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 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, one immunosuppressive agent and one biologic 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.