Abstract Background: Microsatellite stable (MSS) rectal cancer is intrinsically resistant to immunotherapy. Although radiotherapy is often combined with immune checkpoint inhibitor (ICIs) to enhance immunotherapy responses, many immunologically cold tumors remain unresponsive. Methods: The radiotherapy-induced antitumor immunity effect of SIRT1 inhibition was assessed in murine models. The mechanisms of SIRT1 and DDX5 in tumor immune escape with radiotherapy were determined in vitro and in vivo. The synergistic effects of SIRT1 inhibitor or aspirin plus radiotherapy with PD-1 blockade were also investigated. Results: We observed a significant increase in electron transport chain activity, acetyl-CoA level, and pan-acetylation level in patients achieving pathologic complete response (pCR) following immunotherapy administered after radiotherapy. Through transcriptomic screening and in vivo experiments, we found that SIRT1, a key executor in regulating protein acetylation, restricted the immunostimulatory effects of radiotherapy. Mechanistically, SIRT1 deacetylates DDX5, promoting its unwinding of irradiation-induced R-loop and inhibiting the accumulation of cytoplasmic RNA: DNA hybrids, thereby suppressing the cGAS/STING pathway and T cell infiltration. Interestingly, we found that radiotherapy forced the tryptophan-SIRT1-SLC36A4 positive feedback, further enhancing SIRT1 activity and competitively uptaking tryptophan in the microenvironment, thereby inhibiting TLS formation and leading to poor efficacy of radioimmunotherapy. Finally, we discovered that both SIRT1 inhibitor and aspirin plus radiotherapy converts ICI-unresponsive rectal cancer into immunogenic tumors that are hyper-sensitive to ICI. Conclusions: SIRT1 is a potential biomarker and therapeutic target to overcome radioimmunotherapy resistance. Citation Format: Yunxing Shi, Zongfeng Wu, Shaoru Liu, Renhan Luo, Liang Kang. SIRT1 inhibits T cell infiltration and tertiary lymphoid structure formation to promote radioimmunotherapy resistance abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2568.
Shi et al. (Fri,) studied this question.