In vivo CRISPR screens identify CBX4 as an epigenetic regulator for cancer immunotherapy.

Epigenetic dysregulation is associated with immune evasion and immune checkpoint blockade (ICB) resistance. Here, using in vivo CRISPR/Cas9 screens targeting epigenetics-related factors in mouse tumor models treated with ICB, we identified chromobox 4 (CBX4) as a key negative regulator of the immune tumor microenvironment (TME). Single-cell RNA-seq and spatial transcriptomics analyses of patients receiving neoadjuvant anti-programmed cell death protein 1 (anti-PD-1) therapy revealed high CBX4 expression in both tumor cells and immunosuppressive tumor-associated macrophage subpopulations, with preferential accumulation in nonresponders. Deficiency of CBX4 in macrophages or tumor cells induced robust antitumor immunity and increased infiltration and the cytotoxic activity of CD8+ T cells and NK cells, thereby heightening the sensitivity of ICB treatment. Mechanistically, CBX4 targeted H3K9me3- and H3K27me3-marked endogenous retroelements such as RLTR4-Mm-int. Loss of CBX4 derepressed retrotransposons, activating cytosolic RNA-sensing pathways and triggering the type I IFN response, ultimately leading to a robustly inflamed TME. Moreover, we uncovered a negative correlation between CBX4 expression, immune responses, and retrotransposon levels, and were able to determine the prognosis of patients with hepatocellular carcinoma (HCC) undergoing ICB therapy. Our study establishes CBX4 as an epigenetic immune checkpoint through the epigenetic silencing of retrotransposons, remodeling the immune TME and thus providing a promising therapeutic target to enhance tumor immunogenicity and overcome immunotherapy resistance.