Group 2 innate lymphoid cells in nasal polyposis

Abstract

Objective:

Chronic rhinosinusitis with nasal polyps (CRSwNP) is characterized by a chronic type 2 inflammatory response in the paranasal sinuses. Group 2 innate lymphoid cells (ILC2) are potent innate immune cells that contribute to type 2 inflammation by producing cytokines such as IL-4, IL-5 and IL-13. There is increasing evidence suggesting that ILC2 play an important role in CRSwNP pathogenesis.

Data Sources:

We reviewed published literature obtained through PubMed inquiries. Study Selections: Studies relevant to the presence, function, and activation of ILC2 in CRSwNP were included.

Results:

Nasal polyps (NP) are one of the first tissues in which human ILC2 were discovered and many groups have since reported these cells are significantly elevated in NP. ILC2 in NP are also highly activated and produce type 2 cytokines in vivo . Mediators known to activate ILC2 including RANK-L, TSLP, various lipid mediators (including prostaglandin D 2 and cysteinyl leukotrienes), IL-4, and IL-13 have also been shown to be elevated in NP compared to healthy sinonasal tissue. Other well- known ILC2 activators, IL-25 and IL-33 are sometimes elevated in NP in some countries. Furthermore, activation of ILC2 via four distinct transcriptional pathways (NF-kB, NFAT, STAT5 and STAT6) is needed for the most robust generation of type 2 cytokines.

Conclusion:

ILC2-mediated type 2 inflammation plays a crucial role in the pathogenesis of CRSwNP. Targeting upstream mediators responsible for activating ILC2 as well as downstream products these cells release may play an important role in modifying the inflammatory response and improving clinical outcomes in CRSwNP.

Introduction

Chronic rhinosinusitis (CRS) is a common disease that affects 5–12% of the general population and is characterized by local inflammation of the paranasal sinuses and nose.^{1, 2} Clinically, patients with CRS report two or more of the following symptoms for a duration of at least 12 weeks: nasal congestion, nasal drainage (anterior or posterior), reduced sense of smell, and/or sinus pressure or pain. CRS is frequently divided into the two major phenotypes: CRS with nasal polyps (CRSwNP) and CRS without nasal polyps (CRSsNP). Patients with CRSwNP on average tend to have more severe clinical disease with an estimated $5.7 billion spent annually in healthcare costs in the US.³

Over the past several years, significant advances have been made in understanding the underlying mechanisms contributing to CRSwNP pathogenesis. In Western countries, CRSwNP is overwhelmingly characterized by type 2 inflammation with pronounced eosinophilia and the presence of high levels of type 2 cytokines, such as IL-4, IL-5 and IL-13.^4–6 Indeed, our group recently reported that 87% of patients with CRSwNP had type 2 inflammation in the US.⁷ In addition, Wang et al. found that 85% and 73% of CRSwNP patients had type 2 inflammation in Europe and Australia, respectively.^{8, 9} While still lower than in Western countries, the prevalence of type 2 inflammation in nasal polyps (NP) of patients in Asia is also increasing.^{4, 8}

Given the type 2 inflammatory profile observed in CRSwNP, numerous studies have focused on identifying which specific cell types may have key roles in promoting this response. In general, type 2 inflammatory responses were historically thought to be regulated mainly by CD4⁺ T helper 2 (Th2) cells but mast cells, basophils and group 2 innate lymphoid cells (ILC2) have also been identified as important producers of type 2 cytokines. While all these cell types have been implicated to varying degrees in CRSwNP pathogenesis,¹⁰ this review will specifically focus on ILC2 in NP.

Overview of group 2 innate lymphoid cells

Similar to CD4⁺ helper T cells, innate lymphoid cells (ILC) can be classified into 3 major subsets according to their function and production of cytokines: group 1 ILC (ILC1), group 2 (ILC2) and group 3 (ILC3). ILC2 are distinguished from the other ILC subtypes based on their expression of transcription factors GATA3 and RORκκ as part of their differentiation from a common ILC precursor cell.¹¹ Additionally, ILC2 are distinguished by their production of type 2 cytokines, IL-4, IL-5 and IL-13 upon activation.^{11, 12} Importantly (and unlike CD4⁺ T cells), ILC2 are activated by antigen-independent mechanisms with numerous factors including IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) playing critical roles as discussed below.

Human ILC2 were discovered by Mjosberg et al. in 2011.¹³ Human ILC2 can be identified by flow cytometry as being CD45⁺ lymphoid cells that are Lineage (CD1a, CD3, CD4, CD16, CD19, CD34, CD94, CD303, Fcκ RI) negative and CD127⁺ (also known as IL-7Ra) CD161⁺ CRTH2⁺.^{11, 12} However, ILC2 also display heterogeneity.¹¹ Ricardo-Gonzalez et al. performed single-cell RNA-sequencing (scRNA-Seq) on mouse ILC2 isolated from bone marrow, lung, fat, gut and skin and found variability in surface marker expression. For example, IL-18 receptor (IL-18R1), IL-25 receptor (IL-17RB) and IL-33 receptor (ST2; IL-1RL1) were especially enriched on ILC2 in skin, gut and lung, respectively.¹⁴ The IL-33 receptor is also abundant on bone marrow and fat ILC2. In contrast, other common ILC2 markers such as GATA3, IL-7Ra and TSLP receptor (TSLPR; CRLF2) were expressed more uniformly across all tissues.¹⁴ Liu et al. recently found that some ILC2 subsets in asthmatics lack cell surface expression of CD127 and/or CRTH2 but produce type 2 cytokines.¹⁵ Although the heterogeneity of human ILC2 has become clearer, there are no reports about this variability within CRS. As a result, this review will focus on CD127⁺CRTH2⁺ ILC2.

ILC2 in CRSwNP

Sinonasal mucosae are one of the first tissues in which human ILC2 were discovered.¹³ This initial study also found ILC2 were elevated in NP compared to control sinus tissue in a small cohort in the Netherlands.¹³ After this key finding, many groups confirmed the accumulation of ILC2 in NP tissue. Shaw et al. showed the first evidence in the US that ILC2 are elevated in NP.¹⁶ Subsequently, Walford et al. found that ILC2 are elevated in eosinophilic NP but not in non-eosinophilic NP and that frequency of ILC2 positively correlates with eosinophils in NP tissue in the US.¹⁷ Miljkovic et al. found that ILC2 are significantly enriched in patients with CRSwNP and that ILC2 numbers significantly correlate with Th2 cell frequency in sinus mucosa in Australia.¹⁸ Tojima et al. also showed that ILC2 are elevated in eosinophilic NP but not in non-eosinophilic NP, however the frequency of peripheral blood ILC2 is similar between patients with eosinophilic CRSwNP and non-eosinophilic CRSwNP in Japan.¹⁹ Our group carefully characterized the presence of all major ILC subsets in CRS tissues in the US and found that ILC2 are the predominant ILC in NP and are 100-fold elevated in NP compared to sinus mucosa of control subjects.²⁰ In contrast, the frequency of peripheral blood ILC2 is similar between control subjects and patients with CRSwNP,²⁰ confirming the finding by Tojima et al.¹⁹ These studies suggest that ILC2 are highly elevated in eosinophilic NP worldwide and that this elevation is not systemic but rather is restricted to NP and possibly other inflamed tissue within the sinonasal cavity.

In addition to the accumulation of ILC2 in NP, ILC2 may be more activated in NP compared to the circulation. Several groups have reported that inducible T cell co-stimulator (ICOS) on ILC2 controls type 2 inflammation and is highly elevated on activated ILC2 in mice.^{11, 21} Our group found that cell surface expression of ICOS is up-regulated on ILC2 in NP compared to ILC2 in the peripheral blood.²⁰ We also found that CD127 is down-regulated on NP ILC2 in vivo and that cell surface expression of CD127 on peripheral blood ILC2 can be down-regulated upon activation in vitro.²⁰ In contrast, the levels of side scatter (SSC), that measure the granularity or complexity of the cell, are increased in peripheral blood ILC2 by activation in vitro and are significantly increased in NP ILC2 in vivo.²⁰ Furthermore, ILC2 sorted from NP tissue but not from peripheral blood spontaneously release type 2 cytokines IL-5 and IL-13 without additional stimulation ex vivo.²⁰ Finally, we showed that levels of SSC and CD127 on ILC2 correlated with tissue IL-5 and IL-13 in NP.²⁰ Taken together, these results suggest that accumulated ILC2 are highly activated and contribute to type 2 cytokine production in NP in vivo.

Activation of ILC2

Given that ILC2 are elevated in NP and contribute to the type 2 inflammatory response, studies have focused on identifying what factors are responsible for activating these important innate cells in CRSwNP. Classically, epithelial-derived innate cytokines IL-25, IL-33 and TSLP were identified as key inducers of type 2 cytokines in ILC2. However, it is now clear that ILC2 express a wide variety of receptors on their surface, some of which can induce the production of type 2 cytokines when engaged with their respective ligands.¹¹

Growing evidence also suggests that there are 4 key transcription factors, NF-κB, NFAT, STAT5 and STAT6 that control type 2 cytokine production by ILC2 (Figure 1). Activation of NF-κB and NFAT directly induces type 2 cytokine production in ILC2. In contrast, activation of STAT5 and STAT6 does not seem to directly induce but instead can potently enhance induction of type 2 cytokines in ILC2 when NF-κB and/or NFAT activators are present. In this section, we will discuss key inflammatory mediators that alone or in synergy with other mediators activate ILC2.

Figure 1. Schematic of transcription pathways involved in the activation of ILC2.

IL-1 family cytokines, TNF family cytokines and IL-25 weakly induce type 2 cytokines via activation of NF-κB. Activation of NFAT by lipid mediators also weakly induces type 2 cytokines. IL-2 family and IL-7 family cytokines potently enhance NF-κB- and NFAT-mediated induction of IL-4, IL-5 and IL-13 via the activation of STAT5. Activation of STAT6 by IL-4 and IL-13 also controls production of type 2 cytokines and activation of all 4 transcription factors maximally induces type 2 cytokines in ILC2. Potential therapeutic targets for ILC2 activating factors in NP are also listed in this figure. CysLT1R: cysteinyl leukotriene receptor 1.

IL-25

IL-25 is a member of the IL-17 cytokine family that can activate NF-κB and promote type 2 cytokine production in ILC2 (Figure 1).²² Our group recently examined the presence of IL-25 in NP in the US using a larger cohort than most studies. We found levels of IL-25 were almost undetectable in NP tissue.²³ In contrast, at least 8 studies have reported elevations of IL-25 in NP. Interestingly, most of these observations were in Asian countries and this raises the possibility that IL-25 may only be involved in Asian eosinophilic NP pathogenesis.²³ However, a US study by Kohanski et al. recently found IL-25 to be expressed in a minor subset of epithelial cells called solitary chemosensory cells and these cells are increased in NP.²⁴ This may suggest that the number of solitary chemosensory cells present within the NP may impact how much IL-25 is detected and that IL-25 may play a role in NP even in Western countries. Future studies will be required to more carefully examine the importance of IL-25 in Western NP.

IL-33 and IL-1 family cytokines

IL-33 is a member of the IL-1 cytokine superfamily and can also activate the transcription factor NF-κB to induce ILC2 production of type 2 cytokines (Figure 1).²⁵ However, studies are mixed regarding the presence of IL-33 in CRSwNP. We found IL-33 was detectable but not elevated in NP tissue when compared to sinus tissues from control patients.²³ Furthermore, IL-33 protein was mainly localized in the nuclei of epithelial and endothelial cells suggesting that existing IL-33 may have more limited effector activity in CRSwNP.²³ Although other IL-1 family cytokines (IL-1κ, IL-1κ and IL-18) are able to induce type 2 cytokines in ILC2 through NF-κB,^{26, 27} the presence of IL-18 has not been well characterized to date and there are inconsistent results for IL-1 in Western NP.^{28, 29} In contrast, elevations in IL-1κ and IL-18 are reported in Asian NP.^{30, 31}

TSLP

TSLP is mainly produced by epithelial cells and activates the STAT5 transcription factor. Our group found that TSLP was significantly elevated in NP compared to control sinus tissues.^{11, 23, 32} We also found that TSLP activity in NP tissue was controlled by the post-translational modifications of tissue proteases and that this stable and active TSLP metabolite stimulates ILC2 much more potently than mature TSLP.^{32, 33} Importantly, unlike IL-25 or IL-33, up-regulation of TSLP in eosinophilic NP is highly reproducible through the world.²³

TNF superfamily members

The TNF superfamily (TNFSF) is a large group of cytokines that have diverse regulatory functions during an immune response through activation of NF-κB. Currently, 4 members of the TNFSF including TNF (also known as TNFκ, [TNFSF2]), receptor activator of NF-κB ligand (RANK-L [TNFSF11]), TNF-like cytokine 1A (TL1A [TNFSF15]) and glucocorticoid-induced TNF-related ligand (GITR-L [TNFSF18]) are known to induce production of type 2 cytokines from ILC2.^{11, 34–37}

When screening for these four TNFSF members in CRSwNP, RANK-L was the only one elevated in NP and this membrane bound cytokine was expressed on Th2 cells and CXCL16⁺ dendritic cells (Figure 2).³⁵ We also found the receptor for RANK-L, RANK, was expressed on ILC2 in NP.³⁵ When NP-derived ILC2 (expressing RANK) were co-cultured with NP-derived Th2 cells or with CXCL16⁺ dendritic cells (expressing RANK-L), we observed a significantly enhanced production of IL-5 and IL-13.³⁵ These results suggest that RANK-L may be a key activator of NF-κB and is a critical controller of type 2 cytokine production from ILC2 in NP (Figures 1 and 2).

Figure 2. Potential roles for ILC2 in driving CRSwNP pathogenesis.

In nasal polyps, ILC2 contribute to type 2 inflammation via the production of type 2 cytokines. ILC2 can be activated by a variety of mediators that are elevated in NP including TSLP from epithelial cells; RANK-L from Th2 cells and CXCL16⁺ dendritic cells (DC); and lipid mediators (PGD₂, LTC₄ and LTD₄) from mast cells. An autocrine/paracrine loop of PGD₂ is essential in cytokine-induced ILC2 activation Epithelial derived cytokines IL-25 and IL-33 may also play a role in ILC2 activation and type 2 cytokine production in NP. Following activation, ILC2 produce IL-5 which promotes eosinophilia as well as IL-4 and IL-13 which have a broad range of inflammatory effects increasing the production of eosinophil chemoattractants (CCL11, CCL24, and CCL26) as well as promoting mucus production, fibrin deposition, B cells production of IgE, and epithelial barrier dysfunction. IL-4 and IL-13 can also serve in an autocrine/paracrine loop by further activating ILC2 and enhancing their production of type 2 cytokines. Ag: antigen, FXIII-A: factor XIII-A, tPA: tissue plasminogen activator.

Lipid Mediators

Lipid mediators including prostaglandin D₂ (PGD₂) and cysteinyl leukotrienes (LTC₄, LTD₄ and LTE₄) can induce ILC2 migration and production of type 2 cytokines via the transcription factor NFAT (Figure 1).^{11, 38, 39} Notably, unlike members of the IL-1 and TNF families, PGD₂ and CysLTs potently induce production of IL-4 by ILC2.^{38, 40} In CRSwNP, PGD₂ and cysteinyl leukotrienes were found to be elevated in NP compared to healthy sinonasal tissue.^{41, 42} Additionally, the receptors for PGD₂ and LTC₄, CRTH2 and cysteinyl leukotriene receptor 1 respectively, are highly expressed on human ILC2.^{38, 39}

Synergy among co-factors

It is important to note that cytokines in the IL-2 family (e.g. IL-2) and IL-7 family (IL-7, TSLP) which signal through STAT5 are not potent inducers of type 2 cytokines in ILC2.¹¹ However, these factors can synergistically enhance the production of type 2 cytokines generated through activation of NF-κB (via IL-25, IL-33 and TNFSF) or NFAT (via lipid mediators). Indeed, IL-33, TNF, RANK-L and lipid mediators are able to only minimally induce type 2 cytokines in ILC2 in the absence of IL-2, IL-7 or TSLP.^{34, 35, 39, 43, 44} This suggests that IL-2 family and IL-7 family cytokines are necessary for the robust production of type 2 cytokines from ILC2 in NP (Figure 1). Importantly, TSLP is highly elevated in NP (see above) and may be a key synergistic co-factor for ILC2-mediated type 2 inflammation in NP.

ILC2 in Aspirin Exacerbated Respiratory Disease

Approximately 8–16% of patients with CRSwNP also have comorbid asthma and an intolerance to medications that inhibit the cyclooxygenase-1 enzyme.^{45, 46} Patients with this triad of conditions are clinically referred to as having aspirin-exacerbated respiratory disease (AERD). Within minutes to hours of ingesting non-steroidal anti-inflammatory drugs (NSAIDs), patients with AERD develop significant upper and lower respiratory symptoms including rhinorrhea, nasal congestion, shortness of breath, and wheeze. However, even in the absence of NSAID ingestion, patients with AERD still have on average more severe sinonasal disease than patients with CRSwNP alone.⁴⁶

The underlying mechanisms which lead to the more severe sinonasal disease at baseline as well as the acute symptoms following NSAID ingestion are not fully known. Unlike CRSwNP, AERD is uniquely characterized by a dysregulation in arachidonic acid metabolism that leads to an over-production of cysteinyl leukotrienes, reduction in PGE₂ and elevated levels of PGD₂.^{41, 47–49} Additionally, AERD is also characterized by an enhanced type 2 inflammatory response, with elevated levels of type 2 cytokines, eosinophils, and eosinophil granular proteins compared to healthy sinonasal tissue.^{6, 41}

In the case of ILC2, we found that ILC2 were elevated in NP from patients with AERD (n=8) and CRSwNP (n=29) compared to control sinus tissue and there was no difference between CRSwNP and AERD (unpublished observation). However, when AERD patients underwent aspirin challenges, Eastman et al. found that the frequency of ILC2 in nasal epithelium significantly increased whereas peripheral blood ILC2 numbers decreased at the time of the reaction.⁵⁰ This study suggests that during acute reactions to NSAIDs, ILC2 may be migrating into NP tissue from peripheral blood and that ILC2 numbers in NP may be further escalated. Furthermore, Eastman et al. reported a significant correlation between the fold change in ILC2 numbers in nasal epithelium and patient symptoms suggesting that ILC2 may be important contributors to clinical disease.⁵⁰

In the case of ILC2 activators, our group found that expression of IL-25, IL-33, TSLP and RANK-L is similar between patients with CRSwNP alone and AERD.^{23, 35} In contrast, Liu et al. and Buchheit et al. showed that IL-33 and TSLP are significantly elevated in AERD NP compared to NSAID-tolerant NP.^{51, 52} Factors contributing to the variability in IL-25, IL-33, and TSLP expression in AERD nasal polyps warrant further studies. Finally, urinary PGD₂ metabolites and LTE₄ are elevated in patients with AERD especially during NSAID–induced reactions when compared to NSAID-tolerant CRSwNP patients.^50–53 This elevation in PGD₂ and cysteinyl leukotrienes may lead to the enhanced recruitment of ILC2 into the NP of patients with AERD compared to NSAID-tolerant patients with CRSwNP at baseline as well as during aspirin-induced inflammatory responses. Importantly, Maric et al. showed that human ILC2 also produce PGD₂ by upon stimulation and this autocrine/paracrine production of PGD₂ is essential for type 2 cytokine production in ILC2.⁵⁴ Maric et al. also found that human ILC2 express receptors for PGE₂ (EP2 and EP4) and that PGE₂, decreased in AERD, inhibits proliferation and production of type 2 cytokines in ILC2.⁵⁵ The imbalance of the PGD₂/PGE₂ ratio found in AERD patients may be involved in the worsening of ILC2-mediated type 2 inflammation.

Therapeutics targeting ILC2 in CRSwNP

Glucocorticoids remain the mainstay medical treatment option for patients with CRSwNP.^{1, 2} Despite having wide-ranging anti-inflammatory properties, some patients with CRSwNP, especially those with severe disease, are refractory to treatment and surgery. As a result, alternative medical treatment options are needed. Given that ILC2 are potent innate immune cells capable of promoting type 2 inflammation, these cells could be a promising target for a novel therapeutic. Unfortunately, none are currently available that specifically target only ILC2 on a cellular level. However, significant advances have been made in developing novel therapeutics that more specifically target downstream mediators released from activated ILC2 or upstream mediators involved in activating ILC2 as discussed below.

Glucocorticoids and ILC2

Walford et al. reported the first evidence that systemic glucocorticoids reduce ILC2 frequency in NP.¹⁷ Our group showed that dexamethasone suppresses spontaneous production of IL-5 and IL-13 in NP-derived ILC2 ex vivo.⁴⁴ We also found that IL-33, TNF and RANK-L mediated production of type 2 cytokines from ILC2 was inhibited by dexamethasone.^{34, 35, 44} This suggests that the suppression of ILC2 number and effector function may be two mechanisms explaining why glucocorticoid use is associated with clinical improvement in CRSwNP. However, it is very likely that other anti-inflammatory properties of glucocorticoids also play critical roles in improving clinical disease.

Targeting type 2 inflammatory cytokines produced by ILC2

As mentioned above, levels of the type 2 cytokines including IL-5 and IL-13 are elevated in the nasal polyps of patients with CRSwNP compared to healthy controls. In the past several years, biologics have been developed that target soluble IL-5 (mepolizumab, reslizumab), the IL-5 receptor alpha chain (benrazliumab), or the IL-4 receptor alpha chain (dupilumab). While an exhaustive summary of these biologics is outside the scope of this review, dupilumab is the only agent currently FDA approved for the treatment of CRSwNP.

By blocking IL-4 and IL-13 signaling, dupilumab was shown to significantly reduce nasal polyp size, improve sinonasal opacification on sinus CT scan, and improve sinonasal symptoms in patients with CRSwNP.^{56, 57} The exact mechanism(s) by which dupilumab improves clinical outcomes is currently unclear but there are numerous possibilities since IL-4 and IL-13 are known to increase production of eosinophil chemoattractants as well as promote mucus production, tissue remodeling, fibrin formation, B cell production of IgE, and epithelial barrier dysfunction in CRSwNP (Figure 2).⁴

There has been a more recent study suggesting dupilumab may have direct effects on ILC2. Patel et al. found significantly reduced numbers of peripheral blood ILC2 in asthmatics treated with dupilumab than those who were not.⁵⁸ Additionally, gene expression levels of IL-5 and IL-13 were also significantly dampened in peripheral blood ILC2 of asthmatic patients who received dupilumab compared to those who did not.⁵⁸ Separately, our group showed that a STAT6 inhibitor suppresses ILC2-mediated production of type 2 cytokines in vitro.³⁴ In a mouse model of helminth infection, ILC2 deficient in STAT6 had reduced proliferation when compared to wild type ILC2 containing STAT6.⁵⁹ This suggests that IL-4 and IL-13 are not just produced by activated ILC2 but also may be involved in an autocrine/paracrine pathway that amplifies ILC2 activation through activation of STAT6 (Figures 1 and 2). It may also be that these effects on ILC2 are mediated by IL-4 but not by IL-13⁵⁸ but future careful validation studies will be required to examine how IL-4, IL-13, and dupilumab control ILC2-mediated type 2 cytokine responses in humans.

Targeting inflammatory mediators that activate ILC2

Instead of targeting downstream mediators released from activated ILC2, another approach to regulate ILC2 effector function would be to target upstream mediators as a way to potentially prevent ILC2 activation. Etokimab is a monoclonal antibody that targets soluble IL-33. In a phase 2a clinical trial for patients with severe eosinophilic asthma, a single dose of Etokimab was shown to improve FEV1 as well as asthma symptoms when compared to placebo (NCT03469934. https://www.anaptysbio.com/pipeline/etokimab/). In contrast, in a phase 2b clinical trial for patients with atopic dermatitis, another type 2 inflammatory condition, Etokimab failed to meet the primary clinical endpoint.⁶⁰ Etokimab is currently being evaluated in a phase 2 clinical trial (NCT03614923) for patients with CRSwNP. Additionally, two other biologics targeting IL-33, AMG282 and PF-06817024, are in phase 1 clinical trials to address safety and tolerability among patients with CRSwNP (NCT02170337 and NCT02743871 respectively). At the time of publication, the outcomes of these studies have not yet been reported.

Several other biologics and small molecules that target upstream mediators involved in ILC2 activation are currently in development. Tezepelumab is a human monoclonal antibody targeting TSLP that has been studied in patients with severe asthma⁶¹ but not with CRSwNP to date. Compared to placebo, tezepelumab was associated with a significant reduction in asthma exacerbation rate as well as an improvement in lung function.⁶¹ Additionally, fevipiprant, a CRTH2 antagonist, is being actively studied in a clinical trial for patients with CRSwNP (NCT03681093). Interestingly, this drug did not provide clinical benefit in asthmatics⁶² and it will be interesting to see if this drug (as well as tezepelumab) improves clinical outcomes in CRSwNP and how these agents may impact ILC2. Finally, anti-RANK-L (denosumab) and leukotriene receptor antagonists are commercially available for treatment of osteoporosis and asthma, respectively.^{63, 64} These agents may provide clinical benefit in a subset of patients with CRSwNP and may be worth further investigations (Figure 1).

Conclusion

ILC2 are important innate immune cells associated with type 2 inflammatory responses. In CRSwNP, increasing evidence suggests ILC2 are significantly elevated and activated in NP and contribute to the increased levels of type 2 cytokines including IL-4, IL-5 and IL-13 observed in disease. Several inflammatory mediators and transcription factors have been identified that can activate ILC2 either by themselves or in synergy with others. Targeting upstream mediators responsible for activating ILC2 as well as downstream products these cells release may play an important role in modifying the inflammatory response and improving clinical outcomes in CRSwNP.

Key Messages:

• Type 2 cytokines, IL-4, IL-5 and IL-13 are highly elevated and play a critical role in the pathogenesis of CRSwNP.

• Antigen-independent innate type 2 inflammatory responses are mainly controlled by ILC2 which are highly elevated in nasal polyps.

• ILC2 production of type 2 cytokines is controlled by at least 4 transcription factors: NF-κB, NFAT, STAT5 and STAT6 and activation factors for these pathways are all elevated in nasal polyps.

• Lipid mediators including PGD₂ and cysteinyl leukotrienes are significantly elevated in nasal polyps of patients with AERD and may further enhance ILC2-mediated type 2 inflammation in this condition.

• Targeting upstream mediators responsible for activating ILC2 as well as downstream products these cells release may play an important role in modifying the inflammatory response and improving clinical outcomes in CRSwNP.