Translocator protein (TSPO) expression in neoplastic cells and tumor-associated macrophages in meningiomas.

Meningiomas are the most common primary intracranial tumors and show extensive infiltration of macrophages. The mitochondrial membrane protein translocator protein (TSPO) has been used as an in vivo marker of microglia and macrophage activation to visualize neuroinflammation. However, it is unknown which cell types express TSPO in meningiomas. Immunohistochemistry of 38 WHO grade 1-3 meningiomas was subjected to segmentation and deep learning classification of TSPO expression to either Iba1-positive tumor-associated macrophages (TAMs) or all other (mainly neoplastic) cells. A possible association between clinical data and TSPO expression intensities was also investigated. TAMs accounted for 15.9%-26% of all cells in the meningioma tissue. Mean fluorescence intensity of TSPO was significantly higher in TAMs (p < 0.0001), but the mass of neoplastic cells in the tumors exceeded that of TAMs. Thus, the summed fluorescence intensity of TSPO in meningioma cells was 64.1% higher than in TAMs (p = 0.0003). We observed no correlation between TSPO expression intensity and WHO grade. These results indicate that both macrophage-lineage and neoplastic cells in meningiomas express TSPO and that the SPECT-TSPO signal in meningiomas mainly reflects the latter; TSPO is expressed equally in parenchymal activated and resting macrophage/microglia lineage cells.