Reverse Migration of Neutrophils: Where, When, How, and Why?

MCTR1 ameliorates LPS-induced lung injury by inhibiting neutrophil reverse transendothelial migration

OBJECTIVES

Dysregulated inflammatory responses during sepsis often result in acute lung injury (ALI). Neutrophils activated at the primary site of injury can re-enter the circulation through reverse transendothelial migration (rTEM), subsequently infiltrating other organs and contributing to systemic inflammation and multi-organ damage. The specialized pro-resolving lipid mediator (SPM) maresin conjugate in tissue regeneration 1 (MCTR1) has been shown to mitigate organ injury in sepsis. This study investigated the role of neutrophil rTEM in ALI and examined whether MCTR1 can alleviate ALI by modulating neutrophil rTEM.

METHODS

Lung injury was induced in mice by administrating lipopolysaccharide (LPS). Lung damage was assessed using H&E staining, lung wet-to-dry ratio, inflammatory mediator levels, and protein content in the bronchoalveolar lavage fluid. Neutrophil infiltration in lung tissue was evaluated by immunofluorescence, and flow cytometry was used to quantify rTEM neutrophils. Protein expression of neutrophil elastase (NE) and junctional adhesion molecule-C (JAM-C) was analyzed to assess rTEM activity. The role of CXCR4 in neutrophil rTEM was investigated using the CXCR4 inhibitor AMD3100. Additionally, bone marrow-derived neutrophils were isolated to evaluate the effects of MCTR1 on CXCR4 and GRK2 expression.

RESULTS

MCTR1 alleviated lung injury and inhibited neutrophils rTEM in LPS-induced lung injury. MCTR1 also decreased NE expression and increased JAM-C expression. The CXCR4 inhibitor AMD3100 effectively suppressed neutrophil rTEM and alleviated lung injury. Furthermore, MCTR1 inhibited CXCR4 expression and enhanced GRK2 expression.

CONCLUSIONS

MCTR1 reduces lung damage by upregulating GRK2 to inhibit CXCR4 expression, thereby suppressing neutrophil rTEM in LPS-induced lung injury.

Innate immunity of the lungs in homeostasis and disease

Humans breathe thousands of litres of non-sterile air each day containing bacteria, viruses, and fungi, as well as pollutants, allergens, and other particles. The continual exposure to foreign particles is juxtaposed with the vast surface area of the blood-air-barrier which becomes extremely thin to allow for efficient gas exchange. To prevent infection and injury, the healthy lung relies on a robust innate immune system to protect itself. Critically, this innate immune system must clear insults while maintaining immune tolerance and minimizing inflammation to avoid disrupting the lung's vital gas exchange function. In this review, we discuss how the innate immune system protects the lung from its environment.

Intravacuolar persistence in neutrophils facilitates Listeria monocytogenes spread to co-cultured cells

The bacterium Listeria monocytogenes (Lm) causes listeriosis in humans and ruminants. Acute lesions are predominantly infiltrated by polymorphonuclear neutrophils (PMNs), considered to be the efficient bactericidal arm of innate immunity. However, recent evidence suggests that PMNs cannot achieve antilisterial sterilizing immunity and that Lm may persist within PMNs. Despite this, interactions between PMNs and Lm remain poorly understood. In this study, we characterized the listericidal activity and interaction dynamics of bovine PMNs with Lm ex vivo. Phagocytosed Lm failed to escape into the PMN cytosol and was primarily targeted by phagolysosomal mechanisms. However, PMNs enabled prolonged intravacuolar survival of a resilient Lm subpopulation, largely as viable but non-culturable (VBNC) bacteria. This resilient Lm population could spread from PMNs to a cell line, resuscitate, and complete its canonical life cycle, thereby perpetuating the infection. Therefore, we identify PMNs as a mobile niche for Lm survival and provide evidence that PMNs harbor VBNC bacteria, potentially facilitating Lm dissemination within the host.

IMPORTANCE

Listeria monocytogenes (Lm) is a significant foodborne pathogen responsible for high hospitalization rates in humans, especially vulnerable groups such as the elderly, pregnant women, and immunocompromised individuals. In animals like ruminants, Lm infection leads to severe disease manifestations, notably brainstem encephalitis. This study uncovers a novel mechanism by which bovine neutrophils (PMNs) harbor Lm in a viable but non-culturable (VBNC) state, enabling the bacteria to hide in the host. PMNs, traditionally viewed as bacteria killers, may serve as Trojan horses, allowing Lm to persist and spread within the host. This discovery has broad implications for understanding Lm's persistence, its role in recurrent infections, and the development of new therapeutic strategies targeting VBNC forms of Lm to improve treatment outcomes and disease control.

New insights on extramedullary granulopoiesis and neutrophil heterogeneity in the spleen and its importance in disease

Neutrophils are traditionally viewed as uncomplicated exterminators that arrive quickly at sites of infection, kill pathogens, and then expire. However, recent studies employing modern transcriptomics coupled with novel imaging modalities have discovered that neutrophils exhibit significant heterogeneity within organs and have complex functional roles ranging from tissue homeostasis to cancer and chronic pathologies. This has revised the view that neutrophils are simplistic butchers, and there has been a resurgent interest in neutrophils. The spleen was described as a granulopoietic organ more than 4 decades ago, and studies indicate that neutrophils are briefly retained in the spleen before returning to circulation after proliferation. Transcriptomic studies have discovered that splenic neutrophils are heterogeneous and distinct compared with those in blood. This suggests that a unique hematopoietic niche exists in the splenic microenvironment, i.e., capable of programming neutrophils in the spleen. During severe systemic inflammation with an increased need of neutrophils, the spleen can adapt by producing neutrophils through emergency granulopoiesis. In this review, we describe the structure and microanatomy of the spleen and examine how cells within the splenic microenvironment help to regulate splenic granulopoiesis. A focus is placed on exploring the increase in splenic granulopoiesis to meet host needs during infection and inflammation. Emerging technologies such as single-cell RNA sequencing, which provide valuable insight into splenic neutrophil development and heterogeneity, are also discussed. Finally, we examine how tumors subvert this natural pathway in the spleen to generate granulocytic suppressor cells to promote tumor growth.

Exploring Neutrophil Heterogeneity and Plasticity in Health and Disease

Neutrophils, the most abundant polymorphonuclear leukocytes, are critical first responders to infection, and have historically been underappreciated in terms of their functional complexity within the immune response. Once viewed primarily as short-lived, innate immune cells with limited functional plasticity, recent research has illuminated their considerable heterogeneity and diverse functional roles, which extend beyond their involvement in steady-state immunity. This review seeks to provide an updated analysis of neutrophil development, maturation, heterogeneity, and plasticity, with a focus on how these characteristics influence immune modulation in both healthy and diseased tissues. Beginning with the origin of neutrophils, we explore their maturation into effector cells and their evolving roles in immune defense under homeostatic and disease-associated conditions. We then delve into their heterogeneity, discussing recent breakthroughs in neutrophil research that challenge the traditional view of neutrophils as a uniform population. We address the significant advances that have been made in identifying distinct neutrophil subsets, the emerging complexities of their plasticity, and the challenges that remain in fully understanding their functional diversity. Finally, we highlight future directions and opportunities for continued exploration in this rapidly advancing field, shedding light on how these insights could open new avenues for therapeutic interventions.

Neutrophils in cancer: from biology to therapy

The view of neutrophils has shifted from simple phagocytic cells, whose main function is to kill pathogens, to very complex cells that are also involved in immune regulation and tissue repair. These cells are essential for maintaining and regaining tissue homeostasis. Neutrophils can be viewed as double-edged swords in a range of situations. The potent killing machinery necessary for immune responses to pathogens can easily lead to collateral damage to host tissues when inappropriately controlled. Furthermore, some subtypes of neutrophils are potent pathogen killers, whereas others are immunosuppressive or can aid in tissue healing. Finally, in tumor immunology, many examples of both protumorigenic and antitumorigenic properties of neutrophils have been described. This has important consequences for cancer therapy, as targeting neutrophils can lead to either suppressed or stimulated antitumor responses. This review will discuss the current knowledge regarding the pro- and antitumorigenic roles of neutrophils, leading to the concept of a confused state of neutrophil-driven pro-/antitumor responses.

Neutrophils exhibit distinct migration phenotypes that are regulated by transendothelial migration

The extravasation of polymorphonuclear neutrophils (PMNs) is a critical component of the innate immune response that involves transendothelial migration (TEM) and interstitial migration. TEM-mediated interactions between PMNs and vascular endothelial cells (VECs) trigger a cascade of biochemical and mechanobiological signals whose effects on interstitial migration are currently unclear. To address this question, we cultured human VECs on a fibronectin-treated transwell insert to model the endothelium and basement membrane, loaded PMN-like differentiated HL60 (dHL-60) cells in the upper chamber of the insert, and collected the PMNs that crossed the membrane-supported monolayer from the lower chamber. The 3D chemotactic migration of the TEM-conditioned PMNs through collagen matrices was then quantified. Data collected from over 50,000 trajectories showed two distinct migratory phenotypes, i.e., a high-persistence phenotype and a low-persistence phenotype. These phenotypes were conserved across treatment conditions, and their existence was confirmed in human primary PMNs. The high-persistence phenotype was characterized by more straight trajectories and faster migration speeds, whereas the low-persistence one exhibited more frequent sharp turns and loitering periods. A key finding of our study is that TEM induced a phenotypic shift in PMNs from high-persistence migration to low-persistence migration. Changes in the relative proportion of high-persistence and low-persistence populations correlated with GRK2 expression levels. Inhibiting GRK2 hindered the TEM-induced shift in migratory phenotype and impaired the phagocytic function of PMNs. Overall, our study suggests that TEM-mediated GRK2 signaling primes PMNs for a migration phenotype better suited for spatial exploration and inflammation resolution. These observations provide novel insight into the biophysical impacts of TEM that priming PMNs is essential to conduct sentinel functions.

Neutrophil diversity and function in health and disease

Neutrophils, the most abundant type of granulocyte, are widely recognized as one of the pivotal contributors to the acute inflammatory response. Initially, neutrophils were considered the mobile infantry of the innate immune system, tasked with the immediate response to invading pathogens. However, recent studies have demonstrated that neutrophils are versatile cells, capable of regulating various biological processes and impacting both human health and disease. Cytokines and other active mediators regulate the functional activity of neutrophils by activating multiple receptors on these cells, thereby initiating downstream signal transduction pathways. Dysfunctions in neutrophils and disruptions in neutrophil homeostasis have been implicated in the pathogenesis of numerous diseases, including cancer and inflammatory disorders, often due to aberrant intracellular signaling. This review provides a comprehensive synthesis of neutrophil biological functions, integrating recent advancements in this field. Moreover, it examines the biological roles of receptors on neutrophils and downstream signaling pathways involved in the regulation of neutrophil activity. The pathophysiology of neutrophils in numerous human diseases and emerging therapeutic approaches targeting them are also elaborated. This review also addresses the current limitations within the field of neutrophil research, highlighting critical gaps in knowledge that warrant further investigation. In summary, this review seeks to establish a comprehensive and multidimensional model of neutrophil regulation, providing new perspectives for potential clinical applications and further research.

Inhibition of CCl4-induced liver inflammation and fibrosis by a NEU3 inhibitor

Sialic acids are located on the ends of many glycoconjugates and are cleaved off by enzymes called sialidases (neuraminidases). Upregulation of neuraminidase 3 (NEU3) is associated with intestinal inflammation and colitis, neuroinflammation, and lung fibrosis. Genetic ablation of NEU3 or pharmacological inhibition of NEU3 reduces lung fibrosis in mice. To determine if inhibiting NEU3 can inhibit liver fibrosis in the commonly-used CCl4 model, in this report, we examined the effects of injections of the NEU3 inhibitor 2-acetyl pyridine (2AP). 2AP inhibited CCl4-induced weight loss in female but not male mice. 2AP attenuated CCl4-induced liver inflammation and fibrosis in male and female mice, but did not affect CCl4-induced steatosis. After CCl4 treatment, female but not male mice had significant increases in liver neutrophils, and 2AP attenuated this response. 2AP also reversed CCl4-induced liver desialylation and CCl4-induced increased expression of NEU3. Patients with pulmonary fibrosis have increased desialylation of some serum proteins, and elevated serum levels of NEU3. We find that sera from patients with nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) have elevated desialylation of a serum protein and patients with NAFLD have increased levels of NEU3. These data suggest that elevated levels of NEU3 may be associated with liver inflammation and fibrosis, and that in mice this is ameliorated by injections of a NEU3 inhibitor.

MicroRNA-375 modulates neutrophil chemotaxis via targeting Cathepsin B in zebrafish

Neutrophils are crucial for defense against numerous infections, and their migration and activations are tightly regulated to prevent collateral tissue damage. We previously performed a neutrophil-specific miRNA overexpression screening and identified several microRNAs, including miR-375, as potent modulators for neutrophil activity. Overexpression of miR-375 decreases neutrophil motility and migration in zebrafish and human neutrophil-like cells. We screened the genes downregulated by miR-375 in zebrafish neutrophils and identified that Cathepsin B (Ctsba) is required for neutrophil motility and chemotaxis upon tail wounding and bacterial infection. Pharmacological inhibition or neutrophil-specific knockout of ctsba significantly decreased the neutrophil chemotaxis in zebrafish and survival upon systemic bacterial infection. Notably, Ctsba knockdown in human neutrophil-like cells also resulted in reduced chemotaxis. Inhibiting integrin receptor function using RGDS rescued the neutrophil migration defects and susceptibility to systemic infection in zebrafish with either miR-375 overexpression or ctsba knockout. Our results demonstrate that miR-375 and its target Ctsba modulate neutrophil activity during tissue injury and bacterial infection in vivo, providing novel insights into neutrophil biology and the overall inflammation process.

IRG1/ACOD1 promotes neutrophil reverse migration and alleviates local inflammation

Polymorphonuclear neutrophil (PMN) infiltration at inflammatory site plays a critical role in inflammation. PMN reverse migration (rM) describes the phenomenon that PMNs migrate away from inflammatory site back into the vasculature, and its role within inflammatory scenarios remains to be fully determined. This study aimed to investigate the mechanism underlying PMN rM and its role in inflammation. First, we demonstrated PMN rM in a mouse model of lipopolysaccharide-induced acute lung inflammation. By single-cell RNA sequencing, we demonstrated that reverse migrated (rM-ed) PMNs in blood expressed a high level of immune-responsive gene 1 (Irg1), the encoding gene of cis-aconitate decarboxylase (ACOD1). Using a mouse air pouch model, which enabled us to directly track rM-ed PMNs in vivo, we detected higher expression of ACOD1 in the rM-ed PMNs in circulation. Furthermore, mice with Irg1 knockout exhibited decreased PMN rM and higher levels of inflammatory cytokine in inflammatory site. Mechanistically, we found that itaconate, the product of ACOD1 catalyzation, decreased PMN ICAM-1 expression at the inflammation site. Furthermore, inflammatory site showed a high level of shed Cd11a, the ligand of ICAM-1. Neutralization of either ICAM-1 or Cd11a led to increased PMN rM. These findings suggest that the binding of ICAM-1 and shed Cd11a serves as a retaining force to hold PMNs in the site of inflammation, and ACOD1-decreased PMN surface expression of ICAM-1 weakens the retaining force, promoting PMNs to leave the inflammatory site. These results indicate a regulatory role of IRG1 in PMN rM and subsequent contributions to inflammation resolution.

Endothelial Cell-Derived Extracellular Vesicles Promote Aberrant Neutrophil Trafficking and Subsequent Remote Lung Injury

The development of acute respiratory distress syndrome (ARDS) in sepsis is associated with substantial morbidity and mortality. However, the molecular pathogenesis underlying sepsis-induced ARDS remains elusive. Neutrophil heterogeneity and dysfunction contribute to uncontrolled inflammation in patients with ARDS. A specific subset of neutrophils undergoing reverse transendothelial migration (rTEM), which is characterized by an activated phenotype, is implicated in the systemic dissemination of inflammation. Using single-cell RNA sequencing (scRNA-seq), it identified functionally activated neutrophils exhibiting the rTEM phenotype in the lung of a sepsis mouse model using cecal ligation and puncture. The prevalence of neutrophils with the rTEM phenotype is elevated in the blood of patients with sepsis-associated ARDS and is positively correlated with disease severity. Mechanically, scRNA-seq and proteomic analys revealed that inflamed endothelial cell (EC) released extracellular vesicles (EVs) enriched in karyopherin subunit beta-1 (KPNB1), promoting abluminal-to-luminal neutrophil rTEM. Additionally, EC-derived EVs are elevated and positively correlated with the proportion of rTEM neutrophils in clinical sepsis. Collectively, EC-derived EV is identified as a critical regulator of neutrophil rTEM, providing insights into the contribution of rTEM neutrophils to sepsis-associated lung injury.

Neutrophils-biology and diversity

Neutrophils, the most abundant white blood cells in the human circulation, play crucial roles in various diseases, including kidney disease. Traditionally viewed as short-lived pro-inflammatory phagocytes that release reactive oxygen species, cytokines and neutrophil extracellular traps, recent studies have revealed their complexity and heterogeneity, thereby challenging this perception. Neutrophils are now recognized as transcriptionally active cells capable of proliferation and reverse migration, displaying phenotypic and functional heterogeneity. They respond to a wide range of signals and deploy various cargo to influence the activity of other cells in the circulation and in tissues. They can regulate the behavior of multiple immune cell types, exhibit innate immune memory, and contribute to both acute and chronic inflammatory responses while also promoting inflammation resolution in a context-dependent manner. Here, we explore the origin and heterogeneity of neutrophils, their functional diversity, and the cues that regulate their effector functions. We also examine their emerging role in infectious and non-infectious diseases with a particular emphasis on kidney disease. Understanding the complex behavior of neutrophils during tissue injury and inflammation may provide novel insights, thereby paving the way for potential therapeutic strategies to manage acute and chronic conditions. By deciphering their multifaceted role, targeted interventions can be developed to address the intricacies of neutrophil-mediated immune responses and improve disease outcomes.

MPO-DNA Complexes and cf-DNA in Patients with Sepsis and Their Clinical Value

Background/Objectives: Neutrophils, as the first line of defense in the immune response, produce neutrophil extracellular traps (NETs) upon activation, which are significant in the pathogenesis and organ damage in sepsis. This study aims to explore the clinical value of myeloperoxidase-DNA (MPO-DNA) and cell-free DNA (cf-DNA) in sepsis patients. Methods: Clinical data were collected from 106 sepsis patients, 25 non-sepsis patients, and 51 healthy controls. Sequential Organ Failure Assessment (SOFA) scores were calculated, and levels of MPO-DNA) complexes and cf-DNA were measured using specific kits. Correlation analyses assessed relationships between indicators, while logistic regression identified independent risk factors. Receiver operating characteristic (ROC) curves calculated the area under the curve (AUC) to evaluate the diagnostic value of the biomarkers. Results: Sepsis patients exhibited significantly elevated levels of MPO-DNA and cf-DNA compared to non-sepsis patients and healthy controls. In sepsis patients, MPO-DNA and cf-DNA levels correlated with inflammation, coagulation, and organ damage indicators, as well as procalcitonin (PCT) levels and SOFA scores. Both C-reactive protein (CRP) and cf-DNA were identified as independent risk factors for sepsis, demonstrating moderate diagnostic value. ROC analysis showed that the combination of MPO-DNA and CRP (AUC: 0.837) enhances the AUC value of CRP (0.777). Conclusions: In summary, elevated serum levels of MPO-DNA and cf-DNA in sepsis patients correlate with SOFA scores and PCT levels, providing reference value for sepsis diagnosis in clinical settings.

Chemerin attracts neutrophil reverse migration by interacting with C-C motif chemokine receptor-like 2

Neutrophil reverse migration (rM) is a recently identified phenomenon in which neutrophils migrate away from the inflammatory site back into the vasculature following initial infiltration, which involved in the resolution of loci inflammatory response or dissemination of inflammation. Present study was aimed to explore the mechanisms in neutrophil rM. By scRNA-seq on the white blood cells in acute lung injury model, we found rM-ed neutrophils exhibited increased gene expression of C-C motif chemokine receptor-like 2 (Ccrl2), an atypical chemokine receptor. Furthermore, an air pouch model was established to directly track rM-ed neutrophils in vivo. Air pouches were generated by 3 ml filtered sterile air injected subcutaneously for 3 days, and then LPS (2 mg/kg) was injected into the pouches to mimic the inflammatory state. For the rM-ed neutrophil tracking system, cell tracker CMFDA were injected into the air pouch to stain the inflammatory loci cells, and after 6 h, stained cells in blood were regarded as the rM-ed neutrophil. Based on this tracking system, we confirmed that rM-ed neutrophils showed increased CCRL2. We also found that the concentrations of the CCRL2 ligand chemerin in plasma was increased in the late stage. Neutralizing chemerin decreased the rM-ed neutrophil ratio in the blood. These results suggest that circulating chemerin attracts neutrophils to leave inflammatory sites by interacting with CCRL2, which might involve in the dissemination of inflammation.

Endothelial ERG programs neutrophil transcriptome for sustained anti-inflammatory vascular niche

Neutrophils (PMNs) reside as a marginated pool within the vasculature, ready for deployment during infection. However, how endothelial cells (ECs) control PMN extravasation and activation to strengthen tissue homeostasis remains ill-defined. Here, we found that the vascular ETS-related gene (ERG) is a generalized mechanism regulating PMN activity in preclinical tissue injury models and human patients. We show that ERG loss in ECs rewired PMN-transcriptome, enriched for genes associated with the CXCR2-CXCR4 signaling. Rewired PMNs compromise mice survival after pneumonia and induced lung vascular inflammatory injury following adoptive transfer into naïve mice, indicating their longevity and inflammatory activity memory. Mechanistically, EC-ERG restricted PMN extravasation and activation by upregulating the deubiquitinase A20 and downregulating the NFκB-IL8 cascade. Rescuing A20 in EC-Erg -/- endothelium or suppressing PMN-CXCR2 signaling rescued EC control of PMN activation. Findings deepen our understanding of EC control of PMN-mediated inflammation, offering potential avenues for targeting various inflammatory diseases.

Highlights

ERG regulates trans-endothelial neutrophil (PMN) extravasation, retention, and activationLoss of endothelial (EC) ERG rewires PMN-transcriptomeAdopted transfer of rewired PMNs causes inflammation in a naïve mouse ERG transcribes A20 and suppresses CXCR2 function to inactivate PMNs.

In brief/blurb

The authors investigated how vascular endothelial cells (EC) control polymorphonuclear neutrophil (PMN) extravasation, retention, and activation to strengthen tissue homeostasis. They showed that EC-ERG controls PMN transcriptome into an anti-adhesive and anti-inflammatory lineage by synthesizing A20 and suppressing PMNs-CXCR2 signaling, defining EC-ERG as a target for preventing neutrophilic inflammatory injury.

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

Pericytes modulate endothelial inflammatory response during bacterial infection

Pericytes are located around blood vessels, in close contact with endothelial cells. We discovered that pericytes dampen pro-inflammatory endothelial cell responses. Endothelial cells co-cultured with pericytes had markedly reduced expression of adhesion molecules (PECAM-1 and ICAM-1) and proinflammatory cytokines (CCL-2 and IL-6) in response to bacterial stimuli (Brucella ovis, Listeria monocytogenes, or Escherichia coli lipopolysaccharide). Pericyte-depleted mice intraperitoneally inoculated with either B. ovis, a stealthy pathogen that does not trigger detectable inflammation, or Listeria monocytogenes, developed peritonitis. Further, during Citrobacter rodentium infection, pericyte-depleted mice developed severe intestinal inflammation, which was not evident in control mice. The anti-inflammatory effect of pericytes required connexin 43, as either chemical inhibition or silencing of connexin 43 abrogated pericyte-mediated suppression of endothelial inflammatory responses. Our results define a mechanism by which pericytes modulate inflammation during infection, which shifts our understanding of pericyte biology: from a structural cell to a pro-active player in modulating inflammation.

IMPORTANCE

A previously unknown mechanism by which pericytes modulate inflammation was discovered. The absence of pericytes or blocking interaction between pericytes and endothelium through connexin 43 results in stronger inflammation, which shifts our understanding of pericyte biology, from a structural cell to a player in controlling inflammation.

How Neutrophils Shape the Immune Response: Reassessing Their Multifaceted Role in Health and Disease

Neutrophils are the most abundant of the circulating immune cells and are the first to be recruited to sites of inflammation. Neutrophils are a heterogeneous group of immune cells from which are derived extracellular traps (NETs), reactive oxygen species, cytokines, chemokines, immunomodulatory factors, and alarmins that regulate the recruitment and phenotypes of neutrophils, macrophages, dendritic cells, T cells, and B cells. In addition, cytokine-stimulated neutrophils can express class II major histocompatibility complex and the internal machinery necessary for successful antigen presentation to memory CD4+ T cells. This may be relevant in the context of vaccine memory. Neutrophils thus emerge as orchestrators of immune responses that play a key role in determining the outcome of infections, vaccine efficacy, and chronic diseases like autoimmunity and cancer. This review aims to provide a synthesis of current evidence as regards the role of these functions of neutrophils in homeostasis and disease.

Exploring the dynamic behavior of leukocytes with zebrafish

Cell migration is a complex and intricate network of physical, chemical, and molecular events that ultimately leads to cell motility. This phenomenon is involved in both physiological and pathological processes such as proper immune and inflammatory responses. Dysregulation of cell migration machinery in immune cells can have a tremendous impact on the trajectory of inflammation, infection, and resolution. The small vertebrate, the zebrafish, has a remarkable capacity for genetic and pharmacological manipulation aligned to transparency that enables modulation and visualization of cell migration in vivo noninvasively. Such characteristics revolutionized the field of leukocyte biology, particularly neutrophils. In this review, we will focus on leukocyte migration and highlight findings made in the zebrafish that demonstrate how this small vertebrate system is a unique model to perform in vivo imaging and study mechanisms that regulate the dynamic behavior of immune cells in their native environment under homeostasis or upon challenge.

Effects of neutrophil fate on inflammation

INTRODUCTION

Neutrophils are important participants in the innate immune response. They rapidly and efficiently identify and clear infectious agents by expressing large numbers of membrane receptors. Upon tissue injury or pathogen invasion, neutrophils are the first immune cells to reach the site of injury and participate in the inflammatory response.

MATERIALS AND METHODS

A thorough search on PubMed related to neutrophil death or clearance pathways was performed.

CONCLUSION

Inflammatory response and tissue damage can be aggravated when neutrophils are not removed rapidly from the site of injury. Recent studies have shown that neutrophils can be cleared through a variety of pathways, including non-inflammatory and inflammatory death, as well as reverse migration. Non-inflammatory death pathways include apoptosis and autophagy. Inflammatory death pathways include necroptosis, pyroptosis and NETosis. This review highlights the basic properties of neutrophils and the impact of their clearance pathways on the inflammatory response.

Recent advances in the role of neutrophils and neutrophil extracellular traps in acute pancreatitis

Pancreatitis is an inflammatory disease, which is triggered by adverse events in acinar cells of the pancreas. After the initial injury, infiltration of neutrophils in pancreas is observed. In the initial stages of pancreatitis, the inflammation is sterile. It has been shown that the presence of neutrophils at the injury site can modulate the disease. Their depletion in experimental animal models of the acute pancreatitis has been shown to be protective. But information on mechanism of contribution to inflammation by neutrophils at the injury site is not clear. Once at injury site, activated neutrophils release azurophilic granules containing proteolytic enzymes and generate hypochlorous acid which is a strong microbicidal agent. Additionally, emerging evidence shows that neutrophil extracellular traps (NETs) are formed which consist of decondensed DNA decorated with histones, proteases and granular and cytosolic proteins. NETs are considered mechanical traps for microbes, but there is preliminary evidence to indicate that NETs, which constitute a special mechanism of the neutrophil defence system, play an adverse role in pancreatitis by contributing to the pancreatic inflammation and distant organ injury. This review presents the overall current information about neutrophils and their role including NETs in acute pancreatitis (AP). It also highlights current gaps in knowledge which should be explored to fully elucidate the role of neutrophils in AP and for therapeutic gains.

Regulated cell death in neutrophils: From apoptosis to NETosis and pyroptosis

Neutrophils are among the most abundant immune cells, representing about 50%- 70% of all circulating leukocytes in humans. Neutrophils rapidly infiltrate inflamed tissues and play an essential role in host defense against infections. They exert microbicidal activity through a variety of specialized effector mechanisms, including phagocytosis, production of reactive oxygen species, degranulation and release of secretory vesicles containing broad-spectrum antimicrobial factors. In addition to their homeostatic turnover by apoptosis, recent studies have revealed the mechanisms by which neutrophils undergo various forms of regulated cell death. In this review, we will discuss the different modes of regulated cell death that have been described in neutrophils, with a particular emphasis on the current understanding of neutrophil pyroptosis and its role in infections and autoinflammation.

Neutrophil biology in injuries and diseases of the central and peripheral nervous systems

The role of inflammation in nervous system injury and disease is attracting increased attention. Much of that research has focused on microglia in the central nervous system (CNS) and macrophages in the peripheral nervous system (PNS). Much less attention has been paid to the roles played by neutrophils. Neutrophils are part of the granulocyte subtype of myeloid cells. These cells, like macrophages, originate and differentiate in the bone marrow from which they enter the circulation. After tissue damage or infection, neutrophils are the first immune cells to infiltrate into tissues and are directed there by specific chemokines, which act on chemokine receptors on neutrophils. We have reviewed here the basic biology of these cells, including their differentiation, the types of granules they contain, the chemokines that act on them, the subpopulations of neutrophils that exist, and their functions. We also discuss tools available for identification and further study of neutrophils. We then turn to a review of what is known about the role of neutrophils in CNS and PNS diseases and injury, including stroke, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, spinal cord and traumatic brain injuries, CNS and PNS axon regeneration, and neuropathic pain. While in the past studies have focused on neutrophils deleterious effects, we will highlight new findings about their benefits. Studies on their actions should lead to identification of ways to modify neutrophil effects to improve health.

Exosomal PGE2 from M2 macrophages inhibits neutrophil recruitment and NET formation through lipid mediator class switching in sepsis

BACKGROUND

Excess polymorphonuclear neutrophil (PMN) recruitment or excessive neutrophil extracellular trap (NET) formation can lead to the development of multiple organ dysfunction during sepsis. M2 macrophage-derived exosomes (M2-Exos) have exhibited anti-inflammatory activities in some inflammatory diseases to mediate organ functional protection, but their role in treating sepsis-related acute lung injury (ALI) remains unclear. In this study, we sought to investigate whether M2-Exos could prevent potentially deleterious inflammatory effects during sepsis-related ALI by modulating abnormal PMN behaviours.

METHODS

C57BL/6 wild-type mice were subjected to a caecal ligation and puncture (CLP) mouse model to mimic sepsis in vivo, and M2-Exos were administered intraperitoneally 1 h after CLP. H&E staining, immunofluorescence and immunohistochemistry were conducted to investigate lung tissue injury, PMN infiltration and NET formation in the lung. We further demonstrated the role of M2-Exos on PMN function and explored the potential mechanisms through an in vitro coculture experiment using PMNs isolated from both healthy volunteers and septic patients.

RESULTS

Here, we report that M2-Exos inhibited PMN migration and NET formation, alleviated lung injury and reduced mortality in a sepsis mouse model. In vitro, M2-Exos significantly decreased PMN migration and NET formation capacity, leading to lipid mediator class switching from proinflammatory leukotriene B4 (LTB4) to anti-inflammatory lipoxin A4 (LXA4) by upregulating 15-lipoxygenase (15-LO) expression in PMNs. Treatment with LXA4 receptor antagonist attenuated the effect of M2-Exos on PMNs and lung injury. Mechanistically, prostaglandin E2 (PGE2) enriched in M2-Exos was necessary to increase 15-LO expression in PMNs by functioning on the EP4 receptor, upregulate LXA4 production to downregulate chemokine (C-X-C motif) receptor 2 (CXCR2) and reactive oxygen species (ROS) expressions, and finally inhibit PMN function.

CONCLUSIONS

Our findings reveal a previously unknown role of M2-Exos in regulating PMN migration and NET formation through lipid mediator class switching, thus highlighting the potential application of M2-Exos in controlling PMN-mediated tissue injury in patients with sepsis.

The double-edged role of neutrophil heterogeneity in inflammatory diseases and cancers

Neutrophils are important immune cells act as the body's first line of defense against infection and respond to diverse inflammatory cues. Many studies have demonstrated that neutrophils display plasticity in inflammatory diseases and cancers. Clarifying the role of neutrophil heterogeneity in inflammatory diseases and cancers will contribute to the development of novel treatment strategies. In this review, we have presented a review on the development of the understanding on neutrophil heterogeneity from the traditional perspective and a high-resolution viewpoint. A growing body of evidence has confirmed the double-edged role of neutrophils in inflammatory diseases and tumors. This may be due to a lack of precise understanding of the role of specific neutrophil subsets in the disease. Thus, elucidating specific neutrophil subsets involved in diseases would benefit the development of precision medicine. Thusly, we have summarized the relevance and actions of neutrophil heterogeneity in inflammatory diseases and cancers comprehensively. Meanwhile, we also discussed the potential intervention strategy for neutrophils. This review is intended to deepen our understanding of neutrophil heterogeneity in inflammatory diseases and cancers, while hold promise for precise treatment of neutrophil-related diseases.

New genetic and epigenetic insights into the chemokine system: the latest discoveries aiding progression toward precision medicine

Over the past thirty years, the importance of chemokines and their seven-transmembrane G protein-coupled receptors (GPCRs) has been increasingly recognized. Chemokine interactions with receptors trigger signaling pathway activity to form a network fundamental to diverse immune processes, including host homeostasis and responses to disease. Genetic and nongenetic regulation of both the expression and structure of chemokines and receptors conveys chemokine functional heterogeneity. Imbalances and defects in the system contribute to the pathogenesis of a variety of diseases, including cancer, immune and inflammatory diseases, and metabolic and neurological disorders, which render the system a focus of studies aiming to discover therapies and important biomarkers. The integrated view of chemokine biology underpinning divergence and plasticity has provided insights into immune dysfunction in disease states, including, among others, coronavirus disease 2019 (COVID-19). In this review, by reporting the latest advances in chemokine biology and results from analyses of a plethora of sequencing-based datasets, we outline recent advances in the understanding of the genetic variations and nongenetic heterogeneity of chemokines and receptors and provide an updated view of their contribution to the pathophysiological network, focusing on chemokine-mediated inflammation and cancer. Clarification of the molecular basis of dynamic chemokine-receptor interactions will help advance the understanding of chemokine biology to achieve precision medicine application in the clinic.

Competition between chemoattractants causes unexpected complexity and can explain negative chemotaxis

Negative chemotaxis, where eukaryotic cells migrate away from repellents, is important throughout biology, for example, in nervous system patterning and resolution of inflammation. However, the mechanisms by which molecules repel migrating cells are unknown. Here, we use predictive modeling and experiments with Dictyostelium cells to show that competition between different ligands that bind to the same receptor leads to effective chemorepulsion. 8-CPT-cAMP, widely described as a simple chemorepellent, is inactive on its own and only repels cells when it acts in combination with the attractant cAMP. If cells degrade either competing ligand, the pattern of migration becomes more complex; cells may be repelled in one part of a gradient but attracted elsewhere, leading to populations moving in different directions in the same assay or converging in an arbitrary place. More counterintuitively still, two chemicals that normally attract cells can become repellent when combined. Computational models of chemotaxis are now accurate enough to predict phenomena that have not been anticipated by experiments. We have used them to identify new mechanisms that drive reverse chemotaxis, which we have confirmed through experiments with real cells. These findings are important whenever multiple ligands compete for the same receptors.

Neutrophil diversity in inflammation and cancer

Neutrophils are the most abundant circulating leukocytes in humans and the first immune cells recruited at the site of inflammation. Classically perceived as short-lived effector cells with limited plasticity and diversity, neutrophils are now recognized as highly heterogenous immune cells, which can adapt to various environmental cues. In addition to playing a central role in the host defence, neutrophils are involved in pathological contexts such as inflammatory diseases and cancer. The prevalence of neutrophils in these conditions is usually associated with detrimental inflammatory responses and poor clinical outcomes. However, a beneficial role for neutrophils is emerging in several pathological contexts, including in cancer. Here we will review the current knowledge of neutrophil biology and heterogeneity in steady state and during inflammation, with a focus on the opposing roles of neutrophils in different pathological contexts.

Strategies of neutrophil diversification

Neutrophils are formidable defenders. Their vast numbers, constant production, high cytotoxicity and capacity to produce extracellular traps, underlie their ability to efficiently protect in a microorganism-rich world. However, neutrophils are much more than immune sentinels, as evidenced by the expanding repertoire of functions discovered in the context of tissue homeostasis, regeneration or chronic pathologies. In this Perspective, we discuss general functional features of the neutrophil compartment that may be relevant in most, if not all, physiological scenarios in which they participate, including specialization in naïve tissues, transcriptional noise in the bloodstream as a potential strategy for diversification and functional bias in inflammatory sites. We intentionally present the reader with more questions than answers and propose models and approaches that we hope will shed new light onto the biology of these fascinating cells and spark new directions of research.

Prospects for targeting ACKR1 in cancer and other diseases

The chemokine network is comprised of a family of signal proteins that encode messages for cells displaying chemokine G-protein coupled receptors (GPCRs). The diversity of effects on cellular functions, particularly directed migration of different cell types to sites of inflammation, is enabled by different combinations of chemokines activating signal transduction cascades on cells displaying a combination of receptors. These signals can contribute to autoimmune disease or be hijacked in cancer to stimulate cancer progression and metastatic migration. Thus far, three chemokine receptor-targeting drugs have been approved for clinical use: Maraviroc for HIV, Plerixafor for hematopoietic stem cell mobilization, and Mogalizumab for cutaneous T-cell lymphoma. Numerous compounds have been developed to inhibit specific chemokine GPCRs, but the complexity of the chemokine network has precluded more widespread clinical implementation, particularly as anti-neoplastic and anti-metastatic agents. Drugs that block a single signaling axis may be rendered ineffective or cause adverse reactions because each chemokine and receptor often have multiple context-specific functions. The chemokine network is tightly regulated at multiple levels, including by atypical chemokine receptors (ACKRs) that control chemokine gradients independently of G-proteins. ACKRs have numerous functions linked to chemokine immobilization, movement through and within cells, and recruitment of alternate effectors like β-arrestins. Atypical chemokine receptor 1 (ACKR1), previously known as the Duffy antigen receptor for chemokines (DARC), is a key regulator that binds chemokines involved in inflammatory responses and cancer proliferation, angiogenesis, and metastasis. Understanding more about ACKR1 in different diseases and populations may contribute to the development of therapeutic strategies targeting the chemokine network.

Neutrophil-T cell crosstalk and the control of the host inflammatory response

While fundamental in their innate role in combating infection and responding to injury, neutrophils are emerging as key modulators of adaptive immune responses. Such functions are attained via both soluble and nonsoluble effectors that enable at least two major downstream outcomes: first, to mediate and control acute inflammatory responses and second, to regulate adaptive immunity and ultimately promoting the development and maintenance of immune tolerance either by releasing immuno-modulatory factors, including cytokines, or by directly interacting with cells of the adaptive immune system. Herein, we review these novel properties of neutrophils and redefine the pathophysiological functions of these fascinating multi-tasking cells, exploring the different mechanisms through which neutrophils are able to either enhance and orchestrate T cell pro-inflammatory responses or inhibit T cell activity to maintain immune tolerance.

A Nanotherapy of Octanoic Acid Ameliorates Cardiac Arrest/Cardiopulmonary Resuscitation-Induced Brain Injury via RVG29- and Neutrophil Membrane-Mediated Injury Relay Targeting

Treatment of cardiac arrest/cardiopulmonary resuscitation (CA/CPR)-induced brain injury remains a challenging issue without viable therapeutic options. Octanoic acid (OA), a lipid oil that is mainly metabolized in the astrocytes of the brain, is a promising treatment for this type of injury owing to its potential functions against oxidative stress, apoptosis, inflammation, and ability to stabilize mitochondria. However, the application of OA is strictly limited by its short half-life and low available concentration in the target organ. Herein, based on our previous research, an OA-based nanotherapy coated with a neutrophil membrane highly expressing RVG29, RVG29-H-NP_OA, was successfully constructed by computer simulation-guided supramolecular assembly of polyethylenimine and OA. The in vitro and in vivo experiments showed that RVG29-H-NP_OA could target and be distributed in the injured brain focus via the relay-targeted delivery mediated by RVG29-induced blood-brain barrier (BBB) penetration and neutrophil membrane protein-induced BBB binding and injury targeting. This results in enhancements of the antioxidant, antiapoptotic, mitochondrial stability-promoting and anti-inflammatory effects of OA and exhibited systematic alleviation of astrocyte injury, neuronal damage, and inflammatory response in the brain. Due to their systematic intervention in multiple pathological processes, RVG29-H-NP_OA significantly increased the 24 h survival rate of CA/CPR model rats from 40% to 100% and significantly improved their neurological functions. Thus, RVG29-H-NP_OA are expected to be a promising therapeutic for the treatment of CA/CPR-induced brain injury.

The role of G protein-coupled receptor in neutrophil dysfunction during sepsis-induced acute respiratory distress syndrome

Sepsis is defined as a life-threatening dysfunction due to a dysregulated host response to infection. It is a common and complex syndrome and is the leading cause of death in intensive care units. The lungs are most vulnerable to the challenge of sepsis, and the incidence of respiratory dysfunction has been reported to be up to 70%, in which neutrophils play a major role. Neutrophils are the first line of defense against infection, and they are regarded as the most responsive cells in sepsis. Normally, neutrophils recognize chemokines including the bacterial product N-formyl-methionyl-leucyl-phenylalanine (fMLP), complement 5a (C5a), and lipid molecules Leukotriene B4 (LTB4) and C-X-C motif chemokine ligand 8 (CXCL8), and enter the site of infection through mobilization, rolling, adhesion, migration, and chemotaxis. However, numerous studies have confirmed that despite the high levels of chemokines in septic patients and mice at the site of infection, the neutrophils cannot migrate to the proper target location, but instead they accumulate in the lungs, releasing histones, DNA, and proteases that mediate tissue damage and induce acute respiratory distress syndrome (ARDS). This is closely related to impaired neutrophil migration in sepsis, but the mechanism involved is still unclear. Many studies have shown that chemokine receptor dysregulation is an important cause of impaired neutrophil migration, and the vast majority of these chemokine receptors belong to the G protein-coupled receptors (GPCRs). In this review, we summarize the signaling pathways by which neutrophil GPCR regulates chemotaxis and the mechanisms by which abnormal GPCR function in sepsis leads to impaired neutrophil chemotaxis, which can further cause ARDS. Several potential targets for intervention are proposed to improve neutrophil chemotaxis, and we hope that this review may provide insights for clinical practitioners.

Neutrophils in autoimmunity: when the hero becomes the villain

Neutrophils were long considered to be a short-lived homogenous cell population, limited to their role as first responders in anti-bacterial and -fungal immunity. While it is true that neutrophils are first to infiltrate the site of infection to eliminate pathogens, growing evidence suggests their functions could extend beyond those of basic innate immune cells. Along with their well-established role in pathogen elimination, utilizing effector functions such as phagocytosis, degranulation, and the deployment of neutrophil extracellular traps (NETs), neutrophils have recently been shown to possess antigen-presenting capabilities. Moreover, the identification of different subtypes of neutrophils points to a multifactorial heterogeneous cell population with great plasticity in which some subsets have enhanced pro-inflammatory characteristics, while others seem to behave as immunosuppressors. Interestingly, the aberrant presence of activated neutrophils with a pro-inflammatory profile in several systemic and organ-specific autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), multiple sclerosis (MS), and type 1 diabetes (T1D) could potentially be exploited in novel therapeutic strategies. The full extent of the involvement of neutrophils, and more specifically that of their various subtypes, in the pathophysiology of autoimmune diseases is yet to be elucidated.

Neutrophil reverse migration

The behavior of neutrophils is very important for the resolution of inflammation and tissue repair. People have used advanced imaging techniques to observe the phenomenon of neutrophils leaving the injured or inflammatory site and migrating back into blood vessels in transgenic zebrafish and mice, which is called neutrophil reverse migration. Numerous studies have shown that neutrophil reverse migration is a double-edged sword. On the one hand, neutrophil reverse migration can promote the resolution of local inflammation by accelerating the clearance of neutrophils from local wounds. On the other hand, neutrophils re-enter the circulatory system may lead to the spread of systemic inflammation. Therefore, accurate regulation of neutrophil reverse migration is of great significance for the treatment of various neutrophil- mediated diseases. However, the mechanism of neutrophil reverse migration and its relationship with inflammation resolution is unknown. In this review, we reviewed the relevant knowledge of neutrophil reverse migration to elucidate the potential mechanisms and factors influencing reverse migration and its impact on inflammation in different disease processes.

The neutrophil: A key resourceful agent in immune‐mediated vasculitis

The term "vasculitis" refers to a group of rare immune-mediated diseases characterized by the dysregulated immune system attacking blood vessels located in any organ of the body, including the skin, lungs, and kidneys. Vasculitides are classified according to the size of the vessel that is affected. Although this observation is not specific to small-, medium-, or large-vessel vasculitides, patients show a high circulating neutrophil-to-lymphocyte ratio, suggesting the direct or indirect involvement of neutrophils in these diseases. As first responders to infection or inflammation, neutrophils release cytotoxic mediators, including reactive oxygen species, proteases, and neutrophil extracellular traps. If not controlled, this dangerous arsenal can injure the vascular system, which acts as the main transport route for neutrophils, thereby amplifying the initial inflammatory stimulus and the recruitment of immune cells. This review highlights the ability of neutrophils to "set the tone" for immune cells and other cells in the vessel wall. Considering both their long-established and newly described roles, we extend their functions far beyond their direct host-damaging potential. We also review the roles of neutrophils in various types of primary vasculitis, including immune complex vasculitis, anti-neutrophil cytoplasmic antibody-associated vasculitis, polyarteritis nodosa, Kawasaki disease, giant cell arteritis, Takayasu arteritis, and Behçet's disease.

Lung recovery from DNA damage induced by graphene oxide is dependent on size, dose and inflammation profile

BACKGROUND

A key aspect of any new material safety assessment is the evaluation of their in vivo genotoxicity. Graphene oxide (GO) has been studied for many promising applications, but there are remaining concerns about its safety profile, especially after inhalation. Herein we tested whether GO lateral dimension, comparing micrometric (LGO) and nanometric (USGO) GO sheets, has a role in the formation of DNA double strand breaks in mouse lungs. We used spatial resolution and differential cell type analysis to measure DNA damages in both epithelial and immune cells, after either single or repeated exposure.

RESULTS

GO induced DNA damages were size and dose dependent, in both exposure scenario. After single exposure to a high dose, both USGO and LGO induced significant DNA damage in the lung parenchyma, but only during the acute phase response (p < 0.05 for USGO; p < 0.01 for LGO). This was followed by a fast lung recovery at day 7 and 28 for both GOs. When evaluating the chronic impact of GO after repeated exposure, only a high dose of LGO induced long-term DNA damages in lung alveolar epithelia (at 84 days, p < 0.05). Regardless of size, low dose GO did not induce any significant DNA damage after repeated exposure. A multiparametric correlation analysis of our repeated exposure data revealed that transient or persistent inflammation and oxidative stress were associated to either recovery or persistent DNA damages. For USGO, recovery from DNA damage was correlated to efficient recovery from acute inflammation (i.e., significant secretion of SAA3, p < 0.001; infiltration of neutrophils, p < 0.01). In contrast, the persistence of LGO in lungs was associated to a long-lasting presence of multinucleated macrophages (up to 84 days, p < 0.05), an underlying inflammation (IL-1α secretion up to 28 days, p < 0.05) and the presence of persistent DNA damages at 84 days.

CONCLUSIONS

Overall these results highlight the importance of the exposure scenario used. We showed that LGO was more genotoxic after repeated exposure than single exposure due to persistent lung inflammation. These findings are important in the context of human health risk assessment and toward establishing recommendations for a safe use of graphene based materials in the workplace.

Sterile inflammation alters neutrophil kinetics in mice

Neutrophils play a crucial role in establishing inflammation in response to an infection or injury, but their production rates, as well as blood and tissue residence times, remain poorly characterized under these conditions. Herein, using a biomaterial implant model to establish inflammation followed by in vivo tracking of newly formed neutrophils, we determine neutrophil kinetics under inflammatory conditions. To obtain quantifiable information from our experimental observations, we develop an ordinary differential equation-based mathematical model to extract kinetic parameters. Our data show that in the presence of inflammation resulting in emergency granulopoiesis-like conditions, neutrophil maturation time in the bone marrow reduces by around 60% and reduced half-life in the blood, compared with noninflammatory conditions. Additionally, neutrophil residence time at the inflammatory site increases by 2-fold. Together, these data improve our understanding of neutrophil kinetics under inflammatory conditions, which could pave the way for therapies that focus on modulating in vivo neutrophil dynamics.

Opportunities presented by zebrafish larval models to study neutrophil function in tissues

Appropriate neutrophil function is essential for innate immune defence and to avoid inflammatory pathology. Neutrophils can adapt their responses according to their environment and recently, the existence of multiple distinct neutrophil populations has been confirmed in both health and disease. However, the study of neutrophil functions in their tissue environment has remained challenging, and for instance, the relationship between neutrophil maturity and function is not fully understood. Many neutrophil morphological and functional features are highly conserved between mammals and non-mammalian vertebrates. This enables the use of the transparent and genetically tractable zebrafish larvae to study neutrophil biology. We review data on the development and function of zebrafish larval neutrophils and advances zebrafish have brought to studies of neutrophil biology. In addition, we discuss opportunities and aspects to be considered when using the larval zebrafish model to further enhance our understanding of neutrophil function in health and disease.

Immune Cell Plasticity in Inflammation: Insights into Description and Regulation of Immune Cell Phenotypes

Inflammation is a life-saving immune reaction occurring in response to invading pathogens. Nonetheless, inflammation can also occur in an uncontrolled, unrestricted manner, leading to chronic disease and organ damage. Mechanisms triggering an inflammatory response, hindering such a response, or leading to its resolution are well-studied but so far insufficiently elucidated with regard to precise therapeutic interventions. Notably, as an immune reaction evolves, requirements and environments for immune cells change, and thus cellular phenotypes adapt and shift, leading to the appearance of distinct cellular subpopulations with new functional features. In this article, we aim to highlight properties of, and overarching regulatory factors involved in, the occurrence of immune cell phenotypes with a special focus on neutrophils, macrophages and platelets. Additionally, we point out implications for both diagnostics and therapeutics in inflammation research.

Neutrophils: Amoeboid Migration and Swarming Dynamics in Tissues

Neutrophils are key cells of our innate immune response with essential roles for eliminating bacteria and fungi from tissues. They are also the prototype of an amoeboid migrating leukocyte. As one of the first blood-recruited immune cell types during inflammation and infection, these cells can invade almost any tissue compartment. Once in the tissue, neutrophils undergo rapid shape changes and migrate at speeds higher than most other immune cells. They move in a substrate-independent manner in interstitial spaces and do not follow predetermined tissue paths. Instead, neutrophil navigation is largely shaped by the chemokine and chemoattractant milieu around them. This highlights the decisive role of attractant-sensing G-protein coupled receptors (GPCRs) and downstream molecular pathways for controlling amoeboid neutrophil movement in tissues. A diverse repertoire of cell-surface expressed GPCRs makes neutrophils the perfect sentinel cell type to sense and detect danger-associated signals released from wounds, inflamed interstitium, dying cells, complement factors or directly from tissue-invading microbes. Moreover, neutrophils release attractants themselves, which allows communication and coordination between individual cells of a neutrophil population. GPCR-mediated positive feedback mechanisms were shown to underlie neutrophil swarming, a population response that amplifies the recruitment of amoeboid migrating neutrophils to sites of tissue injury and infection. Here we discuss recent findings and current concepts that counteract excessive neutrophil accumulation and swarm formation. In particular, we will focus on negative feedback control mechanisms that terminate neutrophil swarming to maintain the delicate balance between tissue surveillance, host protection and tissue destruction.

Neutrophils in acute inflammation: current concepts and translational implications

Modulation of neutrophil recruitment and function is crucial for targeting inflammatory cells to sites of infection to combat invading pathogens while, at the same time, limiting host tissue injury or autoimmunity. The underlying mechanisms regulating recruitment of neutrophils, 1 of the most abundant inflammatory cells, have gained increasing interest over the years. The previously described classical recruitment cascade of leukocytes has been extended to include capturing, rolling, adhesion, crawling, and transmigration, as well as a reverse-transmigration step that is crucial for balancing immune defense and control of remote organ endothelial leakage. Current developments in the field emphasize the importance of cellular interplay, tissue environmental cues, circadian rhythmicity, detection of neutrophil phenotypes, differential chemokine sensing, and contribution of distinct signaling components to receptor activation and integrin conformations. The use of therapeutics modulating neutrophil activation responses, as well as mutations causing dysfunctional neutrophil receptors and impaired signaling cascades, have been defined in translational animal models. Human correlates of such mutations result in increased susceptibility to infections or organ damage. This review focuses on current advances in the understanding of the regulation of neutrophil recruitment and functionality and translational implications of current discoveries in the field with a focus on acute inflammation and sepsis.

Targeting Neutrophils for Promoting the Resolution of Inflammation

Acute inflammation is a localized and self-limited innate host-defense mechanism against invading pathogens and tissue injury. Neutrophils, the most abundant immune cells in humans, play pivotal roles in host defense by eradicating invading pathogens and debris. Ideally, elimination of the offending insult prompts repair and return to homeostasis. However, the neutrophils` powerful weaponry to combat microbes can also cause tissue damage and neutrophil-driven inflammation is a unifying mechanism for many diseases. For timely resolution of inflammation, in addition to stopping neutrophil recruitment, emigrated neutrophils need to be disarmed and removed from the affected site. Accumulating evidence documents the phenotypic and functional versatility of neutrophils far beyond their antimicrobial functions. Hence, understanding the receptors that integrate opposing cues and checkpoints that determine the fate of neutrophils in inflamed tissues provides insight into the mechanisms that distinguish protective and dysregulated, excessive inflammation and govern resolution. This review aims to provide a brief overview and update with key points from recent advances on neutrophil heterogeneity, functional versatility and signaling, and discusses challenges and emerging therapeutic approaches that target neutrophils to enhance the resolution of inflammation.

Neutrophils as potential therapeutic targets in hepatocellular carcinoma

The success of atezolizumab plus bevacizumab treatment contributed to a shift in systemic therapies for hepatocellular carcinoma (HCC) towards combinations that include cancer immunotherapeutic agents. Thus far, the principal focus of cancer immunotherapy has been on interrupting immune checkpoints that suppress antitumour lymphocytes. As well as lymphocytes, the HCC environment includes numerous other immune cell types, among which neutrophils are emerging as an important contributor to the pathogenesis of HCC. A growing body of evidence supports neutrophils as key mediators of the immunosuppressive environment in which some cancers develop, as well as drivers of tumour progression. If neutrophils have a similar role in HCC, approaches that target or manipulate neutrophils might have therapeutic benefits, potentially including sensitization of tumours to conventional immunotherapy. Several neutrophil-directed therapies for patients with HCC (and other cancers) are now entering clinical trials. This Review outlines the evidence in support of neutrophils as drivers of HCC and details their mechanistic roles in development, progression and metastasis, highlighting the reasons that neutrophils are well worth investigating despite the challenges associated with studying them. Neutrophil-modulating anticancer therapies entering clinical trials are also summarized.

Neutrophil Migratory Patterns: Implications for Cardiovascular Disease

The body's inflammatory response involves a series of processes that are necessary for the immune system to mitigate threats from invading pathogens. Leukocyte migration is a crucial process in both homeostatic and inflammatory states. The mechanisms involved in immune cell recruitment to the site of inflammation are numerous and require several cascades and cues of activation. Immune cells have multiple origins and can be recruited from primary and secondary lymphoid, as well as reservoir organs within the body to generate an immune response to certain stimuli. However, no matter the origin, an important aspect of any inflammatory response is the web of networks that facilitates immune cell trafficking. The vasculature is an important organ for this trafficking, especially during an inflammatory response, mainly because it allows cells to migrate towards the source of insult/injury and serves as a reservoir for leukocytes and granulocytes under steady state conditions. One of the most active and vital leukocytes in the immune system's arsenal are neutrophils. Neutrophils exist under two forms in the vasculature: a marginated pool that is attached to the vessel walls, and a demarginated pool that freely circulates within the blood stream. In this review, we seek to present the current consensus on the mechanisms involved in leukocyte margination and demargination, with a focus on the role of neutrophil migration patterns during physio-pathological conditions, in particular diabetes and cardiovascular disease.

Neutrophils-From Bone Marrow to First-Line Defense of the Innate Immune System

Neutrophils (polymorphonuclear cells; PMNs) form a first line of defense against pathogens and are therefore an important component of the innate immune response. As a result of poorly controlled activation, however, PMNs can also mediate tissue damage in numerous diseases, often by increasing tissue inflammation and injury. According to current knowledge, PMNs are not only part of the pathogenesis of infectious and autoimmune diseases but also of conditions with disturbed tissue homeostasis such as trauma and shock. Scientific advances in the past two decades have changed the role of neutrophils from that of solely immune defense cells to cells that are responsible for the general integrity of the body, even in the absence of pathogens. To better understand PMN function in the human organism, our review outlines the role of PMNs within the innate immune system. This review provides an overview of the migration of PMNs from the vascular compartment to the target tissue as well as their chemotactic processes and illuminates crucial neutrophil immune properties at the site of the lesion. The review is focused on the formation of chemotactic gradients in interaction with the extracellular matrix (ECM) and the influence of the ECM on PMN function. In addition, our review summarizes current knowledge about the phenomenon of bidirectional and reverse PMN migration, neutrophil microtubules, and the microtubule organizing center in PMN migration. As a conclusive feature, we review and discuss new findings about neutrophil behavior in cancer environment and tumor tissue.

Neutrophil Extracellular Traps, Angiogenesis and Cancer

Human neutrophils, the most abundant circulating leukocytes, are fundamental components of the host response against different pathogens. Until a few years ago, neutrophils received limited attention in cancer immunology. Recently, it was discovered that both circulating, and tumor-associated, neutrophils possess functional plasticity when exposed to various inflammatory stimuli and in the tumor microenvironment. Neutrophils and their mediators can exert several pro-tumor activities in cancer and promote metastasis through different mechanisms. Angiogenesis plays a pivotal role in inflammation and tumor growth. Activated human neutrophils release several angiogenic factors [vascular endothelial growth factor-A (VEGF-A), angiopoietin-1 (ANGPT1), CXCL8, hepatocyte growth factor (HGF), and metalloproteinase 9 (MMP-9)] and form neutrophil extracellular traps (NETs). NETs promote tumor growth and metastasis formation through several mechanisms: they can awake dormant cancer cells, capture circulating tumor cells, coat and shield cancer cells, thus preventing CD8⁺- and natural killer (NK) cell-mediated cytotoxicity. ANGPTs released by endothelial and periendothelial mural cells induce platelet-activating factor (PAF) synthesis and neutrophil adhesion to endothelial cells. NETs can directly exert several proangiogenic activities in human endothelial cells and NETs induced by ANGPTs and PAF increase several aspects of angiogenesis in vitro and in vivo. A better understanding of the pathophysiological functions of NETs in cancer and angiogenesis could be of importance in the early diagnosis, prevention and treatment of tumors.

Blocking of inflammatory heparan sulfate domains by specific antibodies is not protective in experimental glomerulonephritis

Glomerulonephritis is an acquired serious glomerular disease, which involves the interplay of many factors such as cytokines, chemokines, inflammatory cells, and heparan sulfate (HS). We previously showed that blocking of inflammatory heparan sulfate domains on cultured glomerular endothelium by specific anti-HS single chain antibodies reduced polymorphonuclear cell (PMN) adhesion and chemokine binding. We hypothesized that injection of anti-HS antibodies in PMN-driven experimental glomerulonephritis should reduce glomerular influx of PMNs and thereby lead to a better renal outcome. In contrast to our hypothesis, co-injection of anti-HS antibodies did not alter the final outcome of anti-glomerular basement membrane (anti-GBM)-induced glomerulonephritis. Glomerular PMN influx, normally peaking 2 hours after induction of glomerulonephritis with anti-GBM IgG was not reduced by co-injection of anti-HS antibodies. Four days after induction of glomerulonephritis, albuminuria, renal function, glomerular hyalinosis and fibrin deposition were similar in mice treated and not treated with anti-HS antibodies. Interestingly, we observed transient effects in mice co-injected with anti-HS antibodies compared to mice that did not receive anti-HS antibodies: (i) a decreased renal function 2 hours and 1 day after induction of glomerulonephritis; (ii) an increased albuminuria after 2 hours and 1 day; (iii) an increased glomerular fibrin deposition after 1 day; (iv) a reduced glomerular macrophage influx after 1 day; (v) a sustained glomerular presence of PMNs at day 1 and 4, accompanied by an increased renal expression of IL-6, CXCL1, ICAM-1, L-selectin, CD11b and NF-κB. The mechanism underlying these observations induced by anti-HS antibodies remains unclear, but may be explained by a temporarily altered glycocalyx and/or altered function of PMNs due to the binding of anti-HS antibodies. Nevertheless, the evaluated anti-HS antibodies do not show therapeutic potential in anti-GBM-induced glomerulonephritis. Future research should evaluate other strategies to target HS domains involved in inflammatory processes during glomerulonephritis.

Rapid orderly migration of neutrophils after traumatic brain injury depends on MMP9/13

Macrophages and granulocytes play an important role in various injuries and post-traumatic repair. Due to the limited number of neutrophils in the brain, their role in traumatic brain injury has rarely been mentioned. Here, neutrophils were found to take over the role of macrophages after brain injury in the absence of macrophages. Neutrophils have the characteristics of long residence time and number advantage to actively remove the apoptotic debris. The number of neutrophils recruited was effectively reduced by inhibiting IL-1β. Interestingly, neutrophils migrated regularly and rapidly to the wound during the early stages of brain injury through three paths. They first infiltrated the wound mainly through blood circulation around the eyes, then became unscrupulous and began to move directly across the brain. In addition, MMP9 and MMP13 were found to be related to the migration of neutrophils, and inhibition of MMP could significantly inhibit the number and speed of neutrophils' migration. Our study showed that neutrophils rely on MMP9 and MMP13 for a rapid and orderly response to brain injury to maintain central nervous system stability in the absence or decrease of macrophages.