GM-CSF but not IL-17 is critical for the development of severe interstitial lung disease in SKG mice

Interstitial lung disease (ILD) is a common complication and sometimes a prognostic factor of connective tissue diseases (CTDs) in humans. However, suitable animal model of severe CTD-associated ILD (CTD-ILD) has been limited. In this study, we showed that zymosan-treated SKG mice developed not only arthritis but also chronic-progressive ILD with high mortality over several months. The pathological and clinical features of ILD in zymosan-treated SKG mice were similar to that of human severe CTD-ILD. ILD in this mouse was characterized by massive infiltration of Th17 cells, GM-CSF-producing CD4 ⁺ T cells, and CD11b⁺ Gr1⁺ neutrophils with fibrosis. Naive SKG T cells were skewed to differentiate into GM-CSF-producing cells, and GM-CSF secreted by T cells enhanced IL-6 and IL-1β production by macrophages, which in turn enhanced differentiation of IL-17A- and/or GM-CSF-producing T cells and infiltration of neutrophils into lung. Neutralization of GM-CSF completely blocked the development of this ILD, and the blocking of IL-6 signaling resulted in partial prevention of it, whereas neutralization of IL-17A did not. In contrast, the progression of arthritis was inhibited by the neutralization of GM-CSF and slightly by the neutralization of IL-17A, but not by the blocking of IL-6 signaling. These data suggested zymosan-treated SKG mice could be a useful mouse model of severe CTD-ILD, and GM-CSF, rather than IL-17A or IL-6, contributed to the development of ILD in zymosan-treated SKG mice, indicating that neutralization of GM-CSF would be a useful therapeutic strategy for severe CTD-ILD.