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

Recurrent airway obstruction (RAO) is a common condition in stabled horses characterised by small airway inflammation and obstruction following exposure of susceptible horses to mouldy hay and straw. The aim of the present study was to investigate whether lung tissue from horses with RAO contains higher numbers of IgE-protein positive (+) cells and mast cells compared to controls after mouldy hay challenge. Furthermore, mast cell subtypes in lung tissue were investigated. ells were detected in most lung tissue samples but no significant differences between RAO-affected and control horses were found. 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.