Lmap-07 potential mechanism of immune escape related to tumor-associated macrophage in glioblastoma after combined therapy with radiotherapy and immune checkpoint blocker

Abstract Immune checkpoint blockers (ICBs) have revolutionized the treatment of cancer. Radiotherapy (RT) has been demonstrated to increase the survival benefit of ICBs in some cancers through its local and systemic immunomodulation, although its efficacy in glioblastoma (GBM) is very limited. With a thorough investigation of the immune responses to single and dual treatment utilizing single-cell RNA-seq analyses on the synergy of RT and anti-PD-L1 antibody (aPD-L1), we studied the potential mechanism of immune escape in GBM even when RT and ICB are combined. Our findings demonstrated a statistically significant longer survival of mice treated with RT and aPD-L1 combined than either single modality of treatment in an orthotopic syngeneic GBM model achieving a 70% cure rate. In the tumor tissues collected from the mouse brain that failed dual treatment, i.e., immune escaped, immune cell profiling still demonstrated much lower T regulatory cells and higher CD8+ cytotoxic T cells infiltrated to tumor, and elevated CD8+/CD4+ and CD8+/Treg ratios in the dual therapy group compared to single therapies, as expected with synergistic effects of dual treatment. RT with aPD-L1 markedly reduced CD8+ and CD4+ T exhaustion cells and elevated T-memory cells in comparison to single modality groups. Nevertheless, compared to the RT alone or no treatment control group, dual therapy failed to decrease the proportion of Arginase1-positive glioma-associated macrophages (GAM) in macrophage/monocytes in tumor microenvironment, which were considerably increased if treated with aPD-L1 only. Moreover, the combination of RT with aPD-L1 did not alter immunosuppressive gene expression in the macrophage/monocyte groups. In conclusion, our research demonstrated that RT and aPD-L1 together may elicit a potent and durable antitumor immune response in the GBM mouse model through regulating lymphocytes but not macrophages. Our findings point to the urgent need for research into GAM-modulating drug and their potential to prevent immunological escape.