Endotoxin: physical requirements for cell activation.

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• Lipopolysaccharide (LPS) is a key component of Gram-negative bacteria outer membranes and a major trigger of innate immunity.
• The biological activity of LPS monomers versus aggregates in immune cell activation remains controversial.
• LPS exists as monomers below its critical micellar concentration (CMC) and forms aggregates above it.

Purpose of the Study:

• To investigate whether LPS monomers or aggregates are the primary activators of immune cells.
• To compare the immune-stimulating activity of defined LPS monomer and aggregate solutions.
• To elucidate the mechanism of LPS-induced immune cell activation.

Main Methods:

• Preparation of distinct LPS aggregate suspensions and monomer solutions using a dialysis setup.
• Stimulation of human mononuclear cells (MNCs) with equal concentrations of Re LPS aggregates and monomers.
• Quantification of TNF-alpha release as a measure of immune cell activation.

Main Results:

• LPS aggregates demonstrated significantly higher TNF-alpha release compared to LPS monomers at identical concentrations.
• This difference in activity was observed both in the absence and presence of serum proteins.
• LPS aggregates are proposed to be detected by membrane-associated LPS-binding protein (LBP) for effective immune cell activation.

Conclusions:

• LPS aggregates are the predominant active form in initiating innate immune responses.
• A model is proposed where LBP facilitates aggregate intercalation into cell membranes, leading to immune cell activation.
• LBP's presence in or at the cell membrane of human macrophages supports this proposed mechanism.