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Tumor-infiltrating lymphocyte (TIL) therapy is FDA-approved for patients with treatment-resistant advanced melanoma, but the TIL subpopulations critical for tumor eradication remains incompletely understood. Using patient-derived TIL-melanoma co-cultures, we identified and characterized a novel subset of CD8 TIL, capable of class I HLA-independent cancer cell lysis. The lymphotoxin β receptor (LTβR) and interferon (IFN) sensing pathways were nominated as key determinants of TIL-mediated cancer cell killing from a whole-genome, loss-of-function CRISPR screen. Validation studies confirmed that dual LTβR and IFN sensing is necessary and sufficient for cancer cell lysis, and that expanded CD8 TIL express high lymphotoxin β () and upregulate lymphotoxin α () upon coculture with cancer cells. Leveraging paired scRNA-seq and scTCR-seq data, we confirmed that enrichment of T cells is associated with clinical response to TIL, and that TIL are expanded from putative neoantigen-reactive, CD8 T cells in resected tumors.