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 (TBYS) 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⁺ TBYS cells comprise a heterogeneous population of TCM, TEM, and TRM subsets defined by distinct chromatin accessibility and transcriptional programs. These subpopulations follow a progressive TCM→TEM→TRM differentiation trajectory during tumor progression, with TRM cells exhibiting superior tissue retention and ultimately dominating the TBYS pool. We further identify TGF-β-derived from regulatory CD4⁺ T cells as the central instructor of this hierarchical differentiation, which promotes TBYS cell accumulation through suppression of KLF2. Our study elucidates a key mechanism of TBYS cell differentiation and maintenance, providing a foundation for the improved immunotherapies targeting this population.