A crucial contribution of T-cells to cancer immunosurveillance and therapy has been intensively studied and documented over the years. Groundbreaking success was recently achieved through the introduction of checkpoint blockade which releases T-cells from the physiological negative feedback mechanisms exploited by tumors to escape immune attack. Blockade of inhibitory receptors such as CTLA-4 and PD-1 expressed on activated T-cells has seen survival of some stage IV melanoma patients dramatically increased. However, it remains the case that not all patients respond, but all may suffer side effects. Further improvements of today’s treatment strategies is still urgently required.In the present proposal, we focus on the numerically minor subset of γδ T-cells, which has a major role in defense against microbial and non-microbial challenges but has been under-investigated in the context of checkpoint blockade in cancer. Results from in vitro and in vivo experiments suggest that these unique T-cells sense infected and/or malignant cells, contribute efficiently to the regulation of immunity and thus to the elimination of these cells. Little is known about the structures which are recognized by γδ T-cells in an MHC-independent manner. Peripheral blood γδ T-cells consist usually of a majority carrying the Vδ2-TCR and a minority of cells expressing Vδ1- and other Vδ-TCRs. The Vδ2 T-cell subset is associated with the expression of semi-invariant public TCRs, which recognize phospho-antigens (pAgs) or pAg-triggered cell surface molecules such as butyrophilin. In contrast, Vδ1 T-cells are focused on a private repertoire with a few high frequency clonotypes.The role of both Vδ1 and Vδ2 T-cells in cancer immunosurveillance is controversial. We and others have reported both, positive and negative associations of γδ T-cell frequencies with melanoma patient survival. Here, we propose a comprehensive analysis of γδ T-cells in stage IV melanoma patients prior to and under PD-1 blockade to investigate their role in a) melanoma rejection and b) the potential direct and indirect immunmodulatory effects of checkpoint blockade (γδ T-cells express checkpoint molecules just as do T-cells). Deep-phenotyping (using CyTOF), investigation of (poly)functionality, proliferative capabilities, and γδ TCR-repertoire analysis of peripheral blood γδ T-cells will be contrasted with investigations of the presence or absence of γδ T-cell subsets infiltrating the tumor, allowing a comprehensive interpretation of resulting data combined with clinical meta-data available for all patients that have been recruited into our biobank. Additionally, we aim to investigate tumor-specific γδ T-cell clones and their amplification by “γδ T-cell boosters” in vitro.The environment of FOR2799 will provide optimal conditions for a sustainable exchange and effective research strategy targeting exploitation of γδ T-cells in future cancer treatment regimes.