A human, high-affinity, tetravalent monospecific agonistic monoclonal antibody targeting glucocorticoid-induced tumor necrosis factor receptor (GITR; tumor necrosis factor superfamily member 18; TNFRSF18; CD357), with potential immune checkpoint modulating activity. Upon administration, anti-GITR agonistic monoclonal antibody ASP1951 binds to and activates GITR, which is expressed on the cell surface of multiple types of T-lymphocytes. This induces both the activation and proliferation of tumor antigen-specific T-effector cells (Teffs), and suppresses the function of activated T-regulatory cells (Tregs), which may promote the elimination of tumor cells. GITR, a member of the TNF receptor superfamily and T-cell receptor co-stimulator, is expressed on the surface of multiple immune cell types, including Tregs, Teffs, B-cells, and natural killer (NK) cells. Inappropriately activated Tregs suppress both Teffs and T-cell receptor (TCR) signaling.