The role of tumor-associated macrophages in cancer development and their significance as prognostic markers

Macrophages are a diverse group of immune cells which have key roles in immune defense, tumor homeostasis, and wound repair. During the last two decades, the role of macrophages as one of the most abundant tumor-infiltrating stromal cells has gradually emerged. The normal function of these tumor-associated macrophages (TAMs) in tumor microenvironment (TME) is to suppress tumor cells through triggering both direct cell cytotoxicity and antibody-mediated immune response. However, they have also been implicated in the progression of cancers. Tumor cells produce chemokines that polarize macrophages into tumor-promoting TAMs. This is the reason why the accumulation of TAMs in TME is correlated with poor prognosis in cancer patients. High plasticity of TAMs makes it feasible to regulate their polarization and adjust the balance between the anti-tumor TAMs and those with pro-tumor phenotypes. In this review, we aim to provide an overview about the origin and polarization of TAMs and their significance as biomarkers for the prediction and prognostication of various cancers.