Synthesis and regulation of accessory/proinflammatory cytokines by intestinal epithelial cells

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Abstract
Intestinal epithelial cells (IEC) have been shown to act as antigen-presenting cells (APC) in vitro and may have this capacity in vivo. In order to determine whether IEC, like other APC, are able to produce accessory cytokines which may play a role in T cell activation, we assessed the accessory cytokine profile of IEC constitutively or after stimulation. We measured expression, production and regulation of accessory cytokines (IL-1β, IL-6, tumour necrosis factor-alpha (TNF-α), transforming growth factor-beta (TGF-β) by the presence of mRNA as well as secreted protein. Freshly isolated IEC from surgical specimens were cultured in the presence or absence of lipopolysaccharide (LPS), interferon-gamma (IFN-γ), IL-1β or TNF-α. mRNA was assessed by a specific RNAse protection assay which controlled for contaminating cell populations while protein secretion was measured by ELISA (IL-1) or bioassay (TNF and IL-6). Neither IL-1β nor TNF-α were detectable in cultured IEC supernatants, supporting the lack of macrophage contamination. All IEC spontaneously secreted IL-6 at levels comparable to those of macro-phages. IEC IL-6 mRNA also increased approximately 200-fold during the first 24 h of culture. LPS, IFN-γ or TNF-α had no effect on spontaneous IL-6 production, and neither resulted in the secretion of IL-1β or TNF-α. However, IL-1β up-regulated IL-6 synthesis by 6–7-fold. IEC express a profile of cytokine mRNAs distinct from conventional APC (low level constitutive IL-6 expression but no detectable IL-1β, TGF-β or TNF-α), adding to their uniqueness as APC.