Chemoradiation-Altered Micromilieu of Glioblastoma Cells Particularly Impacts M1-Like Macrophage Activation

Glioblastoma multiforme (GBM) is a highly aggressive brain tumor with an overall poor prognosis due to its immunosuppressive tumor microenvironment (TME). Microglia and tumor-associated macrophages (TAMs) with pro-tumorigenic properties are dominant populations of immune cells in the GBM TME. To date, several studies targeting TAMs to fight tumor progression in different tumor entities have been initiated. However, the impact of standard therapy schemes of glioblastoma cells on macrophage polarization, activation, and phagocytosis remains controversial. The same applies to the relevance of PD-1/PD-L1 blockade in macrophage and tumor cell interaction. Our study therefore investigated patient-oriented treatment of GBM on the phagocytic capacity of polarized M1- and M2-like macrophages with GL261-luc2 tumor cells as preclinical model system. In addition, we analyzed the expression of activation and immune checkpoint markers on these macrophage subtypes after contact with tumor cells and their microenvironment. These factors were also determined after PD-1 blockade. The analyses revealed that the immunoregulatory M2-like macrophages generally exhibited a higher phagocytosis rate than the pro-inflammatory M1-like macrophages, which was however not influenced by pretreatment of glioblastoma cells with chemo- or radiotherapy. This could not be improved by blocking the PD-1 receptor. Furthermore, there were no modulations in the expression of differentiation, activation, or immune checkpoint molecules of M1- and M2-like macrophages after cell-to-cell contact with glioblastoma cells. But, the medium conditioned by tumor cells strongly altered M1-like macrophages towards a more activated state, whereas M2-like cells were only mildly influenced. This was further enhanced by tumor cell treatment, with the most prominent effect after irradiation. These results suggest an impact of conventional GBM tumor cell treatment on the immunogenic status of macrophage subtypes, which is of relevance for increasing anti-tumor immune response in brain tumors.