The tumor microenvironment is a well-recognized framework, in which myeloid cells play important roles in cancer development from tumor initiation to metastasis. Immune cells present in the tumor microenvironment can promote or inhibit cancer formation and development. Diversity and plasticity are hallmarks of cells of the monocyte-macrophage lineage. In response to distinct signals the cells of the monocyte-macrophage lineage have the ability to display a wide spectrum of activation states; classical Ml or alternative M2 macrophages represent extremes of a continuum of this activation. Tumor-associated macrophages generally acquire an M2-like phenotype that is relevant for their participation in tumor growth and progression. There is now evidence that also neutrophils can be driven towards distinct phenotypes in response to microenvironmental signals. In fact they can interact with distinct cell populations and produce a wide number of cytokines and effector molecules. Therefore, macrophages and neutrophils are both integrated in the regulation of the innate and adaptive immune responses in various inflammatory situations, including cancer. These findings have triggered efforts to target tumor-associated macrophages and neutrophils. In particular, “reeducation” to activate their antitumor potential or elimination of tumor promoting cells is a new strategy undergoing preclinical and clinical evaluation. 1. Introduction The tumor microenvironment plays important roles in cancer development and behaviour [1–4]. The tumor microenvironment consists of various host components such as stromal cells, growing blood vessels, and inflammatory infiltrate that cause a local chronic inflammation. Infiltrating leukocytes in the tumor microenvironment may exert a dual role on tumor development and progression. Tumor cells can be directly eliminated by the immune cells that had developed an antitumoral immune response but can also recruit and instruct immune cells to favour tumor growth and progression. The recruited immune cell consists of many players, including macrophages and neutrophils. Tumor-associated macrophages (TAM) are components of the inflammatory infiltrate of several tumors and produce many mediators (e.g., chemokines) responsible for the activation and maintenance of the chronic inflammatory process [5]. In human peripheral blood neutrophils are the most abundant leukocyte subset and have a primary role in the defence against infective pathogens. They have long been considered to have a negligible role in cancer-related inflammation because of their
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