Pneumonic lesions occurring in calves after respiratory infection with Mycoplasma bovis are characterized by subacute or chronic suppurative bronchopneumonia with multiple foci of necrosis and by persistence of M. bovis antigen, which is frequently associated with phagocytes at the periphery of the necrotic foci. The aims of this study were: (1) to investigate the expression of inducible nitric oxide synthase (iNOS), nitrotyrosine (NT) and manganese superoxide dismutase (Mn-SOD) in the lung lesions of calves infected experimentally with M. bovis, and (2) to analyse the distribution and localization of M. bovis DNA by in-situ hybridization and correlate these findings with the immunohistochemical detection of M. bovis antigen. Phagocytic cells infiltrating the lung tissue were characterized using the markers CD68, S100A8 and S100A9. Lung tissue from 18 infected calves and three non-infected controls were examined. All infected calves had an increased number of cells expressing iNOS, NT and Mn-SOD in the inflamed lung tissue. These molecules were most strongly expressed by macrophages demarcating necrotic areas, by altered bronchiolar epithelial cells and by macrophages within obliterated bronchioles. Co-localization of M. bovis DNA, M. bovis antigen and macrophages expressing iNOS, NT and Mn-SOD was observed. These findings suggest that the generation of reactive oxygen and nitrogen species is involved in the development of severe chronic lung damage in M. bovis infection.
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