REACTIVE OXYGEN INTERMEDIATES AS REGULATORS OF TNF-ALPHA PRODUCTION IN RAT LUNG INFLAMMATION-INDUCED BY SILICA

Exposure to mineral dusts such as silica has been associated with prog ressive pulmonary inflammation and fibrosis. There is evidence that th e release of reactive oxygen intermediates (ROI) and cytokines by alve olar macrophages (AM) is involved in lung injury associated with silic a exposure. However, the chronology and relationship between these two mediators are poorly understood. In this study, an animal model of si licosis has been used, allowing simultaneous follow-up of lung histopa thologic state, AM TNF-alpha production at the protein (biologic assay ) and mRNA (reverse transcriptase-PCR) levels, and the release of ROI (luminol-dependent chemiluminescence), after bronchoalveolar lavages. In particular, it has been shown that intratracheal instillation of si lica (50 mg/kg) in rats led to fibrosis characterized by cellular inte rstitial infiltrates with granulomas, and in AM, it led to 1) an early and continuous increase in 12-O-tetradecanoylphorbol-13-acetate- or z ymosan-triggered ROI production (days 1, 3, 14, and 28 post-treatment) , and 2) a rise of TNF-alpha mRNA expression and protein secretion on days 3 and 14. A free radical scavenger pretreatment (N-ter-butyl-alph a-phenylnitrone) reversed lung histopathologic changes and decreased A M ROI production and TNF-alpha expression at the level of mRNA. These findings suggest that ROI production is an important primary event det ermining the silica-induced inflammatory process. ROI may act in an au tocrine or paracrine manner and regulate TNF-alpha production by a mec hanism promoting gene expression. The critical role of this cytokine i n the pathogenesis of silicosis was confirmed by anti-TNF-alpha Ab tre atment.