Project 1: Active immunotherapy combined with checkpoint modulation for glioblastoma SUMMARY/ABSTRACT The lack of effective treatments for glioblastoma (GBM) patients remains a significant health problem and highlights the need for novel and innovative approaches. Immunotherapy is an appealing strategy because of the potential ability for immune cells to traffic to and destroy infiltrating tumor cells in the brain. Pre-clinical studies and clinical trials of dendritic cell (DC) vaccination for GBM have shown some promising results, but also some treatment failures. The broad overall goals of this research project are to investigate mechanisms of immune evasion following active immunotherapy, and to develop rational combinations of immunotherapeutic strategies to overcome the immunosuppressive milieu of the brain tumor microenvironment. Our new preliminary data strongly suggests that active immunotherapy with DC vaccination may create a pro-inflammatory tumor microenvironment that induces the immigration of immunosuppressive antigen presenting cells (iAPC), which express high levels of PD-L1 and IL-10. We show that these cells are phenotypically similar to the iAPC that dominantly influence the T-cell response to chronic viral infection, and may act to counteract effective T-cell responses induced by DC vaccination via a mechanism involving PDL1/PD-1. Furthermore, inhibition of iAPC using an anti-PD1 mAb (Nivolumab, BMS) or a CNS penetrant inhibitor of CSF-1R (PLX-3397, Plexxikon), in conjunction with tumor lysate-pusled DC vaccination (DC-Vax-L), resulted in significantly prolonged survival in tumor-bearing animals with well-established intracranial (i.c.) gliomas. We therefore postulate that clinically relevant anti-tumor immunity to glioblastoma (GBM) must have two cellular components: 1) significant infiltration of tumor-specific tumor-infiltrating lymphocytes (TIL); and 2) blockade of immune-regulatory antigen presenting cell (iAPC) function within the tumor microenvironment. As such, our hypothesis is that the local cellular interactions between iAPC and T lymphocytes within the brain tumor microenvironment is a critical factor influencing the efficacy of immunotherapies in glioblastoma patients. A better understanding of the biology of these cellular interactions will provide insight into more effective ways to induce therapeutic anti-tumor immune responses for this deadly type of brain tumor. In Aim 1, we will study the mechanisms by which iAPC limit glioma- specific anti-tumor immune responses in vitro and in vivo. In Aim 2, we will evaluate the efficacy of combining tumor lysate-pulsed DC vaccination (to induce T-cell infiltration into tumors) with immune checkpoint inhibition and other novel immunoregulatory targets (to block iAPC function) in pre-clinical syngeneic animal models of glioblastoma, and explore the use of a novel PET tracers as non-invasive imaging biomarkers of immune response. Finally, in Aim ...