Macrophage Checkpoint Blockade in Cancer — Back to the Future

Macrophages are an essential component of the tumor microenvironment and have been ascribed a role in cancer-promoting inflammation.1,2 Phagocytic cells (a component of innate immunity) mediate first-line resistance against pathogens and, when appropriately activated, can mediate extracellular killing and phagocytosis of tumor cells.1-3 However, tumor progression is associated with subversion of immunity, and signals in the tumor microenvironment originating from cancer cells, stroma, and other immunocompetent cells set tumor-associated macrophages to a tumor-promoting mode (