Formulated Phospholipids as Non-Canonical TLR4 Agonists

Area of Science:

• Immunology
• Pharmacology
• Biochemistry

Background:

• Adjuvants are critical for vaccine efficacy, often activating Toll-like receptor (TLR) pathways.
• TLR4 is a key receptor, primarily known for lipopolysaccharide (LPS) but also responsive to other molecules like phospholipids.
• Phospholipid-based pharmaceutical formulations may trigger unintended TLR4 signaling, complicating bioactivity assessments.

Purpose of the Study:

• To investigate the TLR4-activating potential of individual components within a stable emulsion (SE) vaccine adjuvant.
• To determine if the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) signals through murine and human TLR4.
• To understand how phospholipid characteristics influence TLR4 activation.

Main Methods:

• Analysis of a clinical-stage oil-in-water vaccine adjuvant emulsion (SE).
• Testing the ability of SE components, particularly DMPC, to activate murine and human TLR4.
• Evaluating immune phenotypes elicited by DMPC and comparing them to canonical TLR4 agonists.

Main Results:

• The phospholipid DMPC was found to activate TLR4 in both murine and human systems.
• DMPC-induced immune phenotypes resembled those of known TLR4 agonists.
• Phospholipid activation of TLR4 was dependent on saturation, size, and headgroup.
• While DMPC showed effects on human cells, the emulsion's oil components had a more significant impact.

Conclusions:

• Phospholipids, specifically DMPC, can act as TLR4 agonists, potentially causing off-target immune responses.
• Understanding phospholipid-TLR4 interactions is vital for pharmaceutical development, impacting preclinical and clinical outcomes.
• The findings necessitate careful consideration of phospholipid composition in drug formulations to avoid unintended innate immune activation.