Of Inflammasomes and Alarmins: IL-1β and IL-1α in Kidney Disease

Abstract

Kidney injury implies danger signaling and a response by the immune system. The inflammasome is a central danger recognition platform that triggers local and systemic inflammation. In immune cells, inflammasome activation causes the release of mature IL-1β and of the alarmin IL-1α. Dying cells release IL-1α also, independently of the inflammasome. Both IL-1α and IL-1β ligate the same IL-1 receptor (IL-1R) that is present on nearly all cells inside and outside the kidney, further amplifying cytokine and chemokine release. Thus, the inflammasome-IL-1α/IL-β-IL-1R system is a central element of kidney inflammation and the systemic consequences. Seminal discoveries of recent years have expanded this central paradigm of inflammation. This review gives an overview of arising concepts of inflammasome and IL-1α/β regulation in renal cells and in experimental kidney disease models. There is a pipeline of compounds that can interfere with the inflammasome-IL-1α/IL-β-IL-1R system, ranging from recently described small molecule inhibitors of NLRP3, a component of the inflammasome complex, to regulatory agency–approved IL-1–neutralizing biologic drugs. Based on strong theoretic and experimental rationale, the potential therapeutic benefits of using such compounds to block the inflammasome-IL-1α/IL-β-IL-1R system in kidney disease should be further explored.

The immune system has a central role in maintaining homeostasis and in regaining homeostasis after injury. Infectious and noninfectious triggers of injury have an identical capacity to initiate inflammation, e.g., a gouty or bacterial arthritis both present as clinically indistinguishable acute joint inflammation. In the last 15 years the research community has unraveled the molecular mechanisms of danger signaling but this area remains a source of unexpected discoveries. Numerous data have accumulated since the Journal of the American Society of Nephrology published a first overview about inflammasomes in kidney disease in 2011.¹ This brief review provides an update on inflammasome biology and extends the discussion to the alarmin IL-1α. A comprehensive view on the expression and biologic effects of the inflammasome-IL-1 axis in renal cells and its functional contribution to experimental and human kidney disease is required to appreciate the potential of inflammasome-IL-1–related drugs in this evolving area of translational nephrology.

Update on IL-1 Biology

Since its first cloning in 1984 the knowledge on the biology of the IL-1 family of cytokines has expanded to considerable complexity as reviewed elsewhere in detail (Figure 1).^2–4 As the system is of enigmatic importance for the regulation of systemic and tissue inflammation, pro- and anti-inflammatory factors balance the system at all levels, i.e., the receptor ligands, the transmembrane cell surface receptors, and the signaling pathways (Figure 1).^4,5

Figure 1.

The families of IL-1 cytokines and cytokine receptors activate innate and adaptive immunity. Activated or dying cells release all sorts of cytokines of the IL-1 family that specifically interact with several transmembrane surface receptors present on most cell types of the body. Some induce cell activation (IL-1R, IL-18R, IL-36R), others inhibit cell activation (IL-33R, TIGIRR, SIGIRR). This way the family elicits numerous regulatory effects on renal cells, immune cells of the innate and adaptive immune system, either activating or inhibiting their respective cell type–specific functions. AG, antigen; G-CSF, granulocyte colony-stimulating factor; MФ, macrophage; NETs, neutrophil extracellular traps; NK cell, natural killer cell; TIR domain, Toll/interleukin-1 receptor (TIR) homology domain.

IL-1α

IL-1α is constitutively present in keratinocytes and other epithelia including tubular epithelial cells, whereas macrophages, granulocytes, endothelial cells, fibroblasts, and mesangial cells express the IL-1α precursor only upon activation.⁵ The 31 kDa IL-1α precursor lacks a signal peptide fragment and is already biologically as active as the processed 18 kDa “mature” form.⁶ Therefore, cells constitutively expressing IL-1α were considered “a loaded gun” that can at any time release a proinflammatory alarmin upon cell necrosis.^6–8 This way necrotic cells alert surrounding tissues and set up local tissue inflammation.

IL-1β

IL-1β is not constitutively expressed and its secretion is largely restricted to circulating monocytes that are activated for enzymatic cleavage of the inactive 266 amino acid precursor into the 153 amino acid mature form of IL-1β.⁵ Therefore, IL-1β mainly contributes to systemic inflammation by initiating acute phase response proteins in the liver such as C-reactive protein, by activating endothelial cells, by triggering fever, by causing neutrophil mobilization from the bone marrow (leukocytosis), and by activating all classes of leukocytes and renal cells (Figure 1).⁹ Within tissues, mainly resident dendritic cells and infiltrating macrophages and neutrophils can release large amounts of IL-1β, whereas parenchymal cells may release only small amounts under certain circumstances.⁴

Enzymatic Processing

Pro–IL-1α is a substrate for the calcium-dependent, nonlysosomal cysteine protease calpain (Figure 2),^5,10,11 but little is known about how external triggers regulate calpain activity. Calcium release from intracellular stores seems sufficient to activate calpain-driven pro–IL-1α processing.¹⁰ Necrotic cells release both the 31 and 18 kDa forms of IL-1α passively from intracellular stores to alarm surrounding cells.⁶ In those cells that coexpress IL-1α and IL-1β, IL-1α secretion can also be inflammasome-dependent, implying that IL-1α and IL-1β are released together.¹²

Figure 2.

Inflammasome activation in dendritic cells and macrophages involves numerous elements. Dendritic cells first need to induce the expression of the inflammasome components and of pro–IL-1α and pro–IL-1β. This can occur via cytokine receptors or Toll-like receptors (TLRs). Activation of inflammasome assembly can occur upon numerous intracellular danger signals such as mitochondrial reactive oxygen species release, lysosomal protease leakage, and potassium efflux or calcium influx. The multiprotein inflammasome complex forms a wheel-like structure to trigger caspase-1–driven IL-1β (and IL-18) enzymatic activation. Inflammasome fibrils grow in size to a single macromolecular complex called ASC speck. Calcium activates calpain, which cleaves pro–IL-1α to IL-1α, but in contrast to pro–IL-1β, IL-1α is already biologically active, and hence an alarmin. IL-1α, IL-1β, and IL-18 together with other NF-κB–dependent cytokines and chemokines activate cytokine and chemokine receptors in an autocrine, paracrine, or systemic manner. Noncanonical inflammasome signaling, e.g., triggered by cytosolic LPS, involves caspase-11 (mice) and caspase 4/5 (humans), which cleave gasdermin D. The cleaving product activates NLRP3. Noncanonical inflammasome signaling is a recognized trigger for pyroptosis, an immunogenic form of cell death that leads to the release of numerous intracellular components that have the capacity to activate a plethora of pattern recognition receptors and close the vicious cycle of necroinflammation. CCL, CC chemokine ligand; CCR, CC-chemokine receptor.

Pro–IL-1β is a true precursor that requires enzymatic processing for activation. Extrinsic or intrinsic danger signals that initiate the canonical inflammasome–dependent activation of caspase-1 have been extensively studied in mononuclear phagocytes.¹³ In other immune cells other proteases may activate IL-1β, e.g., proteinase-3 in neutrophils, granzyme A in NK cells, chymase in mast cells, and metalloendopeptidase meprin A in epithelial cells.^5,14 Inflammasomes are cytosolic wheel-like complexes composed by the assembly of NACHT, LRR and PYD domains-containing proteins (NLRP), the linker molecule apoptosis-associated specklike protein (ASC), and caspase-1 (Figure 2).¹⁵ Different inflammasomes are defined by the different NLRs that serve as sensors for intracellular danger signals.¹⁵ Whereas the NLRC4, NLRP6, -7, -12, or AIM2 inflammasomes recognize preferably pathogen-associated molecular patterns (PAMPs), NLRP1 and NLRP3 are more promiscuous and have the capacity to translate a large variety of different cytosolic danger signals into the caspase-1–dependent secretion of IL-1β.¹⁵ Many of these may also occur during kidney injury, such as mitochondrial release of reactive oxygen species and pore forming toxins, ATP, or osmotic pressure that affect intracellular potassium and calcium concentrations.^15–17 Other danger-associated molecular patterns (DAMPs) that contribute to NLRP3-driven renal inflammation include uromodulin particles,¹⁸ biglycan,¹⁹ extracellular histones,²⁰ oxalate, or urate crystals that destabilize lysosomes for cathepsin leakage into the cytosol like other phagocytosed microparticles.^21–23 Cytosolic LPS is a known trigger for noncanonical inflammasome signaling involving caspase-11 in mice and caspases-4 and -5 in humans.^24–27 Caspase-11/4/5 activation leads to cleavage of the cytosolic protein gasdermin,^24,28 and gasdermin’s N–terminally cleaved product p70 subsequently activates the NLRP3 inflammasome for caspase-1–dependent pro–IL-1β processing.²⁴

Unlike macrophages, dendritic cells do not constitutively express pro–IL-1β and the inflammasome components. Dendritic cells first require a priming signal via Toll-like receptors or cytokine receptors that induce NF-ĸB–dependent transcription of NLRP3, ASC, caspase-1, pro–IL-1α, and IL-1β.¹⁵ It is of note that also other enzymes have been found to cleave pro–IL-1β, such as serine proteases in neutrophil granules.²⁹

The activation of the NLRP3 inflammasome is tightly regulated.³⁰ A20-driven ubiquitination of NLRP3, PYRIN domain-only protein POP1-dependent inhibition of inflammasome assembly, or reactive oxygen species–driven suppression of caspase-1 are just three such regulatory mechanisms.^31–33

IL-1 Receptor

IL-1R1 is expressed on nearly all cells and hardly regulated.⁴ Its activation induces the release of the soluble IL-1 receptor antagonist that can neutralize the biologic effects of IL-1α and IL-1β.^9,34 IL-1R2 has the same inhibitory effect.^4,35 IL-1R1 ligation signals via the adaptor protein MyD88 to the kinases IRAK-2 and -4 and uses TRAF6 to involve NF-ĸB, p38, Janus kinase, and ERK for initiating the transcription of inflammatory cytokines including pro–IL-1α and pro–IL-1β.⁴

Consequences of Inflammasome Activation or NLRP3/ASC Induction Other Than IL-1 Release

Inflammasome activation also induces the secretion of mature IL-18 (Figure 1).³⁶ IL-18 is clearly expressed by tubular epithelial cells in addition to infiltrating immune cells,³⁷ but if the tubular expression of IL-18 is inflammasome-dependent or not and its contribution to kidney injury remains under debate.^38,39 The strong induction of IL-18 inside the kidney upon injury has raised much attention as IL-18 could be a useful urinary biomarker of kidney injury as discussed in detail elsewhere.⁴⁰ Inflammasome activation by intracellular bacteria or LPS can induce necrotic cell death, referred to as pyroptosis.^41,42 In murine macrophages this process is largely caspase-11–dependent (caspase-4/5 in humans).^24,26 Pyroptosis re-exposes intracellular bacteria to extracellular host defense elements and increases local inflammation by DAMP and alarmin release including IL-1α (Figure 2).⁴¹ Whether NLRP3 agonists other than cytosolic LPS induce pyroptosis in cells other than macrophages is still under debate but human immunodeficiency virus infection depletes CD4 T cells in a caspase-1–dependent manner.⁴³ Recently, it was shown that Syk- and Jnk-dependent Tyr144 phosphorylation of ASC induces ASC speck aggregate formation.⁴⁴ ASC specks grow to fibril-like macromolecular structures inside the cytosol that are eventually released into the extracellular space, e.g., upon pyroptosis.^45,46 Such extracellular ASC specks elicit DAMP-like proinflammatory effects conceptually similar to crystals and other microparticles once they get phagocytosed and induce lysosomal leakage and activate the NLRP3 inflammasome of the phagocyte.^46,47 Finally, several studies suggest that NLRP3 and ASC have other, inflammasome-independent biologic functions, such as facilitating the anti-inflammatory and profibrotic effects of TGF-βR signaling.^48,49

Data in Renal Cells: Expression, Release

Kidney injury exposes renal cells to numerous inflammasome activators, such as ATP, uric acid, histones, uromodulin, oxalate or cystine crystals, and matrix degradation products.^{17,18,20,50–53} But are the inflammasome components expressed at all in the kidney and, if so, do they contribute to canonical inflammasome signaling^54,55? IL-1α and IL-1β are both induced by NF-ĸB signaling but still display heterogeneous expression patterns in solid organs of healthy mice.⁵⁶ Especially IL-1β can be induced in tubular epithelial cells in kidneys of young mice and the expression of both IL-1α and IL-1β increases with age.⁵⁶ Essential elements of the NLRP3 inflammasome have meanwhile been found to be expressed in most renal parenchymal cell types (Table 1) but their functional roles inside the kidney and species-specific differences remain under debate.^57–59 For example, albumin overload induces NLRP3 expression in proximal tubular cells of rats and in cultured HK-2 cells, and IL-1β release could be demonstrated but only in cell lines in vitro.⁵⁹ Indeed, the roles of NLRP3, ASC, and caspase-1 for canonical inflammasome signaling in macrophages and dendritic cells are consistent in literature.²¹ The published data become conflicting when reporting pro–IL-1β processing and IL-1β release from renal parenchymal cells.^60,61 Only a few studies demonstrate the secretion of mature IL-1β from immune cell–free tissue specimens or from nonimmune renal cell cultures by ELISA and western blot. Those that do, obtain inconclusive results.^60–64 Convincing evidence was demonstrated for renal fibroblasts.⁶⁵ Others conclude on inflammasome activation from immunostaining or transcript analysis,^66–68 which is inadequate given the post-transcriptional nature of inflammasome activity. Eventually, confocal laser microscopy might be a feasible way to prove intrarenal inflammasome activation by documenting ASC speck formation,^46,69 but ASC speck complexes have not yet been reported inside the kidney. In fact, recent reports on renal cell or kidney tissue NLRP3/ASC immunostaining depict diffuse cytoplasmic positivity rather than ASC specks. In addition, Nlrp3/Asc−/− control sections were not used to prove specificity of immunolabeling.^66–68 However, renal parenchymal cells may process pro–IL-1β into mature IL-1β via the metalloendopeptidase meprin A.^5,14 Data on caspase-11 and renal cell pyroptosis are still sparse. Yang, et al. proposed tubule cell pyroptosis to occur upon ischemia-reperfusion injury based on an association with caspase-11 activity,⁷⁰ but a functional role of caspase-11 was not proven and others did not find any effect on postischemic tubular necrosis with pan-caspase inhibitor treatment.^71,72 IL-1α has potent immunostimulatory effects on renal cells⁷³ but little is known about the capacity of renal parenchymal cells to release IL-1α from intracellular stores and elicit alarmin-like effects on surrounding cells.⁶

Table 1.

Expression and function of inflammasome components in renal parenchymal cells

Component	NLRP3, ASC Expression	Speck Formation	IL-1β Release	Pyroptosis	IL-1α Release
Endothelial cells	Yes61,112	?	61	?	?
	No63
Mesangial cells	No63	?	—	?	113
	Yes113,114
Podocytes	Yes61,66,95	?	Yes61,115	?	?
	No63		No63
Tubular epithelial cells	Yes49,59,62,76,78,116–119	?		Yes70
			No21,62	No71
Fibroblasts	?	?	65	?	65

?, unknown; —, absent.

Role in Animal Models of Kidney Disease

AKI Models

Necroinflammation occurs mainly in AKI, e.g., in infective pyelonephritis, thrombotic microangiopathy, necrotizing GN, and tubular necrosis (Figure 3).⁷⁴ Up to now only the latter two have been studied using mice deficient for Nlrp3, Asc, Casp1/11, Il1α, or Il1β (Table 2). Whenever tested, mutant mice were consistently protected from renal necroinflammation.^21,75 However, postischemic tubular necrosis depends on NLRP3^76–79 but not on ASC.⁷⁶ This finding could imply an additional, inflammasome-independent, biologic effect of ASC in postischemic AKI. Studies performed on the model of acute oxalosis suggest inflammasome signaling to be restricted to intrarenal dendritic cells.²¹ The independent contribution of IL-1α to AKI has so far been addressed only by Lee et al.⁸⁰ Il1α-deficient mice were protected from cisplatin-induced AKI but, unexpectedly, markers of inflammation were not different from wild-type control mice.⁸⁰ However, not all studies document a consistent blockade of intrarenal inflammation and tubular necrosis in postischemic or toxic AKI in models upon blockade of the IL-1 axis.^81–83 These findings may imply that IL-1 is not a universal mediator of AKI and its role is context-dependent.

Figure 3.

The inflammasome/IL-1 system contributes to renal necroinflammation. Necroinflammation can occur in the glomerulus (A) or the tubulointerstitial compartment (B). The primary event can be intravascular NETosis, as in ANCA vasculitis, or renal cell necrosis, such as in ischemic tubular injury. Histone, DAMP, and alarmin (IL-1α) released from dying cells activate the NLRP3 inflammasome (IL-1α/β release) and induce IL-1R signaling, which implies local inflammation. Especially histones also kill other cells, resulting in a crescendo of tissue inflammation and necrosis, i.e., necroinflammation. The consequences in the glomerulus and the tubules are illustrated. Both lead to a drop out of nephrons and the clinical syndrome of AKI.

Table 2.

AKI models in Nlrp3-, Asc-, Casp1/11-, Il-1α­–, and Il-β–deficient mice

Disease Model	Gene	Inflammation	Renal Function	Reference
Severe GN
Heterologous anti-GBM GN	Nlrp3−/−	—	—	63
	Asc−/−	—	—	63
	Casp1/11−/−	—	—	63
Autologous anti-GBM GN	Nlrp3−/−	↓	Better	86
	Asc−/−	↓	Better	86
	IL1β−/−	↓	Better	84,85
Sepsis
LPS-induced	Casp1/11−/−	↓	Better	120
Tubular necrosis
Ischemia- reperfusion	Nlrp3−/−	—	Better	76,77,79
	Asc−/−	—	—	76
	Casp1/11−/−	↓	Better	121
	IL1α−/−	↓	?	122
	IL1β−/−	↓	?	122
Cisplatin	Nlrp3−/−	—	—	79
	Asc−/−	?	?	123
	Casp1/11−/−	↓	Better	118
	IL1α−/−-	—	Better	80
Rhabdomyolysis	Nlrp3−/−	↓	Better	75
	Asc−/−	↓	Better	75
	Casp1/11−/−	↓	Better	75
	IL1β−/−	↓	Better	75
Acute oxalosis	Nlrp3−/−	↓	Better	21
	Asc−/−	↓	Better	21
	Casp1/11−/−	↓	Better	21

Anti-GBM GN, anti-glomerular basement membrane GN; —, absent; ↓, suppressed; ?, unknown.

Models of acute glomerular injury pose new questions. Autologous anti-GBM disease is a model of GN triggered by in situ immune complex formation. Ten years after Timoshanko et al. had demonstrated that autologous anti-GBM GN is attenuated in Il1β-deficient mice,^84,85 Andersen et al. identified the NLRP3/ASC inflammasome as the mechanism of glomerular IL-1β production in this model.⁸⁶ However, Lichtnekert et al. could not find any phenotype of mice deficient for Nlrp3, Asc, and Casp1/11 with heterologous anti-GBM GN, a model with little leukocyte infiltration.⁶³ In fact, in 2004 Timoshanko et al. already identified infiltrating leukocytes as the only source of glomerular IL-1β release.⁸⁵ Altogether, AKI involves canonical inflammasome–mediated IL-1β secretion mostly from the tubulointerstitial network of dendritic cells and infiltrating leukocytes as part of the autoamplification loop of necroinflammation (Figure 3). The role of leukocyte pyroptosis, NETosis, and IL-1α in this context remains poorly studied. In ANCA vasculitis neutrophil serine proteases such as cathepsin G, elastase, and proteinase 3 mediate intrarenal IL-1β processing.²⁹

CKD Models

Glomerular pathology drives CKD progression in most cases, diabetic nephropathy being the clinically most prevalent disease category (Figure 4). Shahzad et al. provided a comprehensive data set documenting a role of the NLRP3/ASC inflammasome for the progression of STZ-induced diabetic nephropathy (Table 3).⁶¹ Their analysis also included bone marrow transplant studies documenting a significant contribution of NLRP3-mediated caspase-1 activation inside nonimmune cells to podocyte loss, albuminuria, and glomerulosclerosis.^60,61 As their experiments involved conventional knockout mice it remains unclear if this relates to NLRP3 in nonimmune cells in- or outside the kidney. Reports on other CKD models such as hypertensive nephrosclerosis, Western diet-, toxin-, or oxalate crystal–induced nephropathy or ureter obstruction were only conducted using Nlrp3-deficient mice, but consistently displayed a protected phenotype (Table 3). Bone marrow–derived vehicle cells overexpressing IL-1Ra suppress interstitial inflammation in obstructive nephropathy.⁸⁷ However, in lupus nephritis of C57BL/6-(Fas)lpr mice lack of NLRP3 and ASC consistently presented an aggravated phenotype.⁴⁸ As the same was not found for lack of IL-1R or IL-18 an inflammasome-independent effect was postulated.⁴⁸ For example, NLPR3 and ASC mediate TGFR-dependent SMAD phosphorylation,⁴⁸ a signaling pathway known to suppress lupus nephritis via the known immunoregulatory effects of TGF-β signaling on autoimmunity.⁸⁸

Figure 4.

The inflammasome/IL-1 system promotes CKD. CKD is devoid of tissue necrosis and currently there is little evidence for renal cell necroptosis or other forms of regulated cell necrosis being involved in CKD progression. There are data supporting a role of the NLRP3 inflammasome and IL-1 in CKD but the mechanisms are unclear. Most likely systemic release of IL-1, e.g., in diabetes or systemic lupus, contributes to systemic endothelial dysfunction (ED), a process also promoting leukocyte adhesion and vascular leakage (microalbuminuria) in the kidney. Furthermore, intrarenal inflammasome activation in immune cells, and eventually also in renal parenchymal cells, may contribute to local inflammation, cell stress, and cell loss. For example, podocyte detachment promotes albuminuria, hypertrophy of the remaining podocytes, FSGS, and subsequently first focal-global and later diffuse glomerulosclerosis-related nephron loss. In this process the inflammasome components NLRP3 and ASC may contribute to SMAD phosphorylation downstream of TGFR signaling during epithelial-mesenchymal transition (EMT) of parietal epithelial cells (PEC), and induce extracellular matrix production by tubular epithelial cells (not depicted). Whether the same also occurs in interstitial fibroblasts is currently unknown (not depicted). A role of NLRP3 and ASC has also been reported on systemic autoimmunity where that assures the immunosuppressive effect of TGFR signaling on immune cells (not depicted).

Table 3.

CKD models in Nlrp3-, Asc-, caspase-1/11–, Il-1α–, and Il-β–deficient mice

Disease Model	Gene	Inflammation	Renal Function	Fibrosis	Reference
Glomerulopathy
Lupus nephritis	Nlrp3−/−	↑	Worse	?	48
	Asc−/−	↑	Worse	?	48
	Casp1/11−/−	↑	Worse	?	48
Diabetic nephropathy	Nlrp3−/−	↓	Better	↓	61
	Asc−/−	↓	Better	↓	61
	Casp1/11−/−	↓	Better	↓	61
Toxic glomerulopathy	Nlrp3−/−	↓	Better	↓	124
Hypertensive nephrosclerosis	Asc−/−	↓	?	↓	125
Western diet nephropathy	Nlrp3−/−	↓	?	↓	106
Tubulopathy
Obstructive nephropathy	Nlrp3−/−	↓/—	N/A	↓/—	62,126
Chronic oxalosis	Nlrp3−/−	↓	Better	?	107

↑, increased; ?, unknown; ↓, suppressed; —, absent; N/A, not applicable.

IL-1–Related Drugs and Clinical Data

The relevance of the NLRP3 inflammasome for human disease becomes obvious from the cryopyrin-associated periodic syndrome (CAPS), a group of genetic disorders of periodic fever and multifocal tissue inflammation caused by gain-of-function mutations in the NLRP3 gene.⁸⁹ Genetic variants in IL-1–related genes associate with the risk for ESRD.^90,91 The pathogenic concept of CAPS was verified by dramatic clinical responses to IL-1 inhibitors such as the IL-1β neutralizing antibody canakinumab (Ilaris), the soluble decoy receptor rilonacept (Arcalyst), and the recombinant human IL-1Ra (Kineret).⁸⁹ Important for nephrologists is renal amyloidosis secondary to CAPS or any other hereditary systemic (auto-)inflammatory disorder that responds well to therapeutic IL-1 inhibition (Table 4).^92–96 Numerous other IL-1β–, IL-1α–, or IL-1R1–specific biologic drug candidates and one oral caspase-1 inhibitor are currently in clinical trials.⁷ In 2013, canakinumab was approved in Europe for preventing remittent gouty arthritis based on its capacity to suppress painful inflammation in acute gout attacks faster than intramuscular steroid injection.⁹⁷ Unlike most other drugs used in gout, canakinumab can also be used in patients with CKD.⁹⁸ Interestingly, renal function seems to improve when gout is treated with IL-1 inhibition.⁹⁹ Also, safety and pharmacokinetics of rilonacept are not affected by CKD or even ESRD.¹⁰⁰ Now, this finally allows for the testing of the old theory that IL-1 induction seen in hemodialysis patients contributes to dialysis-related systemic inflammation and complications.¹⁰¹ A first interventional study showed that anakinra can significantly improve C-reactive protein, IL-6, and serum albumin levels in patients on hemodialysis.¹⁰² These data create hopes that blocking IL-1–driven systemic inflammation could improve nutritional status and body wasting, and eventually dampen accelerated cardiovascular disease in ESRD.¹⁰³ A trial with monoclonal anti–IL-1β IgG gevokizumab in diabetic kidney disease is ongoing.¹⁰⁴ Clinical trials testing whether IL-1 blockade can also improve outcomes in diseases such as GN, vasculitis, or AKI are still awaited.

Table 4.

Clinical reports and ongoing trials of IL-1 blockade in patients with presumed or manifest kidney disease

Drug	Disease	Trial Phase	NCT Identifier	Reference
Canakinumab	CAPS-related (renal) amyloidosis	Case reports		93
	Gout in CKD	3	NCT01029652	97
			NCT01080131
	Proliferative diabetic retinopathy	1	NCT01589029
Anakinra	CAPS-related (renal) amyloidosis	Case reports		93
	Gout in CKD	2–3	NCT02578394
	Inflammation in CKD	2	NCT00420290	102
	Nutrition, insulin resistance, and inflammation in CKD	2	NCT02278562
	Resistance and inflammation in CKD	2	NCT02278562
	Henoch–Schönlein purpura	Case report		127
Rilonacept	Vascular dysfunction in CKD	2a	NCT01663103
	Inflammation in CKD	2a	NCT00897715
Gevokizumab	Type 2 diabetic kidney disease	2a	EudraCT2013–003610–41	104

NCT, ClinicalTrials.gov identifier; EudraCT, European Union Clinical Trials register.

Perspectives for Targeting the Inflammasome-IL-1 Axis in Kidney Disease

Which types of human kidney disease would be eligible for IL-1 blockade? In analogy to its clinical effectiveness in gout and the role of the NLRP3-IL-1 axis in crystal- and microparticle-related tissue inflammation, IL-1 blockade may also be instrumental in crystalline nephropathies. Primary hyperoxaluria, cystinosis, light chain–related Fanconi syndrome, fibrillary GN, or crystalloglobulinemia are all as rare as CAPS but secondary oxalosis-related AKI is more common.¹⁰⁵ Timing is probably an issue, because the diagnosis of AKI, based on its current definition, is not recognized before the kidney is largely destroyed. Therefore, IL-1 blockade might be more feasible in chronic disorders for which some experimental evidence exists such as primary hyperoxaluria or diabetes.^61,106,107 The ongoing Canakinumab Anti-Inflammatory Thrombosis Outcomes Study aims to evaluate whether IL-1β inhibition can reduce rates of recurrent myocardial infarction, stroke, and cardiovascular death among high risk patients with persistent elevations of CRP.¹⁰⁸

Several small molecule–based NLRP3 antagonists have been validated in preclinical studies. Arglabin, a sesquiterpene lactone from the Chinese herb Artemisia myriantha, can specifically inhibit cholesterol crystal–induced IL-1β, but not IL-6 and IL-12, production in vitro and markedly reduce vascular wall inflammation and atherosclerosis in ApoE2.Ki mice.¹⁰⁹ Coll et al. described MCC950 as a selective small molecule inhibitor of the NLRP3 (but not the AIM2, NLRC4, or NLRP1) inflammasome, which can attenuate experimental multiple sclerosis and a mouse model of CAPS.¹¹⁰ The ketone body and short-chain fatty acid β-hydroxybutyrate is another compound that can block NLRP3-IL-1–mediated inflammatory disease.¹¹¹ β-hydroxybutyrate can be administered in a low carbohydrate ketogenic diet and can block murine CAPS or uric acid crystal–induced peritoneal inflammation. Now it is necessary also to test these compounds in animal models of kidney disease.

Summary and Perspective

Inflammasomes turn a broad variety of danger signals into the release of inflammatory cytokines that rapidly set off inflammation for host defense. Upon activation, the inflammasome assembles to a single ASC speck macromolecular complex per cell that can induce canonical caspase-1 or noncanonical caspase-11/5 activation. The subsequent release of cytokines and immunogenic cell death promotes local and systemic inflammation. Inside the kidney mainly myeloid cells express the NLRP3 inflammasome and secrete IL-1 upon activation but renal parenchymal cells may contribute in certain settings. In many cases, IL-1α release is probably more relevant inside the kidney, whereas IL-1β elicits systemic inflammation. In addition, NLRP3 and ASC have biologic effects unrelated to IL-1 release, e.g., they promote STAT phosphorylation downstream of the TGF-βR, which implies anti-inflammatory and profibrotic effects. As IL-1–dependent inflammation is involved in the pathogenesis of numerous acute and chronic kidney diseases, the inflammasome-IL-1 axis is an evolving therapeutic target. Nephrologists can already use IL-1 antagonists for patients with CAPS-related amyloidosis and relapsing gout. More clinical trials are ongoing that hold the perspective for broader clinical applications, e.g., in diabetes, cardiovascular disease, or CKD-related systemic inflammation. Whether the inflammasome-IL-1 axis holds further promises to better control intrarenal inflammation in immune-mediated kidney diseases remains to be explored.

Disclosures

None.