Experimental airway hypersensitivity in sheep : a model for asthma : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Pathology at Massey University

Abstract

This study aimed to establish an animal model for human bronchial asthma using locally bred Romney sheep. It was then planned to determine whether or not morphological and inflammatory factors in the ovine respiratory tract are associated with a predisposition to allergic bronchial hypersensitivity induced by inhaled Ascaris suum antigen. The skin and airway responses to a commercial A. suum antigen were tested in adult Romney sheep from two local farms with and without previous exposure to pigs. Ninety percent of 101 adult sheep tested showed an immediate skin reaction, and about 70% of 43 adult sheep with positive skin reactions showed an immediate airway response, reflected as a significant increase in airway resistance and/or decreased dynamic lung compliance. Among these 43 sheep, 21 showed changes in both airway resistance and dynamic lung compliance (Group A); ten only in dynamic lung compliance (Group B) and 12 were non-responders (Group C). No significant changes were recorded when the same animals were given an aerosol of phosphate buffered saline. Although the sheep with previous exposure to pigs showed significantly greater skin reactions than those without exposure to pigs, they showed no significant differences in airway response to antigen inhalation. In addition, there was no correlation between the degree of skin reaction and the magnitude of bronchoconstriction. Since no information was available on the respiratory tract-associated lymphoid tissue and cells in healthy sheep, study of this tissue and its associated epithelium was a prerequisite for studies of the morphological and inflammatory mechanisms involved in the development of allergic airway hypersensitivity. The ovine respiratory tract has five forms of lymphoid tissue; intra-luminal, intraepithelial, scattered forms, and dense and nodular aggregations; the dense and nodular aggregations being confined to the pharyngeal tonsil and bronchioles. Morphologically well-developed lymphoepithelium (M cells) is present only in the pharyngeal tonsil region, and absent in the lower respiratory tract. The M cell of the ovine pharyngeal tonsil is ultrastructurally and functionally similar to that in other mucosal tissues of this and other species, but its development and maturation takes place earlier than the bronchus-associated lymphoid tissue. Mast cells in the lower respiratory tract of normal sheep are morphologically heterogeneous, and both formalin-sensitive and formalin-resistant types can be identified. The morphological and histochemical features of formalin-sensitive mast cells are similar to those from the human respiratory tract in several respects which enhances the use of the sheep model in the study of human allergic respiratory disease. A morphometric comparison of airway structure and inflammatory components was conducted between the three groups of sheep with varying airway hypersensitivity. The epithelium of the small airways was significantly thinner and contained fewer goblet cells in the hypersensitive sheep (Groups A and B; than in non-reacting sheep (Group C). Mast cells from the hypersensitive sheep had a significantly greater volume density of secretory granules than those from non-reacting sheep. However, no morphological difference was found in the epithelial integrity of airways between hypersensitive and non-reacting sheep, and the permeability of tracheobronchial epithelium to horseradish peroxidase was of the same order in all groups. Similarly, the airway wall was not significantly thicker in hypersensitive sheep than in non-reacting sheep, and the shortening of smooth muscle required to cause complete airway closure was similar. The numerical density of mast cells, eosinophils, neutrophils and lymphocytes in the airways and lung was not significantly different between the groups. These observations indicate that the Ascaris-induced airway response seen in Romney sheep is similar in several respects to that seen in human asthmatics and these sheep can therefore be used as an animal model to study human asthma. The current findings suggest that the presence of relatively low goblet cell density, thin epithelium, and high volume density of mast cell secretory granules in the small airways and lung may be important inherent factors responsible for the development of airway hypersensitivity in these sheep. It is concluded that most of the other morphological features observed in asthmatics and animal models are likely to be the result of allergic airway reactions rather than a fundamental difference between potentially allergic and non-allergic subjects.