Innate Type-2 Cytokines: From Immune Regulation to Therapeutic Targets

IL-25

IL-25, also known as IL-17E, holds a unique position within the IL-17 family as it has the distinctive capability to directly induce type-2 immunity (6). IL-25 is secreted by various cell types, including lung epithelial cells and synovial fibroblasts, in response to external stimuli such as helminth infections or allergens. Subsequently, IL-25 binds to the heterodimeric IL-25R receptor, composed of IL-17RA and IL-17RB, elucidating the nuanced relationship between IL-25 release and its binding to the receptor following diverse stimuli (7, 8, 9). Upon secretion, IL-25 initially exists in an inactive disulfide-linked homodimeric form, undergoing post-translational maturation processes to become bioactive (Fig. 1). Notably, matrix metalloproteinase-7-mediated cleavage of IL-25 enhances the pathogenic production of type-2 cytokines in allergen-challenged lungs (10).

The crucial role of IL-25 in asthma is underscored by observations in mouse models, where intratracheal administration of IL-25 induces lung eosinophilia, IgE production, and airway hyperreactivity (AHR) (11, 12). These responses involve, in part, type-2 ILCs (ILC2s), which secrete IL-5 and IL-13 in response to IL-25. Notably, IL-25 cannot induce these cells to produce IL-4 (12). Additionally, pathogenic CD4⁺ invariant NKT (iNKT) cells expressing the IL-17RB component of IL-25R produce significant amounts of type-2 cytokines, such as IL-13, upon stimulation with IL-25 in vivo and in vitro (Fig. 1) (13, 14).

IL-25 also plays a crucial role in anti-parasite defense; mice deficient in IL-25 cannot mount Th2 responses against parasites such as Trichuris muris (15). Conversely, intraperitoneal administration of recombinant IL-25 promotes the expulsion of Nippostrongylus brasiliensis from the gut (11).

IL-33

IL-33, a member of the IL-1 family, is primarily expressed by various tissue cell types, including epithelial cells, fibroblasts, and endothelial cells. Additionally, dendritic cells (DCs) and mast cells can release IL-33 in inflammatory conditions (16). This cytokine binds to a receptor complex consisting of the signaling transmembrane isoform of ST2 (ST2L) and the promiscuous accessory protein IL-1 receptor accessory protein (IL-1RAcP). Constitutively expressed by macrophages, ILCs, mast cells, basophils, and eosinophils, this receptor complex initiates signaling cascades, such as NF-κB and AP-1, leading to the expression of inflammatory molecules, including type-2 cytokines (17). Under homeostatic conditions, the full-length 270 amino-acid IL-33 protein is sequestered in the nucleus of epithelial and endothelial cells. Upon exposure to allergens, bacteria, or viruses, immature IL-33 is released into the extracellular space. Proteases released by innate effector cells cleave the immature IL-33, generating mature IL-33 fragments (18, 19, 20). These mature forms interact with immune cells bearing the ST2L receptor (21, 22, 23, 24), displaying markedly increased activity in generating type-2 immune responses compared to the immature form (Fig. 1).

IL-33 plays a crucial role in allergic inflammation. IL-33-deficient mice exhibit resistance to allergen-induced AHR (25). Additionally, intranasal delivery of soluble ST2, functioning as a decoy receptor for IL-33, using adenovirus effectively blocks allergic airway inflammation in animal models (26). Notably, individuals with allergic conjunctivitis, rhinitis, and atopic dermatitis display elevated serum levels of IL-33 compared to healthy counterparts (27).

TSLP

TSLP, a member of the IL-2 cytokine family, is primarily produced by epithelial and stromal cells in various tissues, including the skin, intestine, and lungs (28). TSLP exhibits a dual regulatory role in immune responses, mediated by its two distinct isoforms: short-form TSLP (sfTSLP) and long-form TSLP (lfTSLP) (29). Under homeostatic conditions, sfTSLP is constitutively expressed, acting as a sentinel for tissue integrity and contributing to immune tolerance and barrier function. This prevents unwarranted inflammatory responses. However, in inflammatory environments, lfTSLP is expressed, serving as a potent alarm signal that alerts the immune system to the presence of danger, particularly during infections or tissue damage (29). These isoforms influence various immune cells expressing the TSLP receptor, a heterodimer of the IL-7 receptor alpha chain and the TSLPR chain. DCs, for instance, undergo phenotypic changes and upregulate costimulatory molecules such as OX40L, CD80, and CD86 when stimulated with TSLP. Consequently, TSLP plays a pivotal role in DC Ag presentation to CD4+ T cells, activating them into cells that produce type-2 cytokines (30). TSLP also directly activates innate immune cells, including mast cells, ILC2s, NKT cells, epithelial cells, macrophages, and basophils, prompting the production of type-2 cytokines (5) (Fig. 1). These cytokines, in turn, initiate type-2 immune responses combating parasitic infections, mediating allergies and fibrosis, and inducing anti-obesity adipocyte metabolism (Fig. 2). Thus, TSLP serves as a bridge between the innate and adaptive arms of the immune system, orchestrating the emergence of a complex web of type-2 immune responses.

Figure 2
Effects of innate type-2 cytokines in protective and pathogenic immune responses.
(A) Helminth infections often lead to tissue damage and the release of alarmins such as IL-25 and IL-33 by the necrotic cells. The alarmins bind to local innate-immune cells such as ILC2s, which prompts them to rapidly produce classical type-2 cytokines such as IL-5 and IL-13. IL-5 induces mast cells to degranulate; the released proteases then damage the helminths. IL-5 also induces eosinophilia, which is crucial for controlling parasitic infections, while IL-13 promotes eosinophil recruitment to the infected site. IL-33 also initiates goblet-cell hyperplasia in the lung and intestine, thereby creating an unfavorable environment for helminths. (B) Allergens damage epithelial cells and induce them to release the alarmins IL-25 and IL-33. These directly activate ILC2s and iNKT cells, which release type-2 cytokines and thereby trigger allergic reactions such as asthma and atopic dermatitis. For example, IL-5 and IL-13 induce eosinophilia and local eosinophil recruitment, respectively. IL-13 also generates goblet-cell hyperplasia in the lung, thereby inducing the mucus overproduction that characterizes allergic diseases. (C) Fibrosis is mediated by IL-13/IL-4-secreting ILC2s and iNKT cells. These cytokines directly induce fibroblasts to differentiate into myofibroblasts, which produce vast amounts of collagen and fibronectin. They also potently contract the extracellular matrix. Both activities result in stiffly remodeled tissue that is a hallmark of fibrosis and interferes with normal tissue functions. IL-4 from ILC2s/iNKT cells also activates macrophages and induces eosinophil recruitment. These effects further promote myofibroblast differentiation. (D) Adipose-tissue metabolism is driven by macrophages. Alternatively-activated (M2) macrophages promote adipocyte browning, which releases energy in the form of heat. The differentiation of adipose-tissue M2 macrophages is initiated by hormones, exercise, and environmental stimuli such as cold, which induce the release of the alarmins IL-25 and IL-33 in adipose tissue. These alarmins activate local ILC2s, iNKT cells, and type-II NKT cells, which secrete IL-5 and IL-13. IL-5 induces eosinophilia and IL-13 recruits these cells into the adipose tissue. The eosinophils produce IL-4, which converts the resident macrophages into M2 macrophages that release mediators that promote adipocyte browning. When the innate immune cell-eosinophil-M2 macrophage axis is disrupted, classically-activated macrophages (M1) predominate and the adipocytes accumulate fat and induce obesity.

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NKT cells

NKT cells represent a unique cell type with both the surface markers and functional characteristics of conventional T cells and NK cells. Crucially, NKT cells express TCRs capable of recognizing glycolipid Ags presented by CD1d molecules, which are non-polymorphic MHC class I-like proteins (31). Additionally, they constitutively express markers associated with activated or memory T cells, endowing NKT cells with the remarkable ability to rapidly produce type-2 cytokines, including IL-4 and IL-13, as well as type-1/type-17 cytokines within minutes to hours of TCR activation (31, 32, 33, 34).

NKT cells can be broadly categorized into type-I and type-II NKT cells based on their TCR-repertoire diversity (31). Type-I NKT cells, also known as iNKT cells, express semi-invariant TCRβ chains paired with Vα14 chains in mice and Vα24 chains in humans. They primarily recognize glycolipid Ags such as α-galactosyl ceramide, derived from marine sponges. In contrast, type-II NKT cells exhibit more diverse TCRs and respond to various lipid Ags (e.g., sulfatide) (35).

Understanding the role of NKT cells in type-2 immunity requires an examination of their development and differentiation, predominantly occurring within the thymus. Two developmental models, namely the linear maturation (36) and lineage-differentiation models (37, 38, 39), provide insight. The latter categorizes iNKT cells into distinct subsets based on their expression of transcription factors and cytokine production. iNKT1 cells express T-bet and produce Th1 cytokines (IFN-γ and TNF-α), iNKT2 cells express GATA3 and type-2 (Th2) cytokines (IL-4 and IL-13), and iNKT17 cells express RAR-related orphan receptor-alpha (RORα) and produce Th17 cytokines (IL-17A and IL-22) (37, 38, 39). Recent single-cell RNA sequencing analyses further categorized iNKT2 cells into two subsets based on their expression of Icos and the Promyelocytic Leukemia Zinc Finger-encoding gene Zbtb16 (40).

Notably, the production of type-2 cytokines by iNKT2 cells depends not only on TCR stimulation by CD1d-expressing Ag-presenting cells (APCs) but also on their expression of β-catenin (41). However, the underlying mechanisms remain poorly understood.

ILCs

ILCs emerged in the intricate landscape of the immune system in 2010, adding a layer of complexity to our understanding (42). These cells, predominantly found in barrier tissues, distinguish themselves from other immune cells by lacking markers characteristic of the myeloid and lymphoid lineages and, notably, lacking Ag-specific receptors (43). Unlike T cells, ILCs are promptly activated by engaging their constitutively expressed surface receptors with signals from the tissue microenvironment, including cytokines, microbial products, nutrient components, lipid mediators, and neuronal transmitters (43). This direct activation mechanism allows ILCs to respond swiftly to environmental cues, primarily through cytokine production.

ILCs can be categorized into three subsets (ILC1, ILC2, and ILC3) based on the transcription factors governing their activities and the cytokines they produce. This subdivision closely mirrors the differentiation of Th cells into Th1, Th2, and Th17/22 cells, positioning ILCs as innate counterparts to these T-cell subsets (43). ILC1s, akin to Th1 cells, constitutively express T-bet, which is encoded by Tbx21 and plays a crucial role in the early development of ILC1s (44). Consequently, ILC1s bear receptors for key cytokines such as IL-12 and IL-18 that induce them to produce IFN-γ and TNF-α (45). ILC2s, the innate equivalent of Th2 cells, are prevalent in mucosal tissues, particularly the lungs. Their development and expansion are dependent on specific transcription factors, including RORα and GATA3 (46). They bear the ST2 receptor and are exquisitely sensitive to IL-33: once triggered in this fashion, ILC2s release copious amounts of type-2 cytokines, including IL-5, IL-9, and IL-13 (47). ILC3s, paralleling Th17/22 cells, are governed by the RORγt and aryl hydrocarbon receptor transcription factors. They express receptors for IL-1 and IL-23 and secrete IL-17 and IL-22 (48).

Multiple studies, including those on mouse models, suggest the crucial role of ILC2s in the pathophysiology of asthma and allergic inflammation. For instance, patients with severe eosinophilic asthma show an accumulation of IL-5- and IL-13-producing ILC2s in their sputum after an allergen challenge (49). ILC2s contribute to tissue damage in the lung and skin, primarily through an IL-13-dependent mechanism, and promote allergic inflammation through intricate communications with local DCs and Th2 cells (50). In homeostatic tissues, ILC2s modulate the amplitude of type-2 responses, ensuring a calibrated reaction to environmental threats (51). Understanding the triggers leading to excessive or deranged ILC2 responses holds promise for the development of targeted therapies for type 2-associated diseases.

Granulocytes (mast cells, eosinophils, and basophils)

Granulocytes, encompassing mast cells, eosinophils, and basophils, swiftly respond to triggers by producing substantial amounts of type-2 cytokines. Beyond this, they reciprocate to these cytokines and other stimuli by releasing preformed mediators, including cytokines, proteases, bioactive lipids, and histamine, thereby playing integral roles in fostering airway inflammation (23, 52, 53).

Mast cells, enduring participants in immune activation, engage not only in the immediate response to microbial threats but also contribute to later infection stages. Their functions extend to promoting helminth clearance through protease release and acting as effectors in protective memory immune responses upon secondary infection (54). Well-recognized for their involvement in asthma, allergy, and anaphylaxis, mast cells, when activated, release cytokines such as IL-4 and IL-13, modulating both innate and adaptive immune responses. Additionally, they release prostaglandin D2, activating ILC2s and inducing various mechanisms to enhance type-2 immune responses (55).

Basophils, which share some functional features with tissue-resident mast cells, respond to various stimuli, including cytokines such as IL-3, IL-18, IL-33, and TSLP. Notably, they display the classical features of APCs, including the expression of peptide-MHC-II complexes, which they acquire from DCs in draining lymph nodes via trogocytosis (56). Since basophils also produce relatively high levels of IL-4, they are major promoters of Th2-cell differentiation (29, 53). These cells play such an important pathogenic role in allergies that they are the subject of the Basophil Activation Test, which is a widely employed in vitro assay for characterizing and diagnosing allergic responses in patients (56).

Eosinophils, integral in allergic inflammation and airway injury/repair processes alongside mast cells and basophils, release granule proteins like major basic protein (MBP) and eosinophil peroxidase (EPO) upon activation. This release contributes to airway injury, and the subsequent release of growth factors and cytokines, including IL-5, further amplifies inflammation (57). The migration of eosinophils from the blood to the airways, a critical step influencing asthma severity, is orchestrated by type-2 cytokines (58). These cytokines include IL-4, which upregulates VCAM-1 on eosinophils: this helps the eosinophils adhere to blood vessels, thereby facilitating their migration to the lung and airways (58, 59). Moreover, IL-13 promotes eosinophil migration by inducing epithelial cells to release chemokines such as eotaxin. Furthermore, IL-5, a potent eosinophilopoietic cytokine, supports eosinophil survival (56, 60).

Innate type-2 cytokines in host defense from helminths

Innate type-2 immunity stands as a critical defender against helminth infections, suggesting its evolution as a specialized response tailored to counter parasitic threats (61). The defense mechanism starts when helminths breach the skin or mucosal barriers, inducing tissue damage that prompts the release of alarmins, notably IL-25 and IL-33 (5). These alarmins initiate the innate immune response by triggering local innate immune cells, especially ILC2s, to release key cytokines, such as IL-5 and IL-13 (62). IL-5, in turn, promotes eosinophilia by inducing eosinophil proliferation in the bloodstream and activating these cells. IL-13 recruits eosinophils to the infection site, where activated eosinophils release granule proteins like MBP and EPO, exhibiting direct cytotoxicity against the parasite (63).

This eosinophilic response is pivotal for controlling helminth infections, such as the lung stage of N. brasiliensis (11). Within the infection site, activated eosinophils not only unleash granule proteins with direct parasite toxicity but also sustain their own inflammation and induce inflammation in other innate-immune cells by releasing IL-5. Furthermore, IL-13 derived from eosinophils and other innate-immune cells triggers goblet-cell hyperplasia in the intestine and lung, effectively impeding the early attachment of the parasite to the intestinal/lung wall. IL-13 also stimulates mast cells to release proteases that actively expel parasites from the intestine (64, 65) (Fig. 2).

While innate type-2 cytokines predominantly govern the protective response against helminths, murine schistosomiasis is associated with activated iNKT cells producing both IL-4 and the type-1 cytokine IFN-γ. This dual cytokine production may result from signaling by DCs presenting specific lipid Ags on CD1d molecules. When DCs are sensitized with schistosome eggs in vitro, they can induce iNKT cells to secrete both IFN-γ and IL-4 (66). The exact role of IFN-γ in schistosomiasis, typically associated with downregulation of Th2 responses, remains unclear.

Innate type-2 cytokines in allergic responses

Allergic reactions, manifested in various forms such as hay fever, asthma, and skin rashes, share a common underlying mechanism—a heightened immune response to typically harmless environmental substances (67, 68). This intricate process involves complex interactions among various cellular players and molecules, with crucial roles assigned to the alarmins IL-33 and IL-25. Released by epithelial or other structural/immune cells upon encountering triggering allergens, these cytokines initiate downstream allergic responses (15, 27). Notably, both alarmins strongly activate ILC2s, which inherently express receptors for these molecules and rapidly produce type-2 cytokines, setting off the cascade of events characteristic of allergic reactions (62).

Specifically, IL-5 induces eosinophilia, while IL-13 recruits eosinophils to the lungs. IL-13 also induces goblet-cell hyperplasia in the lungs, leading to mucus hypersecretion (69) (Fig. 2). Given the integral roles of eosinophilia and mucus hypersecretion in the pathogenesis of allergic diseases like asthma and atopic dermatitis, alarmins, ILC2s, and their innate type-2 cytokines emerge as pivotal players in allergic diseases, supported by numerous lines of evidence. For instance, a genome-wide association study has revealed that polymorphisms in genes encoding IL-33 and the IL-33 receptor (ST2) are associated with allergic asthma (70). Additionally, patients with asthma exhibit higher IL-33 levels and ILC2 frequencies in their peripheral blood and sputum compared to control subjects (71).

iNKT cells and their type-2 cytokines may also participate in allergic disease. Terashima et al. (13) showed that IL-25 activates a specific lung-resident iNKT-cell subset expressing both CD4 and IL-17RB. These cells then produce type-2 cytokines and chemokines, leading to AHR. The significance of these IL-17RB⁺ CD4⁺ iNKT cells was underscored by the observation that their specific depletion in mice attenuated IL-25-induced AHR (13). There is also evidence that IL-33 can induce murine and human iNKTs to secrete type-2 cytokines (72, 73). Further studies, including the characterization of ST2⁺ iNKT cells and their role in type-2 immune responses, are warranted.

Innate type-2 cytokines in fibrosis

Fibrosis, characterized by the excessive accumulation of extracellular matrix components in tissues, profoundly affects organ function in various organs, including the lungs, liver, and skin (74). A pivotal contributor to these fibrotic disorders is IL-13, originating from ILC2s, iNKT cells, and other immune cells. This potent cytokine activates fibroblasts, the primary producers of extracellular matrix (2), prompting their differentiation into myofibroblasts. Myofibroblasts, known for producing abundant collagen and fibronectin, are highly contractile, driving tissue contraction, stiffening, and remodeling—a hallmark of fibrosis that ultimately impairs normal tissue function (2, 75). Another type-2 cytokine, IL-4, also contributes to fibrotic responses (76) by inducing myofibroblast differentiation. Additionally, IL-4 promotes fibrosis by recruiting inflammatory cells like macrophages and eosinophils, which release profibrotic mediators (e.g., TGF-β), exacerbating the fibrotic process (77). In the lungs, chronic injuries and insults can lead to conditions such as asthma, chronic obstructive pulmonary disease, and idiopathic pulmonary fibrosis, all resulting in airway remodeling and eventual fibrotic scarring (78). IL-13 and IL-4 are implicated in driving these fibrotic responses, often elevated in pulmonary diseases and correlating with fibrosis severity (79, 80).

Similarly, innate type-2 cytokines may contribute to liver fibrosis induced by chronic infections like schistosomiasis and liver flukes. These infections trigger local production of IL-4, IL-5, and IL-13, associated with the formation of fibroproliferative lesions in the liver (81). Type-2 cytokine-driven liver fibrosis also plays a role in the pathology of hepatitis C and hereditary or infectious liver diseases, including primary sclerosing cholangitis, primary biliary cirrhosis, autoimmune hepatitis, biliary atresia, and non-alcoholic steatohepatitis (82).

Innate type-2 cytokines in metabolism and obesity

Obesity, characterized by abnormal fat accumulation in white adipose tissue (WAT), is a multifaceted metabolic disorder influenced by genetic and environmental factors (83). WAT's primary function is to store excess energy in the form of fat, but it also exhibits adaptive responses to various stimuli, including hormones, exercise, and prolonged exposure to cold temperatures. A crucial adaptation is the beigeing or browning of adipocytes, primarily occurring in subcutaneous WAT. This process involves the uncoupling of the mitochondrial electron transport chain, facilitating thermogenesis (83). Obesity associates with the loss of this adaptive energy-regulating mechanism.

Remarkably, alarmin IL-33, ILC2s, and type-2 cytokines play beneficial roles in countering obesity by promoting WAT browning. Although the mechanisms are not fully elucidated, they involve stromal cells in WAT producing IL-33 (84). This activates local ILC2s, inducing them to release IL-5 and IL-13. These innate type-2 cytokines promote the accumulation of eosinophils, which, in turn, produce IL-4. IL-4 converts WAT-resident macrophages into alternatively activated (M2) PPARγ⁺ macrophages. These M2 macrophages release molecules, possibly catecholamines, and/or anti-inflammatory molecules, inducing WAT browning (85). Disruptions in this delicate balance, such as the deletion of PPARγ in macrophages, are associated with insulin resistance and weight gain (85, 86) (Fig. 2).

iNKT cells contribute to WAT homeostasis and browning by producing IL-4 and IL-10, inducing the alternative activation of macrophages. Conversely, iNKT cells can also produce IFN-γ, and IFN-γ-expressing iNKT cells have been implicated in local WAT inflammation. This inflammation induces the local accumulation of proinflammatory M1 macrophages, neutrophils, mast cells, and T cells, associated with insulin resistance (87). Additionally, treating obese mice with IL-25 has been shown to induce weight loss and elevate the numbers of IL-13-expressing type-II NKT cells, the other category of NKT cells. Treating obese mice with IL-25 induced weight loss, elevated the numbers of IL-13-expressing type-II NKT cells in WAT, and adoptively transferring splenic type-II NKT cells from IL-25-treated normal mice into obese mice induced more effective weight loss and improved glucose tolerance compared to transferring ILC2 and iNKT cells (88). This finding suggests a potential hierarchy of action, highlighting the crucial role of type-II NKT cells.

In summary, innate type-2 cytokines and their downstream immune responses play crucial roles in both defense against helminths and maintaining metabolic homeostasis in adipose tissue. However, dysregulation can lead to severe pathogenic effects, contributing to common and devastating diseases such as allergies and fibrosis. Understanding the mechanisms that induce or disturb innate type-2 cytokine production holds the potential for valuable insights into developing therapeutic strategies.

Targeting TSLP, IL-25, and IL-33

Given their position at the top of the immune cascades that lead to type-2 immune diseases, particularly allergies, targeting the TSLP, IL-25, and IL-33 alarmins may be a promising way to manage these diseases (5). With regard to TSLP, this approach would neutralize its ability to activate mast cells, basophils, and other cells that contribute to type-2 immune diseases, including DCs, monocytes, NKT cells, and, in some settings, CD4⁺ Th2 cells (28). Similarly, blocking IL-25 and IL-33 would reduce their ability to induce type-2 immune responses by ILCs and other innate-immune cells (5, 12, 27). The potential of this alarmin-targeting strategy is supported by studies demonstrating that mice lacking adaptive type-2 immune responses (B and T cells) still efficiently develop type-2 inflammation in response to triggers such as allergens (47, 62). Moreover, treatment with alarmins directly induces type-2 immune responses: for example, IL-25 administration induces lung eosinophilia, IgE production, and AHR in mice (11, 12). Similarly, intravenous IL-25 administration along with a suboptimal dose of allergen generates iNKT cell-mediated AHR (13). Conversely, the development of oxazolone-induced colitis by mice is inhibited by neutralizing Abs to IL-25 or its receptor; this effect associates with reduced intestinal infiltration and IL-13 production of iNKT cells (89). Therefore, targeting alarmin signaling holds promise for alleviating type-2 inflammation-related diseases, especially those affecting mucosal barrier tissues.

To date, clinical trials have primarily examined the ability of TSLP, IL-25, and IL-33 blockers to improve allergic diseases. These drugs include the IL-33/ST2-pathway inhibitors astegolimab, itepekimab, and tozorakimab. The trials on these drugs are currently in phase-3. There are also the TSLP-pathway inhibitors tezepelumab, which has received Food and Drug Administration (FDA)-approval for asthma, and ecleralimab, an inhaled drug, is in phase-2 clinical trials (5). Moreover, Abs that specifically target TSLP have shown promise in reducing allergen-induced bronchoconstriction and airway eosinophilia in individuals with allergies, both before and after allergen exposure (90).

Despite these promising developments, the clinical value of blocking IL-25, IL-33, or TSLP remains uncertain. Thorough investigations into specific mechanisms, disease relevance, and translational research are essential to establish the full therapeutic potential and safety of these interventions in allergic diseases.

Targeting IL-4 and IL-13 signaling

Targeting IL-4 and IL-13 signaling emerges as a promising therapeutic approach, considering their pivotal roles in driving key pathogenic features in type-2 immune diseases (2). A spectrum of interventions has been developed, including antagonists, antisense oligonucleotides, microRNAs, and humanized monoclonal Abs against IL-4Rα, IL-13Rα1, GATA3, and the cytokines themselves (91). However, the clinical trials targeting IL-13 or IL-4 have yielded mixed outcomes. While initial results with anti-IL-13 Abs like lebrikizumab and tralokinumab were promising, phase-3 trials showed poor outcomes (92, 93). This discrepancy may arise from the functional overlap of IL-4 and IL-13, as they can signal through the same receptor (the IL-4Rα/IL-13Rα1 heterodimer). Despite this, IL-4 can also signal through the IL-4Rα/common gamma chain heterodimer, contributing to distinctive functions in type-2 immune diseases. Specifically, IL-13 acts as a primary disease-inducing effector cytokine, while IL-4 amplifies type-2 immunity by facilitating the expansion of the CD4⁺ Th2 cell population in secondary lymphoid organs (94). Consequently, blocking both IL-4 and IL-13 may yield more promising outcomes. Dupilumab, which binds to IL-4Rα and disrupts signaling by both cytokines, has demonstrated efficacy in reducing asthma exacerbations and received FDA approval for moderate-severe uncontrolled asthma (95). Dupilumab also shows effectiveness in other type-2 immune diseases, including allergic rhinitis (96), nasal polyposis (97), and atopic eczema (98).

However, caution is warranted, as targeting both IL-13 and IL-4 may not always be safe. Depleting both cytokines in chronic schistosomiasis leads to a skewed and lethal type-1-driven inflammatory response in the liver and intestines (99). Selectively blocking IL-13 signaling preserves protective features of the type-2 immune response while significantly reducing pathological fibrosis, resulting in less morbidity and mortality during chronic infection (100). These observations underscore the need for tailored therapeutic strategies aligned with specific disease contexts and their immunological dynamics.

Targeting IL-5 and eosinophils

Another promising therapeutic strategy for diseases associated with type-2 immunity involves targeting IL-5 and eosinophils. IL-5, produced by alarmin-activated innate-immune cells, predominantly ILC2s, strongly stimulates the activation and proliferation of eosinophils. As these cells are rapidly recruited to sites of inflammation and collectively produce substantial amounts of IL-4 and IL-13, they play critical roles in the pathophysiology of various type 2-driven diseases, including asthma, eosinophilic esophagitis, and hyper-eosinophilic disorders (3, 63). This eosinophilic response is often amplified by IL-5 that is produced by CD4⁺ Th2 cells in the affected tissue (3).

Several drugs regulating the development, recruitment, and survival of eosinophils are currently under investigation. These include humanized monoclonal Abs blocking the binding of IL-5 to IL-5Rα, which have advanced to phase-2 and phase-3 clinical trials (101). Another intervention, benralizumab, an IL-5Rα-specific monoclonal Ab, eliminates eosinophils through Ab-dependent cellular cytotoxicity. Combining IL-5-neutralizing Abs like mepolizumab and reslizumab with benralizumab effectively reduces eosinophil numbers, leading to the approval of this intervention for severe eosinophilic asthma (102, 103).

While preliminary findings appear promising, it remains imperative to ascertain whether other cell types, including ILC2s, iNKT cells, basophils, and mast cells might compensate for the reduction in eosinophils, as observed in select preclinical studies (104, 105). Exploring the potential for sustained protection achieved through the targeting of IL-5 and eosinophils is an area warranting further investigation.