Clinical and immunological implications of lymphocyte-activation gene 3 expression in metastatic colorectal cancer

Background: Lymphocyte-activation gene 3 (LAG3), an inhibitory immune checkpoint receptor, has emerged as a potential therapeutic target in various cancer, including colorectal cancer (CRC). However, its prognostic role and immunologic context in metastatic CRC (mCRC) remain unclear. This study investigated the prognostic impact of LAG3 expression and its biological role within the mCRC immune microenvironment. Methods: LAG3 expression was evaluated by immunohistochemistry in primary and/or metastatic tumors from a retrospective mCRC cohort. Associations between LAG3 expression, clinicopathological features, and overall survival were analyzed and validated using data from The Cancer Genome Atlas (TCGA) COADREAD cohort. Transcriptomic data from TCGA and single-cell RNA sequencing data from 23 Korean CRC samples were used to explore pathway enrichment and cell–cell interactions. Results: A total of 144 mCRC patients were included. Primary tumors showed higher LAG3 expression than metastatic tumors, with right-sided and liver metastases exhibiting the highest level. Both univariate and multivariate analyses revealed that LAG3 expression was an independent prognostic biomarker of mCRC, validated in the TCGA COADREAD cohort. Transcriptomic analysis revealed significant correlations between LAG3 expression and angiogenesis, epithelial–mesenchymal transition, and TGF-β signaling pathways, which were linked to immunosuppressive microenvironment and poor prognosis. Cell–cell communication analysis showed CD8⁺ T cells interact with various cell subpopulations, with Galectin-9–TIM-3 signaling uniquely present in LAG3⁺CD8⁺ T cells. Conclusions: LAG3 expression is an independent prognostic biomarker of mCRC. The distinct Galectin-9–TIM-3 signaling in LAG3⁺CD8⁺ T cells may contribute to limited therapeutic efficacy of LAG3 blockade, suggesting potential combinational immunotherapy strategies.