Differential effects of Staphylococcal enterotoxin B-mediated immune activation on intestinal defensins.

In the small intestine members of both the alpha-defensin (DEFA5 and DEFA6) and beta-defensin (DEFB1 and DEFB2) family contribute to the anti-microbial barrier against infection. The aim of this study was to determine whether Staphylococcal enterotoxin B (SEB)-mediated immune activation and proinflammatory cytokines play a role in the regulation of intestinal defensin expression. Defensin mRNA and peptide secretion was studied after ex vivo tissue culture of duodenal biopsies over 24 h. Immune (T cell and macrophage) activation was induced by SEB, and in separate experiments exogenous proinflammatory cytokines were added individually. Defensin mRNA levels were quantified by reverse transcription-polymerase chain reaction, and peptide release into culture supernatants was quantified by immuno dot blot or enzyme-linked immunosorbent assay. Increasing concentrations of SEB down-regulated DEFA5, DEFA6 and DEFB1 mRNA in a dose-dependent manner but increased DEFB2 simultaneously. The down-regulation of alpha-defensins was reversed by dexamethasone. DEFA5 and DEFB2 peptide secretion levels were altered in parallel with mRNA. Interferon-gamma and interleukin (IL)-1beta exhibited a dose-dependent down-regulation of alpha-defensin mRNA, IL-6 significantly down-regulated only DEFA6; in contrast, tumour necrosis factor-alpha and IL-4 had no significant effect. Immune cell activation and proinflammatory cytokines down-regulated the constitutively expressed DEFA5, DEFA6 and DEFB1 defensins, and up-regulated DEFB2 in intact human intestinal tissue explants in short-term culture. The effect of local immune activation on innate defence may explain the reduced alpha-defensin expression noted in inflammatory T cell-mediated enteropathies.