The roles of microglia/macrophages in tumor progression of brain cancer and metastatic disease

Malignant brain tumors and brain metastases are highly aggressive diseases that are often resistant to treatment. Consequently, the current prognosis of patients with brain tumors and metastases is dismal. Activated microglia and macrophages are often observed in close proximity to or within the malignant tumor masses, suggesting that microglia/macrophages play an important role in brain tumor progression. Microglia, being resident macrophages of the central nervous system, form a major component of the brain immune system. They exhibit anti-tumor functions by phagocytosis and the release of cytotoxic factors. However, these microglia/macrophages can be polarized into becoming tumor-supportive and immunosuppressive cells by certain tumor-derived soluble factors, thereby promoting tumor maintenance and progression. The activated microglia/macrophages also participate in the process of tumor angiogenesis, metastasis, dormancy, and relapse. In this review, we discuss the recent literature on the dual roles of microglia/macrophages in brain tumor progression. We have also reviewed the effect of several well-known microglia/macrophages-derived molecules and signals on brain tumor progression and further discussed the potential therapeutic strategies for targeting the pro-tumor and metastatic functions of microglia/macrophages.