Immune-mediated Engagement of T Regulatory Cells with Tumor Cells Results in Trogocytosis and Tumor Cell Killing.

Despite advances in treatment, >50% of patients with advanced melanoma are unresponsive to current therapies. Using the B78-D14 melanoma model (GD2+/MHC-I-/MHC-II+), we can cure mice with a regimen that includes radiation therapy (RT) in combination with immunocytokine (IC; anti-GD2 monoclonal antibody linked to IL-2) while establishing immunological memory. We interrogated the role of T cells in the antitumor and memory responses following RT+IC. We show a requirement for CD4, but not CD8 T cells, to achieve both the initial and memory responses. Upon IC-induced cell-cell contact, subsets of CD4 T cells, including Foxp3 T regulatory cells, trogocytose GD2 from tumor cells, acquire cytotoxic granules, and kill tumor cells. These results were confirmed using human tumor cell lines. These findings reveal that CD4 T regulatory cells, upon immunologically-induced binding to tumor cells, can trogocytose tumor antigens and directly kill tumor cells, redefining their potential role in antitumor immunity.