Controlled Release of a Mast Cell Agonist for Glioblastoma Immunotherapy

Glioblastoma (GBM) is a devastating illness that claims the lives of nearly a quarter of a million people annually. The aggressive and immunologically complex nature of GBM, as well as its location behind the blood-brain barrier, restricts the efficacy of many cutting-edge therapies. While new immunotherapeutic strategies have made significant advances for certain patient subsets, there is a critical and unmet need for better treatments. One emerging player in the tumor microenvironment (TME) is the mast cell. A granulocyte of the innate immune system, the mast cell has established pro- and anti-tumor functions across many cancers, with emerging evidence for a role in GBM. Herein we establish MCs as an overrepresented population within two immunocompetent murine GBM models, when compared to healthy contols. We further identify the effects a GBM standard-of-care treatment (SOC), resection, has on MC populations. We identify and characterize a 2nd generation MC agonist, MP12W, for its in vitro safety profile, comparing MP12W to known MC activators, including another well-studied 2nd generation agonist and a standard degranulating control. Next, we produced two scaffolds for the interstitial delivery of a MC-agonist in GBM using the acetalated-dextran (Ace-DEX) controlled release platform. These scaffolds were then investigated for their effects on survival and MC phenotypes in two models of murine GBM. We observed successful in vivo MC activation but no tumor control in either model. To understand the mechanism of treatment failure, we completed multi-organ immunophenotyping of the T-cell and myeloid compartments. Our results show that MC agonism in GBM elicits broad immunosuppression. Further, when this agonism is sustained, as is achieved with Ace-DEX scaffolds, we observed increased exhausted CD4 T-cells. Altogether this work establishes MCs as an elevated immune population in GBM impacted by the SOC. Local delivery of a MC agonist impacts regional MC populations, but encapsulation of the agonist within an Ace-DEX scaffold results in phenotypic changes in systemic MC populations. However, we identify that activation of MCs in GBM dampens local and systemic immune populations. Our work further suggests that sustained MC agonism may push key populations within the TME to adopt classic pro-tumor phenotypes.