Chemotherapy-induced CA-repeat DNA fragments in breast cancer trigger antitumor immune responses.

Damage-associated molecular patterns generated by cancer treatment can modulate antitumor immunity, but the underlying mechanisms of this effect are unclear. Here we show that CA-enriched DNA fragments resulting from DNA-damaging chemotherapy in MSH2-low tumors preferentially bind cGAS with strong affinity and form biomolecular condensates by phase separation in the cytoplasm, resulting in antitumor immunity. However, classical CA-poor DNAs released from MSH2-high tumor cells engage AIM2, resulting in immunosuppression by upregulating PD-L1 and IDO. Intratumoral administration of CA-rich DNA fragments enhanced antitumor immunity in syngrafted PyMT tumors. Clinically, CA-rich DNA abundance in breast cancer following chemotherapy was associated with increased tumor-antigen-reactive T cells and better chemotherapeutic responses. Therefore, different tumor DNA fragments can trigger opposing immune responses depending on their preference for differential sensors. This study highlights another mechanistic link between genome instability and immune modulation and the therapeutic potential of CA-rich DNAs to enhance antitumor immunity.