IMPS-08CONSTITUTIVE CELL INTRINSIC OVEREXPRESSION OF PD-L1 AND PD-L2 IN GLIOMA

High-grade Gliomas (HGG) are the most common primary brain tumors and continue to carry a poor overall survival. Despite extensive molecular characterization, the “mutation-to-drug” paradigm has not been successfully applied to patients with glioblastoma as it has in other cancers. Due to the (a) success of checkpoint blockade immunotherapy, such as inhibition of the PD-1/PD-L1 pathway, in other tumor types and (b) the known immunologic comprise these patients exhibit, there has been tremendous enthusiasm regarding immune-based treatments in this setting. Although clinical trials incorporating the inhibition of the PD-1/PD-L1 pathway are ongoing, the biology underlying PD-L1 expression in glioma remains unclear. Moreover, there is a limited understanding of the underlying functional consequences of the expression of the related PD-1 ligand, PD-L2. To focus exclusively on the cell autonomous regulation of these ligands, we interrogated the Cancer Cell Line Encyclopedia, a Broad Institute/Novartis compendium of passaged cell lines independent of effects of the tumor microenvironment. We found that a subset of glioma cell lines harbor high constitutive PD-L1 and PD-L2 mRNA levels. Consistent with the mRNA levels, we showed that both PD-L1 and PD-L2 cell surface protein is overexpressed by flow cytometry. Two subsequent studies are ongoing. First, to identify the transcriptional basis of ligand upregulation, we have identified a suite of transcription factors using in silico analysis that harbor consensus binding sites in suspected promoter/enhancer regions and have modeled this using endogenous promoter/enhancer luciferase assays. Secondly, to determine the functional consequences of PD-L1 and/or PD-L2 overexpression in the anti-glioma immune response, we ectopically expressed these ligands in the GL-261 murine glioma cell line and tested their influence on T cell recognition of a model antigen. Together, these studies will clarify the regulation of the PD-L1/PD-L2 ligands and identify additional methods with which to decrease their immunosuppressive functions.