Abstract C039: Role of recent thymic emigrants in antitumor responses

Abstract T cells often fail to sustain effector activity within the tumor microenvironment (TME), limiting immunotherapy efficacy. Declining thymic output correlates with cancer risk and mortality. Yet the role of recently generated T cells—recent thymic emigrants (RTEs)—in tumor control remains unclear. We hypothesize that increasing RTE generation and function can enhance antitumor immunity. To track RTEs long term, we created a tamoxifen(TMX)-inducible RAG1(Cre-ERT-RFP) CAG(YFP) mouse (RAG1-YFP). RAG1 expression in developing thymocytes enables transient RFP labeling, while TMX-induced Cre activation drives stable YFP expression. Using this novel model, we profiled RTEs across 23 lymphoid and non-lymphoid tissues at steady state, revealing distinct tissue-specific RTE phenotypes. Across solid tumor models, including B16-F10 melanoma and MC38 colorectal cancer, we identified a phenotypically distinct population of tumor-infiltrating RTEs (TI-RTEs). In B16-F10 tumors, TI-RTEs exhibited lower exhaustion markers PD-1 and CTLA-4 expression than non-RTE TILs. CD8+ TI-RTEs showed increased Granzyme B and IFN-γ production, while CD4+ TI-RTEs skewed toward Th1/Th17 differentiation. Single-cell RNA sequencing revealed elevated glycolytic gene expression (Gapdh, Pkm, Ldha) in TI-RTEs. Using in vivo live imaging, we visualized RTE trafficking from the vasculature into tumors, and immunohistochemistry confirmed robust TI-RTE infiltration. To define RTE function in situ, we selectively depleted RTEs using diphtheria toxin–treated RAG1(Cre-ERT-RFP)CAG(YFP-iDTR) mice after tumor engraftment. RTE depletion accelerated tumor progression, demonstrating their requirement for antitumor activity. Because effective immune checkpoint inhibition (ICI) therapy depends on stem-like, less-exhausted T cells, we next tested the impact of RTEs on dual ICI( anti–PD-1 and anti–CTLA-4). Selective RTE depletion significantly reduced ICI efficacy in B16-F10 tumors, indicating that RTEs contribute to ICI responses. To further evaluate RTE function in TME, we tested exogenous interleukin-7 (IL-7), which enhances RTE output. IL-7–pretreated RAG1-YFP mice bearing B16 tumors showed significantly reduced tumor progression compared to untreated controls, indicating that boosting RTE outputs improves tumor regression. Together, these findings identify RTEs as a mechanistically distinct, therapy-responsive T-cell population essential for effective antitumor immunity. IL-7–mediated enhancement of thymic output and RTE-dependent modulation of ICI efficacy elaborate thymic regeneration and RTE-targeted strategies as potential approaches to improve immunotherapy in solid tumors. Citation Format: Mina Yousefelahiyeh, Nastaran Abbasizadeh. Role of recent thymic emigrants in antitumor responses [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C039.