Rethinking Neutrophils and Eosinophils in Chronic Rhinosinusitis

Abstract

Chronic rhinosinusitis (CRS) often is characterized by an eosinophilic inflammatory pattern, nowadays referred to as type 2 inflammation, although the mucosal inflammation is dominated by neutrophils in about a third of the patients. Neutrophils are typically predominant in 50% of CRS without nasal polyps (CRSsNP), but also are found to play a role in severe type 2 CRS with nasal polyp (CRSwNP) disease. This review aims at summarizing the current understanding of the eosinophilic and neutrophilic inflammation in CRS pathophysiology, and provides a discussion of their reciprocal interactions and the clinical impact of the mixed presentation in severe type 2 CRSwNP patients. A solid understanding of these interactions is of utmost importance when treating uncontrolled severe CRSwNP with biologicals that are preferentially directed towards type 2 inflammation. We here focus on recent findings on both eosinophilic and neutrophilic granulocytes, their subgroups and the activation status, and their interactions in CRS.

Heterogeneity of CRS

Chronic rhinosinusitis (CRS) is an increasing health problem affecting up to 15% of the population in western countries. CRS patients are phenotypically classified as CRS with nasal polyps (CRSwNP) and CRS without nasal polyps (CRSsNP). (1, 2) While phenotyping of CRS is well established, it has been demonstrated that both CRS phenotypes can be further differentiated into endotypes, based on the underlying immune responses and cellular differentiation. (3-5) CRSwNP is in general the most severe phenotype with high rates of recurrence and comorbid asthma, and is traditionally characterized by a strong type 2 biased eosinophilic inflammation and Staphylococcus (S.) aureus colonization rates of 67%. (5-8) CRSsNP on the other hand has long been considered a type 1 – type 17 inflammation, with increased levels of IFN-γ, TNF-α, IL-17 and IL-21, and a predominant presence of neutrophils. (9-11) While this dichotomous type 1 – type 2 classification is still valid in general, CRS endotyping has stressed the complexity of CRS with a frequent presentation of mixed inflammatory patterns and cellular diversity, making it obvious that type 1 – type 2 differentiation alone is not sufficient to explain the pathophysiology. (3, 5, 12-15)

Eosinophilic-neutrophilic inflammation has long been presented as black and white, almost implying mutual exclusion in CRS. Recent endotype-focused studies have challenged this traditional image, showing a more versatile picture than was anticipated based on cytokine profiles in both CRSsNP and CRSwNP patients. (12, 13) The most severe CRSsNP patients have a predominant eosinophilic inflammation and the majority of severe CRSwNP patients display a mixed pattern of eosinophilic-neutrophilic inflammation.

Despite the heterogeneity of CRS, there is a clear association between type 2 immune responses and the severity of clinical features in both CRSsNP and CRSwNP. CRS patients with a type 2 immune response have higher rates of recurrence and comorbid asthma, and a more frequent and severe presentation of clinical symptoms, while the presence of a type 1 or type 17 inflammation is inversely correlated with recurrence and associated with a milder clinical picture. (5, 12, 16-18) Interestingly, the fraction of CRS patients with a type 2 inflammation is significantly increasing in both Caucasian and Asian populations. (15, 19, 20) Because type 2 inflammation is the decisive factor for disease severity in both CRSsNP and CRSwNP, this review will be focused on eosinophilic and neutrophilic inflammation, their interplay and the clinical relevance in CRS with a type 2 immune response.

Type 2 CRSsNP

About 50% of the Caucasian CRSsNP patients show a mild to moderate – in some cases mixed – type 2 inflammation, similar but less pronounced compared to those in severe CRSwNP. (5, 12, 16) Type 2 immune responses in CRSsNP are – just like their CRSwNP counterparts – characterized by increased levels of interleukin (IL)-4, IL-5, total IgE, S. aureus enterotoxin specific (SE)-IgE and eosinophil cationic protein (ECP), and elevated numbers of eosinophils in both the blood and nasal mucosa. (12) In addition, eosinophil extracellular traps (EETs) and Charcot-Leyden crystal (CLC, the crystalized form of galectin-10) deposition are also present in CRSsNP, and strongly associated with the underlying type 2 inflammation. (12) The number of neutrophils, on the other hand, are unaltered in the blood and mucosa of type 2 CRSsNP patients. (12)

Interestingly, the presentation of an eosinophilic type 2 inflammation in CRSsNP patients is associated with a worse clinical outcome, indicated by increased ratios of comorbid asthma, headache and NP recurrence over 12 years, and reduced smell/taste. (12, 16) In addition, tissue eosinophilia in CRSsNP is associated with disease severity, defined by CT, endoscopy and Smell Identification Test (SIT) scores, and reduced improvement in disease-specific and general quality of life after surgery. (21, 22)

Type 2 CRSwNP

The most severe CRSwNP patients display a type 2 inflammatory pattern – a phenomenon observed internationally in Europe, the US and Asia–, associated with profound eosinophilic inflammation. (5, 16, 20) This eosinophilic inflammation is characterized by increased eosinophil infiltration and the presence of EETs in association with S. aureus colonization and CLCs, mainly subepithelially at sites where the epithelial barrier is damaged. (7, 12, 20, 23-25)

Interestingly, several studies over the last decade report the existence of a mixed eosinophilic-neutrophilic inflammation in CRSwNP patients. (4, 26) Chinese studies found that 35.8% of the CRSwNP patients displayed a mixed phenotype, associated with type 2 inflammation. (27, 28) In the western population, 26% of CRSwNP patients have been reported to display a mixed inflammation pattern. (29) Indeed, a substantial neutrophilic inflammation co-occurring with – and affected by – eosinophilia was recently demonstrated in the majority of severe type 2 CRSwNP patients (figure 1). (13, 23) In addition, a CRS cluster analysis demonstrated elevated levels of neutrophil-related proteins, such as IL-6, IL-8 and MPO, in highly type 2 eosinophilic CRSwNP patients, with a severe clinical outcome. (5)

Figure 1:

Mixed presence of eosinophils and neutrophils in CRSwNP. Eosinophils (left) and neutrophils (right) in the mucosa of CRSwNP via immunohistochemistry staining of major basic protein (MBP) and elastase, respectively.

For asthma it is well-known that the presentation of a mixed type 2 – type 17 inflammatory pattern is associated with a mixed eosinophilic – neutrophilic picture, as observed in the more severe and difficult to control asthma phenotype. (30, 31) In contrast, in CRS, the most severe patient group shows a predominant type 2 inflammation with high levels of neutrophil-related proteins, but low IL-17 levels, not linking type 17 to neutrophilia and disease severity. (5, 32) This indicates that while there is more of a predominant type 2 inflammation than a mixed type 2 – type 17 inflammation, the most severe CRSwNP patients do have a mixed eosinophilic – neutrophilic inflammation. Despite the fact that IL-17 is traditionally considered a major driving force of neutrophilia, these findings show that IL-17 levels are not a good indicator for the presence of neutrophils in the tissue of severe type 2 CRSwNP. (5, 13) However, this only seems to be the case specifically for CRSwNP patients with a severe type 2 inflammation, as some non-endotype-based studies still observed a regulatory role for IL-17 on neutrophils in CRSwNP. (33) Interestingly, CLCs could be functional to orchestrate neutrophilia specifically in this severe CRSwNP group, as we will discuss later in this review. (13, 23, 24)

Eosinophilic inflammation

Increased eosinophilic inflammation is a hallmark of severe CRSwNP in Caucasian patients, while CRSsNP patients with a type 2 immune response also display a certain degree of eosinophilic inflammation – albeit far less severe compared to CRSwNP. (7, 12, 13) Recruitment of eosinophils in the tissue of CRS is well-known to be mediated by IL-5, RANTES and eotaxins, but recent studies also demonstrated increased eosinophil migration towards S. aureus at subepithelial regions in CRSwNP tissue. (4, 7, 34-37) Interestingly, eosinophils have increased potential to affect the CRS pathophysiology due to their prolonged survival in CRS tissue, supported by elevated levels of IL-5, IL-33 and TSLP, protecting the eosinophils from apoptosis. (36-42) IL-33 and TSLP could also stimulate eosinophils and their recruitment towards impaired epithelium indirectly, by inducing IL-5 secretion of ILC2s. (7, 43)

Increased CD69 expression – a marker for eosinophil activation – was observed in eosinophils present in the nasal polyps compared to those in the blood of CRSwNP patients. (44-46) Interestingly, eosinophils are already in a priming state in the blood of CRSwNP patients, which indicate that while eosinophils are in an increased state of preparedness in the blood of CRSwNP patients, they are locally activated in the nasal polyp tissue. (47)

Once activated, eosinophils are known to secrete cytotoxic granule proteins with a primary role in protective immunity, but are very toxic at high concentrations, contributing to tissue damage and remodeling. (48-52) Besides protein mediators, eosinophils also produce pro-inflammatory lipids as cysteinyl leukotrienes that can further promote eosinophil recruitment, mucus secretion and increased vascular permeability in CRSwNP. (53-55) Eosinophils have also been reported to produce anti-inflammatory prostaglandin E₂(PGE₂) and pro-inflammatory prostaglandin D₂(PGD₂), but levels of PGE₂ are decreased, while levels of PGD₂ are increased in CRSwNP. (53, 56)

Another way by which eosinophils can cause tissue damage is through formation of EETs, as observed in the tissue of both CRSsNP and CRSwNP patients with a type 2 inflammation. (7, 12) These EETs consist of extracellular DNA and contain large amounts of granule proteins, which can facilitate capturing and killing of pathogens. (57) In CRSwNP, EETs are mainly observed in subepithelial regions with epithelial barrier defects, leading to the entrapment of S. aureus. (7, 25) EETs are also highly present in mucus from patients with eosinophilic CRS, increasing the mucus viscosity. (58) Interestingly, EET formation is closely associated with disease severity in chronic rhinosinusitis, regardless of the presence of NP. (59)

EET formation lays at the basis of CLC deposition, the crystalized form of galectin-10. (24, 60) CLCs are abundantly present in the mucosa and mucus of both patients with CRSsNP and CRSwNP, and are associated with type 2 inflammation. (12, 23, 24) Recent discoveries have demonstrated that CLCs are more than a degradation product of eosinophils, as they affect the epithelial barrier and sustain a neutrophilic inflammation in CRSwNP. (23) An enhanced neutrophil migration caused by CLCs was observed both in vitro and in mouse models and their association was confirmed in the tissue of severe type 2 CRSwNP patients. (13, 23, 24) CLCs also contribute to inflammation by inducing neutrophilic inflammation and activate the NLRP3 inflammasome after uptake by macrophages, causing IL-1β-driven inflammation. (23, 61) However, only the crystalized form of galectin-10 elicits those pro-inflammatory effects in CRSwNP, while soluble galectin-10 displayed anti-inflammatory effects. (23) Recently developed CLC-dissolving antibodies suppressed airway inflammation, goblet-cell metaplasia, bronchial hyperreactivity and IgE synthesis, induced by CLC or by house dust mite inhalation in a humanized mouse model. (24)

Type 2 CRSsNP, characterized by tissue eosinophilia with EET formation and CLC deposition, had significantly higher rates of asthma and recurrence, and reduced improvement in quality of life (based on RSDI and CSS scores), compared to CRSsNP without an eosinophilic type 2 response. (22, 62) However, recurrence among type 2 CRSsNP patients was still remarkably lower than that described in patients with CRSwNP (19% vs. 79% after 12 years). (6, 63) Also in CRSwNP patients, elevated numbers of activated eosinophils in the mucosa, and increased Gal10 mRNA expression in the mucus are associated with higher rates of recurrence. (64-66) Moreover, the presentation of an eosinophilic inflammation in CRSwNP was reflected by disease severity, defined by comorbid asthma, olfactory dysfunction, nasal polyp size and degree of sinonasal inflammation on CT scan. (5, 44, 55, 65, 67-69)

Interestingly, increased CLC mRNA expression was the only eosinophil-related mediator that correlates with olfactory loss in CRSsNP and CRSwNP patients. (70) This matches the observation that the presence of EETs in the tissue of CRS patients was associated with reduced olfactory function and higher Lund-Mackay scores, regardless the presence of nasal polyps. (59) These data indicated that the activation of eosinophils, more than the presence alone is decisive for the clinical outcome in both CRSsNP and CRSwNP. Identification of reliable eosinophil activation markers are thus essential for appropriate therapy assignment.

There are additional studies that may shed some light on how eosinophilic inflammation might be locally regulated within the nasal mucosa via production of endogenous factors. For example, the highly glycosylated protein DMBT1 (deleted in malignant brain tumor 1, also known as gp-340 and salivary agglutinin), produced within nasal mucosal glands and secreted into the nasal passage, is highly over-expressed in CRSwNP. (71) This has taken on greater significance with the recent discovery that a subset of nasal DBMT1 is decorated with unique and specific sialylated and sulfated glycan ligands for Siglec-8, a receptor selectively expressed on eosinophils and whose engagement causes eosinophils to die (so-called DMBT1^S8). Whether the levels of DMBT1^S8 are altered in various forms of CRS is currently being investigated. (72, 73)

Finally, it is important to recognize that there are some reports, such as those employing anti-eosinophil pharmacologic approaches, that raise some interesting conundrums regarding the role of eosinophils in CRS. In one sizable study, an oral agent called dexpramipexole, which causes a gradual but marked reduction in eosinophils, was administered to explore its impact on signs and symptoms of CRSwNP. As expected, use of the drug resulted in a ≈95% reduction in blood and NP eosinophils, but unexpectedly this resulted in no clinical improvement, and in fact an increase in tissue mast cells was observed. (74) A similar pattern was observed in a case report during treatment with reslizumab, an anti-IL-5 antibody. (75) This is confusing because of favorable results in phase three trials of anti-IL-5 and anti-IL-5R antibody treatments (see below).

Neutrophilic inflammation

Neutrophil infiltration has been found to be elevated in severe type 2 CRSwNP patients. (13) As discussed above, neutrophilia can occur independent from IL-17 in several CRS endotypes, especially in severe type 2 immune responses. Interestingly, CLCs can orchestrate a neutrophilic inflammation and increased neutrophil infiltration correlated significantly with markers of severe eosinophilia as EETs and CLCs in severe type 2 CRSwNP patients. (13) These findings imply that CLCs overrule IL-17 in the regulation of the increased neutrophil infiltration in severe type 2 immune responses. However, increased IL-8 production caused by CLCs indicate a potential indirect role for CLCs in the regulation of neutrophil recruitment, like earlier demonstrated for neutrophil elastase. (23, 76) S. aureus colonization is also linked to increased neutrophil migration in CRSwNP and could therefore have a prominent role in vivo triggering neutrophilia in CRSwNP. (8, 77-79)

Increased neutrophil survival has been described in patients with severe asthma. (80-84) Interestingly, neutrophil survival was only associated with tissue levels of IL-17 in CRSsNP, but not in type 2 CRSwNP, underlining the IL-17 independency of neutrophilic inflammation in severe type 2 CRSwNP. (13) GM-CSF, G-CSF, TNF-α and IL-4 stimulate the generation of long-living populations of neutrophils, however, the involvement of these cytokines in neutrophil survival, nor increased neutrophil survival has yet been described in type 2 CRSwNP. (13, 84, 85)

While mature neutrophils are dominant in the blood of CRSwNP patients, a significant shift of activated neutrophils is observed in the tissue of CRSwNP, indicating that neutrophils get activated once they enter the CRSwNP microenvironment. (13, 86, 87) Activated neutrophils contribute to the anti-bacterial cascade via phagocytosis of S. aureus and oxidative burst, and are involved in the development of airway hyperreactivity. (88, 89) Multiple studies reported increased proteolytic activity of both elastase and cathepsin G – granule proteins secreted by activated neutrophils – in the tissue of type 2 CRSwNP patients. (13, 26, 87) Once secreted, elastase and cathepsin G are less effective in microbial killing, but are able to enhance secretion and activation of IL-1 family cytokines as IL-1β, IL-33 and IL-36γ in an extremely efficient manner. (90) IL-1β and IL-33 are key players in the induction of type 2 responses in eosinophilic nasal polyps. In contrast, IL-36γ promotes the secretion of IL-8 and IL-17 from tissue neutrophils, reinforcing a positive feedback loop on their own recruitment. (26, 91-93) Substrates for neutrophil proteases as elastin, collagen and fibronectin are major components of the extracellular matrix, and their degradation is linked to tissue remodeling. (94) In addition, neutrophil serine proteases have a direct negative effect on the nasal epithelial barrier integrity and elastase can initiate goblet cell metaplasia and increased mucus production. (83, 95, 96) These findings indicate that neutrophils are not only more frequent in a severe type 2 environment, but they also affect the local inflammation via increased (proteolytic) activity.

Neutrophil extracellular traps (NETs), generally consisting of neutrophil DNA associated with granule proteins, are present in secretions of CRSwNP and at subepithelial regions in tissue of both CRSsNP and CRSwNP patients. (13, 97-99) The pathway of NET formation is highly dependent on the individual micro-organism identity, pathogen size and additional stimuli. (99-101) CLCs evoke neutrophil death in vitro, making it likely that CLCs in tissue and secretions might contribute to tissue damage in CRS patients. (23, 99) It should be noted that neutrophil cytolysis and NET formation are different phenomena with different underlying molecular mechanisms. (102) Moreover, S. aureus – present in the majority of CRSwNP – has been found to degrade NETs, which then promotes its own survival. (103) This could explain the increased presence of NETs in CRSsNP where NETs were shown to be associated with bacterial colonization, while CLC deposition is more pronounced in CRSwNP. (12) In secretions of eosinophilic CRSwNP patients, NETs were found to increase the mucus viscosity, leading to plug formation, hampering mucociliary clearance and eventually airway damage. (104) Moreover, NETs could have pro-inflammatory effects on macrophages and stimulate tissue remodeling of the extracellular matrix. (99, 101, 105)

Interestingly, research over the last decade demonstrated the involvement of neutrophils in the establishment of a type 2 response, as dsDNA associated with NETs may directly contribute to the pathogenesis by inducing a type 2 immune response. (106) IL-33 treatment of neutrophils resulted in a polarization of the cells and also to the elective production of type 2 cytokines, like IL-4, IL-5, IL-9 and IL-13. (107) The expression of IL-9 in a subgroup of neutrophils was recently described in the tissue of CRSwNP patients. (108) Understanding the heterogeneity of neutrophils in CRS, caused by micro-environment or tissue specific stimuli could help in understanding their contribution across endotypes. So far, subsetting of neutrophils in CRS resulted in the identification of an activated subset (CD16^high CD62L^dim) and IL-9 expressing neutrophils. (13, 86, 108) In asthma, in which NETs have been identified under in vivo conditions, it has been postulated that CXCR4^high neutrophils are more prone to form NETs, however, no evidence on that has been found in the tissue of CRS so far. (109, 110)

Markers of severe or moderate neutrophilic inflammation are elevated in patients with difficult to treat CRS and increased presence of neutrophils in subepithelial regions of nasal polyps is associated with severe refractoriness of CRS. (111-113) Neutrophils produce high amounts of MMP-9 in CRSwNP tissue, which is linked to poor wound healing quality and regeneration of tissue after FESS. (114-116) In spite of these insights, the contribution of neutrophils in the pathophysiology and persistence of Chronic rhinosinusitis, especially in a type 2 context, remains largely unknown and needs attention to enable further improvement of treatments and endotyping of patients.

Mixed eosinophilic-neutrophilic inflammation

Based on the mixed presence described above, eosinophilic and neutrophilic inflammation cannot be seen as separate processes. Neutrophil infiltration was associated with EET formation and CLC-deposition – hallmarks of eosinophilic inflammation – in severe type 2 CRSwNP patients and an increased neutrophil migration towards epithelial cells was observed upon CLC stimulation in vitro. (13, 23) In addition, it is known that activated neutrophils can enhance eosinophil transmigration, and that IL-8 - mediated neutrophil recruitment induces an accumulation of eosinophils. (88, 117, 118) Interestingly, recent studies reported the expression of IL-5R and IL-9 on tissue neutrophils in asthmatic CRSwNP and the potential of neutrophils to initiate a type 2 response that could lay the basis for eosinophil infiltration. (96, 108, 110, 119) Also in mice models, neutrophil proteases elastase and cathepsin G have been reported to induce eosinophil degranulation in a Ca⁺²-dependent manner in vitro. (120) On the other hand, PGD2 released by activated eosinophils can – in synergy with leukotriene E4 – enhance Th2 responses and induce the production of non-classical Th2 inflammatory mediators, including IL-8 and GM-CSF at concentrations that would be sufficient to affect neutrophil migration, survival and activation. (121) Moreover, eosinophil-derived MBP has the potential to activate neutrophils and stimulate its O2⁻ production. (122, 123) These findings indicate that, especially in severe type 2 CRSwNP patients, interplay between eosinophils and neutrophils could be essential in the maintenance of the chronicity of the disease – even after targeting the eosinophilic inflammation – by stimulating each other’s influx. (Figure 2)

Figure 2:

Pathophysiology of a mixed eosinophilic-neutrophilic inflammation in severe type 2 CRSwNP. Both an eosinophilic inflammation (left side) and a neutrophilic inflammation (right side) contribute to the pathophysiology in the mucosa of severe type 2 CRSwNP. Both responses impact the course of the disease and reciprocal interactions between eosinophils and neutrophils contribute to the persistency and severity of the disease. EETs, eosinophil extracellular traps; NETs, neutrophil extracellular traps; CLCs, Charcot-Leyden crystals, MBP, major basic protein; ECP, eosinophil cationic protein; EDN, eosinophil-derived neurotoxin; TSLP, thymic stromal lymphopoietin; RANTES, regulated on activation, normal cell expressed and secreted.

In severe asthma, it is well known that the mixed presence of eosinophils and neutrophils is associated with disease severity and a harder to treat phenotype, defined by glucocorticoid resistance and more pronounced airway obstruction and hyperreactivity compared to predominantly eosinophilic inflammation. (124-126) Recently, the same observations were made in Caucasian CRSwNP patients, where an increased neutrophilic inflammation in association with eosinophilia was demonstrated in the most severe, difficult to treat high type 2 CRSwNP patients that suffer reduction or loss of smell, severe nasal obstruction and increased prevalence of asthma. (5, 13) CRSwNP patients with a mixed granulocytic phenotype have increased severity of tissue inflammation with a greater overall inflammatory burden, reflected by worse CT-scores, olfactory function, disease-specific quality of life and higher symptom burden, compared to predominantly eosinophilic or neutrophilic CRSwNP patients. (29, 68) The same observation was done in Asian CRS patients regarding recurrence. (112) A mixed granulocytic presence in the sputum of asthmatics was associated with severe comorbid CRS, as evaluated by the Lund-Mackay score. (127) Eosinophils and neutrophils can thus stimulate each other’s influx in CRSwNP that results in a mixed inflammation and the establishment of a more persistent and severe pathogenesis of CRSwNP.

Treatment strategies for eosinophilic – neutrophilic inflammation in CRS

Glucocorticosteroids (GCS) do target type 2 inflammatory responses better than non-type 2 responses, however, GCS resistance has been observed even in patients with type 2 CRSwNP. (27, 128) This non-responsiveness could be partially explained by the presence of a neutrophilic inflammation in CRSwNP as neutrophil-low polyps had significantly greater reductions in bilateral polyp scores, nasal congestion scores and total symptom scores, compared to neutrophil-high patients. (27) Indeed, corticosteroid treatment decreases the eosinophilic inflammation, while the neutrophilic inflammation remains unaltered or even increases. (27, 86, 98, 129-132)

Due to the predominant type 2 inflammatory pattern in severe CRS patients, current therapies target the eosinophilic/type 2 inflammation via anti-IL-4R alpha, anti-IgE, anti-IL-5 or IL-5Rα approaches. (3, 133, 134) Phase 3 trials have recently been reported with mepolizumab (anti-IL-5) and benralizumab (anti-IL-5Rα); dupilumab (anti-IL-4R) and xolair (anti-IgE) are already approved for CRSwNP in the United States and Europe. However, these innovative drugs reduce the polyp score only in about 30 to 70% of patients. (133, 135-140) Recent studies demonstrated the presence of functional IL-5Rα on a number of other cells relevant in CRS, including neutrophils, plasma cells and epithelial cells. (119, 141, 142) However, it is still unknown if non-responders are more neutrophilic than responders, but we do have evidence that neutrophils remain present in the tissue and keep affecting the pathogenesis in patients with a mixed eosinophilic-neutrophilic inflammation. (143) In fact, we recently observed the disappearance of eosinophils, EETs and CLCs in subjects with CRSwNP, who were treated for asthma with mepolizumab and benralizumab for more than 6 months, but underwent surgery for resistant nasal polyps, while neutrophils were abundant in these nasal polyp tissues (personal observation). Interestingly, treatment of bronchiectasis patients with brensocatib – an oral inhibitor of dipeptidyl peptidase 1 (DDP-1) that is responsible for neutrophil serine protease activation – was associated with improvements in clinical outcomes in a 24-week trial. (144)

Asthmatic patients diagnosed with predominant eosinophilic inflammation can be well-controlled with GCS or anti-IL-5 biologics. Patients with neutrophilic asthma, on the other hand, are often steroid insensitive, and to date the most effective treatment for this group is macrolides since there are currently no biologics approved for neutrophilic asthma. However, like CRSwNP, the most severe asthmatic patients have a mixed granulocytic inflammation and are difficult-to-control as they show a poor response to GCS. (30, 126, 145) Moreover, gene signatures of neutrophils did not show any significant change after treatment with benralizumab (anti-IL5Ra) in asthmatic patients. (146) These associations between CRSwNP and asthma indicates that these diseases are driven by comparable mechanisms, therefore future insights in the mixed inflammation of severe CRSwNP patients can be extrapolated to get a better understanding of asthma as well.

Conclusion and future clinical implications

While CRS is a complex disease with heterogeneous inflammatory patterns, there is a clear link between the presence of type 2 immunity and severity and persistency in both CRSsNP and CRSwNP. While severe type 2 CRSsNP is characterized by a predominant eosinophilic inflammation, severe type 2 CRSwNP patients display a mixed eosinophilic-neutrophilic inflammation. In the latter, the mixed inflammation is established by – among others – a wide range of reciprocal interactions between eosinophils and neutrophils themselves, establishing a difficult-to-manage disease. Treatments of predominant eosinophilic or neutrophilic inflammations are well-established in airway disease and still in development, while patients with a mixed inflammation may be less responsive to these specific treatments. Therefore, new treatment options, targeting the mixed inflammation by a combination of biologics may be helpful in certain patients with refractory severe and uncontrolled CRSwNP. As this phenomenon is also observed in severe asthmatics, new insights in the treatment of mixed inflammations in CRSwNP could be extrapolated for treatment of severe asthmatics, and vice versa.

Abbreviations

CRS

Chronic Rhinosinusitis

CRSsNP

Chronic rhinosinusitis without nasal polyps

CRSwNP

Chronic rhinosinusitis with nasal polyps

S. aureus

Staphylococcus aureus

IFN-γ

Interferon-gamma

IL

Interleukin

IgE

Immunoglobulin E

SE-IgE

Staphylococcus enterotoxin-specific Immunoglobulin E

ECP

Eosinophil cationic protein

EETs

Eosinophil extracellular traps

NETs

Neutrophil extracellular traps

CLCs

Charcot-Leyden crystals

SIT

Smell Identification Test

MPO

Myeloperoxidase

RANTES

Regulated on Activation, Normal T cell Expressed and Secreted

TSLP

Thymic Stromal Lymphopoietin

ILC2

Group 2 Innate Lymphoid cell

CD

Cluster of Differentiation

PGE₂

Prostaglandin E₂

Gal10

Galectin 10

NLRP3

NLR family pyrin domain containing 3

RSDI

Rhinosinusitis Disability Index

CSS

Chronic Sinusitis Survey

DMBT1

Deleted in Malignant Brain Tumor 1

GM-CSF

Granulocyte/Macrophage Colony-Stimulating Factor

G-CSF

Granulocyte Colony-Stimulating Factor

TNF-α

Tumor Necrosis Factor alpha

dsDNA

Double-stranded DNA

CXCR4

C-X-C chemokine receptor 4

MMP-9

Matrix metalloproteinase 9

FESS

Functional Endoscopic Sinus Surgery

MBP

Major Basic Protein

GCS

Glucocorticosteroids

DDP-1

dipeptidyl peptidase 1

EDN

Eosinophil-derived neurotoxin