Tumor-associated macrophages: Potential therapeutic targets for anti-cancer therapy

The macrophage is known to be a multifunctional antigen presenting cells and playing a central role in inflammation. Macrophages infiltrate into malignant tumor tissues in high numbers (the so-called tumor-associated macrophages [TAMs]) and many studies over the past decade have demonstrated that macrophages have protumor functions and are closely related to tumor progression. It has been shown that protumor macrophages that have differentiated through interaction with tumor cells are involved in stem cell niches, immunosuppression, invasion, and metastasis. Consistent with these functions, studies using human tumor samples have demonstrated that a higher density of macrophages, especially macrophages with the M2 phenotype, is closely associated with worse clinical prognosis in many kinds of malignant tumors. Infiltrating TAMs themselves or polarization pathway of TAMs are considered as new therapeutic targets for the therapy of malignant tumors.