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Tolerance, function and homeostasis of human T cells, including Tregs, developing in pig thymus xenografts

Hannes Kalscheuer, Takashi Onoe, Hiroyuki Tahara, Yong-Guang Yang, Megan Sykes

Columbia Center for Translational Immunology, Department of Medicine, Surgery and Microbiology/Immunology, College of Physicians and Surgeons, Columbia University, New York, NY, United States

Xenogeneic thymus transplantation allows tolerance induction across highly disparate xenogeneic barriers in pig-to-mouse as well as pig-to-baboon models. Previously, we have used a humanized mouse model to demonstrate that porcine thymic tissue supports the development of a diverse repertoire of human T cells which are specifically unresponsive to the donor pig. We modified the humanized (HU) mouse model and achieved sustained, high levels of all critical components of the human immune system in immunodeficient mice. Using this model, we have now assessed regulatory T cell development, phenotype and function as well as T cell antigen-specific responsiveness and homeostasis. We show that human T cells generated in porcine thymus grafts are functional, self-tolerant (to the human hematopoietic stem cell donor) and specifically tolerant towards the porcine donor and the murine recipient. Additionally, we show that regulatory T cells (Treg) develop normally in the porcine thymus and that they participate in the SLA-specific nonresponsiveness. Both “naïve” and “memory”-type conventional T cells and Tregs are detected in the periphery of HU mice generated with porcine or human thymic grafts. However, rapid lymphopenia-driven expansion and effector/memory cell differentiation upon transfer to mice with autologous human APCs but lacking T cells is diminished for T cells generated in porcine compared to human thymus grafts. In pig compared to human thymus-grafted HU mice, the level of Treg activation/memory phenotypic conversion is diminished in proportion to a modest diminution in suppressive function in some, but not all experiments. Finally, in vitro responsiveness to tetanus toxoid following in vivo immunization is diminished in HU mice with porcine compared to human thymus grafts. We are exploring a novel strategy to facilitate autologous human MHC contributions to thymocyte selection in porcine thymus grafts to enhance immune function following thymic transplantation for xenograft tolerance induction.