The role of NLRP3 inflammasome in type 2 inflammation related diseases

Abstract

Type 2 inflammation related diseases, such as atopic dermatitis, asthma, and allergic rhinitis, are diverse and affect multiple systems in the human body. It is common for individuals to have multiple co-existing type 2 inflammation related diseases, which can impose a significant financial and living burden on patients. However, the exact pathogenesis of these diseases is still unclear. The NLRP3 inflammasome is a protein complex composed of the NLRP3 protein, ASC, and Caspase-1, and is activated through various mechanisms, including the NF-κB pathway, ion channels, and lysosomal damage. The NLRP3 inflammasome plays a role in the immune response to pathogens and cellular damage. Recent studies have indicated a strong correlation between the abnormal activation of NLRP3 inflammasome and the onset of type 2 inflammation. Additionally, it has been demonstrated that suppressing NLRP3 expression effectively diminishes the inflammatory response, highlighting its promising therapeutic applications. Therefore, this article reviews the role of NLRP3 inflammasome in the development and therapy of multiple type 2 inflammation related diseases.

Keywords:

• NLRP3
• inflammasome
• type 2 inflammation related diseases

1. Introduction

Type 2 inflammation related diseases is a broad term used to describe a group of diseases that are caused by type 2 inflammation. Clinically, these diseases include asthma, atopic dermatitis (AD), and allergic rhinitis, and affect the respiratory, gastrointestinal, and dermatological systems. They have emerged as chronic conditions that significantly impact human well-being [1–3]. However, the underlying mechanisms of Type 2 inflammation related diseases remain unclear.

An essential member of the NOD-like receptor (NLR) family, NOD-like receptor thermal protein domain associated protein 3 (NLRP3) binds to pro-caspase-1 and an apoptosis-associated speck-like protein (ASC) in the cytoplasm to form an inflammasome [4,5]. Previous studies have shown that NLRP3 plays a role in the development of diseases like atherosclerosis, obesity, and type 2 diabetes. In the context of type 2 inflammation related diseases, NLRP3 has also been found to play a crucial role in disease progression and is a target for many drugs. Despite the increasing attention given to the NLRP3 inflammasome in recent years, there is still a lack of comprehensive review on its role in type 2 inflammatory related diseases [6–10]. Therefore, the article aims to review the significance of NLRP3 inflammasome in type 2 inflammation related diseases and its potential as a therapeutic target.

2. NLRP3 inflammasome activation

NLRP3, encoded by cold induced autoinflammatory syndrome 1 (CIAS1), is a cytoplasmic nod-like receptor (NLR). The NLRP3 inflammasome is formed when the cell is activated along with apoptosis-associated speck-like protein (ASC) and pro-caspase-1 precursor [11–13]. The generation of the NLRP3 inflammasome involves two signals: priming and activation. In the priming signal, cells recognise danger signals from pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) through TLRs, leading to activation of the NF-κB pathway. This results in the up-regulation of various inflammasome-associated components (pro IL-1β, and pro IL-18), including NLRP3 [11,13–18].

The second signal of activation involves the assembly of NLRP3 inflammasomes and the secretion of IL-1β and IL-18 [19–21]. Current studies indicate that agonists do not directly affect the assembly of inflammasomes, but rather they affect NLRP3 through the activation of common pathways. There are three main mechanisms that regulates the activation of NLRP3 inflammasome (Figure 1).

Figure 1. The composition and activation of the NLRP3 inflammasome involve two main signals. In the priming signal, Toll-like receptors (TLRs) recognise pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), leading to the activation of the NF-κB pathway. This activation results in the increased expression of various components associated with the inflammasome. In the activation signal, the NLRP3 inflammasome is triggered by factors such as lysosomal damage, reactive oxygen species (ROS), and potassium efflux. Once activated, the inflammasome activates caspase 1, which then cleaves pro-IL-1β and pro-IL-18, generating active IL-1β and IL-18. Additionally, caspase 1 cleaves gasdermin D (GSDMD), and its N-terminal fragment (GSDMD-N) can create pores in the cell membrane, and induce pyroptosis. PAMPs,pathogen-associated molecular patterns; DAMPs,damage-associated molecular patterns; TLR, Toll-like receptor; IL-1β, interleukin-1β; IL-18, interleukin-18; ASC, associated speck-like protein; P2X7, P2X purinoceptor 7; ROS, reactive oxygen species.

2.1. Mechanisms associated with ion channels and calcium signalling

In studies involving NLRP3 agonists such as ATP, Nigerian bacteriocins, particulate matter, perfringolysin O, and streptolysin O, it has been observed that a decrease in intracellular potassium ion concentration is a critical requirement for NLRP3 inflammasome activation [22,23]. Extracellular ATP can activate the P2X7 receptor (P2X7R), which is an ATP-ligand-gated ion channel located on the cell membrane. This activation allows intracellular potassium ions to enter the extracellular compartment, thereby reducing the intracellular potassium ion concentration [24,25]; Bacterial toxins such as perfringolysin O and streptolysin O can form membrane pores on the cell membrane surface, resulting in potassium ion efflux and inflammasome activation [23,26].

In addition to potassium channels, chloride channels, and calcium signalling may also be involved. In the absence of exogenous ATP, CASR agonists can activate the NLRP3 inflammasome, leading to the release of calcium ions from stores and the production of the inflammasome. At the same time, CASR can also decrease intracellular cyclic AMP (cAMP) levels, which promotes inflammasome production. This is because cAMP can bind to NLRP3 and inhibit inflammasome assembly [27,28]. Furthermore, previous studies have shown that chloride intracellular channel proteins (CLICS) 1-4 are involved in NLRP3 inflammasome activation. These proteins facilitate chloride efflux, which is a process downstream of ROS activation and is associated with the production of IL-1β [29,30].

2.2. Elevated reactive oxygen species (ROS) and mitochondrial damage

The mechanism of ROS action is not fully understood, but it is clear that thioredoxin interacting protein (TXNIP), a ligand for NLRP3, is sensitive to ROS. Under normal physiological conditions, the oxidoreductase thioredoxin (TRX) binds to TXNIP and inhibits its activity. When ROS levels are elevated, the TRX-TXNIP complex dissociates, allowing TXNIP to bind to NLRP3 (mainly the structural domain of LRRs), which in turn activates NLRP3 inflammasome. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, as a common activator of ROS, promotes the activation of inflammasomes in this form [31]. Paraquat and silymarin both act through a similar mechanism to NADPH, with paraquat upregulating the production of ROS and TXINP, inducing the formation of NLRP3 inflammasome and leading to cell death, while silymarin inhibits the toxicity of paraquat by increasing the production of TRX and antioxidant enzymes [32].

Apart from ROS agonists, Rongbin Zhou et al. demonstrated that inhibition of mitosis/autophagy leads to the production and accumulation of ROS. This, in turn, activates the formation of the NLRP3 inflammasome. Conversely, mitochondria are an important source of ROS, dysregulation of mitochondrial activity, achieved by inhibiting voltage-gated anion channels, prevents ROS activation and NLRP3 inflammasome production [33,34]. All of this highlights the importance of ROS production in NLRP3 inflammasome activation.

2.3. Lysosomal damage

Lysosomes, which are organelles found in eukaryotic cells, contain a diverse range of hydrolytic enzymes including protease, nuclease, phosphatase, and lipase. These enzymes are responsible for breaking down biological macromolecules such as proteins, nucleic acids, and polysaccharides. When exogenous substances like silica and asbestos enter the human body, macrophages engulf them and convert them into crystals or particles. These substances can cause damage to lysosomes, leading to the release of cathepsin and activation of NLRP3 inflammasomes. This mechanism of lysosomal damage is also influenced by the efflux of potassium ions and is associated with various particulate matter like uric acid, silica, and cholesterol [35–37].

3. The role of NLRP3 inflammasome in type 2 inflammatory-related diseases

Type 2 inflammation is an adaptive immune response triggered by antigenic stimuli, such as bacteria, viruses, parasites, etc. This response occurs through antigen presentation by dendritic cells (DCs) and activation of ICL2 cells, which induce Th2 lymphocytes to secrete type 2 cytokines like IL-4, IL-5, IL-13, etc. IL-4 prompts B cells to differentiate into plasma cells that produce specific IgE antibodies and bind to high-affinity receptors (FcRI) on the surface of mast cells and basophils, which sensitises the body [2]. Upon exposure to allergens again, IgE recognises antigenic molecules, leading to the activation and degranulation of mast cells and basophils. This process results in the release of enzymes, lipids, various cytokines, and other agents, leading to pathophysiological processes like pruritus, exudation, apoptosis, and allergic diseases such as atopic dermatitis, asthma, allergic rhinitis, and food allergy [38–42]. Type 2 inflammation related diseases including atopic dermatitis, contact allergic dermatitis (ACD), asthma, and allergic rhinitis, is a general term for a range of diseases that are due to type 2 inflammation. Multiple studies have found that NLRP3 inflammasome plays a vital role in type 2 inflammation related diseases. Therefore, we will summarise the important role of NLRP3 inflammasome in the pathogenesis and treatment of atopic dermatitis, contact allergic dermatitis (ACD), asthma, and allergic rhinitis in the following work.

3.1. Atopic dermatitis (AD)

Atopic dermatitis (AD) is an inflammatory skin disease that arises from a combination of genetic and environmental factors, as well as epidermal barrier dysfunction and skin flora disorders. The activation of Th2 cells and the subsequent release of cytokines like IL-4, IL-13, and IL-31 play a crucial role in inducing an inflammatory response. This response involves various processes such as the activation of keratinocytes, the induction of pruritus, and the chemotaxis of inflammatory cells [43–45]. As early as 2011, it was shown that house dust mites (HDM) stimulate keratinocytes to recruit NLRP3, ASC, and caspase-1 to the perinuclear region, leading to the formation of inflammasomes. This process results in the secretion of IL-1β and IL-18 in an inflammasome-dependent manner [46–48]. Furthermore, it has been observed that UVB exposure to the eyes of mice increases plasma levels of ACTH and the inflammasome NLRP3, thereby exacerbating AD. These findings collectively support a connection between increased NLRP3 expression and AD [49]. The current study revealed that the activation of NLRP3 inflammasomes in keratinocytes was linked to the production of reactive oxygen species (ROS) and the activation of the NF-κB signalling pathway [50]. Subsequent investigations demonstrated that the phosphorylation of p38, ERK, and JNK in keratinocytes triggered the activation of the MAPK pathway. This, in turn, stimulated the intranuclear translocation of NF-κB through the activation of AP-1, resulting in the activation of inflammasomes via the MAPK/AP-1/NF-κB pathway [51,52]. Apart from skin lesions, the onset of AD is often accompanied by mood disorders. NLRP3 inflammasomes serve as a crucial target for psychiatric conditions like anxiety and depression. A recent study observed a significant increase in NLRP3 inflammasomes and caspase-1 levels in the hippocampus of mice in a DNFB-induced animal model of AD. These levels were found to be correlated with the duration of psychiatric disordered behaviour in mice [53,54]. Interestingly, however, the NLRP3 inflammasome does not play an entirely negative role: M Niebuhr et al. showed that Th2 cytokines (IL-4, IL-13, etc.) downregulated NLRP3 inflammasome production in primary keratinocytes, whereas Th1-type cytokines (IFN-γ) upregulated its expression, a phenomenon associated with a propensity for co-infection with Staphylococcus aureus in AD patients [55]. In addition to the inflammasome, NLRP3 can also function independently in AD. In this case, NLRP3 interacts with the transcription factor IPF4 to bind to the IL-33-specific promoter in the nucleus of the keratinocyte, resulting in increased secretion of IL-33 and worsening of AD [56].

Targeting the NLRP3 inflammasome has been shown to have a therapeutic effect on AD in several studies. Additionally, non-targeted drugs or herbal extracts have also demonstrated benefits by inhibiting NLRP3 inflammasome. For instance, Mdivi-1 inhibits mitochondrial fission in keratinocytes, blocking the NF-κB pathway and consequently inhibiting inflammasome activation and cytokine release. Another traditional herbal ingredient, Cicadidae Periostracum (CP), also exhibits therapeutic effects by inhibiting inflammasome activation [51,52 ,57–61]. It is important to note that apart from directly inhibiting the NLRP3 inflammasome, heparinoids have been found to reduce IL-1β expression in keratinocytes by inhibiting the activation of extracellular signal-regulated kinases and the p38 pathway. This reduction in expression of inflammasome-related components, without affecting caspase-1 and inflammasome activation, leads to a therapeutic effect [62]. Recent years have witnessed a gradual increase in therapeutic research, further highlighting the therapeutic potential of targeting the NLRP3 inflammasome.

3.2. Contact allergic dermatitis (ACD)

Contact Allergic Dermatitis (ACD) is a commonly occurring occupational disease that is classified as a type IV hypersensitivity reaction. This reaction consists of two phases: the sensitisation phase and the activation phase. During the sensitisation phase, semi-antigen activation triggers the release of DAMPs from epidermal cells. These DAMPs are then recognised by pattern recognition receptors (PRRs) like Toll-like receptors (TLRs) and Nod-like receptors (NLRs) in dendritic cells. Subsequently, the dendritic cells migrate to the lymph nodes, where they activate naive T cells to differentiate into cytotoxic T lymphocytes (CTLs) and helper cells (THs), which then migrate to various peripheral sites. The activation phase occurs when the body is re-exposed to the sensitiser, leading to T-cell recruitment and an immune response [63,64]. In ACD, it is well-known that allergens trigger the release of DAMPs (such as mtROS, ATP, cardiolipin, etc.), which in turn activate the NF-κB inflammatory pathway in keratinocytes. This activation induces the NLRP3 inflammasome to release cytokines like IL-1β, IL-18, and caspase-1 and promotes dendritic cell migration, recruitment, and maturation in living organisms. However, it should be noted that the DAMPs produced by the same allergen are not always isolated. For example, TNCB can cause the release of ATP, uric acid, and cathepsins. The interaction of multiple DAMPs further enhances the expression of the NLRP3 inflammasome, leading to the onset of ACD [65–67].

In terms of therapeutic approaches, Mohamed F Balaha et al. discovered that epimedine A inhibited keratinocyte pyroptosis and improved DNFB-induced ACD in mice by targeting the NF-κB/NLRP3 pathway [67]. Additionally, pterostilbene has been reported to inhibit NLRP3 inflammasome-related IL-1β, providing a potential treatment for hexavalent chromium-induced dermatitis [68]. Although many experiments have proved the therapeutic effect of inhibiting NLRP3 on ACD, specific drug research is still limited.In addition, due to the variety of allergens in ACD, the common pathway of different allergens just proves its importance. In the future, it is worth further exploring drugs.

3.3. Asthma

Asthma is a common allergic respiratory disease that typically begins in childhood. This chronic inflammatory disease is characterised by reversible airflow limitation, leading to recurrent symptoms such as wheezing, shortness of breath, chest tightness, and cough. These symptoms often worsen at night and/or early in the morning. Pathologically, asthma is characterised by eosinophilic inflammation and T-lymphocyte inflammation with CD4 aggregates [69]. A 2009 study identified an association between rs4612666, one of 15 NLRP3 single nucleotide polymorphisms (SNPs), and aspirin-induced asthma [70,71]. Subsequent research found that the expression of the NLRP3 inflammasome and its downstream products, caspase-1 and IL-1β, was increased in bronchoalveolar lavage fluid in a mouse model of asthma. The activation of the NLRP3 inflammasome and IL-1β was closely linked to Th2 cell activation, type 2 cytokine secretion, chemokine secretion, and steroid resistance, but the effects may vary between allergens [72–78]; additionally, IL-1β and caspase-1 can worsen the disease by increasing the secretion of IL-17A in (TCR)β(+) Th17 cells, as well as inducing IL-1β production and raising the amount of ICL3 in the lungs [79–81]. In DCs, High mobility protein 1 (HMGB1) expression and secretion are induced by the NLRP3 inflammasome, which mediates airway inflammation through the ATP/P2X7-NLRP3 axis and increases Th2 and Th17 inflammatory responses in a mouse model of asthma [82]. Interestingly, in contrast to atopic dermatitis, NLRP3 inflammasome-activated caspase 1 reduces IL-33 expression, which inhibits lung inflammation induced by house dust mites [83]. Furthermore, NLRP3 can bind to the IL-4 promoter and co-activate it with the IL-4 transcription factor IRF4 in CD4+ T cells, promoting Th2 polarisation and contributing to the onset of asthma [4]. NLRP3 also promotes M2 cell differentiation in monocytes through a similar mechanism, further promoting inflammation [84].

Therapeutically, Sheng-Jie Yu et al. discovered that in addition to NLRP3 inhibitors, the cell-permeable peptide of human eosinophil cationic protein (CPPecp) inhibited house dust mite-induced pneumonia by suppressing inflammasome activation [85–87]. Similarly, Xue Liu et al. found that Yupingfeng San (YPFS) had beneficial therapeutic effects by inhibiting the activation of the NLRP3 inflammasome and its components. Treatment of asthmatic mice with YPFS resulted in reduced clinical symptoms and inflammatory cell infiltration in lung tissue [88]. Lixia Wang et al. demonstrated that sevoflurane alone can improve asthma by inhibiting NLRP3 expression without involving the inflammasome [89]. Furthermore, suhuang antitussive capsule, schisandrin B, haem oxygenase-1 (HO-1), and certain ethnic traditional remedies have also shown promising efficacy in inhibiting the NLRP3 inflammasome and its related components [90–96]. Based on current research on mechanisms and treatments, although the activation mechanism of NLRP3 in asthma is not fully understood, it is certain that the NLRP3 inflammasome and related pathways play an important role in the pathogenesis of asthma. In addition, recent research on asthma has mainly focused on drug treatment, whether it is the exploration of the mechanism of new drugs or the supplement of the mechanism of old drugs, which can all show the important role of NLRP3 in the pathogenesis of asthma. This also indirectly proves the results of mechanism research. It is believed that future targeted drugs that precisely target NLRP3 will bring good therapeutic effects to patients with asthma.

3.4. Allergic rhinitis

In recent decades, there has been a significant increase in the prevalence of allergic rhinitis, making it a global health concern. This condition occurs when Th2 cells are stimulated to release cytokines like IL-4 and IL-5, leading to the development and expression of IgE antibodies by B cells, and subsequent activation of mast cells [97]. The activation of mast cells triggers a series of events, including vasodilation, increased permeability, itching, nasal discharge, and mucus secretion [98,99]. A Månsson et al. discovered that patients with diseases such as chronic sinusitis exhibit elevated expression of NLR receptors, including NLRP3, which can be reduced through topical steroid treatment [100]; similar findings were reported by J Bogefors, who confirmed that NLRP3 Inflammasome expression is increased in allergic rhinitis and correlates with disease severity [101]. Previous studies have primarily focused on patient biopsies, but subsequent research has made significant progress by exploring cellular and animal models. These studies have revealed that reactive oxygen species (ROS) in macrophages play a crucial role in activating the NLRP3 inflammasome. Activation of the NLRP3 inflammasome leads to increased expression of various cytokines including IL-1β, IL-18, caspase-1, IL-6, IL-10, IL-12, IL-13, IL-17, and inhibits RBCK1. This, in turn, results in macrophage scorching, disease exacerbation, and tissue damage [102–106]. In addition, recent studies have found that dendritic cell pyroptosis induced by allergens promotes the development of allergic rhinitis through GSDMD-N-mediated cell pyroptosis, further supplementing the role of the NLRP3 inflammasome in allergic rhinitis [107]. However, allergic rhinitis is a disease process involving multiple cell types, and the mechanisms involved in other cell types still need to be further explored.

In recent years, there has been a growing interest in the treatment of allergic rhinitis through the inhibition of NLRP3 inflammasome expression. Wo et al. found that human placenta extract can modulate macrophage polarisation by reducing NLRP3 inflammasome and immunity-related GTPase M (IRGM) expression. This leads to the suppression of M1 macrophages and the enhancement of M2 macrophages, providing mucosal protection [108]. Another study showed that the pneumococcal mutant Δ pep27 can reduce inflammation by inhibiting the activation of the NLRP3 inflammasome through Toll-like receptor expression [109]. MicroRNA research is also of great importance in this field, as previous studies have demonstrated that reducing miR-205-5p or overexpressing miR-224-5p can suppress NLRP3 inflammasome expression for therapeutic purposes [110,111]. We can observe that inhibitors targeting NLRP3 exhibit promising therapeutic effects, and numerous drugs also demonstrate good therapeutic potential in the NLRP3-related pathway. However, this is not primarily a study of the drug’s primary mechanism, but rather a complement to the mechanism of drug treatment. Its specific therapeutic role still needs to be further explored in disease models, and in the future, deeper research on marketable drugs will also be required.

3.5. Parasitic infections

Type 2 immune responses are important for protecting against parasites and promoting tissue repair, controlling inflammation, and expelling parasites [112,113]. In the context of parasitic infections, NLRP3 inflammasomes have a dual role. In Trypanosoma cruzi infections, NLRP3 inflammasomes, dependent on caspase-1, produce NO to control the disease [114]. This is similar to Toxoplasma gondii infections [115]. However, in mice infected with the intestinal parasite Heligmosomoides polygyrus, IL-1β secreted by the NLRP3 inflammasome inhibits ILC2 and suppresses IL-25 expression, leading to an incomplete expression of type 2 inflammatory response. This creates an environment in the intestinal mucosa that is suitable for parasite growth, reduces parasite clearance, and induces chronic infection [116]. Similarly, the release of IL-18 by the NLRP3 inflammasome in Trichuris-infected mice reduces the type 2 inflammatory response, resulting in reduced production of Th2 cytokines and exacerbating the infection [117]. NLRP3 can function independently of the inflammasome in mice infected with Ancylostoma braziliense. It promotes exacerbation by suppressing the type 2 inflammatory response, pulmonary neutrophils, and parasite death [118].

On the other hand, the role of NLRP3 may vary in infections with the same parasite. In Trichinella infection, the NLRP3 inflammasome can have a defensive role in reducing larval burden by activating DCs to produce higher levels of IL-4, IL-10, and TGF-β [119]; however, it has also been observed that in Trichinella infection, the NLRP3 inflammasome in macrophages reduces the expression of Th2 cytokines while increasing the expression of the Th1 cytokine INFγ, resulting in an increased burden of adult and larval parasites in muscle [120]. NLRP3 inflammasomes act as insecticides in Leishmania infections by stimulating macrophages to produce IL-1β and reactive oxygen species (ROS) [121], however, the induced secretion of IL-18 by NLRP3 inflammasomes can disrupt the balance of the Th1/Th2 immune response, reducing Th2 cytokines and the resistance of mice to Leishmania protozoa [122]. Given the variability of consequences across different pathogens and sites, it is important to consider the role of NLRP3 in parasitic infections on a case-by-case basis.

Several studies have reported therapeutic effects by inhibiting NLRP3 inflammasome expression through infection-related mechanisms. For instance, Xin Liu et al. found that taurine can reduce NLRP3-dependent liver damage in schistosome infection [123]; praziquantel has also been shown to attenuate M1 macrophage activity by inhibiting NLRP3 inflammasome [124]. Additionally, Xuemin Jin et al. demonstrated that shiitake polysaccharide can reduce parasite burden in mice in an NLRP3-dependent manner [125]. These findings highlight the potential of NLRP3 inflammasome as a target for the treatment of parasitic diseases.

4. Conclusion

In this article, we summarise the process of NLRP3 inflammasome assembly and activation. It aggravates type 2 inflammation related diseases such as AD, asthma, and allergic rhinitis by promoting the release of inflammatory factors, pyroptosis, and inhibiting mucosal immunity, etc. On the therapeutic front, several drugs, both previously used and novel agents, have demonstrated effectiveness in animal models, highlighting the potential for targeting the NLRP3 inflammasome in type 2 inflammation related diseases. Although there is currently no targeted drug available for NLRP3, the therapeutic effects of current drugs are enough to highlight the clinical significance of studying targeted drugs. Therefore, it is important to continue our research on the NLRP3 inflammasome. Future research should focus on investigating the mechanism of the NLRP3 inflammasome in different cell types and developing new drugs that specifically target the human NLRP3 inflammasome for the benefit of patients.

Author contributions

All authors contributed to the conception, writing and revision of this article.

Acknowledgements

I would like to thank my supervisor for his guidance through each stage of the process.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.