This study will test the safety and effects of SBT777101 when given as a single dose to subjects with rheumatoid arthritis. It is the first study of this treatment being done in humans. Increasing dose levels will be given after the safety at lower dose levels is shown. Following completion of the initial single ascending dose cohorts, the study may evaluate alternative administration regimens of SBT777101, including split-dose administration. In the split-dose cohort, participants may receive two intravenous infusions of SBT777101 derived from a single manufacturing process, administered approximately 6 weeks apart. The study will continue to evaluate safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary clinical activity.

The study evaluates the safety and effects of a novel regulatory CARTreg cell-based autoimmune and inflammatory disease therapy for the treatment of rheumatoid arthritis. The therapy is an autologous (using the patient's own cells) Treg cell therapy that targets proteins in the inflamed, disease-associated tissue, with the aim to dampen inflammation and restore balance to the immune system.

Inclusion Criteria:

• Body mass index (BMI) <35 kg/m^2, inclusive
• Adult-onset, moderate-to-severe rheumatoid arthritis (RA)
• Moderate-to-severe active disease
• b/tsDMARD failure criterion (≥2)
• Swollen joint count ≥4
• Clinical and/or ultrasound evidence of synovitis
• Prior inadequate response to or unable to tolerate available RA therapies
• Stable doses of RA medications for at least 30 days
• Use of highly effective methods of contraception

Exclusion Criteria:

• Major surgery within 12 weeks prior to screening or planned within 12 months after dosing
• Uncontrolled cardiovascular, pulmonary, renal, hepatic, endocrine, or gastrointestinal disease
• Recurrent infections or active infection
• Active or untreated latent tuberculosis
• Primary or secondary immunodeficiency
• History of or current inflammatory joint disease other than RA