Background Silicosis is an occupational disease mainly characterized by pulmonary progressive fibrosis induced by the accumulation of free silica (SiO₂) in the lungs due to long-term exposure to SiO₂ dust. It has been shown that neutrophil extracellular traps (NETs) are increased in the lung tissues of silicotic mice after 28 d SiO₂ exposure, but it is unclear how the levels of NETs change throughout entire progression of silicosis in mice.

Objective To observe the levels of NETs and pathological changes in the lungs of silicotic mice after different duration of SiO₂ exposure, and to confirm the possible role and significance of NETsin the development of SiO₂-induced pulmonary fibrosis.

Methods A total of 28 SPF male C57BL/6J mice were randomly divided into a control group, and a model group, and the model group was subdivided into, a 2 d model group, a 7 d model group, and a 28 d model group, with 7 mice in each group. The mice in the model groups were given intratracheal instillation with 10 mg SiO₂ suspension (50 μL), and the mice in the control group were received same volume of saline. Mice were sacrificed and samples were collected at designed time points. The pathological changes of lung tissues of mice were observed after hematoxylin-eosin (HE) and Van Gieson (VG) staining. Immunofluorescence was used to observe the NETs markers citrullination histone H3 (CitH3) and myeloperoxidase (MPO) in bronchoalveolar lavage fluid (BALF), and the percentage of NETs-positive cells was calculated. PicoGreen fluorescent dye kit was used to detect the content of extracelluar DNA (ex-DNA) in mouse BALF, and the expression levels of fibrosis-related proteins α-smooth muscle actin (α-SMA) and fibronectin (FN) and NETs marker CitH3 in lung tissues of mice were detected by Western blot (WB).

Results Compared with the control group, inflammatory cells accumulation, alveolar wall thickening, and collagen deposition were obviously observed in the lungs of the silicosis model groups, and a large number of silicone nodules were recorded in the lung tissues in the 28 d group. Compared with the control group, the expressions of α-SMA and FN in the lung tissue of the 28 d group were significantly increased (P<0.05). The percentages of NETs in BALF increased significantly in the 2 d and the 7 d model group, then decreased in the 28 d model group (P<0.05). Compared with the control group (7.434±0.258) ng·mL⁻¹, the ex-DNA levels in BALF of mice in the 2 d (35.110±6.331) ng·mL⁻¹, the 7 d (39.491±6.948) ng·mL⁻¹, and the 28 d (23.360±4.809) ng·mL⁻¹ model groups were increased (P<0.05), and the increase of ex-DNA in the 2 d and the 7 d model groups were statistically significant (P<0.05). In comparison with the control group, the protein level of CitH3 was significantly increased in the lung tissues of mice in the 7 d model group (P<0.05).

Conclusion The content of NETs increases significantly and reaches a peak in the early inflammatory stage of silicosis, and decreases as the disease progresses to the fibrotic stage, suggesting that NETs may play a role in early stage of silicosis.