Expression of functional D299G.T399I polymorphic variant of TLR4 depends more on coexpression of MD-2 than does...

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Toll-like receptor 4 (TLR4) is crucial for innate immunity, recognizing bacterial lipopolysaccharide (endotoxin).
• Specific missense variants (D299G and T399I) in TLR4 are found in European populations and associated with reduced endotoxin responsiveness.
• The precise impact of these ectodomain (ecd) variants on TLR4 function remains incompletely understood.

Purpose of the Study:

• To investigate how the D299G and T399I TLR4 variants affect TLR4 function, interaction with endotoxin complexes, and cellular responsiveness.
• To elucidate the role of MD-2 (myeloid differentiation factor 2) in mediating the effects of these TLR4 polymorphisms.

Main Methods:

• Utilized radiolabeled endotoxin-CD14 and endotoxin-MD-2 complexes to quantify binding interactions.
• Measured the expression of functional TLR4 and MD-2-TLR4 complexes.
• Assessed cellular endotoxin responsiveness in cells expressing wild-type and variant TLR4, with and without MD-2 coexpression.

Main Results:

• Both wild-type and D299G.T399I TLR4 ectodomains exhibited high-affinity binding to endotoxin-CD14 and endotoxin-MD-2 complexes.
• Functional TLR4 levels were reduced up to 2-fold when D299G.T399I TLR4 was coexpressed with MD-2.
• A significant reduction of over 10-fold in functional TLR4 was observed when D299G.T399I TLR4 was expressed without MD-2.

Conclusions:

• The D299G.T399I TLR4 haplotype significantly impacts the expression of functional TLR4, especially in the absence of MD-2.
• Cells expressing TLR4 without MD-2 are disproportionately affected by these TLR4 polymorphisms, leading to markedly reduced endotoxin responsiveness.
• These findings highlight the critical role of MD-2 in TLR4 signaling and the functional consequences of TLR4 genetic variations.