Tumor-associated macrophages: foe or accomplice of tumors?

Macrophages are predominantly involved in the immune and inflammatory processes of solid tumors. Macrophages infiltrated into a tumor have an ambivalent relationship with the tumor, because they are innately very flexible and adaptable depending on the microenvironment of the tissue and the tissue-derived factors. The relationship between tumor-associated macrophages (TAMs) and a tumor is extremely complicated and has not yet been clearly elucidated. Now, the reintegration of biological knowledge including immunology, pathology and oncology is indispensable for the clinical application of TAMs to cancer therapy. In this review, we focus on the range of pro- and anti-tumor functions performed by TAMs and outline a new class of cancer therapies that aims at controlling the complex functions of TAMs.