Eosinophilia and Chronic Airway Diseases

Asthma has long been associated with an eosinophilic trait. Some early studies have shown a relationship between circulating eosinophil numbers and bronchial hyperresponsiveness. Soon after the use of bronchoscopy and induced sputum as a research tool in mild-to-moderate asthma, it was shown that airway eosinophilia, demonstrated by bronchoalveolar lavage, bronchial biopsies, and sputum cell counts, is related to disease severity. Sputum studies have established the concept of asthma inflammatory phenotypes, with approximately 50% of patients having significant airway eosinophilia. Airway eosinophilia is not sufficient to cause asthma, but is a risk factor for poorly controlled disease. Chronic obstructive pulmonary disease, a disease typically associated with prominent neutrophilic inflammation, sometimes also exhibits a significant eosinophilic trait, even if it is less frequent than in asthma and not associated with disease severity. However, both in asthma and chronic obstructive pulmonary disease, eosinophilic inflammation predicts a good response to inhaled or oral corticoids and to anti-interleukin-5.