Cop1 Controls Macrophage Infiltration and Antitumor Immunity in TNBC

Although immune checkpoint blockade (ICB) has revolutionized the standard of care for several cancer types, the clinical benefit of ICB remains limited for patients with triple-negative breast cancer (TNBC). The immune system is critical in controlling tumor growth, but immune cells in the tumor microenvironment, including macrophages, can be reprogrammed to promote tumor progression and immune evasion. To identify regulators of antitumor immunity, Wang, Tokheim, Gu, Wang, and colleagues developed an in vivo CRISPR knockout screen using a syngeneic model in which mice were injected with murine TNBC cells expressing a lentiviral library of single guide RNAs (sgRNA) targeting more than 4,500 genes associated with tumor initiation, progression, and immune modulation. Comparing sgRNA abundance in tumors harvested 16 days after orthotopic transplantation into immunocompetent versus immunocompromised mice, the screen showed selection for the loss of known regulators of immune evasion, including Cd274 (encodes PD-L1) and Brca2 and Pms2 (key DNA repair pathway genes) as well as depletion of Cop1, which encodes an E3 ubiquitin ligase previously shown to mediate protein degradation of various transcription factors. Following a second in vivo screen focusing on the initial screen's top hits, Cop1 was the most significantly depleted gene in tumors of immunocompetent mice, with Cop1 knockout in TNBC cells decreasing tumor volume and prolonging survival alone, as well as in combination with anti–PD-1 therapy. Cop1 knockout repressed cytokines and chemokines that mediate macrophage recruitment and activation in vitro, and decreased immunosuppressive M2 macrophage infiltration in vivo. Integration of RNA-seq and ATAC-seq analysis highlighted the CEBP family of transcription factors as putative COP1 substrates. Specifically, Cop1 knockout was found to increase CEBPD protein levels via the substrate adaptor protein TRIB2, suppressing CEBPD-mediated expression of macrophage cytokines and immune response genes. In summary, this work highlights Cop1 as a therapeutic target that can enhance response to ICB by controlling immunosuppressive macrophage infiltration in the tumor microenvironment of TNBC.

Wang X, Tokheim C, Gu SS, Wang B, Tang Q, Li Y, et al. In vivo CRISPR screens identify the E3 ligase Cop1 as a modulator of macrophage infiltration and cancer immunotherapy target. Cell 2021 Sep 27 [Epub ahead of print].

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