Summary Cancer immunotherapy restores and/or enhances effector function of CD8+ T cells in the tumor microenvironment1,2. CD8+ T cells activated by cancer immunotherapy execute tumor clearance mainly by inducing cell death through perforin-granzyme- and Fas/Fas ligand-pathways3,4. Ferroptosis is a form of cell death that differs from apoptosis and results from iron-dependent lipid peroxide accumulation5,6. Although it was mechanistically illuminated in vitro7,8, emerging evidence has shown that ferroptosis may be implicated in a variety of pathological scenarios9,10. However, the involvement of ferroptosis in T cell immunity and cancer immunotherapy is unknown. Here, we find that immunotherapy-activated CD8+ T cells enhance ferroptosis-specific lipid peroxidation in tumor cells, and in turn, increased ferroptosis contributes to the anti-tumor efficacy of immunotherapy. Mechanistically, interferon gamma (IFNγ) released from CD8+ T cells downregulates expression of SLC3A2 and SLC7A11, two subunits of glutamate-cystine antiporter system xc-, restrains tumor cell cystine uptake, and as a consequence, promotes tumor cell lipid peroxidation and ferroptosis. In preclinical models, depletion of cyst(e)ine by cyst(e)inase in combination with checkpoint blockade synergistically enhances T cell-mediated anti-tumor immunity and induces tumor cell ferroptosis. Expression of system xc- is negatively associated with CD8+ T cell signature, IFNγ expression, and cancer patient outcome. Transcriptome analyses before and during nivolumab therapy reveal that clinical benefits correlate with reduced expression of SLC3A2 and increased IFNγ and CD8. Thus, T cell-promoted tumor ferroptosis is a novel anti-tumor mechanism. Targeting tumor ferroptosis pathway constitutes a therapeutic approach in combination with checkpoint blockade.

A challenge in oncology is to rationally and effectively integrate immunotherapy with traditional modalities including radiotherapy. Here, we demonstrate that radiotherapy induces tumor cell ferroptosis. Ferroptosis agonists augment and ferroptosis antagonists limit radiotherapy efficacy in tumor models. Immunotherapy sensitizes tumors to radiotherapy by promoting tumor cell ferroptosis. Mechanistically, IFNγ derived from immunotherapy-activated CD8+ T cells and radiotherapy-activated ATM independently, yet synergistically repress SLC7A11, a unit of the glutamate-cystine antiporter xc−, resulting in reduced cystine uptake, enhanced tumor lipid oxidation and ferroptosis, and improved tumor control. Thus, ferroptosis is an unappreciated mechanism and focus for the development of effective combinatorial cancer therapy.

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