Abstract PO087: Radiotherapy treatment in combination with Dendritic Cell Immunotherapy promotes a microglia activation and a disruption of the SIRPα-CD47 signaling axis in the GL261 glioma model
Authors: Pellegatta Serena , Di Ianni Natalia , Maffezzini Martina , Fumagalli Maria Luisa , Pinzi Valentina , Musio Silvia , Fariselli Laura , Finocchiaro Gaetano 
Published: February 2021

Abstract
Although some progress has been made in understanding GBM biology, treatment remains a challenge. There is increasing evidence that radiotherapy (RT) not only provides immunomodulatory effects on tumor microenvironment but also influences systemic immune response, supporting the advantage of combinatorial strategies with immunotherapy (IT). Using immune-competent mice in which syngeneic glioma cells are grown intracranially and treated with local fractionated radiation, we assessed the effects of RT on both local innate and adaptive immune cells. GL261-glioma bearing mice were locally irradiated with a total dose of 15 Gy in three consecutive fractions of 5 Gy on day 7, 8, and 9 after the tumor implantation. Irradiation was delivered both as exclusive (RT) and concomitant treatment in combination with dendritic cell (DC) immunotherapy (RT-IT). DCs were injected subcutaneously on day 16, 23, and 30 after tumor implantation. The potential role of RT in reprogramming the glioma-associated microglia is still poorly characterized. Microglia were isolated and enriched using CD11b microbeads from both the brain tumor hemisphere and contralateral hemisphere of RT, RT-IT, and control mice. A gene expression signature was analyzed on isolated microglia. Expression of mmp14 and trem2, involved in enhancing glioma proliferation, decreased in microglia isolated from RT mice (2.3 and 2.0-fold lower than in controls, day 16 P=0.01). The frequency of CD45dim/CD11b+/CD172A+ microglia showed an early increase at day 16 in the tumor mass and contralateral hemisphere in RT mice, but not in controls (p&lt;0.01). At later stages, the percentage of activated microglia was similar in all groups, including RT-IT mice. SIRPa can interact with CD47 on tumor cells inducing a “don’t me” signal and preventing phagocytosis. The percentage of CD47+ cells, investigated by flow cytometry, was high in gliomas from control mice (62.0±3.6%), and significantly decreased in gliomas from RT and RT-IT mice (14.9±2.2% and 9.3±0.8%, respectively, P&lt;0.005 vs controls). In RT-IT gliomas the CD47 decreased expression correlated with a robust infiltration of CD8+ T cells (65.7±3.2% vs. 22.3±0.8 in RT, 16.9±1.3% in controls, P&lt;0.005). Glioma-bearing mice treated with the combination of RT and IT survived longer than RT mice or controls (median of survival: 60 vs 19.5 vs 17 days respectively, P&lt;0.004). These preliminary results suggest a key role of radiotherapy in activating microglia and favoring an anti-tumor response induced by dendritic cells. The depletion of the CD47 expression encourages us to investigate the potential role of radiotherapy in re-educating microglia to phagocytize glioma cells.
Citation Format: Serena Pellegatta, Natalia Di Ianni, Martina Maffezzini, Maria Luisa Fumagalli, Valentina Pinzi, Silvia Musio, Laura Fariselli, Gaetano Finocchiaro. Radiotherapy treatment in combination with Dendritic Cell Immunotherapy promotes a microglia activation and a disruption of the SIRPα-CD47 signaling axis in the GL261 glioma model [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr PO087.