Oxidised phospholipid inhibition of toll-like receptor (TLR) signalling is restricted to TLR2 and TLR4 - roles for CD14, LPS-binding protein and MD2 as targets for specificity of inhibition

The generation of reactive oxygen species is a central feature of inflammation that results in the oxidation of host phospholipids. Oxidised phospholipids, such as 1-palmitoyl-2-arachidonyl-<i>sn</i>-glycero-3-phosphorylcholine (OxPAPC), have been shown to inhibit signalling induced by bacterial lipopeptide (BLP) or LPS, yet the mechanisms responsible for the inhibition of TLR-signalling by OxPAPC remain incompletely understood. Here, we examined the mechanisms by which OxPAPC inhibits TLR-signalling induced by diverse ligands in macrophages, smooth muscle cells and epithelial cells. OxPAPC inhibited TNF-α production, IκBα degradation, p38 MAPK phosphorylation and NF-κB dependent reporter activation induced by stimulants of TLR2 and TLR4 (Pam3CSK4 and LPS), but not by stimulants of other TLRs (PolyI:C, Flagellin, Loxoribine, ssRNA or CpG DNA) in macrophages and HEK-293 cells transfected with respective TLRs, and significantly reduced inflammatory responses in mice injected subcutaneously or intraperitoneally with Pam<sub>3</sub>CSK<sub>4</sub>. Serum proteins, including CD14 and LBP, were identified as key targets for the specificity of TLR-inhibition since supplementation with excess serum, or recombinant CD14 or LBP, reversed TLR2 inhibition by OxPAPC, while serum accessory proteins, or expression of membrane CD14, potentiated signalling via TLR2 and TLR4, but not other TLRs. Binding experiments and functional assays identified MD2 as a novel additional target of OxPAPC inhibition of LPS signalling. Synthetic phospholipid oxidation products 1-palmitoyl-2-(5-oxovaleryl)-<i>sn</i>-glycero-3-phosphocholine and 1-palmitoyl-2-glutaryl-<i>sn</i>-glycero-3-phosphocholine inhibited TLR2-signalling from around ∼30μM. Taken together, these results suggest that oxidised phospholipid mediated inhibition of TLR-signalling occurs mainly by competitive interaction with accessory proteins that interact directly with bacterial lipids to promote signalling via TLR2 or TLR4