Structural insight into TLR4/MD-2 activation by synthetic LPS mimetics with distinct binding modes

Area of Science:

• Immunology
• Structural Biology
• Drug Discovery

Background:

• Toll-like receptor 4 (TLR4) recognizes pathogen-associated molecular patterns, notably lipopolysaccharide (LPS) from Gram-negative bacteria.
• TLR4 activation is crucial for innate and adaptive immunity, making TLR4 agonists valuable for disease management and vaccine adjuvants.
• LPS presents challenges including complexity, toxicity, and species-specific activity, necessitating alternative agonists.

Purpose of the Study:

• To elucidate the structural basis for recognition of synthetic Disaccharide Lipid A Mimetics (DLAMs) by human and mouse TLR4/MD-2 complexes.
• To understand how DLAMs induce TLR4 dimerization and activation, distinct from LPS.
• To provide a foundation for designing novel TLR4-targeting immunotherapeutics and adjuvants.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of six dimeric [TLR4/MD-2/ligand]2 complexes.
• Structures were resolved at high resolution (2.2-3.1 Å) to visualize ligand-protein interactions.
• Comparative analysis of DLAM-bound structures versus LPS-bound structures.

Main Results:

• Detailed cryo-EM structures reveal specific binding modes of DLAMs to human and mouse TLR4/MD-2.
• DLAMs induce TLR4/MD-2 dimerization by acting as a molecular bridge, facilitated by carbohydrate structure-relevant interactions.
• The binding mode of DLAMs is distinct from LPS, contributing to their species-independent agonistic activity.

Conclusions:

• DLAMs represent a novel class of TLR4 agonists with species-independent activity.
• The distinct binding and dimerization mechanism of DLAMs offers a rational basis for developing potent TLR4-targeting immunotherapeutics and vaccine adjuvants.
• Structural insights pave the way for innovative drug design targeting TLR4 signaling pathways.