Introduction: One of the reasons for limited success with immunotherapy in glioblastoma has been the negative regulation of activated T cells. PD-L1, otherwise known as B7-H1, the third member of a transmembrane glycoprotein of the B7 family of costimulatory molecules, has been reported to be responsible for glioma immunosuppression.

Methods: We assessed the expression of PD-L1 on human glioma cancer stem cells in immunodeficient mice and in culture in the presence or absence of interferon gamma. 8 glioma cancer stem cell lines and their daughter cells were tested for PD-L1 expression in the presence and absence of interferon gamma. Co-cultures with human CD4 and CD8 cells were performed with each of the CSC lines and their differentiated daughter cells to determine the effect of PD-L1 expression on T cell survival.

Results: PD-L1 is clearly expressed by glioma cells during their infiltration into the brain tissues of non-obese diabetic (NOD) mice that were injected with undifferentiated glioma cells. We observed higher expression of PD-L1 in differentiated glioma cells than in undifferentiated cells after addition of proinflammatory cytokine, IFN-gamma. Only differentiated glioma CSCs induce T cell killing preferentially of CD8 cytotoxic T cells more than CD4 helper T cells (p <0.05). The T cell survival was restored when an anti-PD-L1 antibody was incubated with the glioma cells prior to coculture with T cells.

Conclusions: PD-L1 is highly expressed in differentiated glioma cells in comparison to undifferentiated cell brain cancer stem-like cells. In the setting of inflammation, PD-L1 is overexpressed and induces CD8 > CD4 T cell killing. This mechanism of T cell suppression is invoked by daughter cells rather than cancer stem cells and may make CSCs more susceptible to T cell killing if CSC antigens can be differentially targeted.

Patient Care: By understanding the mechanism of T cell immunosuppression by glioma cancer stem cells and their daughter cells, strategies to target these tumors may be developed and implemented to improve patient outcomes in cancer vaccine strategies.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of immunosuppression in glioma immunotherapy 2) Discuss the role of PD-L1 expression in glioma immunosuppression 3) Identify an effective treatment strategy to target glioma cancer stem cells.

References: 1. Yuan X, Curtin J, Xiong Y, Liu G, Waschsmann-Hogiu S, Farkas DL, Black KL, Yu JS, Isolation of cancer stem Cells from adult glioblastoma multiforme. Oncogene 23(58):9392-400, 2004. 2. Liu G, Yuan X, Zeng Z, Tunici P, Ng H, Abdulkadir IR, Lu L, Irvin D, Black KL, Yu JS. Analysis of gene expression and chemoresistance of CD133+ cancer stem cells in glioblastoma. Mol Cancer. 5:67, 2006. 3. Xu Q, Liu G, Yuan X, Xu M, Wang H, Ji J, Konda B, Black KL, Yu JS. Antigen-Specific T Cell Response from Dendritic Cell Vaccination Using Cancer Stem-like Cell-Associated Antigens. Stem Cells. 27(8):1734-40, 2009.