Mast cells and IgE-bearing cells in lungs of RAO-affected horses

In the wall of the bronchi and bronchioli of both RAO-affected and control horses, mainly chymase+ mast cells (MCC) were present (85% in the bronchial wall and 77% in the wall of the bronchioli), while 73% of the mast cells (MC) around blood vessels were tryptase+ mast cells (MCT). No double stained MCs were detected. RAO-affected horses had significantly more MCC than controls in the wall of the bronchi (median = 7.6 and 1.7 cell/mm2, respectively, P ≤ 0.05). They also showed a tendency for more MCC in the wall of the bronchioli than controls (median = 21 and 2.9 cells/mm2, respectively, P = 0.07) but there were no differences in MCT numbers. The data suggest an involvement of MCC in the pathogenesis of RAO. Independently of the clinical diagnosis, there was a significant relationship between high MCC numbers in the bronchial wall and lung fibrosis, suggesting that these MCC may be involved in tissue remodelling. Furthermore, high MCC numbers were also associated with increased infiltration with lymphocytes and neutrophils.