This study has been designed as a 18-month, open-label randomized controlled clinical trial. The study aims to compare the efficacy and safety of two treatment strategies in active IgG4-RD patients with internal organ involvement during maintenance remission period: low dose mycophenolate mofetil group and leflunomide group.

60 active IgG4-RD patients with internal organ involvement are enrolled in this study and accept the combination treatment of glucocorticoids and mycophenolate mofetil in remission induction period (6 months), during which glucocorticoids are tapered regularly and discontinued in 6 months. During remission maintenance period, pateints are randomly divided into two groups at a 1:1 ratio: patients in group I are treated with low dose mycophenolate mofetil (1-1.5g/day) and patients in group Ⅱ accept the treatment of leflunomide (20 mg/day). Patients in two groups will be followed up for another 12 months, and clinical evaluations, laboratory tests, image examinations and IgG4-RD responder index (RI) will be recorded during follow up. The primary endpoint is the difference of relapse rate between two groups at 18 months. The secondary endpoints are relapse time, response rate and side effects.

Inclusion Criteria:

• 1. Fulfillment of the 2019 American College of Rheumatology/European League against Rheumatology (ACR/EULAR) IgG4-related disease classification criteria; 2. Newly-onset or relapsed patients who requires initiation or continuation of glucocorticoids treatment.

Exclusion Criteria:

• 1. Patients who were diagnosed as other autoimmune diseases; 2. Patients who were diagnosed as malignant diseases; 3. Pregnant and lactating women； 4. Active infection: HIV, HCV, HBV, TB； 5. Severe irreversible damage of organ function; 6. Receipt of any biologic therapy or immunosuppressive agent other than GCs.