Circulating and Tumor-Associated Neutrophils in the Era of Immune Checkpoint Inhibitors: Dynamics, Phenotypes, Metabolism, and Functions

Simple Summary Neutrophils are the most abundant leukocytes in the circulation, represent the first line of defense in the immune system and mediate inflammation. Increasing evidence suggests that neutrophils constitute a large population of cells with phenotypic and functional heterogeneity. In this review, we summarize and discuss new findings delineating that both circulating neutrophils and tumor-associated neutrophils have a role in tumor prognosis and resistance to immune checkpoint inhibitors. Neutrophils are the most abundant myeloid cells in the blood and are a considerable immunological component of the tumor microenvironment. However, their functional importance has often been ignored, as they have always been considered a mono-dimensional population of terminally differentiated, short-living cells. During the last decade, the use of cutting-edge, single-cell technologies has revolutionized the classical view of these cells, unmasking their phenotypic and functional heterogeneity. In this review, we summarize the emerging concepts in the field of neutrophils in cancer, by reviewing the recent literature on the heterogeneity of both circulating neutrophils and tumor-associated neutrophils, as well as their possible significance in tumor prognosis and resistance to immune checkpoint inhibitors.