Innate sensing of oxidation-specific epitopes in health and disease

Abstract

Ageing, infections and inflammation result in oxidative stress that can irreversibly damage cellular structures. The oxidative damage of lipids in membranes or lipoproteins is one of these deleterious consequences that not only alters lipid function but also leads to the formation of neo-self epitopes - oxidation-specific epitopes (OSEs) - which are present on dying cells and damaged proteins. OSEs represent endogenous damage-associated molecular patterns that are recognized by pattern recognition receptors and the proteins of the innate immune system, and thereby enable the host to sense and remove dangerous biological waste and to maintain homeostasis. If this system is dysfunctional or overwhelmed, the accumulation of OSEs can trigger chronic inflammation and the development of diseases, such as atherosclerosis and age-related macular degeneration. Understanding the molecular components and mechanisms that are involved in this process will help to identify individuals with an increased risk of developing chronic inflammation, and will also help to indicate novel modes of therapeutic intervention.

Figure 1. The generation of OSEs.

Tissue damage, cellular stress and cell death result in increased oxidative stress, which promotes lipid peroxidation. Lipid peroxidation can occur through non-enzymatic mechanisms, such as reactive oxygen species (ROS), and through enzymatic mechanisms, including myeloperoxidases, 12/15-lipoxygenases, cyclooxygenases and cytochrome P450. The oxidation of sn-2 polyunsaturated fatty acids (PUFAs) of membrane phospholipids leads to fragmentation and the generation of highly reactive breakdown products, such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE)^,,,. In addition, different types of oxidized phospholipids (OxPLs) can be generated from different phospholipid backbones, including oxidized phosphatidylcholine, oxidized phosphatidylethanolamine (OxPE), oxidized phosphatidylserine (OxPS) and oxidized cardiolipin (OxCL). The newly generated breakdown products and the oxidized and truncated residual core OxPLs can in turn react with free amino groups of protein side chains or lipids that are localized in their vicinity, forming stable covalent adducts and creating oxidation-specific epitopes (OSEs). Because phosphocholine (PC) as an OSE is only presented as an epitope in the context of OxPL, these epitopes are termed PC-OxPLs for clarity. The PC moiety can also be a component of the capsular polysaccharide of bacteria, where it is not part of a phospholipid and is constitutively presented as an epitope. In addition, an adduct between an oxidative fragment of docosahexaenoic acid, (E)-4-hydroxy-7-oxohept-5-enoic acid and lysines of proteins (or aminophospholipids) can lead to the formation of 2-(ω-carboxyethyl) pyrrole (CEP). OSE-modified proteins or lipids are sensed by innate immune responses and represent a unique class of damage-associated molecular patterns (DAMPs). PowerPoint slide

Figure 2. Role of innate immune responses targeting OSEs.

Oxidized low-density lipoproteins (OxLDL), apoptotic cells, microvesicles and cellular debris display various types of oxidation-specific epitopes (OSEs)^,,,,. Components of cellular and humoral immune responses sense OSEs and mediate sterile inflammation or clearance and neutralization depending on the availability and context. Macrophages sense OSEs through various pattern recognition receptors (PRRs) that are expressed on their surface. Scavenger receptor CD36 preferentially binds and mediates the uptake of the phosphocholine (PC) of OxPL (PC-OxPL), oxidized phosphatidylserine (OxPS) and 2-(ω-carboxyethyl)pyrrole (CEP). Scavenger receptor SRB1 recognizes PC-OxPL; SRA1 andlectin-like oxidized LDL receptor 1 (LOX1) bind malondialdehyde (MDA); and LOX1 recognizes 4-hydroxynonenal (4-HNE)^,. Toll-like receptors (TLRs) mediate pro-inflammatory signals. PC-OxPL transmit inflammatory signals through a heterotrimer of TLR4–TLR6–CD36, as well as through TLR2, and CEP signals through TLR2 in cooperation with CD36 (Refs 33,46). Humoral immune responses to OSEs include natural IgM antibodies, C-reactive protein (CRP), members of complement and other soluble PRRs that are involved in the clearance of apoptotic cells. Soluble PRRs inhibit the recognition of OSEs by cellular PRRs and mediate uptake via alternative pathways. Natural IgM bound to PC-OxPL, MDA and 4-HNE epitopes are taken up by macrophages by C1q–calreticulin–CD91-dependent or mannose-binding lectin (MBL)- and MBL receptor-dependent mechanisms. CRP bound to PC-OxPL is taken up by C1q–calreticulin–CD91-dependent mechanisms^,. Alternative recognition of C1q is mediated by CD93 (not shown). Complement factor H (CFH) bound to MDA provides cofactor activity for the cleavage of C3b into iC3b opsonins that mediate anti-inflammatory clearance via complement receptor 3 (CR3). Milk fat globule-epidermal growth factor 8 (MFG-E8) bound to OxPS or OxPE mediate clearance via αvβ3 integrins^,. PowerPoint slide

Figure 3. Sensing of OSEs in atherosclerosis.

Endothelial cells sense oxidation-specific epitopes (OSEs) present on microvesicles and oxidized low-density lipoprotein (OxLDL)^,. This results in the expression of adhesion molecules and the secretion of chemoattractants leading to monocyte recruitment to the intima of the artery wall. Macrophages internalize OxLDL via scavenger receptors such as scavenger receptor A1 (SRA1), lectin-like oxidized LDL receptor 1 (LOX1), SRB1 and CD36 (Ref. 37), and in cooperation with Toll-like receptor 2 (TLR2) and TLR4–TLR6 receive pro-inflammatory signals from OSEs. The enhanced uptake of OxLDL via scavenger receptors leads to the excess accumulation of intracellular cholesterol and the formation of lipid-laden foam cells, as well as the secretion of cytokines and chemokines. Excessive free cholesterol accumulation induces cholesterol crystal formation that triggers lysosome rupture and activation of the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome primed by pro-inflammatory OSE-induced signalling. Free intracellular cholesterol also induces endoplasmic reticulum (ER) stress, leading to macrophage apoptosis. As a consequence of this, and impaired efferocytosis, late-stage apoptotic cells accumulate, contributing — together with OxLDL and microvesicles — to a growing pool of OSEs inside the plaque. Natural IgM specific for OSEs blocks scavenger receptor-mediated uptake and neutralizes the pro-inflammatory effects of OSEs by promoting their complement-dependent clearance. Complement factor H (CFH) blocks the pro-inflammatory effects of malondialdehyde (MDA) and facilitates the anti-inflammatory clearance of MDA-decorated surfaces through Factor I-dependent iC3b generation. The milk fat globule-epidermal growth factor 8 (MFG-E8) is also involved in the clearance of OSEs by recognizing oxidized phosphatidylserine (OxPS). Impaired functions of these humoral immune responses as a result of low abundance or decreased binding capacities, as well as excessive accumulation of OxLDL, microvesicles and apoptotic cells, favours the recognition of OSEs by macrophage receptors, leading to sustained inflammation. CRP, C-reactive protein; CXCL8, CXC-chemokine ligand 8; HNE, 4-hydroxynonenal; IL-1β, interleukin-1β; NF-κB, nuclear factor-κB; PC-OxPL, phosphocholine (PC) of oxidized phospholipid. PowerPoint slide