| Summary: | <p>Abstract</p> <p>Background</p> <p>Human intestinal epithelial cells (IECs) secrete the chemokine CCL20 in response to infection by various enteropathogenic bacteria or exposure to bacterial flagellin. CCL20 recruits immature dendritic cells and lymphocytes to target sites. Here we investigated IEC responses to various pathogenic and commensal bacteria as well as the modulatory effects of commensal bacteria on pathogen-induced CCL20 secretion. HT-29 human IECs were incubated with commensal bacteria (<it>Bifidobacterium infantis </it>or <it>Lactobacillus salivarius</it>), or with <it>Salmonella typhimurium</it>, its flagellin, <it>Clostridium difficile, Mycobacterium paratuberculosis</it>, or <it>Mycobacterium smegmatis </it>for varying times. In some studies, HT-29 cells were pre-treated with a commensal strain for 2 hr prior to infection or flagellin stimulation. CCL20 and interleukin (IL)-8 secretion and nuclear factor (NF)-κB activation were measured using enzyme-linked immunosorbent assays.</p> <p>Results</p> <p>Compared to untreated cells, <it>S. typhimurium, C. difficile, M. paratuberculosis</it>, and flagellin activated NF-κB and stimulated significant secretion of CCL20 and IL-8 by HT-29 cells. Conversely, <it>B. infantis, L. salivarius </it>or <it>M. smegmatis </it>did not activate NF-κB or augment CCL20 or IL-8 production. Treatment with <it>B. infantis</it>, but not <it>L. salivarius</it>, dose-dependently inhibited the baseline secretion of CCL20. In cells pre-treated with <it>B. infantis, C. difficile-, S. typhimurium-</it>, and flagellin-induced CCL20 were significantly attenuated. <it>B. infantis </it>did not limit <it>M. Paratuberculosis-</it>induced CCL20 secretion.</p> <p>Conclusion</p> <p>This study is the first to demonstrate that a commensal strain can attenuate CCL20 secretion in HT-29 IECs. Collectively, the data indicate that <it>M. paratuberculosis </it>may mediate mucosal damage and that <it>B. infantis </it>can exert immunomodulatory effects on IECs that mediate host responses to flagellin and flagellated enteric pathogens.</p>
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