A115 ROLE OF GUT SEROTONIN IN ANTIMICROBIAL PEPTIDE PRODUCTION

Antimicrobial peptides (AMPs) constitute an armory of innate regulators of crucial importance in the gastrointestinal (GI) tract. It is becoming evident that AMPs, especially human β-defensins (hBDs) produced by colonocytes, shape the composition of the microbiota and help maintain gut homeostasis. Defective bacterial clearance and subsequent modulation of gut microbiota, due to abnormal defensin expression, is associated with pathogenesis of various GI diseases including Inflammatory Bowel Disease (IBD). Serotonin or 5-hydroxytrytophan (5-HT) is a neurotransmitter and hormone that contributes to the regulation of various physiological functions in GI tract. Changes in gut 5-HT signaling are observed in IBD as well as in experimental colitis. Enterochromaffin (EC) cells of the human GI tract are the largest producers of 5-HT, and 5-HT biosynthesis depends on the rate-limiting tryptophan hydroxylase 1 (Tph1). We have previously demonstrated Tph1-deficient (Tph1-/-) mice, which have significantly lower gut 5-HT levels, exhibited reduced severity of colitis compared to their wild-type (Tph1+/+) littermates; while replenishing 5-HT led to increased severity of colitis. As EC cells are situated in the epithelial layer and epithelial cells express 5-HT receptors, we hypothesize that 5-HT can influence β-defensin production directly by acting on the adjoining epithelial cells in relation to innate defense in the GI tract. To elucidate the role of 5-HT in β-defensin production from intestinal epithelial cells in the context of regulating innate immune response. We utilized human colonic HT-29 cells to assess the level of hBD expression upon stimulation with 5-HT (10–3, 10–7, 10–11 M). Quantitative PCR (qPCR) and conventional ELISA were performed to measure gene expression and protein levels in culture supernatant, respectively. Additionally, we measured mouse β-defensin levels in colonic tissues of naïve Tph1+/+ and Tph1-/-. 5-HT, in a dose dependent manner, regulates hBD production by HT-29 cells, where higher 5-HT concentrations down-regulated both mRNA and protein levels. Moreover, we observed significantly higher levels of total mBD in the colonic tissues of Tph1-/- compared to Tph1+/+, suggesting an inhibitory role of 5-HT in defensin production. Our results illustrate that 5-HT released from EC cells can modulate hBD production from intestinal epithelial cells. These results exemplify novel information on the interaction between 5-HT and hBD in relation to intestinal innate immune response and corroborate with previous findings of attenuated hBD expression in colonic CD patients. CCC