Despite reports of striking outcomes, immunotherapy efficacy in melanoma is limited to subsets of patients 1, 2. Combining immunotherapies with other modalities has yielded limited improvements but also adverse events requiring cessation of treatment 1. In addition to ineffective patient stratification, efficacy can be impaired by paucity of tumor-infiltrating lymphocytes (TILs). Thus, effective strategies to safely increase TILs are urgently needed to improve immunotherapies 3. Here, we report that dietary administration of the sugar L-fucose triggers CD4+T cell-mediated increases in TILs, anti-tumor immunity, and enhanced immune checkpoint blockade responses. This is induced by the fucosylation and cell surface enrichment of the MHC-II protein HLA-DRB1 in melanoma. Single-cell immunofluorescent staining analysis of patient melanoma specimens demonstrates that fucosylation and fucosylated HLA-DRB1 is associated with intratumoral T cell abundance and anti-PD1 responder status. Our findings demonstrate that fucosylation is a key mediator of anti-tumor immunity, via regulation of melanoma cell surface HLA-DRB1 and induction of anti-tumor immunity, suggesting use of melanoma fucosylation as a novel strategy to stratify patients for immunotherapies. Importantly, our study suggests that L-fucose represents a powerful, non-toxic agent for safely increasing anti-tumor immunity and immunotherapy efficacy in melanoma.
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