Apical TLR ligation of intestinal epithelial cells drives a Th1-polarized regulatory or inflammatory type effector response in vitro

Intestinal epithelial cells (IEC) separate the mucosal immune system from the external milieu. Under inflammatory conditions, Toll-like receptor (TLR) expression by IEC is increased. In a transwell co-culture model immune modulation by IEC upon TLR ligation was studied. Human IEC (HT-29 and T84) grown on filters were apically or basolaterally exposed to TLR4 or TLR9 ligands and co-cultured with CD3/CD28-activated healthy donor PBMC in the basolateral compartment. TLR4 ligation of IEC (HT-29) enhanced the production of TNF-α and IEC-derived MDC and decreased numbers of Foxp3+ regulatory T cells. Neutralization of TSLP abrogated TLR4-induced TNF-α secretion. In contrast, apical TLR9 ligation of IEC (HT-29 and T84) enhanced IFN-γ and IL-10 secretion and increased the number of activated Th1 cells. The increase in IFN-γ secretion depended on the presence of IEC. Furthermore, CD14 expression on monocytes was reduced coinciding with enhanced intracellular IL-10 and decreased TNF-α production. However, basolateral TLR9 ligand exposure of HT-29 cells resulted in enhanced IFN-γ, IL-6 and TNF-α, while IL-10 secretion remained unaltered. TLR4 and TLR9 ligands reduced IL-13 secretion in presence and absence of apically exposed IEC and enhanced IL-12 secretion in presence of IEC. These data suggest that TLR4 ligation of IEC drives an inflammatory, while apical TLR9 ligation drives a regulatory Th1 effector immune response in vitro in a polarized manner. IEC may be important modulators of the mucosal effector immune response.