Abstract Body: Introduction
Accumulating evidence suggests that the transplantation of pluripotent stem cell-derived cardiomyocytes (PSC-CMs) regenerates injured hearts in animal studies. Currently, several clinical studies are underway, transplanting allogeneic human PSC-CMs. In these clinical studies, patients have been treated with the same combination of immunosuppressants used after heart transplantation, including steroids, calcineurin inhibitors (CNIs), and mycophenolate mofetil (MMF). However, the optimal treatment following PSC-CM transplantation has yet to be established.
The purpose of this study is to determine the optimal treatment for grafted PSC-CM survival by regulating the immune response using an allogeneic non-human primate transplantation model.

Methods and Results
We prepared 9 male cynomolgus monkeys as recipient animals. Donor cardiomyocytes were generated from induced pluripotent stem (iPS) cells derived from another monkey, with MHC class I and class II types differing from those in the recipients. The recipient animals underwent ischemia/reperfusion injury of the left anterior descending artery two weeks before iPSC-CMs transplantation. Initially, animals were treated with the same combination of immunosuppressants (steroid/CNIs/MMF: N=3), and grafted PSC-CMs survived without apparent immune rejection. Subsequently, we conducted experiments aiming to reduce immunosuppressants. The animals were treated with the same combination, but steroids were terminated at 4 weeks post-transplantation (N=4). Histological analysis at 8 or 12 weeks post-transplantation revealed that grafted cardiomyocytes were rejected, with a significant infiltration of CD3-positive lymphocytes. When recipients were treated with another combination of immunosuppressants (steroid/abatacept/MMF), and steroids were terminated at 4 weeks post-transplantation, grafted cardiomyocytes survived without apparent infiltration of CD3-positive lymphocytes at 8 weeks post-transplantation (N=2).

Conclusion
A different combination of immunosuppressants from heart transplantation allows transplanted allogeneic PSC-CMs to survive without the long-term use of steroids.