Short fatty acid sulfatides are endogenous ligands for TLR4/MD-2
2Department of Chemistry, Triangle Regional Research and Development Center, Israel
3Center for the Genetics of Host Defense, University of Texas Southwestern Medical Center, USA
Introduction: TLR4/MD-2 is the established receptor for lipopolysaccharide (LPS) derived from Gram-negative bacteria. TLR4/MD-2 is also allegedly activated by many endogenous molecules, leading to autoinflammation and autoimmunity, but conclusive evidence supporting direct receptor activation by endogenous ligands are lacking. Due to the chemical similarity of sulfatides (3-O-sulfogalactosylceramides) with the lipid A moiety of LPS, we hypothesized that sulfatides can mimic LPS and directly act at TLR4/MD-2.
Results: Short fatty acid chain sulfatides trigger MyD88- and TRIF-dependent signaling, and stimulate tumor necrosis factor α (TNFα) and type I interferon (IFN) secretion in mouse macrophages in a TLR4-dependent manner. The sulfatides activate NFκB-dependent transcriptional activity in mouse macrophages as well as in SW620 cells over-expressing the intact mouse receptor, including both TLR4 and MD-2. In contrast, the same sulfatides antagonize TLR4/MD-2 activation by LPS in human macrophage-like cells. Both the agonistic and antagonistic activities of the sulfatides require the presence of the sulfate group. The crystal structure of mouse TLR4/MD-2 in complex with C16-sulfatide revealed that three C16-sulfatide molecules bound to the MD-2 hydrophobic pocket, mimicked the detailed interactions of lipid A to induce an active dimer conformation of the receptor complex.
Conclusions: Several species of sulfatides, natural membrane glycolipids in mammals, are bona fide ligands for TLR4 and its coreceptor MD-2. Saturated sulfatides with lipid chains containing 12 or 16 carbon atoms are agonists of mouse TLR4 and antagonists of human TLR4. Our results suggest that sulfatides may mediate sterile inflammation or suppress LPS-stimulated inflammation.