Abstract Responsiveness to immunotherapies is limited in melanoma to subsets of patients. Despite attempts to improve responsiveness to immunotherapies by testing combination therapies, patients often experience significant adverse events that require the termination of treatment. Although causes of nonresponsiveness remain unclear and the subject of active investigation, a commonality among refractory tumors is low pretreatment levels of tumor-infiltrating lymphocytes (TILs). Thus, studies aiming to elucidate the interactions between melanoma and immune cells and to increase TIL levels are anticipated to improve therapies. We previously reported that fucosylation (the post-translational modification of proteins with the dietary plant sugar L-fucose), is altered in melanoma. Increasing fucosylation levels in melanoma—genetically or with oral L-fucose—suppresses tumor growth, metastasis, and increases TILs in immune competent mouse models of melanoma (Lau et al., 2015). The mechanisms underlying how melanoma fucosylation appears to trigger antitumor immunity are unclear. Here, we identified TIL subpopulations triggered by fucosylation to mediate tumor suppression and a key molecular mechanism. In NRAS- and BRAF-mutant melanoma models, dietary L-fucose suppressed tumor growth by ~50-60% and increased TILs by ~10-50-fold. Of all TIL subpopulations examined, CD3+ T cells were most increased by L-fucose, doubling or increasing by 15-fold, in NRAS- or BRAF-mutant models, respectively. Depletion of CD4+ T cells abrogated L-fucose-triggered TIL increases and tumor suppression, indicating that CD4+ T cells are central for antitumor immune effects of L-fucose. We identified the Class II MHC protein HLA-DRB1 as expressed and fucosylated in melanoma cells and confirmed its requirement for L-fucose-triggered, CD4+ T cell-dependent tumor suppression in vivo. Roles of HLA-DRB1 fucosylation in melanoma, effects on CD4+ T cell biology, and how L-fucose can enhance immunotherapies will be discussed. Our studies highlight how L-fucose supplementation can render tumors “immune hot” and represents a potential therapeutic strategy for improving immunotherapies. Citation Format: Daniel K. Lester, Matt Mercurio, Pasquale Innamorato, Kodumudi Krithika, Williamson Danial, Watson Gregory, Pilon-Thomas Shari, Messina Jane, Kerri L. Thomas, Susan McCarthy, Joseph Markowitz, Robert Haltiwanger, Eric Lau. Using L-fucose to render melanomas immune hot: Roles of melanoma HLA-DRB1 and CD4+T cell-mediated antitumor immunity [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B24.