Sulfatides are Endogenous Inhibitors of the Gram-Positive Bacteria Sensor TLR2
2Biochemistry, Triangle Regional Research and Development Center
Introduction: Toll-like receptor 2 (TLR2) senses pathogen-associated molecular pattern (PAMP) molecules derived from Gram-positive bacteria, e.g. lipopeptides, and combines with TLR1 or TLR6 to initiates inflammation. Additionally, TLR2 is also thought to recognize endogenous danger-associated molecular pattern (DAMP) molecules released from stressed or dying cells, leading to autoinflammation and autoimmunity. Yet, the identity of such endogenous TLR2 ligands is still poorly understood. We recently reported that three molecules of the endogenous lipid C16-sulfatide (3-O-sulfogalactosylceramide) can activate mouse TLR4/MD-2 by mimicking LPS, its known ligand – a PAMP derived from Gram-negative bacteria. In contrast, C16-sulfatide inhibits LPS activity in human macrophages. In the current research, we examined the activity of sulfatides toward TLR2.
Results: Sulfatides competitively inhibit the activity of the TLR2/6 agonist Pam2Cys in human macrophages. The inhibitory activity of C16-sulfatide towards hTLR2 occurs at concentrations that are an order of magnitude lower than those required for hTLR4 inhibition (Ki = 0.15 and 1.45 μM, respectively). A wide range of sulfatide variants can serve as TLR2 inhibitors, including sulfatides having short and long FA chains, unsaturated FA chains, and dehydro and phyto derivatives. TLR2 inhibition by sulfatides is conserved in human and mouse cells. Based on the observed competition and the partial chemical similarity with the lipopeptide, we hypothesized that sulfatides could mimic the TLR2 agonist. Indeed, molecular docking simulation has shown that a single sulfatide molecule fits into the hydrophobic pocket of TLR2, similarly to the lipopeptide.
Conclusions: Sulfatides, natural membrane glycosphingolipids in mammals, can act as potent endogenous antagonists of TLR2 in human and mouse macrophages. This activity may suppress inflammation induced by PAMPs and DAMPs acting at TLR2. We propose to define sulfatide as an ‘inhibitory DAMP’.