A Study to Evaluate the Safety and Preliminary Efficacy of ATA3219 in Participants With Systemic Lupus Erythematosus

This is a Phase 1, multi-centered, open-labeled, dose escalation study to evaluate the safety and preliminary efficacy of ATA3219 (as monotherapy) in participants with LN following LD (Cohort LN) and in participants with extrarenal SLE (ERL) without LD (Cohort ERL). This study is planned to be conducted in the United States, Canada, and Australia. For each cohort, up to 3 dose levels (DLs) will be explored in the dose escalation portion of the study and if needed a lower dose may be explored. Prior to undergoing any screening procedure, prospective participants must undergo the ATA3219 inventory check assessments to ensure availability of an appropriate partially human leukocyte antigen (HLA)-matched ATA3219 product lot. Before administration of ATA3219, participants will receive LD treatment (Cohort LN) or methylprednisolone treatment (Cohort ERL). For all enrolled participants, hospitalization during and following ATA3219 dosing is mandatory. Participants will receive a single dose intravenous (IV) infusion of ATA3219 (monotherapy) on Day 1. Participants will remain inpatient for a minimum of 1 week post ATA3219 dosing, where they will be frequently monitored. Clinical responses will be assessed by the investigator on Day 28 (+ 5 days) following each dose of ATA3219. For each cohort, during dose escalation, up to 3 DLs of ATA3219 are planned to be evaluated sequentially and a lower dose may be added. At least 3 and up to 6 dose-limiting toxicity (DLT)-evaluable participants, those who complete the 28-day DLT observation period, will be assessed at each DL. Within each DL, treatment will be staggered to allow appropriate safety monitoring by an independent Data Safety Monitoring Committee (DSMC). Enrolled participants who do not receive ATA3219 for any cause (e.g., rapid deterioration) will be replaced. Participants who experience an adverse event (AE) during the 28-day DLT observation and complete the observation period will not be replaced. In addition, if a participant is treated with ATA3219 and discontinues for any reason other than due to a DLT prior to completing the 28-day DLT observation period, an additional participant may be enrolled at the dose level to ensure that the recommended phase 2 dose (RP2D) can be determined, with a goal not to exceed 6 participants treated/dose level. In rare situations, retreatment of participants with inadequate renal response may be considered. After treatment is completed or discontinued, participants will be followed for safety and clinical response for up to 24 months from the last dose of ATA3219. A separate long-term follow-up study will be conducted to follow participants for up to a total of 15 years after their last dose of ATA3219.