Interstitial lung disease (ILD) is a common pulmonary manifestation of idiopathic inflammatory myopathy (IIM). The overall 5-year mortality is 50%. The prognosis is poor and the treatment is challenging.At present, according to the consensus of IIM-ILD experts, glucocorticoids as first-line treatment are often used in high doses and have a variety of adverse reactions. Previous studies have shown that cyclophosphamide (CYC) is effective for IIM-ILD and tends to be used in rapidly progressive interstitial lung disease（RP-ILD）or refractory ILD. However, CYC is an alkylating agent with many toxic and side effects. It is prone to gonadal inhibition, infection, tumor, hemorrhagic cystitis and other risks. At present, Mycophenolate mofetil (MMF) has been widly used in the treatment of IIM, systemic lupus erythematosus (SLE), ANCA associated vasculitis (AAV). The observational research on MMF in the treatment of IIM-ILD shows that it can delay the progress of pulmonary fibrosis and can be used as the first-line treatment of IIM-ILD. Moreover, immune tolerance caused by defects in the number and/or quality of regulatory T cells (Treg) is considered to be a key source of autoimmune diseases. However, it is unclear whether MMF can improve the immune status of IIM-ILD by increasing Treg cells. The aim of this study was to evaluate the effect of MMF for IIM-ILD and its effcts on Treg through a prospective open single arm study, and provide a theoretical basis for the individualized treatment of IIM-ILD, which has important clinical significance.

Interstitial lung disease (ILD) is a common pulmonary manifestation of idiopathic inflammatory myopathy (IIM), with an incidence ranging from 23.1 to 65%. The treatment of IIM patients with ILD is challenging, and the overall 5-year mortality is 50%. The disease process of ILD from stable or slow to rapid. Increased mortality and poor prognosis were observed, which needs active treatment. At present, according to the consensus of IIM-ILD experts, glucocorticoids as first-line treatment are often used in high doses, but have a variety of adverse reactions. Previous studies have shown that cyclophosphamide (CYC) is effective for IIM-ILD and tends to be used in rapidly progressive interstitial lung disease（RP-ILD）or refractory ILD. However, CYC is an alkylating agent with many toxic and side effects. It is prone to gonadal inhibition, infection, tumor, hemorrhagic cystitis and other risks. Mycophenolate mofetil (MMF) has been widly used in the treatment of IIM, systemic lupus erythematosus (SLE), ANCA associated vasculitis (AAV). The observational research on MMF in the treatment of IIM-ILD shows that it can delay the progress of pulmonary fibrosis and can be used as the first-line treatment of IIM-ILD. Moreover, immune tolerance caused by defects in the number and / or quality of regulatory T cells (Treg) is considered to be a key source of autoimmune diseases. In transplant patients, studies have found that the combination of MMF and tacrolimus can increase the number of Treg cells in peripheral blood, suggesting that MMF has a certain regulatory effect on Treg cells. Therefore, it is unclear whether MMF can improve the immune status of IIM-ILD by increasing Treg cells, so as to improve the prognosis of the disease. However, it is unclear whether MMF can improve the immune status of IIM-ILD by increasing Treg cells. The aim of this study was to evaluate the effect of MMF for IIM-ILD and its effcts on Treg through a prospective open single arm study, and provide a theoretical basis for the individualized treatment of IIM-ILD.This was a single-arm open-label pilot observational study. Patients were received prednisone (0.5mg-1.0mg/kg/d ) combined with MMF(1.5g-2.0g/d) for 12 months, 4 weeks later, reduced 5mg every two weeks to 10mg/d orally for 3 months, gradually reduced to 7.5mg/ day until 12 months.

Inclusion Criteria:

• meet the PM/DM diagnostic criteria according to Bohan-Peter
• consistent with ILD diagnosis
• predicted forced vital capacity (FVC) of at least 50%
• patients who did not use immunosuppress agents (including but not limited to cyclophosphamide, cyclosporine, leflunomide, azathioprine, tacrolimus, etc.) at the time of screening, or who had stopped taking drugs for ≥3 months.

Exclusion Criteria:

• Anti MDA5 antibody positive DM and necrotizing myopathy
• patients if they had other connective tissue diseases, an underlying cancer, a concomitant active infection.