In situ neutrophil efferocytosis shapes T cell immunity to influenza infection

LECs regulate neutrophil clearance through IL-17RC/CMTM4/NF-κB axis at sites of inflammation or infection

The lymphatic system plays a vital role in the regulation of tissue fluid balance and the immune response to inflammation or infection. The effects of lymphatic endothelial cells (LECs) on the regulation of neutrophil migration have not been well-studied. In three murine models: imiquimod-induced skin inflammation, Staphylococcus aureus-induced skin infection, and ligature-induced periodontitis, we show that numerous neutrophils migrate from inflamed or infected tissues to the draining lymph nodes via lymphatic vessels. Moreover, inflamed or infected tissues express a high level of interleukin (IL)-17A and tumor necrosis factor (TNF)-α, simultaneously with a significant increase in the release of neutrophil attractors, including CXCL1, CXCL2, CXCL3, and CXCL5. Importantly, in vitro stimulation of LECs with IL-17A plus TNF-α synergistically promoted these chemokine secretions. Mechanistically, tetra-transmembrane protein CMTM4 directly binds to IL-17RC in LECs. IL-17A plus TNF-α stimulates CXC chemokine secretion by promoting nuclear factor-kappa B signaling. In contrast, knockdown of CMTM4 abrogates IL-17A plus TNF-α activated nuclear factor-kappa B signaling pathways. Lastly, the local administration of adeno-associated virus for CMTM4 in Prox1-CreERT2 mice, mediating LEC-specific overexpression of CMTM4, promotes the drainage of neutrophils by LECs and alleviates immune pathological responses. Thus, our findings reveal the vital role of LECs-mediated neutrophil attraction and clearance at sites of inflammation or infection.

Immune Cell Migration to Cancer

Immune cell migration is required for the development of an effective and robust immune response. This elegant process is regulated by both cellular and environmental factors, with variables such as immune cell state, anatomical location, and disease state that govern differences in migration patterns. In all cases, a major factor is the expression of cell surface receptors and their cognate ligands. Rapid adaptation to environmental conditions partly depends on intrinsic cellular immune factors that affect a cell's ability to adjust to new environment. In this review, we discuss both myeloid and lymphoid cells and outline key determinants that govern immune cell migration, including molecules required for immune cell adhesion, modes of migration, chemotaxis, and specific chemokine signaling. Furthermore, we summarize tumor-specific elements that contribute to immune cell trafficking to cancer, while also exploring microenvironment factors that can alter these cellular dynamics within the tumor in both a pro and antitumor fashion. Specifically, we highlight the importance of the secretome in these later aspects. This review considers a myriad of factors that impact immune cell trajectory in cancer. We aim to highlight the immunotherapeutic targets that can be harnessed to achieve controlled immune trafficking to and within tumors.

Extracellular Matrix Scaffold-Assisted Tumor Vaccines Induce Tumor Regression and Long-Term Immune Memory

Injectable scaffold delivery is a strategy to enhance the efficacy of cancer vaccine immunotherapy. The choice of scaffold biomaterial is crucial, impacting both vaccine release kinetics and immune stimulation via the host response. Extracellular matrix (ECM) scaffolds prepared from decellularized tissues facilitate a pro-healing inflammatory response that promotes local cancer immune surveillance. Here, an ECM scaffold-assisted therapeutic cancer vaccine that maintains an immune microenvironment consistent with tissue reconstruction is engineered. Several immune-stimulating adjuvants are screened to develop a cancer vaccine formulated with decellularized small intestinal submucosa (SIS) ECM scaffold co-delivery. It is found that the STING pathway agonist cyclic di-AMP most effectively induces cytotoxic immunity in an ECM scaffold vaccine, without compromising key interleukin 4 (IL-4) mediated immune pathways associated with healing. ECM scaffold delivery enhances therapeutic vaccine efficacy, curing 50-75% of established E.G-7OVA lymphoma tumors in mice, while none are cured with soluble vaccine. SIS-ECM scaffold-assisted vaccination prolonged antigen exposure is dependent on CD8+ cytotoxic T cells and generates long-term antigen-specific immune memory for at least 10 months post-vaccination. This study shows that an ECM scaffold is a promising delivery vehicle to enhance cancer vaccine efficacy while being orthogonal to characteristics of pro-healing immune hallmarks.

Integrating single-cell RNA sequencing data to genome-wide association analysis data identifies significant cell types in influenza A virus infection and COVID-19

With the global pandemic of COVID-19, the research on influenza virus has entered a new stage, but it is difficult to elucidate the pathogenesis of influenza disease. Genome-wide association studies (GWASs) have greatly shed light on the role of host genetic background in influenza pathogenesis and prognosis, whereas single-cell RNA sequencing (scRNA-seq) has enabled unprecedented resolution of cellular diversity and in vivo following influenza disease. Here, we performed a comprehensive analysis of influenza GWAS and scRNA-seq data to reveal cell types associated with influenza disease and provide clues to understanding pathogenesis. We downloaded two GWAS summary data, two scRNA-seq data on influenza disease. After defining cell types for each scRNA-seq data, we used RolyPoly and LDSC-cts to integrate GWAS and scRNA-seq. Furthermore, we analyzed scRNA-seq data from the peripheral blood mononuclear cells (PBMCs) of a healthy population to validate and compare our results. After processing the scRNA-seq data, we obtained approximately 70 000 cells and identified up to 13 cell types. For the European population analysis, we determined an association between neutrophils and influenza disease. For the East Asian population analysis, we identified an association between monocytes and influenza disease. In addition, we also identified monocytes as a significantly related cell type in a dataset of healthy human PBMCs. In this comprehensive analysis, we identified neutrophils and monocytes as influenza disease-associated cell types. More attention and validation should be given in future studies.

Dynamic Multiplex Tissue Imaging in Inflammation Research

Inflammation is a highly dynamic process with immune cells that continuously interact with each other and parenchymal components as they migrate through tissue. The dynamic cellular responses and interaction patterns are a function of the complex tissue environment that cannot be fully reconstructed ex vivo, making it necessary to assess cell dynamics and changing spatial patterning in vivo. These dynamics often play out deep within tissues, requiring the optical focus to be placed far below the surface of an opaque organ. With the emergence of commercially available two-photon excitation lasers that can be combined with existing imaging systems, new avenues for imaging deep tissues over long periods of time have become available. We discuss a selected subset of studies illustrating how two-photon microscopy (2PM) has helped to relate the dynamics of immune cells to their in situ function and to understand the molecular patterns that govern their behavior in vivo. We also review some key practical aspects of 2PM methods and point out issues that can confound the results, so that readers can better evaluate the reliability of conclusions drawn using this technology.

Monocyte differentiation within tissues: a renewed outlook

When recruited to mammalian tissues, monocytes differentiate into macrophages or dendritic cells (DCs). In the past few years, the existence of monocyte-derived DCs (moDCs) was questioned by the discovery of new DC populations with overlapping phenotypes. Here, we critically review the evidence for monocyte differentiation into DCs in tissues and highlight their specific functions. Recent studies have shown that monocyte-derived macrophages (moMacs) with distinct life cycles coexist in tissues, both at steady state and upon inflammation. Integrating studies in mice and humans, we highlight specific features of moMacs during inflammation and tissue repair. We also discuss the notion of monocyte differentiation occurring via a binary fate decision. Deciphering monocyte-derived cell properties is essential for understanding their role in nonresolving inflammation and how they might be targeted for therapies.

Phenotypic and functional differences of dendritic cells in tumor

Dendritic cells (DCs) are a unique class of immune cells vital to the immune system, functioning as antigen-presenting cells that play a key role in launching both cellular and humoral immune responses. They are crucial in preventing infectious diseases and regulating tumor growth. DCs can be categorized based on various criteria such as phenotype, function, and tissue location, resulting in several subgroups. Generally, DCs are divided into two primary groups: plasmacytoid DCs (pDCs) and conventional DCs (cDCs), which are further classified into Type I classical DCs (cDC1) and Type II classical DCs (cDC2). cDC1 cells are distinguishable by specific gene programs and associated markers, while cDC2 cells display more diversity. Moreover, there is an ongoing debate surrounding a recently identified subgroup called DC3, and whether it can be considered a distinct cell type in the maturation process of DCs remains uncertain. Most of these DC subgroups rely on the growth factor Fms-like tyrosine kinase 3 ligand (FLT3L) for differentiation from a common DC precursor (CDP), guided by various cytokines. Although the general classification of DC subgroups is similar in both humans and mice, numerous phenotypic and functional variations exist within each subgroup. Therefore, comprehending these differences between DC subgroups in humans and mice holds the potential to significantly advance relevant research.

Efferocytosis: a double-edged sword in microbial immunity

Efferocytosis is characterized as the rapid and efficient process by which dying or dead cells are removed. This type of clearance is initiated via "find-me" signals, and then, carries on by "eat-me" and "don't-eat-me" ones. Efferocytosis has a critical role to play in tissue homeostasis and innate immunity. However, some evidence suggests it as a double-edged sword in microbial immunity. In other words, some pathogens have degraded efferocytosis by employing efferocytic mechanisms to bypass innate immune detection and promote infection, despite the function of this process for the control and clearance of pathogens. In this review, the efferocytosis mechanisms from the recognition of dying cells to phagocytic engulfment are initially presented, and then, its diverse roles in inflammation and immunity are highlighted. In this case, much focus is also laid on some bacterial, viral, and parasitic infections caused by Mycobacterium tuberculosis (M. tb), Mycobacterium marinum (M. marinum), Listeria monocytogenes (L. monocytogenes), Chlamydia pneumoniae (CP), Klebsiella pneumoniae (KP), Influenza A virus (IAV), human immunodeficiency virus (HIV), and Leishmania, respectively.

Fluorescence-amplified nanocrystals in the second near-infrared window for in vivo real-time dynamic multiplexed imaging

Optical imaging in the second near-infrared window (NIR-II, 1,000-1,700 nm) holds great promise for non-invasive in vivo detection. However, real-time dynamic multiplexed imaging remains challenging due to the lack of available fluorescence probes and multiplexing techniques in the ideal NIR-IIb (1,500-1,700 nm) 'deep-tissue-transparent' sub-window. Here we report on thulium-based cubic-phase downshifting nanoparticles (α-TmNPs) with 1,632 nm fluorescence amplification. This strategy was also validated for the fluorescence enhancement of nanoparticles doped with NIR-II Er3+ (α-ErNPs) or Ho3+ (α-HoNPs). In parallel, we developed a simultaneous dual-channel imaging system with high spatiotemporal synchronization and accuracy. The NIR-IIb α-TmNPs and α-ErNPs facilitated the non-invasive real-time dynamic multiplexed imaging of cerebrovascular vasomotion activity and the single-cell-level neutrophil behaviour in mouse subcutaneous tissue and ischaemic stroke model.

ST3GAL1 and βII-spectrin pathways control CAR T cell migration to target tumors

Adoptive transfer of genetically engineered chimeric antigen receptor (CAR) T cells is becoming a promising treatment option for hematological malignancies. However, T cell immunotherapies have mostly failed in individuals with solid tumors. Here, with a CRISPR-Cas9 pooled library, we performed an in vivo targeted loss-of-function screen and identified ST3 β-galactoside α-2,3-sialyltransferase 1 (ST3GAL1) as a negative regulator of the cancer-specific migration of CAR T cells. Analysis of glycosylated proteins revealed that CD18 is a major effector of ST3GAL1 in activated CD8+ T cells. ST3GAL1-mediated glycosylation induces the spontaneous nonspecific tissue sequestration of T cells by altering lymphocyte function-associated antigen-1 (LFA-1) endocytic recycling. Engineered CAR T cells with enhanced expression of βII-spectrin, a central LFA-1-associated cytoskeleton molecule, reversed ST3GAL1-mediated nonspecific T cell migration and reduced tumor growth in mice by improving tumor-specific homing of CAR T cells. These findings identify the ST3GAL1-βII-spectrin axis as a major cell-intrinsic program for cancer-targeting CAR T cell migration and as a promising strategy for effective T cell immunotherapy.

Axl receptor induces efferocytosis, dampens M1 macrophage responses and promotes heart pathology in Trypanosoma cruzi infection

Adaptive immunity controls Trypanosoma cruzi infection, but the protozoan parasite persists and causes Chagas disease. T cells undergo apoptosis, and the efferocytosis of apoptotic cells might suppress macrophages and exacerbate parasite infection. Nonetheless, the receptors involved in the efferocytosis of apoptotic lymphocytes during infection remain unknow. Macrophages phagocytose apoptotic cells by using the TAM (Tyro3, Axl, Mer) family of receptors. To address how the efferocytosis of apoptotic cells affects macrophage-mediated immunity, we employ here Axl receptor- and Mer receptor-deficient mouse strains. In bone marrow-derived macrophages (BMDMs), both Axl and Mer receptors play a role in the efferocytosis of proapoptotic T cells from T. cruzi-infected mice. Moreover, treatment with a TAM receptor inhibitor blocks efferocytosis and upregulates M1 hallmarks induced by immune T cells from infected mice. Remarkably, the use of Axl^-/- but not Mer^-/- macrophages increases T-cell-induced M1 responses, such as nitric oxide production and control of parasite infection. Furthermore, infected Axl^-/- mice show reduced peak parasitemia, defective efferocytosis, improved M1 responses, and ameliorated cardiac inflammation and fibrosis. Therefore, Axl induces efferocytosis, disrupts M1 responses, and promotes parasite infection and pathology in experimental Chagas disease. Axl stands as a potential host-direct target for switching macrophage phenotypes in infectious diseases.

Mechanisms regulating neutrophil responses in immunity, allergy, and autoimmunity

Neutrophil granulocytes, or neutrophils, are the most abundant circulating leukocytes in humans and indispensable for antimicrobial immunity, as exemplified in patients with inborn and acquired defects of neutrophils. Neutrophils were long regarded as the foot soldiers of the immune system, solely destined to execute a set of effector functions against invading pathogens before undergoing apoptosis, the latter of which was ascribed to their short life span. This simplistic understanding of neutrophils has now been revised on the basis of insights gained from the use of mouse models and single-cell high-throughput techniques, revealing tissue- and context-specific roles of neutrophils in guiding immune responses. These studies also demonstrated that neutrophil responses were controlled by sophisticated feedback mechanisms, including directed chemotaxis of neutrophils to tissue-draining lymph nodes resulting in modulation of antimicrobial immunity and inflammation. Moreover, findings in mice and humans showed that neutrophil responses adapted to different deterministic cytokine signals, which controlled their migration and effector function as well as, notably, their biologic clock by affecting the kinetics of their aging. These mechanistic insights have important implications for health and disease in humans, particularly, in allergic diseases, such as atopic dermatitis and allergic asthma bronchiale, as well as in autoinflammatory and autoimmune diseases. Hence, our improved understanding of neutrophils sheds light on novel therapeutic avenues, focusing on molecularly defined biologic agents.

Dynamics of phagocytosis mediated by phosphatidylserine

Phagocytosis triggered by the phospholipid phosphatidylserine (PS) is key for the removal of apoptotic cells in development, tissue homeostasis and infection. Modulation of PS-mediated phagocytosis is an attractive target for therapeutic intervention in the context of atherosclerosis, neurodegenerative disease, and cancer. Whereas the mechanisms of target recognition, lipid and protein signalling, and cytoskeletal remodelling in opsonin-driven modes of phagocytosis are increasingly well understood, PS-mediated phagocytosis has remained more elusive. This is partially due to the involvement of a multitude of receptors with at least some redundancy in functioning, which complicates dissecting their contributions and results in complex downstream signalling networks. This review focusses on the receptors involved in PS-recognition, the signalling cascades that connect receptors to cytoskeletal remodelling required for phagocytosis, and recent progress in our understanding of how phagocytic cup formation is coordinated during PS-mediated phagocytosis.

On-Demand Fully Enclosed Superhydrophobic-Optofluidic Devices Enabled by Microstereolithography

Superhydrophobic surface-based optofluidics have been introduced to biosensors and unconventional optics with unique advantages, such as low light loss and power consumption. However, most of these platforms were made with planar-like microstructures and nanostructures, which may cause bonding issues and result in significant waveguide loss. Here, we introduce a fully enclosed superhydrophobic-based optofluidics system, enabled by a one-step microstereolithography procedure. Various microstructured cladding designs with a feature size down to 100 μm were studied and a "T-type" overhang design exhibits the lowest optical loss, regardless of the excitation wavelength. Surprisingly, the optical loss of superhydrophobic-based optofluidics is not solely decided by the solid area fraction at the solid/water/air interface, but also the cross-section shape and the effective cladding layer composition. We show that this fully enclosed optofluidic system can be used for CRISPR-labeled quantum dot quantification, intended for in vitro and in vivo CRISPR therapeutics.

Drugging the efferocytosis process: concepts and opportunities

The daily removal of billions of apoptotic cells in the human body via the process of efferocytosis is essential for homeostasis. To allow for this continuous efferocytosis, rapid phenotypic changes occur in the phagocytes enabling them to engulf and digest the apoptotic cargo. In addition, efferocytosis is actively anti-inflammatory and promotes resolution. Owing to its ubiquitous nature and the sheer volume of cell turnover, efferocytosis is a point of vulnerability. Aberrations in efferocytosis are associated with numerous inflammatory pathologies, including atherosclerosis, cancer and infections. The recent exciting discoveries defining the molecular machinery involved in efferocytosis have opened many avenues for therapeutic intervention, with several agents now in clinical trials.

The EGFR/ErbB inhibitor neratinib modifies the neutrophil phosphoproteome and promotes apoptosis and clearance by airway macrophages

Dysregulated neutrophilic inflammation can be highly destructive in chronic inflammatory diseases due to prolonged neutrophil lifespan and continual release of histotoxic mediators in inflamed tissues. Therapeutic induction of neutrophil apoptosis, an immunologically silent form of cell death, may be beneficial in these diseases, provided that the apoptotic neutrophils are efficiently cleared from the tissue. Previous research in our group identified ErbB inhibitors as able to induce neutrophil apoptosis and reduce neutrophilic inflammation both in vitro and in vivo. Here, we extend that work using a clinical ErbB inhibitor, neratinib, which has the potential to be repurposed in inflammatory diseases. We show that neratinib reduces neutrophilic migration o an inflammatory site in zebrafish larvae. Neratinib upregulates efferocytosis and reduces the number of persisting neutrophil corpses in mouse models of acute, but not chronic, lung injury, suggesting that the drug may have therapeutic benefits in acute inflammatory settings. Phosphoproteomic analysis of human neutrophils shows that neratinib modifies the phosphorylation of proteins regulating apoptosis, migration, and efferocytosis. This work identifies a potential mechanism for neratinib in treating acute lung inflammation by upregulating the clearance of dead neutrophils and, through examination of the neutrophil phosphoproteome, provides important insights into the mechanisms by which this may be occurring.

ROS-activated CXCR2+ neutrophils recruited by CXCL1 delay denervated skeletal muscle atrophy and undergo P53-mediated apoptosis

Neutrophils are the earliest master inflammatory regulator cells recruited to target tissues after direct infection or injury. Although inflammatory factors are present in muscle that has been indirectly disturbed by peripheral nerve injury, whether neutrophils are present and play a role in the associated inflammatory process remains unclear. Here, intravital imaging analysis using spinning-disk confocal intravital microscopy was employed to dynamically identify neutrophils in denervated muscle. Slice digital scanning and 3D-view reconstruction analyses demonstrated that neutrophils escape from vessels and migrate into denervated muscle tissue. Analyses using reactive oxygen species (ROS) inhibitors and flow cytometry demonstrated that enhanced ROS activate neutrophils after denervation. Transcriptome analysis revealed that the vast majority of neutrophils in denervated muscle were of the CXCR2 subtype and were recruited by CXCL1. Most of these cells gradually disappeared within 1 week via P53-mediated apoptosis. Experiments using specific blockers confirmed that neutrophils slow the process of denervated muscle atrophy. Collectively, these results indicate that activated neutrophils are recruited via chemotaxis to muscle tissue that has been indirectly damaged by denervation, where they function in delaying atrophy.

Live animal imaging experiments reveal how rapid recruitment of a subset of immune cells helps prevent muscle wasting after peripheral nerve injury. Such injuries take considerable time to heal, and there are no therapies that reliably prevent wasting of muscle lacking nervous innervation. Researchers led by JunJian Jiang and Jianguang Xu at Fudan University, Shanghai, China, have used intravital microscopy to record the cellular and molecular events that follow nerve damage in live mice. They observed heightened production of chemicals that summon immune cells known as neutrophils to the site of the injury. Even though the surrounding muscle cells were initially undamaged in this animal model, the recruited neutrophils delayed subsequent muscle wasting. This neutrophil recruitment was transient, but therapies that elicit a more sustained response could provide durable protection against muscle wasting.

Neutrophil subsets and their differential roles in viral respiratory diseases

Neutrophils play significant roles in immune homeostasis and as neutralizers of microbial infections. Recent evidence further suggests heterogeneity of neutrophil developmental and activation states that exert specialized effector functions during inflammatory disease conditions. Neutrophils can play multiple roles during viral infections, secreting inflammatory mediators and cytokines that contribute significantly to host defense and pathogenicity. However, their roles in viral immunity are not well understood. In this review, we present an overview of neutrophil heterogeneity and its impact on the course and severity of viral respiratory infectious diseases. We focus on the evidence demonstrating the crucial roles neutrophils play in the immune response toward respiratory infections, using influenza as a model. We further extend the understanding of neutrophil function with the studies pertaining to COVID-19 disease and its neutrophil-associated pathologies. Finally, we discuss the relevance of these results for future therapeutic options through targeting and regulating neutrophil-specific responses.

Macrophages secrete murinoglobulin-1 and galectin-3 to regulate neutrophil degranulation after myocardial infarction

Inflammation presides early after myocardial infarction (MI) as a key event in cardiac wound healing. Ischemic cardiomyocytes secrete inflammatory cues to stimulate infiltration of leukocytes, predominantly macrophages and neutrophils. Infiltrating neutrophils degranulate to release a series of proteases including matrix metalloproteinase (MMP)-9 to break down extracellular matrix and remove necrotic myocytes to create space for the infarct scar to form. While neutrophil to macrophage communication has been explored, the reverse has been understudied. We used a proteomics approach to catalogue the macrophage secretome at MI day 1. Murinoglobulin-1 (MUG1) was the highest-ranked secreted protein (4.1-fold upregulated at MI day 1 vs. day 0 pre-MI cardiac macrophages, p = 0.004). By transcriptomics evaluation, galectin-3 (Lgals3) was 2.2-fold upregulated (p = 0.008) in MI day 1 macrophages. We explored the direct roles of MUG1 and Lgals3 on neutrophil degranulation. MUG1 blunted while Lgals3 amplified neutrophil degranulation in response to phorbol 12-myristate 13-acetate or interleukin-1β, as measured by MMP-9 secretion. Lgals3 itself also stimulated MMP-9 secretion. To determine if MUG1 regulated Lgals3, we co-stimulated neutrophils with MUG1 and Lgals3. MUG1 limited degranulation stimulated by Lgals3 by 64% (p < 0.001). In vivo, MUG1 was elevated in the infarct region at MI days 1 and 3, while Lgals3 increased at MI day 7. The ratio of MUG1 to Lgals3 positively correlated with infarct wall thickness, revealing that MUG1 attenuated infarct wall thinning. In conclusion, macrophages at MI day 1 secrete MUG1 to limit and Lgals3 to accentuate neutrophil degranulation to regulate infarct wall thinning.

Efferocytosis and Its Role in Inflammatory Disorders

Efferocytosis is the effective clearance of apoptotic cells by professional and non-professional phagocytes. The process is mechanically different from other forms of phagocytosis and involves the localization, binding, internalization, and degradation of apoptotic cells. Defective efferocytosis has been demonstrated to associate with the pathogenesis of various inflammatory disorders. In the current review, we summarize recent findings with regard to efferocytosis networks and discuss the relationship between efferocytosis and different immune cell populations, as well as describe how efferocytosis helps resolve inflammatory response and modulate immune balance. Our knowledge so far about efferocytosis suggests that it may be a useful target in the treatment of numerous inflammatory diseases.

CCR7-guided neutrophil redirection to skin-draining lymph nodes regulates cutaneous inflammation and infection

Neutrophils are the first nonresident effector immune cells that migrate to a site of infection or inflammation; however, improper control of neutrophil responses can cause considerable tissue damage. Here, we found that neutrophil responses in inflamed or infected skin were regulated by CCR7-dependent migration and phagocytosis of neutrophils in draining lymph nodes (dLNs). In mouse models of Toll-like receptor-induced skin inflammation and cutaneous Staphylococcus aureus infection, neutrophils migrated from the skin to the dLNs via lymphatic vessels in a CCR7-mediated manner. In the dLNs, these neutrophils were phagocytosed by lymph node-resident type 1 and type 2 conventional dendritic cells. CCR7 up-regulation on neutrophils was a conserved mechanism across different tissues and was induced by a broad range of microbial stimuli. In the context of cutaneous immune responses, disruption of CCR7 interactions by selective CCR7 deficiency of neutrophils resulted in increased antistaphylococcal immunity and aggravated skin inflammation. Thus, neutrophil homing to and clearance in skin-dLNs affects cutaneous immunity versus pathology.

The Immune Response to Respiratory Viruses: From Start to Memory

Biomedical research has long strived to improve our understanding of the immune response to respiratory viral infections, an effort that has become all the more important as we live through the consequences of a pandemic. The disease course of these infections is shaped in large part by the actions of various cells of the innate and adaptive immune systems. While these cells are crucial in clearing viral pathogens and establishing long-term immunity, their effector mechanisms may also escalate into excessive, tissue-destructive inflammation detrimental to the host. In this review, we describe the breadth of the immune response to infection with respiratory viruses such as influenza and respiratory syncytial virus. Throughout, we focus on the host rather than the pathogen and try to describe shared patterns in the host response to different viruses. We start with the local cells of the airways, onto the recruitment and activation of innate and adaptive immune cells, followed by the establishment of local and systemic memory cells key in protection against reinfection. We end by exploring how respiratory viral infections can predispose to bacterial superinfection.

The spatio-temporal control of effector T cell migration

Effector T cells leave the lymph nodes armed with specialized functional attributes. Their antigenic targets may be located anywhere in the body, posing the ultimate challenge: how to efficiently identify the target tissue, navigate through a complex tissue matrix and, ultimately, locate the immunological insult. Recent advances in real-time in situ imaging of effector T cell migratory behaviour have revealed a great degree of mechanistic plasticity that enables effector T cells to push and squeeze their way through inflamed tissues. This process is shaped by an array of 'stop' and 'go' guidance signals including target antigens, chemokines, integrin ligands and the mechanical cues of the inflamed microenvironment. Effector T cells must sense and interpret these competing signals to correctly position themselves to mediate their effector functions for complete and durable responses in infectious disease and malignancy. Tuning T cell migration therapeutically will require a new understanding of this complex decision-making process.

Ly6G deficiency alters the dynamics of neutrophil recruitment and pathogen capture during Leishmania major skin infection

Neutrophils represent one of the first immune cell types recruited to sites of infection, where they can control pathogens by phagocytosis and cytotoxic mechanisms. Intracellular pathogens such as Leishmania major can hijack neutrophils to establish an efficient infection. However the dynamic interactions of neutrophils with the pathogen and other cells at the site of the infection are incompletely understood. Here, we have investigated the role of Ly6G, a homolog of the human CD177 protein, which has been shown to interact with cell adhesion molecules, and serves as a bona fide marker for neutrophils in mice. We show that Ly6G deficiency decreases the initial infection rate of neutrophils recruited to the site of infection. Although the uptake of L. major by subsequently recruited monocytes was tightly linked with the concomitant uptake of neutrophil material, this process was not altered by Ly6G deficiency of the neutrophils. Instead, we observed by intravital 2-photon microscopy that Ly6G-deficient neutrophils entered the site of infection with delayed initial recruitment kinetics. Thus, we conclude that by promoting neutrophils’ ability to efficiently enter the site of infection, Ly6G contributes to the early engagement of intracellular pathogens by the immune system.

Neutrophils in respiratory viral infections

Viral respiratory infections are a common cause of severe disease, especially in infants, people who are immunocompromised, and in the elderly. Neutrophils, an important innate immune cell, infiltrate the lungs rapidly after an inflammatory insult. The most well-characterized effector mechanisms by which neutrophils contribute to host defense are largely extracellular and the involvement of neutrophils in protection from numerous bacterial and fungal infections is well established. However, the role of neutrophils in responses to viruses, which replicate intracellularly, has been less studied. It remains unclear whether and, by which underlying immunological mechanisms, neutrophils contribute to viral control or confer protection against an intracellular pathogen. Furthermore, neutrophils need to be tightly regulated to avoid bystander damage to host tissues. This is especially relevant in the lung where damage to delicate alveolar structures can compromise gas exchange with life-threatening consequences. It is inherently less clear how neutrophils can contribute to host immunity to viruses without causing immunopathology and/or exacerbating disease severity. In this review, we summarize and discuss the current understanding of how neutrophils in the lung direct immune responses to viruses, control viral replication and spread, and cause pathology during respiratory viral infections.

Complement Decay-Accelerating Factor is a modulator of influenza A virus lung immunopathology

Clearance of viral infections, such as SARS-CoV-2 and influenza A virus (IAV), must be fine-tuned to eliminate the pathogen without causing immunopathology. As such, an aggressive initial innate immune response favors the host in contrast to a detrimental prolonged inflammation. The complement pathway bridges innate and adaptive immune system and contributes to the response by directly clearing pathogens or infected cells, as well as recruiting proinflammatory immune cells and regulating inflammation. However, the impact of modulating complement activation in viral infections is still unclear. In this work, we targeted the complement decay-accelerating factor (DAF/CD55), a surface protein that protects cells from non-specific complement attack, and analyzed its role in IAV infections. We found that DAF modulates IAV infection in vivo, via an interplay with the antigenic viral proteins hemagglutinin (HA) and neuraminidase (NA), in a strain specific manner. Our results reveal that, contrary to what could be expected, DAF potentiates complement activation, increasing the recruitment of neutrophils, monocytes and T cells. We also show that viral NA acts on the heavily sialylated DAF and propose that the NA-dependent DAF removal of sialic acids exacerbates complement activation, leading to lung immunopathology. Remarkably, this mechanism has no impact on viral loads, but rather on the host resilience to infection, and may have direct implications in zoonotic influenza transmissions.

Exacerbated complement activation and immune deregulation are at the basis of several pathologies induced by respiratory viruses. Here, we report that complement decay-accelerating factor (DAF), which inhibits complement activation in healthy cells, increases disease severity upon influenza A virus (IAV) infection. Remarkably, DAF interaction with IAV proteins, hemagglutinin (HA) and neuraminidase (NA), resulted in excessive complement activation and recruitment of innate and adaptive immune cells, without affecting viral loads. Furthermore, we observed that viral NA directly cleaves DAF and promotes complement activation, providing a possible link between IAV-DAF interaction and pathology. Therefore, our results unveil a novel pathway that could modulate disease severity, which may help to understand the increased pathogenicity of zoonotic and pandemic IAV infections.

The Neutrophil

Neutrophils are immune cells with unusual biological features that furnish potent antimicrobial properties. These cells phagocytose and subsequently kill prokaryotic and eukaryotic organisms very efficiently. Importantly, it is not only their ability to attack microbes within a constrained intracellular compartment that endows neutrophils with antimicrobial function. They can unleash their effectors into the extracellular space, where, even post-mortem, their killing machinery can endure and remain functional. The antimicrobial activity of neutrophils must not be misconstrued as being microbe specific and should be viewed more generally as biotoxic. Outside of fighting infections, neutrophils can harness their noxious machinery in other contexts, like cancer. Inappropriate or dysregulated neutrophil activation damages the host and contributes to autoimmune and inflammatory disease. Here we review a number of topics related to neutrophil biology based on contemporary findings.

Antigen presentation by mouse monocyte-derived cells: Re-evaluating the concept of monocyte-derived dendritic cells

Antigen presentation is a key feature of classical dendritic cells (cDCs). Numerous studies have also reported in mouse that, upon inflammation, monocytes enter tissues and differentiate into monocyte-derived DCs (mo-DC) that have the ability to present antigens to T cells. However, a population of inflammatory cDCs sharing phenotypic features with mo-DC has been recently described, challenging the existence of in vivo-generated mo-DC. Here we review studies describing mouse mo-DC in the light of these findings, and evaluate the in vivo evidence for monocyte-derived antigen-presenting cells. We examine the strategies used to demonstrate the monocytic origin of these cells. Finally, we propose that mo-DC play a complementary role to cDCs, by presenting antigens to effector T cells locally in tissues.

The Outcome of Neutrophil-T Cell Contact Differs Depending on Activation Status of Both Cell Types

Neutrophils and T cells exist in close proximity in lymph nodes and inflamed tissues during health and disease. They are able to form stable interactions, with profound effects on the phenotype and function of the T cells. However, the outcome of these effects are frequently contradictory; in some systems neutrophils suppress T cell proliferation, in others they are activatory or present antigen directly. Published protocols modelling these interactions in vitro do not reflect the full range of interactions found in vivo; they do not examine how activated and naïve T cells differentially respond to neutrophils, or whether de-granulating or resting neutrophils induce different outcomes. Here, we established a culture protocol to ask these questions with human T cells and autologous neutrophils. We find that resting neutrophils suppress T cell proliferation, activation and cytokine production but that de-granulating neutrophils do not, and neutrophil-released intracellular contents enhance proliferation. Strikingly, we also demonstrate that T cells early in the activation process are susceptible to suppression by neutrophils, while later-stage T cells are not, and naïve T cells do not respond at all. Our protocol therefore allows nuanced analysis of the outcome of interaction of these cells and may explain the contradictory results observed previously.

Bite of the wolf: innate immune responses propagate autoimmunity in lupus

The etiopathogenesis of systemic lupus erythematosus (SLE), a clinically heterogeneous multisystemic syndrome that derives its name from the initial characterization of facial lesions that resemble the bite of a wolf, is considered a complex, multifactorial interplay between underlying genetic susceptibility factors and the environment. Prominent pathogenic factors include the induction of aberrant cell death pathways coupled with defective cell death clearance mechanisms that promote excessive externalization of modified cellular and nuclear debris with subsequent loss of tolerance to a wide variety of autoantigens and innate and adaptive immune dysregulation. While abnormalities in adaptive immunity are well recognized and are key to the pathogenesis of SLE, recent findings have emphasized fundamental roles of the innate immune system in the initiation and propagation of autoimmunity and the development of organ damage in this disease. This Review focuses on recent discoveries regarding the role of components of the innate immune system, specifically neutrophils and interferons, in promoting various aspects of lupus pathogenesis, with potential implications for novel therapeutic strategies.