Introduction

Heart failure (HF) is a complex clinical condition characterized by abnormal heart structure and/or function, and confirmed by elevated natriuretic peptide levels and/or objective evidence of pulmonary or systemic congestive [1]. The morbidity and mortality of HF will further increase with the aging of the population and the continuous rise of the number of cardiovascular diseases. The latest epidemiological survey data revealed that nearly 6.2 million adults suffer from HF in America and more than 37.7 million individuals worldwide [2, 3]. The prevalence of HF among Chinese patients is 1.3% (about 13.7 million patients), and the total prevalence has increased by 44% in the last 15 years [4]. HF is the fatal and terminal stage of numerous cardiovascular diseases. These patients have a high risk of rehospitalization, poor quality of life, and a heavy economic burden. It is one of the most important chronic cardiovascular diseases of the twenty-first century.

The basic pathological mechanism of the occurrence and development of HF is ventricular remodeling. Inhibiting the excessive activation of neuroendocrine has become the therapeutic basis for delaying and reversing ventricular remodeling. Therefore, the treatment of HF mainly starts by improving symptoms, enhancing the quality of life, improving prognosis, preventing and delaying ventricular remodeling, and reducing the rates of rehospitalization and mortality [5]. At present, the main therapeutic regimes for HF treatment include antagonizing sympathetic and neuroendocrine anti-heart failure drugs, cardiac resynchronization therapy (CRT), implantable cardioverter-defibrillators (ICDs), ventricular assist devices, and heart transplants. Among them, the drug treatment of HF has changed from the original “Golden Triangle”—angiotensin-converting enzyme inhibitor (ACEI)/angiotensin receptor neprilysin inhibitor (ARNI), beta-blockers, and mineralocorticoid receptor antagonists (MRA)—to “New Quadruple”—ACEI/ARNI, beta-blockers, MRA, and sodium-glucose cotransporter 2 inhibitor (SGLT2i). Currently, the treatment of chronic heart failure (CHF) has stepped into a new era of multi-mechanism and multi-target, bringing more benefits to the majority of HF patients [6]. Although remarkable achievements have been made in the pathological mechanism and prevention of HF in recent years, the overall prognosis of HF is poor. A study showed that the 1-year rehospitalization rates of inpatients and stable HF patients were 44% and 32%, respectively, and the mortality of HF patients during hospitalization was still as high as 4.1% [7, 8]. How to seek new therapeutic targets for HF, further reduce the mortality and rehospitalization rate, ameliorate symptoms, and improve the prognosis of patients has become the hotspot of modern medical research and a great challenge for global medicine. As we all know, in addition to the classic neurohumoral mechanism and sympathetic nervous system mechanism, inflammation takes on a significant influence on the development and prognosis of HF. As the key note of the chronic inflammatory response, nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) is the inflammasome with the most definite structure and function at present, which is expected to provide a new intervention mediator for the therapy of HF [9, 10]. This paper synopsizes the research progress of NLRP3 inflammasome in HF, to provide scientific reference for clinical prevention and treatment of HF.

The Importance of Inflammation in Heart Failure

Inflammation is the organism’s first immune defense against harmful stimuli, which occurs when the organism receives exogenous infection and endogenous cell tissue damage [11, 12]. Various inflammatory cytokines released by inflammatory cells induce cardiomyocyte apoptosis, decrease myocardial systolic and diastolic function, further reduce cardiac ejection and filling capacity, cause ventricular remodeling, and then exacerbate the deterioration of HF. Elevated inflammatory biomarkers are a hallmark feature of CHF and can better predict prognosis in patients with HF [13]. Many pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6), and C-reactive protein (CRP) are elevated in HF patients, and elevated levels of them are strongly associated with severity and poor prognosis of HF [14]. These cytokines can regulate ventricular remodeling, apoptosis, myocardial fibrosis, and other structural changes. TNF-α is a pro-inflammatory cytokine that has been implicated in the pathogenesis of HF. TNF-α can promote myocardial cell protein synthesis, weaken myocardial contractility, and promote cell pyroptosis and myocardial hypertrophy, thereby accelerating the process of HF [15]. A CHF immunomodulation trial improved patient clinical endpoints by reducing TNF-α levels, suggesting that elevated TNF-α levels in HF patients were associated with worse outcomes [16]. Animal studies also showed that the TNF-α was significantly elevated in aortic tissue in the HFpEF pig model [17]. A substantial body of experimental evidence indicates that IL-1 is involved in the pathogenesis of HF and the occurrence of systolic dysfunction [18]. It has been reported that IL-1β can accumulate myocardial collagen, increase reactive oxygen species, and accelerate the process of HF in rats with HF [19]. However, the potential effectiveness of IL-1 targeting in patients with HF has not yet been determined. IL-6 is derived from monocytes, which can increase the production of intracellular superoxide anion (O2 -), reduce the expression of nitric oxide (NO), induce endoplasmic reticulum stress, and cause cardiomyocyte apoptosis. Clinical studies have confirmed that IL-6 promotes cardiomyocyte apoptosis, the expression of IL-6 mRNA in cardiomyocytes in patients with HF increases, and the level of plasma IL-6 increases [20]. CRP is a sensitive marker of the nonspecific inflammatory response, which can bind to receptors on monocytes, macrophages, and neutrophils, and promotes phagocytosis by activating the classical complement pathway [21]. The level of hs CRP in HF patients is higher than that of healthy people, and the level of hs CRP is positively correlated with the severity of inflammation [22].

Furthermore, other inflammatory markers such as nuclear factor κB (NF-κB) [23] and monocyte chemoattractant protein 1 (MCP-1) [24] have been reported. A large amount of evidence above suggests that immune and inflammatory responses are taking part in the pathological process of CHF via inflammatory cytokines.

NLRP3 Inflammasome

As the critical first line of defense against exogenous or endogenous stimuli, the innate immune system can recognize danger signals and initiate immune defense reactions in the hosts. The inflammasome is an intracellular pattern recognition receptor that discriminates various stimuli such as pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Signals that induce immune and inflammation-related gene expression play a major role in defending against infection and maintaining immune homeostasis [25]. Inflammasomes include nucleotide-binding oligomerization domain-like receptors (NLRs) family and pyrin and HIN domain-containing (PYHIN) family. Among the NLRs family, the NLRP3 inflammasome is the most widely and comprehensively characterized. NLRP3 inflammasome is a significant innate immune sensor that recognizes a variety of unsafe signals, induces inflammatory responses, and participates in the immune response of the body [26].

Structure and Function

NLRP3 inflammasome is a complex composed of a variety of proteins, mainly composed of receptor protein NLRP3 (a sensor), apoptosis-associated speck-like protein containing a CARD (ASC, an adaptor), and pro-caspase-1 (an effector) [27]. The NLRP3 protein includes three regions; the C-terminal leucine-rich repeat domain (LRR) can recognize and bind PAMPs and DAMPs. The central terminal is the nucleotide-binding oligomerization domain (NOD), responsible for mediating nucleic acid ligation, and protein oligomerization. And an N-terminal effector domain consists of a caspase recruitment domain (CARD) or a pyrin domain (PYD), which is responsible for transmitting the signal downstream. The N-terminal PYD and the C-terminal CARD form the adaptor protein ASC, which connects NLRP3 and caspase-1. Pro-caspase-1 is the precursor molecule of the effector protein caspase-1, which generates active caspase-1 through self-cleavage, which promotes the maturation and secretion of IL-1β and IL-18 [28,29,30].

Activation Mechanism

Normally, NLRP3 inflammasome activation takes two steps: The first is to upregulate the expression of each component and downstream factors. DAMPs or PAMPs activate TLRs located on the cell membrane so that nuclear factor-κB (NF-κB) can upregulate the NLRP3 protein expression at the transcriptional level [31]. The second step is to activate NLRP3 to promote the assembly of multi-protein complexes. The receptor protein recognizes PAMPs or DAMPs through LRR, and through PYD and the PYD site of ASC is connected to form a complex next. Subsequently, ASC recruits pro-caspase-1 through CARD to activate the effector protein [32]. After the effector protein is activated, dependent on its proteolytic hydrolysis, active caspase-1 cleaves into pro-IL-1β and pro-IL-18, promotes the maturation and secretion of downstream IL-1β and IL-18, and initiates the downstream inflammatory response [33, 34]. Additionally, activated caspase-1 cleaves and decomposes GSDMD (gasdermin D), making it into pores in the cell membrane, inducing cell rupture, and triggering pyroptosis [35]. Currently, it is considered that the activation pathways of the NLRP3 inflammasome mainly include the following three: The first is the efflux of potassium ions. Studies have shown that potassium efflux is a trigger for the activation of the NLRP3 inflammasome. Under the stimulation of extracellular adenosine triphosphate (ATP), intracellular potassium ion is activated by the P2X7 receptor and pannexin-1 pore, causing the activation of NLRP3 inflammasome [36]. The second is the generation of reactive oxygen species (ROS). The NLRP3 ligand thioredoxin-interacting protein (TXNIP) is activated in a ROS-sensitive manner upon ROS accumulation. The binding of free TXNIP to NLRP3 protein makes the NLRP3 inflammasome active. However, some studies have shown that ROS is involved in the sensitization process of NLRP3, but not in the activation of NLRP3 [37]. The third is the lysosome damage. After macrophages phagocytose microbial toxins and other substances, the continuous accumulation causes lysosomal swelling, leakage, and release of contents such as cathepsin B, thereby activating the NLRP3 inflammasome. Furthermore, cathepsin B can also exert downstream effects by activating TXNIP [38, 39]. Thus, investigating the activation pathway of the NLRP3 inflammasome will help to further understand the pathogenesis of HF.

The Role of NLRP3 Inflammasome in the Pathogenesis of Heart Failure

Inflammasome has significant implications for chronic inflammation and affects the process of HF. Activation of NLRP3 inflammasome can stimulate the expression and secretion of IL-1β and IL-18, thereby aggravating the occurrence and development of HF [10].

Experimental Research Progress

Experimental studies suggest that compared to wild-type mice, the levels of NLRP3 and pro-caspase-1 in myocardial tissue, and IL-1β in serum are significantly increased in cardiac-specific transgenic heterozygous (CNTg) mice overexpressing phosphatase. Myocardial inflammation and myocardial contractility are significantly improved in NLRP3-knockout mice with HF and IL-1β antagonist intervention in CNTg mice. This experiment demonstrates that the NLRP3 inflammasome in CNTg mice leads to myocarditis and cardiac systolic dysfunction by activating IL-1β, thereby triggering HF [40]. The experiment carried out two mouse models of HF: pressure overload and LAD ligation; hematopoietic or myeloid Tet2 deficiency exacerbates cardiac remodeling and dysfunction in mice while increasing IL-1 expression. Simultaneously, the therapy with NLRP3 inflammasome inhibitor slowed the HF process, and there is no significant difference in cardiac parameters between Tet2-deficient and wild-type mice [41]. The NLRP3 inflammasome is significantly increased in a model of ventricular hypertrophy induced by transverse aortic constriction (TAC) and is associated with inflammatory mediators, pro-fibrotic factors, and cardiac dysfunction [42]. Expression of NLRP3, IL-1β, and IL-18 in myocardial infarction mice with left coronary artery ligation (CAL) is significantly decreased after MCC950 treatment, indicating that NLRP3 inhibitors can reduce myocardial fibrosis and delay the development of HF after myocardial infarction (MI) [43]. Similarly, it has been shown that coronary ligation and doxorubicin were used to establish ischemic and non-ischemic myocardial injury models in mice, and after intervention with NLRP3 inflammasome inhibitors, the results showed that the left ventricular systolic function of the two myocardial injury models was improved, and protected primary and cultured cardiomyocytes from the deleterious effects of inflammasome activation [44]. Cardiomyocyte calmodulin-dependent protein kinase IIδ (CaMKIIδ)-dependent activation of the NLRP3 inflammasome mediates fibrosis during HF, while NLRP3 knockout also reduces macrophage accumulation, which attenuated the inflammatory response and the development of fibrosis (Table 1) [45].

Table 1 Role of NLRP3 inflammasome in heart failure—experimental research
Full size table

Clinical Research Progress

Clinical studies have found that serum inflammatory factors and chemokines are proportional to the deterioration of cardiac function. Stable coronary artery disease (SCAD) patients with elevated CRP, fibrinogen, and leukocytes are associated with the incidence of HF [46]. A study of 7 patients with HF who received 2-week anakinra (IL-1 antagonist) injections showed that the treatment group significantly increased oxygen consumption, decreased carbon dioxide retention, and improved exercise tolerance, and the serum IL-1β levels decreased by 89% [47]. Comparing the plasma IL-18 levels of 48 CHF patients and 10 healthy people who died unexpectedly, the results described that compared with the normal control group, the plasma IL-18 level in patients with CHF was upwards. After treatment, the IL-18 level with improved symptoms of HF decreased significantly, and the IL-18 level was found to be significantly different in CHF patients with different cardiac functions and increased with the severity of HF [48]. In addition, studies have found that IL-18 levels have no significant correlation with the underlying cause of CHF, but are positively correlated with the severity of CHF [49]. IL-18 may accelerate the progression of HF by promoting myocardial fibrosis [50]. A prospective study of 54 patients with HF showed that patients with HF had a downward trend in ASC methylation levels and IL-1β and ASC mRNA levels trended upward, while the exact opposite results were obtained after exercise. The reason for this result may be that the changes in ASC methylation and expression have a connection to the decline of IL-1β in exercisers [51]. Many studies have presented that targeting the NLRP3 inflammasome and downstream IL-1 and IL-18 therapy has promise as a new direction for the treatment of HF (Table 2).

Table 2 Role of NLRP3 inflammasome in heart failure—clinical trial
Full size table

Therapeutic Strategies for Heart Failure Associated with the NLRP3 Inflammasome

Study shows the NLRP3 inflammasome has been proposed as a potential intervention mediator to treat multiple inflammatory diseases [52]. With further research of its activation mechanism, the development of drugs acting on NLRP3 or related signaling pathways has become a research hotspot of NLRP3 inflammasome inhibitors, and targeting the NLRP3 inflammasome supplies a novel idea for the therapy of HF (Table 3).

Table 3 Therapeutic effect of specific NLRP3 inhibitors in heart failure
Full size table

NLRP3 Inhibitors

Direct Inhibitors

MCC950

MCC950, a diarylsulfonylurea-containing compound, can not only blockade the in-depth handling of IL-1β by caspase-1 but can also block classical and non-classical NLRP3 inflammasome activation and IL-1β secretion by eliminating ASC oligomerization [53]. MCC950 specifically acts on the NLRP3 inflammasome, but not activation of the NLRP1, NLRC4, or AIM [54]. Recent studies have found that MCC950 acts through direct binding to the Walker B motif of the NLRP3 NACHT domain preventing ASC oligomerization and NLRP3 inflammasome composition [54]. Studies have shown that in a porcine MI model, MCC950 can reduce the level of IL-1β in the myocardium, obviously decline the size of MI, and improve cardiac function [55]. MCC950 effectively improves diabetic kidney injury by inhibiting the NLRP3/caspase-1/IL-1β pathway [56]. MCC950 can delay the progression of HF caused by Tet2 mutation in hematopoietic cells by reducing IL-1β levels [41].

OLT1177

OLT1177 is an active β-sulfonyl nitrile molecule, and its two most defined roles are specific inhibition of the NLRP3 inflammasome and reversal of the metabolic cost of inflammation. OLT1177 blocks classical and non-classical of the NLRP3 inflammasome. It also acts directly with NLRP3 and blockades ATPase activity [57]. Meanwhile, OLT1177 cuts down IL-18 and IL-1β secretion, with no effect on NLRC4 or AIM2 inflammasomes. The study found that OLT1177 preserves β-adrenergic responsiveness and prevents left ventricular diastolic dysfunction in a mouse model of non-reperfused anterior MI [58]. At present, OLT1177 is undergoing clinical trials, and phase I trials of OLT1177 in healthy subjects showed good tolerability and a favorable safety profile. OLT1177 reduces joint pain in patients with acute gouty arthritis [59]. Furthermore, it has longer half-lives and the subjects did not show any toxicity (organ/blood) despite the different doses [60]. In a phase 1B double-blind trial, patients with stable HFrEF showed significant improvement in LVEF and exercise time after 14 days of treatment with OLT1177, which was well tolerated and safe [61]. Therefore, OLT1177 has the potential to be developed into NLRP3-targeting therapeutics against various NLRP3-related diseases.

CY90

CY90, as an analog of CFTR(inh)-172 (C172), inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) channel [62]. Studies have shown that CY90 compounds can directly act on the NLRP3 protein and specifically prevent the composition of the NLRP3 inflammasome and the secretion of IL-1β. CY90 achieves the purpose of blocking ATPase activity by directly binding to the NLRP3 Walker A motif, and it does not affect NOD2/RIG, NLRP1, and NLRC4 [63]. After receiving CY90 treatment in diabetic stroke model mice, the cardiac dysfunction of the model mice was significantly improved [64]. Furthermore, CY90 also has good oral bioavailability, safety, and stability, and the drug development targeting CY90 has great potential. However, there is no direct evidence for the relationship between CY90 and HF, and further investigation of its anti-inflammatory effect as a potential NLRP3 inhibitor for the treatment of HF is warranted.

Other NLRP3 direct inhibitors include tranilast [65], 3,4-methylenedioxy-beta-nitrostyrene (MNS) [66], and oridonin [67]. By directly targeting the NLRP3 inflammasome, they can more accurately prevent and treat NLRP3-related diseases.

Indirect Inhibitors

Glyburide

Glyburide is a sulfonylurea drug mainly used to treat type II diabetes, which blockades ATP-sensitive K-channels (KATP) channels by stimulating insulin release [68]. The study found that glyburide can inhibit the NLRP3 inflammasome, thereby reducing the inflammatory response in fetal mice with acute lung injury induced by hyperoxia exposure [69]. Glyburide prevents the activation of the NLRP3 inflammasome and has a specific inhibitory effect on the NLRP3 inflammasome, and makes no difference to other inflammasomes (NLRC4, AIM2, or NLRP1) [68]. Glyburide can treat cardiovascular abnormalities caused by Cantu syndrome by inhibiting the K ATP channel. However, the relationship between glyburide and HF is not yet clear, and further research is needed [70].

16673-34-0

16673-34-0 is an intermediate formed in the synthesis of glyburide, but it lacks cyclohexylurea, which is associated with insulin release, and the lack of the cyclohexylurea moiety is inessential for the inhibition of NLRP3 inflammasome activity. Consequently, it has a specific inhibitory effect on the NLRP3 inflammasome but has no huge influence on glucose metabolism, and can also hinder the aggregation of the structure by interfering with NLRP3 activation or aggregation with ASC [71]. The pericardial effusion and pericardial thickening in experimental pericarditis model mice were markedly decreased by the effect of 16673-34-0 [72]. 16673-34-0 prevents cardiac insufficiency in obese mice induced by a high-sugar, high-fat diet (Western diet), and dietary treatment with 16673-34-0 prevents systolic and diastolic dysfunction [73].

Caspase-1, IL-1, and IL-18 Inhibitors

Direct blockade of caspase-1, IL-1β, and IL-18 can overlap with inhibition of NLRP3. Nevertheless, they are not exclusive to the NLRP3 inflammasome, so blocking caspase-1, IL-1β, and IL-18 may affect the normal physiological function of the organism.

VX-765 is a highly selective caspase-1 inhibitor combined with a P2Y 12 receptor antagonist for the ischemia-reperfusion rat model which can further reduce the size of myocardial infarction and improve cardiac function in rats [74]. VX-765 helps reduce cognitive impairment and AD severity in mice [75]. However, the exact inhibitory mechanism of HF effects is not fully understood, and further studies are required to fully define its inhibitory potential. The inflammatory response induced by cytokines of the IL-1 family further exacerbates the development of HF. Anakinra was previously approved for the treatment of rheumatoid arthritis [76]. Recent studies have shown that in the acute decompensated heart failure (ADHF) patients, acute inflammatory responses were significantly reduced with anakinra initiated within 24 h of admission compared with placebo, and LVEF improved after 14 days of treatment [77]. In another study hospitalized for ADHF, anakinin was taken at discharge. After 12 weeks, it was observed that the expression of N-terminal pro-B-type natriuretic peptide (NT proBNP) decreased, and cardiopulmonary function and quality of life improved [78]. Canakinumab has anti-inflammatory effects and is beneficial for IL-1β-mediated inflammatory diseases, including acute gouty arthritis [79] and type 2 diabetes [80]. A recently published analysis of the CANTOS trial found, when compared with placebo, participants who responded to canakinumab experienced a significant 38% reduction in HF hospitalization and a combined 32% reduction in HF hospitalization and all-cause mortality after IL-1β blockade [81]. IL-18 can promote myocardial fibrosis and may be involved in the occurrence and development of HF [50]. Compared with controls, the infarct size of the ischemia-reperfusion injury mice pretreated with IL-18 antibody was significantly reduced [82]. Recombinant IL-18 binding protein (IL-18BP) can improve right ventricular function in mice with chronic alveolar hypoxia [83]. IL-18BP prevents left ventricular systolic dysfunction in mice treated with plasma from decompensated HF patients [84]

Innovation and Limitations

In inhibiting inflammatory response, identifying and effectively intervening in new inflammatory pathways are one of the important means to delay the development of HF. Due to many redundant and compensatory reactions in this process, anti-inflammatory drug treatment may lead to the impairment of body defense function or the amplification of the inflammatory process. Therefore, targeting specific inflammatory pathways, such as the NLRP3 inflammasome, can more precisely reduce harmful inflammation in HF and protect host defense barriers. Furthermore, this novel therapy that inhibits the NLRP3 inflammasome for the treatment of HF may be a novel mechanism of investigation for many existing drugs, with high applicability in the clinical environment, and may broaden the therapeutic field of HF. Some limitations should be fully considered before recommending NLRP3 inhibitors for clinical practice. First of all, most data on inflammation are obtained in laboratory acute animal models; how to translate experimental animal data into human medicine is still in the research stage, and HF is accompanied by complex multi-system lesions; gender and age differences, early and late differences in inflammation, and so on should be considered, which is difficult to simulate in the laboratory animal model. Second, we tend to overlook whether the detected inflammatory parameters are the cause of cardiovascular pathology or simple markers, and the reliability of these biomarkers in clinical settings or human clinical trials is also worth considering. In addition, correlation does not reflect causality; and finally, how to avoid the side effects of these NLRP3 inhibitors. In conclusion, in-depth studies should be carried out to investigate the exact role of NLRP3 in animal models and HF patients.

Future Development Directions

In recent years, small molecules have been developed as specific NLRP3 inhibitors and have been confirmed in animal and cell experiments, but are less studied in clinical trials and need to be proven effective and safe in more patients. The future research direction should focus on clinical trials. In addition, the future can also focus on the development of compound derivatives with improved pharmacokinetic characteristics to enhance the specificity of targeting NLRP3 inhibitors. Finally, the complex disease of HF cannot adopt a unified treatment method. It is necessary to master individualized strategies, but also to go beyond the heart, focus on comorbidities, and provide new treatment opportunities for the prevention and management of HF.

Conclusions

After the NLRP3 inflammasome receives stimulatory signals, it induces inflammatory responses by upregulating the expression of inflammatory factors such as IL-1β and IL-18. NLRP3 inflammasome is inseparable from the formation and deterioration of HF. Therefore, by intervening in the downstream molecules of NLRP3 inflammasome and its signal transduction pathway, the activation of NLRP3 inflammasome can be effectively inhibited, providing a new idea for the prophylaxis and cure of HF (Fig. 1). Although the research on inflammation-modulating drugs is still in the preliminary stage, it is believed that with advancing science and research, targeted drug therapy for the NLRP3 inflammasome is expected to provide new ideas and new solutions for the diagnosis and treatment of HF.

Fig. 1
figure 1

The NLRP3 inflammasome plays an important role in the pathogenesis of HF, and inhibiting its downstream factors and signaling pathways is expected to provide a new therapeutic target for the treatment of HF. ASC, apoptosis-associated speck-like protein containing a CARD; CARD, caspase recruitment domain; DAMPs, danger-associated molecular patterns; GSDMD, Gasdermin D; GSDMD-NT, N-terminal fragment of Gasdermin D; IL-1β, interleukin-1β; IL-18, interleukin-18; IL-18BP, interleukin-18 binding protein; LRR, leucine-rich repeat domain; NF-kB, nuclear factor κB; NLRP3, nucleotide-binding oligomerization domain-like receptor protein 3; PAMPs, pathogen-associated molecular patterns; Pro-IL-1β, pro-interleukin 1β; Pro-IL-18, pro-interleukin 18; PYD, pyrin domain; TLR4, toll-like receptor4.

Full size image