Combination of local radiotherapy and anti-glucocorticoid-induced tumor necrosis factor receptor (GITR) therapy augments PD-L1 blockade-mediated anti-tumor effects in murine breast cancer model

Purpose: In this study, we investigated whether local radiotherapy (RT) and an anti-glucocorticoid-induced tumor necrosis factor receptor (GITR) agonist could increase the efficacy of PD-L1 blockade. Methods and Materials: We analyzed a breast cancer dataset from the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) to determine the role of GITR in breast cancer. We used the 4T1 murine TNBC model (primary and secondary tumors) to investigate the efficacy of PD-L1 blockade, local RT, anti-GITR agonist, and their combinations. We assessed tumor growth by tumor volume measurements, in vivo bioluminescence imaging, and metastatic lung nodule counts to evaluate the effects of these treatments. Flow cytometry and immunohistochemistry determined the proportions and phenotypes of CD8+ T-cells and regulatory T-cells (Tregs) in the tumors and spleen. Plasma cytokine levels were measured by enzyme-linked immunosorbent assay. Results: In the METABRIC cohort, patients with high expression of TNFRSF18, which encodes GITR, had significantly better survival than those with low expression. Adding local RT or anti-GITR agonist to PD-L1 blockade did not significantly augment efficacy compared to PD-L1 blockade alone; however, adding both to PD-L1 blockade significantly reduced tumor growth and lung metastasis. The benefits of the triple combination were accompanied by increased CD8+ T-cells and decreased Tregs in the tumor microenvironment and spleen. Conclusions: The combination of local RT and an anti-GITR agonist significantly enhanced the anti-tumor immune responses induced by PD-L1 blockade. These results provide the preclinical rationale for the combination of therapy.