GITR Research in Tumor Immunotherapy

Investigation into new potential targets for tumor immunotherapy has been ongoing now for over a decade. One protein that has long been studied in this framework is GITR (TNFRSF18), a member of the TNF superfamily of proteins that is expressed on T cells. Its ligand, GITRL, is expressed on antigen presenting cells such as dendritic cells. This pathway modulates the T cell immune response through a dual mechanism¹ that primarily enhances the proliferation of regulatory T cells (Tregs)² and also enhances the metabolism of effector T cells³. Because of these dual effects, understanding how best to target GITR in the setting of immunotherapy has remained challenging. Luckily, advances have recently been seen, and the stimulatory effect on regulatory T cells does not appear to be a factor in the cancer setting.

Various GITR agonists are used to target this pathway for therapeutic purposes. In a study of multiple murine tumor models, a GITRL fusion protein was delivered in conjunction with a tumor vaccine. Depending on the tumor model, monotherapy with the fusion protein or combination with the vaccine led to an increase in the number of tumor-specific effector T cells and a depletion of Tregs. Additionally, this treatment increased the levels of GITR on tumor-specific effector T cells, making this population of cells more responsive to treatment⁴. In a similar study, antibody-induced agonism of GITR increased the proliferation, interferon gamma production, and glycolysis of effector T cells in both the tumor microenvironment and draining lymph node of a tumor³. Most importantly, GITR agonism in combination therapy leads to tumor eradication or delayed tumor progression⁵. Another role of GITR agonist antibodies is the depletion of Tregs, and this has been shown in both murine tumor models and human tumor models in humanized mice^{6, 7}. The specific population of Tregs that appears to be depleted by this treatment is a highly suppressive subtype that expresses both CD44 and ICOS. Depletion of Tregs, in turn, allows intratumoral T cells to downregulate expression of PD-1 and LAG3 to become more effector-like⁷.

Besides impacting T cells, GITR agonism also affects B cells, and this effect is crucial for therapeutic efficacy. In a mouse model, treatment with an agonistic antibody targeting GITR led to an increase in B cell activation, differentiation, and production of antibodies. Most importantly, B cells were required for this treatment to have any effect on effector T cells, as shown in B cell depletion studies⁸.

More recently, GITR agonism has made its way into the clinic. The anti-GITR antibody MK-4166 is in clinical trials as both a monotherapy and a combination therapy with checkpoint blockade for solid tumors, including melanoma⁹. An earlier clinical trial using a different anti-GITR antibody, TRX518, as a monotherapy depleted both circulating and intratumoral Tregs and increased the ratio of effector to regulatory T cells in patients with various solid tumors. However, clinical responses including changes in tumor burden or survival were not observed. This antibody is now in clinical trials in combination with anti-PD-1^10. With the recent migration of GITR from the lab to the clinic, yet with questions still to be answered about the basic biology of targeting this pathway, studying GITR in the context of cancer immunotherapy remains an exciting and valuable area of research.