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.