The goal of this clinical trial is to learn if IMM0306 works to reduce the risk of disease relapse in participants with IgG4-related disease (IgG4-RD). It will also learn about the safety and tolerability of IMM0306. The main questions it aims to answer are:

• Does IMM0306 reduce the risk of disease relapse in participants with IgG4-RD?
• What medical problems do participants have when receiving IMM0306?
• How does IMM0306 behave in the body, and does the body develop anti-drug antibodies against IMM0306? In the Phase II part, all participants will receive IMM0306. In the Phase III part, researchers will compare IMM0306 with placebo to evaluate whether IMM0306 reduces the risk of disease relapse in participants with IgG4-RD.

Participants will:

• Receive IMM0306 or placebo by intravenous infusion once weekly for 4 consecutive weeks, with the same treatment repeated 6 months later
• Start a planned glucocorticoid taper from Day 1 and gradually reduce oral prednisone or equivalent until complete discontinuation after 8 weeks
• Visit the study site for efficacy and safety assessments, including disease relapse assessment, IgG4-RD responder index assessment, laboratory tests, imaging examinations, electrocardiograms, pharmacokinetic sampling, immunogenicity sampling, biomarker sampling, and adverse event assessment.

Inclusion Criteria:

• Clinical diagnosis of IgG4-RD;
• Meeting 2019 ACR/EULAR classification criteria with an inclusion score ≥20;
• At least two organs/sites involved during the disease course;
• Prior or recent IgG4-RD flare requiring initiation or continuation of GC treatment at informed consent.

Exclusion Criteria:

• Fibrotic manifestation as the only clinical manifestation of the current relapse;
• Significant hematologic or hepatic abnormalities;
• Recent B-cell-depleting therapy, alkylating agents, DMARDs or immunosuppressants;
• Other chronic active immune diseases requiring long-term use of immunosuppressants;
• Active malignancy or active malignancy within 10 years;
• Significant cardiac disease;
• Active infection or active TB;
• Severe pulmonary disease;
• Recent opportunistic infection;
• Alcohol/drug abuse;
• Pregnancy, lactation or failure to meet contraception requirements.