Mast cells in human airways: the culprit?

By virtue of their undisputed role in allergy, the study of airway mast cells has focused on nasal and bronchial mast cells and their involvement in allergic rhinitis and asthma. However, recent mechanistic and human studies suggest that peripheral mast cells also have an important role in asthma, as well as chronic obstructive pulmonary disease, respiratory infections and lung fibrosis. Pathogenic roles include immune-modulatory, pro-inflammatory and pro-fibrotic activities. Importantly, mast cells also actively downregulate inflammation and participate in the defence against respiratory infections. Another complicating factor is the notorious mast cell heterogeneity, where each anatomical compartment of the lung harbours site-specific mast cell populations. Alveolar mast cells stand out as they lack the cardinal expression of the high affinity IgE receptor. Supporting the emerging concept of alveolar inflammation in asthma, alveolar mast cells shift to a highly FcϵRI-expressing phenotype in uncontrolled asthma. Site-specific and disease-associated mast cell changes have also recently been described in most other inflammatory conditions of the lung. Thus, in the exploration of new anti-mast cell treatment strategies the search has widened to include the lung periphery and the delicate task of identifying which of the countless potential roles are the critical disease modifying ones in a given clinical situation.