Steroid-sparing immunosuppression for MHC-mismatched allogeneic cardiomyocyte cell therapy in rhesus macaque

Background: Cardiac cell therapy with human pluripotent stem cell (hPSC)-derived cardiomyocytes (CMs) is an emerging strategy to treat myocardial infarction (MI), but long-term immunogenicity of the allogeneic transplantation is unknown. Seeking to achieve long-term engraftment without harsh immunosuppression, we transplanted allogenic CMs differentiated from rhesus macaque induced PSCs (iPSCs) into major histocompatibility complex (MHC)-mismatched rhesus recipients. Methods: A survey was conducted to assess cardiologists’ perceptions about hPSC-CM therapy and immunosuppression. Allogeneic iPSCs from a male rhesus macaque ( Macaca mulatta ) were differentiated into highly enriched cardiomyocytes (riPSC-CMs). 200×10 ⁶ riPSC-CMs were transplanted into 16 MHC-mismatched rhesus recipients with or without MI. Immune modulating interventions included genetically deleting β2-microglobulin (β2M) to create MHC class I-deficient allogeneic CMs, as well as testing four steroid-sparing immunosuppression regimens to determine the minimal immunosuppression required to support long-term engraftment. Results: The majority of cardiologists are likely to offer hPSC-CM therapy to their post-MI patients but only if no or weak chronic immunosuppression is required (Fig. 1). In the absence of immunosuppression, MHC-mismatched riPSC-CM allografts survived for two weeks without significant rejection but were fulminantly rejected by eight weeks (Fig. 2). β2M knockout and resulting MHC I-deficiency did not prevent rejection. Single-agent immunosuppression with abatacept (ABT) or tacrolimus (TAC) led to severe rejection. Mycophenolate mofetil (MMF) combined with TAC also resulted in severe rejection. In contrast, a combination of TAC + ABT supported four of five transplanted grafts for 16 weeks without significant cellular rejection, with a fifth heart showing focal moderate rejection (Fig. 3). Conclusions: A minimal chronic immunosuppression regimen is necessary to translate hPSC-CM therapy. Mismatched allogeneic cardiomyocyte cell therapy is less immunogenic than orthotopic heart transplantation, and graft rejection can be prevented using low intensity immunosuppression with TAC + ABT. Immune-engineering to prevent immunoreactivity will require more than MHC I deletion alone.