Tim-3 mediates T cell trogocytosis to limit antitumor immunity

T cell immunoglobulin mucin domain-containing protein 3 (Tim-3) negatively regulates innate and adaptive immunity in cancer. To identify the mechanisms of Tim-3 in cancer immunity, we evaluated the effects of Tim-3 blockade in human and mouse melanoma. Here, we show that human programmed cell death 1–positive (PD-1⁺) Tim-3⁺CD8⁺ tumor-infiltrating lymphocytes (TILs) upregulate phosphatidylserine (PS), a receptor for Tim-3, and acquire cell surface myeloid markers from antigen-presenting cells (APCs) through transfer of membrane fragments called trogocytosis. Tim-3 blockade acted on Tim-3⁺ APCs in a PS-dependent fashion to disrupt the trogocytosis of activated tumor antigen–specific CD8⁺ T cells and PD-1⁺Tim-3⁺ CD8⁺ TILs isolated from patients with melanoma. Tim-3 and PD-1 blockades cooperated to disrupt trogocytosis of CD8⁺ TILs in 2 melanoma mouse models, decreasing tumor burden and prolonging survival. Deleting Tim-3 in dendritic cells but not in CD8⁺ T cells impeded the trogocytosis of CD8⁺ TILs in vivo. Trogocytosed CD8⁺ T cells presented tumor peptide–major histocompatibility complexes and became the target of fratricide T cell killing, which was reversed by Tim-3 blockade. Our findings have uncovered a mechanism Tim-3 uses to limit antitumor immunity.