Possible involvement of the M2 anti‐inflammatory macrophage phenotype in growth of human gliomas

Within tumours, many non-neoplastic cells such as fibroblasts, endothelial cells, and macrophages assist tumour growth by producing various growth factors and pro-angiogenic cytokines. Various tumour-derived molecules drive tumour-associated macrophages towards an anti-inflammatory phenotype (M2) and thus promoting tumour growth. Here we investigated microglia/macrophage differentiation in glioma tissues by means of immunostaining of paraffin-embedded glioma samples. The number of microglia/macrophages with positive staining for CD163 and CD204, which are believed to be markers for M2 macrophages, was correlated with the histological grade of the gliomas. The ratio of M2 macrophages in the tumour-associated microglia/macrophages was also associated with the histological grade. Culture supernatant from the glioma cell line can stimulate macrophages to develop into the M2 phenotype in vitro. Macrophage colony-stimulating factor (M-CSF), which strongly induces M2 polarization of macrophages, was significantly correlated with histological malignancy and with the proportion of M2 microglia/macrophages in vivo. In addition, the proportion of M2 microglia/macrophages and M-CSF expression in tumour cells correlated well with proliferation of glioblastoma cells. These results suggest that tumour-derived M-CSF induces a shift of microglia/macrophages towards the M2 phenotype, which influences tumour growth. Evaluation of the proportion of M2 microglia/macrophages and M-CSF expression in tumour tissue would be useful for assessment of microglia/macrophage proliferative activity and the prognosis of patients with gliomas.