Agonistic and Antagonistic Effects of LPS on TLR4

Bacterial lipopolysaccharide (LPS) is the major structural component of the outer wall of all Gram-negative bacteria and a potent activator of the immune system.
LPS can lead to pathological reactions such as the induction of septic shock. LPS is recognized by Toll-like receptor 4 (TLR4) which interacts with three different extracellular proteins: LPS binding protein (LBP), CD14 and, myeloid differentiation protein 2 (MD-2), to induce a signaling cascade leading to the activation of NF-κB and the production of proinflammatory cytokines.

Agonist and antagonist activities of Lipid A

LPS structure

LPS consists of a polysaccharide region that is anchored in the outer bacterial membrane by a specific carbohydrate lipid moiety termed lipid A. Lipid A, also known as endotoxin, is responsible for the immunostimulatory activity of LPS. Lipid A is a glucosamine disaccharide linked to hydroxy fatty acids that are further substituted by nonhydroxylated fatty acids. The number of fatty acids is a major determinant of the immunogenicity of endotoxin. The most active form of lipid A contains six fatty acyl groups and is found in pathogenic bacteria such as Escherichia coli and Salmonella species. Underacylated lipid A structures, containing four or five fatty acids, induce markedly less host defense responses and can inhibit in a dose-dependent manner the strong endotoxic response triggered by hexa-acylated LPS. Such antagonist LPS have been isolated from Rhodobacter sphaeroides, Porphyromonas gingivalis and an E. coli strain bearing a mutation in the msbB gene^[1].
LPS antagonists have received significant attention as potential therapeutic agents to treat septic shock long before their antagonist mechanism was known.

Structures of agonist hexa-acylated lipid A from E.coli and antagonist penta-acylated lipid A from R.sphaeroides

TLR4 activation pathway by LPS

According to the current model, LPS is delivered to CD14 by LBP and transferred to MD-2 to form a monomeric endotoxin:MD-2 complex that binds and activates TLR4 ^{[2, 3]}. TLR4 activation can occur without LPB and CD14 but requires several orders of magnitude more endotoxin. Canonical lipid A binds MD-2 and induces conformational changes that trigger TLR4 oligomerization and signaling.
Underacylated lipid A seem to utilize at least two distinct mechanisms to block LPS-dependent activation of TLR4. The main mechanism consists of direct competition between under-acylated LPS and hexa-acylated LPS for the same binding site on MD-2, while the secondary mechanism involves the ability of under-acylated LPS:MD-2 complexes to inhibit hexa-acylated endotoxin:MD-2 complexes function at TLR4 ^[1-4].

Understanding the molecular mechanism of LPS-induced TLR4 activation is key for the development of therapeutic lipid A antagonists. HEK293 cells transfected to stably express TLR4, MD-2 and CD14 is the model of choice to study TLR4 activation.
InvivoGen provides such cell line with or without a convenient reporter system to monitor TLR4-induced NF-κB activation, as well as a large collection of TLR4 agonists and antagonists.

References:

1. Coats SR. et al., 2005. MD-2 mediates the ability of tetra-acylated and penta-acylated lipopolysaccharides to antagonize Escherichia coli lipopolysaccharide at the TLR4 signaling complex. J Immunol.;175(7):4490-8.
2. Teghanemt A. et al., 2005. Molecular basis of reduced potency of underacylated endotoxins. J Immunol. 175(7):4669-76.
3. Visintin A. et al., 2005. Pharmacological inhibition of endotoxin responses is achieved by targeting the TLR4 coreceptor, MD-2. J Immunol. 175(10):6465-72.
4. Saitoh S. et al., 2004. Lipid Aantagonist, lipid IVa, is distinct from lipid A in interaction with Toll-like receptor 4 (TLR4)-MD-2 and ligand-induced TLR4 oligomerization. Int Immunol. 16(7):961-9.