Abstract Text: Human immune system (HIS) mice generated using human CD34+ stem cells, serve as a pivotal experimental model for the in vivo evaluation of immunotherapies. Yet, these mice have posse certain limitations including small size. In contrast, rats, due to their size and comprehensive immune system, hold promise for experiments that require assessment of a complete immune response. While immunodeficient rats have been produced, humanizing them with CD34+ hematopoietic stem cells even when expressing human SIRPalpha remains inefficient and suboptimal. In this study, we introduce an efficacious method for long-term immune humanization, achieved through intrahepatic injection of human CD34+ cells in irradiated newborn immunodeficient (Rag1 and Il2rg deficient) rats expressing human SIRP. By 12 weeks post-transplantation, significant immune reconstitution was evident in the blood, further increasing over 18 and 24 weeks. Importantly, and in contrast to HIS mice, the proportion of human immune mononuclear cell subsets mirrored that of human blood: predominantly T cells (52-98%), followed by B (5-25%), NK (~3%) and monocytes (~2%). These included diverse T cells subsets (naïve, memory, Treg), as well as a normal range of B cell subsets. At sacrifice, immune humanization was also high in spleen, thymus, bone marrow and lymph nodes. In conclusion, we describe for the first time a method to efficiently generate HIS rats. Furthermore, HIS rats show the development of a human immune system that more closely resembles humans compared to HIS mice. HIS rats have the potential to be a useful model for translational immunology.
