Neutrophils in cancer: from immune defense to tumor promotion

Neutrophils are the protagonists of the host immune response, possessing potent antimicrobial and inflammatory capacities. The neutrophil reservoir as well as the development, mobilization, chemotaxis, pro-inflammatory activity, and clearance of neutrophils are strictly regulated to prevent inflammation-induced tissue damage. Inflammation pervades almost every type of cancer. However, there is growing awareness that although the tumor microenvironment has the capacity to recruit neutrophils, the functions are diverse and include roles other than that of sentinels in cancer. This review highlights the heterogeneity of neutrophils in tumors, discusses the dual role of neutrophils as angels and demons in tumorigenesis, invasion, and metastasis, and examines the potential of neutrophils as targets in clinical therapy.