A homogenous hexameric agonist fusion protein composed of the extracellular domain (ECD) of the T-cell costimulatory receptor human glucocorticoid-induced tumor necrosis factor receptor (tumor necrosis factor superfamily, member 18; TNFRSF18; GITR) ligand (GITRL) and an immunoglobulin (Ig) G1 Fc domain, with potential immunomodulating and antineoplastic activities. Upon administration, efgivanermin alfa binds to and activates GITR found on multiple types of T-cells, thereby inducing both the activation and proliferation of tumor antigen-specific T effector cells. This abrogates the suppression of T effector cells which is induced by inappropriately activated T regulatory cells (Tregs), suppresses Tregs and decreases Treg tumor infiltration, and activates the immune system to help eradicate tumor cells. GITR, a member of the TNF receptor superfamily, is expressed on the surface of multiple types of immune cells, including Tregs, effector T-cells, B-cells, dendritic cells (DCs) and natural killer (NK) cells.