Anti-PD-L1 antibodies target programmed cell death ligand 1 (PD-L1). Signals from PD-L1 on tumor cells and in tumor microenvironment help those tumors avoid immune attack and elimination by preventing activation of tumor specific effector T-cells. Anti-PD-L1 antibodies are designed to block that signal permitting effector T-cells to attack the cancer. Anti-GITR antibodies target glucocorticoid-induced tumor necrosis factor receptor related protein (GITR), which is regularly expressed on the surface of regulatory T-cells (Tregs) and is expressed on the surface of effector T-cells after their activation. Modulation of GITR with agonistic antibodies has been shown to amplify the antitumor immune responses in animal models via multiple mechanisms.  Anti-GITR antibodies are designed to activate the GITR receptor thereby increasing the proliferation and function of effector T cells.  At the same time, ligation of GITR on surface of Tregs could abrogate suppressive function of these cells on tumor specific effector T-cells thus further augmenting T-cell immune response. While targeting PD-1/PD-L1 axes alone has already demonstrated impressive anticancer efficacy and durable responses in humans, its efficacy appears to be limited to certain patients.  It is believed the effects of anti-PD-L1 intervention can be enhanced by utilizing a co-stimulatory antibody, like one targeting GITR, that can turn on tumor specific effector T-cells. Combining immunotherapies like anti-PD-L1 that counters the tumor's immune-evading defense system with an anti-GITR that activates effector T-cells, represents a rational approach to use the body's own immune system to help fight cancer. Pre-clinical research on the combination of the two approaches has yielded very encouraging results to support synergistic potential of this combination.