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 progressive pulmonary inflammation and fibrosis. There is evidence that the release of reactive oxygen intermediates (ROI) and cytokines by alveolar macrophages (AM) is involved in lung injury associated with silica exposure. However, the chronology and relationship between these two mediators are poorly understood. In this study, an animal model of silicosis has been used, allowing simultaneous follow-up of lung histopathologic 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 silica (50 mg/kg) in rats led to fibrosis characterized by cellular interstitial infiltrates with granulomas, and in AM, it led to 1) an early and continuous increase in 12-O-tetradecanoylphorbol-13-acetate- or zymosan-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-alpha-phenylnitrone) reversed lung histopathologic changes and decreased AM ROI production and TNF-alpha expression at the level of mRNA. These findings suggest that ROI production is an important primary event determining the silica-induced inflammatory process. ROI may act in an autocrine or paracrine manner and regulate TNF-alpha production by a mechanism promoting gene expression. The critical role of this cytokine in the pathogenesis of silicosis was confirmed by anti-TNF-alpha Ab treatment.