Abstract
The Toll-like receptor 4 (TLR4)/myeloid differentiation protein-2 (MD-2) complex is considered the major receptor of the innate immune system to recognize lipopolysaccharides (LPSs). However, some atypical LPSs with different lipid A and core saccharide moiety structures and compositions than the well-studied enterobacterial LPSs can induce a TLR2-dependent response in innate immune cells. Ochrobactrum intermedium, an opportunistic pathogen, presents an atypical LPS. In this study, we found that O. intermedium LPS exhibits a weak inflammatory activity compared to Escherichia coli LPS and, more importantly, is a specific TLR4/TLR2 agonist, able to signal through both receptors. Molecular docking analysis of O. intermedium LPS predicts a favorable formation of a TLR2/TLR4/MD-2 heterodimer complex, which was experimentally confirmed by fluorescence resonance energy transfer (FRET) in cells. Interestingly, the core saccharide plays an important role in this interaction. This study reveals for the first time TLR4/TLR2 heterodimerization that is induced by atypical LPS and may help to escape from recognition by the innate immune system.
Highlights
The innate immune system provides a first line of defense against a broad spectrum of pathogens
We compared the production of inflammatory cytokines in J774 macrophages treated with E. coli and O. intermedium LPS at different doses
Levels of proinflammatory cytokines tumor necrosis factor (TNF)-a and IL-6 induced by O. intermedium were much lower than those induced by E. coli LPS, even at 100-fold higher doses (Figure 1A)
Summary
The innate immune system provides a first line of defense against a broad spectrum of pathogens. Macrophages, dendritic cells, and neutrophils contain a variety of pattern recognition receptors (PRRs) that sense the presence of pathogens [1]. Upon binding to conserved pathogen-associated molecular patterns (PAMPs) [2], PRRs trigger a cascade of signals that induce the production of proinflammatory mediators determinant for pathogen killing and activation of the adaptive immune system. The lipopolysaccharide (LPS) present in the Gram-negative cell wall is a well-described inducer of the innate immune response. Toll-like receptor 4 (TLR4) complexed with myeloid differentiation protein-2 (MD-2) recognizes the lipid A portion of the LPS molecule. The Escherichia coli LPS is one of the most potent agonists of TLR4, containing a lipid A with six acyl chains, where five of them are buried inside the MD-2 pocket and the sixth
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