Targeting Galectin-9 to overcome immunosuppression and potentiate ATR inhibitor therapy.

Pharmacological targeting of ATR (ataxia telangiectasia and Rad3-related kinase), the master regulator of replication stress response, is emerging as a promising anticancer strategy. Despite the documented immune-modulatory effects of ATR inhibitors (ATRi), the immune evasion mechanisms constraining their therapeutic efficacy remain undefined. Here, we demonstrate that ATRi upregulates Galectin-9 (Gal-9), a ligand for the TIM-3 immune checkpoint, in tumor cells and host antigen-presenting cells (dendritic cells/macrophages) via STING-type I interferon (IFN-I) innate immune pathway. Notably, combining Gal-9 blockade with ATRi ceralasertib elicits potent anti-tumor effects and induces durable immunologic memory in syngeneic mouse models. In immune checkpoint-refractory lung cancer, the triple combination of ATRi, anti-Gal-9 and anti-PD-1 demonstrates superior efficacy. Mechanistically, Gal-9 blockade synergizes with ATRi to activate dendritic cells/macrophages and promote CD8 T cell differentiation toward stem-like memory phenotypes with enhanced functional capacity. CD8 T cell depletion completely abrogates the anti-tumor effects, suggesting their essential role in mediating therapeutic responses. These findings establish Gal-9 upregulation as a critical adaptive immune resistance mechanism constraining ATRi efficacy, providing a compelling rationale for clinical translation of ceralasertib/anti-Gal-9 combinations.