Background We previously reported preliminary safety and efficacy results from a multi-center Phase 1 trial of CART-PSMA-TGF b RDN T-cells (TmPSMA-01; NCT04227275) in patients (pts) with metastatic castration resistant prostate cancer (mCRPC). Nine pts were dosed with TmPSMA-01 across three doses ranging from 1x10 7 -3x10 8 CAR+ cells. Evidence of clinical activity was observed, however, two pts developed severe immune-mediated toxicity and experienced Grade (Gr) 5 events. These pts had no known clinically relevant risk-fac-tors and demonstrated disparate pre- and post-infusion clinical courses. Translational research efforts to evaluate the mecha-nisms of immune-mediated toxicity to TmPSMA-01 were undertaken. Results are summarized herein. Methods Pt samples collected per clinical protocol were sub-jected to correlative analysis platforms. Longitudinal serum and peripheral blood specimens were evaluated for soluble factors by immunoassays and TmPSMA-01 kinetics by molecu-lar techniques, respectively. Biomarker comparisons between patients with Gr5 events and those without were made. As available, pt tissues (tumor and autopsy samples) were evaluated for TmPSMA-01 infiltration by RNA-ISH. Additionally, an in vitro model of immune toxicity was applied to evaluate TmPSMA-01 product potency and new CAR candidates. Results Serum cytokines all pts showed patterns consis-tent with an immune-effector response but pts with