SIRT1 Inhibits T-cell Infiltration and Tertiary Lymphoid Structure Formation to Promote Radioimmunotherapy Resistance

Microsatellite stable (MSS) rectal cancer exhibits intrinsic resistance to immunotherapy. Although radiotherapy is frequently combined with immune checkpoint inhibitors (ICIs) to augment immunotherapy responses, numerous immunologically cold tumors remain unresponsive. In this study, we observed a significant increase in electron transport chain activity, acetyl-CoA level, and global lysine acetylation level in patients achieving a pathologic complete response (pCR) following immunotherapy administered after radiotherapy. Transcriptomic screening and in vivo experiments revealed that SIRT1, a key regulator of protein acetylation, restricted the immunostimulatory effects of radiotherapy. Mechanistically, SIRT1 deacetylated DDX5, promoting unwinding of irradiation-induced R-loops and inhibiting accumulation of cytoplasmic RNA:DNA hybrids to suppress cGAS/STING pathway activation and T cell infiltration. Moreover, radiotherapy induced a tryptophan-SIRT1-SLC36A4 positive feedback loop that enhanced SIRT1 activity and promoted competitive tryptophan uptake from the microenvironment, thereby inhibiting tertiary lymphoid structure (TLS) formation and radioimmunotherapy efficacy. Finally, combining both SIRT1 inhibitor and aspirin with radiotherapy converted ICI-unresponsive rectal cancer into immunogenic tumors that were sensitive to ICI. Together, this study identifies SIRT1 as a potential biomarker and therapeutic target to overcome radioimmunotherapy resistance in MSS rectal cancer.