EXTH-79. BREAKING THE BARRIER: ADOPTIVE CELLULAR THERAPY HALTS GLIOMA IMMUNOSUPPRESSION BY TARGETING MYELOID-DERIVED SUPPRESSOR CELL MIGRATION AND CELL CYCLE MECHANISMS

Abstract INTRODUCTION Our group previously demonstrated that adoptive cellular therapy (ACT) significantly increased overall survival in mice across several brain cancer models. ACT remodels the tumor microenvironment (TME) in a pleiotropic manner, and specifically depletes immunosuppressive cells like myeloid-derived suppressor cells (MDSCs) which are a significant barrier to efficacious immunotherapy responses. This led us to hypothesize that ACT depletes MDSCs from the glioma microenvironment by inhibiting MDSC proliferation and migration, thereby overcoming glioma immunosuppression. METHODS We quantified MDSC cell cycle, apoptosis, and localization in the TME and peripheral lymphoid tissues using flow cytometry as well as chemokines in the TME from glioma-bearing mice treated with ACT. C57BL/6J mice intracranially received KR158b-luciferase glioma and were treated with ACT. After treatment, MDSC cell cycle was quantified using bromodeoxyuridine and 7-aminoactinomycin to define cell cycle phases. MDSC apoptosis and cell death states were quantified by measuring annexin-V and propidium iodide. Anatomic localization of MDSCs was measured by comparing host (CD45.2) and transferred HSC-derived cell markers (CD45.1) in the TME and spleen. A multiplex array was performed using tumor lysates to quantify cytokine and chemokine levels within the TME. RESULTS We observed that MDSCs in the TME were significantly less proliferative and exhibited significantly increased levels of apoptosis and cell death after ACT. HSC-derived MDSCs were significantly enriched in the spleen of ACT-treated mice, but both transferred HSC-derived MDSCs and host MDSCs were depleted in the TME. Finally, ACT significantly decreased several chemokines in the TME, including interleukin-16 and monocyte chemoattractant protein-5. CONCLUSION These results indicate that ACT shifts MDSC cell cycle states away from proliferation towards cell death, and inhibits peripheral recruitment to the TME. Our investigation revealed novel, myeloproliferative chemokines that are decreased by ACT, representing a novel mechanism underlying disrupted MDSC migration to the glioma microenvironment and ultimately to overcoming glioma-mediated immunosuppression.