Damage-associated molecular patterns (DAMPs) [1] are molecules within cells that are a component of the innate immune response released from damaged or dying cells due to trauma or an infection by a pathogen. [2] They are also known as danger signals, and alarmins because they serve as warning signs to alert the organism to any damage or infection to its cells. DAMPs are endogenous danger signals that are discharged to the extracellular space in response to damage to the cell from mechanical trauma or a pathogen. [3] Once a DAMP is released from the cell, it promotes a noninfectious inflammatory response by binding to a pattern recognition receptor (PRR). [4] Inflammation is a key aspect of the innate immune response; it is used to help mitigate future damage to the organism by removing harmful invaders from the affected area and start the healing process. [5] As an example, the cytokine IL-1α is a DAMP that originates within the nucleus of the cell which, once released to the extracellular space, binds to the PRR IL-1R, which in turn initiates an inflammatory response to the trauma or pathogen that initiated the release of IL-1α. [3] In contrast to the noninfectious inflammatory response produced by DAMPs, pathogen-associated molecular patterns (PAMPs) initiate and perpetuate the infectious pathogen-induced inflammatory response. [6] Many DAMPs are nuclear or cytosolic proteins with defined intracellular function that are released outside the cell following tissue injury. [7] This displacement from the intracellular space to the extracellular space moves the DAMPs from a reducing to an oxidizing environment, causing their functional denaturation, resulting in their loss of function. [7] Outside of the aforementioned nuclear and cytosolic DAMPs, there are other DAMPs originated from different sources, such as mitochondria, granules, the extracellular matrix, the endoplasmic reticulum, and the plasma membrane. [3]
DAMPs and their receptors are characterized as: [3]
Origin | Major DAMPs | Receptors | |
---|---|---|---|
Extracellular matrix | Biglycan | TLR2, TLR4, NLRP3 | |
Decorin | TLR2, TLR4 | ||
Versican | TLR2, TLR6, CD14 | ||
LMW hyaluronan | TLR2, TLR4, NLRP3 | ||
Heparan sulfate | TLR4 | ||
Fibronectin (EDA domain) | TLR4 | ||
Fibrinogen | TLR4 | ||
Tenascin C | TLR4 | ||
Intracellular compartments | Cytosol | Uric Acid | NLRP3, P2X7 |
S100 proteins | TLR2, TLR4, RAGE | ||
Heat-shock proteins | TLR2, TLR4, CD91 | ||
ATP | P2X7, P2Y2 | ||
F-actin | DNGR-1 | ||
Cyclophilin A | CD147 | ||
Aβ | TLR2, NLRP1, NLRP3, CD36, RAGE | ||
Nuclear | Histones | TLR2, TLR4 | |
HMGB1 | TLR2, TLR4, RAGE | ||
HMGN1 | TLR4 | ||
IL-1α | IL-1R | ||
IL-33 | ST2 | ||
SAP130 | Mincle | ||
DNA | TLR9, AIM2 | ||
RNA | TLR3, TLR7, TLR8, RIG-I, MDA5 | ||
Mitochondria | mtDNA | TLR9 | |
TFAM | RAGE | ||
Formyl peptide | FPR1 | ||
mROS | NLRP3 | ||
Endoplasmic reticulum | Calreticulin | CD91 | |
Granule | Defensins | TLR4 | |
Cathelicidin (LL37) | P2X7, FPR2 | ||
Eosinophil-derived neurotoxin | TLR2 | ||
Granulysin | TLR4 | ||
Plasma membrane | Syndecans | TLR4 | |
Glypicans | TLR4 |
Two papers appearing in 1994 anticipated the deeper understanding of innate immune reactivity, pointing towards the subsequent understanding of the nature of the adaptive immune response. The first [8] came from transplant surgeons who conducted a prospective randomized, double-blind, placebo-controlled trial. Administration of recombinant human superoxide dismutase (rh-SOD) in recipients of cadaveric renal allografts demonstrated prolonged patient and graft survival with improvement in both acute and chronic rejection events. They speculated that the effect was related to SOD's antioxidant action on the initial ischemia/reperfusion injury of the renal allograft, thereby reducing the immunogenicity of the allograft. Thus, free radical-mediated reperfusion injury was seen to contribute to the process of innate and subsequent adaptive immune responses. [9]
The second study [10] suggested the possibility that the immune system detected "danger", through a series of what is now called damage-associated molecular pattern molecules (DAMPs), working in concert with both positive and negative signals derived from other tissues. Thus, these papers anticipated the modern sense of the role of DAMPs and redox, important, apparently, for both plant and animal resistance to pathogens and the response to cellular injury or damage. Although many immunologists had earlier noted that various "danger signals" could initiate innate immune responses, the "DAMP" was first described by Seong and Matzinger in 2004. [1]
DAMPs vary greatly depending on the type of cell (epithelial or mesenchymal) and injured tissue, but they all share the common feature of stimulating an innate immune response within an organism. [2]
DAMPs in plants have been found to stimulate a fast immune response, but without the inflammation that characterizes DAMPs in mammals. [34] Just as with mammalian DAMPs, plant DAMPs are cytosolic in nature and are released into the extracellular space following damage to the cell caused by either trauma or pathogen. [35] The major difference in the immune systems between plants and mammals is that plants lack an adaptive immune system, so plants can not determine which pathogens have attacked them before and thus easily mediate an effective immune response to them. To make up for this lack of defense, plants use the pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) pathways to combat trauma and pathogens. PTI is the first line of defense in plants and is triggered by PAMPs to initiate signaling throughout the plant that damage has occurred to a cell. Along with the PTI, DAMPs are also released in response to this damage, but as mentioned earlier they do not initiate an inflammatory response like their mammalian counterparts. The main role of DAMPs in plants is to act as mobile signals to initiate wounding responses and to promote damage repair. A large overlap occurs between the PTI pathway and DAMPs in plants, and the plant DAMPs effectively operate as PTI amplifiers. The ETI always occurs after the PTI pathway and DAMP release, and is a last resort response to the pathogen or trauma that ultimately results in programmed cell death. The PTI- and ETI-signaling pathways are used in conjunction with DAMPs to rapidly signal the rest of the plant to activate its innate immune response and fight off the invading pathogen or mediate the healing process from damage caused by trauma. [36]
Plant DAMPs and their receptors are characterized as: [35]
Category | DAMP | Molecular structure or epitope | Source or precursor | Receptor or signaling regulator | Species |
---|---|---|---|---|---|
Epidermis cuticle | Cutin monomers | C16 and C18 hydroxy and epoxy fatty acids | Epidermis cuticle | Unknown | Arabidopsis thaliana, Solanum lycopersicum |
Cell wall polysaccharide fragments or degrading products | OGs | Polymers of 10–15 α-1-4-linked GalAs | Cell wall pectin | WAK1 (A. thaliana) | A. thaliana, G. max, N. tabacum |
Cellooligomers | Polymers of 2–7 β-1,4-linked glucoses | Cell wall cellulose | Unknown | A. thaliana | |
Xyloglucan oligosaccharides | Polymers of β-1,4-linked glucose with xylose, galactose, and fructose side chains | Cell-wall hemicellulose | Unknown | A. thaliana, Vitis vinifera | |
Methanol | Methanol | Cell wall pectin | Unknown | A. thaliana, Nicotiana tabacum | |
Apoplastic peptides and proteins | CAPE1 | 11-aa peptide | Apoplastic PR1 | Unknown | A. thaliana, S. lycopersicum |
GmSUBPEP | 12-aa peptide | Apoplastic subtilase | Unknown | Glycine max | |
GRIp | 11-aa peptide | Cytosolic GRI | PRK5 | A. thaliana | |
Systemin | 18-aa peptide (S. lycopersicum) | Cytosolic prosystemin | SYR1/2 (S. lycopersicum) | Some Solanaceae species | |
HypSys | 15-, 18-, or 20-aa peptides | Apoplastic or cytoplasmic preproHypSys | Unknown | Some Solanaceae species | |
Peps | 23~36-aa peptides (A. thaliana) | Cytosolic and vacuolar PROPEPs | PEPR1/2 (A. thaliana) | A. thaliana, Zea mays, S. lycopersicum, Oryza sativa | |
PIP1/2 | 11-aa peptides | Apoplastic preproPIP1/2 | RLK7 | A. thaliana | |
GmPep914/890 | 8-aa peptide | Apoplastic or cytoplasmic GmproPep914/890 | Unknown | G. max | |
Zip1 | 17-aa peptide | Apoplastic PROZIP1 | Unknown | Z. mays | |
IDL6p | 11-aa peptide | Apoplastic or cytoplasmic IDL6 precursors | HEA/HSL2 | A. thaliana | |
RALFs | ~50-aa cysteine-rich peptides | Apoplastic or cytoplasmic RALF precursors | FER (A. thaliana) | A. thaliana, N. tabacum, S. lycopersicum | |
PSKs | 5-aa peptides | Apoplastic or cytoplasmic PSK precursors | PSKR1/2 (A. thaliana) | A. thaliana, S. lycopersicum | |
HMGB3 | HMGB3 protein | Cytosolic and nuclear HMGB3 | Unknown | A. thaliana | |
Inceptin | 11-aa peptide | Chloroplastic ATP synthase γ-subunit | INR [37] | Vigna unguiculata | |
Extracellular nucleotides | eATP | ATP | Cytosolic ATP | DORN1/P2K1 (A. thaliana) | A. thaliana, N. tabacum |
eNAD(P) | NAD(P) | Cytosolic NAD(P) | LecRK-I.8 | A. thaliana | |
eDNA | DNA fragments < 700 bp in length | Cytosolic and nuclear DNA | Unknown | Phaseolus vulgaris, P. lunatus, Pisum sativum, Z. mays | |
Extracellular sugars | Extracellular sugars | Sucrose, glucose, fructose, maltose | Cytosolic sugars | RGS1 (A. thaliana) | A. thaliana, N. tabacum, Solanum tuberosum |
Extracellular amino acids and glutathione | Proteinogenic amino acids | Glutamate, cysteine, histidine, aspartic acid | Cytosolic amino acids | GLR3.3/3.6 or others (A. thaliana) | A. thaliana, S. lycopersicum, Oryza sativa |
Glutathione | Glutathione | Cytosolic glutathione | GLR3.3/3.6 (A. thaliana) | A. thaliana |
Many mammalian DAMPs have DAMP counterparts in plants. One example is with the high-mobility group protein. Mammals have the HMGB1 protein, while Arabidopsis thaliana has the HMGB3 protein. [38]
Preventing the release of DAMPs and blocking DAMP receptors would, in theory, stop inflammation from an injury or infection and reduce pain for the affected individual. [39] This is especially important during surgeries, which have the potential to trigger these inflammation pathways, making the surgery more difficult and dangerous to complete. The blocking of DAMPs also has theoretical applications in therapeutics to treat disorders such as arthritis, cancer, ischemia reperfusion, myocardial infarction, and stroke. [39] These theoretical therapeutic options include:
DAMPs can be used as biomarkers for inflammatory diseases and potential therapeutic targets. For example, increased S100A8/A9 is associated with osteophyte progression in early human osteoarthritis, suggesting that S100 proteins can be used as biomarkers for the diagnosis of the progressive grade of osteoarthritis. [40] Furthermore, DAMP can be a useful prognostic factor for cancer. This would improve patient classification, and a suitable therapy would be given to patients by diagnosing with DAMPs. The regulation of DAMP signaling can be a potential therapeutic target to reduce inflammation and treat diseases. For example, administration of neutralizing HMGB1 antibodies or truncated HMGB1-derived A-box protein ameliorated arthritis in collagen-induced arthritis rodent models. Clinical trials with HSP inhibitors have also been reported. For nonsmall-cell lung cancer, HSP27, HSP70, and HSP90 inhibitors are under investigation in clinical trials. In addition, treatment with dnaJP1, which is a synthetic peptide derived from DnaJ (HSP40), had a curative effect in rheumatoid arthritis patients without critical side effects. Taken together, DAMPs can be useful therapeutic targets for various human diseases, including cancer and autoimmune diseases. [3]
DAMPs can trigger re-epithelialization upon kidney injury, contributing to epithelial–mesenchymal transition, and potentially, to myofibroblast differentiation and proliferation. These discoveries suggest that DAMPs drive not only immune injury, but also kidney regeneration and renal scarring. For example, TLR2-agonistic DAMPs activate renal progenitor cells to regenerate epithelial defects in injured tubules. TLR4-agonistic DAMPs also induce renal dendritic cells to release IL-22, which also accelerates tubule re-epithelialization in acute kidney injury. Finally, DAMPs also promote renal fibrosis by inducing NLRP3, which also promotes TGF-β receptor signaling. [41]
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system. The innate immune system provides a preconfigured response to broad groups of situations and stimuli. The adaptive immune system provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use molecules and cells to perform their functions.
Macrophages are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris, and foreign substances, which do not have proteins that are specific to healthy body cells on their surface. This process is called phagocytosis, which acts to defend the host against infection and injury.
Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses. The TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, and TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. The receptors TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.
Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed by phagocytes. Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens, cancer cells, aged cells, dead or dying cells, excess synapses, or protein aggregates. Opsonins help clear pathogens, as well as dead, dying and diseased cells.
Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes, but not present in the host. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. This allows the innate immune system to recognize pathogens and thus, protect the host from infection.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed mainly by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils, as well as by epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
The innate immune system or nonspecific immune system is one of the two main immunity strategies in vertebrates. The innate immune system is an alternate defense strategy and is the dominant immune system response found in plants, fungi, prokaryotes, and invertebrates.
Pyroptosis is a highly inflammatory form of lytic programmed cell death that occurs most frequently upon infection with intracellular pathogens and is likely to form part of the antimicrobial response. This process promotes the rapid clearance of various bacterial, viral, fungal and protozoan infections by removing intracellular replication niches and enhancing the host's defensive responses. Pyroptosis can take place in immune cells and is also reported to occur in keratinocytes and some epithelial cells.
Toll-like receptor 4 (TLR4), also designated as CD284, is a key activator of the innate immune response and plays a central role in the fight against bacterial infections. TLR4 is a transmembrane protein of approximately 95 kDa that is encoded by the TLR4 gene.
Toll-like receptor 6 is a protein that in humans is encoded by the TLR6 gene. TLR6 is a transmembrane protein, member of toll-like receptor family, which belongs to the pattern recognition receptor (PRR) family. TLR6 acts in a heterodimer form with toll-like receptor 2 (TLR2). Its ligands include multiple diacyl lipopeptides derived from gram-positive bacteria and mycoplasma and several fungal cell wall saccharides. After dimerizing with TLR2, the NF-κB intracellular signalling pathway is activated, leading to a pro-inflammatory cytokine production and activation of innate immune response. TLR6 has also been designated as CD286.
Toll-like receptor 9 is a protein that in humans is encoded by the TLR9 gene. TLR9 has also been designated as CD289. It is a member of the toll-like receptor (TLR) family. TLR9 is an important receptor expressed in immune system cells including dendritic cells, macrophages, natural killer cells, and other antigen presenting cells. TLR9 is expressed on endosomes internalized from the plasma membrane, binds DNA, and triggers signaling cascades that lead to a pro-inflammatory cytokine response. Cancer, infection, and tissue damage can all modulate TLR9 expression and activation. TLR9 is also an important factor in autoimmune diseases, and there is active research into synthetic TLR9 agonists and antagonists that help regulate autoimmune inflammation.
High mobility group box 1 protein, also known as high-mobility group protein 1 (HMG-1) and amphoterin, is a protein that in humans is encoded by the HMGB1 gene.
Nucleotide-binding oligomerization domain-like receptor (NLR) pyrin domain (PYD)-containing protein 12 is a protein that in humans is encoded by the NLRP12 gene.
NLRC5, short for NOD-like receptor family CARD domain containing 5, is an intracellular protein that plays a role in the immune system. NLRC5 is a pattern recognition receptor implicated in innate immunity to viruses potentially by regulating interferon activity. It also acts as an innate immune sensor to drive inflammatory cell death, PANoptosis. In humans, the NLRC5 protein is encoded by the NLRC5 gene. It has also been called NOD27, NOD4, and CLR16.1.
Apoptotic-cell associated molecular patterns (ACAMPs) are molecular markers present on cells which are going through apoptosis, i.e. programmed cell death. The term was used for the first time by C. D. Gregory in 2000. Recognition of these patterns by the pattern recognition receptors (PRRs) of phagocytes then leads to phagocytosis of the apoptotic cell. These patterns include eat-me signals on the apoptotic cells, loss of don’t-eat-me signals on viable cells and come-get-me signals ) secreted by the apoptotic cells in order to attract phagocytes. Thanks to these markers, apoptotic cells, unlike necrotic cells, do not trigger the unwanted immune response.
Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.
The danger model of the immune system proposes that it differentiates between components that are capable of causing damage, rather than distinguishing between self and non-self.
Immunogenic cell death is any type of cell death eliciting an immune response. Both accidental cell death and regulated cell death can result in immune response. Immunogenic cell death contrasts to forms of cell death that do not elicit any response or even mediate immune tolerance.
Seung-Yong Seong is a South Korean immunologist and microbiologist known for his study of innate immune system response and his development of the damage-associated molecular pattern (DAMP) model of immune response initiation in collaboration with Polly Matzinger. Seong is also known for his research on the bacterium Orientia tsutsugamushi and his research on immunological adjuvant when he was a student. Since 2013 he has served as Director of the Wide River Institute of Immunology – Seoul National University in conjunction with his Professor position in the Microbiology and Immunology department of Seoul National University College of Medicine. In 2012, he became Editor in Chief of the World Journal of Immunology.
Inflammaging is a chronic, sterile, low-grade inflammation that develops with advanced age, in the absence of overt infection, and may contribute to clinical manifestations of other age-related pathologies. Inflammaging is thought to be caused by a loss of control over systemic inflammation resulting in chronic overstimulation of the innate immune system. Inflammaging is a significant risk factor in mortality and morbidity in aged individuals.