The immune suppressive microenvironment of human gliomas depends on the accumulation of bone marrow-derived macrophages in the center of the lesion.

Background. Systemic and local immune suppression plays a significant role in glioma progression. Glioma microenvironment contains both brain-resident microglial cells (MG) and bone marrow-derived macrophages (BMDM), but the study of their functional and immune regulatory activity requires accurate identification and isolation to collect pure populations. Methods. Myeloid and lymphoid infiltrate were characterized in grade II, III and IV gliomas by multicolor flow cytometry, along with the composition of the cell subsets of circulating myeloid cells. Macrophages were sorted and tested for their immunosuppressive ability. Moreover, following preoperative administration of 5-aminolevulinic acid to patients, distinct areas of tumor lesion were surgically removed and analyzed, based on protoporphyrin IX fluorescence emission. Results. The immune microenvironment of grade II to grade IV gliomas contains a large proportion of macrophages and a small proportion of lymphocytes expressing markers of dysfunctional activity. BMDM and resident MG cells were identified through a combination of markers, thus permitting their geographical identification in the lesions, their sorting and subsequent characterization of the functional characteristics. The infiltration of BMDM has the highest percentages in grade IV gliomas, and it increases from the periphery to the center of the lesion, where it exerts a strong immunosuppression that is, instead, absent in the marginal area. By contrast, MG shows little or no suppression. Functional differences, such as iron metabolism and phagocytosis, characterize resident versus blood-derived macrophages. Significant alterations of circulating monocytes are present in grade IV patients, correlating with accumulation of tumor macrophages. Conclusions. Grade IV gliomas have an alteration of both circulating and tumor-associated myeloid cells and, differently from grade II and III gliomas, show a significant presence of blood-derived immune suppressive macrophages. BMDM and MG show different functional properties.