Characterization of differential Toll-like receptor responses below the optical diffraction limit

Area of Science:

• Cellular biology
• Immunology
• Microscopy

Background:

• Membrane receptors form nanoscale clusters upon ligand activation, initiating cell signaling pathways, including innate immunity.
• Pathogenic bacteria like Yersinia pestis can evade innate immune detection, unlike microbes such as Escherichia coli.
• Lipopolysaccharides (LPS) on bacterial cell walls are recognized by the Toll-like receptor 4 (TLR4), and differences in LPS structure are implicated in immune evasion.

Purpose of the Study:

• To investigate nanoscale differences in the spatial clustering of TLR4 upon binding LPS from Y. pestis versus E. coli.
• To visualize TLR4 distributions at resolutions below the optical diffraction limit (<30 nm) in intact cells.
• To compare LPS-TLR4 cluster sizes and ligand/receptor colocalization between immunostimulatory and immunoevasive bacterial LPS.

Main Methods:

• Utilized a novel, dual-color stochastic optical reconstruction microscopy (STORM) technique for super-resolution imaging.
• Visualized TLR4 distributions on intact cells with resolutions below 30 nm.
• Distinguished between TLR4 receptors with and without bound LPS at the nanoscale.

Main Results:

• Demonstrated significantly larger LPS-TLR4 cluster sizes when bound to LPS from immunostimulatory bacteria compared to immunoevasive LPS.
• Observed nearly twofold greater ligand/receptor colocalization with immunostimulatory LPS.
• Provided the first nanoscale visualizations of TLR4 distributions on intact cells using dual-color STORM.

Conclusions:

• Nanoscale differences in TLR4 clustering correlate with the immune response to bacterial LPS.
• Super-resolution microscopy (STORM) is a powerful tool for resolving nanoscale receptor-ligand interactions in innate immunity.
• The study provides insights into bacterial immune evasion mechanisms at the molecular level.