Abstract
The GBM microenvironment is composed of numerous non-neoplastic cells, among which tumor-associated myeloid cells (TAMs) are the most abundant. TAMs are constituted of several different cell types, including brain microglia and bone-marrow-derived infiltrating cells, which can be further divided into inflammatory monocytes, monocyte-derived macrophages, and polymorphnuclear neutrophils. These myeloid cells are believed to promote tumor growth and immune evasion by expressing immune checkpoint proteins. Remarkable progress has been made in treating lung cancer and melanoma by using immune-checkpoint inhibitors; however, mixed results were obtained for its application in GBM. To examine how molecular-subtypes of GBM influence the composition and immunosuppression of tumor-associated myeloid cells, we used the RCAS/tv-a system, a somatic cell-specific gene transfer system, to generate de novo murine GBM (mGBM). By manipulating different known human oncogenic-driver mutations such as overexpressing PDGFB or EGFRVIII, or silencing NF1, we generated Proneural (PN), Classical (CL) and Mesenchymal (MES) GBMs respectively, which phenotypically resemble their human counterparts. We found that the majority (80%) of tumor-infiltrating myeloid cells in PN GBM are bone-marrow derived macrophages, whereas this number is significantly less in CL and MES mGBM. Interestingly, unique to MES, there is a significant presence (16%) of neutrophils in these tumors than in other subtypes (4%). Further characterization for subtype-specific expression of PD-L1, a druggable immune checkpoint molecule, by flow cytometry shows that MES has the highest expression of PD-L1 (Mean Fluorescent Intensity=13,000 ± 3,000), whereas the expression is lower (MFI=8,100 ± 2,000) in PN and lowest (MFI=5,900 ± 2,000) in CL. The current effort is devoted to examining how myeloid cells isolated from different subtypes can differentially suppress T cell functions; and how mice carry these subtypes respond to anti-PD-L1 therapy. Our findings illuminate the unique composition and functions of tumor-associated myeloid cells in different GBM subtypes, establishing a rationale to target these cells in this heterogeneous neoplasm.