Phase 2 Study of Rapcabtagene Autoleucel in Myositis

This is a Phase 2, randomized, active-controlled study. This study comprises two cohorts: * A lead-in cohort enrolling participants to receive rapcabtagene autoleucel * A randomized cohort with participants receiving either rapcabtagene autoleucel or a comparator option. Participants in the comparator arm whose signs and symptoms are not fully controlled may receive rapcabtagene autoleucel treatment once the participant is confirmed to be eligible After end of study (EOS), participants who received rapcabtagene autoleucel infusion will enter a long-term follow-up (LTFU) period after rapcabtagene autoleucel infusion. This LTFU will be described in a separate study protocol.