Lipopolysaccharide (LPS)-binding protein stimulates CD14-dependent Toll-like receptor 4 internalization and LPS-induced TBK1-IKKϵ-IRF3 axis activation.

Toll-like receptor 4 (TLR4) is an indispensable immune receptor for lipopolysaccharide (LPS), a major component of the Gram-negative bacterial cell wall. Following LPS stimulation, TLR4 transmits the signal from the cell surface and becomes internalized in an endosome. However, the spatial regulation of TLR4 signaling is not fully understood. Here, we investigated the mechanisms of LPS-induced TLR4 internalization and clarified the roles of the extracellular LPS-binding molecules, LPS-binding protein (LBP), and glycerophosphatidylinositol-anchored protein (CD14). LPS stimulation of CD14-expressing cells induced TLR4 internalization in the presence of serum, and an inhibitory anti-LBP mAb blocked its internalization. Addition of LBP to serum-free cultures restored LPS-induced TLR4 internalization to comparable levels of serum. The secretory form of the CD14 (sCD14) induced internalization but required a much higher concentration than LBP. An inhibitory anti-sCD14 mAb was ineffective for serum-mediated internalization. LBP lacking the domain for LPS transfer to CD14 and a CD14 mutant with reduced LPS binding both attenuated TLR4 internalization. Accordingly, LBP is an essential serum molecule for TLR4 internalization, and its LPS transfer to membrane-anchored CD14 (mCD14) is a prerequisite. LBP induced the LPS-stimulated phosphorylation of TBK1, IKKϵ, and IRF3, leading to IFN-ß expression. However, LPS-stimulated late activation of NF-κB or necroptosis were not affected. Collectively, our results indicate that LBP controls LPS-induced TLR4 internalization, which induces TLR adaptor molecule 1 (TRIF)-dependent activation of the TBK1-IKKϵ-IRF3-IFN-ß pathway. In summary, we showed that LBP-mediated LPS transfer to mCD14 is required for serum-dependent TLR4 internalization and activation of the TRIF pathway.

An inhibitory epitope of human Toll-like receptor 4 resides on leucine-rich repeat 13 and is recognized by a monoclonal antibody.

Lipopolysaccharide (LPS)-induced activation of Toll-like receptor 4 (TLR4) elicits the innate immune response and can trigger septic shock if excessive. Two antibodies (HT4 and HT52) inhibit LPS-induced human TLR4 activation via novel LPS binding-independent mechanisms. The HT52 epitope resides on leucine-rich repeat 2 (LRR2) and is a feature of many inhibitory antibodies; antigen specificity of HT4 does not reside in LRR2. Here, we identified an HT4 epitope on LRR13 located close to the TLR4 dimerization interface that plays a role in NFκB activation. HT4 and HT52 mutually enhanced TLR4 inhibition. LRR13 is a novel inhibitory epitope and may be useful for developing anti-TLR4 antibodies. Combination therapy with LRR2 and LRR13 may effectively inhibit TLR4 activation.