Mass General - Division of Clinical Research

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.

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.