Targeting ADAR1 reprograms cold tumors to hot and enhances immunotherapy in bladder cancer

Despite significant advances in immunotherapy for certain solid tumors, bladder cancer (BCa) exhibits limited responsiveness, primarily due to its highly immunosuppressive “cold” tumor microenvironment (TME). The role of Adenosine Deaminase Acting on RNA1 (ADAR1) in regulating BCa sensitivity to immunotherapy remains elusive. Here, we established that targeting ADAR1 in BCa reprogrammed the TME immune landscape by promoting CD8 + T cells infiltration and PD-L1 expression on tumor cells, thereby sensitizing BCa to PD-1 blockade therapy. Mechanistically, “A-to-I” RNA editing-defective mutant ADAR1 E912A promoted CD8 + T cells infiltration by increasing CCL5 expression and secretion. ADAR1 E912A reduced editing of CCL5 mRNA, diminishing endonuclease V mediated degradation and consequently increased CCL5 mRNA stability. Additionally, ADAR1 cooperated with DICER to facilitate the maturation of miR-377-3p independently of its editing function, which mediated repression of PD-L1. Moreover, chemotherapeutic agent fludarabine served as an ADAR1 inhibitor by suppressing both its editing activity and expression. Fludarabine significantly promoted CD8 + T cells infiltration, increased PD-L1 expression and enhanced sensitivity to PD-1 blockade therapy in both vitro and vivo models. Collectively, our results demonstrated that targeting ADAR1 in tumor cells reprogramed the immunosuppressive “cold” TME and validate fludarabine as an ADAR1 inhibitor capable of sensitizing BCa to PD-1 blockade therapy.