IMMU-37. SINGLE-CELL SYSTEMS NEUROIMMUNOLOGY REVEALS IMMUNOSUPPRESSIVE CORRELATES WITH VENTRICULAR STEM CELL NICHE CONTACT IN HUMAN GLIOBLASTOMA

Glioblastomas make up more than 60% of adult primary brain tumors and carry a median survival of less than 15 months despite aggressive standard therapy. Immunotherapy, which is now standard of care for many solid tumors, offers an appealing therapeutic approach that may improve outcomes for glioblastoma patients. Predictive features in glioblastomas that may inform responsiveness to different immunotherapeutic modalities, however, are still lacking. Recent studies have demonstrated that patients whose tumors show radiographic contact with the lateral ventricles, and thus the stem cell niche of the ventricular-subventricular zone (V-SVZ), have reduced survival outcomes compared to patients whose tumors do not contact the V-SVZ. We therefore hypothesized that tumor contact with the V-SVZ engenders a unique, immunosuppressive microenvironment that promotes tumor growth by suppressing anti-tumor immunity. Glioblastoma tumors, obtained in accordance with the Declaration of Helsinki and with institutional IRB approval (#131870, #030372, #181970), were disaggregated into single-cell suspensions and multi-dimensional single-cell mass cytometry was performed to interrogate >30 immune parameters in thirteen immune populations infiltrating human glioblastomas. Using advanced computational dimensionality-reduction tools (Citrus, t-SNE, FlowSOM, and MEM), we identified distinctions among the abundance and phenotypes of tumor-infiltrating immune cells. Firstly, on the basis of tumor contact with the V-SVZ, Citrus identified differential abundance of five T and myeloid cell subsets among glioblastomas. Secondly, differential expression of five functional immune markers was observed in seven distinct immune cell subsets infiltrating glioblastoma tumors. Further, both immune abundance and marker expression correlated with patient outcome. Manual gating analysis and parallel computational pipelines confirmed that comparable cell subsets could be identified with traditional approaches and unsupervised algorithmic analysis. These results provide key insights into the immune microenvironment of glioblastomas. In addition, several clinically actionable immunotherapeutic targets were uncovered that may be used to optimize treatment strategies for glioblastomas based on V-SVZ contact status.