PCIF1 loss licenses antitumour immunity via cholesterol biosynthesis

Cholesterol biosynthesis is classically viewed as a tumour supportive metabolic programme, yet cholesterol-associated metabolic states can paradoxically coincide with improved responses to immune checkpoint blockade (ICB)1, highlighting an unresolved context-dependent relationship between cholesterol homeostasis and antitumour immunity. Here we identify PCIF1, a cap-specific RNA methyltransferase2, as a tumour-intrinsic brake on the inflammatory potential of cholesterol biosynthesis. PCIF1 depletion had limited effects on tumour cell proliferation in vitro, but suppressed tumour growth in immunocompetent, not immunodeficient, hosts and sensitized immune-refractory syngeneic tumours to ICB. Immune profiling by single-cell RNA sequencing, flow cytometry and multiplex immunofluorescence revealed increased CD8+ T cell infiltration in PCIF1-deficient tumours, and CD8+ T cell depletion restored tumour growth. Mechanistically, PCIF1 loss selectively enhanced translational engagement of SCAP, activated SREBP2-dependent cholesterol biosynthesis and promoted intracellular cholesterol accumulation. Unexpectedly, activation of this cholesterol programme induced a tumour cell inflammatory cytokine state enriched for T cell-recruiting chemokines, including CXCL10. Pharmacological perturbation of the SCAP-cholesterol axis attenuated this inflammatory programme, placing cholesterol sensing upstream of PCIF1 loss-induced tumour cell inflammatory activation. In ICB-treated patient cohorts, high PCIF1 expression in pre- and on-treatment samples was associated with poor clinical response, and across tumour types, PCIF1 expression inversely correlated with cytolytic immune signatures. These findings reveal PCIF1-mediated SCAP translational control as a tumour cell checkpoint that constrains the antitumour inflammatory activity of a canonical growth-associated cholesterol programme and shapes checkpoint therapy responsiveness.