Engineering Poly(ethylene glycol) Hydrogels to Facilitate Ovarian Tissue Transplantation

Abstract

Primary ovarian insufficiency (POI), the premature loss of ovarian function, is a prevalent side effect of common life-saving anti-cancer therapies. POI patients face infertility and insufficient production of ovarian hormones. The premature loss of ovarian hormones places patients at increased risk for a myriad of comorbidities, which, alongside the psychosocial impact of infertility, can significantly impact their quality of life. As cancer survivorship continues to increase, developing treatments that minimize the long-term impact of side effects like POI becomes increasingly important.

Currently, separate treatments are required to restore fertility and ovarian hormones. Hormone restoration is achieved through hormone replacement therapy (HRT), and infertility is addressed using assistive reproductive technologies like ovarian stimulation followed by cryopreservation of the retrieved eggs or fertilized embryos. Although effective, these approaches have key limitations. HRT delivers only a fraction of the hormones produced by the native ovary and does not recapitulate the cyclic hormone production seen in vivo. Oocyte retrieval is unfeasible for prepubertal patients as they cannot produce mature oocytes even after ovarian stimulation. This dissertation explores approaches to employ ovarian tissue transplantation (OTT) as an alternative POI treatment capable of restoring fertility and dynamically delivering the complete milieu of ovarian hormones.

Patients unable to preserve their ovarian tissue before gonadotoxic treatments would rely on allogenic OTT. This approach is still experimental and lacks exploration of the factors governing endocrine function and immune rejection of ovarian allografts. We investigated the impact of donor age on ovarian allograft function and survival by implanting allogeneic ovarian tissue from pre- and peripubertal mice into ovariectomized recipients. We observed a strong correlation between restoration of ovarian endocrine function and the size of the implanted follicular reserve, consistent with clinical observations. Allospecific antibody production was first observed 10-12 days after transplantation, consistent with the timeline of graft revascularization. Peripubertal transplants elicited higher avidity antibodies than prepubertal, suggesting aging ovaries may be more immunogenic. This work deepens our understanding of the relationship between donor age and allogenic OTT outcomes and can help inform allowable criteria for future ovarian tissue donors.

Ovarian tissue cryopreservation and autotransplantation (OTCT) is the most established mode of OTT and only biological fertility preservation option for prepubertal cancer patients with ovaries. Unfortunately, this approach remains contraindicated for up to 40% of prepubertal cancer patients due to the risk of transplanting malignant cells. Transplantation of isolated follicles may significantly reduce the risk of cancer recurrence, while restoring complete ovarian function. However, this approach is challenging because isolated follicles require a supportive scaffold to survive and develop. We report a synthetic, proteolytically degradable poly(ethylene glycol) (PEG) hydrogel that supports the development and hormone production of isolated, human ovarian follicles in vivo. Early-stage, human ovarian follicles encapsulated in biomimetic PEG hydrogels developed to preantral stages over 20 weeks in vivo. Developing follicles produced estradiol and restored estrous activity in ovariectomized mice. This work is the first to leverage a fully synthetic scaffold to support isolated human ovarian follicles and longitudinally monitor restoration of ovarian endocrine function in a mouse model.

This work further substantiates OTT as a novel treatment to restore ovarian endocrine function and fertility. Our results corroborate the correlation between ovarian reserve and graft function, broaden our understanding of ovarian allograft rejection, and demonstrate folliculogenesis and hormone production from isolated human ovarian follicles in a synthetic matrix.