Regulatory T cell-derived TGF-β signaling governs the differentiation and maintenance of tumor-infiltrating bystander CD8+ T cells

While indispensable for antitumor immunity, tumor-specific CD8⁺ T cells are numerically scarce and functionally exhausted in the tumor microenvironment (TME). In contrast, bystander memory CD8⁺ T (T_BYS) cells that recognize pathogen-derived antigens but not tumor antigens are abundant in tumors and maintain polyfunctional effector capacity, yet their differentiation and maintenance mechanisms remain unclear. Here, we demonstrate that CD8⁺ T_BYS cells comprise a heterogeneous population of T_CM, T_EM, and T_RM subsets defined by distinct chromatin accessibility and transcriptional programs. These subpopulations follow a progressive T_CM→T_EM→T_RM differentiation trajectory during tumor progression, with T_RM cells exhibiting superior tissue retention and ultimately dominating the T_BYS pool. We further identify TGF-β-derived from regulatory CD4⁺ T cells as the central instructor of this hierarchical differentiation, which promotes T_BYS cell accumulation through suppression of KLF2. Our study elucidates a key mechanism of T_BYS cell differentiation and maintenance, providing a foundation for the improved immunotherapies targeting this population.