TREX1 limits anti-tumor immunity by degrading short cytosolic DNA molecules.

TREX1 has recently emerged as a promising therapeutic target in cancer owing to its role in degrading cytosolic DNA and suppressing type I interferon signaling. However, the nature of the immunostimulatory DNA species degraded by TREX1 remains elusive. Here, we show that TREX1 deficiency in murine lung and melanoma tumor cells leads to a substantial, MRE11-dependent increase in short cytosolic DNA fragments that originate preferentially from transcriptionally active regions of the genome, without a corresponding increase in micronuclei. These fragments robustly activate interferon-stimulated genes and drive differentiation of pro-inflammatory macrophages. In syngeneic mice, tumors derived from Trex1-knockout cells exhibit delayed growth, increased sensitivity to anti-PD-1 immune checkpoint blockade, and extended survival. These effects depend on an intact immune system and foster durable anti-tumor memory. Together, our results reveal that short, transcription-derived cytosolic DNA fragments are key TREX1 substrates mediating immune activation in cancer, establishing TREX1 inhibition as a compelling strategy to enhance tumor immunogenicity.