CD47 and cancer: Macrophage-mediated phagocytosis is a pivotal process underlying the anti-tumor effect of activated macrophages.331 The signal regulatory protein α (SIRPα) expressed on (e.g., macrophages and DCs) was observed to trigger a cascade of events that inhibited phagocytosis when phagocytes interacted with CD47 highly expressed on cancer cells.630,631 Thereby CD47 appears to serve as a “don’t eat me” signal and a myeloid-specific immune checkpoint.