Safety and Efficacy of Abatacept in IgG4-Related Disease

Inclusion Criteria: 1. Are male or female 18 years of age or older 2. Meet the American College of Rheumatology (ACR)/EULAR 2018 Classification Criteria for IgG4-RD 3. Have active disease based on an IgG4-RD Responder Index (RI) ≥2 at screening with disease manifestation in at least one organ system excluding lymph nodes at screening 4. May or may not have received prior IgG4-RD therapy 5. Must be willing to taper off any systemic corticosteroid therapy within 4 weeks of first dose of trial drug. 6. Must be able and willing to discontinue any immunosuppressive agent at screening (e.g. methotrexate, mycophenolate mofetil, 6-mercaptopurine, tacrolimus, cyclophosphamide or azathioprine). 7. No history of severe allergic reactions to monoclonal antibodies. 8. Are able and willing to complete the entire study according to the study schedule. 9. Are willing to forego other forms of experimental treatment during the study. 10. Are able to provide written informed consent. Exclusion Criteria: 1. History or evidence of a clinically unstable/uncontrolled disorder, condition or disease (including but not limited to cardiopulmonary, oncologic, renal, hepatic, metabolic, hematologic or psychiatric) other than IgG4-RD that, in the opinion of the Investigator, would pose a risk to patient safety or interfere with the study evaluation, procedures or completion. 2. Malignancy within 5 years (except successfully treated in situ cervical cancer, resected squamous cell or basal cell carcinoma of the skin, or prostate cancer with no recurrence ≥3 years following prostatectomy). 3. Liver disease: Acute or chronic non-IgG4-related liver disease deemed sufficiently severe to impair their ability to participate in the trial. 4. Uncontrolled disease: evidence of another uncontrolled condition, including drug and alcohol abuse, which could interfere with participation in the trial according to the protocol. 5. Presence of recurrent or chronic infections, defined as ≥3 infections requiring antimicrobials over the past 6 months prior to screening. 6. Active infection requiring hospitalization or treatment with parenteral antimicrobials within the 30 days prior to randomization. 7. Prior use of rituximab (or other B cell depleting agents) within 6 months of enrollment unless B cells have been demonstrated to have repopulated. 8. Use of any investigational agent within 5 half-lives of the agent (or 6 months if the half-life is unknown) prior to enrollment. 9. White blood cell count \< 2.5 x 103/µL. 10. Absolute neutrophil count (ANC) \< 1.0 x 103/µL. 11. IgG4-related renal disease with serum creatinine \>2.0 mg/dL. 12. Hemoglobin \< 10 g/dL. 13. Platelet count \< 75 x 109/L. 14. Known positive result for HIV I or II antibody, hepatitis B surface antigen, hepatitis B core antibody or hepatitis C antibody. 15. Has received live vaccines within 4 weeks of enrollment. 16. Inability to communicate reliably with the investigator. 17. Patient is pregnant or breast feeding, or planning to become pregnant while enrolled in the study, up to end of study (EOS) visit. 18. Positive pregnancy test at screening or during the study. 19. Subjects who do not agree to use medically acceptable methods of contraception. 20. Male patient with a pregnant partner who is not willing to use a condom during the treatment and up to end of study (EOS)visit. 21. Known or suspected sensitivity to mammalian cell-derived products or any components of the study drug. 22. History of alcohol and/or substance abuse within 12 months prior to screening. 23. Unable or unwilling to partake in follow-up assessments or required protocol procedures.