Effect of stress on pulmonary host defences of cattle
The lung is continuously exposed to inhaled opportunistic pathogens; yet efficient gas exchange requires the absence of pulmonary infection and inflammation. Enabling this are myriad proteins within the epithelial-lining fluid of the lung, which are dynamically regulated to maintain or restore homeostasis. Stressors existent in the management cycle of feedlot cattle are known to enhance susceptibility to bacterial bronchopneumonia and may do so by altering the expression of pulmonary proteins. This thesis endeavoured to characterize proteomic changes in the lungs of cattle exposed to natural stress or a glucocorticoid-induced model of stress. Initially, multiple neuroendocrine mediators of the stress response were tested for their ability to alter antimicrobial peptide expression in cultured tracheal epithelial cells using real-time reverse transcription-PCR and mass spectrometry. Glucocorticoids were found to inhibit the lipopolysaccharide (LPS)-induced expression of tracheal antimicrobial peptide (TAP) in cultured cells. Bronchial biopsies from glucocorticoidtreated calves had significantly lower expression of TAP and lingual antimicrobial peptide (LAP) compared to saline-treated controls. Two-dimensional electrophoresis and mass spectrometry were used to survey bronchoalveolar lavage fluid for changes in epithelial-lining fluid protein expression occurring in glucocorticoid-treated cattle. Significant expression changes were found for proteins involved in the acute phase response ([alpha]-l-acid glycoprotein, [alpha]-l-antitrypsin, [alpha]-1-antichymotrypsin, [alpha]-2-HS-glycoprotein) and protection against oxidative damage (adipocyte-fatty acid binding protein, odorant binding protein [OBP]). In addition, [alpha]-enolase and cofilin-1 were increased while immunoglobulin J chain was reduced in glucocorticoid-treated calves compared to saline-injected controls. Applying a similar proteomic approach in cattle exposed to the stress of weaning, transportation and castration revealed expression changes in proteins implicated in the acute phase response ([alpha]-1-antichymotrypsin, [alpha]-2-HS-glycoprotein) and the regulation of oxidative stress (annexin A1, isocitrate dehydrogenase, albumin, haptoglobin, fibrinogen, heme-binding protein 1 and OBP). Abundant in bronchoalveolar lavage fluid and altered by glucocorticoids and stress, the cellular expression of OBP in the lung was characterized by immunohistochemistry. Functional testing of OBP revealed a putative anti-inflammatory role by inhibiting neutrophil chemotaxis towards culture supernatants from LPS-stimulated macrophages. These findings identify stress- and glucocorticoid-induced expression changes in pulmonary proteins involved in host defence, the acute phase response and protection against oxidative stress.