Defining the role of mast cells in guinea pig models of asthma

Abstract: Asthma is a common respiratory disease characterized by several pathophysiological features, such as allergen induced bronchoconstriction (in allergic asthma), airway hyperresponsiveness (AHR), airway inflammation, airway remodeling and mast cell hyperplasia. An increase of mast cells has been found in asthma patients. However, how these cells are involved in the development of asthma are not well defined. To investigate the role of mast cells in the pathophysiological characteristics of asthma, we established asthma models in guinea pigs, which have many similarities with humans, by exposing the animals to human relevant allergens: house dust mite (HDM) and cat dander extract (CDE). The involvement of mast cells in asthma-like features was investigated either by the addition of mast cell mediator antagonists or inhibitors, or inducing mast cell death. In paper I, we repeatedly exposed guinea pigs to HDM via intranasal instillation for seven weeks and successfully recaptured the antigen induced bronchoconstriction, the production of HDM specific immunoglobulins, AHR, eosinophilic inflammation with an increase of IL-13, airway remodeling (e.g., subepithelial collagen deposition and goblet cell hyperplasia) and mast cell hyperplasia. This model can be further used to study the role of mast cells in asthma. In paper II, we exposed guinea pigs to HDM or CDE intranasally for different time. Both HDM and CDE induce airway inflammation and airway remodeling after 4 weeks’ antigen exposures. These increases maintained after 8- and 12-week exposures. Exposing to both antigens for 8 weeks and 12 weeks induced a clear expansion of mast cells which is predominated by mast cells expressing tryptase. An increase of mast cells expressing both tryptase and chymase were also observed. In paper III, we isolated guinea pig trachea for comparing the effect of different mast cell agonists (HDM and Compound 48/80 (C48/80)) on airway smooth muscle responses and mediator release. We found that histamine, prostaglandins and 5- lipoxygenase products mediated the bronchoconstriction induced by HDM and C48/80. Both agonists induced a release of histamine, prostaglandin D2 and leukotriene B4. However, distinct of lipid mediator profiles were observed. The leukotriene E4 was only elevated by HDM, whereas C48/80 induced a broader release of lipid mediators. In paper IV and V, we identified an antibiotic monensin that can induce mast cell death. To examine if monensin can be a tool for investigating the role of mast cells in asthma, we cultured guinea pig tracheal segments from HDM sensitized guinea pigs and human bronchi with different concentrations of monensin for different time. We found that monensin has robust effects on causing mast cell death and totally blocked the HDM (in guinea pig trachea) and anti-IgE (in human bronchi) induced bronchoconstriction after 2 to 72h exposure without affecting the general tissue viability at low concentration. In the in vivo investigations, we exposed the guinea pigs to HDM repeatedly with or without monensin interventions. Monensin reduced the AHR, airway inflammation and mast cell hyperplasia in the HDM induced guinea pig model. In conclusion, exposing to human relevant allergens (HDM and CDE) are suitable for modeling of allergic asthma in guinea pigs. The increase of mast cells by HDM and CDE helps to investigate the role of mast cells in asthma models. Mast cells in guinea pig airways can respond differently to antigen and non-antigen agonists. Monensin can be a robust tool to induce mast cell death. The antigen induced bronchoconstriction by HDM in guinea pig trachea and anti-IgE in human bronchi are purely mast cell mediated. Our findings emphasize that mast cells have important roles in the development of AHR and airway inflammation in the guinea pig model used in this PhD study. The findings in this thesis highlight the importance of mast cells in asthma and the models we developed can be used as important tools for defining the mechanisms behind asthma.