Strategies of neutrophil diversification

Gasdermin D drives focal crystalline thrombotic microangiopathy by accelerating immunothrombosis and necroinflammation

ABSTRACT

Thrombotic microangiopathy (TMA) is characterized by immunothrombosis and life-threatening organ failure but the precise underlying mechanism driving its pathogenesis remains elusive. In this study, we hypothesized that gasdermin D (GSDMD), a pore-forming protein that serves as the final downstream effector of the pyroptosis/interleukin-1β (IL-1β) pathway, contributes to TMA and its consequences by amplifying neutrophil maturation and subsequent necrosis. Using a murine model of focal crystalline TMA, we found that Gsdmd deficiency ameliorated immunothrombosis, acute tissue injury, and failure. Gsdmd-/- mice exhibited a decrease in mature IL-1β, as well as in neutrophil maturation, β2-integrin activation, and recruitment to TMA lesions, in which they formed reduced neutrophil extracellular traps in both arteries and interstitial tissue. The GSDMD inhibitor disulfiram dose-dependently suppressed human neutrophil pyroptosis in response to cholesterol crystals. Experiments with GSDMD-deficient, human-induced, pluripotent stem cell-derived neutrophils confirmed the involvement of GSDMD in neutrophil β2-integrin activation, maturation, and pyroptosis. Both prophylactic and therapeutic administration of disulfiram protected the mice from focal TMA, acute tissue injury, and failure. Our data identified GSDMD as a key mediator of focal crystalline TMA and its consequences, including ischemic tissue infarction and organ failure. GSDMD could potentially serve as a therapeutic target for the systemic forms of TMA.

Blocking HXA3-mediated neutrophil elastase release during S. pneumoniae lung infection limits pulmonary epithelial barrier disruption and bacteremia

Streptococcus pneumoniae (Sp), a leading cause of community-acquired pneumonia, can spread from the lung into the bloodstream to cause septicemia and meningitis, with a concomitant three-fold increase in mortality. Limitations in vaccine efficacy and a rise in antimicrobial resistance have spurred searches for host-directed therapies that target pathogenic immune processes. Polymorphonuclear leukocytes (PMNs) are essential for infection control but can also promote tissue damage and pathogen spread. The major Sp virulence factor, pneumolysin (PLY), triggers acute inflammation by stimulating the 12-lipoxygenase (12-LOX) eicosanoid synthesis pathway in epithelial cells. This pathway is required for systemic spread in a mouse pneumonia model and produces a number of bioactive lipids, including hepoxilin A3 (HXA3), a hydroxy epoxide PMN chemoattractant that has been hypothesized to facilitate breach of mucosal barriers. To understand how 12-LOX-dependent inflammation promotes dissemination during Sp lung infection and dissemination, we utilized bronchial stem cell-derived air-liquid interface (ALI) cultures that lack this enzyme to show that HXA3 methyl ester (HXA3-ME) is sufficient to promote basolateral-to-apical PMN transmigration, monolayer disruption, and concomitant Sp barrier breach. In contrast, PMN transmigration in response to the non-eicosanoid chemoattractant fMLP did not lead to epithelial disruption or bacterial translocation. Correspondingly, HXA3-ME but not fMLP increased release of neutrophil elastase (NE) from Sp-infected PMNs. Pharmacologic blockade of NE secretion or activity diminished epithelial barrier disruption and bacteremia after pulmonary challenge of mice. Thus, HXA3 promotes barrier disrupting PMN transmigration and NE release, pathological events that can be targeted to curtail systemic disease following pneumococcal pneumonia.

The different impact of drug-resistant Leishmania on the transcription programs activated in neutrophils

Drug resistance threatens the effective control of infections, including parasitic diseases such as leishmaniases. Neutrophils are essential players in antimicrobial control, but their role in drug-resistant infections is poorly understood. Here, we evaluated human neutrophil response to clinical parasite strains having distinct natural drug susceptibility. We found that Leishmania antimony drug resistance significantly altered the expression of neutrophil genes, some of them transcribed by specific neutrophil subsets. Infection with drug-resistant parasites increased the expression of detoxification pathways and reduced the production of cytokines. Among these, the chemokine CCL3 was predominantly impacted, which resulted in an impaired ability of neutrophils to attract myeloid cells. Moreover, decreased myeloid recruitment when CCL3 levels are reduced was confirmed by blocking CCL3 in a mouse model. Collectively, these findings reveal that the interplay between naturally drug-resistant parasites and neutrophils modulates the infected skin immune microenvironment, revealing a key role of neutrophils in drug resistance.

Decoding the biology and clinical implication of neutrophils in intracranial aneurysm

OBJECTIVE

Abundant neutrophils have been identified in both ruptured and unruptured intracranial aneurysm (IA) domes, with their function and clinical implication being poorly characterized.

MATERIALS AND METHODS

We employed single-cell RNA sequencing (scRNA-Seq) datasets of both human and murine model, and external bulk mRNA sequencing datasets to thoroughly explore the features and functional heterogeneous of neutrophils infiltrating the IA dome.

RESULTS

We found that both unruptured and ruptured IA dome contain a substantial population of neutrophils, characterized by FCGR3B, G0S2, CSF3R, and CXCR2. These cells exhibited heterogeneity in terms of function and differentiation. Despite similar transcriptional activation, neutrophils in IA dome expressed a repertoire of gene programs that mimicked transcriptomic alterations observed from bone marrow to peripheral blood, showing self-similarity. In addition, the recruitment of neutrophils in unruptured IA was primarily mediated by monocytes/macrophages, and once ruptured, both neutrophils, and a specific subset of inflammatory smooth muscle cells (SMCs) were involved in the process. The receiver operator characteristic curve (ROC) analysis indicated that distinct neutrophil subclusters were associated with IA formation and rupture, respectively. By reviewing current studies, we found that neutrophils play a detrimental role to IA wall integrity through secreting specific ligands, ferroptosis driven by ALOX5AP and PTGS2, and the formation of neutrophil extracellular traps (NETs) mediated by PADI4.

INTERPRETATION

This study delineated the biology and potential clinical implications of neutrophils in IA dome and provided a reliable basis for future researches.

Peripheral priming induces plastic transcriptomic and proteomic responses in circulating neutrophils required for pathogen containment

Neutrophils rapidly respond to inflammation and infection, but to which degree their functional trajectories after mobilization from the bone marrow are shaped within the circulation remains vague. Experimental limitations have so far hampered neutrophil research in human disease. Here, using innovative fixation and single-cell-based toolsets, we profile human and murine neutrophil transcriptomes and proteomes during steady state and bacterial infection. We find that peripheral priming of circulating neutrophils leads to dynamic shifts dominated by conserved up-regulation of antimicrobial genes across neutrophil substates, facilitating pathogen containment. We show the TLR4/NF-κB signaling-dependent up-regulation of canonical neutrophil activation markers like CD177/NB-1 during acute inflammation, resulting in functional shifts in vivo. Blocking de novo RNA synthesis in circulating neutrophils abrogates these plastic shifts and prevents the adaptation of antibacterial neutrophil programs by up-regulation of distinct effector molecules upon infection. These data underline transcriptional plasticity as a relevant mechanism of functional neutrophil reprogramming during acute infection to foster bacterial containment within the circulation.

Neutrophils seeking new neighbors: radiotherapy affects the cellular framework and the spatial organization in a murine breast cancer model

Neutrophils are known to contribute in many aspects of tumor progression and metastasis. The presence of neutrophils or neutrophil-derived mediators in the tumor microenvironment has been associated with poor prognosis in several types of solid tumors. However, the effects of classical cancer treatments such as radiation therapy on neutrophils are poorly understood. Furthermore, the cellular composition and distribution of immune cells in the tumor is of increasing interest in cancer research and new imaging technologies allow to perform more complex spatial analyses within tumor tissues. Therefore, we aim to offer novel insight into intra-tumoral formation of cellular neighborhoods and communities in murine breast cancer. To address this question, we performed image mass cytometry on tumors of the TS/A breast cancer tumor model, performed spatial neighborhood analyses of the tumor microenvironment and quantified neutrophil-extracellular trap degradation products in serum of the mice. We show that irradiation with 2 × 8 Gy significantly alters the cellular composition and spatial organization in the tumor, especially regarding neutrophils and other cells of the myeloid lineage. Locally applied radiotherapy further affects neutrophils in a systemic manner by decreasing the serum neutrophil extracellular trap concentrations which correlates positively with survival. In addition, the intercellular cohesion is maintained due to radiotherapy as shown by E-Cadherin expression. Radiotherapy, therefore, might affect the epithelial-mesenchymal plasticity in tumors and thus prevent metastasis. Our findings underscore the growing importance of the spatial organization of the tumor microenvironment, particularly with respect to radiotherapy, and provide insight into potential mechanisms by which radiotherapy affects epithelial-mesenchymal plasticity and tumor metastasis.

Neutrophils are itching to specialize

Neutrophils are heterogeneous, but the mechanisms underlying their ability to polarize remain unclear. In this issue of Immunity, Gour et al. demonstrate that the GPCR Mrgpra1 and the neuropeptide NPFF, molecules involved in pain and itch, direct neutrophil polarization that impacts host defense and pneumonia susceptibility.

Myeloid-Derived Suppressor-like Cells as a Prognostic Marker in Critically Ill Patients: Insights from Experimental Endotoxemia and Intensive Care Patients

Patients admitted to the intensive care unit (ICU) often experience endotoxemia, nosocomial infections and sepsis. Polymorphonuclear and monocytic myeloid-derived suppressor cells (PMN-MDSCs and M-MDSCs) can have an important impact on the development of infectious diseases, but little is known about their potential predictive value in critically ill patients. Here, we used unsupervised flow cytometry analyses to quantify MDSC-like cells in healthy subjects challenged with endotoxin and in critically ill patients admitted to intensive care units and at risk of developing infections. Cells phenotypically similar to PMN-MDSCs and M-MDSCs increased after endotoxin challenge. Similar cells were elevated in patients at ICU admission and normalized at ICU discharge. A subpopulation of M-MDSC-like cells expressing intermediate levels of CD15 (CD15int M-MDSCs) was associated with overall mortality (p = 0.02). Interestingly, the high abundance of PMN-MDSCs and CD15int M-MDSCs was a good predictor of mortality (p = 0.0046 and 0.014), with area under the ROC curve for mortality of 0.70 (95% CI = 0.4-1.0) and 0.86 (0.62-1.0), respectively. Overall, our observations support the idea that MDSCs represent biomarkers for sepsis and that flow cytometry monitoring of MDSCs may be used to risk-stratify ICU patients for targeted therapy.

Whole blood stimulation as a tool for studying the human immune system

The human immune system is best accessible via tissues and organs not requiring major surgical intervention, such as blood. In many circumstances, circulating immune cells correlate with an individual's health state and give insight into physiological and pathophysiological processes. Stimulating whole blood ex vivo is a powerful tool to investigate immune responses. In the context of clinical research, the applications of whole blood stimulation include host immunity, disease characterization, diagnosis, treatment, and drug development. Here, we summarize different setups and readouts of whole blood assays and discuss applications for preclinical research and clinical practice. Finally, we propose combining whole blood stimulation with high-throughput technologies, such as single-cell RNA-sequencing, to comprehensively analyze the human immune system for the identification of biomarkers, therapeutic interventions as well as companion diagnostics.

Dying to Defend: Neutrophil Death Pathways and their Implications in Immunity

Neutrophils, accounting for ≈70% of human peripheral leukocytes, are key cells countering bacterial and fungal infections. Neutrophil homeostasis involves a balance between cell maturation, migration, aging, and eventual death. Neutrophils undergo different death pathways depending on their interactions with microbes and external environmental cues. Neutrophil death has significant physiological implications and leads to distinct immunological outcomes. This review discusses the multifarious neutrophil death pathways, including apoptosis, NETosis, pyroptosis, necroptosis, and ferroptosis, and outlines their effects on immune responses and disease progression. Understanding the multifaceted aspects of neutrophil death, the intersections among signaling pathways and ramifications of immunity will help facilitate the development of novel therapeutic methods.

Neutrophils in Physiology and Pathology

Infections, cardiovascular disease, and cancer are major causes of disease and death worldwide. Neutrophils are inescapably associated with each of these health concerns, by either protecting from, instigating, or aggravating their impact on the host. However, each of these disorders has a very different etiology, and understanding how neutrophils contribute to each of them requires understanding the intricacies of this immune cell type, including their immune and nonimmune contributions to physiology and pathology. Here, we review some of these intricacies, from basic concepts in neutrophil biology, such as their production and acquisition of functional diversity, to the variety of mechanisms by which they contribute to preventing or aggravating infections, cardiovascular events, and cancer. We also review poorly explored aspects of how neutrophils promote health by favoring tissue repair and discuss how discoveries about their basic biology inform the development of new therapeutic strategies.

G-CSF reshapes the cytosolic PCNA scaffold and modulates glycolysis in neutrophils

Cytosolic proliferating cell nuclear antigen (PCNA) is involved in neutrophil survival and function, in which it acts as a scaffold and associates with proteins involved in apoptosis, NADPH oxidase activation, cytoskeletal dynamics, and metabolism. While the PCNA interactome has been characterized in neutrophils under homeostatic conditions, less is known about neutrophil PCNA in pathophysiological contexts. Granulocyte colony-stimulating factor (G-CSF) is a cytokine produced in response to inflammatory stimuli that regulates many aspects of neutrophil biology. Here, we used isolated normal-density neutrophils from G-CSF-treated haemopoietic stem cell donors (GDs) as a model to understand the role of PCNA during inflammation. Proteomic analysis of the neutrophil cytosol revealed significant differences between GDs and healthy donors (HDs). PCNA was one of the most upregulated proteins in GDs, and the PCNA interactome was significantly different in GDs compared with HDs. Importantly, while PCNA associated with almost all enzymes involved in glycolysis in HDs, these associations were decreased in GDs. Functionally, neutrophils from GDs had a significant increase in glycolysis compared with HDs. Using p21 competitor peptides, we showed that PCNA negatively regulates neutrophil glycolysis in HDs but had no effect on GD neutrophils. These data demonstrate that G-CSF alters the PCNA scaffold, affecting interactions with key glycolytic enzymes, and thus regulates glycolysis, the main energy pathway utilized by neutrophils. By this selective control of glycolysis, PCNA can organize neutrophils functionality in parallel with other PCNA mechanisms of prolonged survival. PCNA may therefore be instrumental in the reprogramming that neutrophils undergo in inflammatory or tumoral settings.

Generative modeling of biological shapes and images using a probabilistic α-shape sampler

Understanding morphological variation is an important task in many areas of computational biology. Recent studies have focused on developing computational tools for the task of sub-image selection which aims at identifying structural features that best describe the variation between classes of shapes. A major part in assessing the utility of these approaches is to demonstrate their performance on both simulated and real datasets. However, when creating a model for shape statistics, real data can be difficult to access and the sample sizes for these data are often small due to them being expensive to collect. Meanwhile, the current landscape of generative models for shapes has been mostly limited to approaches that use black-box inference-making it difficult to systematically assess the power and calibration of sub-image models. In this paper, we introduce the α -shape sampler: a probabilistic framework for generating realistic 2D and 3D shapes based on probability distributions which can be learned from real data. We demonstrate our framework using proof-of-concept examples and in two real applications in biology where we generate (i) 2D images of healthy and septic neutrophils and (ii) 3D computed tomography (CT) scans of primate mandibular molars. The α -shape sampler R package is open-source and can be downloaded at https://github.com/lcrawlab/ashapesampler.

Phenotypic and Functional Diversity of Neutrophils in Gut Inflammation and Cancer

Neutrophils [polymorphonuclear leukocytes (PMNs)] execute important effector functions protecting the host against invading pathogens. However, their activity in tissue can exacerbate inflammation and inflammation-associated tissue injury and tumorigenesis. Until recently, PMNs were considered to be short-lived, terminally differentiated phagocytes. However, this view is rapidly changing with the emerging evidence of increased PMN lifespan in tissues, PMN plasticity, and phenotypic heterogeneity. Specialized PMN subsets have been identified in inflammation and in developing tumors, consistent with both beneficial and detrimental functions of PMNs in these conditions. Because PMN and tumor-associated neutrophil activity and the resulting beneficial/detrimental impacts primarily occur after homing to inflamed tissue/tumors, studying the underlying mechanisms of PMN/tumor-associated neutrophil trafficking is of high interest and clinical relevance. This review summarizes some of the key findings from over a decade of work from my laboratory and others on the regulation of PMN recruitment and identification of phenotypically and functionally diverse PMN subtypes as they pertain to gut inflammation and colon cancer.

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.

Androgen receptor inhibition suppresses anti-tumor neutrophil response against bone metastatic prostate cancer via regulation of TβRI expression

Bone metastatic disease of prostate cancer (PCa) is incurable and progression in bone is largely dictated by tumor-stromal interactions in the bone microenvironment. We showed previously that bone neutrophils initially inhibit bone metastatic PCa growth yet metastatic PCa becomes resistant to neutrophil response. Further, neutrophils isolated from tumor-bone lost their ability to suppress tumor growth through unknown mechanisms. With this study, our goal was to define the impact of metastatic PCa on neutrophil function throughout tumor progression and to determine the potential of neutrophils as predictive biomarkers of metastatic disease. Using patient peripheral blood polymorphonuclear neutrophils (PMNs), we identified that PCa progression dictates PMN cell surface markers and gene expression, but not cytotoxicity against PCa. Importantly, we also identified a novel phenomenon in which second generation androgen deprivation therapy (ADT) suppresses PMN cytotoxicity via increased transforming growth factor beta receptor I (TβRI). High dose testosterone and genetic or pharmacologic TβRI inhibition rescued androgen receptor-mediated neutrophil suppression and restored neutrophil anti-tumor immune response. These studies highlight the ability to leverage standard-care ADT to generate neutrophil anti-tumor responses against bone metastatic PCa.

Intracellular Major Histocompatibility Complex Class II and C-X-C Motif Chemokine Ligand 10-Expressing Neutrophils Indicate the State of Anti-Tumor Activity Induced by Bacillus Calmette-Guérin

(1) Background: Inflammatory responses induce the formation of both anti-tumor and pro-tumor neutrophils known as myeloid-derived suppressor cells (MDSCs). Intermittent intravesical infusion of Bacillus Calmette-Guérin (BCG) is an established cancer immunotherapy for non-muscle-invasive bladder cancer (NMIBC). However, the types of neutrophils induced via the inflammatory response to both tumor-bearing and BCG remain unclear. (2) Methods: We therefore analyzed neutrophil dynamics in the peripheral blood and urine of patients with NMIBC who received BCG therapy. Further, we analyzed the effects of BCG in a mouse intraperitoneal tumor model. (3) Results: BCG therapy induced the formation of CXCL10 and MHC class II-positive neutrophils in the urine of patients with NMIBC but did not induce MDSC formation. CXCL10- and MHC class II-expressing neutrophils were detected in peritoneal exudate cells formed after BCG administration. Partial neutrophil depletion using an anti-Ly6G antibody suppressed the upregulation of CXCL10 and MHC class II in neutrophils and reversed the anti-tumor activity of BCG in mouse models. (4) Conclusions: These results indicated that intracellular MHC class II- and CXCL10-expressing neutrophils indicate the state of anti-tumor activity induced via BCG. The status of neutrophils in mixed inflammation of immunosuppressive and anti-tumor responses may therefore be useful for evaluating immunological systemic conditions.

Hemostasis without clot formation: how platelets guard the vasculature in inflammation, infection, and malignancy

Platelets are key vascular effectors in hemostasis, with activation signals leading to fast recruitment, aggregation, and clot formation. The canonical process of hemostasis is well-characterized and shares many similarities with pathological thrombus formation. However, platelets are also crucially involved in the maintenance of vascular integrity under both steady-state and inflammatory conditions by ensuring blood vessel homeostasis and preventing microbleeds. In these settings, platelets use distinct receptors, signaling pathways, and ensuing effector functions to carry out their deeds. Instead of simply forming clots, they mainly act as individual sentinels that swiftly adapt their behavior to the local microenvironment. In this review, we summarize previously recognized and more recent studies that have elucidated how anucleate, small platelets manage to maintain vascular integrity when faced with challenges of infection, sterile inflammation, and even malignancy. We dissect how platelets are recruited to the vascular wall, how they identify sites of injury, and how they prevent hemorrhage as single cells. Furthermore, we discuss mechanisms and consequences of platelets' interaction with leukocytes and endothelial cells, the relevance of adhesion as well as signaling receptors, in particular immunoreceptor tyrosine-based activation motif receptors, and cross talk with the coagulation system. Finally, we outline how recent insights into inflammatory hemostasis and vascular integrity may aid in the development of novel therapeutic strategies to prevent hemorrhagic events and vascular dysfunction in patients who are critically ill.

Neutrophils' Contribution to Periodontitis and Periodontitis-Associated Cardiovascular Diseases

Neutrophils represent the primary defense against microbial threats playing a pivotal role in maintaining tissue homeostasis. This review examines the multifaceted involvement of neutrophils in periodontitis, a chronic inflammatory condition affecting the supporting structures of teeth summarizing the contribution of neutrophil dysfunction in periodontitis and periodontal-related comorbidities. Periodontitis, a pathological condition promoted by dysbiosis of the oral microbiota, is characterized by the chronic inflammation of the gingiva and subsequent tissue destruction. Neutrophils are among the first immune cells recruited to the site of infection, releasing antimicrobial peptides, enzymes, and reactive oxygen species to eliminate pathogens. The persistent inflammatory state in periodontitis can lead to aberrant neutrophil activation and a sustained release of proinflammatory mediators, finally resulting in tissue damage, bone resorption, and disease progression. Growing evidence now points to the correlation between periodontitis and systemic comorbidities. Indeed, the release of inflammatory mediators, immune complexes, and oxidative stress by neutrophils, bridge the gap between local and systemic immunity, thus highlighting neutrophils as key players in linking periodontal inflammation to chronic conditions, including cardiovascular diseases, diabetes mellitus, and rheumatoid arthritis. This review underscores the crucial role of neutrophils in the pathogenesis of periodontitis and the complex link between neutrophil dysfunction, local inflammation, and systemic comorbidities. A comprehensive understanding of neutrophil contribution to periodontitis development and their impact on periodontal comorbidities holds significant implications for the management of oral health. Furthermore, it highlights the need for the development of novel approaches aimed at limiting the persistent recruitment and activation of neutrophils, also reducing the impact of periodontal inflammation on broader health contexts, offering promising avenues for improved disease management and patient care.

CREB1-driven CXCR4hi neutrophils promote skin inflammation in mouse models and human patients

Neutrophils have a pathogenic function in inflammation via releasing pro-inflammatory mediators or neutrophil extracellular traps (NETs). However, their heterogeneity and pro-inflammatory mechanisms remain unclear. Here, we demonstrate that CXCR4hi neutrophils accumulate in the blood and inflamed skin in human psoriasis, and correlate with disease severity. Compared to CXCR4lo neutrophils, CXCR4hi neutrophils have enhanced NETs formation, phagocytic function, neutrophil degranulation, and overexpression of pro-inflammatory cytokines and chemokines in vitro. This is accompanied by a metabolic shift in CXCR4hi neutrophils toward glycolysis and lactate release, thereby promoting vascular permeability and remodeling. CXCR4 expression in neutrophils is dependent on CREB1, a transcription factor activated by TNF and CXCL12, and regulated by de novo synthesis. In vivo, CXCR4hi neutrophil infiltration amplifies skin inflammation, whereas blockade of CXCR4hi neutrophils through CXCR4 or CXCL12 inhibition leads to suppression of immune responses. In this work, our study identifies CREB1 as a critical regulator of CXCR4hi neutrophil development and characterizes the contribution of CXCR4hi neutrophils to vascular remodeling and inflammatory responses in skin.

Whole-genome screens reveal regulators of differentiation state and context-dependent migration in human neutrophils

Neutrophils are the most abundant leukocyte in humans and provide a critical early line of defense as part of our innate immune system. We perform a comprehensive, genome-wide assessment of the molecular factors critical to proliferation, differentiation, and cell migration in a neutrophil-like cell line. Through the development of multiple migration screen strategies, we specifically probe directed (chemotaxis), undirected (chemokinesis), and 3D amoeboid cell migration in these fast-moving cells. We identify a role for mTORC1 signaling in cell differentiation, which influences neutrophil abundance, survival, and migratory behavior. Across our individual migration screens, we identify genes involved in adhesion-dependent and adhesion-independent cell migration, protein trafficking, and regulation of the actomyosin cytoskeleton. This genome-wide screening strategy, therefore, provides an invaluable approach to the study of neutrophils and provides a resource that will inform future studies of cell migration in these and other rapidly migrating cells.

Canonical and atypical chemokine receptors in the neutrophil life cycle

Chemokines are mainly studied for their local function in the control of leukocyte extravasation in homeostatic and inflammatory conditions. However, they have additional roles at the systemic level including the regulation of the hematopoietic process and leukocyte differentiation. Due to the redundancy and pleiotropicity of the chemokine system, chemokines have often multiple and complex roles in neutrophil differentiation ranging from retention and control of proliferation of progenitors to the mobilization of mature cells from the bone marrow (BM) to the bloodstream and their further differentiation in tissues. Atypical chemokine receptors (ACKRs) are regulators of the chemokine system by controlling chemokine bioavailability and chemokine receptor function. Even though ACKRs bind a wide range of chemokines, they appear to have a selective role in the process of neutrophil production and differentiation. The aim of this review is to give an overview of the current evidence regarding the role of chemokines and chemokine receptors in the life of neutrophils with a focus on the regulation exerted by ACKRs.

Neutrophil diversity and plasticity: Implications for organ transplantation

Neutrophils, as the first defenders against external microbes and stimuli, are highly active and finely regulated innate immune cells. Emerging evidence has challenged the conventional dogma that neutrophils are a homogeneous population with a short lifespan that promotes tissue damage. Recent findings on neutrophil diversity and plasticity in homeostatic and disease states have centered on neutrophils in the circulation. In contrast, a comprehensive understanding of tissue-specialized neutrophils in health and disease is still lacking. This article will first discuss how multiomics advances have contributed to our understanding of neutrophil heterogeneity and diversification in resting and pathological settings. This discussion will be followed by a focus on the heterogeneity and role of neutrophils in solid organ transplantation and how neutrophils may contribute to transplant-related complications. The goal of this article is to provide an overview of the research on the involvement of neutrophils in transplantation, with the aim that this may draw attention to an underappreciated area of neutrophil research.

Olfactomedin-4 + neutrophils exacerbate intestinal epithelial damage and worsen host survival after Clostridioides difficile infection

Neutrophils are key first responders to Clostridioides difficile infection (CDI). Excessive tissue and blood neutrophils are associated with worse histopathology and adverse outcomes, however their functional role during CDI remains poorly defined. Utilizing intestinal epithelial cell (IEC)-neutrophil co-cultures and a pre-clinical animal model of CDI, we show that neutrophils exacerbate C. difficile -induced IEC injury. We utilized cutting-edge single-cell transcriptomics to illuminate neutrophil subtypes and biological pathways that could exacerbate CDI-associated IEC damage. As such, we have established the first transcriptomics atlas of bone marrow (BM), blood, and colonic neutrophils after CDI. We found that CDI altered the developmental trajectory of BM and blood neutrophils towards populations that exhibit gene signatures associated with pro-inflammatory responses and neutrophil-mediated tissue damage. Similarly, the transcriptomic signature of colonic neutrophils was consistent with hyper-inflammatory and highly differentiated cells that had amplified expression of cytokine-mediated signaling and degranulation priming genes. One of the top 10 variable features in colonic neutrophils was the gene for neutrophil glycoprotein, Olfactomedin 4 (OLFM4). CDI enhanced OLFM4 mRNA and protein expression in neutrophils, and OLFM4 + cells aggregated to areas of severe IEC damage. Compared to uninfected controls, both humans and mice with CDI had higher concentrations of circulating OLFM4; and in mice, OLFM4 deficiency resulted in faster recovery and better survival after infection. Collectively, these studies provide novel insights into neutrophil-mediated pathology after CDI and highlight the pathogenic role of OLFM4 + neutrophils in regulating CDI-induced IEC damage.

One Sentence Summary

Utilizing single-cell transcriptomics, IEC-epithelial co-cultures, and pre-clinical models of CDI, we have identified a subset of neutrophils that are marked by OLFM4 expression as pathogenic determinants of IEC barrier damage after CDI.

Evaluation of the innate immune response of caprine neutrophils against Mycobacterium avium subspecies paratuberculosis in vitro

Neutrophils constitute an essential component of the innate immune response, readily killing most bacteria through phagocytosis, degranulation, and the release of neutrophil extracellular traps (NETs) among other mechanisms. These cells play an unclear role in mycobacterial infections such as Mycobacterium avium subspecies paratuberculosis (Map), the etiological agent of paratuberculosis, and its response is particularly understudied in ruminants. Herein, a wide set of techniques were adapted, or newly developed, to study the in vitro response of caprine neutrophils after Map infection. Immunofluorescence was used to demonstrate, simultaneously, chemotaxis, phagocytosis, degranulation, and NETs. The quantification of neutrophil phagocytic activity against Map at a 1:10 multiplicity of infection (MOI), through flow cytometry, showed values that varied from 4.54 to 5.63% of phagocyting neutrophils. By immunofluorescence, a 73.3 ± 14.5% of the fields showed NETs, and the mean release of DNA, attributable to NETosis, calculated through a fluorometric method, was 16.2 ± 3.5%. In addition, the RNA expression of TGF-β, TNF and IL-1β cytokines, measured through reverse transcription qPCR, was significantly higher in the two latter. Overall, neutrophil response was proportional to the number of bacteria. This work confirms that the simultaneous study of several neutrophil mechanisms, and the combination of different methodologies, are essential to reach a comprehensive understanding of neutrophil response against pathogens, demonstrates that, in vitro, caprine neutrophils display a strong innate response against Map, using their entire repertoire of effector functions, and sets the basis for further in vitro and in vivo studies on the role of neutrophils in paratuberculosis.

Mycobacterium tuberculosis in a Trap: The Role of Neutrophil Extracellular Traps in Tuberculosis

Mycobacterium tuberculosis complex causes tuberculosis (TB), a disease that causes pulmonary inflammation but can also affect other tissues. Despite macrophages having a defined role in TB immunopathogenesis, other innate immune cells, such as neutrophils, are involved in this process. These cells have high phagocytic ability and a microbial-killing machine comprised of enzymes, antimicrobial peptides, and reactive oxygen species. In the last two decades, a new neutrophil immune response, the neutrophil extracellular traps (NETs), has been intensely researched. NETs comprise DNA associated with histones, enzymes, and antimicrobial peptides. These structures are related to antimicrobial immune response and some immuno-pathogenesis mechanisms. This mini review highlights the role of NETs in tuberculosis and how they can be helpful as a diagnostic tool and/or therapeutic target.

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.