Patrolling monocytes promote intravascular neutrophil activation and glomerular injury in the acutely inflamed glomerulus

Innate lymphoid cells in kidney diseases

It is well known that innate immune cells, including dendritic cells, macrophages, and natural killer cells, contribute to pathogenesis and protection in various kidney diseases. The understanding of innate immunity has been advanced recently by the discovery of a new group of innate lymphoid cells (ILCs), including ILC1, ILC2, and ILC3. ILCs lack adaptive antigen receptors, yet can be triggered by various pathogens and rapidly provide an abundant source of immunomodulatory cytokines to exert immediate immune reactions and direct subsequent innate and adaptive immune responses. ILCs play critical roles in immunity, tissue homeostasis, and pathological inflammation. In this review, we highlight the biological function of ILC subpopulations in the normal kidney, and their important roles in acute and chronic kidney diseases, thus demonstrating the emerging importance of ILC-regulated immunity in this special organ and providing insights for future research directions and therapeutic interventions.

The relative abundance of monocyte subsets determines susceptibility to perinatal hepatic inflammation

The devastating consequences of perinatal liver inflammation contribute to a pressing need to develop therapeutics for the diseases that underly this condition. Biliary atresia (BA) is a perinatal inflammatory disease of the liver that results in obliterative cholangiopathy and rapidly progresses to liver failure, requiring transplantation. The ability to develop targeted therapies requires an understanding of the immune mechanisms that mitigate perinatal liver inflammation. This article reviews our recent findings demonstrating that in a murine model of perinatal hepatic inflammation, Ly6c^Lo non-classical monocytes express a pro-reparative transcriptomic profile and that the relative abundance of Ly6c^Lo monocytes promotes resolution of perinatal liver inflammation, rendering neonatal pups resistant to disease. We also examine the lineage relationship between monocyte subsets, reviewing data that suggests classical monocytes are a precursor for non-classical monocytes, and the alternative possibility that separate progenitors exist for each subset. Although a precursor-product relationship between classical and non-classical monocytes might exist in certain environments, we argue that they may also arise from separate progenitors, which is evident by sustained Ly6c^Lo non-classical monocyte expansion when Ly6c^Hi monocytes are absent. An improved understanding of monocyte subsets and their developmental trajectories during perinatal hepatic inflammation will provide insight into how therapies directed at controlling monocyte function may help alleviate the devastating consequences of diseases like BA.

Live Imaging of Monocyte Subsets in Immune Complex-Mediated Glomerulonephritis Reveals Distinct Phenotypes and Effector Functions

BACKGROUND

Immune complexes within glomerular capillary walls cause crescentic GN (CrGN). Monocytes and macrophages are important in mediating CrGN, but little work has been done to phenotype the subpopulations involved and determine their respective contributions to glomerular inflammation.

METHODS

Live glomerular imaging using confocal microscopy monitored intravascular monocyte subset behavior during nephrotoxic nephritis (NTN) in a novel WKY-hCD68-GFP monocyte/macrophage reporter rat strain. Flow cytometry and qPCR further analyzed ex vivo the glomerular leukocyte infiltrate during NTN.

RESULTS

Non-classical monocytes surveyed the glomerular endothelium via lymphocyte function-associated antigen 1 (LFA-1) in the steady state. During NTN, non-classical monocytes were recruited first, but subsequent recruitment and retention of classical monocytes was associated with glomerular damage. Monocytes recruited to the glomerular vasculature did not undergo transendothelial migration. This finding suggests that inflammation in immune complex-mediated CrGN is predominantly intravascular, driven by dynamic interactions between intravascular blood monocytes and the endothelium. Glomerular endothelium and non-classical monocytes overexpressed a distinct chemokine axis, which may orchestrate inflammatory myeloid cell recruitment and expression of damage mediators. Reduced classical monocyte recruitment in Lewis rats during NTN confirmed a role for CD16 in mediating glomerular damage.

CONCLUSIONS

Monocyte subsets with distinct phenotypes and effector functions may be important in driving inflammation in experimental CrGN resulting from immune complexes formed within the glomerular capillary wall. LFA-1-dependent endothelial surveillance by non-classical monocytes may detect immune complexes through CD16, orchestrating the inflammatory response through intravascular retention of classical monocytes, which results in glomerular damage and proteinuria.

Autoimmunity and organ damage in systemic lupus erythematosus

Impressive progress has been made over the last several years toward understanding how almost every aspect of the immune system contributes to the expression of systemic autoimmunity. In parallel, studies have shed light on the mechanisms that contribute to organ inflammation and damage. New approaches that address the complicated interaction between genetic variants, epigenetic processes, sex and the environment promise to enlighten the multitude of pathways that lead to what is clinically defined as systemic lupus erythematosus. It is expected that each patient owns a unique 'interactome', which will dictate specific treatment.

Ly6cLo non-classical monocytes promote resolution of rhesus rotavirus-mediated perinatal hepatic inflammation

Perinatal hepatic inflammation can have devastating consequences. Monocytes play an important role in the initiation and resolution of inflammation, and their diverse functions can be attributed to specific cellular subsets: pro-inflammatory or classical monocytes (Ly6c^Hi) and pro-reparative or non-classical monocytes (Ly6c^Lo). We hypothesized that inherent differences in Ly6c^Hi classical monocytes and Ly6c^Lo non-classical monocytes determine susceptibility to perinatal hepatic inflammation in late gestation fetuses and neonates. We found an anti-inflammatory transcriptional profile expressed by Ly6c^Lo non-classical monocytes, and a physiologic abundance of these cells in the late gestation fetal liver. Unlike neonatal pups, late gestation fetuses proved to be resistant to rhesus rotavirus (RRV) mediated liver inflammation. Furthermore, neonatal pups were rendered resistant to RRV-mediated liver injury when Ly6c^Lo non-classical monocytes were expanded. Pharmacologic inhibition of Ly6c^Lo non-classical monocytes in this setting restored susceptibility to RRV-mediated disease. These data demonstrate that Ly6c^Lo monocytes promote resolution of perinatal liver inflammation in the late gestation fetus, where there is a physiologic expansion of non-classical monocytes, and in the neonatal liver upon experimental expansion of these cells. Therapeutic strategies directed towards enhancing Ly6c^Lo non-classical monocyte function may mitigate the detrimental effects of perinatal liver inflammation.

VCAM1/VLA4 interaction mediates Ly6Clow monocyte recruitment to the brain in a TNFR signaling dependent manner during fungal infection

Monocytes exist in two major populations, termed Ly6C^hi and Ly6C^low monocytes. Compared to Ly6C^hi monocytes, less is known about Ly6C^low monocyte recruitment and mechanisms involved in the recruitment of this subset. Furthermore, the role of Ly6C^low monocytes during infections is largely unknown. Here, using intravital microscopy, we demonstrate that Ly6C^low monocytes are predominantly recruited to the brain vasculature following intravenous infection with Cryptococcus neoformans, a fungal pathogen causing meningoencephalitis. The recruitment depends primarily on the interaction of VCAM1 expressed on the brain endothelium with VLA4 expressed on Ly6C^low monocytes. Furthermore, TNFR signaling is essential for the recruitment through enhancing VLA4 expression on Ly6C^low monocytes. Interestingly, the recruited Ly6C^low monocytes internalized C. neoformans and carried the organism while crawling on and adhering to the luminal wall of brain vasculature and migrating to the brain parenchyma. Our study reveals a substantial recruitment of Ly6C^low monocytes to the brain and highlights important properties of this subset during infection.

Monocytes are white blood cells, circulating in the bloodstream and playing important roles during infections. There are two subsets of monocytes in mice: Ly6C^hi and Ly6C^low monocytes. In contrast to the recruitment of Ly6C^hi monocytes shown in other infection models, we observed the predominant recruitment of Ly6C^low monocytes to the brain post-capillary venules during intravenous infection with C. neoformans, a fungal pathogen causing brain infection. The recruitment is mainly mediated by the interaction of VCAM1 and VLA4, which are expressed on the brain endothelium and monocytes, respectively. We further demonstrate that TNFR signaling plays an essential role during Ly6C^low monocyte recruitment through enhancing VLA4 expression on monocytes. We also observed that Ly6C^low monocytes internalize C. neoformans and, together with the ingested organism, crawl along the luminal wall of brain vasculatures and migrate to the brain parenchyma. Thus, VCAM1/VLA4 interaction mediates Ly6C^low monocyte recruitment to the brain in a TNFR signaling dependent manner during fungal infection.

Cellular Stress and General Pathological Processes

From the viewpoint of the general pathology, most of the human diseases are associated with a limited number of pathogenic processes such as inflammation, tumor growth, thrombosis, necrosis, fibrosis, atrophy, pathological hypertrophy, dysplasia and metaplasia. The phenomenon of chronic low-grade inflammation could be attributed to non-classical forms of inflammation, which include many neurodegenerative processes, pathological variants of insulin resistance, atherosclerosis, and other manifestations of the endothelial dysfunction. Individual and universal manifestations of cellular stress could be considered as a basic element of all these pathologies, which has both physiological and pathophysiological significance. The review examines the causes, main phenomena, developmental directions and outcomes of cellular stress using a phylogenetically conservative set of genes and their activation pathways, as well as tissue stress and its role in inflammatory and para-inflammatory processes. The main ways towards the realization of cellular stress and its functional blocks were outlined. The main stages of tissue stress and the classification of its typical manifestations, as well as its participation in the development of the classical and non-classical variants of the inflammatory process, were also described. The mechanisms of cellular and tissue stress are structured into the complex systems, which include networks that enable the exchange of information with multidirectional signaling pathways which together make these systems internally contradictory, and the result of their effects is often unpredictable. However, the possible solutions require new theoretical and methodological approaches, one of which includes the transition to integral criteria, which plausibly reflect the holistic image of these processes.

Intravital Microscopy of the Beating Murine Heart to Understand Cardiac Leukocyte Dynamics

Cardiovascular disease is the leading cause of worldwide mortality. Intravital microscopy has provided unprecedented insight into leukocyte biology by enabling the visualization of dynamic responses within living organ systems at the cell-scale. The heart presents a uniquely dynamic microenvironment driven by periodic, synchronous electrical conduction leading to rhythmic contractions of cardiomyocytes, and phasic coronary blood flow. In addition to functions shared throughout the body, immune cells have specific functions in the heart including tissue-resident macrophage-facilitated electrical conduction and rapid monocyte infiltration upon injury. Leukocyte responses to cardiac pathologies, including myocardial infarction and heart failure, have been well-studied using standard techniques, however, certain questions related to spatiotemporal relationships remain unanswered. Intravital imaging techniques could greatly benefit our understanding of the complexities of in vivo leukocyte behavior within cardiac tissue, but these techniques have been challenging to apply. Different approaches have been developed including high frame rate imaging of the beating heart, explantation models, micro-endoscopy, and mechanical stabilization coupled with various acquisition schemes to overcome challenges specific to the heart. The field of cardiac science has only begun to benefit from intravital microscopy techniques. The current focused review presents an overview of leukocyte responses in the heart, recent developments in intravital microscopy for the murine heart, and a discussion of future developments and applications for cardiovascular immunology.

Immune mechanisms of salt-sensitive hypertension and renal end-organ damage

Immune mechanisms have been recognized to have a role in the pathogenesis of hypertension, vascular disease and kidney damage in humans and animals for many decades. Contemporary advances in experimentation have permitted a deeper understanding of the mechanisms by which inflammation and immunity participate in cardiovascular disease, and multiple observations have demonstrated strong correlations between the discoveries made in animals and those made in patients with hypertension. Of note, striking phenotypic similarities have been observed in the infiltration of immune cells in the kidney and the development of end-organ damage in patients and animal models with sodium-sensitive hypertension. The available data suggest that an initial salt-induced increase in renal perfusion pressure, which is likely independent of immune mechanisms, induces the infiltration of immune cells into the kidney. The mechanisms mediating immune cell infiltration in the kidney are not well understood but likely involve tissue damage, the direct influence of salt to stimulate immune cell activation, sympathetic nerve stimulation or other factors. The infiltrating cells then release cytokines, free radicals and other factors that contribute to renal damage as well as increased retention of sodium and water and vascular resistance, which lead to the further development of hypertension.

Proinflammatory Differentiation of Macrophages Through Microparticles That Form Immune Complexes Leads to T- and B-Cell Activation in Systemic Autoimmune Diseases

Patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) demonstrate increased circulating microparticles (MP). These vesicles, primarily those that form immune complexes (MP-IC), may activate monocytes. We evaluated the effect of MP and MP-IC in the differentiation of monocytes to macrophages (monocyte-derived macrophages; MDM) and for consequences in autologous lymphocyte activation. Monocytes from healthy controls (HC) and patients with RA and SLE that differentiated into MDM in the presence of MP-IC showed a proinflammatory (M1-like) profile, which was more evident using MP-IC from patients with RA than those from patients with SLE. Notably, MDM from HC and patients with RA that differentiated with MP-IC were more prone to M1-like profile than those from patients with SLE. In HC and patients with RA, monocyte differentiation using MP-IC decreased the frequency of MDM that bound/internalized latex beads. The M1-like profile did not completely revert following IL-4 treatment. The effect of M1-like MDM on T lymphocytes stimulated with phytohemagglutinin was further evaluated. MDM differentiated with MP enhanced the proliferation of T cells obtained from patients with RA compared with those differentiated with MP-IC or without vesicles. Neither MP nor MP-IC induced interferon (IFN)-γ+ and tumor necrosis factor (TNF)-α+ T cells in patients with RA. Conversely, unlike MDM differentiated with or without MP, MP-IC enhanced the proliferation and increased the frequencies of IFN-γ+CD4+ T, TNF-α+CD4+ T, and IFN-γ+CD8+ T cells in patients with SLE. The co-culture of B cells with MDM obtained from patients with RA and SLE and differentiated with MP-IC increased the expression of B-cell activation markers and prevented B lymphocyte death. Strikingly, only for patients with SLE, these responses seemed to be associated with a significant increase in B-cell activating factor levels, high plasmablast frequency and immunoglobulin production. These results showed that MP-IC from patients with systemic autoimmune diseases favored the polarization of MDM into a proinflammatory profile that promotes T-cell activation, and additionally induced B-cell activation and survival. Therefore, the effect of MP-IC in mononuclear phagocytes may be an important factor for modulating adaptive responses in systemic autoimmune diseases.

Macrophages Are Key Regulators of Stem Cells during Skeletal Muscle Regeneration and Diseases

Muscle regeneration is a closely regulated process that involves a variety of cell types such as satellite cells, myofibers, fibroadipogenic progenitors, endothelial cells, and inflammatory cells. Among these different cell types, macrophages emerged as a central actor coordinating the different cellular interactions and biological processes. Particularly, the transition of macrophages from their proinflammatory to their anti-inflammatory phenotype was shown to regulate inflammation, myogenesis, fibrosis, vascularization, and return to homeostasis. On the other hand, deregulation of macrophage accumulation or polarization in chronic degenerative muscle disorders was shown to impair muscle regeneration. Considering the key roles of macrophages in skeletal muscle, they represent an attractive target for new therapeutic approaches aiming at mitigating various muscle disorders. This review aims at summarizing the novel insights into macrophage heterogeneity, plasticity, and functions in skeletal muscle homeostasis, regeneration, and disease.

Current Concepts on 6-sulfo LacNAc Expressing Monocytes (slanMo)

The human mononuclear phagocytes system consists of dendritic cells (DCs), monocytes, and macrophages having different functions in bridging innate and adaptive immunity. Among the heterogeneous population of monocytes the cell surface marker slan (6-sulfo LacNAc) identifies a specific subset of human CD14^- CD16⁺ non-classical monocytes, called slan⁺ monocytes (slanMo). In this review we discuss the identity and functions of slanMo, their contributions to immune surveillance by pro-inflammatory cytokine production, and cross talk with T cells and NK cells. We also consider the role of slanMo in the regulation of chronic inflammatory diseases and cancer. Finally, we highlight unresolved questions that should be the focus of future research.

Inflammation research sails through the sea of immunology to reach immunometabolism

Inflammation occurs as a result of acute trauma, invasion of the host by different pathogens, pathogen-associated molecular patterns (PAMPs) or chronic cellular stress generating damage-associated molecular patterns (DAMPs). Thus inflammation may occur under both sterile inflammatory conditions including certain cancers, autoimmune or autoinflammatory diseases (Rheumatic arthritis (RA)) and infectious diseases including sepsis, pneumonia-associated acute lung inflammation (ALI) or acute respiratory distress syndrome (ARDS). The pathogenesis of inflammation involves dysregulation of an otherwise protective immune response comprising of various innate and adaptive immune cells and humoral (cytokines and chemokines) mediators secreted by these immune cells upon the activation of signaling mechanisms regulated by the activation of different pattern recognition receptors (PRRs). However, the pro-inflammatory and anti-inflammatory action of these immune cells is determined by the metabolic stage of the immune cells. The metabolic process of immune cells is called immunometabolism and its shift determined by inflammatory stimuli is called immunometabolic reprogramming. The article focuses on the involvement of various immune cells generating the inflammation, their interaction, immunometabolic reprogramming, and the therapeutic targeting of the immunometabolism to manage inflammation.

Patrolling monocytes promote the pathogenesis of early lupus-like glomerulonephritis

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with genetic and environmental contributions. Hallmarks of the disease are the appearance of immune complexes (IC) containing autoreactive Abs and TLR-activating nucleic acids, whose deposition in kidney glomeruli is suspected to promote tissue injury and glomerulonephritis (GN). Here, using a mouse model based on the human SLE susceptibility locus TNFAIP3-interacting protein 1 (TNIP1, also known as ABIN1), we investigated the pathogenesis of GN. We found that GN was driven by TLRs but, remarkably, proceeded independently of ICs. Rather, disease in 3 different mouse models and patients with SLE was characterized by glomerular accumulation of patrolling monocytes (PMos), a cell type with an emerging key function in vascular inflammation. Consistent with such function in GN, monocyte-specific deletion of ABIN1 promoted kidney disease, whereas selective elimination of PMos provided protection. In contrast to GN, PMo elimination did not protect from reduced survival or disease symptoms such as IC generation and splenomegaly, suggesting that GN and other inflammatory processes are governed by distinct pathogenic mechanisms. These data identify TLR-activated PMos as the principal component of an intravascular process that contributes to glomerular inflammation and kidney injury.

Nonclassical Monocytes in Health and Disease

Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14+CD16−in humans and Ly6Chiin mice), intermediate (CD14+CD16+in humans and Ly6C+Treml4+in mice), and nonclassical (CD14−CD16+in humans and Ly6Cloin mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.

Capturing the Fantastic Voyage of Monocytes Through Time and Space

Monocytes are a subset of cells that are categorized together with dendritic cells (DCs) and macrophages in the mononuclear phagocyte system (MPS). Despite sharing several phenotypic and functional characteristics with MPS cells, monocytes are unique cells with the ability to function as both precursor and effector cells in their own right. Before the development of hematopoietic stem cells (HSCs) in utero, monocytes are derived from erythro-myeloid precursors (EMPs) in the fetal liver that are important for populating the majority of tissue resident macrophages. After birth, monocytes arise from bone marrow (BM)-derived HSCs and are released into the circulation upon their maturation, where they survey peripheral tissues and maintain endothelial integrity. Upon sensing of microbial breaches or inflammatory stimuli, monocytes migrate into tissues where their plasticity allows them to differentiate into cells that resemble macrophages or DCs according to the environmental niche. Alternatively, they may also migrate into tissues in the absence of inflammation and remain in an undifferentiated state where they perform homeostatic roles. As monocytes are typically on the move, the availability of intravital imaging approaches has provided further insights into their trafficking patterns in distinct tissue compartments. In this review, we outline the importance of understanding their functional behavior in the context of tissue compartments, and how these studies may contribute towards improved vaccine and future therapeutic strategies.

Neutrophils disturb pulmonary microcirculation in sepsis-induced acute lung injury

The lung is highly vulnerable during sepsis, yet its functional deterioration accompanied by disturbances in the pulmonary microcirculation is poorly understood. This study aimed to investigate how the pulmonary microcirculation is distorted in sepsis-induced acute lung injury (ALI) and reveal the underlying cellular pathophysiologic mechanism.

Using a custom-made intravital lung microscopic imaging system in a murine model of sepsis-induced ALI, we achieved direct real-time visualisation of the pulmonary microcirculation and circulating cells in vivo. We derived the functional capillary ratio (FCR) as a quantitative parameter for assessing the fraction of functional microvasculature in the pulmonary microcirculation and dead space.

We identified that the FCR rapidly decreases in the early stage of sepsis-induced ALI. The intravital imaging revealed that this decrease resulted from the generation of dead space, which was induced by prolonged neutrophil entrapment within the capillaries. We further showed that the neutrophils had an extended sequestration time and an arrest-like dynamic behaviour, both of which triggered neutrophil aggregates inside the capillaries and arterioles. Finally, we found that Mac-1 (CD11b/CD18) was upregulated in the sequestered neutrophils and that a Mac-1 inhibitor restored the FCR and improved hypoxaemia.

Using the intravital lung imaging system, we observed that Mac-1-upregulated neutrophil aggregates led to the generation of dead space in the pulmonary microcirculation that was recovered by a Mac-1 inhibitor in sepsis-induced ALI.

Neutrophils induce dead space in the pulmonary microcirculation in sepsis-induced ALI, recovered by a Mac-1 inhibitorhttp://ow.ly/vUzO30nbUyU

Unraveling the host's immune response to infection: Seeing is believing

It has long been appreciated that understanding the interactions between the host and the pathogens that make us sick is critical for the prevention and treatment of disease. As antibiotics become increasingly ineffective, targeting the host and specific bacterial evasion mechanisms are becoming novel therapeutic approaches. The technology used to understand host‐pathogen interactions has dramatically advanced over the last century. We have moved away from using simple in vitro assays focused on single‐cell events to technologies that allow us to observe complex multicellular interactions in real time in live animals. Specifically, intravital microscopy (IVM) has improved our understanding of infection, from viral to bacterial to parasitic, and how the host immune system responds to these infections. Yet, at the same time it has allowed us to appreciate just how complex these interactions are and that current experimental models still have a number of limitations. In this review, we will discuss the advances in vivo IVM has brought to the study of host‐pathogen interactions, focusing primarily on bacterial infections and innate immunity.

Advances in Renal Cell Imaging

A great variety of cell imaging technologies are used routinely every day for the investigation of kidney cell types in applications ranging from basic science research to drug development and pharmacology, clinical nephrology, and pathology. Quantitative visualization of the identity, density, and fate of both resident and nonresident cells in the kidney, and imaging-based analysis of their altered function, (patho)biology, metabolism, and signaling in disease conditions, can help to better define pathomechanism-based disease subgroups, identify critical cells and structures that play a role in the pathogenesis, critically needed biomarkers of disease progression, and cell and molecular pathways as targets for novel therapies. Overall, renal cell imaging has great potential for improving the precision of diagnostic and treatment paradigms for individual acute kidney injury or chronic kidney disease patients or patient populations. This review highlights and provides examples for some of the recently developed renal cell optical imaging approaches, mainly intravital multiphoton fluorescence microscopy, and the new knowledge they provide for our better understanding of renal pathologies.

Platelet retention in inflamed glomeruli occurs via selective prolongation of interactions with immune cells

Platelet-leukocyte interactions promote acute glomerulonephritis. However, neither the nature of the interactions between platelets and immune cells nor the capacity of platelets to promote leukocyte activation has been characterized in this condition. We used confocal intravital microscopy to define the interactions of platelets with neutrophils, monocytes, and endothelial cells in glomerular capillaries in mice. In the absence of inflammation, platelets underwent rapid on/off interactions with immune cells. During glomerulonephritis induced by in situ immune complex formation, platelets that interacted with neutrophils or monocytes, but not with other intraglomerular cells, were retained in the glomerulus for prolonged durations. Depletion of platelets inhibited both neutrophil recruitment and activation. Inhibition of platelet activating factor reduced neutrophil recruitment without impacting reactive oxygen species generation, while blocking CXC chemokine ligand 7 (CXCL7) reduced both responses. In contrast, inhibition of the adenosine diphosphate and thromboxane A2 pathways inhibited neutrophil reactive oxygen species generation without affecting neutrophil adhesion. Thus, platelet retention in glomerular capillaries following immune complex deposition stems from prolongation of platelet interactions with immune cells but not other substrates. Pro-inflammatory mediators play divergent roles in promoting neutrophil retention and activation in glomerular capillaries.

Organ-Specific Mechanisms of Transendothelial Neutrophil Migration in the Lung, Liver, Kidney, and Aorta

Immune responses are dependent on the recruitment of leukocytes to the site of inflammation. The classical leukocyte recruitment cascade, consisting of capture, rolling, arrest, adhesion, crawling, and transendothelial migration, is thoroughly studied but mostly in model systems, such as the cremasteric microcirculation. This cascade paradigm, which is widely accepted, might be applicable to many tissues, however recruitment mechanisms might substantially vary in different organs. Over the last decade, several studies shed light on organ-specific mechanisms of leukocyte recruitment. An improved awareness of this matter opens new therapeutic windows and allows targeting inflammation in a tissue-specific manner. The aim of this review is to summarize the current understanding of the leukocyte recruitment in general and how this varies in different organs. In particular we focus on neutrophils, as these are the first circulating leukocytes to reach the site of inflammation. Specifically, the recruitment mechanism in large arteries, as well as vessels in the lungs, liver, and kidney will be addressed.

Recording the wild lives of immune cells

Intravital microscopic imaging can uncover fundamental aspects of immune cell behavior in real time in both healthy and pathological states. Here, we discuss approaches for single-cell imaging of adaptive and innate immune cells to explore how they migrate, communicate, and mediate regulatory or effector functions in various tissues throughout the body. We further review how intravital single-cell imaging can be used to study drug effects on immune cells.

Intracapillary immune complexes recruit and activate slan-expressing CD16+ monocytes in human lupus nephritis

Lupus nephritis is a major cause of morbidity in patients with systemic lupus erythematosus. Among the different types of lupus nephritis, intracapillary immune complex (IC) deposition and accumulation of monocytes are hallmarks of lupus nephritis class III and IV. The relevance of intracapillary ICs in terms of monocyte recruitment and activation, as well as the nature and function of these monocytes are not well understood. For the early focal form of lupus nephritis (class III) we demonstrate a selective accumulation of the proinflammatory population of 6-sulfo LacNAc+ (slan) monocytes (slanMo), which locally expressed TNF-α. Immobilized ICs induced a direct recruitment of slanMo from the microcirculation via interaction with Fc γ receptor IIIA (CD16). Interestingly, intravenous immunoglobulins blocked CD16 and prevented cell recruitment. Engagement of immobilized ICs by slanMo induced the production of neutrophil-attracting chemokine CXCL2 as well as TNF-α, which in a forward feedback loop stimulated endothelial cells to produce the slanMo-recruiting chemokine CX3CL1 (fractalkine). In conclusion, we observed that expression of CD16 equips slanMo with a unique capacity to orchestrate early IC-induced inflammatory responses in glomeruli and identified slanMo as a pathogenic proinflammatory cell type in lupus nephritis.

Neutrophil exocytosis induces podocyte cytoskeletal reorganization and proteinuria in experimental glomerulonephritis

Acute glomerulonephritis is characterized by rapid glomerular neutrophil recruitment, proteinuria, and glomerular hypercellularity. The current study tested the hypothesis that the release of neutrophil granule contents plays a role in both the loss of filtration barrier leading to proteinuria and the increase in glomerular cells. Inhibition of neutrophil exocytosis with a peptide inhibitor prevented proteinuria and attenuated podocyte and endothelial cell injury but had no effect on glomerular hypercellularity in an experimental acute glomerulonephritis model in mice. Cultivation of podocytes with neutrophil granule contents disrupted cytoskeletal organization, an in vitro model for podocyte effacement and loss of filtration barrier. Activated, cultured podocytes released cytokines that stimulated neutrophil chemotaxis, primed respiratory burst activity, and stimulated neutrophil exocytosis. We conclude that crosstalk between podocytes and neutrophils contributes to disruption of the glomerular filtration barrier in acute glomerulonephritis. Neutrophil granule products induce podocyte injury but do not participate in the proliferative response of intrinsic glomerular cells.

Invariant Natural Killer T Cells Shape the Gut Microbiota and Regulate Neutrophil Recruitment and Function During Intestinal Inflammation

Invariant natural killer T (iNKT) cells and neutrophils play an increasingly important part in the pathogenesis of inflammatory diseases, but their precise roles in modulating colitis remain unclear. Previous studies have shown important interplays between host immune system and the gut microbiota, and the resulting modulation of inflammation. However, the interactions between iNKT cells, neutrophil and gut microbiota in regulating colitis pathology are poorly understood. Here, we show iNKT cell-deficient Jα18^−/− mice display reduced dextran sodium sulfate (DSS)-induced colonic inflammation compared to their wild-type (WT) counterparts. We reveal that there is a distinct gut microbiota shaped by the absence of iNKT cells, which comprises of microorganisms that are associated with protection from colonic inflammation. Additionally, the reduced inflammation in Jα18^−/− mice was correlated with increased expressions of neutrophil chemoattractant (Cxcl1 and Cxcl2) and increased neutrophil recruitment. However, these neutrophils were recruited to the colon at day 3 of our model, prior to observable clinical signs at day 5. Further analysis shows that these neutrophils, primed by the microbiota shaped by the lack of iNKT cells, exhibit anti-inflammatory and immune-modulatory properties. Indeed, depletion of neutrophils in DSS-treated Jα18^−/− mice demonstrates that neutrophils confer an anti-colitogenic effect in the absence of iNKT cells. Thus, our data supports a changing dogma that neutrophils possess important regulatory roles in inflammation and highlights the complexity of the iNKT cell–microbiota–neutrophil axis in regulating colonic inflammation.

Monocytes enhance neutrophil-induced blister formation in an ex vivo model of bullous pemphigoid

BACKGROUND

Lesions of bullous pemphigoid (BP), an autoimmune subepidermal blistering disease characterized by the presence of tissue-bound and circulating autoantibodies to hemidesmosomal antigens, harbor a mixed inflammatory cellular infiltrate. In various models, neutrophils, eosinophils, mast cells, monocytes as well as B and T cells have been shown to be involved in the pathogenesis of BP. However, their interactions with and effective role in blister formation remain uncertain. This study was aimed at investigating the effect of monocyte/neutrophil interaction on blister formation in an ex vivo BP model.

METHODS

Skin cryosections were incubated with purified human neutrophils and monocytes, in the presence or absence of BP autoantibodies. Production of reactive oxygen species (ROS), degranulation, mediator release (neutrophil elastase [NE], myeloperoxidase [MPO], matrix metalloproteinase-9 [MMP-9]), binding of Fcγ receptor (CD16, CD32, CD64), and cell adhesion (CD18, ICAM-1) was investigated using appropriate inhibitors. Dermal-epidermal separation (DES) was assessed by light microscopy and quantified by Fiji software.

RESULTS

Monocytes and neutrophils synergistically interact resulting in a significantly higher DES compared to either monocytes or neutrophils separately (P < .0001). Monocyte/neutrophil-induced DES was associated with increased ROS production and was dependent on adhesion and FcγRIII binding. Upon stimulation by the granule-poor fraction of monocyte supernatants, neutrophils increased their release of MMP-9, thereby also DES at the dermal-epidermal junction of skin cryosections.

CONCLUSION

Our observations suggest that the interaction of cells, as shown here for monocytes and neutrophils, enhances mediator release resulting in an increased subepidermal blister formation. Thus, blocking intercellular cross talk promises a new therapeutic approach for blocking tissue damage in BP.

New insights into crosstalk in the kidney

PURPOSE OF REVIEW

The kidney is a highly complex organ and renal function depends on many factors, both extrinsic to the kidney and intrinsic. The kidney responds both to systemic hormonal and neuronal signals and to autocrine and paracrine factors produced within the renal tissue. Recently, there has been an increased emphasis on crosstalk in and between different compartments in the kidney.

RECENT FINDINGS

Crosstalk in the kidney between different cellular compartments has added new and important understanding of renal function and the development of kidney disease.

SUMMARY

Most of the literature cited concern glomerular crosstalk but also tubular and interstitial crosstalk are being reviewed. Mechanistic insight into the communication between the cells may help us find new targets for treating kidney disease.

Partners in crime: neutrophils and monocytes/macrophages in inflammation and disease

Neutrophils are becoming recognized as highly versatile and sophisticated cells that display de novo synthetic capacity and potentially prolonged lifespan. Emerging concepts such as neutrophil heterogeneity and plasticity have revealed that, under pathological conditions, neutrophils may differentiate into discrete subsets defined by distinct phenotypic and functional characteristics. Indeed, these newly described neutrophil subsets will undoubtedly add to the already complex interactions between neutrophils and other immune cell types for an effective immune response. The interactions between neutrophils and monocytes/macrophages enable the host to efficiently defend against and eliminate foreign pathogens. However, it is also becoming increasingly clear that these interactions can be detrimental to the host if not tightly regulated. In this review, we will explore the functional cooperation of neutrophil and monocytes/macrophages in homeostasis, during acute inflammation and in various disease settings. We will discuss this in the context of cardiovascular disease in the form of atherosclerosis, an autoimmune disease mainly occurring in the kidneys, as well as the unique intestinal immune response of the gut that does not conform to the norms of the typical immune system.

Effector CD4+ T cells recognize intravascular antigen presented by patrolling monocytes

Although effector CD4⁺ T cells readily respond to antigen outside the vasculature, how they respond to intravascular antigens is unknown. Here we show the process of intravascular antigen recognition using intravital multiphoton microscopy of glomeruli. CD4⁺ T cells undergo intravascular migration within uninflamed glomeruli. Similarly, while MHCII is not expressed by intrinsic glomerular cells, intravascular MHCII-expressing immune cells patrol glomerular capillaries, interacting with CD4⁺ T cells. Following intravascular deposition of antigen in glomeruli, effector CD4⁺ T-cell responses, including NFAT1 nuclear translocation and decreased migration, are consistent with antigen recognition. Of the MHCII⁺ immune cells adherent in glomerular capillaries, only monocytes are retained for prolonged durations. These cells can also induce T-cell proliferation in vitro. Moreover, monocyte depletion reduces CD4⁺ T-cell-dependent glomerular inflammation. These findings indicate that MHCII⁺ monocytes patrolling the glomerular microvasculature can present intravascular antigen to CD4⁺ T cells within glomerular capillaries, leading to antigen-dependent inflammation.

Monocytes constitutively adhere and crawl along the glomerular endothelium and are thought to contribute to glomerulonephritis. Here the authors use multiphoton microscopy to show local antigen presentation by MHCII⁺ monocytes to T cells in glomerular capillaries of mice.

Measuring and interpreting platelet-leukocyte aggregates

Platelets, besides their specialised role in haemostasis and atherothrombosis, actively modulate innate and adaptive immune responses with crucial roles in immune surveillance, inflammation and host defence during infection. An important prerequisite for platelet-mediated changes of immune functions involves direct engagement with different types of leukocytes. Indeed, increased platelet-leukocyte aggregates (PLAs) within the circulation and/or locally at the site of inflammation represent markers of many thrombo-inflammatory diseases, such as cardiovascular diseases, acute lung injury, renal and cerebral inflammation. Therefore, measurement of PLAs could provide an attractive and easily accessible prognostic and/or diagnostic tool for many diseases. To measure PLAs in different (patho-)physiological settings in human and animal models flow cytometric and microscopic approaches have been applied. These techniques represent complementary tools to study different aspects relating to the involvement of leukocyte subtypes and molecules, as well as location of PLAs within tissues, dynamics of their interactions and/or dynamic changes in leukocyte and platelet behaviour. This review summarises various approaches to measure and interpret PLAs and discusses potential experimental factors influencing platelet binding to leukocytes. Furthermore, we summarise insights gained from studies regarding the underlying mechanism of platelet-leukocyte interactions and discuss implications of these interactions in health and disease.

Renal outcomes in primary IgA nephropathy patients with segmental glomerular necrosis: a case-control study

The renal prognosis and treatment of primary IgA nephropathy (IgAN) patients with segmental glomerular necrosis (SGN) remain controversial. Patients with primary IgAN confirmed by renal biopsy were enrolled. Patients with SGN on renal biopsy were selected as the necrosis group, and a propensity score matching method was used to match a control group according to age, gender, weight, height and follow-up time. A total of 825 IgAN patients were enrolled in the present study. Seventy-three (8.8%) patients with SGN were selected as the necrosis group, and 292 patients without SGN were matched as the control group. Compared to the control group, a significantly increased serum fibrinogen level (3.97 g/L vs 3.54 g/L, P=.002) and proportion of patients with macroscopic hematuria (35.6% vs 14.7%, P<.001) was observed in the necrosis group. According to the new IgA pathological classification system, crescent formation was more pronounced in the necrosis group (P=.001). The average estimated glomerular filtration rate was obviously higher in the necrosis group and decreased more slowly during follow-up. However, the time-averaged urine protein-to-creatinine ratio remained low in the necrotic group, whereas it gradually increased in the control group. SGN suggests an active renal inflammatory state, but it was not an independent risk factor for a poor renal outcome in patients treated with immunosuppressive therapy. Furthermore, patients with SGN had a more stable renal function and low urinary protein excretion during follow-up, which may be attributable to aggressive immunotherapy.

Macrophage extracellular trap formation promoted by platelet activation is a key mediator of rhabdomyolysis-induced acute kidney injury

Rhabdomyolysis is a serious syndrome caused by skeletal muscle injury and the subsequent release of breakdown products from damaged muscle cells into systemic circulation. The muscle damage most often results from strenuous exercise, muscle hypoxia, medications, or drug abuse and can lead to life-threatening complications, such as acute kidney injury (AKI). Rhabdomyolysis and the AKI complication can also occur during crush syndrome, an emergency condition that commonly occurs in victims of natural disasters, such as earthquakes, and man-made disasters, such as wars and terrorism. Myoglobin released from damaged muscle is believed to trigger renal dysfunction in this form of AKI. Recently, macrophages were implicated in the disease pathogenesis of rhabdomyolysis-induced AKI, but the precise molecular mechanism remains unclear. In the present study, we show that macrophages released extracellular traps (ETs) comprising DNA fibers and granule proteins in a mouse model of rhabdomyolysis. Heme-activated platelets released from necrotic muscle cells during rhabdomyolysis enhanced the production of macrophage extracellular traps (METs) through increasing intracellular reactive oxygen species generation and histone citrullination. Here we report, for the first time to our knowledge, this unanticipated role for METs and platelets as a sensor of myoglobin-derived heme in rhabdomyolysis-induced AKI. This previously unknown mechanism might be targeted for treatment of the disease. Finally, we found a new therapeutic tool for prevention of AKI after rhabdomyolysis, which might rescue some sufferers of this pathology.

Illuminating the covert mission of mononuclear phagocytes in their regional niches

Monocytes, dendritic cells (DCs) and macrophages have been classically categorized into the mononuclear phagocyte system (MPS) based on their similar functional and phenotypic characteristics. While an increasing amount of research has revealed substantial ontogenic and functional differences among these cells, the reasons behind their heterogeneity and strategic positioning in specific niches throughout the body are yet to be fully elucidated. In this review, we outline how recent advances in intravital imaging studies have dissected this phenomenon and have allowed us to appreciate how MPS cells exploit their regional niches to specialize and maximize their functional properties. Understanding their cellular behavior in each of their specialized microenvironment will eventually allow us to target specific cells and their behavioral patterns for improved vaccine and therapeutic purposes.

Patrolling Mechanics of Non-Classical Monocytes in Vascular Inflammation

Non-classical monocytes have emerged as the preeminent vascular housekeepers. Continuous intravascular screening is enabled by slow patrolling on the endothelium and allows a rapid response to local perturbations. Intravital imaging has been crucial to elucidate the molecular mechanisms and migratory phenotype of patrolling. In this review, we discuss technical requirements of intravital microscopy such as imaging modalities, labeling strategies, and data analysis. We further focus on patrolling kinetics and adhesion receptors in different organs and vascular beds including arteries during homeostasis and vascular inflammation and define pertinent questions in the field.

Re-Examining Neutrophil Participation in GN

Significant advances in understanding the pathogenesis of GN have occurred in recent decades. Among those advances is the finding that both innate and adaptive immune cells contribute to the development of GN. Neutrophils were recognized as key contributors in early animal models of GN, at a time when the prevailing view considered neutrophils to function as nonspecific effector cells that die quickly after performing antimicrobial functions. However, advances over the past two decades have shown that neutrophil functions are more complex and sophisticated. Specifically, research has revealed that neutrophil survival is regulated by the inflammatory milieu and that neutrophils demonstrate plasticity, mediate microbial killing through previously unrecognized mechanisms, demonstrate transcriptional activity leading to the release of cytokines and chemokines, interact with and regulate cells of the innate and adaptive immune systems, and contribute to the resolution of inflammation. Therefore, neutrophil participation in glomerular diseases deserves re-evaluation. In this review, we describe advances in understanding classic neutrophil functions, review the expanded roles of neutrophils in innate and adaptive immune responses, and summarize current knowledge of neutrophil contributions to GN.

Imaging Leukocyte Responses in the Kidney

The kidney can be negatively affected by a range of innate and adaptive immune responses, resulting in alterations in the functions of the kidney and, in some cases, progression to renal failure. In many of these responses, infiltration of blood-borne leukocytes into the kidney is central to the response. In addition, a large population of mononuclear phagocytes resident in the kidney can modulate these responses. A great deal of research has investigated both the mechanisms of leukocyte recruitment to the kidney and the actions of immune cells resident within the kidney. Because of the dynamic nature of the processes whereby leukocytes enter sites of inflammation, in vivo imaging has been one of the key approaches used for understanding leukocyte recruitment as it occurs throughout the body, and this is also true for kidney. However, imaging this organ and its complicated microvasculature during different forms of renal pathology presents a unique set of challenges. In this review, we examine the approaches used for intravital imaging of the kidney and summarize the insights gained from these studies regarding the mechanisms of leukocyte entry into the kidney during inflammation and the actions of immune cells within this organ.

Endothelial Protective Monocyte Patrolling in Large Arteries Intensified by Western Diet and Atherosclerosis

Supplemental Digital Content is available in the text.

Rationale:

Nonclassical mouse monocyte (CX3CR1^high, Ly-6C^low) patrolling along the vessels of the microcirculation is critical for endothelial homeostasis and inflammation. Because of technical challenges, it is currently not established how patrolling occurs in large arteries.

Objective:

This study was undertaken to elucidate the molecular, migratory, and functional phenotypes of patrolling monocytes in the high shear and pulsatile environment of large arteries in healthy, hyperlipidemic, and atherosclerotic conditions.

Methods and Results:

Applying a new method for stable, long-term 2-photon intravital microscopy of unrestrained large arteries in live CX3CR1-GFP (green fluorescent protein) mice, we show that nonclassical monocytes patrol inside healthy carotid arteries at a velocity of 36 μm/min, 3× faster than in microvessels. The tracks are less straight but lead preferentially downstream. The number of patrolling monocytes is increased 9-fold by feeding wild-type mice a Western diet or by applying topical TLR7/8 (Toll-like receptor) agonists. A similar increase is seen in CX3CR1^+/GFP/apoE^−/− mice on chow diet, with a further 2- to 3-fold increase on Western diet (22-fold over healthy). In plaque conditions, monocytes are readily captured onto the endothelium from free flow. Stable patrolling is unaffected in CX3CR1-deficient mice and involves the contribution of LFA-1 (lymphocyte-associated antigen 1) and α₄ integrins. The endothelial damage in atherosclerotic carotid arteries was assessed by electron microscopy and correlates with the number of intraluminal patrollers. Abolishing patrolling monocytes in Nr4a1^−/− apoE^−/− mice leads to pronounced endothelial apoptosis.

Conclusions:

Arterial patrolling is a prominent new feature of nonclassical monocytes with unique molecular and kinetic properties. It is highly upregulated in hyperlipidemia and atherosclerosis in a CX3CR1-independent fashion and plays a potential role in endothelial protection.

Effects of dyslipidaemia on monocyte production and function in cardiovascular disease

Monocytes are heterogeneous effector cells involved in the maintenance and restoration of tissue integrity. Monocytes and macrophages are involved in cardiovascular disease progression, and are associated with the development of unstable atherosclerotic plaques. Hyperlipidaemia can accelerate cardiovascular disease progression. However, monocyte responses to hyperlipidaemia are poorly understood. In the past decade, accumulating data describe the relationship between the dynamic blood lipid environment and the heterogeneous circulating monocyte pool, which might have profound consequences for cardiovascular disease. In this Review, we explore the updated view of monocytes in cardiovascular disease and their relationship with macrophages in promoting the homeostatic and inflammatory responses related to atherosclerosis. We describe the different definitions of dyslipidaemia, highlight current theories on the ontogeny of monocyte heterogeneity, discuss how dyslipidaemia might alter monocyte production, and explore the mechanistic interface linking dyslipidaemia with monocyte effector functions, such as migration and the inflammatory response. Finally, we discuss the role of dietary and endogenous lipid species in mediating dyslipidaemic responses, and the role of these lipids in promoting the risk of cardiovascular disease through modulation of monocyte behaviour.

CX3CL1/CX3CR1 Axis, as the Therapeutic Potential in Renal Diseases: Friend or Foe?

The fractalkine receptor chemokine (C-X3-C motif) receptor 1 (CX3CR1) and its highly selective ligand CX3CL1 mediate chemotaxis and adhesion of immune cells, which are involved in the pathogenesis and progression of numerous inflammatory disorders and malignancies. The CX3CL1/CX3CR1 axis has recently drawn attention as a potential therapeutic target because it is involved in the ontogeny, homeostatic migration, or colonization of renal phagocytes. We performed a Medline/PubMed search to detect recently published studies that explored the relationship between the CX3CL1/CX3CR1 axis and renal diseases and disorders, including diabetic nephropathy, renal allograft rejection, infectious renal diseases, IgA nephropathy, fibrotic kidney disease, lupus nephritis and glomerulonephritis, acute kidney injury and renal carcinoma. Most studies demonstrated its role in promoting renal pathopoiesis; however, several recent studies showed that the CX3CL1/CX3CR1 axis could also reduce renal pathopoiesis. Thus, the CX3CL1/CX3CR1 axis is now considered to be a double-edged sword that could provide novel perspectives into the pathogenesis and treatment of renal diseases and disorders.