Aggregated eosinophils and neutrophils characterize the properties of mucus in chronic rhinosinusitis

Epithelial cell dynamics: Key drivers of type 2 inflammation in eosinophilic chronic rhinosinusitis

Sinonasal mucosal epithelial cells act not only as a physical barrier, but also as dynamic regulators of immune responses through innate and acquired immunity. These cells play a key role in detecting environmental stimuli, such as pathogens, allergens, and pollutants, and in initiating the inflammatory cascade that shapes the overall immune response. By releasing cytokines, such as interleukin (IL)-25 and IL-33, and thymic stromal lymphopoietin, epithelial cells interact with immune cells and promote type 2 inflammation. The pathogenesis of eosinophilic chronic rhinosinusitis (ECRS), which is driven by type 2 inflammation, is heterogeneous. While immune cells have traditionally been considered to be central to the disease pathogenesis, emerging evidence has indicated the critical role of epithelial cells. Furthermore, novel biologics targeting the IL-4/IL-13 signaling pathway have shown potential in alleviating epithelial dysfunction and inflammation. Eosinophilic mucins that accumulate in the sinuses impair mucociliary function, and especially eosinophil extracellular trap cell death (EETosis) stimulate epithelial cells and amplify eosinophilic inflammation. Eosinophilic mucin formation has been shown to significantly increase viscosity through EETosis, and novel biologics targeting the IL-5 signaling pathway hold promise for effectively mitigating this process. To develop targeted interventions, it is important to explore the role of epithelial subpopulations, such as basal cells and tuft cells, in maintaining the balance between tissue repair and chronic inflammation. Single-cell transcriptomics and spatial transcriptomics technologies have provided significant insights into the complexity of epithelial cell-derived inflammation in ECRS. The heterogeneity of the pathogenesis of CRS with nasal polyps and ECRS across patient populations complicates the development of universal therapies, underscoring the need for stratified medicine approaches. Potential future therapeutic strategies include the restoration of epithelial integrity and immune balance by disrupting aberrant crosstalk between epithelial and immune cells, particularly in patients unresponsive to current treatments.

Mucus and mucin: changes in the mucus barrier in disease states

In this review we discuss mucus, the viscoelastic secretion from goblet or mucous producing cells that covers and protects all non-keratinized wet epithelial surfaces. In addition to the surface of organs directly contacting with the external environment such as the eyes, this layer provides protection to the underlying gastrointestinal, respiratory and female reproductive tracts by trapping pathogens, irritants, environmental fine particles and potentially harmful foreign substances. Mucins, the primary structural components of mucus, form structurally different mucus layers at different sites in a process regulated by a variety of factors. Currently, more and more studies have shown that the mucus barrier is not only closely related to various intestinal mucus diseases, but also involved in the occurrence and development of various airway diseases and mucus-related diseases, thus it may become a new target for the treatment of various related diseases in the future. Since the dysfunction of the mucous layer is closely related to various pathological processes, in-depth understanding of its molecular mechanism and physiological role is of great theoretical and practical significance for disease prevention and treatment. Here, we discuss different aspects of the mucus layer by focusing on its chemical composition, synthetic pathways, and some of the characteristics of the mucus layer in physiological and pathological situations.

Clinical Characteristics of Difficult-To-Treat Allergic Bronchopulmonary Aspergillosis and Its Prediction Score

BACKGROUND AND OBJECTIVE

Administration of oral corticosteroids and/or azole antifungals for 4-6 months remains the standard treatment for allergic bronchopulmonary aspergillosis (ABPA). This study investigated the clinical characteristics of patients with difficult-to-treat ABPA who failed to achieve clinical remission within 6 months.

METHODS

Among the participants of a nationwide survey conducted in Japan in 2020, treatment-naïve patients with ABPA who satisfied Asano's criteria were enrolled in this study. Clinical remission was defined as stable disease without exacerbation for ≥ 6 months under minimal treatment (oral prednisolone: ≤ 5 mg/day and no antifungal medication). A risk prediction score for difficult-to-treat ABPA was developed and validated in an independent cohort comprising patients with ABPA from a prospective registration study in Japan.

RESULTS

In total, 316 treatment-naïve patients with ABPA were enrolled in the study. The median time to minimal treatment status was 4.8 months in the group receiving standard treatment. The clinical remission rate at 6 months after standard treatment was 51%. Age ≤ 50 years at onset of ABPA (p = 0.04), serum A. fumigatus-specific IgE titer of ≥ 20 UA/mL (p = 0.006), positive culture for Aspergillus spp. in the sputum/bronchial lavage fluid (p = 0.05), and presence of high attenuation mucus (HAM; p = 0.10) were associated with difficult-to-treat ABPA. The number of positive indicators indicated the risk of failure of standard treatment to yield clinical remission within 6 months in the derivation (n = 87, p < 0.001) and validation (n = 64, p = 0.009) cohorts.

CONCLUSION

Multiple components, including age at onset, allergic sensitization, airway fungal burden, and HAM, were associated with difficult-to-treat ABPA.

Allergic Fungal Rhinosinusitis and Eosinophilic Chronic Rhinosinusitis Have Different Phenotypes in Japan

BACKGROUND

Allergic fungal rhinosinusitis (AFRS) is a subtype of chronic rhinosinusitis driven by Types 1 and 3 allergies to fungi. In Japan, it is relatively rare and characterized by prominent eosinophilic infiltration of the sinonasal mucosa, together with eosinophilic mucin containing scattered fungi in the sinus cavity. Eosinophilic chronic rhinosinusitis (eCRS) involves similar eosinophilic infiltration and shares some clinical features with AFRS. However, the clinical differences between eCRS and AFRS remain to be fully elucidated. The aim of this study was to clarify the phenotypes of eCRS and AFRS.

METHODS

This multicenter retrospective study enrolled patients with AFRS and eCRS and compared their clinical parameters. A cluster analysis was conducted to determine the phenotypes of the two diseases.

RESULTS

AFRS patients had a younger age of onset and exhibited milder computed tomography and nasal polyp scores than eCRS patients. Total IgE was significantly higher in AFRS patients than in eCRS patients, while mucosal eosinophil counts were similar. Olfactory disturbances were significantly less severe in AFRS patients compared with eCRS patients. The cluster analysis revealed three phenotypes for AFRS: one that was distinct and independent from eCRS, representing the more classically described AFRS patients, and more the other two that shared characteristics with eCRS.

CONCLUSIONS

AFRS exhibits unique clinical features compared with eCRS. Cluster analysis identified three distinct AFRS phenotypes characterized by CT findings, eosinophilic inflammation, and specific IgE levels against inhaled antigens. These findings underscore the importance of differential diagnosis and personalized treatment strategies for AFRS and eCRS.

Comparative Insights on IL-5 Targeting with Mepolizumab and Benralizumab: Enhancing EGPA Treatment Strategies

Eosinophilic granulomatosis with polyangiitis (EGPA) is a necrotizing vasculitis characterized by extravascular granulomas and eosinophilia in both blood and tissues. Eosinophils, which play a critical role in the pathophysiology of EGPA, require interleukin (IL)-5 for maturation in the bone marrow and migration to tissues. Glucocorticoids and immunosuppressants have been the cornerstone of treatment; however, their side effects have imposed a significant burden on many patients. Mepolizumab, an antibody that binds to and neutralizes IL-5, demonstrated efficacy in controlling disease activity in EGPA in the MIRRA trial conducted in 2017. In 2024, benralizumab, an IL-5 receptor alpha antagonist, was shown to be non-inferior to mepolizumab in efficacy against EGPA in the MANDARA trial. Both drugs were originally used for severe asthma and have benefited EGPA by reducing eosinophil counts. Due to differences in pharmacological structure and pharmacokinetics, the degree of eosinophil suppression varies between the two agents, and recent studies suggest that they may also affect inflammatory and homeostatic eosinophils differently. This review summarizes the latest insights into the pathophysiology of EGPA, highlights the similarities and differences between the two drugs, and discusses future treatment strategies for EGPA based on current clinical unmet needs, including drug selection.

Treatment of allergic bronchopulmonary aspergillosis with biologics

Patients with allergic bronchopulmonary aspergillosis (ABPA) respond well to standard treatments (oral corticosteroids and/or antifungals); however, approximately in half of the patients, the condition recurs during tapering or early after treatment discontinuation. To avoid the adverse effects of long-term treatment, biologics targeting immunoglobulin E (IgE), eosinophils, or type 2 immune responses have been used in refractory ABPA. Omalizumab, an anti-IgE antibody, as well as mepolizumab and benralizumab targeting eosinophils has been consistently shown to decrease co-morbid asthma exacerbation and dose of oral corticosteroids. Furthermore, mepolizumab and benralizumab effectively improved chest radiographic abnormalities, such as mucus plugs in the bronchi. Data on dupilumab and tezepelumab are limited; however, they may be effective in patients who are resistant to treatment with omalizumab/mepolizumab/benralizumab. Future studies examining the effects of these biologics in preventing the recurrences/exacerbations of ABPA are warranted.

Allergic Bronchopulmonary Aspergillosis/Mycosis: An Allergic Disease or an Eosinophilic Disease?

Allergic bronchopulmonary aspergillosis/mycosis (ABPA/ABPM) is characterized by increased serum levels of total and fungi-specific immunoglobulin E (IgE) and eosinophilic mucus plugs in the airways. Its classification as either an allergic or eosinophilic disease remains controversial. In the present review, we explored this topic based on three clinical studies that analyzed the clinical characteristics of ABPA/ABPM using a cluster analysis, factor analysis, and comparison between ABPM caused by Schizophyllum commune and ABPA. We also compared therapeutic responses to biologics targeting either IgE (omalizumab) or eosinophils (mepolizumab/benralizumab) to elucidate the role of these components in the pathogenesis of ABPA/ABPM. Based on these analyses, eosinophilic mucus plug formation in the airways is considered a cardinal feature of the development of ABPA/ABPM, whereas IgE responses to fungi are important factors that modulate disease manifestation.

Eosinophil extracellular vesicles and DNA traps in allergic inflammation

Eosinophil granulocytes, a specialized subset of white blood cells, have traditionally been associated with allergic responses and parasitic infections. However, recent research has unveiled their versatile roles in immune regulation beyond these classical functions. This review highlights the emerging field of eosinophil biology, with a particular focus on their release of extracellular vesicles (EVs) and extracellular DNA traps (EETs). It further explores potential implications of eosinophil-derived EVs and EETs for immune responses during inflammatory diseases. The release of EVs/EETs from eosinophils, which also affects the eosinophils themselves, may influence both local and systemic immune reactions, affecting the pathophysiology of conditions such as airway inflammation, chronic rhinosinusitis and atopic dermatitis.

Mucus plugging on computed tomography and the sputum microbiome in patients with asthma, chronic obstructive pulmonary disease, and asthma-COPD overlap

BACKGROUND

Despite clinical implications, the pathogenesis of mucus plugging in asthma, chronic obstructive pulmonary disease (COPD), and asthma-COPD overlap (ACO) remains unclear. We hypothesized that distinct airway microbiomes might affect mucus plugging differently among ACO, asthma, and COPD and among different extents of airway eosinophilic inflammation.

METHODS

The sputum microbiome, sputum cell differential count, and mucus plug score on computed tomography were cross-sectionally evaluated in patients with chronic airflow limitation.

RESULTS

Patients with ACO, asthma, or COPD were enrolled (n = 56, 10, and 25). Higher mucus plug scores were associated with a greater relative abundance of the phylum Proteobacteria (rho = 0.29) only in patients with ACO and a greater relative abundance of the phylum Actinobacteria (rho = 0.46) only in patients with COPD. In multivariable models including only patients with ACO, the presence of mucus plugs was associated with a greater relative abundance of the phylum Proteobacteria and the genus Haemophilus, independent of smoking status, airflow limitation, and emphysema severity. Moreover, the mucus score was associated with a greater relative abundance of the genus Streptococcus (rho = 0.46) in patients with a high sputum eosinophil count (n = 22) and with that of the genus Haemophilus (rho = 0.46) in those with a moderate sputum eosinophil count (n = 26).

CONCLUSIONS

The associations between mucus plugging and the microbiome in ACO differed from those in COPD and asthma. Greater relative abundances of the phylum Proteobacteria and genus Haemophilus may be involved in mucus plugging in patients with ACO and moderate airway eosinophilic inflammation.

Associations of fractional exhaled nitric oxide with airway dimension and mucus plugs on ultra-high-resolution computed tomography in former smokers and nonsmokers with asthma

BACKGROUND

Associations of fractional exhaled nitric oxide (FeNO) with airway wall remodeling and mucus plugs remain to be explored in smokers and nonsmokers with asthma. Ultra-high-resolution computed tomography (U-HRCT), which allows accurate structural quantification of airways >1 mm in diameter, was used in this study to examine whether higher FeNO was associated with thicker walls of the 3rd to 6th generation airways and mucus plugging in patients with asthma.

METHODS

The retrospective analyses included consecutive former smokers and nonsmokers with asthma who underwent U-HRCT in a hospital. The ratio of wall area to summed lumen and wall area was calculated as the wall area percent (WA%). Mucus plugging was visually scored.

RESULTS

Ninety-seven patients with asthma (including 59 former smokers) were classified into low (<20 ppb), middle (20-35 ppb), and high (>35 ppb) FeNO groups (n = 24, 26, and 47). In analysis including all patients and subanalysis including nonsmokers or former smokers, WA% in the 6th generation airways was consistently higher in the high FeNO group than in the low FeNO group, whereas WA% in the 3rd to 5th generation airways was not. In multivariable models, WA% in the 6th generation airways and the rate of mucus plugging were higher in the high FeNO group than in the low FeNO group after adjusting for age, sex, body mass index, smoking status, lung volume, and allergic rhinitis presence.

CONCLUSIONS

Higher FeNO may reflect the inflammation and remodeling of relatively peripheral airways in asthma in both former smokers and nonsmokers.

Eosinophilic mucus diseases

Eosinophilic inflammation is primarily characterized by type 2 immune responses against parasitic organisms. In the contemporary human being especially in developed countries, eosinophilic inflammation is strongly associated with allergic/sterile inflammation, and constitutes an undesired immune reaction. This situation is in stark contrast to neutrophilic inflammation, which is indispensable for the host defense against bacterial infections. Among eosinophilic inflammatory disorders, massive accumulation of eosinophils within mucus is observed in certain cases, and is often linked to the distinctive clinical finding of mucus with high viscosity. Eosinophilic mucus is found in a variety of diseases, including chronic allergic keratoconjunctivitis, chronic rhinosinusitis encompassing allergic fungal sinusitis, eosinophilic otitis media, eosinophilic sialodochitis, allergic bronchopulmonary aspergillosis/mycosis, eosinophilic plastic bronchitis, and eosinophilic asthma. In these pathological conditions, chronic inflammation and tissue remodeling coupled with irreversible organ damage due to persistent adhesion of toxic substances and luminal obstruction may impose a significant burden on the body. Eosinophils aggregate in the hyperconcentrated mucus together with cell-derived crystals, macromolecules, and polymers, thereby affecting the biophysical properties of the mucus. This review focuses on the clinically significant challenges of mucus and discusses the consequences of activated eosinophils on the mucosal surface that impact mucus and persistent inflammation.