Sulfatides are endogenous antagonists of toll-like receptor 2 (TLR2)

Introduction: Toll-like receptor 2 (TLR2) senses pathogen-associated molecular pattern (PAMP) molecules derived from Gram-positive bacteria, e.g., lipopeptides, and heterodimerizes with either TLR1 or TLR6 to initiate inflammation. In addition, 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 C16-sulfatide (3-O-sulfogalactosylceramide) can bind together and 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 towards TLR2.

Results: Short (C16) and long (C24) fatty acid chain sulfatides competitively inhibit the activity of the TLR2/6 agonist Pam2Cys in human macrophages. The inhibitory activity towards TLR2 is also demonstrated in mouse cells, and it occurs at concentrations that are more than an order of magnitude lower than those required for TLR4 ligation by C16-sulfatide. 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.

Conclusion: Sulfatides, natural membrane glycolipids in mammals, can act as potent endogenous antagonists of TLR2 in human and mouse macrophages. This activity may serve to suppress inflammation induced by PAMPs and DAMPs acting at TLR2.