Neonatal and pediatric thymic grafts generate similar human T-cell chimerism in humanized mice

Human immune system (HIS) humanized mice, particularly those incorporating human thymic tissue and hematopoietic stem/progenitor cell (HSPC) co-transplantation, provide a powerful in vivo platform to study human T-cell development, tolerance, and effector function in the context of a full human immune system. The thymus is a lymphoid organ that is indispensable for generating a diverse and self-tolerant T-cell repertoire, and its inclusion in HIS models is essential for generating human major histocompatibility complex-restricted T cells. The bone marrow-liver-thymus (BLT) HIS model is made using human fetal tissues of a very narrow gestational age, typically 17-22 weeks. We previously developed the NeoThy™ humanized immune system mouse model, which is created using cord blood HSPCs and donated thymus tissue from neonatal and pediatric donors, as a non-fetal alternative to the BLT model. While the BLT model uses tissue of relatively consistent gestational age, further study is needed to assess the impact of the thymic donor age in the NeoThy model, since thymic tissue donors can range from a few days old to many years old. Here, we created multiple cohorts of NeoThy mice using thymic tissue from donors of various ages and compared them to each other and to cohorts of BLT mice. In the NeoThy mouse cohorts, we confirmed de novo thymopoiesis within transplanted human thymic grafts. Across multiple independent experiments encompassing both neonatal and pediatric thymic donors, we observed similar T-cell chimerism and function, regardless of thymic donor age. Inter-donor variability, an inherent feature of all HIS mouse models, accounted for observed differences in immune engraftment kinetics. T cells developing in the presence of a transplanted thymus exhibited gene expression signatures consistent with more mature functional responses to mRNA vaccine challenge than those observed in mice receiving cord blood only. These findings highlight the importance of including thymic transplantation in this context and confirm that both neonatal and pediatric thymus donor tissue are capable of generating robust chimeric immune systems in HIS mice.