TLR Signalling Pathways Diverge in Their Ability to Induce PGE(2)

PGE(2) is a lipid mediator abundantly produced in inflamed tissues that exerts relevant immunoregulatory functions. Dendritic cells (DCs) are key players in the onset and shaping of the inflammatory and immune responses and, as such, are well known PGE(2) targets. By contrast, the precise role of human DCs in the production of PGE(2) is poorly characterized. Here, we asked whether different ligands of Toll-like receptors (TLRs), a relevant family of pathogen-sensing receptors, could induce PGE(2) in human DCs. The only active ligands were LPS (TLR4 ligand) and R848 (TLR7-8 ligand) although all TLRs, but TLR9, were expressed and functional. While investigating the molecular mechanisms hindering the release of PGE(2), our experiments highlighted so far oversight differences in TLR signalling pathways in terms of MAPK and NF-κB activation. In addition, we identified that the PGE(2)-limiting checkpoint downstream TLR3, TLR5, and TLR7 was a defect in COX2 induction, while TLR1/2 and TLR2/6 failed to mobilize arachidonic acid, the substrate for the COX2 enzyme. Finally, we demonstrated the in vivo expression of PGE(2) by myeloid CD11c(+) cells, documenting a role for DCs in the production of PGE(2) in human inflamed tissues.