Alveolar mast cells shift to an FcεRI-expressing phenotype in mild atopic asthma: a novel feature in allergic asthma pathology

Background: A unique feature of alveolar mast cells is their low high-affinity IgE receptor (Fc epsilon RI) expression. Recent discoveries in uncontrolled asthma suggest that the appearance of Fc epsilon RI-expressing alveolar mast cells may be a novel disease-specific feature of allergic asthma. This study investigates whether increased Fc epsilon RI-expressing alveolar mast cells are present in patients with mild allergic asthma or even in non-asthmatic allergic rhinitis patients (AR) who have developed bronchial hyperactivity (BHR). Methods: Bronchial and alveolar tissues were obtained from healthy controls, AR patients with or without BHR, and AR patients with concurrent asthma. Samples were processed for immunohistochemical identification of MCT and MCTC and expression of Fc epsilon RI and surface-bound IgE. Results: Bronchial mast cell expression of Fc epsilon RI was high in all groups. In contrast, in the alveolar tissue, the expression of Fc epsilon RI on mast cells was low in healthy controls and in the AR patient groups, whereas a high expression was present in AR patients with concurrent asthma (P = 0.006 compared to controls). The asthmatics had a 29-fold increase in numbers (P = 0.006) and a 19-fold increase in proportion (P = 0.007) of alveolar mast cells that expressed surface-bound IgE. Conclusions: The present data show that alveolar mast cells in patients with mild atopic asthma, but not atopic patients with AR, have turned into a highly Fc epsilon RI- and IgE-expressing phenotype. These data support the hypothesis that increased Fc epsilon RI expression on alveolar mast cells is a novel disease-specific feature of allergic asthma that is important for understanding asthma phenotypes and designing new therapeutic strategies.