α2,6-Sialoglycan-regulated anti-tumor immunity of tumor-infiltrating CD8+ T cells in the tumor microenvironment

Cell-surface sialoglycans overexpressed on cancer cells suppress the activation of tumor-infiltrating immune cells (TIICs), and have emerged as a promising target for cancer immunotherapy. However, the functional roles of sialylation in TIICs remain incompletely understood. Here, we develop the single-cell correlative analysis of α2,3- and α2,6-linked sialoglycans and transcriptomes (scCAST_3&6) strategy to correlate cell-surface sialylation in a linkage-specific manner with gene expression of the same cells by single-cell RNA sequencing. Applying scCAST_3&6, we discover that α2,3- and α2,6-sialoglycans are distinctively regulated in specific TIIC subpopulations. Importantly, downregulated α2,6-sialoglycan is demonstrated as a cell-surface marker for labeling and isolating CD8+ T cells with high reactivity to tumor-specific antigens (TSAs) from the tumor. Mechanistically, α2,6-sialyltransferase ST6Gal-I is transcriptionally suppressed upon the activation of intratumoral CD8+ T cells with high-avidity T cell receptors towards tumor antigens, which downregulates α2,6-sialylation and exposes galectin-1 ligands. Functionally, increased galectin-1 binding to TSA-reactive CD8+ T cells promotes T cell apoptosis and cancer immune evasion. This work underscores the functional significance and therapeutic promise of specific sialoglycans on TIICs.