Significance and Potential Role of Eosinophils in Non-Cystic Fibrosis Bronchiectasis

Bronchiectasis is a complex and heterogeneous disease with a myriad of pulmonary and extrapulmonary etiologies. Bronchiectasis has a predominantly neutrophilic inflammatory profile. However, eosinophilic inflammation has also been documented in both the airways and the systemic circulation. Various diseases (eg, asthma, allergic bronchopulmonary aspergillosis, chronic rhinosinusitis with nasal polyps) characterized by heightened type 2 airway inflammatory responses, including blood or sputum eosinophilia, may coexist with bronchiectasis. Apart from those eosinophilic etiologies or comorbidities related to bronchiectasis, around 20% of patients with bronchiectasis have peripheral eosinophilia (at least 3% or 300 eosinophils/μL) with no identified concomitant disease (also termed "eosinophilic bronchiectasis"), whose roles have not been fully understood. The two key points regarding these observations are that eosinophils confer both bactericidal and antiviral properties against common pathogenic microorganisms that are usually detected in bronchiectasis, and that eosinophilic bronchiectasis has been associated with better therapeutic response to inhaled corticosteroids and other anti-TH2 profile treatments. In this review, we summarize the most significant evidence regarding the role of eosinophils in patients with bronchiectasis, including the association of bronchiectasis with eosinophilic diseases (as etiologies or comorbidities), and existing data on eosinophilic bronchiectasis not related to eosinophilic disorders. Bronchiectasis is a complex and heterogeneous disease with a myriad of pulmonary and extrapulmonary etiologies. Bronchiectasis has a predominantly neutrophilic inflammatory profile. However, eosinophilic inflammation has also been documented in both the airways and the systemic circulation. Various diseases (eg, asthma, allergic bronchopulmonary aspergillosis, chronic rhinosinusitis with nasal polyps) characterized by heightened type 2 airway inflammatory responses, including blood or sputum eosinophilia, may coexist with bronchiectasis. Apart from those eosinophilic etiologies or comorbidities related to bronchiectasis, around 20% of patients with bronchiectasis have peripheral eosinophilia (at least 3% or 300 eosinophils/μL) with no identified concomitant disease (also termed "eosinophilic bronchiectasis"), whose roles have not been fully understood. The two key points regarding these observations are that eosinophils confer both bactericidal and antiviral properties against common pathogenic microorganisms that are usually detected in bronchiectasis, and that eosinophilic bronchiectasis has been associated with better therapeutic response to inhaled corticosteroids and other anti-TH2 profile treatments. In this review, we summarize the most significant evidence regarding the role of eosinophils in patients with bronchiectasis, including the association of bronchiectasis with eosinophilic diseases (as etiologies or comorbidities), and existing data on eosinophilic bronchiectasis not related to eosinophilic disorders.