Tumors evolve to avoid immune destruction and establish an immunosuppressive microenvironment. Syngeneic mouse tumor models are critical for understanding tumor immune evasion and testing cancer immunotherapy. Derived from established mouse tumor cell lines that can already evade the immune system, these models cannot simulate early phases of immunoediting during initial tumorigenesis. We developed a syngeneic mouse teratoma model derived from noncancerous mouse embryonic stem cells and conducted a genome-wide CRISPR screen to identify genes that impact early phases of cancer immunoediting. We found that loss of pro-apoptotic tumor suppressor genes, including Trp53, increased necrosis in teratomas, releasing APOE lipid particles into the extracellular milieu. Infiltrating T cells drawn to tumor necrotic regions accumulated lipids and became dysfunctional. Blocking lipid uptake in T cells or reducing necrosis in teratomas by inactivating the mitochondrial permeability transition pore (mPTP) restored immunosurveillance. Because mouse teratomas were highly enriched for brain tissues, we next examined the tumor-immune interaction in human glioblastoma (GBM). Indeed, infiltrating T cells in TP53-mutated human GBM accumulated APOE and were dysfunctional. Anti-APOE and anti-PDCD1 antibodies synergistically boosted anti-GBM immunity and prolonged survival in mice. Our results link mPTP-mediated tumor necrosis to immune evasion and suggest that targeting the uptake of lipids released by necrotic tumor cells by infiltrating immune cells can enhance cancer immunotherapy.