Circadian Control of Tumor Immunosuppression Impacts Efficacy of Immune Checkpoint Blockade

The circadian clock is a critical regulator of immunity, and this circadian control of immune modulation plays an essential role in host defense as well as tumor immunosurveillance. Using a single cell RNA-sequencing approach in a genetic model of colorectal cancer (CRC), we identified clock-dependent changes to the immune landscape that dictate the abundance of immunosuppressive cells and consequent suppression of cytotoxic CD8+ T cells. Of these immunosuppressive cell types, PD-L1 expressing myeloid-derived suppressor cells (MDSCs) peak in abundance in a rhythmic manner. Mechanistically, we identified that disruption of the epithelial cell clock regulates the secretion of cytokines and chemokines that promote heightened inflammation, recruitment of neutrophils, and the subsequent development of MDSCs. We leveraged these findings to demonstrate that time-of-day delivery of anti-PD-L1 immunotherapy is most effective when synchronized with the abundance of immunosuppressive MDSCs. Collectively, our results indicate that circadian gating of tumor immunosuppression informs the timing and optimal efficacy of immune checkpoint inhibitors (ICIs). Four C57BL/6 genotypes (wild type, Bmal1-/-, Apc+/-, and Dko (Apc+/-;Bmal1-/-)) were used to study how circadian disruption in the intestine affects the tumor microenvironment of colorectal cancer. Adult mouse intestines were collected, digested for single cell suspension, and then FACS sorted for CD45+ immune cells. scRNA-seq was performed on these immune cells and analyzed using Seurat.