A Study of ATG-201 in Adult Participants With Autoimmune Diseases

Inclusion Criteria: 1. Participants who give consent to this study participation and sign informed consent form. 2. Participants at least 18 years of age inclusive at screening visit. 3. Must be able to adhere to the study visit schedule and other protocol .requirements. 4. Female of childbearing potential must agree to use adequate contraceptive measures from 28 days before starting study treatment to until 48 weeks after last dose of treatment, not breastfeeding, and have a negative serum pregnancy test at screening. 5. Male participants must agree to use an effective barrier method of contraception during sexual contact with a female of childbearing potential during screening and throughout the study until 48 weeks after the final dose of study treatment. Exclusion Criteria: 1. Intolerance or allergy to the investigational product, or to any component thereof, or to any concomitant medications that may be used in the study (e.g., tocilizumab). 2. Presence of central nervous system disorders of either autoimmune or non-autoimmune etiology, including but not limited to: epilepsy, psychotic disorders, organic brain syndrome, cerebrovascular accident, encephalitis, or central nervous system vasculitis. 3. Catastrophic or severe antiphospholipid syndrome within 12 months prior to screening or during screening. 4. Severe (requiring oxygen support) or rapidly progressing interstitial lung disease. 5. Uncontrolled other diseases: history or clinical evidence of active significant acute or chronic diseases (i.e., cardiovascular, pulmonary, untreated hypertension, anemia, gastrointestinal, hepatic, renal, neurological, cancer, or infectious diseases) which, in the opinion of the investigator, could confound the results of the study or put the participant at undue risk. 6. History of a major organ transplant (e.g. heart, lung, renal, liver) or hematopoietic stem cell/marrow transplant. 7. Have a planned surgical procedure, or a history of any other medical disease, or laboratory abnormalities, or conditions which would make the participant (in the opinion of the investigator) unsuitable for the study. 8. Received intravenous immunoglobulin, plasmapheresis, hemodialysis, intravenous cyclophosphamide, or high dose prednisone and its equivalents (\>60 mg/day) within 2 months prior to screening. 9. Live or attenuated vaccine within 28 days prior to screening or during screening. 10. Participants who have evidence of serious suicide risk including any history of suicidal behavior in the last 6 months or who, in the investigator's opinion, pose a significant suicide risk. 11. The participant has an abnormality on 12-lead electrocardiogram at screening which is clinically significant in the opinion of the investigator, and which could confound the results of the study.