Mediators of innate immune recognition of bacteria concentrate in lipid rafts and facilitate lipopolysaccharide-induced cell activation

TLDR

The plasma membrane contains lipid rafts—cholesterol‑ and glycosphingolipid‑rich microdomains that mediate membrane sorting and signal transduction. This study examined the role of lipid raft formation in innate immune recognition of bacteria. The authors used biochemical assays and fluorescence imaging to assess raft involvement. Receptor molecules CD14, hsp70, hsp90, CXCR4, GDF5, and TLR4 localize to lipid rafts upon LPS stimulation, and disruption of raft integrity by nystatin or MCD blocks LPS‑induced TNF‑α secretion, indicating that CD14‑mediated bacterial recognition and recruitment of these signaling proteins within rafts drive cellular activation.

Abstract

The plasma membrane of cells is composed of lateral heterogeneities, patches and microdomains. These membrane microdomains or lipid rafts are enriched in glycosphingolipids and cholesterol and have been implicated in cellular processes such as membrane sorting and signal transduction. In this study we investigated the importance of lipid raft formation in the innate immune recognition of bacteria using biochemical and fluorescence imaging techniques. We found that receptor molecules that are implicated in lipopolysaccharide (LPS)-cellular activation, such as CD14, heat shock protein (hsp) 70, 90, Chemokine receptor 4 (CXCR4), growth differentiation factor 5 (GDF5) and Toll-like receptor 4 (TLR4), are present in microdomains following LPS stimulation. Lipid raft integrity is essential for LPS-cellular activation, since raft-disrupting drugs, such as nystatin or MCD, inhibit LPS-induced TNF-alpha secretion. Our results suggest that the entire bacterial recognition system is based around the ligation of CD14 by bacterial components and the recruitment of multiple signalling molecules, such as hsp70, hsp90, CXCR4, GDF5 and TLR4, at the site of CD14-LPS ligation, within the lipid rafts.