| Summary: | The tumor microenvironment of glioblastoma is extremely immunosuppressive due to high amount of myeloid cells leading to low levels of lymphocytes. This immunosuppressive scenario makes difficult the development of curative therapies for this devastating tumor. Our group has engineered Delta-24-ACT, an oncolytic adenovirus armed with the costimulatory ligand 4-1BBL which is capable to trigger the activation of T cells and thereby, increasing the antitumor response. Since, monotherapies have not elicited survival benefit in glioblastoma, in this project; we evaluated the antitumor effect of Delta-24-ACT combined with different modulators of the immune system (targeting both lymphoid and myeloid cell populations).
Delta-24-ACT was able to infect and kill murine (GL261-5 and CT-2A) and human (U87-MG and U251-MG) glioma cell lines, while maintaining its replication in the latter. Of importance, after infection with Delta-24-ACT, the 4-1BBL not only was detected on the membrane of glioma cells but also it was able to stimulate CD8 T cells in vitro. These data suggested its potential to trigger an effective immune response. Furthermore, in vivo, Delta-24-ACT significantly increased the median survival and led to long-term survivors in three different orthotopic glioma models. However, the virus did not generate antiglioma memory immune response. In order to improve the survival and to generate immune memory, we combined the virus with different immune modulating approaches (dendritic cell vaccination, IDO inhibitor, macrophages inhibitor etc). However, no differences in survival benefit were observed when compared to Delta-24-ACT.
Since Delta-24-ACT treatment led to the expression of higher levels of PD-1 in T cells, we combined Delta-24-ACT with an anti-PD-L1 antibody. This combination not only alleviated the exhaustion phenotype showed by the T-cells but in addition, resulted in an increase survival and the acquisition of antiglioma memory immune response. In summary, our data demonstrated that Delta-24-ACT exerts a potent antitumor response in vitro and in vivo. Moreover, the different proposed combination strategies induce survival benefit in mice as a result of the recruitment of immune cell populations modulating the immunosuppressive tumor microenvironment of gliomas. Of importance, Delta-24-ACT in combination with PD-L1 blockade displayed the best therapeutic effect.
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