Fig. 1

The yin and yang of TAMs in response to cancer immunotherapy. Macrophages have bimodal, yin and yang roles in orchestrating immune responses, and can either hamper (left-hand side), or foster (right-hand side) the effectiveness of cancer immunotherapy strategies. The two macrophage subtypes are defined as M1 and M2. M1 macrophages, also known as ‘killer’ macrophages, were previously referred to as classically activated macrophages; In contrast, the M2 macrophages, also known as “repair” macrophage, are referred to as the alternatively activated macrophages. The M2 macrophages contribute to constructive processes, including wound healing and tissue repair. In addition, the M2 macrophage prevents immune surveillance system by producing immuno-suppressing cytokines. This differential polarization is under the control of many stimuli that alters the differentiated state of the macrophages. At present, TAMs are believed to promote cancer initiation and malignant progression by stimulating tumor-associated angiogenesis, promoting tumor cell metastasis, invasion and intravasation, chemotherapeutic resistance as well as suppressing the response to antitumor immunity