Modulating ILC2 function for treatment of type 2 airway diseases

Abstract

Type 2 airway diseases including chronic rhinosinusitis, allergic rhinitis, and asthma remain a major health concern. These disorders are largely characterized by an uncontrolled type 2 immune response with elevated cytokines of IL-4, IL-5 and IL-13, eosinophilic inflammation, goblet cell hyperplasia as well as tissue remodeling. In the last few decades, critical potential roles of innate lymphoid cells (ILCs) in type 2 human diseases have emerged. Unlike their lymphocyte counterpart T cells, ILCs lack antigen-specific receptors and are largely tissue resident. Specifically, group 2 innate lymphoid cells (ILC2s) respond to airway epithelium-derived alarmins (TSLP, IL-33) and secrete high levels of type 2 cytokines. ILC2 responses can bypass the activation of T cells as well as develop corticosteroid-resistance. Currently approved biologics targeting the alarmin thymic stromal lymphopoietin (TSLP) or the IL-4/IL-13 receptor may reduce ILC2 activation, though novel treatments of type 2 airway diseases remain needed. In this review, we briefly discuss the pathogenesis of ILC2-mediated airway diseases followed by their current and potential treatments.

1. Introduction

Type 2 airway diseases including chronic rhinosinusitis with and without nasal polyps, allergic rhinitis, and most forms of asthma are common chronic medical conditions [1–4]. These chronic diseases significantly affect the quality of life of patients and leads to high economic health care burden. While current medications including nasal and inhaled steroid medications provide long term control for many patients, a substantial portion continue to remain uncontrolled and develop more severe/persistent airway disease. Importantly, group 2 innate lymphoid cells (ILC2s) can also develop corticosteroid resistance under some conditions which implicates their involvement in steroid-resistant and refractory type 2 airway diseases [5–8]. Thus, targeting ILC2 activation may be an important way to treat type 2 airway diseases.

2. ILC2s in type 2 airway diseases

The type 2 immune response in the airway is orchestrated by Th2 cells, ILC2s, IgE-producing B cells, mast cells, basophils, and eosinophils. Type 2 cytokines (IL-4, IL-5, IL-13) drive eosinophil development and chemotaxis, IgE class switch and production, Th2 cell differentiation, tissue repair and remodeling, airway hyperresponsiveness (AHR) and mucus production [9]. CD4⁺ Th2 cells and ILC2s are the major source of type 2 cytokines. IL-4 induces class switching of B cells to IgE which then attach to FcER1 on mast cells and basophils.

When allergens bind to allergen-specific IgE present on mast cells and basophils, IgE cross linking occurs leading to degranulation that characterizes type 1 hypersensitivities [10]. IL-5 promotes infiltration of eosinophils that release a wide range of inflammatory mediators including granule proteins (e.g. ECP, EDN, EPO, MBP), cytokines (e.g. IL-1β, TGF-β, GM-CSF), chemokines (e.g. RANTES), lipid mediators (leukotrienes and prostaglandins), and superoxide [11]. Eosinophils have been identified to release DNA to form eosinophil extracellular traps in some airway diseases [12–15]. This extracellular form of DNA is a potent inflammatory stimulator, and it may be associated with the severity of the diseases [16, 17]. Eosinophils may also be considered as a critical feature in diagnosis [18, 19]. IL-13 shares a receptor with IL-4 (both using the IL-4Rα) and is a central mediator of airway hyper responsiveness, remodeling, mucus production and contributes to type 2 inflammatory cell influx.

ILC2s were first characterized in 2010 and are a population of “lineage-negative” lymphoid cells that do not express antigen-specific receptors. They respond to tissue-derived alarmins, such as IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) and act as tissue resident sentinels [9]. Once activated, ILC2s secrete large amounts of IL-5 and IL-13 (and IL-4 in some contexts), and promote type 2 inflammation, eosinophilia, mucus production, and tissue remodeling. The alarmin IL-33 is pre-synthesized and is released rapidly in response to epithelial damage [11, 20]. TSLP is another alarmin that amplifies ILC2 responses, especially in synergy with other mediators such as IL-33. Thus, the alarmins remain viable targets to reduce ILC2 activation that contributes to type 2 airway inflammation.

3. Current medications for type 2 airway diseases and effects on ILC2s

Currently, there are several medications available to treat type 2 airway diseases that fortunately help many patients with rhinosinusitis and asthma symptoms (Table 1). Some therapies function as symptom relivers upon acute symptom onset, such as H1 antihistamines, bronchodilators, and mast cell stabilizers. Others are used as controller therapy and include leukotriene modifiers, intranasal and inhaled corticosteroids. Bronchodilators include beta-2 agonists and muscarinic antagonists and are either short or long acting. These are often used in combination with inhaled corticosteroids as either reliever and/or maintenance therapy. Interestingly, beta-2 agonists and muscarinic antagonists have shown some ILC2 suppressive activity in mouse models and theoretically might contribute to their efficacy beyond bronchodilation [21–24]. The leukotriene receptor antagonist montelukast blocks the binding of cysteinyl leukotrienes (mainly LTC4 and LTD4) to CysLT1 receptor. In mice and humans, cysteinyl leukotrienes (CysLTs) demonstrate clear ILC2-activating properties and enhance alarmin-induced responses [25–27]. Intranasal and inhaled, and sometimes systemic, corticosteroids are the most common used medication for long term control of type 2 airway diseases. They have broad effects including induction of immune cell apoptosis, suppression of both cytokine secretion from T cells and antibody production from B cells, and reductions in dendritic cell function [28]. Under conditions of alarmin activation by IL-33 and TSLP, ILC2s develop a corticosteroid resistance though the effects of alarmin activation are not universally observed and may also be related to dose [7, 8, 29–32]. These established treatments may have some effect in partially reducing ILC2 activation on a certain level, but despite these treatments including systemic corticosteroids, activated ILC2s are still recovered from the airways of patients with severe and refractory airway disease [6, 8, 29, 30].

Table 1.

Type 2 disease therapies and their potential to target ILC2.

Medication	Mechanism of action	Effects on ILC2 function?
Currently approved
β2 agonists	Relaxes the smooth muscle by binding to beta-adrenergic receptors within the bronchioles (Salbutamol, Terbutaline, Salmeterol, and Formoterol)	β2AR negatively regulates ILC2 proliferation and effector function [21]
Muscarinic antagonists	Decreases bronchoconstriction by binding to and blocking neural signals from parasympathetic muscarinic receptors M1, M2, and M3 (Ipratropium and Tiotropium)	Suppresses IL-4 production from basophils and, subsequently, regulates ILC2 activation [22] Suppresses ILC2 by blocking acetylcholine signaling of ILC2 [23, 24]
Leukotriene receptor antagonists	Inhibits the cysteinyl leukotriene CysLT1 receptor (Montelukast and Zafirlukast) Inhibits the synthesis of leukotrienes, LTB4, LTC4, LTD4, and LTE4, by antagonizing 5-lipoxygenase (Zileuton)	Suppresses ILC2 function by reducing cytokine production and cell migration [25, 26]
Corticosteroids	Modulates the inflammatory and anti-inflammatory gene expression by glucocorticoid receptors (Prednisone, Methylprednisolone, and Dexamethasone)	Potentially suppresses ILC2s through MEK/JAK-STAT signalling pathways [41]
αIL-4Rα	Binds to the receptor subunit IL-4Rα to block IL-4 and IL-13 signaling (Dupilumab)	Suppresses ILC2 function by reducing cytokine production and cell proliferation [42]
αIL-5	Binds to IL-5 to block IL-5 signaling (Mepolizumab)	No direct effects on ILC2s [34]
αIL-5Rα	Binds to the receptor subunit IL-5Rα to block IL-5 signaling and contributes to ADCC (Benralizumab)	No direct effects on ILC2s [34]. May trigger ADCC for IL-5R+ILC2 [6], and indirect regulation may occur, such as via eosinophils or basophils [43, 44]
αTSLP	Binds to human TSLP at the TSLP receptor to block TSLP signaling (Tezepelumab-ekko)	Prevents ILC2 activation via TSLP [45]
Under investigation
CRTH2 antagonists	Binds to PGD2 receptor CRTH2 to prevent PGD2 signaling (GB001, Fevipiprant)	Binds to CRTH2 on ILC2s to prevent activation [39, 46, 47]
αIL-33	Binds to human IL-33 to prevent IL-33 signaling (Itepekimab)	Binds to IL-33 to prevent ILC2 activation [36, 37]
αST-2	Binds to IL-33 receptor ST-2 to prevent IL-33 signaling (Astegolimab)	Binds to ST2 receptor on ILC2s to prevent activation [38]
GATA-3 DNA enzyme (DNAzyme)	DNAzyme that cleaves and inactivates GATA-3 messenger RNA (SB010)	Could attenuate ILC2 differentiation via GATA-3 inactivation [40]

The recent approval of biologics that target type 2 airway diseases (asthma and chronic rhinosinusitis with nasal polyps) has improved the lives of countless patients. Currently approved biologics include dupilumab (αIL-4Rα), mepolizumab and reslizumab (αIL-5), benralizumab (αIL-5Rα), omalizumab (αIgE), and tezepelumab (αTSLP) (Table 1). Precisely how these biologics regulate ILC2 responses is unknown, but their mechanisms of action may be unmasked by mouse models and in vitro studies. The IL-4Rα is expressed by ILC2s and binds to both IL-4 and IL-13 and induces STAT6 signaling that also regulates ILC2 proliferation in vivo in mice [33]. Dupilumab which is approved for asthma, atopic dermatitis, eosinophilic esophagitis, and nasal polyposis binds to IL-4Rα and could potentially limit ILC2 proliferation and possibly activation. Human ILC2s have also been reported to express IL-5R but this remains controversial as other studies have found no expression of IL-5R [6, 34]. Benralizumab induces apoptosis of IL-5R-expressing cells and reslizumab and mepolizumab bind to IL-5 preventing effects at IL-5R. The anti-IL-5/IL-5R biologics likely regulate ILCs predominately by reducing eosinophils that are key sources of lipid mediators PGD2 and CysLTs that activate ILC2s [35]. Recently, tezepelumab (anti-TSLP) was approved in the US for treatment of severe asthma uncontrolled by other therapies. As TSLP is a critical alarmin regulator of ILC2s, the expectation of anti-TSLP therapy would be to reduce ILC2 activation. Overall, current therapy for type 2 airway diseases may reduce ILC2 activation through varying mechanisms.

4. Investigational treatments that may reduce ILC2 responses

Investigational therapies have the potential to regulate ILC2s in airway diseases. Currently, there are promising phase 2 clinical trials targeting IL-33 signaling with anti-IL-33 antibody itepekimab demonstrating positive effects on asthma and chronic obstructive pulmonary disease (COPD) treatment [36, 37]. A clinical trial against IL-33 receptor ST-2 astegolimab also passed the phase 2 with positive results [38]. There are numerous studies aiming to target CRTH2 which is highly expressed on human ILC2s. Unfortunately, CRTH2 antagonists have been largely disappointing in trials though the PGD2 antagonist GB001 has shown potential [39]. Finally, a novel therapy to antagonize the master type 2 transcription factor GATA-3 has shown positive proof of principle outcomes in an allergen challenge study in asthmatics [40].

5. Conclusion

Type 2 airway diseases including asthma and chronic rhinosinusitis are a significant public health problem globally. Current treatments improve the lives of many patients, but novel treatments are needed for those with more severe disease. ILC2s are critical sources of type 2 cytokines and current therapies may have varying effects on ILC2 function. However, an understanding of ILC2 biology could lead to novel targets of therapy for those with refractory type 2 airway diseases.