Silica accelerated systemic autoimmune disease in lupus-prone New Zealand mixed mice

The genetic backgrounds of lupus-prone murine models are a valuable resource for studying the influence of environmental exposure on autoimmune diseases in sensitive populations. Epidemiological studies have shown associations between silica exposure and several autoimmune diseases, including scleroderma and systemic lupus erythematosus. To determine whether silica exposure can exacerbate systemic autoimmunity in genetically predisposed animals, New Zealand mixed mice were intranasally instilled twice with saline or saline suspensions of 1 mg silica or 500 μg TiO₂, a dose equivalent in surface area, and were evaluated with respect to health and immune status. Survival in silica exposed NZM mice was decreased compared to saline and TiO₂ exposed mice. Proteinuria levels were elevated in silica exposed mice. Levels of circulating immune complexes, autoantibodies to nuclear antigen (ANA), histone, and double stranded DNA were measured every two weeks by ELISA. Circulating immune complexes showed a trend towards an increased acceleration in levels in the silica exposed mice compared to saline and TiO₂ exposed mice. ANA levels were significantly higher in silica exposed animals compared to saline and TiO₂ exposed animals (0.237 ± 0.03 versus 0.140 ± 0.029 and 0.125 ± 0.03, P < 0.05) 16 weeks postexposure. Autoantibodies to histone were also significantly elevated after 16 weeks in silica exposed animals compared to saline and TiO₂ exposed animals (0.227 ± 0.03 versus 0.073 ± 0.015 and 0.05 ± 0.03, P < 0.05). In contrast, serum IgG levels were decreased in silica exposed NZM mice compared to the saline controls, however, IgM levels were unaffected. Lungs of the silica-exposed mice had increased inflammatory infiltrates as well as fibrotic lesions characterized by excess collagen deposition. Therefore, although NZM mice are susceptible to SLE, silica exposure significantly exacerbated the course of disease.