The relevance of eosinophils in chronic obstructive pulmonary disease: inflammation, microbiome and clinical outcomes.

Chronic obstructive pulmonary disease (COPD) is caused by the inhalation of noxious particles such as cigarette smoke. The pathophysiological features include airway inflammation, alveolar destruction and poorly reversible airflow obstruction. A sub-group of COPD patients have higher blood eosinophil counts (BECs), associated with an increased response to inhaled corticosteroids and increased biomarkers of pulmonary type 2 (T2) inflammation. Emerging evidence shows that COPD patients with increased pulmonary eosinophil counts have an altered airway microbiome. Higher BECs are also associated with increased lung function decline, implicating T2 inflammation in progressive pathophysiology in COPD. We provide a narrative review of the role of eosinophils and T2 inflammation in the pathophysiology of COPD, encompassing the lung microbiome, pharmacological targeting of T2 pathways in COPD, and the clinical use of BEC as a COPD biomarker.