Microglia-specificity of different markers is overridden in glioblastoma specimens

Microglia are the resident immune cells of the central nervous system. When glioblastoma develops, microglia and peripheral macrophages accumulate within the tumor tissue. Since both cell populations presumably exhibit different functional properties with anti- and pro-tumor characteristics, it is essential to distinguish between these cell populations accurately. The widespread and extensive use of single-cell technologies allowed the discovery of novel microglial markers like Sall1, Tmem119, P2ry12 and Hexb that were used for discrimination of populations. In our study, these markers were tested on protein level using immunofluorescence staining. This method permits easy identification by co-staining with IBA1 as lineage marker for myeloid cells. Bone marrow chimeras served as differentiation control. Staining showed ubiquitous marker expression of microglia in both murine naïve brains and human epilepsy specimens. However, experiments using a murine glioblastoma model demonstrated that macrophages can also express these markers after infiltrating the brain and tumor tissue. Furthermore, the level of expression varies spatially, decreasing towards the intratumoral area. In vitro experiments confirmed positive staining results for both microglia and macrophages. Furthermore, cultivation with tumor-conditioned medium led to downregulation of microglial markers. Consequently, the concept that SALL1, TMEM119, P2RY12, and HEXB are exclusive markers for microglia depends strongly on the experimental and pathological conditions, and should not be generalized. Under neuroinflammatory conditions, cell specificity of the examined markers largely diminishes and both microglia and macrophages show ubiquitous expression. Considering this fact should lead to a context dependent application when using these markers to identify myeloid cell populations within the brain.