Mucosal immunity in asthma and chronic obstructive pulmonary disease: a role for immunoglobulin A?

Despite our knowledge on the role of IgA in mucosal homeostasis and host defense and clinical evidence suggesting deficient first-line defense mechanisms in chronic airway disorders, little is known regarding its role in asthma and chronic obstructive pulmonary disease (COPD). Studies suggest that the mucosal IgA response is impaired in COPD, and a deficient transport of IgA across the bronchial epithelium in COPD has been identified, possibly involving neutrophil proteinases, which may degrade the Ig receptor mediating this transepithelial routing. In contrast, the IgA response to allergens in patients with asthma may play a pathogenic role through eosinophil activation. Thus, secretory IgA can induce eosinophil degranulation in vitro, a feature in keeping with the correlations observed in vivo between airway IgA levels and eosinophil cationic protein during late asthmatic responses. Selective IgA deficiency is associated with an increased prevalence of atopy, and a protective role of IgA has been seen in murine models of asthma, delineating the complexity of the IgA system in the airway mucosa. Future studies will hopefully yield better knowledge of IgA biology and lung mucosal immunity and help to use more efficiently the mucosal route for immunotherapy or target specific genes in inflamed airways.