RESEARCH ARTICLE The PD-1/B7-H1 Pathway Modulates the Natural Killer Cells versus Mouse Glioma Stem Cells

Purpose Glioblastoma multiforme (GBM) is the most malignant primary type of brain tumor in adults. There has been increased focus on the immunotherapies to treat GBM patients, the thera-peutic value of natural killer (NK) cells is still unknown. Programmed death-1 (PD-1) is a major immunological checkpoint that can negatively regulate the T-cell-mediated immune response. We tested the combination of the inhibiting the PD-1/B7H1 pathway with a NK-cell mediated immune response in an orthotopic mouse model of GBM. Methods and Materials Mouse glioma stem cells (GL261GSCs) and mouse NK cells were isolated and identified. A lactate dehydrogenase (LDH) assay was perfomed to detect the cytotoxicity of NK cells against GL261GSCs. GL261GSCs were intracranially implanted into mice, and the mice were stratified into 3 treatment groups: 1) control, 2) NK cells treatment, and 3) PD-1 inhib-ited NK cells treatment group. Overall survival was quantified, and animal magnetic reso-nance imaging (MRI) was performed to determine tumor growth. The brains were harvested after the mice were euthanized, and immunohistochemistry against CD45 and PCNA was performed. Results The mouse NK cells were identified as 90 % CD3- NK1.1+CD335+ by flow cytometric analy-sis. In the LDH assay, the ratios of the damaged GL261GSCs, with the E:T ratios of 2.5:1