The role of the host—Neutrophil biology

Abstract Neutrophilic polymorphonuclear leukocytes (neutrophils) are myeloid cells packed with lysosomal granules (hence also called granulocytes) that contain a formidable antimicrobial arsenal. They are terminally differentiated cells that play a critical role in acute and chronic inflammation, as well as in the resolution of inflammation and wound healing. Neutrophils express a dense array of surface receptors for multiple ligands, ranging from integrins to support their egress from bone marrow into the circulation and from the circulation into tissues, to cytokine/chemokine receptors that drive their navigation to the site of infection or tissue damage and also prime them for a second stimulus, to pattern recognition receptors and immunoglobulin receptors to facilitate the destruction and removal of infective agents or debridement of damaged tissues. When afferent neutrophil signals are proportionate and coordinated they will phagocytose opsonized and unopsonized bacteria, activating the nicotinamide adenine dinucleotide phosphate oxidase (respiratory burst) to generate reactive oxygen species, which augment the proteolytic destruction of microbes secured within the phagosome. A highly orchestrated process of apoptosis follows, forming membrane‐bound substructures that are removed by macrophages. Neutrophils are capable of various other forms of programmed cell death, such as NETosis and pyroptotic cell death, as well as nonprogrammed cell death by necrosis. In recent years, research has revealed that neutrophils are capable of far more subtle cell‐cell interactions than previously thought possible. This includes synthesis of various inflammatory mediators and also myeloid cell training within bone marrow, where epigenetic and metabolic signals associated with returning neutrophils that undergo reverse egress from tissues into the vasculature and back to bone marrow program a hyperreactive subset of neutrophils during myelopoiesis that are capable of hypersensitive reactions to microbial aggressors. These characteristics are evident in various neutrophil subsets/subpopulations, creating broad heterogeneity in the behavior and biological repertoire of these seemingly schizophrenic immune cells. Moreover, neutrophils are critical effector cells of adaptive and innate immunity, binding to opsonized bacteria and destroying them by extracellular and intracellular methods. The former creates substantial collateral host tissue damage, as they are less specific than T‐cytotoxic cell‐killing mechanisms, and in conditions such as peri‐implantitis, where plasma cells and neutrophils dominate the immune infiltrate, bone and tissue destruction are rapid and appear relentless. Finally, the role of neutrophils as conduits for periodontal‐systemic disease connections and for oxidative damage to act as a causal link between the two has only recently been realized. In this chapter, we attempt to expand on these issues, emphasizing the contributions of European scientists throughout a detailed appraisal of the benefits and side effects of neutrophilic inflammation and immune function.