The purpose of this study is to find out what dose of CABA-201 ("study drug") can be safely administered to patients with Systemic Lupus Erythematosus and Lupus Nephritis. CABA-201 is a CAR T cell therapy drug, which uses your own T cells (immune cells), and modifies them in order to find and remove other immune cells (B cells) that cause this disease.

The cells will be collected through a process called leukapheresis, then modified in a lab. This modified version will be infused into the body.

The purpose of this study is to test an experimental approach called cell therapy for Systemic Lupus Erythematosus (SLE).CAR T-cells have been used to treat some cancers, but these cells, known as CART19 cells, are an experimental treatment for lupus. The CAR T-cells are made by collecting T-cells from patients and changing them to find and remove B-cells, including ones that are involved in causing lupus. Participants will be given CART19 cells through an IV and then monitored for up to 24 months.

Participants will also be asked to participate in a separate, long term follow-up study to monitor their health for up to 15 years after their last CART19 treatment.