P18.09.B REPLACEMENT OF MICROGLIA AND TUMOR-ASSOCIATED MACROPHAGES IN EXPERIMENTAL MODELS OF BRAIN MALIGNANCIES

Abstract BACKGROUND Tumor-associated macrophages (TAM) represent a main component of the tumor microenvironment of brain malignancies and promote a pro-tumorigenic, immunosuppressive state. Modifying this myeloid cell population might hold therapeutic potential. Here we investigated methods for high-level integration of grafted hematopoietic myeloid cells into the developing tumor microenvironment of murine experimental models of high-grade glioma and brain metastasis. MATERIAL AND METHODS C57BL/6J mice underwent bone marrow transplantations (BMT) using GFP-labeled donor cells. Replacement of endogenous microglia with donor-derived myeloid cells was achieved through additional pharmacological inhibition of the colony stimulating factor 1 receptor (CSF1R). Transplanted mice were then orthotopically inoculated with syngeneic tumor cell lines of glioma or breast cancer metastasis. The extent of donor cell integration was determined with immunofluorescent analysis. The transcriptomic profile of donor-derived myeloid cells was further characterized using single cell RNA sequencing. RESULTS We found strong integration of donor-derived myeloid cells into the tumor core in all BMT animals across all tumor models. Additional CSF1R inhibition led to an increase in the chimerism of donor-derived myeloid cells in peritumoral and distant brain areas. Single cell RNA sequencing demonstrated that graft-derived myeloid cells adapt transcriptional states equivalent to cells of endogenous origin including a TAM cluster, which we confirmed histologically. We observed a small but significant survival extension in glioma animals with high-level microglia replacement [BMT with CSF1R inhibition] compared to animals with BMT only (median survival: 26 vs 22 days, p = 0.014 log-rank test). CONCLUSION We demonstrated that BMT was sufficient to promote a virtually complete integration of graft-derived cells in the tumor microenvironment, while augmented microglia replacement increased extratumoral engraftment. The latter might be of functional relevance, as we observed slightly improved survival with combined BMT and CSF1R inhibitor treatment preceding glioma inoculation.