Sustained Accumulation of Blood-Derived Macrophages in the Immune Microenvironment of Patients with Recurrent Glioblastoma after Therapy

Simple Summary Glioblastoma (GBM) is the most aggressive type of brain cancer and, despite standard treatments, recurrence is inevitable. The immunosuppressive microenvironment, characterized by an intense recruitment of myeloid cells and a low frequency of anti-tumor lymphocytes, hampers the success of new immunological therapies. Thus, understanding how treatments impact the tumor microenvironment is crucial to limit recurrences. In this study, we compared the immune profile in the central and in the marginal areas of resected tumors on a cohort of patients with primary or relapsing GBM and identified a different immune composition according to tumor localization. In addition, levels of four subsets of myeloid-derived suppressor cells were determined before and after standard therapy. Significant correlations were obtained by combining data collected from tumor and blood, thus reinforcing the notion that immunosuppression should be evaluated both in the circulation and in the tumor microenvironment, to circumvent this phenomenon from a therapeutic point of view. The cell composition of the glioblastoma (GBM) microenvironment depends on the recruitment of myeloid cells from the blood, promoting tumor progression by inducing immunosuppression. This phenomenon hampers immunotherapies and investigating its complexity may help to tailor new treatments. Peripheral blood and tissue specimens from the central and marginal tumor areas were collected from 44 primary and 19 recurrent GBM patients. Myeloid and lymphoid cell subsets and the levels of immunosuppressive markers were defined by multiparametric flow cytometry. Multiplexed immunohistochemistry was used to confirm the differences in the immune infiltrate and to analyze the cell spatial distribution. Relapsing GBM showed an increased presence of blood-derived macrophages in both tumor areas and a higher frequency of infiltrating lymphocytes, with a high level of exhaustion markers. The expansion of some myeloid-derived suppressor cell (MDSC) subsets in the blood was found in both primary and recurrent GBM patients. A significant inverse correlation between infiltrating T cells and an MDSC subset was also found. In patients with recurrent GBM after standard first-line therapy, the immune-hostile tumor microenvironment and the levels of some MDSC subsets in the blood persisted. Analysis of the immune landscape in GBM relapses aids in the definition of more appropriate stratification and treatment.