In vivo CRISPR screens identify E3 ligase COP1 as a modulator of macrophage infiltration and cancer immunotherapy target

Despite remarkable clinical efficacy of immune checkpoint blockade (ICB) in cancer treatment, ICB benefits for triple-negative breast cancer (TNBC) remain limited. Through pooled in vivo CRISPR knockout (KO) screens in syngeneic TNBC mouse models, we found that deletion of the E3 ubiquitin ligase Cop1 in cancer cells decreases secretion of macrophage-associated chemokines, reduces tumor macrophage infiltration, enhances anti-tumor immunity, and strengthens ICB response, Transcriptomics, epigenomics, and proteomics analyses revealed that Cop1 functions through proteasomal degradation of the C/ebp delta protein. The Cop1 substrate Trib2 functions as a scaffold linking Cop1 and C/ebp delta, which leads to polyubiquitination of C/ebp delta. In addition, deletion of the E3 ubiquitin ligase Cop1 in cancer cells stabilizes C/ebp delta to suppress expression of macrophage chemoattractant genes. Our integrated approach implicates Cop1 as a target for improving cancer immunotherapy efficacy in TNBC by regulating chemokine secretion and macrophage infiltration in the tumor microenvironment.