Expansion of Interferon- γ –Producing Lung Lymphocytes in Mouse Silicosis

Abstract

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.

Attenuation of Lung Inflammation and Fibrosis in Interferon- γ –Deficient Mice after Intratracheal Bleomycin

“…Similar to this hypothesized role for IFN-␥ in bleomycin-induced pulmonary fibrosis, the fibrotic response to intratracheal silica (another lung injury-inducing agent) exposure in rodents has also been shown to be dependent on IFN-␥, IL-12, and TNF-␣ (29).…”

“…Our findings that IFN-␥ may play a profibrotic role under certain conditions in experimental bleomycin-induced pulmonary toxicity have possible implications for human disease. First, these observations suggest that IFN-␥ could play a profibrotic role in the lung fibrosis that occurs in diseases associated with enhanced chronic production of IFN-␥ such as sarcoidosis, chronic beryllium disease, silicosis, and hypersensitivity pneumonitis (29,(39)(40)(41)(42). In these diseases, chronic lung injury secondary to the proinflammatory effects of IFN-␥ could theoretically result in progressive lung fibrosis (43).…”

Pulmonary overexpression of IL-10 augments lung fibrosis and Th2 responses induced by silica particles

“…Both Th1 and Th2 polarization during experimental silicosis have already been reported. Although IL-4 expression was unchanged or decreased, Davis and colleagues (12) showed an increased IFN-␥ response in the lung of mice with silicosis, suggesting a Th1-like lymphocyte-mediated immune response. Opposite results showed that in wild-type animals, silica-induced fibrosis was correlated with markers of a Th2-like response such as upregulation of IL-4 levels in lung tissue and an increased immunoglobulin IgG1/IgG2a ratio in BAL (24).…”

Type 2 immune response associated with silicosis is not instrumental in the development of the disease

“…Both Th1 (8,13,14,17,18,22,51) and Th2 responses (1,4,10,22) have been found to be associated with the development of silicosis. Thus, in our study, we examined the potential role of type 2 immune polarization by investigating the functions of IL-4 and IL-13 in the fibrotic process in response to silica particles in C57BL/6 wild-type, IL-4 Ϫ/Ϫ , and IL-4R␣ Ϫ/Ϫ mice.…”

“…IL-4 expression was also shown to be increased in the lungs of silica-treated rats (10). By contrast, overproduction of IFN-␥ by lung and thoracic lymph node T lymphocytes was reported in mice and rats developing silicosis (13,14,17,18,22). A predominance of Th1 immune response in silicosis was also evidenced by enhanced levels of serum IgG 2a (51) but reduced serum IgG 1 concentrations in silica-treated mice (8).…”