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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.

Despite initial responses1-3, most melanoma patients develop resistance4 to immune checkpoint blockade (ICB). To understand the evolution of resistance, we studied 37 tumor samples over 9 years from a patient with metastatic melanoma with complete clinical response to ICB followed by delayed recurrence and death. Phylogenetic analysis revealed co-evolution of seven lineages with multiple convergent, but independent resistance-associated alterations. All recurrent tumors emerged from a lineage characterized by loss of chromosome 15q, with post-treatment clones acquiring additional genomic driver events. Deconvolution of bulk RNA sequencing and highly multiplexed immunofluorescence (t-CyCIF) revealed differences in immune composition among different lineages. Imaging revealed a vasculogenic mimicry phenotype in NGFRhi tumor cells with high PD-L1 expression in close proximity to immune cells. Rapid autopsy demonstrated two distinct NGFR spatial patterns with high polarity and proximity to immune cells in subcutaneous tumors versus a diffuse spatial pattern in lung tumors, suggesting different roles of this neural-crest-like program in different tumor microenvironments. Broadly, this study establishes a high-resolution map of the evolutionary dynamics of resistance to ICB, characterizes a de-differentiated neural-crest tumor population in melanoma immunotherapy resistance and describes site-specific differences in tumor-immune interactions via longitudinal analysis of a patient with melanoma with an unusual clinical course.