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.