Refining the Role of Tumor-Associated Macrophages in Oral Squamous Cell Carcinoma

In the tumor microenvironment, various immune and stromal cells, such as fibroblasts and vascular endothelial cells, contribute to tumor growth and progression by interacting with cancer cells. Tumor-associated macrophages (TAMs) have attracted attention as major players in the tumor microenvironment. The origin of TAMs is believed to be the infiltration of monocytes derived from bone marrow progenitor cells into tumor tissues and their differentiation into macrophages, whereas tissue-resident macrophages derived from yolk sacs have recently been reported. TAMs infiltrating tumor tissues act in a tumor-promoting manner through immunosuppression, angiogenesis, and the promotion of cancer cell invasion. Reflecting the nature of TAMs, increased TAM invasion and TAM-specific gene expression in tumor tissues may be the new biomarkers for cancer. Moreover, new therapeutic strategies targeting TAMs, such as transformation into immunostimulatory macrophages, suppression of TAM infiltration, and promotion of phagocytosis, are being investigated, and many clinical trials are underway. As the origin and function of TAMs are further elucidated, TAM-targeted therapy is expected to become a new option for the immunotherapy of various cancers, including oral cancers.