Neutrophils have traditionally been thought of as simple foot soldiers of the innate immune system with a restricted set of pro-inflammatory functions. More recently, it has become apparent that neutrophils are, in fact, complex cells capable of a vast array of specialized functions. Although neutrophils are undoubtedly major effectors of acute inflammation, several lines of evidence indicate that they also contribute to chronic inflammatory conditions and adaptive immune responses. Here, we discuss the key features of the life of a neutrophil, from its release from bone marrow to its death. We discuss the possible existence of different neutrophil subsets and their putative anti-inflammatory roles. We focus on how neutrophils are recruited to infected or injured tissues and describe differences in neutrophil recruitment between different tissues. Finally, we explain the mechanisms that are used by neutrophils to promote protective or pathological immune responses at different sites.

Neutrophil recruitment and function in health and inflammation

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

https://biblio.ugent.be/publication/8555786/file/8555788.pdf

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018.

Ischemia and reperfusion-elicited tissue injury contributes to morbidity and mortality in a wide range of pathologies, including myocardial infarction, ischemic stroke, acute kidney injury, trauma, circulatory arrest, sickle cell disease and sleep apnea. Ischemia-reperfusion injury is also a major challenge during organ transplantation and cardiothoracic, vascular and general surgery. An imbalance in metabolic supply and demand within the ischemic organ results in profound tissue hypoxia and microvascular dysfunction. Subsequent reperfusion further enhances the activation of innate and adaptive immune responses and cell death programs. Recent advances in understanding the molecular and immunological consequences of ischemia and reperfusion may lead to innovative therapeutic strategies for treating patients with ischemia and reperfusion-associated tissue inflammation and organ dysfunction.

/pdf/ischemia-and-reperfusion-from-mechanism-to-translation-1v2cagcnsb.pdf

Ischemia and reperfusion—from mechanism to translation

Neutrophils have long been viewed as the final effector cells of an acute inflammatory response, with a primary role in the clearance of extracellular pathogens. However, more recent evidence has extended the functions of these cells. The newly discovered repertoire of effector molecules in the neutrophil armamentarium includes a broad array of cytokines, extracellular traps and effector molecules of the humoral arm of the innate immune system. In addition, neutrophils are involved in the activation, regulation and effector functions of innate and adaptive immune cells. Accordingly, neutrophils have a crucial role in the pathogenesis of a broad range of diseases, including infections caused by intracellular pathogens, autoimmunity, chronic inflammation and cancer.

/pdf/neutrophils-in-the-activation-and-regulation-of-innate-and-2ccbalvc3n.pdf

Neutrophils in the activation and regulation of innate and adaptive immunity

Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

/pdf/cell-biology-of-ischemia-reperfusion-injury-1rdtb8k8os.pdf

Cell Biology of Ischemia/Reperfusion Injury

Neutrophils are recruited from the blood to sites of sterile inflammation, where they contribute to wound healing but may also cause tissue damage. By using spinning disk confocal intravital microscopy, we examined the kinetics and molecular mechanisms of neutrophil recruitment to sites of focal hepatic necrosis in vivo. Adenosine triphosphate released from necrotic cells activated the Nlrp3 inflammasome to generate an inflammatory microenvironment that alerted circulating neutrophils to adhere within liver sinusoids. Subsequently, generation of an intravascular chemokine gradient directed neutrophil migration through healthy tissue toward foci of damage. Lastly, formyl-peptide signals released from necrotic cells guided neutrophils through nonperfused sinusoids into the injury. Thus, dynamic in vivo imaging revealed a multistep hierarchy of directional cues that guide neutrophil localization to sites of sterile inflammation.

https://science.sciencemag.org/content/sci/330/6002/362.full.pdf

Intravascular danger signals guide neutrophils to sites of sterile inflammation.

https://aasldpubs.onlinelibrary.wiley.com/doi/pdf/10.1002/hep.28003

Imaging the dynamic platelet‐neutrophil response in sterile liver injury and repair in mice

Over the last several decades, a new paradigm of platelet function has evolved. Platelets, long forgotten hemostatic cells, were demonstrated to be versatile immune effector cells engaged in every compartment of the immune system. Platelets express multiple immune receptors, such as toll-like receptors, allowing them to sense both pathogen- and danger-associated signals. Upon activation, platelets release a large array of biologically active molecules, like cytokines, chemokines, and growth factors, many of which are delivered in platelet-derived microparticles. Rich platelet transcriptome allows for signal-dependent translation and protein synthesis, and platelets are considered the main source of circulating microRNA. Platelets avidly interact with immune cells, endothelial cells, neutrophils, and monocytes in particular. Platelets were shown to propagate and modulate the inflammatory response of other immune cells in sterile inflammatory diseases, such as atherosclerosis and metabolic diseases, acute lung injury, ischemia reperfusion, and autoimmune diseases. Platelets are directed to the sites of infection and directly interact with pathogens. They contribute to elimination of pathogens by phagocytosis, release of microbicidal peptides, and signaling to other immune cells. Their interaction with neutrophils triggers release of neutrophil extracellular traps, a potent mechanism to entrap bacteria in flowing blood. Platelets were recently shown to be involved in adaptive immunity and possibly link innate and adaptive responses together. Fast flowing in the bloodstream, readily available for cellular interactions, able to sense signals from pathogens and damaged tissue, and packed with multiple immune mediators, platelets arise as immune sentinels forming an integral part of our immune system.

Ingrid Slaba

Papers

Intravascular danger signals guide neutrophils to sites of sterile inflammation.

Imaging the dynamic platelet‐neutrophil response in sterile liver injury and repair in mice

Platelets and Immunity