Eosinophilic Otitis Media: the Aftermath of Eosinophil Extracellular Trap Cell Death

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.

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

BACKGROUND

Airway obstruction caused by viscous mucus is an important pathophysiologic characteristic of persistent inflammation, which can result in organ damage.

OBJECTIVE

We investigated the hypothesis that the biophysical characteristics of accumulating granulocytes affect the clinical properties of mucus.

METHODS

Surgically acquired nasal mucus samples from patients with eosinophilic chronic rhinosinusitis and neutrophil-dominant, noneosinophilic chronic rhinosinusitis were evaluated in terms of computed tomography density, viscosity, water content, wettability, and protein composition. Isolated human eosinophils and neutrophils were stimulated to induce the formation of extracellular traps, followed by the formation of aggregates. The biophysical properties of the aggregated cells were also examined.

RESULTS

Mucus from patients with eosinophilic chronic rhinosinusitis had significantly higher computed tomography density, viscosity, dry weight, and hydrophobicity compared to mucus from patients with noneosinophilic chronic rhinosinusitis. The levels of eosinophil-specific proteins in mucus correlated with its physical properties. Eosinophil and neutrophil aggregates showed physical and pathologic characteristics resembling those of mucus. Cotreatment with deoxyribonuclease and heparin, which slenderizes the structure of eosinophil extracellular traps, efficiently induced reductions in the viscosity and hydrophobicity of both eosinophil aggregates and eosinophilic mucus.

CONCLUSIONS

The present study elucidated the pathogenesis of mucus stasis in infiltrated granulocyte aggregates from a novel perspective. These findings may contribute to the development of treatment strategies for eosinophilic airway diseases.

Proteomic Study of Middle Ear Effusion and Its Clinical Application for Otitis Media with Effusion

Background

Proteins found in middle ear effusion play crucial roles in the physiological and pathological processes of otitis media with effusion (OME), influencing the etiology and clinical characteristics of this disease. The qualitative and quantitative composition of these proteins depending on the underlying pathogenesis of middle ear effusion. Understanding their physiological and pathological functions is of great importance.

Methods

We collected samples from 19 volunteers diagnosed with OME. After offline separation using high-pH reversed-phase liquid chromatography (RPLC), the pooled sample was subjected to LC-MS/MS analysis to obtain a comprehensive profile of the OME proteome. Functional analysis was performed using Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Ingenuity Pathway Analysis (IPA) annotations. Data-independent acquisition (DIA) technology was utilized to analyze samples and fix whether the OME proteome could replicate the pathophysiological features associated with this disease. We conducted a differential proteomic analysis between patients with simple OME and patients who had received radiotherapy. The radiotherapy-reduced group was further divided into two subgroups: nasopharyngeal carcinoma (NPC) and other types of carcinoma. Parallel reaction monitoring (PRM) technology was used for validation of 36 differentially expressed proteins (DEPs).

Results

A number of 732 proteins were identified in the OME proteome database. Among them, 527 proteins were quantified using peak intensity-based semi-quantification (iBAQ), covering a wide dynamic range of approximately 8 orders of magnitude. Based on the functional analysis, we proposed a hypothetical mechanism of OME.

Conclusion

This study managed to put up an inclusive analysis of the OME proteome, establishing the first human OME proteome database. We focused on differential proteomic analysis among different groups to gain a more comprehensive concept of the OME proteome and search for meaningful biomarkers.

Intractable otitis media - Pathogenesis and treatment of Eosinophilic otitis media (EOM) and otitis media with Antineutrophil cytoplasmic antibody (ANCA) -associated vasculitis (OMAAV)

Intractable otitis media is resistant to antimicrobial therapy, tympanostomy ventilation tube insertion, and surgery. In children, intractable acute otitis media, pathological tympanic membrane due to prolonged otitis media with effusion (OME), tympanic membrane atelectasis, and adhesive otitis media are common. Contrarily, in adults, otitis media caused by drug-resistant pathogens, tuberculous otitis media, cholesterol granuloma, malignant otitis externa (skull base osteomyelitis), eosinophilic otitis media (EOM), and otitis media with antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (OMAAV) are common and require differentiation. Among them, EOM is increasing along with eosinophilic upper respiratory tract diseases, such as bronchial asthma and eosinophilic chronic rhinosinusitis (ECRS), a subgroup of chronic rhinosinusitis with nasal polyps (CRSwNP). EOM is associated with adult-onset bronchial asthma and is characterized by viscous middle ear effusion and middle ear mucosa thickness with eosinophilic infiltration, which requires treatment with glucocorticoids according to disease activity and symptoms. Recently, OMAAV was proposed because of the similarities in clinical features and therapeutic effects. The clinical course of OMAAV is characterized by a relatively rapid increase in the bone conductive hearing threshold, which progresses over 1-2 months, without response to antimicrobial agents or tympanostomy ventilation tube insertion, and in some cases, is complicated by facial paralysis and hypertrophic pachymeningitis. This new concept may explain the pathogenesis and clinical presentation of many cases of intractable otitis media, the cause of which was previously unknown. Although making a diagnosis of OMAAV is relatively easy based on the clinical course, such as vascular dilatation of the tympanic membrane and positive ANCA titer, it is often difficult because the ANCA titer becomes negative with previous administration of glucocorticoids. In adults with intractable otitis media, ANCA titers must be measured before glucocorticoid administration. Treatment consisted of remission induction therapy with a combination of glucocorticoids and immunosuppressive drugs.

Presence of eosinophil extracellular trap cell death in the affected skin of eosinophilic granulomatosis with polyangiitis

Eosinophil activation in tissue might be associated with disease severity. Eosinophil cytolysis, a process of active cell death, has been referred to as eosinophil extracellular trap cell death (EETosis). In the present study, the authors investigated EETosis in the affected skin of four patients with eosinophilic granulomatosis with polyangiitis (EGPA) using an immunofluorescence staining method. Immunofluorescence staining for myelin basic protein, galectin-10, and DNA revealed various degrees of EETosis and Charcot-Leyden crystals in skin tissue, suggesting the different degree of eosinophil activation status. The histopathological characteristic features may help physicians establish an earlier diagnosis of intractable eosinophilic-related disease including EGPA. Furthermore, ETotic eosinophil infiltration in perineurium of skin tissue might play a primary role in peripheral neuropathy of this disorder.

Eosinophilic Otitis Media: Modern Aspects of Pathogenesis, Clinical Features, Diagnosis and Treatment

Eosinophilic otitis media (EOM) is an inflammatory chronic disease of the middle ear, characterized by the presence of a particularly viscous effusion with a high content of protein toxins of eosinophilic origin in the middle ear cavity. The pathology has relationship with bronchial asthma, allergic rhinitis and chronic rhinosinusitis with nasal polyps. EOM is characterized by a sluggish course, a tendency to relapse, which can lead to a gradual hearing decrease up to complete deafness. In this paper, we reviewed the international literature with special attention to pathogenesis and treatment management.

The Quantification of Extracellular Trap Cell Death-Derived Products as Diagnostic Biomarkers for Otitis Media With Antineutrophil Cytoplasmic Antibody-Associated Vasculitis and Eosinophilic Otitis Media

OBJECTIVE

This study aimed to quantify the cell-free deoxyribonucleic acid (DNA), citrullinated-histone H3 (cit-H3)-DNA complex, and myeloperoxidase (MPO)-DNA complex as extracellular trap cell death (ETosis)-derived products in the middle ear fluid, and to identify diagnostic biomarkers for the discrimination of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (OMAAV) from eosinophilic otitis media (EOM).

STUDY DESIGN

Prospective study.

SETTING

Tertiary referral center.

PATIENTS

OMAAV patients were eligible for inclusion in this analysis. Patients with EOM were examined as controls.

INTERVENTION

All samples were obtained from the middle ear fluid in patients with OMAAV or EOM. The fluid samples were aspirated from the middle ear through the anterior-inferior portion of the tympanic membrane using a 1-ml tuberculin syringe with a 24- or 26-gauge needle under a microscope.

MAIN OUTCOME MEASURES

The levels of cell-free DNA, cit-H3-DNA complex and MPO-DNA complex in the fluid samples were quantified using an enzyme-linked immunosorbent assay.

RESULTS

Patients with OMAAV showed significantly higher levels of MPO-DNA complex compared to patients with EOM, regardless of the serum ANCA status at the time of sampling (p < 0.001 and p < 0.001, respectively). Meanwhile, there were no significant differences in the values of cell-free DNA or cit-H3-DNA complex between the OMAAV and EOM patients.

CONCLUSION

The findings of this study suggest that the detection and quantification of MPO-DNA complex in the otitis media fluid can be utilized to discriminate OMAAV, especially in cases of eosinophilic granulomatosis with polyangiitis, from EOM regardless of the serum ANCA status. It should be noted that it is possible for cell-free DNA and cit-H3-DNA complex in fluid samples to be derived from dead cells other than neutrophils that undergo ETosis.

Eosinophil-mediated inflammation in the absence of eosinophilia

The increase of eosinophil levels is a hallmark of type-2 inflammation. Blood eosinophil counts act as a convenient biomarker for asthma phenotyping and the selection of biologics, and they are even used as a prognostic factor for severe coronavirus disease 2019. However, the circulating eosinophil count does not always reflect tissue eosinophilia and vice versa. The mismatch of blood and tissue eosinophilia can be seen in various clinical settings. For example, blood eosinophil levels in patients with acute eosinophilic pneumonia are often within normal range despite the marked symptoms and increased number of eosinophils in bronchoalveolar lavage fluid. Histological studies using immunostaining for eosinophil granule proteins have revealed the extracellular deposition of granule proteins coincident with pathological conditions, even in the absence of a significant eosinophil infiltrate. The marked deposition of eosinophil granule proteins in tissue is often associated with cytolytic degranulation. Recent studies have indicated that extracellular trap cell death (ETosis) is a major mechanism of cytolysis. Cytolytic ETosis is a total cell degranulation in which cytoplasmic and nuclear contents, including DNA and histones that act as alarmins, are also released. In the present review, eosinophil-mediated inflammation in such mismatch conditions is discussed.

Omalizumab Restores Response to Corticosteroids in Patients with Eosinophilic Chronic Rhinosinusitis and Severe Asthma

Eosinophilic chronic rhinosinusitis (ECRS), which is a subgroup of chronic rhinosinusitis with nasal polyps, is characterized by eosinophilic airway inflammation extending across both the upper and lower airways. Some severe cases are refractory even after endoscopic sinus surgery, likely because of local steroid insensitivity. Although real-life studies indicate that treatment with omalizumab for severe allergic asthma improves the outcome of coexistent ECRS, the underlying mechanisms of omalizumab in eosinophilic airway inflammation have not been fully elucidated. Twenty-five patients with ECRS and severe asthma who were refractory to conventional treatments and who received omalizumab were evaluated. Nineteen of twenty-five patients were responsive to omalizumab according to physician-assessed global evaluation of treatment effectiveness. In the responders, the levels of peripheral blood eosinophils and fractionated exhaled nitric oxide (a marker of eosinophilic inflammation) and of CCL4 and soluble CD69 (markers of eosinophil activation) were reduced concomitantly with the restoration of corticosteroid sensitivity. Omalizumab restored the eosinophil-peroxidase-mediated PP2A inactivation and steroid insensitivity in BEAS-2B. In addition, the local inflammation simulant model using BEAS-2B cells incubated with diluted serum from each patient confirmed omalizumab’s effects on restoration of corticosteroid sensitivity via PP2A activation; thus, omalizumab could be a promising therapeutic option for refractory eosinophilic airway inflammation with corticosteroid resistance.

Eosinophilic otitis media and comorbid asthma

PURPOSE OF REVIEW

Eosinophilic otitis media (EOM) is an intractable otitis media characterized by numerous eosinophils infiltrating the middle ear cavity, which is part of the upper airway. EOM shows a high rate of comorbidity with asthma. They are considered to have a 'one airway, one disease' relationship. Here, we summarize our current knowledge regarding the characteristics of EOM, EOM's relationship with asthma and the efficacy of optimal treatments for EOM.

RECENT FINDINGS

The greater the severity of asthma, the more pronounced the development of EOM. Asthma control is usually inadequate in asthmatics who develop EOM, and appropriate strengthening of asthma inhalation therapy leads to improvement in the EOM. EOM severity can be divided into mild, moderate, and severe. Intratympanic infusion therapy using a topical steroid such as triamcinolone acetone is effective for mild EOM, whereas moderate EOM requires a systemic steroid in addition to triamcinolone acetone, and severe EOM forms granulation tissue that requires surgical removal. Recently, the effectiveness of molecularly targeted drugs is being reported, but more data need to be accumulated.

SUMMARY

EOM and asthma are closely related. Optimal asthma treatment is important for treating EOM. Treatments commensurate with the severity of EOM are being developed.

How to detect eosinophil ETosis (EETosis) and extracellular traps

Eosinophils are short-lived and comprise only a small population of circulating leukocytes; however, they play surprisingly multifunctional roles in homeostasis and various diseases including allergy and infection. Recent research has shed light on active cytolytic eosinophil cell death that releases eosinophil extracellular traps (EETs) and total cellular contents, namely eosinophil extracellular trap cell death (EETosis). The pathological contribution of EETosis was made more cogent by recent findings that a classical pathological finding of eosinophilic inflammation, that of Charcot-Leyden crystals, is closely associated with EETosis. Currently no gold standard methods to identify EETosis exist, but “an active eosinophil lysis that releases cell-free granules and net-like chromatin structure” appears to be a common feature of EETosis. In this review, we describe several approaches that visualize EETs/EETosis in clinical samples and in vitro studies using isolated human eosinophils. EETs/EETosis can be observed using simple chemical or fluorescence staining, immunostaining, and electron microscopy, although it is noteworthy that visualization of EETs is greatly changed by sample preparation including the extracellular space of EETotic cells and shear flow. Considering the multiple aspects of biological significance, further study into EETs/EETosis is warranted to give a detailed understanding of the roles played in homeostasis and disease pathogenesis.

Periostin in middle ear mucosa according to eosinophilic otitis media severity: Middle ear pathology-based treatment

OBJECTIVE

Eosinophilic otitis media (EOM) is an intractable disorder associated with bronchial asthma and chronic rhinosinusitis with nasal polyposis. Periostin is an extracellular matrix protein secreted by fibroblasts in response to interleukin (IL)-4 and/or IL-13 and is a known marker for eosinophilic disorders. We assessed serum periostin levels and expression of periostin in the middle ear mucosa according to three grade of EOM severity (grade1 to 3).

METHODS

68 patients of blood and serum samples were corrected by whom diagnose bilateral EOM in Jichi Medical University Saitama Medical Center from January 2015 to June 2017.Immunohistochemical evaluation was performed to 18 EOM middle ears mucosa samples, which cauterized in tree groups and compared to that of chronic otitis media (COM).

RESULTS

Serum periostin levels was significantly higher in EOM patients than in COM patients (EOM, 125.0 ± 45.5 ng/mL; COM, 79.4 ± 38.3 ng/mL; P<0.0001). The expression of periostin immunopositivity in the EOM middle ear mucosa was significantly greater in severe cases (grade3 samples) than others (grade1 and grade2 samples) (P <0.001 and P = 0.011, respectively). Periostin was expressed at the lamina propria especially in severe EOM cases and the cases had little response to glucocorticoids treatment.

CONCLUSION

This study showed that periostin in the middle ear mucosa was correlated with EOM severity, and EOM with highly expressed periostin had difficulty in glucocorticoids treatment.

Cellular Immune Response against Nontypeable Haemophilus influenzae Infecting the Preinflamed Middle Ear of the Junbo Mouse

Nontypeable Haemophilus influenzae (NTHi) is a major pathogen causing acute otitis media (AOM). The pathology of AOM increases during long-term infection in the middle ear (ME), but the host cellular immune response to bacterial infection in this inflamed environment is poorly understood. Using the Junbo mouse, a characterized NTHi infection model, we analyzed the cellular response to NTHi infection in the Junbo mouse middle ear fluid (MEF). NTHi infection increased the total cell number and significantly decreased the proportion of live cells in the MEF at day 1, and this further decreased gradually on each day up to day 7. Flow cytometry analysis showed that neutrophils were the dominant immune cell population in the MEF and that NTHi infection significantly increased their proportion whereas it decreased the monocyte, macrophage, and dendritic cell proportions. Neutrophil and macrophage numbers increased in blood and spleen after NTHi infection. The T-cell population was dominated by T-helper (Th) cells in noninoculated MEF, and the effector Th (CD44⁺) cell population increased at day 2 of NTHi infection with an increase in IL-12p40 levels. Sustained NTHi infection up to 3 days increased the transforming growth factor β levels, decreasing the effector cell population and increasing the T-regulatory (T-reg) cell population. In the preinflamed ME environment of the Junbo mouse, neutrophils are the first responder to NTHi infection followed by T-reg immune suppressive cells. These data indicate that sustained NTHi infection in the ME induces the immune suppressive response by inducing the T-reg cell population and reducing immune cell infiltration, thus promoting longer-term infection.

Charcot-Leyden Crystals in Eosinophilic Inflammation: Active Cytolysis Leads to Crystal Formation

PURPOSE OF REVIEW

Charcot-Leyden crystals (CLCs), slender bipyramidal hexagonal crystals, were first described by Jean-Martin Charcot in 1853, predating Paul Ehrlich's "discovery" of eosinophils by 26 years. To date, CLCs are known as a classical hallmark of eosinophilic inflammation. CLC protein expresses palmitate cleaving lysophospholipase activity and is a member of the family of S-type lectins, galectin-10. We summarize current knowledge regarding the pathological observations of CLCs and their mechanism of generation focusing on eosinophil cell death.

RECENT FINDINGS

The presence of CLCs in vivo has been consistently associated with lytic eosinophils. Recent evidence revealed that cytolysis represents the occurrence of extracellular trap cell death (ETosis), an active non-apoptotic cell death process releasing filamentous chromatin structure. Galectin-10 is a predominant protein present within the cytoplasm of eosinophils but not stored in secretory granules. Activated eosinophils undergo ETosis and loss of galectin-10 cytoplasmic localization results in intracellular CLC formation. Free galectin-10 released following plasma membrane disintegration forms extracellular CLCs. Of interest, galectin-10-containing extracellular vesicles are also released during ETosis. Mice models indicated that CLCs could be a novel therapeutic target for Th2-type airway inflammation. The concept of ETosis, which represents a major fate of activated eosinophils, expands our current understanding by which cytoplasmic galectin-10 is crystalized/externalized. Besides CLCs and free galectin-10, cell-free granules, extracellular chromatin traps, extracellular vesicles, and other alarmins, all released through the process of ETosis, have novel implications in various eosinophilic disorders.

Eosinophil extracellular trap formation is closely associated with disease severity in chronic rhinosinusitis regardless of nasal polyp status

Chronic rhinosinusitis (CRS) is a heterogeneous inflammatory airway disease involving non-eosinophilic and eosinophilic phenotypes, which translate to various endotypes. Activated eosinophils and neutrophils are known to generate extracellular traps consisting of DNA and cytotoxic granule proteins. We sought to investigate the presence of eosinophil and neutrophil extracellular traps (EETs and NETs, respectively) in human CRS tissues and to clarify the associations with their clinical features. Nasal polyp (NP) or ethmoid tissue slides of 43 subjects from endoscopic sinus surgery for CRS were analysed. Quantitative analysis of EETs and NETs was performed by confocal microscopy using immunofluorescent staining. For correlation study, the presence of NPs, number of infiltrating tissue eosinophils, preoperative Lund–Mackay scores, and other comorbidities were analysed. EET formation was observed to varying degrees in all CRS groups and was correlated with the number of tissue eosinophils (r  =  0.83, p  < 0.001) regardless of the presence of NPs. Patients with more EETs demonstrated higher Lund–Mackay scores (r  =  0.51, p  = 0.009), blood eosinophilia (r  =  0.80, p  < 0.001), and decreased olfactory function (r  = −0.65, p  < 0.001). No correlation between the extent of EET formation and the presence of atopy or asthma was apparent. However, none of the CRS groups containing neutrophils formed NETs in this study. Eosinophilic CRS indicates the presence of EETs. Formation of EETs could have a role in clinical decision-making and prediction of treatment outcome of CRS, regardless of NP status.

Subversion of host immune responses by otopathogens during otitis media

Otitis media (OM) is one of the most common ear diseases affecting humans. Children are at greater risk and suffer most frequently from OM, which can cause serious deterioration in the quality of life. OM is generally classified into two main types: acute and chronic OM (AOM and COM). AOM is characterized by tympanic membrane swelling or otorrhea and is accompanied by signs or symptoms of ear infection. In COM, there is a tympanic membrane perforation and purulent discharge. The most common pathogens that cause AOM are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis whereas Pseudomonas aeruginosa and Staphylococcus aureus are commonly associated with COM. Innate and adaptive immune responses provide protection against OM. However, pathogens employ a wide arsenal of weapons to evade potent immune responses and these mechanisms likely contribute to AOM and COM. Immunologic evasion is multifactorial, and involves damage to host mucociliary tract, genetic polymorphisms within otopathogens, the number and variety of different otopathogens in the nasopharynx as well as the interaction between the host's innate and adaptive immune responses. Otopathogens utilize host mucin production, phase variation, biofilm production, glycans, as well as neutrophil and eosinophilic extracellular traps to induce OM. The objective of this review article is to discuss our current understanding about the mechanisms through which otopathogens escape host immunity to induce OM. A better knowledge about the molecular mechanisms leading to subversion of host immune responses will provide novel clues to develop effective treatment modalities for OM.

Charcot-Leyden crystal formation is closely associated with eosinophil extracellular trap cell death

Protein crystallization in human tissue rarely occurs. Charcot-Leyden crystals (CLCs) were described in various eosinophilic diseases >150 years ago, but our understanding of CLC formation still remains limited. In this study, we demonstrate that CLCs observed in varied inflamed human tissues are closely associated with eosinophil cell-free granules and nuclear envelope/plasma membrane disintegration with release of filamentous chromatin (extracellular traps), typical morphologies of a regulated pathway of extracellular trap cell death (ETosis). During the process of eosinophil ETosis, eccentrically localized cytoplasmic and perinuclear CLC protein (galectin-10) is homogeneously redistributed in the cytoplasm. Rapid (1-2 minutes) formation of intracytoplasmic CLCs was observed using time-lapse imaging. Plasma membrane rupture enabled the release of both intracellularly formed CLCs and soluble galectin-10 that further contributed to formation of CLCs extracellularly, in parallel with the expulsion of free intact granules and extracellular traps. CLC formation and galectin-10 release were dependent on nicotinamide adenine dinucleotide phosphate oxidase activation. To our knowledge, this is the first demonstration of natural formation of CLCs in association with an active physiological process (ie, ETosis). These results indicate that dynamic changes in intracellular localization and release of galectin-10 contribute to CLC formation in vivo and suggest that CLC/galectin-10 might serve as an indicator of ETosis.

Allergic Bronchopulmonary Aspergillosis-A Luminal Hypereosinophilic Disease With Extracellular Trap Cell Death

Allergic bronchopulmonary aspergillosis (ABPA) is characterized by an early allergic response and late-phase lung injury in response to repeated exposure to Aspergillus antigens, as a consequence of persistent fungal colonization of the airways. Here, we summarize the clinical and pathological features of ABPA, focusing on thick mucus plugging, a key observation in ABPA. Recent findings have indicated that luminal eosinophils undergo cytolytic extracellular trap cell death (ETosis) and release filamentous chromatin fibers (extracellular traps, ETs) by direct interaction with Aspergillus fumigatus. Production of ETs is considered to be an innate immune response against non-phagocytable pathogens using a "trap and kill" mechanism, although eosinophil ETs do not promote A. fumigatus damage or killing. Compared with neutrophils, eosinophil ETs are composed of stable and condensed chromatin fibers and thus might contribute to the higher viscosity of eosinophilic mucus. The major fate of massively accumulated eosinophils in the airways is ETosis, which potentially induces the release of toxic granule proteins and damage-associated molecular patterns, epithelial damage, and further decreases mucus clearance. This new perspective on ABPA as a luminal hypereosinophilic disease with ETosis/ETs could provide a better understanding of airway mucus plugging and contribute to future therapeutic strategies for this challenging disease.

Allergic bronchopulmonary mycosis - pathophysiology, histology, diagnosis, and treatment

Allergic bronchopulmonary mycosis (ABPM) develops mainly in patients with asthma or cystic fibrosis via types I and III hypersensitivity reactions to filamentous fungi. Aspergillus spp., especially Aspergillus fumigatus, is the major causative fungus because of its small conidia, thermophilic hyphae, and ability to secrete serine proteases. The cardinal histological feature of ABPM is allergic (eosinophilic) mucin-harboring hyphae in the bronchi, for which the formation of extracellular DNA trap cell death (ETosis) of eosinophils induced by viable fungi is essential. Clinically, ABPM is characterized by peripheral blood eosinophilia, increased IgE levels in the serum, IgE and IgG antibodies specific for fungi, and characteristic radiographic findings; however, there are substantial differences in the clinical features of this disease between East and South Asian populations. Systemic corticosteroids and/or antifungal drugs effectively control acute diseases, but recurrences are quite common, and development of novel treatments are warranted to avoid adverse effects and emergence of drug-resistance due to prolonged treatment with corticosteroids and/or antifungal drugs.