The beta-galactoside binding immunomodulatory lectin galectin-3 reverses the desensitized state induced in neutrophils by the chemotactic peptide f-Met-Leu-Phe: role of reactive oxygen species generated by the NADPH-oxidase and inactivation of the agonist

Unraveling the role of galectin-3 in cardiac pathology and physiology

Galectin-3 (Gal-3) is a carbohydrate-binding protein with multiple functions. Gal-3 regulates cell growth, proliferation, and apoptosis by orchestrating cell-cell and cell-matrix interactions. It is implicated in the development and progression of cardiovascular disease, and its expression is increased in patients with heart failure. In atherosclerosis, Gal-3 promotes monocyte recruitment to the arterial wall boosting inflammation and atheroma. In acute myocardial infarction (AMI), the expression of Gal-3 increases in infarcted and remote zones from the beginning of AMI, and plays a critical role in macrophage infiltration, differentiation to M1 phenotype, inflammation and interstitial fibrosis through collagen synthesis. Genetic deficiency of Gal-3 delays wound healing, impairs cardiac remodeling and function after AMI. On the contrary, Gal-3 deficiency shows opposite results with improved remodeling and function in other cardiomyopathies and in hypertension. Pharmacologic inhibition with non-selective inhibitors is also protective in cardiac disease. Finally, we recently showed that Gal-3 participates in normal aging. However, genetic absence of Gal-3 in aged mice exacerbates pathological hypertrophy and increases fibrosis, as opposed to reduced fibrosis shown in cardiac disease. Despite some gaps in understanding its precise mechanisms of action, Gal-3 represents a potential therapeutic target for the treatment of cardiovascular diseases and the management of cardiac aging. In this review, we summarize the current knowledge regarding the role of Gal-3 in the pathophysiology of heart failure, atherosclerosis, hypertension, myocarditis, and ischemic heart disease. Furthermore, we describe the physiological role of Gal-3 in cardiac aging.

Detection of Reactive Oxygen Species in Human Neutrophils Under Various Conditions of Exposure to Galectin

Reactive oxygen species (ROS) have been extensively studied in biology in the past years. This class of molecules can be derived from endogenous sources (e.g., phagocytic cells as neutrophils, eosinophils, monocytes, macrophages, and organelles as mitochondria and peroxisomes) and participate in physiological and pathological conditions. The beneficial and harmful effects of ROS depend on redox regulation, which establishes the balance between their production and the activity of antioxidant systems to prevent oxidative stress in vivo. Neutrophils are the immune effectors most well depicted with an intense oxidative burst in response to tissue inflammation. Several proteins and members of the galectin family are involved in this fine modulation of ROS production by neutrophils. Interestingly, studies have indicated that Galectin-1 (Gal-1) can up- or downregulate ROS production by neutrophils even when exposed to N-formyl-Met-Leu-Phe (fMLP) or Phorbol Myristate Acetate (PMA), both of which are potent neutrophil stimulants that trigger high levels of ROS production. Similarly, Galectin-3 (Gal-3) induces ROS in neutrophils from a sterile or nonsterile inflammatory environment, possibly creating a negative loop that could control ROS production. Besides, superoxide production is also induced by Galectin-8 (Gal-8) and Galectin-9 (Gal-9) in neutrophils but in a different manner. We describe herein the luminol and lucigenin-dependent chemiluminescence technique by using a luminometer as a method of assessment to measure ROS production by human neutrophils isolated and exposed to purified human recombinant Gal-1. The protocol described herein could be applied for the investigation of the role of other galectins in the modulation of ROS production by neutrophils.

The Two Formyl Peptide Receptors Differently Regulate GPR84-Mediated Neutrophil NADPH Oxidase Activity

Neutrophils express the two formyl peptide receptors (FPR1 and FPR2) and the medium-chain fatty acid receptor GPR84. The FPRs are known to define a hierarchy among neutrophil G protein-coupled receptors (GPCRs), that is, the activated FPRs can either suppress or amplify GPCR responses. In this study, we investigated the position of GPR84 in the FPR-defined hierarchy regarding the activation of neutrophil nicotine adenine dinucleotide phosphate (NADPH) oxidase, an enzyme system designed to generate reactive oxygen species (ROS), which are important regulators in cell signaling and immune regulation. When resting neutrophils were activated by GPR84 agonists, a modest ROS release was induced. However, vast amounts of ROS were induced by these GPR84 agonists in FPR2-desensitized neutrophils, and the response was inhibited not only by a GPR84-specific antagonist but also by an FPR2-specific antagonist. This suggests that the amplified GPR84 agonist response is achieved through a reactivation of desensitized FPR2s. In addition, the GPR84-mediated FPR2 reactivation was independent of β-arrestin recruitment and sensitive to a protein phosphatase inhibitor. In contrast to FPR2-desensitized cells, FPR1 desensitization primarily resulted in a suppressed GPR84 agonist-induced ROS response, indicating a receptor hierarchical desensitization of GPR84 by FPR1-generated signals. In summary, our data show that the two FPRs in human neutrophils control the NADPH oxidase activity with concomitant ROS production by communicating with GPR84 through different mechanisms. While FPR1 desensitizes GPR84 and by that suppresses the release of ROS induced by GPR84 agonists, amplified ROS release is achieved by GPR84 agonists through reactivation of the desensitized FPR2.

Neuroinflammation and oxidative stress act in concert to promote neurodegeneration in the diabetic retina and optic nerve: galectin-3 participation

Diabetes is a lifelong disease characterized by glucose metabolic imbalance, in which low insulin levels or impaired insulin signaling lead to hyperglycemic state. Within 20 years of diabetes progression, 95% of patients will have diabetic retinopathy, the leading cause of visual defects in working-age people worldwide. Although diabetes is considered a microvascular disease, recent studies have shown that neurodegeneration precedes vascular changes within the diabetic visual system, albeit its mechanisms are still under investigation. Neuroinflammation and oxidative stress are intrinsically related phenomena, since macrophage/microglia and astrocytes are the main sources of reactive oxygen species during central nervous system chronic degenerative diseases, and both pathological processes are increased in the visual system during diabetes. The present review will focus on recent findings of the contribution of oxidative stress derived from neuroinflammation in the early neurodegenerative aspects of the diabetic visual system and their relationship with galectin-3.

Galectin-1 modulation of neutrophil reactive oxygen species production depends on the cell activation state

Here we report the effects of exogenous and endogenous galectin-1 (Gal-1) in modulating the functional responses of human and murine neutrophils at different stages of activation, i.e. naive, primed, and activated. Exposure to Gal-1 did not induce ROS production in either naive or N-formyl-methionyl-leucyl-phenylalanine-primed (fMLP; 10^-9 M) neutrophils. However, Gal-1 elicited a concentration-dependent ROS production in neutrophils activated with fMLP at concentrations ranging from 10^-8 M to 10^-6 M. Additional fMLP (10^-7 M) stimulation of fMLP-activated neutrophils increased ROS production, whose intensity was inversely related to the fMLP concentration used in the first activation step (10^-8 M to 10^-6 M), and was not influenced by the presence of Gal-1. Naive neutrophils treated with Gal-1 and then exposed to fMLP (10^-6 M) or phorbol-12-myristate-13-acetate (10^-7 M) produced less ROS, as compared to naive neutrophils not treated with Gal-1. Interestingly, these in vitro Gal-1 effects were associated with Gal-1 carbohydrate-binding activity and the ability to decrease FPR-1 (formyl peptide receptor 1) expression in naive human neutrophils. Conversely, positive ROS modulation by Gal-1 in activated neutrophils was not associated with FPR-1 expression but it was related to its carbohydrate recognition. In vitro, fMLP stimulation of Gal-1^-/- mouse neutrophils produced more ROS than fMLP stimulation of Gal-1^+/+ neutrophils and this effect may be associated with increased FPR-1 expression. Exogenous Gal-1 induced ROS production in Gal-1^-/- mouse neutrophils more effectively than in Gal-1^+/+ mouse neutrophils. Compared to Gal-1^+/+ mice, Gal-1^-/- mice exhibited lower bacterial load in the peritoneal fluid and peripheral blood, thus indicating a greater bactericidal activity in vivo. These findings demonstrate that endogenous Gal-1 restricts ROS generation that correlates with bacterial killing capacity in inflammatory neutrophils. Thus, endogenous and exogenous Gal-1 may either positively or negatively modulate the effector functions of neutrophils according to the cell activation stage.

The Sweet-Side of Leukocytes: Galectins as Master Regulators of Neutrophil Function

Among responders to microbial invasion, neutrophils represent one of the earliest and perhaps most important factors that contribute to initial host defense. Effective neutrophil immunity requires their rapid mobilization to the site of infection, which requires efficient extravasation, activation, chemotaxis, phagocytosis, and eventual killing of potential microbial pathogens. Following pathogen elimination, neutrophils must be eliminated to prevent additional host injury and subsequent exacerbation of the inflammatory response. Galectins, expressed in nearly every tissue and regulated by unique sensitivity to oxidative and proteolytic inactivation, appear to influence nearly every aspect of neutrophil function. In this review, we will examine the impact of galectins on neutrophils, with a particular focus on the unique biochemical traits that allow galectin family members to spatially and temporally regulate neutrophil function.

Age-Appropriate Functions and Dysfunctions of the Neonatal Neutrophil

Neonatal and adult neutrophils are distinctly different from one another due to well-defined and documented deficiencies in neonatal cells, including impaired functions, reduced concentrations of microbicidal proteins and enzymes necessary for pathogen destruction, and variances in cell surface receptors. Neutrophil maturation is clearly demonstrated throughout pregnancy from the earliest hematopoietic precursors in the yolk sac to the well-developed myeloid progenitor cells in the bone marrow around the seventh month of gestation. Notable deficiencies of neonatal neutrophils are generally correlated with gestational age and clinical condition, so that the least functional neutrophils are found in the youngest, sickest neonates. Interruption of normal gestation secondary to preterm birth exposes these shortcomings and places the neonate at an exceptionally high rate of infection and sepsis-related mortality. Because the fetus develops in a sterile environment, neonatal adaptive immune responses are deficient from lack of antigen exposure in utero. Newborns must therefore rely on innate immunity to protect against early infection. Neutrophils are a vital component of innate immunity since they are the first cells to respond to and defend against bacterial, viral, and fungal infections. However, notable phenotypic and functional disparities exist between neonatal and adult cells. Below is review of neutrophil ontogeny, as well as a discussion regarding known differences between preterm and term neonatal and adult neutrophils with respect to cell membrane receptors and functions. Our analysis will also explain how these variations decrease with postnatal age.

A pepducin designed to modulate P2Y2R function interacts with FPR2 in human neutrophils and transfers ATP to an NADPH-oxidase-activating ligand through a receptor cross-talk mechanism

Several G-protein-coupled receptors (GPCRs) can be activated or inhibited in a specific manner by membrane-permeable pepducins, which are short palmitoylated peptides with amino acid sequences identical to an intracellular domain of the receptor to be targeted. Unlike the endogenous P2Y2R agonist ATP, the P2Y2PalIC2 pepducin, which has an amino acid sequence corresponding to the second intracellular loop of the human ATP receptor (P2Y2R), activated the superoxide anion-generating NADPH-oxidase in neutrophils. In addition to having a direct effect on neutrophils, the P2Y2R pepducin converted naïve neutrophils to a primed state, which secondarily responded to ATP by producing superoxide. A pepducin with a peptide identical to the third intracellular loop of P2Y2R (P2Y2PalIC3) exhibited the same basic functions as P2Y2PalIC2, whereas one with a peptide that was identical to the first intracellular loop (P2Y2PalIC1) lacked these functions. The responses induced in neutrophils by the P2Y2R pepducins were not inhibited by the P2Y2R antagonist AR-C118925, and the receptor desensitization profile suggested the involvement of FPR2 rather than P2Y2R. Accordingly, antagonists/inhibitors of FPR2 attenuated the activities of the P2Y2R pepducins, which also selectively activated FPR2-overexpressing cells. In summary, we show that pepducins supposed to target P2Y2R activate human neutrophils through FPR2. We also show that the P2Y2PalIC2 pepducin can convert ATP from a non-activating agent to a potent neutrophil NADPH-oxidase activator. The molecular basis of this phenomenon involves cross-talk between the receptor/ligand pairs of P2Y2R/ATP and FPR2/P2Y2-pepducin.

The role of galectin-3 in phagocytosis of Candida albicans and Candida parapsilosis by human neutrophils

Candida albicans causes the majority of invasive candidiasis in immunocompromised adults while Candida parapsilosis is a leading cause of neonatal candidiasis. While much work has focused on how the immune system recognizes and responds to C. albicans, less is known about host interaction with C. parapsilosis. This study investigates the human neutrophil phagocytic response to these species. Neutrophils underwent phagocytosis of C. parapsilosis yeast and C. albicans hyphae much more efficiently than C. albicans yeast. Treatment of neutrophils with a galectin-3 (gal3) blocking antibody inhibited phagocytosis of C. parapsilosis yeast and C. albicans hyphae, but not C. albicans yeast. The majority of neutrophil gal3 was expressed intracellularly and was secreted from neutrophils after treatment with C. parapsilosis mannan. When neutrophils were treated with exogenous gal3, phagocytosis of both C. albicans and C. parapsilosis yeast increased. Exposure of neutrophils to C. parapsilosis yeast increased phagocytosis of C. albicans yeast and was inhibited by gal3 blocking antibody. Taken together, these data indicate that gal3 secreted from neutrophils may act as a pro-inflammatory autocrine/paracrine signal in neutrophil phagocytosis and suggest that gal3 has a unique role in neutrophil response to C. parapsilosis yeast and C. albicans hyphae distinct from C. albicans yeast.

Receptor-dependent and -independent immunomodulatory effects of phenol-soluble modulin peptides from Staphylococcus aureus on human neutrophils are abrogated through peptide inactivation by reactive oxygen species

The virulence and pathogenesis mechanisms of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strains depend on a newly described group of phenol-soluble modulin (PSM) peptides (the PSMα peptides) with cytolytic activity. These toxins are α-helical peptides with a formyl group at the N terminus, and they activate neutrophils through formyl peptide receptor 2 (FPR2), a function closely correlated to the capacity of staphylococcal species to cause invasive infections. The effects of two synthetic PSMα peptides were investigated, and we show that they utilize FPR2 and promote neutrophils to produce reactive oxygen species (ROS) which in turn trigger inactivation of the peptides. Independently of FPR2, the PSMα peptides also downregulate the neutrophil response to other stimuli and exert a cytolytic effect to which apoptotic neutrophils are more sensitive than viable cells. The novel immunomodulatory functions of the PSMα peptides were sensitive to ROS generated by the neutrophil myeloperoxidase (MPO)-H(2)O(2) system, suggesting a role for this enzyme system in counteracting bacterial virulence.

A non-peptide receptor inhibitor with selectivity for one of the neutrophil formyl peptide receptors, FPR 1

The neutrophil formyl peptide receptors (FPR1 and FPR2) are members of the G-protein coupled receptor family. The signals generated by occupied FPRs are both pro-inflammatory and anti-inflammatory. Accordingly, these receptors have become a therapeutic target for the development of novel drugs that may be used to reduce injuries in inflammatory diseases including asthma, rheumatoid arthritis, Alzheimer's disease and cardiovascular diseases. To support the basis for a future pharmacological characterization, we have identified a small molecular non-peptide inhibitor with selectivity for FPR1. We used the FPR1 and FPR2 specific ligands fMLF and WKYMVM, respectively, and an earlier described ratio technique, to determine inhibitory activity combined with selectivity. We show that the compound 3,5-dichloro-N-(2-chloro-5-methyl-phenyl)-2-hydroxy-benzamide (BVT173187) fulfills the criteria for an FPR1 inhibitor selective for FPR1 over FPR2, and it inhibits the same functional repertoire in neutrophils as earlier described peptide antagonists. Accordingly, the new inhibitor reduced neutrophil activation with FPR1 agonists, leading to mobilization of adhesion molecules (CR3) and the generation of superoxide anion from the neutrophil NADPH-oxidase. The effects of a number of structural analogs were determined but these were either without activity or less active/specific than BVT173187. The potency of the new inhibitor for reduction of FPR1 activity was the same as that of the earlier described FPR1 antagonist cyclosporine H, but signaling through the C5aR and CXCR (recognizing IL8) was also affected by BVT173187.

Galectin-3 inhibits the chemotaxis of human polymorphonuclear neutrophils in vitro

In the recent years, the participation of the animal lectin galectin (gal)-3 in inflammation and in host defence mechanisms was extensively studied. In vivo studies implied - among others - a role of gal-3 in the recruitment of polymorphonuclear neutrophils (PMN) to sites of bacterial infection. In that context, we asked the question whether gal-3 was chemotactic for PMN. Functional assays revealed that gal-3 was not chemotactic for PMN, but that it inhibited the spontaneous migration and the chemotaxis of PMN towards complement C5a, interleukin (IL)-8, or ATP. Moreover, gal-3 inhibited the shape change and the actin polymerisation of PMN that occurs in response to C5a or IL-8. By use of FITC-labelled gal-3, we found that it attached rapidly to the PMN membrane in a lactose-sensitive manner. In response to gal-3 the MAP kinase p38 was phosphorylated. This kinase is crucial for the migration of PMN towards end-target chemokines, such as C5a, and is activated in response to C5a or IL-8. When PMN were preincubated with gal-3, the C5a-induced p38 phosphorylation was transiently enhanced, but eventually down-modulated. We conclude that by interfering with the chemokine-induced p38 phosphorylation gal-3 inhibits chemotaxis of PMN.

Intracellular generation of superoxide by the phagocyte NADPH oxidase: how, where, and what for?

Professional phagocytes increase their consumption of molecular oxygen during the phagocytosis of microbes or when encountering a variety of nonparticulate stimuli. In these circumstances, oxygen is reduced by the phagocyte NADPH oxidase, and reactive oxygen species (ROS), which are important for the microbicidal activity of the cells, are generated. The structure and function of the NADPH oxidase have been resolved in part by studying cells from patients with chronic granulomatous disease (CGD), a condition characterized by the inability of phagocytes to assemble a functional NADPH oxidase and thus to produce ROS. As a result, patients with CGD have a predisposition to infections as well as a variety of inflammatory symptoms. A long-standing paradigm has been that NADPH oxidase assembly occurs exclusively in the plasma membrane or invaginations thereof (phagosomes). A growing body of evidence points to the possibility that phagocytes are capable of NADPH oxidase assembly in nonphagosomal intracellular membranes, resulting in ROS generation within intracellular organelles also in the absence of phagocytosis. The exact nature of these ROS-producing organelles is yet to be determined, but granules are prime suspects. Recent clinical findings indicate that the generation of intracellular ROS by NADPH oxidase activation is important for limiting inflammatory reactions and that intracellular and extracellular ROS production are regulated differently. Here we discuss the accumulating knowledge of intracellular ROS production in phagocytes and speculate on the precise role of these oxidants in regulating the inflammatory process.

Stable formyl peptide receptor agonists that activate the neutrophil NADPH-oxidase identified through screening of a compound library

The neutrophil formyl peptide receptors (FPR1 and FPR2) are G-protein coupled receptors that can induce pro-inflammatory as well as anti-inflammatory activities when activated. Accordingly, these receptors may become therapeutic targets for the development of novel drugs to be used for reducing the inflammation induced injuries in asthma, rheumatoid arthritis, Alzheimer's disease, cardiovascular diseases and traumatic shock. We screened a library of more then 50K small compounds for an ability of the compounds to induce a transient rise in intracellular Ca(2+) in cells transfected to express FPR2 (earlier called FPRL1 or the lipoxin A(4) receptor). Ten agonist hits were selected for further analysis representing different chemical series and five new together with five earlier described molecules were further profiled. Compounds 1-10 gave rise to a calcium response in the FPR2 transfectants with EC(50) values ranging from 4×10(-9)M to 2×10(-7)M. All 10 compounds activated human neutrophils to release superoxide, and based on the potency of their activity, the three most potent activators of the neutrophil NADPH-oxidase were further characterized. These three agonists were largely resistant to inactivation by neutrophil produced reactive oxygen species and shown to trigger the same functional repertoire in neutrophils as earlier described peptide agonists. Accordingly they induced chemotaxis, granule mobilization and secretion of superoxide. Interestingly, the oxidase activity was largely inhibited by cyclosporine H, an FPR1 selective antagonist, but not by PBP10, an FPR2 selective inhibitor, suggesting that FPR1 is the preferred receptor in neutrophils for all three agonists.

Mutational tuning of galectin-3 specificity and biological function

Galectins are defined by a conserved β-galactoside binding site that has been linked to many of their important functions in e.g. cell adhesion, signaling, and intracellular trafficking. Weak adjacent sites may enhance or decrease affinity for natural β-galactoside-containing glycoconjugates, but little is known about the biological role of this modulation of affinity (fine specificity). We have now produced 10 mutants of human galectin-3, with changes in these adjacent sites that have altered carbohydrate-binding fine specificity but that retain the basic β-galactoside binding activity as shown by glycan-array binding and a solution-based fluorescence anisotropy assay. Each mutant was also tested in two biological assays to provide a correlation between fine specificity and function. Galectin-3 R186S, which has selectively lost affinity for LacNAc, a disaccharide moiety commonly found on glycoprotein glycans, has lost the ability to activate neutrophil leukocytes and intracellular targeting into vesicles. K176L has increased affinity for β-galactosides substituted with GlcNAcβ1–3, as found in poly-N-acetyllactosaminoglycans, and increased potency to activate neutrophil leukocytes even though it has lost other aspects of galectin-3 fine specificity. G182A has altered carbohydrate-binding fine specificity and altered intracellular targeting into vesicles, a possible link to the intracellular galectin-3-mediated anti-apoptotic effect known to be lost by this mutant. Finally, the mutants have helped to define the differences in fine specificity shown by Xenopus, mouse, and human galectin-3 and, as such, the evidence for adaptive change during evolution.

The FPR2-induced rise in cytosolic calcium in human neutrophils relies on an emptying of intracellular calcium stores and is inhibited by a gelsolin-derived PIP2-binding peptide

BACKGROUND

The molecular basis for neutrophil recognition of chemotactic peptides is their binding to specific G-protein-coupled cell surface receptors (GPCRs). Human neutrophils express two pattern recognition GPCRs, FPR1 and FPR2, which belong to the family of formyl peptide receptors. The high degree of homology between these two receptors suggests that they share many functional and signal transduction properties, although they exhibit some differences with respect to signaling. The aims of this study were to determine whether FPR2 triggers a unique signal that allows direct influx of extracellular calcium without the emptying of intracellular calcium stores, and whether the gelsolin-derived PIP2-binding peptide, PBP10, selectively inhibits FPR2-mediated transient rise in intracellular Ca2+.

RESULTS

The transient rise in intracellular Ca2+ induced by agonists for FPR1 or FPR2 in human neutrophils occurred also in the presence of a chelator of Ca2+ (EGTA). PBP10 inhibited not only FPR2-induced oxidase activity, but also the transient rise in intracellular Ca2+.

CONCLUSIONS

Ca2+ signaling mediated via FPR2 follows the same route as FPR1, which involves initial emptying of the intracellular stores. PBP10 inhibits selectively the signals generated by FPR2, both with respect to NADPH-oxidase activity and the transient rise in intracellular Ca2+ induced by agonist exposure.

Galectins in innate immunity: dual functions of host soluble beta-galactoside-binding lectins as damage-associated molecular patterns (DAMPs) and as receptors for pathogen-associated molecular patterns (PAMPs)

The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell- or microorganism-specific 'glycoinformation'. This information is decoded by host glycan-binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin-1, and dendritic cell (DC)-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan 'epitopes'. As they recognize non-self glycans presented by microorganisms, they can be considered as receptors for pathogen-associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection-initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non-classical pathway, the 'leaderless' secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine-like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen-initiated tissue damage signals appear, this lectin family is also considered as potential damage-associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.