PROJECT SUMMARY Immune checkpoint blockade has elicited unprecedented clinical responses in patients with metastatic melanoma and other cancers. A promising new checkpoint under investigation in cancer therapeutic trials is T- cell immunoglobulin and mucin domain 3 (Tim-3). Tim-3 blockade reverses T-cell impairment, thereby reinvigorating antitumor T-cell immunity. However, we found that Tim-3 inhibitors, including those in clinical trials, not only target T-cell-Tim-3, but also have varying affinity for Tim-3 on dendritic cells (DCs), macrophages (MΦs), NK and melanoma cells. Clinical benefit might thus not exclusively rely on antagonism of T-cell-Tim-3, but also on inhibition of these additional Tim-3-expressing cell types. In support, blockade of T- cell-Tim-3 suppressed, while melanoma-directed Tim-3 inhibition enhanced tumor growth in murine melanoma models, thereby counteracting desired efficacy of Tim-3 therapy. Consistently, enforced expression of Tim-3 on melanoma cells suppressed tumorigenesis, metastasis formation, and proliferative pathway activity. Our preliminary studies thus identify melanoma cell-intrinsic, DC-, MΦ-, and NK-cell-Tim-3 as unexpected variables and/or potential confounders of treatment outcome. They further highlight the need to define therapeutic consequences of Tim-3 antibody (Ab) responses at the level of specific cell types. The Tim-3 protein bears multiple N- and O-glycostructures that differ dramatically in composition, size, and charge between cell lineages and which might explain the marked variations in Tim-3 Ab clone reactivity we found between cell types. For example, the clinical Tim-3 trial candidate, TSR-022, avidly bound T-cell- and melanoma-, but not NK-, DC-, or MΦ-Tim-3, while other Tim-3 Abs showed high affinity for Tim-3 on T-cells, MΦs, DCs, and/or NK, but not melanoma cells. Notably, glycan-modifying regimens shifted inhibitor binding towards desired T-cell- Tim-3 recognition and reduced melanoma-Tim-3 reactivity. Our preliminary data highlights the critical need for dissecting immune- vs. melanoma cell-Tim-3 glyco-epitopes, Ab affinity, signaling, and immunobiology. Results will help optimize Tim-3 therapeutic efficacy by validating regimens that preferentially target immune cell-Tim-3 glycans, while avoiding unwanted blockade of melanoma cell-Tim-3. Our aims are to 1) define cell type- associated Tim-3 glycan moieties, ligands, Ab affinities, and signaling networks, 2) examine immune cell- vs. melanoma-intrinsic effects of existing Tim-3 antagonists and their relevance to interpreting therapeutic benefit, and 3) identify new Tim-3 targeting strategies that accentuate immune cell-Tim-3 inhibition. We will use state- of-the-art gain and loss of Tim-3 function and glycan-modifying strategies, Tim-3 inhibitors with variable tissue- associated affinities, and immune and melanoma model systems to define cell type-specific Tim-3 functions and glycomolecular targets. Our initiative also implements cli...