MODL-18. ESTABLISHMENT OF A NOVEL HUMANIZED PERIPHERAL BLOOD MONONUCLEAR CELL (HU-PBMC) MOUSE MODEL TO EVALUATE LYMPHOCYTE INVOLVEMENT IN GLIOBLASTOMA AND ITS TREATMENT

Abstract Glioblastoma (GBM) has traditionally been classified as an ‘immune cold’ tumor, characterized by strong immunosuppression, low mutational burden, and few presentable antigens. However, lack of standard-of-care improvement and the discovery of targetable immune-modulatory biomarkers like PD-1/PD-L1 and CTLA4 in the GBM microenvironment has prompted clinical trials to explore immune checkpoint inhibition (ICI) efficacy in patients, with moderate responses in specific patient subsets. To better characterize ICI in GBM, and to match prospective patients to suitable therapeutics, it is necessary to establish preclinical GBM mouse models that incorporate the lymphocytic milieu as xenograft and syngeneic models are unable to replicate the immune responses seen in humans. We have established a novel humanized PBMC mouse model that recapitulates human GBM pathology in the brain, supports and maintains systemic and intracranial human T cell engraftment in the presence of GBM tumors, and allows for a suitable timeline for preclinical testing of therapeutics prior to onset of GVHD. In our study, orthotopic GBM-bearing NSG mice showed successful human CD45+ engraftment in the blood (~20%) and brain (~5%) 2 weeks post intravenous implantation of hu-PBMCs. Restricted tumor growth was observed, indicating initial responsiveness of human immune cells to GBM. Engraftment remained unaffected upon treatment with temozolomide (GBM standard of care), and persisted over 6 weeks (brain ~40%, blood ~70%) before GVHD onset (week 7). ~90% of the hCD45+ population was CD3+, indicating robust T cell engraftment, which was further characterized into CD4+ and CD8+ subsets and by activation status (HLA-DR, CD45RA/RO, PD-1, FOXP3 etc) within the tumor-bearing brain and blood. Anti-PD-1 inhibitor efficacy was also tested in engrafted animals to observe ICI potential in GBM. This humanized mouse provides a powerful translational model to evaluate T cell responses in tumor pathology and immunotherapy in GBM and potentially other primary or secondary brain malignancies.