a largely STAT-independent up-regulation of MyD88 expression leading to increased secretion of immunomodulatory factors. These data indicate that epithelial responses to flagellin are potentiated by IFNγ, most likely mediated by increased MyD88 expression. This adds to our understanding of the spectrum of effects of this cytokine on gut epithelium that may contribute to bacterial driven inflammation in the gut.
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