Programmed death 1 (PD-1, CD279) and programmed death ligand 1 (PD-L1, CD274) 
are involved in generating tumor-associated immunosuppression by suppression of 
T-cell proliferation and interleukin 2 (IL-2) production and immune checkpoint 
inhibitors targeting these molecules are showing compelling activity against a 
variety of human cancers. PD-L1 expression has shown a positive association with 
response to PD-1 inhibition in noncentral nervous system (CNS) tumors, e.g., 
melanoma or non-small cell lung cancer, and is discussed as a potential 
predictive biomarker for patient selection in these tumor types. This review 
summarizes current knowledge and potential clinical implications of PD-L1 
expression in glioblastoma. At present, the following conclusions are drawn: (a) 
functional data support a role for PD-1/PD-L1 in tumor-associated 
immunosuppression in glioblastoma; (b) the incidence of PD-L1-expressing 
glioblastomas seems to be relatively high in comparison to other tumor types, 
however, the reported rates of glioblastomas with PD-L1 protein expression vary 
and range from 61 to 88%; (c) there is considerable variability in the 
methodology of PD-L1 assessment in glioblastoma across studies with 
heterogeneity in utilized antibodies, tissue sampling strategies, 
immunohistochemical staining protocols, cut-off definitions, and evaluated 
staining patterns; (d) there are conflicting data on the prognostic role and so 
far no data on the predictive role of PD-L1 gene and protein expression in 
glioblastoma. In summary, the ongoing clinical studies evaluating the activity 
of PD-1/PD-L1 inhibitors in glioblastoma need to be complemented with well 
designed and stringently executed studies to understand the influence of 
PD-1/PD-L1 expression on therapy response or failure and to develop robust means 
of PD-L1 assessment for meaningful biomarker development.