Neutrophils control the magnitude and spread of the immune response in a thromboxane A2-mediated process

Progress in cytokine research for ARDS: A comprehensive review

Introduction

Acute respiratory distress syndrome (ARDS) is a critical form of acute respiratory failure characterized by diffuse alveolar damage, refractory hypoxemia, and non-cardiogenic pulmonary edema, resulting in high mortality. Dysregulated inflammation, driven by cytokines, is central to ARDS pathogenesis, progression, and prognosis.

Objective

This review synthesizes current knowledge on the role of cytokines in ARDS and evaluates their potential as therapeutic targets, offering new insights for clinical management.

Methods

A comprehensive analysis of recent studies was conducted to explore the roles of pro-inflammatory cytokines (e.g., IL-1β, IL-6, IL-8) and anti-inflammatory cytokines (e.g., IL-10, IL-22) in ARDS pathogenesis and to assess current and emerging therapies targeting these cytokines.

Results

Pro-inflammatory cytokines are crucial in initiating inflammatory responses and lung injury in early ARDS, while anti-inflammatory cytokines help regulate and resolve inflammation. Targeted therapies, such as IL-1 and IL-6 inhibitors, show potential in managing ARDS, particularly in COVID-19, but their clinical efficacy is still debated. Combination therapy strategies may enhance outcomes, but further large-scale, multicenter randomized controlled trials are required to establish their safety and efficacy.

Conclusion

Understanding cytokine regulation in ARDS could lead to innovative therapeutic approaches. Future research should focus on cytokine roles across ARDS subtypes and stages and develop biomarker-driven, individualized treatments.

Complex role of neutrophils in the tumor microenvironment: an avenue for novel immunotherapies

Neutrophils, which originate from the bone marrow and are characterized by a segmented nucleus and a brief lifespan, have a crucial role in the body's defense against infections and acute inflammation. Recent research has uncovered the complex roles of neutrophils as regulators in tumorigenesis, during which neutrophils exhibit a dualistic nature that promotes or inhibits tumor progression. This adaptability is pivotal within the tumor microenvironment (TME). In this review, we provide a comprehensive characterization of neutrophil plasticity and heterogeneity, aiming to illuminate current research findings and discuss potential therapeutic avenues. By delineating the intricate interplay of neutrophils in the TME, this review further underscores the urgent need to understand the dual functions of neutrophils with particular emphasis on the anti-tumor effects to facilitate the development of effective therapeutic strategies against cancer.

Single-Cell RNA Sequencing Reveals Transcriptional Landscape of Neutrophils and Highlights the Role of TREM-1 in EAE

BACKGROUND AND OBJECTIVES

Neutrophils, underestimated in multiple sclerosis (MS), are gaining increased attention for their significant functions in patients with MS and the experimental autoimmune encephalomyelitis (EAE) animal model. However, the precise role of neutrophils in cervical lymph nodes (CLNs), the primary CNS-draining lymph nodes where the autoimmune response is initiated during the progression of EAE, remains poorly understood.

METHODS

Applying single-cell RNA sequencing (scRNA-seq), we constructed a comprehensive immune cell atlas of CLNs during development of EAE. Through this atlas, we concentrated on and uncovered the transcriptional landscape, phenotypic and functional heterogeneity of neutrophils, and their crosstalk with immune cells within CLNs in the neuroinflammatory processes in EAE.

RESULTS

Notably, we observed a substantial increase in the neutrophil population in EAE mice, with a particular emphasis on the significant rise within the CLNs. Neutrophils in CLNs were categorized into 3 subtypes, and we explored the specific roles and developmental trajectories of each distinct neutrophil subtype. Neutrophils were found to engage in extensive interactions with other immune cells, playing crucial roles in T-cell activation. Moreover, our findings highlighted the strong migratory ability of neutrophils to CLNs, partly regulated by triggering the receptor expressed on myeloid cells 1 (TREM-1). Inhibiting TREM1 with LR12 prevents neutrophil migration both in vivo and in vitro. In addition, in patients with MS, we confirmed an increase in peripheral neutrophils with an upregulation of TREM-1.

DISCUSSION

Our research provides a comprehensive and precise single-cell atlas of CLNs in EAE, highlighting the role of neutrophils in regulating the periphery immune response. In addition, TREM-1 emerged as an essential regulator of neutrophil migration to CLNs, holding promise as a potential therapeutic target in MS.

Sphingosine-1-Phosphate Receptor 4 links neutrophils and early local inflammation to lymphocyte recruitment into the draining lymph node to facilitate robust germinal center formation

The successful development of germinal centers (GC) relies heavily on innate mechanisms to amplify the initial inflammatory cascade. In addition to their role in antigen presentation, innate cells are essential for the redirection of circulating lymphocytes toward the draining lymph node (dLN) to maximize antigen surveillance. Sphingosine-1-Phosphate (S1P) and its receptors (S1PR1-5) affect various aspects of immunity; however, the role of S1PR4 in regulating an immune response is not well understood. Here we use a footpad model of localized TH1 inflammation to carefully monitor changes in leukocyte populations within the blood, the immunized tissue, and the dLN. Within hours of immunization, neutrophils failed to adequately mobilize and infiltrate into the footpad tissue of S1PR4-/- mice, thereby diminishing the local vascular changes thought to be necessary for redirecting circulating cells toward the inflamed region. Neutrophil depletion with anti-Ly6G antibodies significantly reduced early tissue edema as well as the redirection and initial accumulation of naïve lymphocytes in dLN of WT mice, while the effects were less prominent or absent in S1PR4-/- dLN. Adoptive transfer experiments further demonstrated that the lymphocyte homing deficiencies in vivo were not intrinsic to the donor S1PR4-/- lymphocytes, but were instead attributed to differences within the S1PR4-deficient host. Reduced cell recruitment in S1PR4-/- mice would seed the dLN with fewer antigen-respondent lymphocytes and indeed, dLN hypertrophy at the peak of the immune response was severely diminished, with attenuated GC and activation pathways in these mice. Histological examination of the S1PR4-/- dLN also revealed an underdeveloped vascular network with reduced expression of the leukocyte tethering ligand, PNAd, within high endothelial venule regions, suggesting inadequate growth of the dLN meant to support a robust GC response. Thus, our study reveals that S1PR4 may link early immune modulation by neutrophils to the initial recruitment of circulating lymphocytes and downstream expansion and maturation of the dLN, thereby contributing to optimal GC development during an adaptive response.

Roles of prostaglandins in immunosuppression

Prostaglandins (PGs) play a crucial and multifaceted role in various physiological processes such as intercellular signaling, inflammation regulation, neurotransmission, vasodilation, vasoconstriction, and reproductive functions. The diversity and biological significance of these effects are contingent upon the specific types or subtypes of PGs, with each PG playing a crucial role in distinct physiological and pathological processes. Particularly within the immune system, PGs are essential in modulating the function of immune cells and the magnitude and orientation of immune responses. Hence, a comprehensive comprehension of the functions PG signaling pathways in immunosuppressive regulation holds substantial clinical relevance for disease prevention and treatment strategies. The manuscript provides a review of recent developments in PG signaling in immunosuppressive regulation. Furthermore, the potential clinical applications of PGs in immunosuppression are also discussed. While research into the immunosuppressive effects of PGs required further exploration, targeted therapies against their immunosuppressive pathways might open new avenues for disease prevention and treatment.

Neutrophils at the Crossroads: Unraveling the Multifaceted Role in the Tumor Microenvironment

Over the past decade, research has prominently established neutrophils as key contributors to the intricate landscape of tumor immune biology. As polymorphonuclear granulocytes within the innate immune system, neutrophils play a pivotal and abundant role, constituting approximately ∼70% of all peripheral leukocytes in humans and ∼10-20% in mice. This substantial presence positions them as the frontline defense against potential threats. Equipped with a diverse array of mechanisms, including reactive oxygen species (ROS) generation, degranulation, phagocytosis, and the formation of neutrophil extracellular traps (NETs), neutrophils undeniably serve as indispensable components of the innate immune system. While these innate functions enable neutrophils to interact with adaptive immune cells such as T, B, and NK cells, influencing their functions, they also engage in dynamic interactions with rapidly dividing tumor cells. Consequently, neutrophils are emerging as crucial regulators in both pro- and anti-tumor immunity. This comprehensive review delves into recent research to illuminate the multifaceted roles of neutrophils. It explores their diverse functions within the tumor microenvironment, shedding light on their heterogeneity and their impact on tumor recruitment, progression, and modulation. Additionally, the review underscores their potential anti-tumoral capabilities. Finally, it provides valuable insights into clinical therapies targeting neutrophils, presenting a promising approach to leveraging innate immunity for enhanced cancer treatment.

Beyond host defense and tissue injury: the emerging role of neutrophils in tissue repair

Neutrophils, the most abundant immune cells in human blood, play a fundamental role in host defense against invading pathogens and tissue injury. Neutrophils carry potentially lethal weaponry to the affected site. Inadvertent and perpetual neutrophil activation could lead to nonresolving inflammation and tissue damage, a unifying mechanism of many common diseases. The prevailing view emphasizes the dichotomy of their function, host defense versus tissue damage. However, tissue injury may also persist during neutropenia, which is associated with disease severity and poor outcome. Numerous studies highlight neutrophil phenotypic heterogeneity and functional versatility, indicating that neutrophils play more complex roles than previously thought. Emerging evidence indicates that neutrophils actively orchestrate resolution of inflammation and tissue repair and facilitate return to homeostasis. Thus, neutrophils mobilize multiple mechanisms to limit the inflammatory reaction, assure debris removal, matrix remodeling, cytokine scavenging, macrophage reprogramming, and angiogenesis. In this review, we will summarize the homeostatic and tissue-reparative functions and mechanisms of neutrophils across organs. We will also discuss how the healing power of neutrophils might be harnessed to develop novel resolution and repair-promoting therapies while maintaining their defense functions.

Lymphatic vessel: origin, heterogeneity, biological functions, and therapeutic targets

Lymphatic vessels, comprising the secondary circulatory system in human body, play a multifaceted role in maintaining homeostasis among various tissues and organs. They are tasked with a serious of responsibilities, including the regulation of lymph absorption and transport, the orchestration of immune surveillance and responses. Lymphatic vessel development undergoes a series of sophisticated regulatory signaling pathways governing heterogeneous-origin cell populations stepwise to assemble into the highly specialized lymphatic vessel networks. Lymphangiogenesis, as defined by new lymphatic vessels sprouting from preexisting lymphatic vessels/embryonic veins, is the main developmental mechanism underlying the formation and expansion of lymphatic vessel networks in an embryo. However, abnormal lymphangiogenesis could be observed in many pathological conditions and has a close relationship with the development and progression of various diseases. Mechanistic studies have revealed a set of lymphangiogenic factors and cascades that may serve as the potential targets for regulating abnormal lymphangiogenesis, to further modulate the progression of diseases. Actually, an increasing number of clinical trials have demonstrated the promising interventions and showed the feasibility of currently available treatments for future clinical translation. Targeting lymphangiogenic promoters or inhibitors not only directly regulates abnormal lymphangiogenesis, but improves the efficacy of diverse treatments. In conclusion, we present a comprehensive overview of lymphatic vessel development and physiological functions, and describe the critical involvement of abnormal lymphangiogenesis in multiple diseases. Moreover, we summarize the targeting therapeutic values of abnormal lymphangiogenesis, providing novel perspectives for treatment strategy of multiple human diseases.

Inhibition of neutrophil extracellular trap formation alleviates vascular dysfunction in type 1 diabetic mice

While neutrophil extracellular traps (NETs) have previously been linked to some diabetes-associated complications, such as dysfunctional wound healing, their potential role in diabetic vascular dysfunction has not been studied. Diabetic Akita mice were crossed with either Elane-/- or Pad4-/- mice to generate NET-deficient diabetic mice. By 24 weeks of age, Akita aortae showed markedly impaired relaxation in response to acetylcholine, indicative of vascular dysfunction. Both Akita-Elane-/- mice and Akita-Pad4-/- mice had reduced levels of circulating NETs and improved acetylcholine-mediated aortic relaxation. Compared with wild-type aortae, the thromboxane metabolite TXB2 was roughly 10-fold higher in both intact and endothelium-denuded aortae of Akita mice. In contrast, Akita-Elane-/- and Akita-Pad4-/- aortae had TXB2 levels similar to wild type. In summary, inhibition of NETosis by two independent strategies prevented the development of vascular dysfunction in diabetic Akita mice. Thromboxane was up-regulated in the vessel walls of NETosis-competent diabetic mice, suggesting a role for neutrophils in driving the production of this vasoconstrictive and atherogenic prostanoid.

The C5a-C5aR1 complement axis is essential for neutrophil recruitment to draining lymph nodes via high endothelial venules in cutaneous leishmaniasis

Neutrophils are specialized innate immune cells known for their ability to fight pathogens. However, the mechanisms of neutrophil trafficking to lymph nodes are not fully clear. Using a murine model of dermal infection with Leishmania parasites, we observe a transient neutrophil influx in draining lymph nodes despite sustained recruitment to the infection site. Cell-tracking experiments, together with intravital two-photon microscopy, indicate that neutrophil recruitment to draining lymph nodes occurs minimally through lymphatics from the infected dermis, but mostly through blood vessels via high endothelial venules. Mechanistically, neutrophils do not respond to IL-1β or macrophage-derived molecules. Instead, they are guided by the C5a-C5aR1 axis, using L-selectin and integrins, to extravasate into the draining lymph node parenchyma. We also report that C5, the C5a precursor, is locally produced in the draining lymph node by lymphatic endothelial cells. Our data establish and detail organ-specific mechanisms of neutrophil trafficking.

Neutrophil Functional Heterogeneity and Implications for Viral Infections and Treatments

Evidence suggests that neutrophils exert specialized effector functions during infection and inflammation, and that these cells can affect the duration, severity, and outcome of the infection. These functions are related to variations in phenotypes that have implications in immunoregulation during viral infections. Although the complexity of the heterogeneity of neutrophils is still in the process of being uncovered, evidence indicates that they display phenotypes and functions that can assist in viral clearance or augment and amplify the immunopathology of viruses. Therefore, deciphering and understanding neutrophil subsets and their polarization in viral infections is of importance. In this review, the different phenotypes of neutrophils and the roles they play in viral infections are discussed. We also examine the possible ways to target neutrophil subsets during viral infections as potential anti-viral treatments.

Determinants of Innate Immunity in Visceral Leishmaniasis and Their Implication in Vaccine Development

Leishmaniasis is endemic to the tropical and subtropical regions of the world and is transmitted by the bite of an infected sand fly. The multifaceted interactions between Leishmania, the host innate immune cells, and the adaptive immunity determine the severity of pathogenesis and disease development. Leishmania parasites establish a chronic infection by subversion and attenuation of the microbicidal functions of phagocytic innate immune cells such as neutrophils, macrophages and dendritic cells (DCs). Other innate cells such as inflammatory monocytes, mast cells and NK cells, also contribute to resistance and/or susceptibility to Leishmania infection. In addition to the cytokine/chemokine signals from the innate immune cells, recent studies identified the subtle shifts in the metabolic pathways of the innate cells that activate distinct immune signal cascades. The nexus between metabolic pathways, epigenetic reprogramming and the immune signaling cascades that drive the divergent innate immune responses, remains to be fully understood in Leishmania pathogenesis. Further, development of safe and efficacious vaccines against Leishmaniasis requires a broader understanding of the early interactions between the parasites and innate immune cells. In this review we focus on the current understanding of the specific role of innate immune cells, the metabolomic and epigenetic reprogramming and immune regulation that occurs during visceral leishmaniasis, and the strategies used by the parasite to evade and modulate host immunity. We highlight how such pathways could be exploited in the development of safe and efficacious Leishmania vaccines.

Inhibition of elastase enhances the adjuvanticity of alum and promotes anti-SARS-CoV-2 systemic and mucosal immunity

We report that suppression of the serine protease elastase reshapes innate responses induced by injected vaccines containing alum adjuvant. This reprogramming improves the induction of protective antibodies in the bloodstream and stimulates innate signals, which support the development of antibody responses in mucosal tissues. Our findings identify elastase as the innate regulator that blunts the adjuvant activity of alum. They also demonstrate that vaccination via mucosal routes is not an absolute requirement for antibody responses in mucosal tissues and secretions. Supplementation of an alum-based vaccine containing SARS-CoV-2 spike protein subunit 1 as antigen increased anti–SARS-CoV-2 immunity in the blood and mucosal secretions in mice. Thus, this strategy could help in the development of future protein-based vaccines against SARS-CoV-2.

Alum, used as an adjuvant in injected vaccines, promotes T helper 2 (Th2) and serum antibody (Ab) responses. However, it fails to induce secretory immunoglobulin (Ig) A (SIgA) in mucosal tissues and is poor in inducing Th1 and cell-mediated immunity. Alum stimulates interleukin 1 (IL-1) and the recruitment of myeloid cells, including neutrophils. We investigated whether neutrophil elastase regulates the adjuvanticity of alum, and whether a strategy targeting neutrophil elastase could improve responses to injected vaccines. Mice coadministered a pharmacological inhibitor of elastase, or lacking elastase, developed high-affinity serum IgG and IgA antibodies after immunization with alum-adsorbed protein vaccines, including the spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2). These mice also developed broader antigen-specific CD4⁺ T cell responses, including high Th1 and T follicular helper (Tfh) responses. Interestingly, in the absence of elastase activity, mucosal SIgA responses were induced after systemic immunization with alum as adjuvant. Importantly, lack or suppression of elastase activity enhanced the magnitude of anti–SARS-CoV-2 spike subunit 1 (S1) antibodies, and these antibodies reacted with the same epitopes of spike 1 protein as sera from COVID-19 patients. Therefore, suppression of neutrophil elastase could represent an attractive strategy for improving the efficacy of alum-based injected vaccines for the induction of broad immunity, including mucosal immunity.

Role of Neutrophils in Cardiac Injury and Repair Following Myocardial Infarction

Neutrophils are first-line responders of the innate immune system. Following myocardial infarction (MI), neutrophils are quickly recruited to the ischemic region, where they initiate the inflammatory response, aiming at cleaning up dead cell debris. However, excessive accumulation and/or delayed removal of neutrophils are deleterious. Neutrophils can promote myocardial injury by releasing reactive oxygen species, granular components, and pro-inflammatory mediators. More recent studies have revealed that neutrophils are able to form extracellular traps (NETs) and produce extracellular vesicles (EVs) to aggravate inflammation and cardiac injury. On the contrary, there is growing evidence showing that neutrophils also exert anti-inflammatory, pro-angiogenic, and pro-reparative effects, thus facilitating inflammation resolution and cardiac repair. In this review, we summarize the current knowledge on neutrophils’ detrimental roles, highlighting the role of recently recognized NETs and EVs, followed by a discussion of their beneficial effects and molecular mechanisms in post-MI cardiac remodeling. In addition, emerging concepts about neutrophil diversity and their modulation of adaptive immunity are discussed.

Analysis of a mathematical model of immune response to fungal infection

Fungi are cells found as commensal residents, on the skin, and on mucosal surfaces of the human body, including the digestive track and urogenital track, but some species are pathogenic. Fungal infection may spread into deep-seated organs causing life-threatening infection, especially in immune-compromised individuals. Effective defense against fungal infection requires a coordinated response by the innate and adaptive immune systems. In the present paper we introduce a simple mathematical model of immune response to fungal infection consisting of three partial differential equations, for the populations of fungi (F), neutrophils (N) and cytotoxic T cells (T), taking N and T to represent, respectively, the innate and adaptive immune cells. We denote by [Formula: see text] the aggressive proliferation rate of the fungi, by [Formula: see text] and [Formula: see text] the killing rates of fungi by neutrophils and T cells, and by [Formula: see text] and [Formula: see text] the immune strengths, respectively, of N and T of an infected individual. We take the expression [Formula: see text] to represent the coordinated defense of the immune system against fungal infection. We use mathematical analysis to prove the following: If [Formula: see text], then the infection is eventually stopped, and [Formula: see text] as [Formula: see text]; and (ii) if [Formula: see text] then the infection cannot be stopped and F converges to some positive constant as [Formula: see text]. Treatments of fungal infection include anti-fungal agents and immunotherapy drugs, and both cause the parameter I to increase.

Complex and Controversial Roles of Eicosanoids in Fungal Pathogenesis

The prevalence of fungal infections has increased in immunocompromised patients, leading to millions of deaths annually. Arachidonic acid (AA) metabolites, such as eicosanoids, play important roles in regulating innate and adaptative immune function, particularly since they can function as virulence factors enhancing fungal colonization and are produced by mammalian and lower eukaryotes, such as yeasts and other fungi (Candida albicans, Histoplasma capsulatum and Cryptococcus neoformans). C. albicans produces prostaglandins (PG), Leukotrienes (LT) and Resolvins (Rvs), whereas the first two have been well documented in Cryptococcus sp. and H. capsulatum. In this review, we cover the eicosanoids produced by the host and fungi during fungal infections. These fungal-derived PGs have immunomodulatory functions analogous to their mammalian counterparts. Prostaglandin E₂ (PGE₂) protects C. albicans and C. parapsilosis cells from the phagocytic and killing activity of macrophages. H. capsulatum PGs augment the fungal burden and host mortality rates in histoplasmosis. However, PGD₂ potentiates the effects and production of LTB₄, which is a very potent neutrophil chemoattractant that enhances host responses. Altogether, these data suggest that eicosanoids, mainly PGE₂, may serve as a new potential target to combat diverse fungal infections.

Neutrophils in the Pathogenesis of Rheumatoid Arthritis and Systemic Lupus Erythematosus: Same Foe Different M.O

Dysregulated neutrophil activation contributes to the pathogenesis of autoimmune diseases including rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Neutrophil-derived reactive oxygen species (ROS) and granule proteases are implicated in damage to and destruction of host tissues in both conditions (cartilage in RA, vascular tissue in SLE) and also in the pathogenic post-translational modification of DNA and proteins. Neutrophil-derived cytokines and chemokines regulate both the innate and adaptive immune responses in RA and SLE, and neutrophil extracellular traps (NETs) expose nuclear neoepitopes (citrullinated proteins in RA, double-stranded DNA and nuclear proteins in SLE) to the immune system, initiating the production of auto-antibodies (ACPA in RA, anti-dsDNA and anti-acetylated/methylated histones in SLE). Neutrophil apoptosis is dysregulated in both conditions: in RA, delayed apoptosis within synovial joints contributes to chronic inflammation, immune cell recruitment and prolonged release of proteolytic enzymes, whereas in SLE enhanced apoptosis leads to increased apoptotic burden associated with development of anti-nuclear auto-antibodies. An unbalanced energy metabolism in SLE and RA neutrophils contributes to the pathology of both diseases; increased hypoxia and glycolysis in RA drives neutrophil activation and NET production, whereas decreased redox capacity increases ROS-mediated damage in SLE. Neutrophil low-density granulocytes (LDGs), present in high numbers in the blood of both RA and SLE patients, have opposing phenotypes contributing to clinical manifestations of each disease. In this review we will describe the complex and contrasting phenotype of neutrophils and LDGs in RA and SLE and discuss their discrete roles in the pathogenesis of each condition. We will also review our current understanding of transcriptomic and metabolomic regulation of neutrophil phenotype in RA and SLE and discuss opportunities for therapeutic targeting of neutrophil activation in inflammatory auto-immune disease.

Tumor-associated neutrophils (TANs) in human carcinoma-draining lymph nodes: a novel TAN compartment

Objectives

The role of tumor‐associated neutrophils (TANs) in the nodal spread of cancer cells remains unexplored. The present study evaluates the occurrence and clinical significance of human nodal TANs.

Methods

The relevance, derivation, phenotype and interactions of nodal TANs were explored via a large immunohistochemical analysis of carcinoma‐draining lymph nodes, and their clinical significance was evaluated on a retrospective cohort of oral squamous cell carcinomas (OSCC). The tumor‐promoting function of nodal TAN was probed in the OSCC TCGA dataset combining TAN and epithelial‐to‐mesenchymal transition (EMT) signatures.

Results

The pan‐carcinoma screening identified a consistent infiltration (59%) of CD66b⁺  TANs in tumor‐draining lymph nodes (TDLNs). Microscopic findings, including the occurrence of intra‐lymphatic conjugates of TANs and cancer cells, indicate that TANs migrate through lymphatic vessels. In vitro experiments revealed that OSCC cell lines sustain neutrophil viability and activation via release of GM‐CSF. Moreover, by retrospective analysis, a high CD66b⁺ TAN density in M‐TDLNs of OSCC (n = 182 patients) predicted a worse prognosis. The analysis of the OSCC‐TCGA dataset unveiled that the expression of a set of neutrophil‐specific genes in the primary tumor (PT) is highly associated with an EMT signature, which predicts nodal spread. Accordingly, in the PT of OSCC cases, CD66b⁺TANs co‐localised with PDPN⁺S100A9⁻ EMT‐switched tumor cells in areas of lymphangiogenesis. The pro‐EMT signature is lacking in peripheral blood neutrophils from OSCC patients, suggesting tissue skewing of TANs.

Conclusion

Our findings are consistent with a novel pro‐tumoral TAN compartment that may promote nodal spread via EMT, through the lymphatics.

Bacterial antigen is directly delivered to the draining lymph nodes and activates CD8+ T cells during Staphylococcus aureus skin infection

Staphylococcus aureus is one of the most common causes of community- and hospital-acquired bacterial infection worldwide. While neutrophils play an important role in anti-S. aureus immune defense, the role of adaptive immunity is less clear. In this study, we generated a model antigen-expressing S. aureus strain to investigate the dynamics and magnitude of T cell immune responses against this pathogen. We demonstrate that S. aureus is delivered to the draining lymph nodes (LNs) by lymphatic flow immediately after intradermal inoculation. There, the bacterium initiates CD8⁺ cytotoxic T lymphocyte (CTL) proliferation via activating LN-resident dendritic cells. Large numbers of neutrophils are recruited to the draining LNs to engulf bacteria; however, neutrophil depletion did not impact on CTL proliferation, despite increasing bacterial burden. Tissue-resident memory T cells were formed in the skin at bacteria-inoculated sites. Yet, blood and tissue-resident memory T cells failed to prevent secondary cutaneous S. aureus infection. Our study defines the delivery kinetics of S. aureus from the skin and suggests that CTLs are dispensable for protection against skin infections.

The Role of Neutrophils in Brucellosis

Brucellosis is a bacterial disease of domestic animals and humans. The pathogenic ability of Brucella organisms relies on their stealthy strategy and their capacity to replicate within host cells and to induce long-lasting infections. Brucella organisms barely induce neutrophil activation and survive within these leukocytes by resisting microbicidal mechanisms. Very few Brucella-infected neutrophils are found in the target organs, except for the bone marrow, early in infection. Still, Brucella induces a mild reactive oxygen species formation and, through its lipopolysaccharide, promotes the premature death of neutrophils, which release chemokines and express "eat me" signals. This effect drives the phagocytosis of infected neutrophils by mononuclear cells that become thoroughly susceptible to Brucella replication and vehicles for bacterial dispersion. The premature death of the infected neutrophils proceeds without NETosis, necrosis/oncosis, or classical apoptosis morphology. In the absence of neutrophils, the Th1 response exacerbates and promotes bacterial removal, indicating that Brucella-infected neutrophils dampen adaptive immunity. This modulatory effect opens a window for bacterial dispersion in host tissues before adaptive immunity becomes fully activated. However, the hyperactivation of immunity is not without a price, since neutropenic Brucella-infected animals develop cachexia in the early phases of the disease. The delay in the immunological response seems a sine qua non requirement for the development of long-lasting brucellosis. This property may be shared with other pathogenic alphaproteobacteria closely related to Brucella We propose a model in which Brucella-infected polymorphonuclear neutrophils (PMNs) function as "Trojan horse" vehicles for bacterial dispersal and as modulators of the Th1 adaptive immunity in infection.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.

Neutrophils Recirculate through Lymph Nodes to Survey Tissues for Pathogens

The adaptive immune function of lymph nodes is dependent on constant recirculation of lymphocytes. In this article, we identify neutrophils present in the lymph node at steady state, exhibiting the same capacity for recirculation. In germ-free mice, neutrophils still recirculate through lymph nodes, and in mice cohoused with wild microbiome mice, the level of neutrophils in lymph nodes increases significantly. We found that at steady state, neutrophils enter the lymph node entirely via L-selectin and actively exit via efferent lymphatics via an S1P dependent mechanism. The small population of neutrophils in the lymph node can act as reconnaissance cells to recruit additional neutrophils in the event of bacterial dissemination to the lymph node. Without these reconnaissance cells, there is a delay in neutrophil recruitment to the lymph node and a reduction in swarm formation following Staphylococcus aureus infection. This ability to recruit additional neutrophils by lymph node neutrophils is initiated by LTB4. This study establishes the capacity of neutrophils to recirculate, much like lymphocytes via L-selectin and high endothelial venules in lymph nodes and demonstrates how the presence of neutrophils at steady state fortifies the lymph node in case of an infection disseminating through lymphatics.

Mouse Models and Tools for the in vivo Study of Neutrophils

Neutrophils are the most abundant leukocytes in human blood and critical actors of the immune system. Many neutrophil functions and facets of their activity in vivo were revealed by studying genetically modified mice or by tracking fluorescent neutrophils in animals using imaging approaches. Assessing the roles of neutrophils can be challenging, especially when exact molecular pathways are questioned or disease states are interrogated that alter normal neutrophil homeostasis. This review discusses the main in vivo models for the study of neutrophils, their advantages and limitations. The side-by-side comparison underlines the necessity to carefully choose the right model(s) to answer a given scientific question, and exhibit caveats that need to be taken into account when designing experimental procedures. Collectively, this review suggests that at least two models should be employed to legitimately conclude on neutrophil functions.

The regulatory roles of neutrophils in adaptive immunity

Neutrophils have long been considered as cells playing a crucial role in the immune defence against invading pathogens. Accumulating evidence strongly supported the direct and indirect regulatory effects of neutrophils on adaptive immunity. Exogenous cytokines or cytokines produced in an autocrine manner as well as a cell-to-cell contact between neutrophils and T cells could induce the expression of MHC-II and costimulatory molecules on neutrophils, supporting that neutrophils may function as antigen-presenting cells (APCs) in respects of presenting antigens and activating T cells. In addition to the inflammatory roles, neutrophils also have the propensity and ability to suppress the immune response through different mechanisms. In this review, we will mainly highlight the heterogeneity and functional plasticity of neutrophils and the antigen-presenting capacity of different neutrophil subsets. We also discuss mechanisms relevant to the regulatory effects of neutrophils on adaptive immunity. Understanding how neutrophils modulate adaptive immunity may provide novel strategies and new therapeutic approaches for diseases associated with neutrophils.

Extracellular traps in kidney disease

During the past decade the formation of neutrophil extracellular traps (NETs) has been recognized as a unique modality of pathogen fixation (sticky extracellular chromatin) and pathogen killing (cytotoxic histones and proteases) during host defense, as well as collateral tissue damage. Numerous other triggers induce NET formation in multiple forms of sterile inflammation, including thrombosis, gout, obstruction of draining ducts, and trauma. Whether neutrophils always die along with NET release, and if they do die, how, remains under study and is most likely context dependent. In certain settings, neutrophils release NETs while undergoing regulated necrosis-for example, necroptosis. NETs and extracellular traps (ETs) released by macrophages also have been well documented in kidney diseases-for example, in various forms of acute kidney injury. Histones released from ETs and other sources are cytotoxic and elicit inflammation, contributing to necroinflammation of the early-injury phase of acute tubular necrosis in antineutrophil cytoplasmic antibody-related renal vasculitis, anti-glomerular basement membrane disease, lupus nephritis, and thrombotic microangiopathies. Finally, acute kidney injury-related releases of dying renal cells or ETs promote remote organ injuries-for example, acute respiratory distress syndrome. In this review, we summarize what is known about the release of ETs from neutrophils and macrophages in the kidney, the available experimental evidence, and ongoing discussions in the field.

Spatiotemporal Modeling of the Key Migratory Events During the Initiation of Adaptive Immunity

Initiation of adaptive immunity involves distinct migratory cell populations coming together in a highly dynamic and spatially organized process. However, we lack a detailed spatiotemporal map of these events due to our inability to track the fate of cells between anatomically distinct locations or functionally identify cell populations as migratory. We used photo-convertible transgenic mice (Kaede) to spatiotemporally track the fate and composition of the cell populations that leave the site of priming and enter the draining lymph node to initiate immunity. We show that following skin priming, the lymph node migratory population is principally composed of cells recruited to the site of priming, with a minor contribution from tissue resident cells. In combination with the YAe/Eα system, we also show that the majority of cells presenting antigen are CD103⁺CD11b⁺ dendritic cells that were recruited to the site of priming during the inflammatory response. This population has previously only been described in relation to mucosal tissues. Comprehensive phenotypic profiling of the cells migrating from the skin to the draining lymph node by mass cytometry revealed that in addition to dendritic cells, the migratory population also included CD4⁺ and CD8⁺ T cells, B cells, and neutrophils. Taking our complex spatiotemporal data set, we then generated a model of cell migration that quantifies and describes the dynamics of arrival, departure, and residence times of cells at the site of priming and in the draining lymph node throughout the time-course of the initiation of adaptive immunity. In addition, we have identified the mean migration time of migratory dendritic cells as they travel from the site of priming to the draining lymph node. These findings represent an unprecedented, detailed and quantitative map of cell dynamics and phenotypes during immunization, identifying where, when and which cells to target for immunomodulation in autoimmunity and vaccination strategies.

Prostaglandin-cytokine crosstalk in chronic inflammation

Chronic inflammation underlies various debilitating disorders including autoimmune, neurodegenerative, vascular and metabolic diseases as well as cancer, where aberrant activation of the innate and acquired immune systems is frequently seen. Since non-steroidal anti-inflammatory drugs exert their effects by inhibiting COX and suppressing PG biosynthesis, PGs have been traditionally thought to function mostly as mediators of acute inflammation. However, an inducible COX isoform, COX-2, is often highly expressed in tissues of the chronic disorders, suggesting an as yet unidentified role of PGs in chronic inflammation. Recent studies have shown that in addition to their short-lived actions in acute inflammation, PGs crosstalk with cytokines and amplify the cytokine actions on various types of inflammatory cells and drive pathogenic conversion of these cells by critically regulating their gene expression. One mode of such PG-mediated amplification is to induce the expression of relevant cytokine receptors, which is typically observed in Th1 cell differentiation and Th17 cell expansion, events leading to chronic immune inflammation. Another mode of amplification is cooperation of PGs with cytokines at the transcription level. Typically, PGs and cytokines synergistically activate NF-κB to induce the expression of inflammation-related genes, one being COX-2 itself, which makes PG-mediated positive feedback loops. This signalling consequently enhances the expression of various NF-κB-induced genes including chemokines to macrophages and neutrophils, which enables sustained infiltration of these cells and further amplifies chronic inflammation. In addition, PGs are also involved in tissue remodelling such as fibrosis and angiogenesis. In this article, we review these findings and discuss their relevance to human diseases.

Neutrophils Which Migrate to Lymph Nodes Modulate CD4+ T Cell Response by a PD-L1 Dependent Mechanism

It is well known that neutrophils are rapidly recruited to a site of injury or infection and perform a critical role in pathogen clearance and inflammation. However, they are also able to interact with and regulate innate and adaptive immune cells and some stimuli induce the migration of neutrophils to lymph nodes (LNs). Previously, we demonstrated that the immune complex (IC) generated by injecting OVA into the footpad of OVA/CFA immunized mice induced the migration of OVA⁺ neutrophils to draining LNs (dLNs). Here we investigate the effects of these neutrophils which reach dLNs on CD4⁺ T cell response. Our findings here strongly support a dual role for neutrophils in dLNs regarding CD4⁺ T cell response modulation. On the one hand, the CD4⁺ T cell population expands after the influx of OVA⁺ neutrophils to dLNs. These CD4⁺ T cells enlarge their proliferative response, activation markers and IL-17 and IFN-γ cytokine production. On the other hand, these neutrophils also restrict CD4⁺ T cell expansion. The neutrophils in the dLNs upregulate PD-L1 molecules and are capable of suppressing CD4⁺ T cell proliferation. These results indicate that neutrophils migration to dLNs have an important role in the homeostasis of adaptive immunity. This report describes for the first time that the influx of neutrophils to dLNs dependent on IC presence improves CD4⁺ T cell response, at the same time controlling CD4⁺ T cell proliferation through a PD-L1 dependent mechanism.

Imaging the neutrophil: Intravital microscopy provides a dynamic view of neutrophil functions in host immunity

Neutrophils are the first cellular responders of the immune system. They employ their impressive arsenal of microbicidal molecules to provide rapid and efficient defense against pathogens. However, the role of neutrophils extends far beyond microbial destruction to include tissue repair and remodeling, provision of signals to the adaptive immune system and body homeostasis. Intravital imaging has allowed the visualization of neutrophils in their native environment in both health and disease and provided crucial insights into their mechanisms of action. In the last few years the power of intravital imaging has been considerably extended by the introduction of photoconvertible proteins and intracellular signaling reporter mice. This review will highlight recent advances in our understanding of neutrophil biology based on the use of intravital microscopy to visualize their modus operandi in vivo including migration in and out of inflamed tissues, host-pathogen interactions and cell fate.

Glucocorticoid-Induced Leucine Zipper Promotes Neutrophil and T-Cell Polarization with Protective Effects in Acute Kidney Injury

The glucocorticoid-induced leucine zipper (GILZ) mediates anti-inflammatory effects of glucocorticoids. Acute kidney injury (AKI) mobilizes immune/inflammatory mechanisms, causing tissue injury, but the impact of GILZ in AKI is not known. Neutrophils play context-specific proinflammatory [type 1 neutrophil (N1)] and anti-inflammatory [type 2 neutrophil (N2)] functional roles. Also, regulatory T lymphocytes (Tregs) and regulatory T-17 (Treg17) cells exert counterinflammatory effects, including the suppression of effector T lymphocytes [e.g., T-helper (Th) 17 cells]. Thus, utilizing cell preparations of mice kidneys subjected to AKI or sham operation, we determined the effects of GILZ on T cells and neutrophil subtypes in the context of its renoprotective effect; these studies used the transactivator of transcription (TAT)-GILZ or the TAT peptide. AKI increased N1 and Th-17 cells but reduced N2, Tregs, and Treg17 cells in association with increased interleukin (IL)-17⁺ but reduced IL-10⁺ cells accompanied with the disruption of mitochondrial membrane potential (ψ _m) and increased apoptosis/necrosis compared with sham kidneys. TAT-GILZ, compared with TAT, treatment reduced N1 and Th-17 cells but increased N2 and Tregs, without affecting Treg17 cells, in association with a reduction in IL-17⁺ cells but an increase in IL-10⁺ cells; TAT-GILZ caused less disruption of ψ _m and reduced cell death in AKI. Importantly, TAT-GILZ increased perfusion of the ischemic-reperfused kidney but reduced tissue edema compared with TAT. Utilizing splenic T cells and bone marrow-derived neutrophils, we further showed marked reduction in the proliferation of Th cells in response to TAT-GILZ compared with response to TAT. Collectively, the results indicate that GILZ exerts renoprotection accompanied by the upregulation of the regulatory/suppressive arm of immunity in AKI, likely via regulating cross talk between T cells and neutrophils.

Cyclooxygenase-1 and -2 Play Contrasting Roles in Listeria-Stimulated Immunity

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) activity and are commonly used for pain relief and fever reduction. NSAIDs are used following childhood vaccinations and cancer immunotherapies; however, how NSAIDs influence the development of immunity following these therapies is unknown. We hypothesized that NSAIDs would modulate the development of an immune response to Listeria monocytogenes-based immunotherapy. Treatment of mice with the nonspecific COX inhibitor indomethacin impaired the generation of cell-mediated immunity. This phenotype was due to inhibition of the inducible COX-2 enzyme, as treatment with the COX-2-selective inhibitor celecoxib similarly inhibited the development of immunity. In contrast, loss of COX-1 activity improved immunity to L. monocytogenes Impairments in immunity were independent of bacterial burden, dendritic cell costimulation, or innate immune cell infiltrate. Instead, we observed that PGE₂ production following L. monocytogenes is critical for the formation of an Ag-specific CD8⁺ T cell response. Use of the alternative analgesic acetaminophen did not impair immunity. Taken together, our results suggest that COX-2 is necessary for optimal CD8⁺ T cell responses to L. monocytogenes, whereas COX-1 is detrimental. Use of pharmacotherapies that spare COX-2 activity and the production of PGE₂ like acetaminophen will be critical for the generation of optimal antitumor responses using L. monocytogenes.

Vaccine priming is restricted to draining lymph nodes and controlled by adjuvant-mediated antigen uptake

The innate immune mechanisms by which adjuvants enhance the potency and protection of vaccine-induced adaptive immunity are largely unknown. We introduce a model to delineate the steps of how adjuvant-driven innate immune activation leads to priming of vaccine responses using rhesus macaques. Fluorescently labeled HIV-1 envelope glycoprotein (Env) was administered together with the conventional aluminum salt (alum) adjuvant. This was compared to Env given with alum with preabsorbed Toll-like receptor 7 (TLR7) ligand (alum-TLR7) or the emulsion MF59 because they show superiority over alum for qualitatively and quantitatively improved vaccine responses. All adjuvants induced rapid and robust immune cell infiltration to the injection site in the muscle. This resulted in substantial uptake of Env by neutrophils, monocytes, and myeloid and plasmacytoid dendritic cells (DCs) and migration exclusively to the vaccine-draining lymph nodes (LNs). Although less proficient than monocytes and DCs, neutrophils were capable of presenting Env to memory CD4⁺ T cells. MF59 and alum-TLR7 showed more pronounced cell activation and overall higher numbers of Env⁺ cells compared to alum. This resulted in priming of higher numbers of Env-specific CD4⁺ T cells in the vaccine-draining LNs, which directly correlated with increased T follicular helper cell differentiation and germinal center formation. Thus, strong innate immune activation promoting efficient vaccine antigen delivery to infiltrating antigen-presenting cells in draining LNs is an important mechanism by which superior adjuvants enhance vaccine responses.

Expression of the TPα and TPβ isoforms of the thromboxane prostanoid receptor (TP) in prostate cancer: clinical significance and diagnostic potential

The prostanoid thromboxane (TX) A₂ plays a central role in haemostasis and is increasingly implicated in cancer progression. TXA₂ signals through two T Prostanoid receptor (TP) isoforms termed TPα and TPβ, with both encoded by the TBXA2R gene. Despite exhibiting several functional and regulatory differences, the role of the individual TP isoforms in neoplastic diseases is largely unknown.

This study evaluated expression of the TPα and TPβ isoforms in tumour microarrays of the benign prostate and different pathological (Gleason) grades of prostate cancer (PCa). Expression of TPβ was significantly increased in PCa relative to benign tissue and strongly correlated with increasing Gleason grade. Furthermore, higher TPβ expression was associated with increased risk of biochemical recurrence (BCR) and significantly shorter disease-free survival time in patients post-surgery. While TPα was more variably expressed than TPβ in PCa, increased/high TPα expression within the tumour also trended toward increased BCR and shorter disease-free survival time. Comparative genomic CpG DNA methylation analysis revealed substantial differences in the extent of methylation of the promoter regions of the TBXA2R that specifically regulate expression of TPα and TPβ, respectively, both in benign prostate and in clinically-derived tissue representative of precursor lesions and progressive stages of PCa. Collectively, TPα and TPβ expression is differentially regulated both in the benign and tumourigenic prostate, and coincides with clinical pathology and altered CpG methylation of the TBXA2R gene. Analysis of TPβ, or a combination of TPα/TPβ, expression levels may have significant clinical potential as a diagnostic biomarker and predictor of PCa disease recurrence.

The adjuvant system AS01 up-regulates neutrophil CD14 expression and neutrophil-associated antigen transport in the local lymphatic network

The liposome-based adjuvant system AS01 is under evaluation for use in several vaccines in clinical development. We have shown previously that AS01 injected with hepatitis B surface antigen (HBsAg) induces a distinct cellular signature within the draining lymphatics that enhances local lymphocyte recruitment and antigen-specific humoral immunity. Here, we show that AS01-induced neutrophil recruitment is associated with increased expression of CD14 and enhanced antigen uptake capacity in neutrophils from both afferent and efferent lymphatic compartments during the first 48 h after vaccination. Significant and transient increases in CD14 expression on systemic neutrophils were also observed following primary and boost vaccination with HBsAg-AS01; however, they were not observed following additional encounter with HBsAg-alone or HBsAg-alum. These results show that following immunization with AS01, neutrophils expressing higher levels of CD14 are both more abundant and efficient at antigen uptake, warranting further investigation into the role of neutrophil-associated CD14 in the adjuvanticity of AS01.

Strategies to Improve Vaccine Efficacy against Tuberculosis by Targeting Innate Immunity

The global tuberculosis epidemic is the most common cause of death after infectious disease worldwide. Increasing numbers of infections with multi- and extensively drug-resistant variants of the Mycobacterium tuberculosis complex, resistant even to newly discovered and last resort antibiotics, highlight the urgent need for an efficient vaccine. The protective efficacy to pulmonary tuberculosis in adults of the only currently available vaccine, M. bovis BCG, is unsatisfactory and geographically diverse. More importantly, recent clinical studies on new vaccine candidates did not prove to be better than BCG, yet. Here, we propose and discuss novel strategies to improve efficacy of existing anti-tuberculosis vaccines. Modulation of innate immune responses upon vaccination already provided promising results in animal models of tuberculosis. For instance, neutrophils have been shown to influence vaccine efficacy, both, positively and negatively, and stimulate specific antibody secretion. Modulating immune regulatory properties after vaccination such as induction of different types of innate immune cell death, myeloid-derived suppressor or regulatory T cells, production of anti-inflammatory cytokines such as IL-10 may have beneficial effects on protection efficacy. Incorporation of lipid antigens presented via CD1 molecules to T cells have been discussed as a way to enhance vaccine efficacy. Finally, concepts of dendritic cell-based immunotherapies or training the innate immune memory may be exploitable for future vaccination strategies against tuberculosis. In this review, we put a spotlight on host immune networks as potential targets to boost protection by old and new tuberculosis vaccines.

Regulation of inflammation by lipid mediators in oral diseases

Lipid mediators (LM) of inflammation are a class of compounds derived from ω-3 and ω-6 fatty acids that play a wide role in modulating inflammatory responses. Some LM possess pro-inflammatory properties, while others possess proresolving characteristics, and the class switch from pro-inflammatory to proresolving is crucial for tissue homeostasis. In this article, we review the major classes of LM, focusing on their biosynthesis and signaling pathways, and their role in systemic and, especially, oral health and disease. We discuss the detection of these LM in various body fluids, focusing on diagnostic and therapeutic applications. We also present data showing gender-related differences in salivary LM levels in healthy controls, leading to a hypothesis on the etiology of inflammatory diseases, particularly Sjögren's syndrome. We conclude by enumerating open areas of research where further investigation of LM is likely to result in therapeutic and diagnostic advances.

Deep insight into neutrophil trafficking in various organs

Neutrophils are professional phagocytes that constitute the first line of defense in humans. The primary function of neutrophils is to eliminate invading pathogens through oxidative and nonoxidative mechanisms. Because neutrophils rapidly migrate into inflammatory foci via diapedesis and chemotaxis, neutrophil recruitment has long been considered a hallmark of inflammation. Recent advances in intravital microscopic technologies using animal model systems have enabled researchers to directly visualize neutrophil trafficking. Consequently, the specific mechanisms of neutrophil transmigration have been identified, and even the reverse migration of neutrophils can be verified visually. Moreover, the detailed phenomena of neutrophil infiltration into various organs, such as the liver, lymphoid organs, and CNS have been identified. This progress in the study of neutrophil migration from the blood vessels to organs results in a deeper understanding of these immune cells' motility and morphology, which are closely related to the spatiotemporal regulation of the overall immune response. In this review, we discuss our current understanding of neutrophil trafficking in various organs.

IVIG regulates the survival of human but not mouse neutrophils

Intravenous immunoglobulin (IVIG) are purified IgG preparations made from the pooled plasma from thousands of healthy donors and are being tested in preclinical mouse models. Inherent challenges, however, are the pluripotency of IVIG and its xenogeneicity in animals. IVIG can alter the viability of human neutrophils via agonistic antibodies to Fas and Siglec-9. In this study, we compared the effects of IVIG on human and mouse neutrophils using different death assays. Different commercial IVIG preparations similarly induced cytokine-dependent death in human neutrophils, whereas they had no effects on the survival of either peripheral blood or bone marrow neutrophils from C57BL/6 or BALB/c mice. F(ab’)₂ but not Fc fragments of IVIG induced death of human neutrophils, whereas neither of these IVIG fragments, nor agonistic monoclonal antibodies to human Fas or Siglec-9 affected the viability of mouse neutrophils. Pooled mouse IgG, which exhibited a different immunoprofile compared to IVIG, also had no effect on mouse cells. Together, these observations demonstrate that effects of IVIG on neutrophil survival are not adequately reflected in current mouse models, despite the key role of these cells in human inflammatory and autoimmune diseases.

Neutrophils as protagonists and targets in chronic inflammation

Traditionally, neutrophils have been acknowledged to be the first immune cells that are recruited to an inflamed tissue and have mainly been considered in the context of acute inflammation. By contrast, their importance during chronic inflammation has been studied in less depth. This Review aims to summarize our current understanding of the roles of neutrophils in chronic inflammation, with a focus on how they communicate with other immune and non-immune cells within tissues. We also scrutinize the roles of neutrophils in wound healing and the resolution of inflammation, and finally, we outline emerging therapeutic strategies that target neutrophils.

Neutrophils acquire the capacity for antigen presentation to memory CD4+ T cells in vitro and ex vivo

Neutrophils are critical cells of the innate immune system and rapidly respond to tissue injury and infection. Increasing evidence also indicates that neutrophils have versatile functions in contributing to adaptive immunity by internalizing and transporting antigen and influencing antigen-specific responses. Here, we demonstrate that freshly isolated human neutrophils can function as antigen-presenting cells (APCs) to memory CD4⁺ T cells. Neutrophils pulsed with the cognate antigens cytomegalovirus pp65 or influenza hemagglutinin were able to present the antigens to autologous antigen-specific CD4⁺ T cells in a major histocompatibility complex class II (MHC-II; HLA-DR)-dependent manner. Although myeloid dendritic cells and monocytes showed superior presenting ability, neutrophils consistently displayed antigen presentation capability. Upregulation of HLA-DR on neutrophils required the presence of the antigen-specific or activated T cells whereas exposure to innate stimuli such as Toll-like receptor ligands was not sufficient. Neutrophils sorted from vaccine-draining lymph nodes from rhesus macaques also showed expression of HLA-DR and were capable of presenting vaccine antigen to autologous antigen-specific memory CD4⁺ T cells ex vivo. Altogether, the data demonstrate that neutrophils can adapt a function as APCs and, in combination with their abundance in the immune system, may have a significant role in regulating antigen-specific T-cell responses.

Neutrophils are dispensable in the modulation of T cell immunity against cutaneous HSV-1 infection

Neutrophils rapidly infiltrate sites of inflammation during peripheral infection or tissue injury. In addition to their well described roles as pro-inflammatory phagocytes responsible for pathogen clearance, recent studies have demonstrated a broader functional repertoire including mediating crosstalk between innate and adaptive arms of the immune system. Specifically, neutrophils have been proposed to mediate antigen transport to lymph nodes (LN) to modulate T cell priming and to influence T cell migration to infected tissues. Using a mouse model of cutaneous herpes simplex virus type 1 (HSV-1) infection we explored potential contributions of neutrophils toward anti-viral immunity. While a transient, early influx of neutrophils was triggered by dermal scarification, we did not detect migration of neutrophils from the skin to LN. Furthermore, despite recruitment of neutrophils into LN from the blood, priming and expansion of CD4⁺ and CD8⁺ T cells was unaffected following neutrophil depletion. Finally, we found that neutrophils were dispensable for the migration of effector T cells into infected skin. Our study suggests that the immunomodulatory roles of neutrophils toward adaptive immunity may be context-dependent, and are likely determined by the type of pathogen and anatomical site of infection.

Innate Immune Responses after Airway Epithelial Stimulation with Mycobacterium bovis Bacille-Calmette Guérin

Mycobacterium bovis bacilli Calmette-Guerin (BCG) is used as a benchmark to compare the immunogenicity of new vaccines against tuberculosis. This live vaccine is administered intradermal, but several new studies show that changing the route to mucosal immunisation represents an improved strategy. We analysed the immunomodulatory functions of BCG on human neutrophils and primary airway epithelial cells (AECs), as the early events of mucosal immune activation are unclear. Neutrophils and the primary epithelial cells were found to express the IL-17A receptor subunit IL-17RA, while the expression of IL-17RE was only observed on epithelial cells. BCG stimulation specifically reduced neutrophil IL-17RA and epithelial IL-17RE expression. BCG induced neutrophil extracellular traps (NETs), but did not have an effect on apoptosis as measured by transcription factor forkhead box O3 (FOXO3). BCG stimulation of AECs induced CXCL8 secretion and neutrophil endothelial passage towards infected epithelia. Infected epithelial cells and neutrophils were not found to be a source of IL-17 cytokines or the interstitial collagenase MMP-1. However, the addition of IFNγ or IL-17A to BCG stimulated primary epithelial cells increased epithelial IL-6 secretion, while the presence of IFNγ reduced neutrophil recruitment. Using our model of mucosal infection we revealed that BCG induces selective mucosal innate immune responses that could lead to induction of vaccine-mediated protection of the lung.

Neutrophils in the tumor microenvironment: trying to heal the wound that cannot heal

Neutrophils are the first responders to infection and injury and are critical for antimicrobial host defense. Through the generation of reactive oxidants, activation of granular constituents and neutrophil extracellular traps, neutrophils target microbes and prevent their dissemination. While these pathways are beneficial in the context of trauma and infection, their off-target effects in the context of tumor are variable. Tumor-derived factors have been shown to reprogram the marrow, skewing toward the expansion of myelopoiesis. This can result in stimulation of both neutrophilic leukocytosis and the release of immature granulocytic populations that accumulate in circulation and in the tumor microenvironment. While activated neutrophils have been shown to kill tumor cells, there is growing evidence for neutrophil activation driving tumor progression and metastasis through a number of pathways, including stimulation of thrombosis and angiogenesis, stromal remodeling, and impairment of T cell-dependent anti-tumor immunity. There is also growing appreciation of neutrophil heterogeneity in cancer, with distinct neutrophil populations promoting cancer control or progression. In addition to the effects of tumor on neutrophil responses, anti-neoplastic treatment, including surgery, chemotherapy, and growth factors, can influence neutrophil responses. Future directions for research are expected to result in more mechanistic knowledge of neutrophil biology in the tumor microenvironment that may be exploited as prognostic biomarkers and therapeutic targets.

The Lymphatic Immune Response Induced by the Adjuvant AS01: A Comparison of Intramuscular and Subcutaneous Immunization Routes

The liposome-based adjuvant AS01 incorporates two immune stimulants, 3-O-desacyl-4'-monophosphoryl lipid A and the saponin QS-21. AS01 is under investigation for use in several vaccines in clinical development. i.m. injection of AS01 enhances immune cell activation and dendritic cell (DC) Ag presentation in the local muscle-draining lymph node. However, cellular and Ag trafficking in the lymphatic vessels that connect an i.m. injection site with the local lymph node has not been investigated. The objectives of this study were: 1) to quantify the in vivo cellular immune response induced by AS01 in an outbred ovine model, 2) to develop a lymphatic cannulation model that directly collects lymphatic fluid draining the muscle, and 3) to investigate the function of immune cells entering and exiting the lymphatic compartments after s.c. or i.m. vaccination with AS01 administered with hepatitis B surface Ag (HBsAg). We show that HBsAg-AS01 induces a distinct immunogenic cellular signature within the blood and draining lymphatics following both immunization routes. We reveal that MHCII(high) migratory DCs, neutrophils, and monocytes can acquire Ag within muscle and s.c. afferent lymph, and that HBsAg-AS01 uniquely induces the selective migration of Ag-positive neutrophils, monocytes, and an MHCII(high) DC-like cell type out of the lymph node via the efferent lymphatics that may enhance Ag-specific immunity. We report the characterization of the immune response in the lymphatic network after i.m. and s.c. injection of a clinically relevant vaccine, all in real time using a dose and volume comparable with that administered in humans.

BAFF-secreting neutrophils drive plasma cell responses during emergency granulopoiesis

Harris and collaborators show that neutropenia results in increased formation of plasma cells and elevated antibody production.

Prolonged infections or adjuvant usage can trigger emergency granulopoiesis (EG), leading to dysregulation in neutrophil blood counts. However, the impact of EG on T and B cell function remains largely unknown. In this study, to address this question, we used a mouse model of neutropenia and studied immune activation after adjuvant administration. The initial neutropenic state fostered an environment of increased dendritic cell activation and T cell–derived IL-17 production. Interestingly, neutropenic lysozyme 2–diphtheria toxin A mice exhibited striking EG and amplified neutrophil recruitment to the lymph nodes (LNs) that was dependent on IL-17–induced prostaglandin activity. The recruited neutrophils secreted a B cell–activating factor that highly accelerated plasma cell generation and antigen-specific antibody production. Reduction of neutrophil functions via granulocyte colony-stimulating factor neutralization significantly diminished plasma cell formation, directly linking EG with the humoral immune response. We conclude that neutrophils are capable of directly regulating T cell–dependent B cell responses in the LN.

Neutrophil heterogeneity: implications for homeostasis and pathogenesis

Neutrophils are polymorphonuclear leukocytes of the phagocytic system that act as first line of host defense against invading pathogens but are also important mediators of inflammation-induced injury. In contrast to other members of the innate immune system, neutrophils are classically considered a homogenous population of terminally differentiated cells with a well-defined and highly conserved function. Indeed, their short lifespan, the absent proliferative capacity, their limited ability to produce large amounts of cytokines, and the failure to recirculate from the tissue to the bloodstream have sustained this idea. However, increasing evidence over the last decade has demonstrated an unexpected phenotypic heterogeneity and functional versatility of the neutrophil population. Far beyond their antimicrobial functions, neutrophils are emerging as decision-shapers during innate and adaptive immune responses. These emerging discoveries open a new door to understand the role of neutrophils during homeostatic but also pathogenic immune processes. Thus, this review details novel insights of neutrophil phenotypic and functional heterogeneity during homeostasis and disease.