Beyond Tissue Injury—Damage-Associated Molecular Patterns, Toll-Like Receptors, and Inflammasomes Also Drive Regeneration and Fibrosis

Abstract

Tissue injury initiates an inflammatory response through the actions of immunostimulatory molecules referred to as damage-associated molecular patterns (DAMPs). DAMPs encompass a group of heterogenous molecules, including intracellular molecules released during cell necrosis and molecules involved in extracellular matrix remodeling such as hyaluronan, biglycan, and fibronectin. Kidney-specific DAMPs include crystals and uromodulin released by renal tubular damage. DAMPs trigger innate immunity by activating Toll-like receptors, purinergic receptors, or the NLRP3 inflammasome. However, recent evidence revealed that DAMPs also trigger re-epithelialization upon kidney injury and contribute to epithelial-mesenchymal transition and, potentially, to myofibroblast differentiation and proliferation. Thus, these discoveries suggest that DAMPs drive not only immune injury but also kidney regeneration and renal scarring. Here, we review the data from these studies and discuss the increasingly complex connection between DAMPs and kidney diseases.

Toxins, ischemia, and trauma trigger inflammation just like pathogens, but why? Inflammation was first defined by rubor, calor, dolor, tumor, and function laesa, which all represent responses of the body to injury.¹ The discovery of pathogenic bacteria as a cause of inflammation 150 years ago led to the assumption that pathogens trigger most forms of inflammation. The fields of immunology and microbiology evolved and generated the popular concept that the immune system developed from the everlasting competition between host and pathogens, with inflammation being the battlefield.² However, this concept did not cover numerous clinical observations, including toxin-, ischemia-, or trauma-related inflammation as well as autoimmune or autoinflammatory disorders.³ Nephrologists should have been at the forefront of this debate because sterile inflammation drives the majority of kidney disorders.^4–6 But how do sterile injuries trigger kidney inflammation?

The last decade revealed that injured cells release intracellular molecules that activate innate immunity just like pathogen-associated molecular patterns (PAMPs).⁷ Accordingly, such molecules were named damage-associated molecular patterns (DAMPs). PAMPs and DAMPs activate identical pattern recognition receptors including Toll-like receptors (TLRs) and inflammasomes, a process that induces kidney inflammation and immunopathology.^8–11 This review provides an update on the different modes of DAMP generation and how this contributes to tissue remodeling in kidney disease.

DAMP Generation inside the Kidney

DAMP Release from Dying Cells

Cell death may or may not activate immunity.¹² Apoptosis is a silent cell death because apoptosis maintains membrane integrity and DAMP release (Figure 1). Apoptosis is important for homeostatic cell clearance (e.g., of autoreactive lymphocytes during negative selection in the thymus or of senescent blood cells).¹² Apoptosis involves a complex series of signaling events to induce surface expression of find-me and eat-me signals that foster phagocytic clearance.¹³ Uptake of apoptotic cells settles the phagocyte’s host defense modus and rather induces an anti-inflammatory phenotype, which enforces homeostasis.^14,15 It has long been thought that tissue injury also involves apoptotic cell death, often based on terminal deoxynucleotidyl transferase–mediated digoxigenin-deoxyuridine nick-end labeling positivity, but this also identifies DNA breaks during cell necrosis or DNA repair.¹⁶ In fact, it has now become evident that injury-induced cell death mostly involves regulated necrosis (Figure 1).¹⁷ For example, necroptosis is a receptor-interacting serine/threonine-protein kinase RIP1/RIP3–mediated form of necrosis that is triggered by genotoxic stress as well as by ligands to TLR3/TLR4 and surface receptors of the TNF receptor superfamily.¹⁸ Necroptosis was documented to contribute to acute tubular necrosis in several AKI models.^19–23 Cyclophilin D–mediated disruption of the mitochondrial transmembrane potential is another form of regulated necrosis involved in postischemic AKI.²⁰ Remarkably, mice lacking both RIP3 and cyclophilin D no longer develop AKI, even upon extended times of renal ischemia.²⁰ Ferroptosis is a glutathione peroxidase 4–mediated form of regulated necrosis specifically triggered by oxidative stress.²⁴ Some types of regulated necrosis seem restricted to immune cells such as NETosis, a controlled explosion of activated neutrophils. NETosis supports bacterial entrapment and killing during host defense.²⁵ NETosis has not yet been demonstrated in infective pyelonephritis but occurs in renal vasculitis with crescentic GN.²⁶ Pyroptosis is a caspase-1– dependent and caspase-11–dependent necrosis of infected macrophages.^27,28 Currently, no functional in vivo data document pyroptosis contributing to kidney inflammation.^29,30 All of these forms of necrosis have the potential to release DAMPs from different intracellular compartments into the extracellular space (Figure 1, Table 1).

Figure 1.

The type of cell death defines DAMP release. Most cells of the body undergo periodic replacement by renewal just like the blood cells. In addition, lymphocyte maturation involves induced cell death to sort out cells with autoreactivity. All such cells undergo apoptosis, a programmed form of cell death that maintains inner and outer membranes to avoid DAMP release. By contrast, cell death induced by injury leads to necrosis. Because of the numerous types of injury (genotoxic stress, toxins, cytokines, oxidative stress), several pathways exist to induce programmed necrosis. Some seem to occur only in specific cell types such as neutrophils (N) or macrophages (MØ). The graph lists essential signaling molecules that trigger this form of cell death. Necrosis implies the disruption of inner and outer membranes, which leads to the release of intracellular DAMPs from various compartments as listed on the right. U1snRNP, U1 small nuclear ribonucleoprotein; NADPH, NAD phosphate dehydrogenase; CYPD, cyclophilin D, GPX-4, glutathione peroxidase 4; HSP, heat shock protein.

Table 1.

DAMP effects in kidney disease (models) (receptor studies not included)

DAMP	Kidney Disease	Receptor and Effects	References
Intracellular DAMPs
Nucleus
HMGB1	Sepsis, AKI	TLR2/TLR4-induced inflammation	142–145
Histones	Sepsis, AKI	TLR2/TLR4-induced inflammation	89,100
DNA/RNA/U1snRNP	IC-GN, inflammation in HD	TLR3-induced mesangial cell activation TLR3/TLR7/TLR9 DC and B cell activation	156,157,169,170,172–177
Mitochondrial
DNA	—	TLR9	178
ATP	Fibrosis, hypertension, metabolic syndrome	P2X7-mediated renal inflammation	95,179–181
Cytosol
Uric acid	DN	NLRP3	82,182
S100A, S100B	DN	RAGE-induced inflammation	183
Heat shock proteins	—	TLR2/TLR4-induced inflammation	184
Extracellular DAMPs
Biglycan	Renal fibrosis, lupus nephritis, DN, GN	NLRP3-, P2 X7-induced inflammation,	38,42,95,165
		DC/macrophage activation via TLR2/TLR4	185,186
	Sepsis	TLR2/TLR4, NLRP3-, P2X7	90,185,187
Decorin	Sepsis	TLR2/TLR4	78
Fibrinogen	FSGS, MPGN	Podocyte activation via TLR2/TLR4	90,187,188
	Interstitial fibrosis	Fibroblast activation via TLR2TLR4
Fibronectin	—	TLR4	189
Hyaluronan	—	TLR2/TLR4-induced inflammation, not confirmed in mesangial cells	124,189–191
	AKI, lupus nephritis	TLR4/CD44	6,163,192
	DN	—	163
Heparan sulfate	DN, CKD	TLR4-induced DC maturation	62, 193,194
Versican	CKD	TLR2	195,196
Amyloid-β	—	NLRP3-activation in DCs/macrophages	197
HDL	CVD in uremia in absence of uremia	Endothelial dysfunction and inflammation via TLR2	198–200
Crystals	Oxalate nephropathy	NLRP3-activation in renal DC	84,97,171
	Adenine nephropathy	TLR2/4-NLRP3 activation
Kidney-specific DAMPs
Uromodulin	AKI	?, cytokine+DAMPs clearance from kidney?	80,81,201
	?	TLR4 and NLRP3-induced inflammation

IRI, ischemia-reperfusion injury; IC-GN, immune complex GN; HD, hemodialysis; RAGE, receptor for advanced glycation end products; MPGN, membranoproliferative GN; EDA, extra domain A; CVD, cardiovascular disease.

DAMP Generation during Extracellular Matrix Remodeling

The extracellular matrix (ECM) is another source of DAMPs.³¹ The renal ECM consists of collagens and elastic fibers, proteoglycans, hyaluronan (HA), and assorted glycoproteins, which undergo a constant turnover.^31–36 Enzymatic degradation can turn immunologically quiescent ECM components into fragments that ligate TLRs, purinergic receptors, inflammasomes, or integrins of infiltrating and resident renal cells (Table 1).^33,37–45 As a second mechanism, macrophages and renal resident cells stimulated by TGF-β^46–50 and proinflammatory cytokines^42,51 de novo synthesize soluble DAMPs^38,42,52 (Figure 2). For example, HA exists under physiologic conditions as a polymer with a high molecular mass of ≥10⁶ Da. HA accumulates in kidneys during AKI,^53,54 allograft rejection,^54,55 interstitial nephritis,^54,56 and lupus nephritis.^54,57 During inflammation and fibrosis, HA is depolymerized by hyaluronidases, which generates low molecular weight fragments that interact with TLRs and the NLR family, pyrin domain–containing 3 (NLRP3) inflammasome (Figure 2, Table 1).^33–35 Inflammation also breaks down the glycosaminoglycan heparan sulfate (HS).⁵⁸ Heparanase-mediated HS degradation is a key process in diabetic nephropathy^59–61 and CKD⁶² (Table 1). However, the majority of extracellular DAMPs^63–67 are released by matrix metalloproteinases.^40,68–72 Among those, the TGF-β–binding small leucine-rich proteoglycans biglycan and decorin, in their soluble form, can promote sterile as well as pathogen-induced inflammation.^33,37,40,73 Numerous inflammatory and fibrotic kidney disorders are associated with decorin and biglycan induction.^{33,37,38,40,57,74,75}. Transient overexpression of soluble biglycan in mice demonstrates that biglycan triggers inflammation in healthy kidneys and potentiates renal inflammation and fibrosis (Table 1).^38,74–76 Finally, there is overwhelming evidence for an antifibrotic activity of soluble decorin directly interacting with another crucial members of the TGF-β superfamily, such as connective tissue growth factor (CTGF), and inhibiting apoptosis of renal tubular epithelial cells (TECs) via the IGF type I receptor/Akt-signaling pathway (Figure 2).^{33,48,49,73,77,78}

Figure 2.

ECM-related DAMPs are secreted or originate from ECM cleavage. Under tissue stress or injury, resident cells (e.g., TECs and fibroblasts) release MMPs and HYALs to cleave ECM components. Soluble biglycan and decorin and low molecular weight HA act as extracellular DAMPs by interacting with TLR2/TLR4 on the surface of infiltrating mononuclear and resident renal cells. In mononuclear cells, this leads to the activation of NF-κB and production of proinflammatory cytokines and chemokines that, in turn, recruit additional mononuclear cells to the side of injury. Moreover, soluble biglycan and low molecular weight HA activate the NLRP3 inflammasome. Proinflammatory cytokines stimulate mononuclear and renal resident cells to de novo produce DAMPs, thereby creating a positive feedback loop that amplifies the inflammatory response. Besides acting as a DAMP, decorin signals via IGF-IR/Akt in TECs and protects them from apoptosis. LMW, low molecular weight; MMP, matrix metalloproteinase; HYAL, hyaluronidase; IGF-IR, IGF type 1 receptor; PKB, protein kinase B.

Kidney-Specific Modes of DAMP Generation

Tamm–Horsfall protein (renamed uromodulin) is an adhesive, particle-forming protein that is exclusively secreted at the thick ascending limb of the distal tubule. Its adhesive nature coats all particles in the distal tubule such as cells (forming cellular casts) and cell debris (granular casts), crystals (supporting crystal aggregation), bacteria (supporting their clearance), and even serves as a sink for inflammatory cytokines, albumin, and so forth.⁷⁹ Uromodulin is immunologically inert inside the tubular lumen. Tubular injury, however, allows uromodulin leakage into the interstitial compartment where it turns into a DAMP that activates interstitial dendritic cells via TLR4.⁸⁰ The particle nature of uromodulin fosters phagocytosis and endosomal destabilization in dendritic cells, a process that activates the NLRP3 inflammasome resulting in the release of IL-1β.⁸¹ This mechanism is another avenue for activating innate immunity in tubular injury (Figure 3).

Figure 3.

Crystals and uromodulin act as DAMPs to induce renal inflammation. Crystals precipitate in the proximal tubule, the distal tubule, and/or in the interstitial compartment of the kidney. Crystals kill TECs. In addition, crystals can be taken up by interstitial dendritic cells via phagocytosis. Lysosomal leakage and potassium efflux (not shown) provide a signal to activate the NLRP3 inflammasome, which cleaves caspase-1 and subsequently pro-IL-1β and pro-IL-18 (not shown). IL-1β ligates the IL-1 receptor (IL-1R) on renal parenchymal cells as well as immune cells, which triggers NF-κB–dependent cytokine and chemokine release. In distal tubule injury, uromodulin leakage into the interstitial compartment activates dendritic cells via TLR4 and the NLRP3 inflammasome. As uromodulin also binds to crystals, crystal precipitation in the distal tubule is likely to involve both mechanisms.

The kidney is a preferred site of particle formation (e.g., from anorganic mineral-related crystals or proteins aggregates such as myoglobin and light chains). Crystals or crystalline proteins act as DAMPs by activating the NLRP3 inflammasome in dendritic cells and macrophages.⁸² In the distal tubule, uromodulin coats crystals and can trigger immune activation via the aforementioned mechanism.⁸³ Some disorders, including oxalosis, involve diffuse crystallization in the renal interstitium, in which dendritic cells pick up crystals into lysosomal compartments and again activate the NLRP3 inflammasome (Figure 3).⁸⁴ The same mechanism is likely to also contribute to renal inflammation and tubular injury in tumor lysis syndrome and other crystalline nephropathies.⁸³

DAMP Effects on Kidney Injury and Repair

DAMPs Activate Systemic Alloimmunity and Autoimmunity

Circulating DAMPs activate pattern recognition receptors on immune cells in the circulation or in lymphoid organs.^2,7 By activating these receptors, DAMPs mimic PAMPs and convert tolerogenic immune responses into immunogenic immune responses. In particular, the activation of antigen-presenting cells such as dendritic cells and B cells enhances antigen presentation, expansion of antigen-specific lymphocyte subsets, and antibody production.^2,7 This process plays an important role in kidney diseases involving alloimmunity and autoimmunity such as kidney transplantation, immune complex GN, or ANCA vasculitis.^8,85–87

How DAMPs Activate the Kidney’s Innate Immune System

Immunostimulatory “Autoadjuvant” Effects

DAMPs activate TLRs on renal parenchymal cells as well as on resident and infiltrating immune cells (for a review of the involved signaling pathways, see refs.^11,88). TLR-mediated cell activation involves the induction of NF-κB–dependent inflammatory cytokines and chemokines in all cells (Figures 4 and 5). Additional cell type–specific effects include the upregulation of adhesion molecules, increased vascular permeability in renal endothelial cells, and filtration barrier dysfunction in podocytes.^89–93 Resident and infiltrating mononuclear phagocytes also host the NLRP3 inflammasome that integrates DAMP signals such as ATP, histones, HA, biglycan, crystals, or uromodulin into the activation of caspase-1.^81,94–97 Caspase-1 and caspase-11 confer proteolytic cleavage of pro-IL-1β and pro-IL-18 into the mature cytokines, which are then secreted and trigger local inflammation inside the kidney (reviewed in ref.¹⁰).

Figure 4.

DAMP effects in immunopathology. DAMPs activate several classes of pattern recognition receptors, which all induce an immediate activation of innate immunity (i.e., systemic and tissue inflammation). Most DAMPs activate TLR2 and TLR4 at the cell surface. In particular, particles enter the cells via phagocytosis and trigger assembly and activation of the NLRP3 inflammasome. Nucleic acid–related DAMPs activate TLR7 and TLR9 in intracellular endosomes. All pattern recognition receptors finally drive the secretion of proinflammatory cytokines that then activate cytokine receptors on the same cell or on other cells. Extracellular histones also directly kill (endothelial) cells. The molecular mechanism of this process is poorly defined but may involve surface charge. Certain nuclear DAMPs also act as autoantigens in systemic lupus and contribute to lupus nephritis. Their adjuvant-like ability to also activate antigen-presenting cells via TLR7 and TLR9 strongly promotes autoimmunization and the expansion of autoreactive lymphocytes. Local release within the kidney also promotes intrarenal autoantigen recognition (e.g., by circulating autoantibodies and subsequent immune complex GN; not shown). LMW, low molecular weight; HSP, heat shock protein; MAL, MyD88 adaptor–like; IRF, IFN regulatory factor; TRAP, TNF receptor–associated protein; TRAF, TNF receptor–associated factor; TRIF, TIR domain–containing adapter inducing IFN-β; ssRNA, single-stranded RNA; dsDNA, double-stranded RNA; DC, dendritic cell;ASC, apoptotic speck protein; IRAK, IL-1 receptor–associated kinase; TIRAP, Toll IL-1 receptor domain–containing adaptor protein; TCR, T-cell receptor; IFNAR, IFN-α receptor; IL-R, IL receptor; CCR, CC-chemokine receptor.

Figure 5.

How DAMPs trigger immune injury, regeneration and fibrosis. Early injury-induced necrosis leads to DAMP release by renal endothelial and epithelial cells. These DAMPs activate pattern recognition receptors such as TLRs or inflammasomes in intrarenal mononuclear phagocytes such as dendritic cells (DCs) or macrophages. Upon activation, these cells produce inflammatory cytokines and chemokines that cause further renal cell necrosis, (e.g., by TNF-induced necroptosis). This autoamplification loop of injury and inflammation is further accelerated by chemokine-driven leukocyte influx (not shown). DAMP release by necrotic cells also triggers regenerative mechanisms, directly and indirectly. Certain DAMPs activate TLR2 on renal progenitor cells, which accelerates tubular repair. In addition, TLR4 activation of renal dendritic cells triggers IL-22 release, which specifically activates the IL-22 receptor on TECs and accelerates tubular re-epithelialization. DAMPs also trigger fibrosis by activating pericytes, fibroblasts, and mesangial cells (latter not shown). TLR activation induces NLRP3 and ASC expression, which are needed for SMAD2 phosphorylation as a critical step in TGF-β receptor signaling. This was DAMPs drive the transition into myofibroblasts, proliferation, and ECM secretion. The same process also triggers TGF-β receptor–dependent epithelial-mesenchymal transition of renal epithelial cells. ASC, apoptotic speck protein; EMT, epithelial-mesenchymal transition; EC, endothelial cell.

DAMPs with Direct Killing Effects

Histones release not only activates TLR2, TLR4, and the NLRP23 inflammasome (Figure 4),^89,94,98,99 but extracellular histones also elicit direct toxic effects on vascular endothelial cells.^89,100 For example, histone injection into the renal artery results in widespread renal necrosis, which is only partially prevented in TLR2/TLR4-deficient mice.⁸⁹ Whether histone-induced cell death is a passive or regulated form of necrosis (or both) is unknown to date.

DAMPs as Autoantigens

Some DAMPs are important autoantigens that drive intrarenal autoimmune disease. For example, when nucleosomes and double-stranded DNA (dsDNA) are released from glomerular cells, they can promote glomerular binding of lupus autoantibodies that trigger lupus nephritis (Figure 4).^89,101–103 Neutrophils undergoing NETosis can also be an important source of the DAMPs in lupus.^104,105 In lupus, secondary necrosis of apoptotic cells exposes hypomethylated dsDNA or U1 small nuclear ribonucleoprotein to TLR7 and TLR9 in dendritic cells and B cells, which drives RNA and DNA autoantibody production, systemic inflammation, and lupus nephritis.

How DAMP Signaling Drives Kidney Regeneration

Recent data now suggest that DAMPs can accelerate tubule regeneration upon injury either in a direct or indirect manner.¹⁰⁶ Renal progenitor cells are scattered along the thick limbs of the proximal and distal tubule segments of the human kidney.^106–112 These cells have a higher stress resistance as terminally differentiated TECs and preferentially survive tubule injury.¹⁰⁷ TLR2-agonistic DAMPs enhance the clonal expansion and differentiation of these progenitor cells within the tubule, which accelerates tubule regeneration (Figure 5).^113,114 As a second mechanism of DAMP-driven kidney regeneration, TLR4-agonistic DAMPs activate interstitial dendritic cells and macrophages to release IL-22.¹¹⁵ IL-22, in turn, activates the IL-22 receptor, which is exclusively expressed on TECs (Figure 5). IL-22 receptor signaling accelerates tubule re-epithelialization from surviving TECs in the recovery phase of AKI.¹¹⁵ Whereas TLR4 blockade in the injury phase abrogates AKI by preventing immunopathology, TLR4 blockade in the regeneration phase delays tubule recovery.¹¹⁵ This dual role of TLR4 agonistic DAMPs in AKI illustrates that DAMPs confer not only immune injury but also wound healing as part of danger control.¹¹⁶ Thus, DAMPs that ligate TLR2 or TLR4 drive kidney regeneration.

How DAMP Signaling Affects Kidney Fibrosis/Sclerosis

Renal fibrosis is considered an aberrant form of injury- or stress-induced wound healing accompanied by excessive ECM deposition.^31,117,118 Leukocytes secrete growth factors and matrix metalloproteinases that activate mesangial cells and interstitial fibroblasts to secrete ECM components.^117,119 In addition, ECM-related DAMPs link renal inflammation and fibrogenesis by ligating TLRs, integrins, purinergic receptors, and inflammasomes, a process that also leads to the release of fibrogenic cytokines¹¹⁸ and chemoattractants^63,120 (Figure 4). It is of particular interest that although intracellular and extracellular DAMPs are structurally unrelated, they often signal via TLR2/TLR4 as shared receptors (Table 1), thereby resulting in different downstream events depending on the initial trigger.³⁷ Histones use TLR2/TLR4 to activate MyD88, NF-κB, and mitogen-activated protein kinase signaling and induction of IL-6 and TNF-α.⁸⁹ Low molecular weight HA acts via TLR2/MyD88/IL receptor–associated kinase/protein kinase C-ζ or TLR4/MyD88 to trigger the production of chemokine ligands CCL3, CCL4, CXCL1, CCL5, and CCL2, as well as IL-12, IL-8, and TNF-α,^121–123 although this could not be confirmed in mesangial cells.¹²⁴ HS signals through the TLR4/MyD88 pathway and promotes dendritic cell maturation and production of proinflammatory IL-6 and IL-12.¹²⁵ HS also induces the release of IL-1α, IL-1β, IL-6, and TNF-α in macrophages.¹²⁶ Biglycan and decorin, endogenous ligands of TLR2/TLR4, activate the mitogen-activated protein kinase p38, extracellular signal-regulated kinase, and NF-κB pathways, leading to the secretion of TNF-α, pro-IL-1β, and a series of chemoattractants for neutrophils, macrophages, and T/B lymphocytes in either a MyD88- or TIR domain–containing adapter inducing IFN-β–dependent manner.^{38,40,42,74,75,78} Importantly, both small leucine-rich proteoglycans are orchestrating signaling pathways of diverse receptors probably in a hierarchical manner to sequentially induce signaling pathways.^37,39,43 For example, biglycan clusters TLR2/TLR4 with the purinergic P2X₇ and P2X₄ receptors, which activates NLRP3⁹⁵ (Figure 4, Table 1). The NLRP3 inflammasome triggers secretion of IL-1β and IL-18, which are involved in renal fibrogenesis.^127–130 NLRP3 activation in TECs also drives epithelial-mesenchymal transition during progressive renal fibrosis,¹³¹ which is associated with progressive renal fibrosis. For example, Nlrp3^−/− mice display reduced tubular injury and interstitial fibrosis upon unilateral ureteral ligation compared with wild-type animals.¹³¹ Another study found an effect on early renal vascular permeability, rather than fibrosis, in this model.¹³² A role for NLRP3 in fibrosis may not necessarily involve only canonical (caspase/IL-1β/IL-18–dependent) inflammasome activation,^98,133 because NLRP3 has additional (noncanonical) roles in SMAD2/SMAD3 phosphorylation of the TGF-β1 receptor signaling pathway.¹³⁴ Whether this mechanism also operates within fibroblasts (as proposed in Figure 5) remains speculative at this point.

However, DAMP signaling in fibrosis is not only restricted to inflammatory pathways. ECM-related DAMPs also modulate crosslinking of matrix components and cytokine signaling in fibrogenesis. For example, the HS-proteoglycan, syndecan interacts via its HS chains with transglutaminase type 2, an enzyme that promotes ECM crosslinking in fibrosis.¹³⁵ Consistently, the lack of syndecan protects from renal fibrosis.¹³⁵ In addition, fibronectin can induce fibroblast differentiation via interaction with α₄β₇ integrin.¹³⁶ As another avenue of DAMPs regulating fibrogenesis, decorin sequesters TGF-β in the ECM¹³⁷ or compete with TGF-β for receptor binding.¹³⁸ Decorin also inhibits CTGF signaling in fibroblasts.⁷⁷ In addition, decorin downregulates microRNA miR-21,⁷⁸ which promotes interstitial fibrosis.¹³⁹ Together, DAMPs also directly modulate ECM crosslinking and renal fibrogenesis (e.g., by modulating TGF-β and CTGF signaling).

DAMPs in Distinct Kidney Disorders

There is currently mostly experimental evidence for a role of DAMPs in kidney disease, as listed in detail in Table 1. Data obtained from mice deficient for DAMP receptors provide only indirect evidence on the role of DAMPs; hence, we do not further discuss this here because numerous reviews on this topic exist.^8,10,140,141

AKI

Tubular Necrosis

Among kidney disorders, AKI is most frequently associated with cell necrosis, which implies DAMP release in acute tubular necrosis. For example, septic, ischemic, or toxic forms of tubular necrosis involve the release of histones and high-mobility group protein B1 (HMGB1), which drive the associated sterile inflammatory and immunopathology that determines organ failure.^{89,100,142–146} In addition, lethality in sepsis relates to the release of HMGB1, histones, decorin, or biglycan.^{42,78,100,144} This process seems to preferentially involve DAMP-mediated endothelial dysfunction via TLR2/TLR4, which increases vascular permeability, shock, and hypoperfusion.^89,100,147 HMGB1 also can facilitate ischemic preconditioning, which implies that HMGB1 exposure protects from subsequent postischemic AKI.¹⁴⁸ This TLR2-mediated process involves the upregulation of Siglec as one of many counterregulatory mediators that limit DAMP-related immunity just like endotoxin tolerance.¹⁴⁹ In addition, NLRP3-deficient mice were protected from postischemic AKI.^150,151 However, apoptotic speck protein or caspase-1 deficiency as well as IL-1/IL-18 blockade were not protective, largely excluding an inflammasome-related role of NLRP3 in postischemic AKI and arguing for yet unknown noncanonical effects.¹⁵²

GN

Necrotizing GN is another form of AKI that involves DAMP release from renal cells.¹⁵³ Data on necrotizing GN are currently limited to models induced in TLR2/TLR4-deficient mice or to serum and tissue expression levels of HMGB1 that correlate with disease activity.^154,155 However, histones are released from netting neutrophils in necrotizing GN,²⁶ and contribute to glomerular necrosis in crescentic GN in a TLR2/TLR4-dependent manner (S. Kumar, unpublished data). Nuclear DAMPs also contribute to lupus nephritis by activating TLR7 and TLR9 outside and inside the kidney,^86,156–158 as well as by activating dendritic cells and B cells, which enhances local as well as systemic autoimmunity.¹⁵⁹

CKD

Diabetic Nephropathy

Inflammatory responses mediated by activation of TLR2, TLR4, and NLRP3 inflammasome play key roles in the progression of diabetic nephropathy (DN).^160,161 Under diabetic conditions, NLRP3 is activated by intracellular reactive oxygen species or extracellular ATP.^160,162 Furthermore, hyperglycemia evokes the expression of the ATP receptor P2X₄ in renal TECs of patients with type 2 DN, and this correlates with IL-1 cytokine release.¹⁶⁰ The production of another proinflammatory molecule (i.e., HA) is also induced by hyperglycemia through a protein kinase C/TGF-β pathway.¹⁶³ Besides HA, several DAMPs, including the glucose-inducible HMGB1,¹⁶⁴ biglycan, and decorin, are overexpressed in diabetic kidneys and may trigger inflammation by activating TLR2/TLR4 receptors and NLRP3 inflammasomes.^47,51,165

Increased renal biglycan levels correlate with enhanced infiltration of macrophages and renal LDL accumulation, and appear to promote kidney injury. By contrast, because of its antiapoptotic effects on TECs and ability to neutralize TGF-β1 and CTGF, decorin is nephroprotective in DN.⁵¹ Whereas HA accumulation is considered as a marker of renal damage during DN and is potentially involved in the development of interstitial fibrosis,¹⁶³ high molecular weight-HA reduces diabetes-induced renal injury.^32,34,166 Thus, decorin and low molecular weight versus high molecular weight HA, orchestrating signaling of various receptors, appear to act in diabetic kidneys in a scenario more complex than canonical DAMPs.

Lupus Nephritis

Lupus nephritis involves immune activation by nuclear DAMPs that share autoantigen and autoadjuvant qualities.¹⁵⁹ Ribonucleoproteins activate TLR7 and hypomethylated dsDNA activates TLR9 (Table 1), which enhances autoantigen presentation and the expansion of autoreactive lymphocytes.⁸⁶ In addition, these TLR7- and TLR9-specific DAMPs trigger plasmacytoid dendritic cells to release IFN-α, which initiates antiviral gene transcription accounting for many of the unspecific (viral infection-like) symptoms of lupus¹⁶⁷ and IFN-related glomerular pathology.¹⁶⁸ Furthermore, TLR7- and TLR9-specific DAMPs activate intrarenal macrophages and dendritic cells toward an M1 phenotype, a process that accelerates immunopathology in lupus nephritis.^156,169,170 In addition, DAMP ligands of TLR2/TLR4 accelerate renal damage in lupus nephritis.³⁸ Biglycan also aggravates lupus nephritis.³⁸ Transient overexpression of soluble biglycan induces CCL2, CCL3, and CCL5 and aggravates murine lupus nephritis, whereas its deficiency suppresses disease activity and renal damage.³⁸

Crystalline Nephropathies

Crystal formation within the kidney creates a DAMP that has the potential to activate renal inflammation and immunopathology via the NLRP3 inflammasome (e.g., in renal dendritic cells).^82,83 For this to occur, crystals need to reach the interstitial compartment, which happens during diffuse crystallization or upon tubular injury from inside the tubular lumen. NLRP3 activation results in IL-1β and IL-18 secretion, which initiates downstream inflammatory effects by activating their respective IL receptors.⁸⁴ This was consistently demonstrated in animal models of acute and chronic oxalate nephropathy as well as adenine overload-induced CKD (Table 1).^84,97,171

Summary

Tissue injury emits DAMPs as danger signals to activate danger control (e.g., inflammation for host defense). DAMPs can either be intracellular molecules that signal cell necrosis (HMGB1, histones, ATP), matrix constituents that signal extensive matrix remodeling (decorin, HA, HS, biglycan), or luminal factors that signal barrier destruction (uromodulin). DAMPs activate TLRs, purinergic receptors, and inflammasomes in parenchymal cells and leukocytes. DAMP-induced inflammation initiates an autoamplification loop by triggering regulated forms of necrotic cell death, which aggravates immunopathology and further DAMP release. Hence, blocking DAMP signaling in the early injury phase of acute disorders limits immunopathology. However, DAMPs have additional effects. Certain nuclear DAMPs (RNA/DNA) combine autoadjuvant and autoantigen qualities and thereby promote systemic lupus and lupus nephritis. TLR2-agonistic DAMPs activate renal progenitor cells to regenerate epithelial defects in injured tubules. TLR4-agonistic DAMPs induce renal dendritic cells to release IL-22, which also accelerates tubule re-epithelialization in AKI. Finally, DAMPs also promote renal fibrosis by inducing NLRP3, which also promotes TGF-β receptor signaling. It is likely that more exciting discoveries are to be made in this area.

Disclosures

None.