A Study of Inebilizumab Efficacy and Safety in IgG4- Related Disease

Key Inclusion Criteria: 1. Male or female adults, ≥ 18 years of age at time of informed consent. 2. Clinical diagnosis of IgG4-RD. 3. Fulfillment of the 2019 ACR/EULAR classification criteria. 4. Experiencing (or recently experienced) an IgG4-RD flare that requires initiation or continuation of glucocorticoid (GC) treatment at the time of informed consent. 5. IgG4-RD affecting at least 2 organs/sites at any time in the course of IgG4-RD 6. Non-sterilized male subjects who are sexually active with a female partner of childbearing potential must use a condom with spermicide from Day 1 through to the end of the study and must agree to continue using such precautions for at least 6 months after the final dose of IP. Females of childbearing potential who are sexually active with a non-sterilized male partner must use a highly effective method of contraception Key Exclusion Criteria: 1. History of solid organ or cell-based transplantation or known immunodeficiency disorder. 2. Active malignancy or history of malignancy that was active within the last 10 years (some specific situations for cervical, skin or prostate cancer are acceptable). 3. Receipt of any biologic B cell-depleting therapy or non-depleting B-cell-directed therapy in the 6 months prior to screening. 4. Receipt of non-biologic DMARD or immunosuppressive agent other than GCs within 4 weeks prior to screening. 5. Active tuberculosis or high risk for tuberculosis; hepatitis C infection in absence of curative treatment; evidence of hepatitis B infection. 6. Receipt of live vaccine or live therapeutic infectious agent within 2 weeks prior to screening. 7. Estimated glomerular filtration rate \< 30 mL/min/1.73 m\^2.