The Role of Lipopolysaccharide-Induced Cell Signalling in Chronic Inflammation

Abstract

Lipopolysaccharide (LPS) is the main structural component of the outer membrane of most Gram-negative bacteria and has diverse immunostimulatory and procoagulant effects. Even though LPS is well described for its role in the pathology of sepsis, considerable evidence demonstrates that LPS-induced signalling and immune dysregulation are also relevant in the pathophysiology of many diseases, characteristically where endotoxaemia is less severe. These diseases are typically chronic and progressive in nature and span broad classifications, including neurodegenerative, metabolic, and cardiovascular diseases. This Review reappraises the mechanisms of LPS-induced signalling and emphasises the crucial contribution of LPS to the pathology of multiple chronic diseases, beyond conventional sepsis. This perspective asserts that new ways of approaching chronic diseases by targeting LPS-driven pathways may be of therapeutic benefit in a wide range of chronic inflammatory conditions.

Keywords: chronic inflammation; coagulation; immunopatholgy; lipopolysaccharide; noncommunicable diseases.

Figure 1.

The structures of LPS and LBP. (a) Simplified structure of LPS from Gram-negative bacteria. The immunostimulatory lipid A is bound to a conserved inner core of sugars such as Kdo, which is bound in turn to a more variable outer core of common hexose sugars (eg glucose and galactose). The highly diverse O-antigen at the distal region of the LPS molecule is composed of repeated units of common hexose sugars. Rough-type LPS lacks an O-antigen. (b) Crystal structure of LBP (Protein Data Bank: 4M4D). The N-terminal portion is the main binding sites for lipid A, with the C-terminal involved in LPS transfer.

Figure 2.

The LPS transfer cascade. The transfer of LPS to TLR4–MD2 through LBP and CD14 massively amplifies the host response to extracellular LPS. (1) LBP binds longitudinally to a LPS micelle. (2) Electrostatic interactions form between LBP (bound to LPS) and CD14. (3) LPS is transferred to CD14 in multiple rounds of transfer. (4) LPS is transferred from CD14 to MD2 by interacting with the LRR13–LRR15 domain of TLR4. (5) The receptor complex of TLR4–MD2 and LPS dimerises. (6) This results in signal activation by TLR4–MD2. (7) Signalling proceeds through MYD88-dependent and MYD88-independent pathways. (8) TLR4 cooperates with various co-receptors. LPS aggregates that have been intercalated into the plasma membrane by LBP may induce mechanical stress on transmembrane proteins and ion channels and contribute to LPS signalling. Adapted from.

Figure 3.

Intracellular sensing of LPS and the non-canonical inflammasome. (1) Binding of PAMPs such as LPS to extracellular pattern-recognition receptors activates signal transduction cascades that induce gene expression of the components required for non-canonical inflammasome activation, a process termed “priming”. (2) Extracellular Gram-negative bacteria or their outer membrane vesicles (OMVs) can deliver LPS into the cytoplasm of host cells through endocytosis. (3) Guanylate-binding proteins (GBP) together with IRGB10 disrupt vacuoles containing pathogens, which enables LPS to be released into the cytosol. (*) LPS from OMVs are released from early endosomes, which does not involve GBPs/IRGB10 mediated lysis. (4) HMGB1 promotes the internalisation of LPS into vacuoles through RAGE. (5) HMGB1 additionally permeabilises this vacuole to release LPS into the cytosol. Cytosolic LPS can be recognised by inflammatory caspases and to provide a “triggering” signal for activation of the non-canonical inflammasome pathway. (6) The lipid A tail of LPS binds to the CARD motif of pro-caspase 4 in a direct interaction. (7) These intracellular LPS–pro-caspase 4 complexes dimerise and (8) oligomerise through the CARD motif, and proximity-induced activation leads to a catalytically active conformation of the non-canonical inflammasome. (9) The activated non-canonical inflammasome cleaves gasdermin D (GSDMD), releasing the amino-terminal fragments, (10) which form pores at the cell membrane. (11) The non-canonical inflammasome also activates the NLRP3 canonical inflammasome through an unknown mechanism, (12) and this inflammasome pathway releases IL-1β and IL-18, which is secreted through GSDMD pores. (13) LPS can additionally be delivered into cells through cell-bound LBP. (14) Intracellular transport of LPS by LBP raises the possibility of a role for LBP in the delivery of LPS to intracellular LPS receptors such as endosomal TLR4 and inflammatory caspases.

Figure 4.

Summary of the inhibitory actions of high-dose LBP. (I) LBP mediates LPS transfer to lipoproteins and (2) chylomicrons, which sequesters LPS, attenuates its stimulatory effects, and leads to its intestinal excretion via the liver–bile duct pathway. (3) LBP supresses the transfer of LPS to membrane-bound CD14 (mCD14) and subsequently to TLR4–MD2. (4) LBP facilitates the binding of a series of phosphatidylinositides (PtdIns) and phosphatidylserine (PS) to mCD14, and this inhibits LPS–mCD14 binding and LPS-induced responses. (5) LBP can also cause the dissociation of LPS from mCD14. (6) The LBP–LPS complex may also remain associated with mCD14 to form a ternary LBP–LPS–mCD14 complex that does not trigger signalling and is eventually internalised. (7) Cellular uptake may also proceed by a CD14-independent pathway. (8) LBP can inhibit the transfer of LPS from soluble CD14 (sCD14) to soluble MD2 (sMD2). (9) LBP can bind and intercalate into LPS aggregates, which has an inhibitory effect. The LBP–LPS complex cannot intercalate into the membrane, cannot bind LPS into the membrane and cannot interact with membrane-bound LBP. LBP can also mediate the crosslinking of several layers of lipid A, thereby decreasing its accessibility. It has also been proposed that by opsonising LPS aggregates at high density, LBP causes steric hindrance to other molecules such as CD14.

Figure 5.

Summarised downstream effects of LPS signalling. LPS influences a range of cell types and physiological processes. The activation of leukocytes initiates the immune response and the release of various inflammatory mediators. LPS also activates specific immune processes such as the maturation and migration of dendritic cell, autophagy in macrophages, and activation of the complement system. In the liver, LPS stimulates the production of acute phase proteins as well as several inflammatory mediators. Similarly, LPS promotes inflammatory reactions in adipose tissue. These mediators as well as the increased activity of enzymes involved in the production of reactive oxygen and nitrogen species contribute to a cellular stress. A major action of LPS is its ability to promote coagulation both by enhancing the expression of molecules that stimulate clotting and through direct interaction with red blood cells and platelets. LPS-mediated changes to the endothelium also promote coagulation.