Engineering Treg-mediated immune tolerance via foxp3a overexpression to evade allograft transplantation barriers in zebrafish

In mammals, regulatory T cells (Treg) cells have been utilized to enhance tolerance in organ transplantation. Although transplanting germline stem cells (GSCs) or gonadal primordia into immunodeficient zebrafish have shown to be an important technique which can bypass the juvenile phase and expedite gamete generation, it remains challenging to raise the immunodeficient fish. Here, we achieved in vivo induction of Tregs by overexpressing the key transcription factor Foxp3a, generating transgenic zebrafish with robust immune tolerance. This was characterized by a significant downregulation of T cell development and homing-related genes, accompanied by a marked upregulation of immunosuppressive factors. Using these immune-tolerant fish as hosts for subcutaneous gonadal primordium transplantation (SGPT) and intraperitoneal GSC transplantation (IGCT), we markedly accelerated germ cell maturation and efficiently established stable transgenic lines. Transcriptomic analyses demonstrated that transgenic hosts demonstrated a phenotypic profile characterized by delayed immune activation, attenuated responsiveness, and enhanced graft survival. Thus, we present a Treg-induction-based approach in fish that resolves the intrinsic conflict between high embryonic transgenesis efficiency and early lethality, while significantly improving overall transgenesis success rates. Moreover, it offers a practical alternative to immunodeficient recipients, avoiding the challenges of complex husbandry and compromised fertility.