Characterizing the functional phenotype of infiltrating macrophages in meningiomas
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Meningiomas are the most common primary brain tumors, yet few successful non-invasive treatment options are available for patients. Immunotherapy has provided new insights into treatments for solid tumors. The immune infiltrate of meningiomas has been broadly characterized, showing a significant monocytic cell content, but little is known about the phenotype and function of these myeloid cells within the tumor environment. As circulating monocytes differentiate into macrophages with highly plastic character within tissue, it remains to be seen how the macrophages in meningiomas are influencing the tumor. As many studies have described the presence of monocytic subpopulations within other solid tumors, we hypothesize that meningiomas contain two populations of myeloid cells: a pro-inflammatory macrophage-like population, and an immunosuppressive myeloid-derived suppressor cell (MDSC)-like population. We collected fresh tumor samples and processed them into a single-cell suspension. The cells were then stained with fluorescently labeled surface marker antibodies commonly found on macrophages and MDSCs. We used flow cytometry to quantify the myeloid populations, sorted the populations with a FACSAria™, and analyzed their gene expression profiles with NanoString® and TaqMan®. Two distinct myeloid populations were found in all analyzed tumor samples, varying in macrophage-like to MDSC-like ratios from tumor to tumor. Gene expression analysis of these populations confirmed the sorting strategy and provided new clues into the identity and function of the myeloid cell populations infiltrating meningiomas. NanoString® results confirmed a high HLA-DR gene expression in the HLA-DR+ sorted populations. The tumor HLA-DR+ population was found to have higher gene expression relative to the HLA-DR- population for chemoattractants such as IL8, CCL3, and CCL4. Compared to a healthy blood monocyte control, tumor myeloid cells expressed higher levels of the genes C3AR1, ROCK2, IL10, NOS2, IL18, and CSF2. Finally, qPCR analysis and NanoString® results showed high expression of the gene IL6 in the non-immune cell tumor cells (CD45-). The IL6 cytokine has been shown to induce MDSCs. These findings may have significant implications in identifying new targets in immunotherapy to stop tumor growth and increase survival outcomes.