Influence of the NECTIN2-TIGIT axis on resistance to PD-L1 blockade combined with chemoradiotherapy in MSS locally advanced rectal cancer.

188 Background: While PD-1/PD-L1 blockade combined with chemoradiotherapy benefits dMMR/MSI-H colorectal cancer (CRC) patients, its efficacy in pMMR/MSS locally advanced rectal cancer (LARC) remains unclear. The mechanisms underlying differential therapeutic responses, particularly immunosuppressive pathways limiting treatment success, require further exploration. Methods: A phase II clinical trial evaluated neoadjuvant chemoradiotherapy (long-course radiotherapy + capecitabine) with Atezolizumab (anti-PD-L1) in 12 MSS LARC patients, followed by total mesorectal excision surgery. Single-cell RNA sequencing was performed on surgical specimens from major pathological responders (MPR) and non-responders (non-MPR), supplemented by spatial transcriptomics, cell-cell communication analysis, and functional validation through in vitro and in vivo testing of TIGIT blockade combined with PD-L1 inhibition and radiotherapy in CRC models. Results: Non-responders exhibited distinct immunosuppressive ecosystems characterized by elevated regulatory T cells, M2-like macrophages, and genomically unstable stem-like epithelial cells. Notably, non-MPR tumors showed significant expansion of functionally exhausted TIGIT+ CD8+ T cells compared to responders, alongside enhanced NECTIN2-TIGIT interactions between cancer-associated fibroblasts and CD8+ T cells. Mechanistically, NECTIN2-TIGIT ligation inhibited the cGAS-STING pathway, suppressing CD8+ T cell cytotoxicity. Therapeutic blockade of TIGIT synergized with PD-L1 inhibition and radiotherapy, effectively reprogramming the tumor immune microenvironment, reducing tumor burden, and extending survival in preclinical models. Conclusions: The NECTIN2-TIGIT axis is a critical immune checkpoint driving resistance to PD-L1 blockade combined with chemoradiotherapy in MSS CRC by inducing CD8+ T cell exhaustion through suppression of the cGAS-STING pathway. Targeting this axis restores anti-tumor immunity, demonstrating that combined TIGIT and PD-L1 blockade with chemoradiotherapy represents a rational therapeutic strategy to overcome resistance in MSS locally advanced rectal cancer.