IMMU-15. HEPARIN INHIBITS THE EXTRACELLULAR VESICLE-MEDIATED INDUCTION OF IMMUNOSUPPRESSIVE MONOCYTES IN GLIOBLASTOMA Free

Glioblastoma (GBM) is the most common and fatal primary brain tumor in adults. The development of novel therapies is critical, as little has changed regarding the standard of care in nearly two decades. Immunotherapy holds much promise, as treatments including chimeric antigen receptor (CAR) T cells and immune checkpoint blockade inhibitors have transformed the treatment of a number of cancers in recent years. However, GBM patients exhibit profound immunosuppression, limiting the efficacy of these therapies. Understanding the mechanisms of GBM-mediated immunosuppression is critical to overcoming this barrier. GBM-derived extracellular vesicles (EVs) have been shown to mediate the induction of immunosuppressive monocytes, which may point to a mechanism of immunosuppression. EVs make initial contact with target cells through interactions between heparan sulfate proteoglycans, and soluble heparin has been shown to inhibit these interactions in some models. We demonstrate that soluble heparin inhibits the binding of GBM-derived EVs to monocytes in a dose-dependent manner, and that heparin treatment reduces the induction of immunosuppressive monocytes upon in vitro conditioning of monocytes with GBM-derived EVs (p< 0.01). Further, we demonstrate that heparin treated EV-conditioned monocytes are functionally less immunosuppressive than untreated EV-conditioned monocytes as measured by T cell proliferation in co-culture studies (p< 0.05). Taken together, these findings underscore the import of tumor-derived EVs in immunosuppression in GBM, and demonstrate the feasibility of targeting EV-monocyte interactions in treating GBM-mediated immunosuppression.