This is a multicenter, phase 2 single-arm trial designed to evaluate the efficacy and safety of FT819 in participants with moderate-to-severe systemic lupus erythematosus (SLE) with Class III/IV lupus nephritis (LN) (with or without concomitant Class V involvement) refractory to at least 2 immunosuppressive therapies prior to trial intervention.

Participants will undergo a screening period of up to 28 days. Following screening, trial intervention will consist of bendamustine administration followed by a single dose of FT819. Efficacy, safety, and exploratory assessments will be conducted at predefined timepoints through Month 24 of post-treatment follow-up (PTFU). Following completion of these scheduled assessments, participants will continue in long-term follow-up (LTFU) for up to 15 years after FT819 administration to monitor ongoing safety and survival.

Efficacy and disease activity will be assessed using standard LN measures, including complete renal response (CRR) and PRR (partial renal response), as well as clinician-reported outcomes, such as the SLEDAI-2K, BILAG, and PGA, performed at specified timepoints.