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Oncolytic viruses have become promising therapeutic candidates to treat gliomas. Our group has developed Delta-24-ACT, an oncolytic adenovirus armed with the positive costimulatory ligand 4-1BBL which is capable to trigger the activation of T cells and thereby increase their antitumor immune response. Here we evaluate the anti-glioma effect of Delta-24-ACT alone or in combination with an immune checkpoint inhibitor. We observed that Delta-24-ACT was able to infect and kill murine glioma (GL261-5 and CT-2A) and also human glioma cell lines (U87-MG and U251-MG), while maintaining its replication in the latter. Of importance, Delta-24-ACT infection resulted in 4-1BBL expression on the membrane of glioma cells. Moreover, this ligand was functional and was able to stimulate CD8 lymphocytes in vitro, suggesting the potential of Delta-24-ACT to trigger an effective immune response. Furthermore, in vivo Delta-24-ACT showed anti-tumour effect in two murine glioma models by significantly increasing the median survival time and leading to long-term survivors. Mechanistic studies demonstrated an increase of the T cell infiltration and the activation of different immune cell populations by flow cytometry and a decrease of proliferative cells and tumour vessels by immunohistochemistry on FFPE brain samples. Importantly, the infiltrating lymphocytes also showed signs of exhaustion increasing the amount of IL-10 and the expression of PD-1. To overcome this exhaustion we combined Delta-24-ACT with an anti-PD-1 antibody. Evaluation of this combination in vivo further increased the median survival time of treated tumor-bearing mice and resulted in 50% long-term survivors. Rechallenge studies with the same cell line showed that combination treatment effectively protected these animals of developing tumors and therefore, the acquisition of immune memory. In summary, our data demonstrated that Delta-24-ACT induces a potent anti-tumour effect in vitro and in vivo as a result of the recruitment of immune cell populations modulating the immunosuppressive tumour microenvironment of glioma.

IMMU-21. THE COMBINATION OF DELTA-24-ACT WITH AN IMMUNE CHECKPOINT INHIBITOR RESULTS IN ANTI-GLIOMA EFFECT AND IMMUNE MEMORY