Document resource
Background Many patients with cancer benefit little from immune checkpoint blockade (ICB), a major obstacle to immunotherapy for decades. Finding alternative immune checkpoints that control CD8 + T-cell exhaustion is urgent if we are to improve the efficacy of immunotherapies, particularly in microsatellite stable (MSS) colorectal cancer (CRC) that is resistant to ICB.Methods Spatial proximity is essential for suppressive ligand-receptor signaling. Here, we mapped the spatial tumor microenvironment of patients with MSS CRC at single-cell resolution and analyzed the cells interacting with exhausted CD8 + T cells to identify immune checkpoint ligand-receptor pairs. To investigate the function of this previously unrecognized immune checkpoint, we performed validation studies spanning cellular experiments, mouse models, and clinical patient samples.Results We found that a subset of MSS CRC exhibits substantial CD8 + T-cell infiltration, but their function is suppressed. We identified CLEC2B (ligand)-KLRB1 (receptor) as a novel inhibiting ligand-receptor pair for CD8+ T cells. KLRB1 acts as an immune checkpoint receptor, increasing CD8+ T-cell exhaustion and facilitating immune escape in various human cancers. Binding of CLEC2B to KLRB1 initiates immunosuppressive signaling in CD8+ T cells. Clinically, CLEC2B-KLRB1 expression correlates positively with cancer progression and poor response to ICB, demonstrating that KLRB1+ CD8+ T cells are a key marker of the poorly responsive ICB subtype. Furthermore, blocking CLEC2B-KLRB1 signaling with antibodies enhances the antitumor function of CD8+ T cells, providing a potential immunotherapy target for ICB non-responders.Conclusions Our study revealed CLEC2B-KLRB1 as a previously unrecognized immune checkpoint axis that drives T-cell exhaustion specifically in ICB poor responsive MSS CRC. Blockade of KLRB1 with a therapeutic antibody reinvigorated CD8 + T-cell antitumor immunity, positioning this axis as a promising target for enhancing immunotherapy efficiency in malignancies, including CRC and other ICB-resistant cancers.