IMMU-04. OVERCOMING IMMUNE EVASION IN GLIOBLASTOMA

The negative phase 3 trial of anti-PD-1 in human glioblastoma (GBM) contradicts preclinical data showing eradication of GL261 mouse brain tumors by checkpoint inhibitors (CPI). To understand whether the tumor microenvironment contributed to this discrepancy, we immunoprofiled 19 human GBM tumors by CyTOF mass cytometry. We observed a macrophage-rich, T cell-poor microenvironment, with low PD-L1 expression. In contrast, GL261 tumors showed substantial T cell infiltration and high PD-L1 expression. Seeking a model that more accurately reflects the immune landscape of human GBM, we immunoprofiled SB28, a C57BL/6-derived transplantable model of GBM driven by PDGFB, NRASG12D, and p53 loss. CyTOF analysis of SB28 revealed a macrophage-rich, T cell-poor, PD-L1-low microenvironment that recapitulates human GBM. Surprisingly, the efficacy of CPI (anti-PD-1 + anti-CTLA-4) treatment in SB28 was site-dependent. Subcutaneous SB28 flank tumors were eradicated by CPI in all mice, whereas intracranial tumors were refractory. CyTOF analysis revealed that MHC-II+ intratumoral macrophages were twice as frequent in subcutaneous versus intracranial tumors. Similarly, dendritic cells were abundant in subcutaneous tumor-draining lymph nodes, but absent in intracranial tumor-draining (cervical) lymph nodes. Mice previously cured of subcutaneous tumors rejected intracranial tumors upon rechallenge. Taken together, these results suggest that subcutaneous tumors allow superior antigen presentation, resulting in priming and sustained immunological memory after CPI. We then investigated FLT3-ligand, an essential cytokine for dendritic cell maturation, as a therapy to improve antigen presentation in mice with intracranial tumors. We administered recombinant FLT3-ligand intraperitoneally for 10 days after engraftment of intracranial tumors in naive hosts. FLT3-ligand dramatically increased the frequency of dendritic cells and CD8+ T cells in spleens and cervical lymph nodes. Notably, FLT3-ligand monotherapy also caused durable rejection of intracranial tumors in 30% of mice. These results indicate that sufficient T cell priming can confer immunological control of GBM, even in a macrophage-rich, T cell-poor microenvironment.