TLR4 Is the Signaling but Not the Lipopolysaccharide Uptake Receptor

Abstract

TLR4 is the primary recognition molecule for inflammatory responses initiated by bacterial LPS (endotoxin). Internalization of endotoxin by various cell types is an important step for its removal and detoxification. Because of its role as an LPS-signaling receptor, TLR4 has been suggested to be involved in cellular LPS uptake as well. LPS uptake was investigated in primary monocytes and endothelial cells derived from TLR4 and CD14 knockout C57BL/6 mice using tritiated and fluorescein-labeled LPS. Intracellular LPS distribution was investigated by deconvolution confocal microscopy. We could not observe any difference in LPS uptake and intracellular LPS distribution in either monocytes or endothelial cells between TLR4−/− and wild-type cells. As expected, CD14−/− monocytes showed a highly impaired LPS uptake, confirming CD14-dependent uptake in monocytes. Upon longer incubation periods, the CD14-deficient monocytes mimicked the LPS uptake pattern of endothelial cells. Endothelial cell LPS uptake is slower than monocyte uptake, LBP rather than CD14 dependent, and sensitive to polyanionic polymers, which have been shown to block scavenger receptor-dependent uptake mechanisms. We conclude that TLR4 is not involved in cellular LPS uptake mechanisms. In membrane CD14-positive cells, LPS is predominantly taken up via CD14-mediated pathways, whereas in the CD14-negative endothelial cells, there is a role for scavenger receptor-dependent pathways.

Mice expressing high levels of soluble CD14 retain LPS in the circulation and are resistant to LPS‐induced lethality

“…Originally cloned as the only known receptor for LPS residing on the surface of cells and in serum, CD14 is now thought to form a complex with MD-2 and TLR4, through which a signal is propagated [22]. In mediating the recognition of LPS by TLR4, CD14 was believed to function mostly as an uptake protein [23]. However, recent reports indicate that the role of CD14 in LPS signaling could be more sophisticated and present evidence that CD14-TLR4 signaling permits a MyD88-independent TLR4 response [24].…”

Pulmonary Lipopolysaccharide (LPS)-Binding Protein Inhibits the LPS-Induced Lung Inflammation In Vivo

“…They concluded that LBP is also part of the LPS clearance mechanism and that this pathway bifurcates after binding to mCD14. A more recent publication delineated that TLR4 is responsible for LPS signaling while LPS neutralization is mediated by other receptors in vitro (30). Using murine blood monocytes and endothelial cells, Dunzendorfer et al (30) demonstrated that LBP contributes substantially to the neutralization of LPS via transfer to CD14 as well as to other anionic structure binding receptors, presumably SRs.…”

Bruton's Tyrosine Kinase Separately Regulates NFκB p65RelA Activation and Cytokine Interleukin (IL)-10/IL-12 Production in TLR9-stimulated B Cells

“…It was also shown recently that Btk was involved in NFB activation in macrophages stimulated by LPS (9,10). However, LPS binds both TLR4 and CD14 (11,12), hence making it ambiguous whether Btk is indeed involved in TLR signaling per se. Moreover, most studies of Btk involvement in TLR signaling had been done in macrophages and dendritic cells (13)(14)(15), and it is not clear if Btk is involved in TLR signaling in B cells.…”