Study to Assess Safety, Efficacy and Persistence of ACE1831, in Subjects With IgG4-Related Disease

Inclusion Criteria: To be eligible for this study, all of the following inclusion criteria must be met:

* Signed Informed Consent
* Male or female ≥ 18 to 75 years of age
* Active IgG4-RD flare at screening with IgG4-RD Responder Index at least 2, confirmed by symptoms, labs, and/or imaging.
* History of IgG4-RD involving at least 2 organs/sites, and current flare involves at least 1 organ/site (excluding lymph nodes) requiring treatment.
* Elevated serum IgG4 above the upper limit of normal at screening.
* Able to receive glucocorticoids for current flare and taper to 0 mg by Day -5.
* Contraception agreement per protocol from screening through 24 weeks after last ACE1831 dose (no LDC) or 12 months after last LDC dose (with LDC).
* For sites in China only: prior treatment failure to glucocorticoids and at least one immunosuppressive agent.

Exclusion Criteria: An individual who meets any of the following criteria will be excluded from participation in this trial.

* Significant conditions that impair ability to receive study treatment or comply.
* Predominant fibrosis in affected organs.
* Active/latent infection that would interfere with therapy (including HBV, HCV, HIV, TB, syphilis) or significant recent infection per protocol.
* Known immunodeficiency state.
* NYHA class III/IV heart disease.
* Severe allergy/hypersensitivity to monoclonal antibodies or relevant study agents.
* Malignancy within 5 years (protocol exceptions apply).
* Recent investigational agent exposure.
* Recent B-cell depleting therapy (anti-CD20/anti-CD19) unless reconstitution per protocol.
* Live/attenuated vaccine within 2 months.
* Pregnant or breastfeeding.
* Inadequate organ function/blood counts per protocol.