Expansion of interferon-gamma-producing lung lymphocytes in mouse silicosis

Silicosis is characterized by mononuclear cell inflammation with macrophage activation, accumulation of lymphocytes, and fibrosis. Interferon-gamma (IFN-gamma) is a lymphocyte cytokine with broad effects, particularly macrophage activation. Mice exposed to an aerosol of cristobalite silica (70 mg/m3, 12 d, 5 h/d) developed diffuse pulmonary pathologic changes with macrophage, lymphocyte, and neutrophil recruitment, and increased lung collagen. IFN-gamma messenger RNA (mRNA) was more abundant by semiquantitative reverse transcription-polymerase chain reaction in the lungs of silica-exposed mice than in control animals. IFN-gamma mRNA transcripts were detected by in situ hybridization with digoxigenin-labeled complementary DNA probes in normal mouse lung tissue within bronchial-associated lymphoid tissues (BALT). In silica- exposed mice, mononuclear cells with IFN-gamma mRNA were more numerous in the silicotic lesions and enlarged BALT structures. Lung-cell suspensions were prepared by enzyme digestion, stained with fluorescent-labeled antibodies against intracellular cytokines, and enumerated by flow cytometry. The percentage of cells producing IFN-gamma was increased in silicotic mice (19% versus 11%). Interleukin (IL)-4 mRNA transcripts were less abundant in the lung tissue from silica-exposed mice than in control mice. Cells staining for IL-4 mRNA were found rarely in either the air-sham or the silica-exposed mouse lungs, and almost all appeared to be within BALT structures. Approximately 3% of cells stained for IL-4 in the digested lungs from both groups. Similar cytokine patterns were observed in mediastinal lymph node/thymus and spleen tissues. The augmented IFN-gamma response, with IL-4 unchanged or decreased, in the lung lesions and lymphoid tissue of mice with silicosis suggests a Th-1-like lymphocyte-mediated immune-inflammatory response.