Eosinophil biology in COPD; from scientific insight to clinical treatment action

Chronic obstructive pulmonary disease (COPD) is a common lung disease that causes significant global morbidity. Strikingly, elevated blood and/or airway eosinophil levels are observed in up to 40% of COPD patients. These innate immune cells are clinically associated with increased exacerbation frequency and risk. Eosinophils are key in COPD symptom management and a drive to uncover their related immunopathology is critical to our understanding of disease progression. Recent biologic treatments targeting eosinophils have had success in reducing symptoms in eosinophilic COPD patients. However, the underlying pathways that lead to these improvements is incompletely understood. Given eosinophils are important in pathogen clearance, mucus viscosity, barrier integrity, and tissue remodelling, any disease modifying treatments will have several proposed mechanisms. In this narrative review, we summarise the developmental, recruitment, activation, and related pathology of eosinophils in the context of eosinophil-depleting treatments to elucidate treatment efficacy. Furthermore, we highlight potential future biological pathways for therapeutics aimed at specific eosinophil mechanisms.