Project Summary With increased diagnostic precision and expanded treatment options, the health and lifespan of children born with chronic diseases has extended significantly into adulthood, drawing attention to preserving their quality of life and reproductive options. Unfortunately, the pathology of many chronic conditions can have negative consequences for the gonad, resulting in infertility and/or premature menopause. Cryopreservation (and later auto-transplantation) of whole ovarian tissue could provide a means of “safe- guarding” ovarian reserve from the gonadotoxic effects of disease pathology, and this approach has been utilized in cancer patients to yield at least 100 live births since early attempts more than a decade ago. Although this approach is effective, the viability and productivity of cryopreserved ovarian tissue is drastically undermined by the inflammatory, ischemic environment within the graft during the acute phase post-transplant. This proposal leverages the potent bioactivity and straightforward delivery of modified mRNA (modRNA) as a therapeutic approach to improving the viability and output of ovarian tissue autografts. Using a xenograft platform developed by our group, we will investigate the potential for modRNA to: 1) improve viability and output of ovarian tissue transplanted for the purpose of fertility preservation; 2) maintain a robust pool of ovarian follicles in a quiescent primordial state that will serve as a long-term reservoir for normalization of endocrine function and oocyte output; and 3) demonstrate the efficacy of this approach in a highly relevant pre-clinical animal model, rhesus macaque. With these experiments we will establish a foundation for robust, repeatable and FDA-approvable methods for ovarian tissue cryopreservation/auto-transplantation in chronically diseased girls that wish to maintain their reproductive options.