Signal integration in lipopolysaccharide (LPS)-stimulated murine macrophages

Area of Science:

• Immunology
• Molecular Biology

Background:

• Lipopolysaccharide (LPS) from Gram-negative bacteria triggers potent inflammatory responses.
• Toll-like receptors (TLRs) are key mediators of innate immunity, recognizing pathogen-associated molecular patterns like LPS.
• Macrophage activation by LPS involves a complex signaling network including TLR4, CD14, and CD11b/CD18.

Purpose of the Study:

• To investigate the roles of CD14, CD11b/CD18, and TLR4 in LPS-induced gene expression in macrophages.
• To compare the signaling pathways activated by Escherichia coli LPS and Porphyromonas gingivalis LPS.
• To elucidate the composition and function of the LPS signaling complex.

Main Methods:

• Gene expression analysis of LPS-inducible genes in wild-type and receptor-deficient macrophages (CD14, CD11b/CD18, TLR4).
• Stimulation with Escherichia coli LPS and Taxol (LPS mimetic).
• Analysis of NF-κB translocation and MAPK phosphorylation.
• Comparison of responses to E. coli LPS and Porphyromonas gingivalis LPS.

Main Results:

• All examined genes were TLR4-dependent for induction by E. coli LPS and Taxol.
• CD14 was required for induction at low LPS/Taxol doses, while CD11b/CD18 regulated a distinct subset of genes (COX-2, IL-12).
• NF-κB and MAPK signaling pathways were dysregulated in receptor-deficient macrophages.
• Porphyromonas gingivalis LPS activated TLR2, leading to differential gene expression compared to E. coli LPS.

Conclusions:

• TLR4 is necessary but not sufficient for full LPS-induced gene expression.
• CD14 and CD11b/CD18 act as co-receptors, facilitating TLR4-dependent signaling for specific gene subsets.
• TLR2 and TLR4 signaling pathways differ quantitatively and qualitatively.
• A functional LPS signaling complex in macrophages minimally involves CD14, CD11b/CD18, and TLR4 for E. coli LPS response.