CD8⁺ CD226^high T cells in liver metastases dictate the prognosis of colorectal cancer patients treated with chemotherapy and radical surgery

CD226 has been reported to participate in the rescue of CD8⁺ T cell dysfunction. In this study, we aimed to assess the prognostic value of CD226 in tumor-infiltrating lymphocytes (TILs) derived from colorectal cancer (CRC) liver metastases treated with chemotherapy and radical surgery. TILs from 43 metastases were isolated and analyzed ex vivo using flow cytometry. CD155 and CD3 levels in the tumor microenvironment were assessed by immunohistochemistry. Exploration and validation of biological processes highlighted in this study were performed by bioinformatics analysis of bulk RNA-seq results for 28 CRC liver metastases pretreated with chemotherapy as well as public gene expression datasets. CD226 expression contributes to the definition of the immune context in CRC liver metastases and primary tumors. CD226 on CD8⁺ T cells was not specifically coexpressed with other immune checkpoints, such as PD1, TIGIT, and TIM3, in liver metastases. Multivariate Cox regression analysis revealed CD226 expression on CD8⁺ T cells to be an independent prognostic factor (p = 0.003), along with CD3 density at invasion margins (p = 0.003) and TIGIT expression on CD4⁺ T cells (p = 0.019). CD155 was not associated with the prognostic value of CD226. Gene expression analysis in a validation dataset confirmed the prognostic value of CD226 in CRC liver metastases but not in primary tumors. Downregulation of CD226 on CD8⁺ TILs in the liver microenvironment was restored by IL15 treatment. Overall, CD226 expression on liver metastasis-infiltrating CD8⁺ T cells selectively contributes to immune surveillance of CRC liver metastases and has prognostic value for patients undergoing radical surgery.