Neutrophils in the tumor microenvironment. Neutrophils can either promote or inhibit tumor growth depending on their polarization states. Antitumor neutrophils can directly kill tumor cells through release of reactive oxygen species (ROS) and reactive nitrogen species (RNS). They can also promote T-cell activation and recruit proinflammatory (M1) macrophages. In contrast, protumor neutrophils can release matrix metalloproteinase 9 (MMP9), which promotes angiogenesis and dissemination of tumor cells. They can also suppress natural killer (NK) cell function. Furthermore, they can recruit anti-inflammatory (M2) macrophages and T-regulatory cells. Finally, suppressor neutrophils, often referred to as polymorphonuclear neutrophil myeloid-derived suppressor cells (PMN-MDSCs), as well as other protumor neutrophils, can suppress CD8 T-cell function.