Tumor-infiltrating CD8 + T lymphocytes are key in controlling cancer cells. The composition of the immune infiltrate in the TME largely determines cancer progression and the cancer's sensitivity to various therapies. These cells, along with the extracellular matrix, and soluble mediators, form the tumor microenvironment (TME). As tumors develop, a tireless cross-talk takes place between heterogeneous malignant cells and neighboring parenchymal, stromal, and recruited immune cells. This oversimplified view has made way for a new perspective. Until the beginning of the Twenty-first century, cancer was considered a disease afflicting a single cell. Introduction to the Role of Myeloid Cells in Cancer In addition, we provide an overview of approaches that are under investigation to deplete myeloid cells or redirect their function, as these hold promise to overcome resistance to current cancer therapies. We review myeloid cells in the TME with a focus on the mechanisms they exploit to support cancer cells. Therefore, tumor-associated myeloid cells have been designated as the culprits in cancer. The key role of myeloid cells in cancer is further evidenced by the fact that they negatively impact on virtually all types of cancer therapy. These studies further show that tumor-derived factors mold these myeloid cells into cells that support cancer initiation and progression, amongst others by enabling immune evasion, tumor cell survival, proliferation, migration and metastasis. Experimental and clinical studies show that most myeloid cells are kept in an immature state in the TME. Consequently, the TME of many cancer types is characterized by high infiltration of monocytes, macrophages, dendritic cells and granulocytes.
Tumor cells frequently produce soluble factors that favor myelopoiesis and recruitment of myeloid cells to the tumor microenvironment (TME).