Resolution of Chronic Granulomatous Inflammation and Fibrosis in a Murine Model of Sarcoidosis

Abstract

Sarcoidosis is a chronic granulomatous disease of unknown cause. Granulomas resolve within 12-26 months in 50% of cases without therapy while 30% of patients have chronic disease that progresses to pulmonary fibrosis. The transcription factor peroxisome proliferator activated receptor gamma (PPAR[gamma]), a negative regulator of pro-inflammatory macrophage activation, is deficient in the alveolar macrophages of sarcoidosis patients. We established a chronic murine model of sarcoidosis in which multiwall carbon nanotubes (MWCNTs) are administered via oropharyngeal instillation. The expression of PPAR[gamma] is also deficient in the murine alveolar macrophages. Granulomatous lesions persist for at least 90-days post installation. Previous studies from our lab have implicated matrix metalloproteinase-12 (MMP12) and osteopontin (OPN) as key factors that work together to promote granuloma formation and persistence. We used a macrophage-specific PPAR[gamm] knockout mouse to study the effects of PPAR[gamma] deficiency on sarcoidosis pathophysiology. As expected, PPAR[gamma] mice had more robust granuloma formation at 60-days post instillation compared to C57BL/6J controls. We hypothesized that PPAR[gamma] mice would have large, persisting granulomatous structures 90-days post instillation and that levels of MMP12 and OPN would be elevated. PPAR[gamma] and C57BL/6J mice were instilled with MWCNT. Controls received PBS alone. Lung tissue from both strains were sectioned, stained, and subjected to granuloma scoring and qualitative evaluation. Tissue from PPAR[gamma] mice was also used for immunohistochemistry (IHC). Surprisingly, at 90-days post instillation, PPAR[gamma] mice had attenuated fibrosis and decreased granuloma size compared to 60-days and C57BL/6J mice. Evaluation of mediators in the canonical TGF[beta] signaling pathway did not explain the changes in fibrosis. Changes in MMP12 and OPN protein levels, however, were apparent. Immunohistochemistry of lung tissue showed decreased expression of both MMP12 and OPN at 90-days post instillation. These results suggest a relationship between MMP12 and OPN in the resolution of granulomatous structures. The decrease in MMP12 and OPN in tissue sections at 90-days is especially important as pulmonary granulomatous structures form and resolve within lung tissue.