SERT inhibits CD8 T cell antitumor immunity through regulating the intratumoral serotonin axis

Identifying new immune checkpoint molecules hindering antitumor T cell responses and devising new therapeutic blockades are key to the development of next-generation cancer immunotherapies. Here, we report the induction of the serotonin transporter (SERT) in solid tumor-infiltrating cytotoxic CD8 T cells, a molecule best known for its role in regulating serotonin neurotransmission in the brain. In solid tumors, CD8 T cells secrete serotonin, which stimulates their antitumor reactivities, while SERT functions as a negative regulator by depleting intratumoral serotonin. Inhibition of SERT using clinically approved selective serotonin reuptake inhibitors (SSRIs), a popular class of antidepressants, significantly suppressed tumor growth and enhanced CD8 T cell-mediated antitumor responses in various preclinical mouse syngeneic and human xenograft tumor models. Importantly, SSRI treatment exhibited significant therapeutic synergy with anti-PD-1 therapy, and clinical data correlation studies negatively associated intratumoral SERT expression with patient survival in a range of cancers. These findings highlight the significance of the intratumoral serotonin axis and identify SERT as a novel immune checkpoint, positioning SSRIs as promising candidates for next-generation combination cancer therapies. Overall design: Gene expression profiling of tumor infiltrating immune cells in an in vivo study using a B16-OVA syngeneic mouse melanoma model treated with fluoxetine or anti-PD-1