239. Induction of Allograft Immune Tolerance with Bioengineered Thymus Organoids

One of the major obstacles in solid organ transplantation is to establish immune tolerance of allografts. While immunosuppressive drugs can prevent graft rejection to a certain degree, their efficacies are limited, impermanent, and associated with severe side effects. As a primary immune organ essential to adaptive immunity, the thymus continuously generates a diverse population of naive T-cells, which can effectively react to invading pathogens, but remains unresponsive to self-antigens. Many factors (e.g. infection, irradiation, drug treatments, or aging) can irreversibly compromise thymic function, resulting in immune deficiency, cancer, autoimmunity and other immunodysregulatory diseases. While numerous efforts have been made to modulate/rejuvenate thymic function, manipulating the thymus, either in vitro or in vivo, proves to be difficult. The major challenge is to reproduce its unique extracellular matrix microenvironment that is critical for the survival and function of thymic epithelial cells (TECs). TECs are the predominant population of thymic stromal cells that are essential for T-cell lineage determination and maturation. TECs cultured in the traditional 2-D culture rapidly lose their molecular properties and fail to grow, which prohibits them from undergoing genetic modification. We have recently developed a thymus decellularization technique, which allows us to reconstruct a functional thymus organoid de novo with isolated TECs. The 3-D thymic scaffolds can support the survival of TECs in vitro, and maintain their unique molecular properties. When transplanted into athymic nude mice, the bioengineered thymus organoids could effectively promote the homing of lymphocyte progenitors and support the development of a diverse, self-tolerant T-cell repertoire. The thymus-reconstructed nude mice could promptly reject skin allografts, and were able to mount antigen-specific humoral responses against ovalbumin upon immunization. Notably, tolerance to allogeneic skin grafts could be achieved by transplanting thymus organoids constructed with either TECs co-expressing both syngeneic and allogenic MHCs, or mixtures of donor and recipient TECs. Our results demonstrate the technical feasibility of inducing donor-specific allogeneic tolerance with bioengineered thymus organoids and highlight the clinical implications of this thymus reconstruction technique in solid organ transplantation and regenerative medicine.