Determining the Bordetella LPS structural features that influence TLR4 downstream signaling

Area of Science:

• Immunology
• Microbiology
• Molecular Biology

Background:

• Bacterial lipopolysaccharide (LPS) recognition by human Toll-like receptor 4 (TLR4) triggers distinct MyD88 (pro-inflammatory) and TRIF (dendritic cell maturation) signaling pathways.
• LPS structural variations can modulate the activation of these TLR4-mediated signaling cascades.
• Understanding these structure-function relationships is key to manipulating immune responses.

Purpose of the Study:

• To investigate how specific structural features of *Bordetella* LPS influence TLR4 signaling via the MyD88 and TRIF pathways.
• To determine the relative importance of LPS O antigen, lipid A acylation, and lipid A glucosamine modification in TLR4 activation.

Main Methods:

• Systematic engineering of *Bordetella* LPS variants with specific structural modifications.
• Quantification of NFκB activation as an indicator of MyD88 pathway activity.
• Quantification of IRF3 activation as an indicator of TRIF pathway activity.

Main Results:

• Glucosamine modification of lipid A demonstrated a dominant role, essential for both MyD88 and TRIF pathway activation, overriding O antigen and acylation effects.
• Absence of glucosamine modification significantly impaired both MyD88 and TRIF pathway activation, highlighting its role in TLR4 dimerization.
• Under-acylated LPS (4-5 acyl chains) partially reduced NFκB activation and abrogated TRIF activation, while hexa- and hepta-acylated LPS robustly activated both pathways. The *Bordetella* O antigen selectively enhanced TRIF pathway signaling.

Conclusions:

• Lipid A glucosamine modification is a critical determinant of TLR4 signaling pathway activation.
• LPS acylation level differentially impacts MyD88 and TRIF pathway activation.
• The *Bordetella* O antigen can selectively bias TLR4 signaling towards the TRIF pathway, offering a mechanism for tailored immune modulation.