Programmed death ligand 1 gene silencing in murine glioma models reveals cell line-specific modulation of tumor growth in vivo.

Glioblastoma is the most common brain tumor in adults and virtually incurable. Therefore, new therapeutic strategies are urgently needed. Immune checkpoint inhibition has not shown activity in various phase III trials and intra- as well as intertumoral expression of programmed death ligand 1 (PD-L1) varies in glioblastoma. We abrogated constitutive PD-L1 gene expression by CRISPR/Cas9 in murine glioma models and characterized the consequences of gene deletion in vitro and in vivo. A heterogeneous expression of Pdl1 mRNA and PD-L1 protein was detected in the glioma cell panel in vitro and in vivo. PD-L1, but not PD-L2, was inducible by interferon β and γ. Co-culture with splenocytes induced PD-L1 expression in GL-261 and SMA-560, but not in CT-2A cells, in an interferon γ-dependent manner. Conversely, Pdl1 gene silencing conferred a survival benefit in CT-2A, but not in the other 2 models. Accordingly, PD-L1 antibody prolonged survival in CT-2A glioma-bearing mice. This activity required PD-L1 expression on tumor rather than host cells, and the survival gain mediated by PD-L1 loss was reproduced in immune-deficient RAG-/- mice. PD-L1 is expressed and interferon-inducible in murine glioma cell lines. PD-L1 has model-specific roles for tumor growth. Future studies need to determine which subset of glioblastoma patients may benefit from PD-L1 antagonism as part of a multimodality therapeutic approach to glioblastoma.