TNF-α sensitizes HT-29 colonic epithelial cells to intestinal lactobacilli

The ability of the proinflammatory cytokine tumor necrosis factor-α (TNF-α) to influence epithelial interleukin (IL)-8 responses to the intestinal bacterium Lactobacillus plantarum 299v was analyzed in the human HT-29 colonic epithelial cell line. In the absence of TNF-α, IL-8 mRNA expression was not detectable by Northern blot analysis in HT-29 cells alone or in HT-29 cells co-cultured with L. plantarum 299v. However, TNF-α induced IL-8 mRNA expression, and co-culture of TNF-αtreated HT-29 cells with L. plantarum 299v significantly increased IL-8 mRNA expression above levels induced by TNF-α alone in an adhesion-dependent manner. The increase in IL-8 mRNA expression was not observed in TNF-α-treated HT-29/L. plantarum 299v cocultures using heat-killed lactobacilli or when L. plantarum adhesion was prevented using mannoside or a trans-well membrane. Paradoxically, IL-8 secretion was decreased in TNF-αtreated HT-29 cells with L. plantarum 299v relative to cells treated with TNF-α alone. TNF-α-mediated responsiveness to L. plantarum 299v was further investigated by analyzing expression of a coreceptor for bacterial cell wall products CD14. HT-29 cells expressed CD14 mRNA and cell-surface CD14; however, TNF-α did not alter CD14 mRNA or cell-surface expression, and blockade of CD14 with monoclonal antibody MY4 did not alter the IL-8 response to L. plantarum 299v in TNF-α-treated HT-29 cells. These results indicate that although TNF-α sensitizes HT-29 epithelial cells to intestinal lactobacilli, the bacteria exert a protective effect by downregulating IL-8 secretion.