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.