Asthma is a common respiratory disease that currently affects 338 million individuals worldwide. The disease’s underlying mechanism of airway inflammation and airway hyperresponsiveness, causing airway wall edema, mucus plugging, airway smooth muscle contraction, and/or airway remodeling, leads to clinical symptoms of chronic, episodic cough, phlegm production, chest tightness, wheezing, and shortness of breath (1). Asthma phenotypes are used to characterize patients based on their clinical features, including symptom severity, a history of exacerbation, and lung function in combination with airway inflammation and genetic profiles (2). Majority of asthma patients with mild to moderate disease are controlled by appropriate treatment with high dose Inhaled corticosteroids/Long-acting beta-2 agonist (ICS/LABA) and comorbidities management. Approximately 5–10% suffer from severe uncontrolled asthma and experience symptoms, persistent airway inflammation, and frequent exacerbations despite high-dose ICS treatment (3). Systemic corticosteroids, which have broad anti-inflammatory effects, remain the mainstay of treatment to manage more severe symptoms but are well known for their adverse effects (4).

Airway inflammation and hyperresponsiveness in asthma occur due to both exogenous and endogenous triggers. The key cytokines involved in the Type 2 inflammatory pathways are IL-5 and IL-13 (5). As a steroid-sparing strategy, anti-T2 monoclonal antibody (mAb) therapies were introduced in the past decade, that have shown modest reduction of asthma exacerbations by 50–60% (6). An optimal, personalized treatment is, therefore needed based on the underlying phenotype and associated endotypes (7). Although blood eosinophil counts are widely used in clinical practice, they may not be sufficient to justify treatment decisions. It is crucial to consider the compartmentalization of airway inflammation in asthma (8). The role of airway eosinophils is further corroborated by a recent cohort study showing sputum eosinophil peroxidase (EPX) is superior to blood eosinophils in understanding patients who remain uncontrolled on anti-IL-5 targeted therapy (9). Again, the modest reduction in exacerbations and persistence of symptoms that remain uncontrolled by anti-IL-5 mAbs (10–12) despite depletion of airway eosinophils suggest alternative/residual airway inflammation (13). The measurement of sputum cytokine in this group of patients revealed raised IL-13, IL-18, and/or IL-1β, indicating alternative pathways beyond IL-5 (14). Furthermore, lack of improvement in asthma symptoms on benralizumab, an eosinophil depleting mAb was associated with raised/residual levels of IL-18 in sputum (13). In severe allergic asthma patients, who typically respond to omalizumab (anti-IgE therapy), high baseline serum free IL-18 levels may predict reduced omalizumab efficacy. A 2-year study showed that significantly more patients showed high baseline serum free IL-18 levels among the patients who developed exacerbation in the second year (incomplete responders) than the complete responder group (15). In non-T2 inflammation, IL-18 works in combination with IL-12, stimulating T helper 1 (Th1) cell development, Natural killer (NK) cells, and Natural killer T (NKT) cells (16). Since IL-18 is primarily produced as an inactive precursor, there is a process to proteolyze pro-IL-18 to an active form. This process is involved in the inflammasome complexes cascade (discussed in detail), which plays an important role in our innate immune systems (17).

This review aims to consolidate current knowledge on IL-18 biology and signaling pathways, particularly the involvement of inflammasome complexes in airway disease to provide active IL-18. We also discuss the role of IL-18 in asthma pathophysiology, autoimmunity, interaction with other cytokines, and its clinical perspective, including the potential therapeutic targets.

IL-18 was initially identified as an interferon-γ (IFN-γ) inducing factor (IGIF). The IGIF is mainly produced as a 193 amino acids precursor and is cleaved to a mature protein of 157 amino acids (18). IL-18 is classified to be a part of the IL-1 cytokine family due to its shared common beta-pleated sheet structure and amino acid sequence (18–20). Similar to the other cytokines in the IL-1 family, IL-18 exerts active functions by binding to the corresponding receptors on the surface of target cells. The IL-18 receptor belongs to IL-1R family members and The Toll/IL-1 receptor (TIR) domain receptors superfamily (21). The extracellular part is conserved to recognize the cytokine while the intracellular part containing a TIR domain orchestrates the downstream signal transduction. The architectural structure of the IL-18 receptor has a second domain that is unique and contributes to the distinct inter-receptor interaction and binding affinity, not present in the other IL-1Rs (22).

The production of IL-18 occurs in a wide array of cell types, including both hematopoietic and non-hematopoietic cells. The IL-18 precursor is present in several cell types, including circulating monocytes, resident macrophages, dendritic cells, endothelial cells, keratinocytes, osteoblasts, intestinal epithelial cells, and mesenchymal cells (23). The IL-18 mRNA or protein is also found in airway epithelial cells (24, 25). Initially synthesized as an inactive precursor, pro-IL-18 undergoes proteolytic cleavage by the intracellular cysteine protease, caspase (Casp)-1, to yield its biologically active form, IL-18, secreted by the cell. This activation process is intricately linked to the canonical inflammasome pathways, particularly Nucleotide-binding oligomerization domain leucine-rich repeat and pyrin domain-containing protein (NLRP) 3, discussed in detail in subsequent sections (17). Additionally, alternative pathways beyond inflammasome are also identified, such as Fas-mediated signaling via Casp-8 in macrophages and dendritic cells (26).

The IL-18 receptor (IL-18R) is expressed in T cells and NK cells. Non-immune cells like neurons and epithelial cells also express IL-18R that may play a role in their cellular differentiation and survival. The receptor comprises of two subunits, IL-18Rα and IL-18Rβ, forming a high-affinity binding heterodimer upon IL-18 stimulation (23). This complex triggers downstream signaling involving myeloid differentiation primary response 88 (MyD88) that activates nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) through association with the signal adaptor IL-1R-associated kinase (IRAK) 1–4 and tumor necrosis factor (TNF) receptor-associated factor (TRAF) 6 (27, 28). Inhibitors of IL-18 signaling include IL-18-binding protein (IL-18BP) and IL-37. IL-18BP can bind soluble mature IL-18 with a higher affinity and prevent IL-18 binding to IL-18R. Free IL-37 binds to IL-18α with a low affinity and then induces the recruitment of IL-1R8 to form a high-affinity complex. This complex does not recruit MyD88, thus inhibiting IL-18 signaling and inducing an anti-inflammatory effect via signal transducer and activator of transcription (STAT) 3 (23, 29, 30).

Inflammasomes are intricate cytosolic complexes that are essential components of the innate immune system. They primarily consist of three components: First, a sensory protein or the pattern recognition receptors (PRRs), including the Nucleotide oligomerization domain (NOD)-like receptors (NLRs); second, an adaptor protein called apoptosis-associated speck-like protein containing a Caspase Activation and Recruitment Domain (CARD) (ASC), and third, an enzymatic effector such as Casp1. The NLR family is characterized by the presence of a central nucleotide-binding and oligomerization (NACHT) domain, which is commonly flanked by C-terminal leucine-rich repeats (LRRs) for ligand sensing and N-terminal effector domain for mediating signal transduction. The NLRs can be further categorized into subfamilies based on the effector domains, including the NLRP with pyrin domain (PYD) and NLRC with CARD domain (46, 47). Several inflammasome moieties have been identified thus far: NLRP1, NLRP3, NLRP6/7/12, NLR family CARD domain-containing protein (NLRC) 4, retinoic acid-inducible gene I (RIG-I), absent in melanoma 2 (AIM-2) and interferon gamma-inducible protein 16 (IFI 16). The specific component and activation mechanisms vary depending on the nature of the individual protein.

As illustrated in Figure 2, when the sensory protein senses the danger signal, including pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPS), and homeostasis-altering molecular processes (HAMPs) (48), the signaling pathway is activated. ASC is recruited and interconnects with pro-Casp1 through the CARD domain, leading to oligomerization. This process triggers autoproteolysis, giving rise to active Casp1. Active Casp1 plays a central role in cleaving pro-IL-1β and pro-IL-18 into their mature forms, facilitating the release of IL-1β, IL-18, high mobility group box 1 (HMGB1) (a known DAMP), and other proteins from the cell. Casp1 also initiates a specific form of highly inflammatory programmed cell death known as pyroptosis, characterized by rapid plasma membrane rupture, leading to the release of intracellular contents. Unlike apoptosis, which is immunologically silent, pyroptosis serves as a defense mechanism against intracellular pathogens. The term “pyroptosis” derives from the Greek words “pyro,” meaning fire or fever, and “ptosis,” meaning falling, emphasizing both its inflammatory nature and the collapse of cellular integrity (49).

The execution of pyroptosis is mediated by the gasdermin family of proteins, particularly gasdermin D (GSDMD). Upon activation by Casp-1, GSDMD undergoes proteolytic cleavage, liberating its N-terminal domain (50). The N-terminal fragment of GSDMD inserts into the plasma membrane, forming large pores known as “pyroptotic pores.” These pores compromise membrane integrity, leading to osmotic imbalance, cellular swelling, and, ultimately, membrane rupture. Pyroptosis serves as a double-edged sword in host defense, eliminating infected cells to limit pathogen replication while also triggering an inflammatory cascade (51).

Innately, inflammasomes are known for protecting against invading pathogens and initiating adaptive immune responses. However, their dysregulation is implicated in several metabolic disorders (52), autoinflammatory diseases (53, 54), neurodegenerative diseases (55, 56) and of recent, the cytokine storm reported in COVID-19 (57). In the past decade, there has been a growing focus on the role of inflammasomes in chronic airway diseases, particularly asthma and COPD. Mutations of the genes in inflammasome pathway have been associated with eosinophilia in patients with asthma (54). Expression of the NLRP3 inflammasome has been associated with acute exacerbations of COPD (58) and neutrophilic airway inflammation, worsening lung function, and poor asthma control (59). Rhinovirus infection activates RIG-I inflammasome in asthma patients and leads to prolonged viral clearance and unresolved inflammation, demonstrated via both in-vitro and in-vivo investigations (60).

Even though IL-18 is not recognized to be a major player in asthma pathobiology, several recent studies have linked IL-18 to diverse immune responses in asthma. As discussed earlier, both clinical and basic science investigations suggest IL-18 to play a significant yet complex role in T2 and non-T2 inflammation, depending on the micro-environmental cues (airways). The studies involving IL-18 with respect to asthma, both in animal models and humans, are summarized in Tables 1, 2.

Autoimmunity arises when the body’s immune system mistakenly identifies its own cells as foreign, leading to the production of autoantibodies against self-structures. This self-reactivity results in sustained self-immune response and tissue damage (107). The consequence of this phenomenon is demonstrated in a variety of diseases ranging from systemic involvement, such as systemic lupus erythematosus, to organ-specific pathology, such as Crohn’s disease and Hashimoto’s thyroiditis. Even though autoimmunity is theoretically related to Th1/Th17 responses, its possible involvement in the pathogenesis/severity of Th2 diseases such as asthma and chronic rhinosinusitis is being extensively studied (107). Chronic inflammation and subsequent inflammasome signaling may underscore the development and sustenance of airway autoimmune responses described in the airways of patients with complex airways disease (107–109).

In patients with severe asthma with increased airway degranulation evident by the presence of free eosinophil granules, autoantibodies to eosinophil granule proteins such as eosinophil peroxidase (EPX) and anti-nuclear/extranuclear antigens (ANAs) have been reported (81, 109). These sputum autoantibodies were observed in an IL-13-rich micro-environment with increased levels of IL-18 (109). The autoantibodies trigger eosinophil extracellular traps (a similar event to NETs) termed EEtosis (109, 110) which is associated with the release of HMGB1 and crystallization of the periplasmic Galectin-10 to form CLCs (111, 112). The DAMPs activate inflammasomes, leading to the subsequent release of active IL-18 (113), and propagate inflammation and tissue damage beyond the canonical IL-5 and IL-13 pathways. This process leads to self-antigens and activation of self-reactive lymphocytes, resulting in the production of autoantibodies (107), which further triggers and perpetuates EETosis, resulting in a vicious cycle of chronic persistent inflammation in severe asthma (107). Inflammasome signaling and associated IL-18 release may also underlie some of the suboptimal responses documented in prototype severe eosinophilic patients treated with anti-IL-5 (10)/IL-5R biologics (12, 13, 114).

Recent research has increasingly focused on the role of IL-18 and inflammasomes in a variety of autoimmune and inflammatory diseases, including chronic lung diseases such as asthma and COPD. The IL-18 and inflammasome cascade play crucial roles in the immune response, with IL-18 being intricately involved in both T2 and non-T2 airway inflammation. This has highlighted IL-18 and related proteins as potential therapeutic targets for severe asthma beyond conventional T2 targets.

Animal models have provided encouraging data, indicating that IL-18 suppression can reduce airway inflammation, AHR, and mucus production (93, 115). Therapeutic strategies aimed at inhibiting IL-18 is either by directly blocking its signaling or target its activation/release by modulating the inflammasome pathway. Various molecules have been studied in diverse inflammatory conditions associated with the IL-18 pathways, offering potential avenues for treatment (tabulated in Table 3). For example, the humanized monoclonal antibody GSK 1070806 has been tested in renal transplant delayed graft function (116), Type 2 diabetes patients (117) and is currently in Phase II developmental pipeline for atopic dermatitis indication (NCT05999799). The search for therapeutic targets has now extended to IL-18R blocking agents and IL-18BP activity-enhancing therapies (118). IL-37, which binds to the IL-18Rα, has also shown potential in asthma treatment. Local administration of IL-37 in asthmatic mouse models reduced eosinophils in the airway and improved AHR (119). Another promising therapeutic, APB-R3, a long-acting recombinant human IL-18BP, has shown effectiveness in reducing liver inflammation and splenomegaly in a model of the macrophage activation syndrome and controlled skin inflammation in atopic dermatitis mice model (120). Tadekinig alfa, a recombinant IL-18 binding protein, has completed Phase II trials in adult-onset Still’s disease, showing early signs of efficacy (121).

Inflammasomes, particularly NLRP3, that cleaves precursor forms of IL-1β and IL-18 into their biologically active forms, have also emerged as therapeutic targets. Numerous agents are under investigation, including Selnoflast, a potent, selective, and reversible NLRP3 blocker, which has completed a Phase Ib study in ulcerative colitis patients (122). Ongoing studies are exploring its potential in diseases such as Parkinson’s (NCT05924243), COPD, and asthma (ISRCTN73873157). Another small-molecule inhibitor, MCC 950 (CP-456,773), a diaryl sulfonylurea-containing compound, that specifically targets NLRP3 inflammasome activation (123) documented significant reductions in NLRP3 and IL-1β production in asthmatic mice (124). Furthermore, sodium houttuyfonate, derived from the Chinese herb Houttuynia cordata, has demonstrated efficacy in reducing the expression of NLRP3, ASC, caspase-1, GSDMD, IL-1β, and IL-18 in the lung tissues of asthmatic mice (125). Similarly, the isoquinoline alkaloid protopine, an anti-inflammatory agent (126), has been shown to reduce airway inflammation in asthmatic rats by inhibiting the TLR4/NF-κB signaling pathways, leading to the suppression of NLRP3, gasdermin D, and caspase-1 activation, along with decreased levels of reactive oxygen species (ROS), IL-1β and IL-18 (127).

Another promising therapeutic approach involves the use of glucagon-like peptide-1 receptor (GLP-1R) agonists, initially approved as anti-diabetic and anti-obesity drugs, have been studied for their potential benefits in obesity-related asthma. Obesity is a major risk factor and disease modifier in asthma (125), and studies in obese asthmatic mouse have revealed that AHR can be NLRP3-dependent. GLP-1R agonists have been shown to suppress peri bronchial inflammation and reduce the expression of NLRP3, activated caspase-1, and IL-1β in lung tissues (128). Additionally, miR-223, a microRNA with evolutionary anti-inflammatory effects, particularly in the lungs, has shown potential as a therapeutic option (129). The microRNA, a non-coding RNA, can function to control the expression of target genes at the post transcriptional level. Overexpression of miR-223 is linked to decreased NLRP3 and NF-κB activity in porcine lungs (130) and bronchial epithelial cells (131). Treatment with miR-223 agomirs in neutrophilic asthmatic mouse models attenuates airway inflammation, reduces NLRP3 levels, and decreases IL-1β release, suggesting miR-223 as a potential therapeutic candidate for severe non-T2 asthma (132).

Ongoing investigations in IL-18 and inflammasomes have revealed their critical roles in the pathophysiology of asthma, particularly in severe forms of the disease. IL-18, traditionally seen as a minor player compared to the key T2 cytokines like IL-5, IL-4, and IL-13, have emerged as a key cytokine involved in both T2 and non-T2 inflammation. Its role in promoting infections, airway inflammation and airway remodeling, as well as its contribution to autoimmune responses within the airways, underscores the complexity of asthma beyond the conventional Th2 paradigm. In this review we have highlighted the multifaceted involvement of IL-18 in asthma, from its interactions with other cytokines to its activation through inflammasome pathways, particularly NLRP3. The evidence linking IL-18 to both airway inflammation, mucus plugging and AHR emphasizes its significance in asthma pathogenesis. Additionally, IL-18’s involvement in autoimmune processes introduces a new dimension to understanding severe asthma, particularly in cases where traditional therapies targeting Th2 inflammation have shown limited effectiveness (107). Furthermore, the current review sheds light on potential therapeutic targets. The identification of novel therapies targeting IL-18 and the associated inflammasome pathways offer promising avenues for treating severe and refractory asthma, addressing the limitations of current biologics. In conclusion, IL-18 is not just an ancillary cytokine in asthma but one of the key components in its pathophysiology, particularly in severe cases. A deeper understanding of its intricate role within the broader network of immune responses can guide the development of more effective treatments, offering new hope for patients with this challenging condition.