Autologous Thymic Organoids Support Functional T-cell Education and Enhance Antitumor Immunity in Humanized Mice with Melanoma Xenografts.

A fundamental limitation of HLA-A matched humanized mice (mHM), generated with donor hematopoietic stem and progenitor cells and bearing patient-derived xenografts, is that the fidelity of the interaction between the immune system and implanted tumors is limited by lack of thymic T-cell education. We created a model with patient-matched thymic and tumor tissue by using patient peripheral blood mononuclear cells reprogrammed into induced pluripotent stem cells to generate stem cell-derived thymic organoids (sTOs). When implanted in thymectomized mHM, which were subsequently engrafted with patient melanoma tissue (autologous, or mHMTA), sTOs expressed thymic tissue markers EPCAM, KRT5, and KRT8 and contained developing double-positive T cells. mHMTA had more activated HLA-DR+ T cells in their spleens. Tumor growth in mHMTA was significantly slower compared with mHM, and both tumor volume and viable melanoma content were significantly decreased in mHMTA. This was associated with an increase of intratumoral activated T cells in mHMTA. Whole-exome sequencing followed by intratumoral heterogeneity analysis identified seven candidate neoantigens that were selected against in the mHMTA model, suggesting more effective identification and T cell-driven tumor clearance. This study shows the feasibility of generating patient-specific, thymus-bearing mice and provides a promising new tool in immunology and immunotherapy development. SIGNIFICANCE: A humanized mouse xenograft model, in which T cells are educated in a patient-derived implanted thymic organoid autologous to the tumor, provides a more faithful and complete environment to study cancer immunity.