Funktionelle Charakterisierung von BAL-Zellen NO2-exponierter Ratten

Macrophages and neutrophile granulocytes have the ability to eliminate pathogenic microorganisms by means of phagocytosis and subsequent production of free radicals, such as superoxide. These effector cells of the innate immune system appear in increased numbers in lungs of patients with COPD, which may suggest a protective role of those cells from bacterial infections in this pulmonary disease. Contradictory to that, however, clinical experience shows that patients with COPD have an increased susceptibility to exacerbations induced by bacterial infections. In order to elucidate the underlying mechanisms of this phenomenon, we aimed at characterizing BAL-cells in the lungs of rats which had been experimentally exposed to an NO2-induced pulmonary inflammation. We hypothesized that BAL-cells in lungs of NO2-exposed rats experience a shift towards an alternative activation pathway. We particularly emphasized the investigation of free-radical metabolism as well as the secretion of cytokines of BAL-cells at different time-points (24 hours, 3 days, 20 days) after NO2-exposure as compared to BAL-cells of healthy control animals. We could demonstrate that zymosan-induced superoxid-production was significantly reduced in BAL-cells of NO2-exposed rats at all measured time-points as compared to control rats. The use of enzyme inhibitors revealed that essential enzymes for the production of superoxid radicals, such as the NADPH-oxidase as well as Complex III of the mitochondrial respiratory chain, were functionally affected by NO2-exposure. Expression analysis of genes involved in free radical production, revealed that mRNA levels of particular isoforms of SOD and GPx were significantly increased in response to NO2-exposure. Furthermore, we investigated the activity of the antioxidative enzymes SOD and GPx in BAL-cells by functional assays. Our data indicate that an NO2-exposure for 20 days results in a significant increase in activity of both enzymes. Another important mechanism for immunological defense and immunmodulation next to the production of free radicals is the secretion of cytokines. We therefore investigated whether the surfactant Protein-A was able to affect the cytokine-production of BAL cells in response to NO2-exposure. We could demonstrate that LPS-induced secretion of TNF-alpha in BAL-cells was significantly reduced at 3 and 20 days after NO2-exposure when compared to control animals. Treatment with SP-A resulted in an additional inhibition of TNF-alpha secretion in BAL-cells after 20 days of NO2-exposure, although this inhibition lacked statistical significance. Regarding the secretion of IL-10 only animals which underwent 3 days of NO2 exposition had the ability to liberate cytokines in appreciable quantity. In tendency, the administration of SP-A evoked an enhanced secretion of this anti inflammatory cytokine. Production of the chemokine MCP-1 by BAL-cells was regulated differently in the respective phases of the inflammatory process caused by NO2-exposure. However, treatment with SP-A resulted in a suppression of secretion of this proinflammatory cytokine independently of a prior NO2-exposure. In conclusion, our experiments demonstrated that BAL cells displayed an activation switch towards an alternative activation mechanism within a NO2 induced inflammation reaction. Here, the changes of the radical metabolism are particularly noteworthy. In our animal model this activational shift could be interpreted as an autoprotective mechanism in response to NO2-induced inflammation. However, a prevailing of alternatively activated BAL-cells with a decreased free radical production increases the susceptibility to bacterial infection. This could possibly explain why patients with COPD suffer from increased numbers of exacerbations triggered by bacterial infections.