The LPS receptor generates inflammatory signals from the cell surface

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Bacterial lipopolysaccharides (LPSs) are key triggers of innate immunity in mammals.
• LPS recognition involves a receptor complex including CD14, Toll-like receptor 4 (TLR4), and MD-2.
• The precise mechanism of TLR4-mediated signaling remains incompletely understood.

Purpose of the Study:

• To elucidate the functional mechanism of Toll-like receptor 4 (TLR4) in lipopolysaccharide (LPS) signaling.
• To investigate the cellular localization and dynamics of the TLR4/CD14/MD-2 complex.
• To determine the role of the Golgi apparatus in LPS-induced cellular responses.

Main Methods:

• Construction and stable expression of chimeric TLR molecules fused to fluorescent proteins.
• Confocal and time-lapse confocal microscopy to visualize TLR4 localization and dynamics.
• Cellular response assays following antibody-mediated TLR4 engagement and Golgi disruption.

Main Results:

• Toll-like receptor 4 (TLR4) is expressed on both the plasma membrane and the Golgi apparatus.
• Rapid recycling of the TLR4/CD14/MD-2 complex between the Golgi and plasma membrane was observed.
• TLR4 engagement on the cell surface by antibody was sufficient to trigger signaling.
• Disruption of the Golgi apparatus did not impede cellular responses to lipopolysaccharide (LPS).

Conclusions:

• Lipopolysaccharide (LPS) signaling is initiated upon recognition by surface-expressed Toll-like receptor 4 (TLR4).
• The cellular response to LPS is independent of LPS trafficking to the Golgi apparatus.
• These findings clarify the early events in TLR4-mediated innate immune activation.