The epitranscriptional factor PCIF1 orchestrates CD8 T cell ferroptosis and activation to control antitumor immunity.

T cell-based immunotherapies have revolutionized cancer treatment, yet durable responses remain elusive. Here we show that PCIF1, an RNA N 2'-O-dimethyladenosine (mA) methyltransferase, negatively regulates CD8 T cell antitumor responses. Whole-body or T cell-specific Pcif1 knockout (KO) reduced tumor growth in mice. Single-cell RNA sequencing shows an increase in the number of tumor-infiltrating cytotoxic CD8 T cells in Pcif1-deficient mice. Mechanistically, proteomic and mA-sequencing analyses pinpoint that Pcif1 KO elevates mA-modified targets, specifically ferroptosis suppressor genes (Fth1, Slc3a2), and the T cell activation gene Cd69, imparting resistance to ferroptosis and enhancing CD8 T cell activation. Of note, Pcif1-deficient mice had enhanced responses to anti-PD-1 immunotherapy, and Pcif1 KO chimeric antigen receptor T cells improved tumor control. Clinically, cancer patients with low PCIF1 expression in T cells have enhanced responses to immunotherapies. These findings suggest that PCIF1 suppresses CD8 T cell activation and targeting PCIF1 is a promising strategy to boost antitumor immunity.