Tumor desialylation surpasses anti-PD-L1 checkpoint therapy in restoring anti-tumor immunity in a murine model for colorectal cancer.

Abnormal levels of tumor-associated glycans are correlated with tumor progression in many types of cancer, including (CRC). Sialic acids, a family of nine-carbon monosaccharides, are key regulators of the anti-tumor immune response via Siglecs, yet the sialic acid-specific effects are highly dependent on the tumor type studied. Therefore, a detailed understanding of sialic acid-mediated immunomodulation in different tumor contexts is warranted. Using CRISPR/Cas9 technology, we generated an isogenic knockout of the N-acylneuraminate cytidylyltransferase (Cmas) gene in CT26 CRC cells, thus creating cells that lack cell surface sialylation. Compared to CT26-MOCK control cells, CT26-CMAS KO cells displayed significantly reduced tumor growth in vivo, resulting in increased survival of the mice. This difference was absent in immunodeficient mice, signifying an immune-dependent effect. High-dimensional profiling of immune cell networks in the tumor microenvironment revealed increased infiltration and differentiation trajectories of lymphoid cells in the CT26-CMAS KO tumors, especially of natural killer (NK) cells and γδ T cells. Strikingly, sialic acid ablation resulted in a stronger immunostimulatory capacity and did not synergize with anti-PD-L1 checkpoint inhibition, suggesting that at least in the CT26 model, sialic acids impose a superior immune inhibitory circuit than the well-known PD-1/PD-L1 pathway. Overall, our findings strengthen the concept of sialic acid-mediated impairment of tumor immune surveillance and reinforce ongoing efforts to target sialic acids for the treatment of cancer.