Methods: Eight allergic sheep received weekly intra-lung challenges of HDM to the left caudal lung for 24 weeks. Eight separate sheep were used as controls. Baseline lung function was assessed in the left caudal segments of all sheep throughout the challenge regime using a wedged-bronchoscope technique. Airway tissue was collected from challenged segments from all sheep, 7 days following the final intra-lung challenge. The airway tissues were immunohistochemically labelled for chymase-mast cells and eosinophils. Collagen and airway smooth muscle content were assessed on Masson’s Trichrome stained sections.
Results: Resting lung function in the left caudal segment is elevated in four out of eight sheep at the end of the repeated allergen challenge regime. Chymase mast cell density was significantly increased in the small bronchial walls of the HDM-challenged group compared to the control group (52 ± 8 vs. 8 ± 4; P<0.01). There were significant increases in bronchial collagen deposition in HDM-exposed segments compared to control segments (0.17 ± 0.02 vs. 0.11 ± 0.02 mm2/BM, P<0.05). A correlation analysis of individual sheep data showed that there was a trend for a direct association between the increases in bronchial collagen deposition and the density of chymase-labelled mast cells (rs=0.71, P=0.088). Eosinophil density in the small bronchial walls of HDM-challenged segments was also significantly increased compared to controls (65 ± 19 vs. 11 ± 3 cells/mm2, P<0.001), but not associated with collagen content. The bronchial smooth muscle content was not different between HDM-challenged and unexposed control segments.
Conclusions: The results show that repeated exposure to allergen results in significant increases in density of chymase-labelled mast cells, together with increased levels of collagen content in the small airways. The segmental challenge protocol allows for a novel approach to characterise the progressive remodelling events occurring in the small airways in chronic asthma.