The "Phagocytic Synapse" and Clearance of Apoptotic Cells

Single-cell topographical profiling of the immune synapse reveals a biomechanical signature of cytotoxicity

Immune cells have intensely physical lifestyles characterized by structural plasticity and force exertion. To investigate whether specific immune functions require stereotyped mechanical outputs, we used super-resolution traction force microscopy to compare the immune synapses formed by cytotoxic T cells with contacts formed by other T cell subsets and by macrophages. T cell synapses were globally compressive, which was fundamentally different from the pulling and pinching associated with macrophage phagocytosis. Spectral decomposition of force exertion patterns from each cell type linked cytotoxicity to compressive strength, local protrusiveness, and the induction of complex, asymmetric topography. These features were validated as cytotoxic drivers by genetic disruption of cytoskeletal regulators, live imaging of synaptic secretion, and in silico analysis of interfacial distortion. Synapse architecture and force exertion were sensitive to target stiffness and size, suggesting that the mechanical potentiation of killing is biophysically adaptive. We conclude that cellular cytotoxicity and, by implication, other effector responses are supported by specialized patterns of efferent force.

Clearing the Path: Exploring Apoptotic Cell Clearance in Inflammatory and Autoimmune Disorders for Therapeutic Advancements

Inflammatory and autoimmune disorders, characterized by dysregulated immune responses leading to tissue damage and chronic inflammation, present significant health challenges. This review uniquely focuses on efferocytosis-the phagocyte-mediated clearance of apoptotic cells-and its pivotal role in these disorders. We delve into the intricate mechanisms of efferocytosis' four stages and their implications in disease pathogenesis, distinguishing our study from previous literature. Our findings highlight impaired efferocytosis in conditions like atherosclerosis and asthma, proposing its targeting as a novel therapeutic strategy. We discuss the therapeutic potential of efferocytosis in modulating immune responses and resolving inflammation, offering a new perspective in treating inflammatory disorders.

Mechanistic insights into the C-type lectin receptor CLEC12A-mediated immune recognition of monosodium urate crystal

CLEC12A, a member of the C-type lectin receptor family involved in immune homeostasis, recognizes MSU crystals released from dying cells. However, the molecular mechanism underlying the CLEC12A-mediated recognition of MSU crystals remains unclear. Herein, we reported the crystal structure of the human CLEC12A-C-type lectin-like domain (CTLD) and identified a unique "basic patch" site on CLEC12A-CTLD that is necessary for the binding of MSU crystals. Meanwhile, we determined the interaction strength between CLEC12A-CTLD and MSU crystals using single-molecule force spectroscopy. Furthermore, we found that CLEC12A clusters at the cell membrane and seems to serve as an internalizing receptor of MSU crystals. Altogether, these findings provide mechanistic insights for understanding the molecular mechanisms underlying the interplay between CLEC12A and MSU crystals.

All-trans retinoic acid and dexamethasone regulate phagocytosis-related gene expression and enhance dead cell uptake in C2C12 myoblast cells

Extensive mechanical stress frequently causes micro-traumas in skeletal muscle, followed by a regeneration period. The effective removal of dead myofibers is a prerequisite for proper regeneration, and several cell types, including professional phagocytes, were reported to be active in this process. Myoblasts express several molecules of the phagocytic machinery, such as BAI1, stabilin-2, and TAM (Tyro3, Axl, Mertk) tyrosine kinase receptors, but these molecules were reported to serve primarily cell fusion and survival, and their role in the phagocytosis was not investigated. Therefore, we aimed to investigate the in vitro phagocytic capacity of the C2C12 mouse myoblast cell line. RNA sequencing data were analyzed to determine the level and changes of phagocytosis-related gene expression during the differentiation process of C2C12 cells. To study the phagocytic capacity of myoblasts and the effect of dexamethasone, all-trans retinoic acid, hemin, and TAM kinase inhibitor treatments on phagocytosis, C2C12 cells were fed dead thymocytes, and their phagocytic capacity was determined by flow cytometry. The effect of dexamethasone and all-trans retinoic acid on phagocytosis-related gene expression was determined by quantitative PCR. Both undifferentiated and differentiated cells engulfed dead cells being the undifferentiated cells more effective. In line with this, we observed that the expression of several phagocytosis-related genes was downregulated during the differentiation process. The phagocytosis could be increased by dexamethasone and all-trans retinoic acid and decreased by hemin and TAM kinase inhibitor treatments. Our results indicate that myoblasts not only express phagocytic machinery genes but are capable of efficient dead cell clearance as well, and this is regulated similarly, as reported in professional phagocytes.

Understanding the diversity and dynamics of in vivo efferocytosis: Insights from the fly embryo

The clearance of dead and dying cells, termed efferocytosis, is a rapid and efficient process and one that is critical for organismal health. The extraordinary speed and efficiency with which dead cells are detected and engulfed by immune cells within tissues presents a challenge to researchers who wish to unravel this fascinating process, since these fleeting moments of uptake are almost impossible to catch in vivo. In recent years, the fruit fly (Drosophila melanogaster) embryo has emerged as a powerful model to circumvent this problem. With its abundance of dying cells, specialist phagocytes and relative ease of live imaging, the humble fly embryo provides a unique opportunity to catch and study the moment of cell engulfment in real-time within a living animal. In this review, we explore the recent advances that have come from studies in the fly, and how live imaging and genetics have revealed a previously unappreciated level of diversity in the efferocytic program. A variety of efferocytic strategies across the phagocytic cell population ensure efficient and rapid clearance of corpses wherever death is encountered within the varied and complex setting of a multicellular living organism.

Topographical analysis of immune cell interactions reveals a biomechanical signature for immune cytolysis

Immune cells live intensely physical lifestyles characterized by structural plasticity, mechanosensitivity, and force exertion. Whether specific immune functions require stereotyped patterns of mechanical output, however, is largely unknown. To address this question, we used super-resolution traction force microscopy to compare cytotoxic T cell immune synapses with contacts formed by other T cell subsets and macrophages. T cell synapses were globally and locally protrusive, which was fundamentally different from the coupled pinching and pulling of macrophage phagocytosis. By spectrally decomposing the force exertion patterns of each cell type, we associated cytotoxicity with compressive strength, local protrusiveness, and the induction of complex, asymmetric interfacial topographies. These features were further validated as cytotoxic drivers by genetic disruption of cytoskeletal regulators, direct imaging of synaptic secretory events, and in silico analysis of interfacial distortion. We conclude that T cell-mediated killing and, by implication, other effector responses are supported by specialized patterns of efferent force.

Toxoplasma gondii in CD36 -/- mice shows lethal infection and poor immunization with probable macrophage immune defects

Experimental toxoplasmosis is an excellent model for adaptive immune response. Gamma-irradiated tachyzoites or soluble tachyzoite antigen extracts (STag) induce protection against experimental toxoplasmosis in mice. Scavenger receptors recognize irradiated proteins, promote their entry into cells, and lead to antigen presentation. CD36 is a specific scavenger receptor involved in intracellular transport of free fatty acid (FFA), cellular recycling, and intracellular trafficking in lipid rafts outside the lysosomal pathways. CD36 is also associated with an altered immune response, as CD36^-/- mice presented some immune defects in the cyst-forming Toxoplasma gondii. We studied T. gondii infection in CD36^-/- mice, naïve or immunized, with irradiated T. gondii STags by investigating protection, antibody production, and primed macrophage transplantation. CD36^-/- mice presented no resistance against the viable RH tachyzoites, even after immunization with gamma-irradiated STags that protected wild-type mice. The animals presented poor humoral responses to both immunogens despite adequate levels of serum immunoglobulins. CD36^-/- mice failed to induce protection against virulent T. gondii infection with inadequate antibody production or an innate response. Irradiated antigens failed to induce antibodies in CD36^-/- mice and only produced adequate levels of immunoglobulin G when transplanted with irradiated STag-primed wild-type macrophages. The CD36 pathway is necessary for humoral response against the irradiated antigen; however, several other pathways are also involved in mounting a humoral response against any antigen. CD36 is a multipurpose molecule for FFA and lipid transport, as well as for the immune response, and gamma radiation mimics the innate response by targeting irradiated antigens of this pathway.

Phospholipid fatty acid remodeling and carbonylated protein increase in extracellular vesicles released by airway epithelial cells exposed to cigarette smoke extract

Cigarette smoke (CS) represents one of the most relevant environmental risk factors for several chronic pathologies. Tissue damage caused by CS exposure is mediated, at least in part, by oxidative stress induced by its toxic and pro-oxidant components. Evidence demonstrates that extracellular vesicles (EVs) released by various cell types exposed to CS extract (CSE) are characterized by altered biochemical cargo and gained pathological properties. In the present study, we evaluated the content of oxidized proteins and phospholipid fatty acid profiles of EVs released by human bronchial epithelial BEAS-2B cells treated with CSE. This specific molecular characterization has hitherto not been performed. After confirmation that CSE reduces viability of BEAS-2B cells and elevates intracellular ROS levels, in a dose-dependent manner, we demonstrated that 24 h exposure at 1% CSE, a concentration that only slight modifies cell viability but increases ROS levels, was able to increase carbonylated protein levels in cells and released EVs. The release of oxidatively modified proteins via EVs might represent a mechanism used by cells to remove toxic proteins in order to avoid their intracellular overloading. Moreover, 1% CSE induced only few changes in the fatty acid asset in BEAS-2B cell membrane phospholipids, whereas several rearrangements were observed in EVs released by CSE-treated cells. The impact of changes in acyl chain composition of CSE-EVs accounted for the increased saturation levels of phospholipids, a membrane parameter that might influence EV stability, uptake and, at least in part, EV-mediated biological effects. The present in vitro study adds new information concerning the biochemical composition of CSE-related EVs, useful to predict their biological effects on target cells. Furthermore, the information regarding the presence of oxidized proteins and the specific membrane features of CSE-related EVs can be useful to define the utilization of circulating EVs as marker for diagnosing of CS-induced lung damage and/or CS-related diseases.

Assessing the Immune Cell Subset and Genetic Mutations in Patients With Palindromic Rheumatism Seronegative for Rheumatoid Factor and Anti-Cyclic Citrullinated Peptide

OBJECTIVE

The etiology underlying cases of palindromic rheumatism (PR) not associated with other rheumatic diseases in patients who are seronegative for rheumatoid factor and anti-cyclic citrullinated peptide (seronegative PR) is unclear. We aimed to investigate the immune cells and genes involved.

METHODS

This was a single-center comparative study of 48 patients with seronegative PR and 48 healthy controls. Mass cytometry and RNA sequencing were used to identify distinct immune cell subsets in blood. Among the 48 seronegative PR patients, plasma samples from 40 patients were evaluated by enzyme-linked immunosorbent assay for cytokine levels, and peripheral blood samples from 25 patients were evaluated by flow cytometry for mononuclear cell subsets. Plasma samples from 21 patients were evaluated by real-time polymerase chain reaction for differential gene and protein expression, and samples from 3 patients were analyzed with whole-exome sequencing for gene mutations.

RESULTS

Immunophenotyping revealed a markedly increased frequency of CD14+CD11b+CD36+ and CD4+CD25-CD69+ cells in seronegative PR patients with active flares compared with healthy controls (P < 0.0001 for both cell subset comparisons). Gene enrichment analyses of RNA-sequencing data from sorted CD14+CD11b+CD36+ and CD4+CD25-CD69+ cells showed involvement of the inflammatory/stress response, phagocytosis, and regulation of apoptosis functional pathways. Up-regulated expression of CXCL16 and IL10RA was observed in monocytes from PR patients. Up-regulation of PFKFB3, DDIT4, and TGFB1, and down-regulation of PDIA6 were found in monocytes and lymphocytes from PR patients with active flares and PR patients in intercritical periods. Plasma levels of S100A8/A9 and interleukin-1β were elevated in PR patients. Whole-exome sequencing revealed novel polygenic mutations in HACL1, KDM5A, RASAL1, HAVCR2, PRDM9, MBOAT4, and JRKL.

CONCLUSION

In seronegative PR patients, we identified a distinct CD14+CD11b+CD36+ cell subset that can induce an inflammatory response under stress and exert antiinflammatory effects after phagocytosis of apoptotic cells, and a CD4+CD25-CD69+ T cell subset with pro- and antiinflammatory properties. Individuals with genetic mutations involving epigenetic modification, potentiation and resolution of stress-induced inflammation/apoptosis, and a dysregulated endoplasmic reticulum stress response could be predisposed to seronegative PR.

From the Catastrophic Objective Irreproducibility of Cancer Research and Unavoidable Failures of Molecular Targeted Therapies to the Sparkling Hope of Supramolecular Targeted Strategies

The unprecedented non-reproducibility of the results published in the field of cancer research has recently come under the spotlight. In this short review, we try to highlight some general principles in the organization and evolution of cancerous tumors, which objectively lead to their enormous variability and, consequently, the irreproducibility of the results of their investigation. This heterogeneity is also extremely unfavorable for the effective use of molecularly targeted medicine. Against the seemingly comprehensive background of this heterogeneity, we single out two supramolecular characteristics common to all tumors: the clustered nature of tumor interactions with their microenvironment and the formation of biomolecular condensates with tumor-specific distinctive features. We suggest that these features can form the basis of strategies for tumor-specific supramolecular targeted therapies.

Nur77 and PPARγ regulate transcription and polarization in distinct subsets of M2-like reparative macrophages during regenerative inflammation

Macrophage polarization is a process whereby macrophages develop a specific phenotype and functional response to different pathophysiological stimuli and tissue environments. In general, two main macrophage phenotypes have been identified: inflammatory (M1) and alternatively activated (M2) macrophages characterized specifically by IL-1β and IL-10 production, respectively. In the cardiotoxin-induced skeletal muscle injury model bone marrow-derived macrophages (BMDMs) play the central role in regulating tissue repair. Bone marrow-derived monocytes arriving at the site of injury differentiate first to M1 BMDMs that clear cell debris and trigger proliferation and differentiation of the muscle stem cells, while during the process of efferocytosis they change their phenotype to M2 to drive resolution of inflammation and tissue repair. The M2 population is formed from at least three distinct subsets: antigen presenting, resolution-related and growth factor producing macrophages, the latest ones expressing the transcription factor PPARγ. Nuclear receptor subfamily 4 group A member 1 (NR4A1; also termed Nur77) transcription factor is expressed as an early response gene, and has been shown to suppress the expression of pro-inflammatory genes during efferocytosis. Here we demonstrate that (1) Nur77 null BMDMs are characterized by elevated expression of PPARγ resulting in enhanced efferocytosis capacity; (2) Nur77 and PPARγ regulate transcription in different subsets of M2 skeletal muscle macrophages during muscle repair; (3) the loss of Nur77 prolongs M1 polarization characterized by increased and prolonged production of IL-1β by the resolution-related macrophages normally expressing Nur77; whereas, in contrast, (4) it promotes M2 polarization detected via the increased number of IL-10 producing CD206⁺ macrophages generated from the PPARγ-expressing subset.

Analysis of Efferocytic Receptor Dynamics and Synapse Formation in a Frustrated Efferocytosis Model

Efferocytes express multiple receptors that mediate the recognition and engulfment of apoptotic cells through a process known as efferocytosis. Ligation of these receptors induces the formation of a structured efferocytic synapse that mediates the engulfment of the apoptotic cell by the efferocyte. The lateral diffusion of these receptors allows for clustering-mediated receptor activation and is central for the formation of the efferocytic synapse. This chapter describes a single particle tracking protocol to analyze the diffusion of efferocytic receptors within a frustrated efferocytosis model. This enables high-resolution tracking of efferocytic receptors throughout synapse formation, allowing the user to simultaneously quantify synapse formation and the dynamics of receptor diffusion as the efferocytic synapse evolves.

Anti-Inflammatory Neutrophil Functions in the Resolution of Inflammation and Tissue Repair

Neutrophils are highly abundant circulating leukocytes that are amongst the first cells to be recruited to sites of infection or sterile injury. Their ability to generate and release powerful cytotoxic products ties with their role in host defence from bacterial and fungal infections. Neutrophilic inflammation is tightly regulated to limit the amount of 'bystander injury' caused. Neutrophils were in the past regarded as short-lived, indiscriminate killers of invading microorganisms. However, this view has changed quite dramatically in recent years. Amongst other insights, neutrophils are now recognised to also have important anti-inflammatory functions that are critical for the resolution of inflammation and return to homeostasis. This minireview focusses on anti-inflammatory neutrophil functions, placing a particular focus on recent findings linked to neutrophil cell death, several types of which may be anti-inflammatory (apoptosis, secondary necrosis, and neutrophil extracellular traps). These are discussed together with features that may further promote the clearance of dead cells by efferocytosis and reprogramming of macrophages to promote resolution and repair.

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

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

Palmitate Inhibits Mouse Macrophage Efferocytosis by Activating an mTORC1-Regulated Rho Kinase 1 Pathway: Therapeutic Implications for the Treatment of Obesity

Every day, billions of our cells die and get cleared without inducing inflammation. When, clearance is improper, uncleared cells undergo secondary necrosis and trigger inflammation. In addition, proper efferocytosis would be required for inducing resolution of inflammation, thus clearance deficiencies in the long term lead to development of various chronic inflammatory diseases. Increasing evidence indicates that obesity, itself being a low-grade inflammatory disease, predisposes to a variety of other chronic inflammatory diseases. Previous studies indicated that this later might be partially related to an impaired efferocytosis induced by increased uptake of circulating saturated fatty acids by macrophages in obese people. Here, we show that palmitate inhibits efferocytosis by bone marrow-derived macrophages in a dose-dependent manner. Palmitate triggers autophagy but also activates an energy-sensing mTORC1/ROCK1 signaling pathway, which interferes with the autophagosome–lysosome fusion, resulting in accumulation of the cellular membranes in autophagosomes. We propose that lack of sufficient plasma membrane supply attenuates efferocytosis of palmitate-exposed macrophages. AMP-activated protein kinase activators lead to mTORC1 inhibition and, consequently, released the palmitate-induced efferocytosis block in macrophages. Thus, they might be useful in the treatment of obesity not only by affecting metabolism thought so far. ROCK1 inhibitors could also be considered.

Involvement of phosphatidylserine receptors in the skeletal muscle regeneration: therapeutic implications

Sarcopenia is a progressive loss of muscle mass and strength with a risk of adverse outcomes such as disability, poor quality of life, and death. Increasing evidence indicates that diminished ability of the muscle to activate satellite cell-dependent regeneration is one of the factors that might contribute to its development. Skeletal muscle regeneration following myogenic cell death results from the proliferation and differentiation of myogenic stem cells, called satellite cells, located beneath the basal lamina of the muscle fibres. Satellite cell differentiation is not a satellite cell-autonomous process but depends on signals provided by the surrounding cells. Infiltrating macrophages play a key role in the process partly by clearing the necrotic cell debris, partly by producing cytokines and growth factors that guide myogenesis. At the beginning of the muscle regeneration process, macrophages are pro-inflammatory, and the cytokines produced by them trigger the proliferation and differentiation of satellite cells. Following the uptake of dead cells, however, a transcriptionally regulated phenotypic change (macrophage polarization) is induced in them resulting in their transformation into healing macrophages that guide resolution of inflammation, completion of myoblast differentiation, myoblast fusion and growth, and return to homeostasis. Impaired efferocytosis results in delayed cell death clearance, delayed macrophage polarization, prolonged inflammation, and impaired muscle regeneration. Thus, proper efferocytosis by macrophages is a determining factor during muscle repair. Here we review that both efferocytosis and myogenesis are dependent on the cell surface phosphatidylserine (PS), and surprisingly, these two processes share a number of common PS receptors and signalling pathways. Based on these findings, we propose that stimulating the function of PS receptors for facilitating muscle repair following injury could be a successful approach, as it would enhance efferocytosis and myogenesis simultaneously. Because increasing evidence indicates a pathophysiological role of impaired efferocytosis in the development of chronic inflammatory conditions, as well as in impaired muscle regeneration both contributing to the development of sarcopenia, improving efferocytosis should be considered also in its management. Again applying or combining those treatments that target PS receptors would be expected to be the most effective, because they would also promote myogenesis. A potential PS receptor-triggering candidate molecule is milk fat globule-EGF-factor 8 (MFG-E8), which not only stimulates PS-dependent efferocytosis and myoblast fusion but also promotes extracellular signal-regulated kinase (ERK) and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

Activatable Fluorophores for Imaging Immune Cell Function

Optical imaging has become an essential tool to study biomolecular processes in live systems with unprecedented spatial resolution. New fluorescent technologies and advances in optical microscopy have revolutionized the ways in which we can study immune cells in real time. For example, activatable fluorophores that emit signals after target recognition have enabled direct imaging of immune cell function with enhanced readouts and minimal background. In this Account, we summarize recent advances in the chemical synthesis and implementation of activatable fluorescent probes to monitor the activity and the role of immune cells in different pathological processes, from infection to inflammatory diseases or cancer. In addition to the contributions that our group has made to this field, we review the most relevant literature disclosed over the past decade, providing examples of different activatable architectures and their application in diagnostics and drug discovery. This Account covers the imaging of the three major cell types in the immune system, that is, neutrophils, macrophages, and lymphocytes. Attracted by the tunability and target specificity of peptides, many groups have designed strategies based on fluorogenic peptides whose fluorescence emission is regulated by the reaction with enzymes (e.g., MMPs, cathepsins, granzymes), or through Förster resonance energy transfer (FRET) mechanisms. Selective imaging of immune cells has been also achieved by targeting different intracellular metabolic routes, such as lipid biogenesis. Other approaches involve the implementation of diversity-oriented fluorescence libraries or the use of environmentally sensitive fluorescent scaffolds (e.g., molecular rotors). Our group has made important progress by constructing probes to image metastasis-associated macrophages in tumors, apoptotic neutrophils, or cytotoxic natural killer (NK) cells against cancer cells, among other examples. The chemical probes covered in this Account have been successfully validated in vitro in cell culture systems, and in vivo in relevant models of inflammation and cancer. Overall, the range of chemical structures and activation mechanisms reported to sense immune cell function is remarkable. However, the emergence of new strategies based on new molecular targets or activatable mechanisms that are yet to be discovered will open the door to track unexplored roles of immune cells in different biological systems. We anticipate that upcoming generations of activatable probes will find applications in the clinic to help assessing immunotherapies and advance precision medicine. We hope that this Account will evoke new ideas and innovative work in the design of fluorescent probes for imaging cell function.

Rationales Design von Phe-BODIPY-Aminosäuren als fluorogene Bausteine für den peptidbasierten Nachweis von Candida-Infektionen im Harntrakt

Pilzinfektionen, die durch Candida‐Arten verursacht werden, gehören zu den häufigsten Infektionen bei Krankenhauspatienten. Die derzeitigen Methoden zum Nachweis von Candida‐Pilzzellen in klinischen Proben beruhen jedoch auf zeitaufwändigen Analysen, die eine schnelle und zuverlässige Diagnose erschweren. In diesem Beitrag beschreiben wir die rationale Entwicklung neuer Phe‐BODIPY‐Aminosäuren als kleine fluorogene Bausteine und ihre Anwendung zur Erzeugung fluoreszierender antimikrobieller Peptide für die schnelle Markierung von Candida‐Zellen im Urin. Mit Hilfe von computergestützten Berechnungen haben wir das fluorogene Verhalten von BODIPY‐substituierten aromatischen Aminosäuren analysiert und Bioaktivitäts‐ und konfokale Mikroskopieexperimente bei verschiedenen Stämmen durchgeführt, um den Nutzen und die Vielseitigkeit von Peptiden mit Phe‐BODIPYs zu bestätigen. Schließlich haben wir einen einfachen und sensitiven fluoreszensbasierten Test zum Nachweis von Candida albicans in menschlichen Urinproben entwickelt.

Rational Design of Phe‐BODIPY Amino Acids as Fluorogenic Building Blocks for Peptide‐Based Detection of Urinary Tract Candida Infections

Fungal infections caused by Candida species are among the most prevalent in hospitalized patients. However, current methods for the detection of Candida fungal cells in clinical samples rely on time‐consuming assays that hamper rapid and reliable diagnosis. Herein, we describe the rational development of new Phe‐BODIPY amino acids as small fluorogenic building blocks and their application to generate fluorescent antimicrobial peptides for rapid labelling of Candida cells in urine. We have used computational methods to analyse the fluorogenic behaviour of BODIPY‐substituted aromatic amino acids and performed bioactivity and confocal microscopy experiments in different strains to confirm the utility and versatility of peptides incorporating Phe‐BODIPYs. Finally, we have designed a simple and sensitive fluorescence‐based assay for the detection of Candida albicans in human urine samples.

SCARF1-Induced Efferocytosis Plays an Immunomodulatory Role in Humans, and Autoantibodies Targeting SCARF1 Are Produced in Patients with Systemic Lupus Erythematosus

Deficiency in the clearance of cellular debris is a major pathogenic factor in the emergence of autoimmune diseases. We previously demonstrated that mice deficient for scavenger receptor class F member 1 (SCARF1) develop a lupus-like autoimmune disease with symptoms similar to human systemic lupus erythematosus (SLE), including a pronounced accumulation of apoptotic cells (ACs). Therefore, we hypothesized that SCARF1 will be important for clearance of ACs and maintenance of self-tolerance in humans, and that dysregulation of this process could contribute to SLE. In this article, we show that SCARF1 is highly expressed on phagocytic cells, where it functions as an efferocytosis receptor. In healthy individuals, we discovered that engagement of SCARF1 by ACs on BDCA1+ dendritic cells initiates an IL-10 anti-inflammatory response mediated by the phosphorylation of STAT1 and STAT3. Unexpectedly, there was no significant difference in SCARF1 expression in samples of patients with SLE compared with healthy donor samples. However, we detected anti-SCARF1 autoantibodies in 26% of patients with SLE, which was associated with dsDNA Ab positivity. Furthermore, our data show a direct correlation of the levels of anti-SCARF1 in the serum and defects in the removal of ACs. Depletion of Ig restores efferocytosis in SLE serum, suggesting that defects in the removal of ACs are partially mediated by SCARF1 pathogenic autoantibodies. Our data demonstrate that human SCARF1 is an AC receptor in dendritic cells and plays a role in maintaining tolerance and homeostasis.

A Bivalent Activatable Fluorescent Probe for Screening and Intravital Imaging of Chemotherapy-Induced Cancer Cell Death

The detection and quantification of apoptotic cells is a key process in cancer research, particularly during the screening of anticancer therapeutics and in mechanistic studies using preclinical models. Intravital optical imaging enables high-resolution visualisation of cellular events in live organisms; however, there are few fluorescent probes that can reliably provide functional readouts in situ without interference from tissue autofluorescence. We report the design and optimisation of the fluorogenic probe Apotracker Red for real-time detection of cancer cell death. The strong fluorogenic behaviour, high selectivity, and excellent stability of Apotracker Red make it a reliable optical reporter for the characterisation of the effects of anticancer drugs in cells in vitro and for direct imaging of chemotherapy-induced apoptosis in vivo in mouse models of breast cancer.

Nrf2 Is Required for Optimal Alveolar-Macrophage-Mediated Apoptotic Neutrophil Clearance after Oxidant Injury

Recognition and clearance of apoptotic cells by phagocytes (also known as efferocytosis), primarily mediated by macrophages, are essential to terminate lung inflammatory responses and promote tissue repair after injury. The Nrf2 transcription factor is crucial for cytoprotection and host defense. Previously, we showed sustained neutrophilic lung inflammation in Nrf2-deficient (Nrf2^−/−) mice after hyperoxia-induced lung injury in vivo, but the mechanisms underlying this abnormal phenotype remain unclear. To examine whether Nrf2 regulates apoptotic neutrophil clearance, we used the alveolar macrophages (AMФs) and bone-marrow-derived macrophages (BMDMФs) of wild-type (WT) and Nrf2^−/− mice. We found that the efferocytic ability of AMФ was impaired in hyperoxia-exposed mice’s lungs, but the effect was more pronounced in Nrf2^−/− mice. Importantly, AMФ-mediated efferocytosis remained impaired in Nrf2^−/− mice recovering from injury but was restored to the basal state in the wild-type counterparts. Hyperoxia affected apoptotic neutrophil binding, not internalization, in both WT and Nrf2^−/− BMDMФs, but the effect was more significant in the latter cells. Augmenting Nrf2 activity restored hyperoxia attenuated efferocytosis in WT, but not in Nrf2^−/− macrophages. However, the loss of Nrf2 in neutrophils affected their uptake by WT macrophages. Collectively, these results demonstrate that Nrf2 is required for optimal macrophage-mediated efferocytosis and that activating Nrf2 may provide a physiological way to accelerate apoptotic cell clearance after oxidant injury.

An Imaging Flow Cytometry Protocol for Studying Immunoregulatory Receptor-Mediated Regulation of Phagocytosis

Advances in flow cytometry have allowed for innovative functional investigations of innate immune cell responses. Imaging flow cytometers combine the imaging capabilities of microscopy with rapid, high-throughput data acquisition attributes of standard flow cytometers. Here, we describe a detailed method for co-expressing stimulatory and inhibitory immunoregulatory receptor-types in AD293 cells and then measuring receptor cross-talk during the regulation of the phagocytic response. Information on reagent selection, imaging flow cytometry calibration, and automated template analyses are included.

A role for the centrosome in regulating the rate of neuronal efferocytosis by microglia in vivo

During brain development, many newborn neurons undergo apoptosis and are engulfed by microglia, the tissue-resident phagocytes of the brain, in a process known as efferocytosis. A hallmark of microglia is their highly branched morphology characterized by the presence of numerous dynamic extensions that these cells use for scanning the brain parenchyma and engulfing unwanted material. The mechanisms driving branch formation and apoptotic cell engulfment in microglia are unclear. By taking a live-imaging approach in zebrafish, we show that while microglia generate multiple microtubule-based branches, they only successfully engulf one apoptotic neuron at a time. Further investigation into the mechanism underlying this sequential engulfment revealed that targeted migration of the centrosome into one branch is predictive of phagosome formation and polarized vesicular trafficking. Moreover, experimentally doubling centrosomal numbers in microglia increases the rate of engulfment and even allows microglia to remove two neurons simultaneously, providing direct supporting evidence for a model where centrosomal migration is a rate-limiting step in branch-mediated efferocytosis. Conversely, light-mediated depolymerization of microtubules causes microglia to lose their typical branched morphology and switch to an alternative mode of engulfment, characterized by directed migration towards target neurons, revealing unexpected plasticity in their phagocytic ability. Finally, building on work focusing on the establishment of the immunological synapse, we identified a conserved signalling pathway underlying centrosomal movement in engulfing microglia.

A Bivalent Activatable Fluorescent Probe for Screening and Intravital Imaging of Chemotherapy-induced Cancer Cell Death

The detection and quantification of apoptotic cells is a key process in cancer research, particularly during the screening of anticancer therapeutics and in mechanistic studies using preclinical models. Intravital optical imaging enables high‐resolution visualisation of cellular events in live organisms; however, there are few fluorescent probes that can reliably provide functional readouts in situ without interference from tissue autofluorescence. We report the design and optimisation of the fluorogenic probe Apotracker Red for real‐time detection of cancer cell death. The strong fluorogenic behaviour, high selectivity, and excellent stability of Apotracker Red make it a reliable optical reporter for the characterisation of the effects of anticancer drugs in cells in vitro and for direct imaging of chemotherapy‐induced apoptosis in vivo in mouse models of breast cancer.

First we eat, then we do everything else: The dynamic metabolic regulation of efferocytosis

Clearance of apoptotic cells, or "efferocytosis," is essential for diverse processes including embryonic development, tissue turnover, organ regeneration, and immune cell development. The human body is estimated to remove approximately 1% of its body mass via apoptotic cell clearance daily. This poses several intriguing cell metabolism problems. For instance, phagocytes such as macrophages must induce or suppress metabolic pathways to find, engulf, and digest apoptotic cells. Then, phagocytes must manage the potentially burdensome biomass of the engulfed apoptotic cell. Finally, phagocytes reside in complex tissue architectures that vary in nutrient availability, the types of dying cells or debris that require clearance, and the neighboring cells they interact with. Here, we review advances in our understanding of these three key areas of phagocyte metabolism. We end by proposing a model of efferocytosis that integrates recent findings and establishes a new paradigm for testing how efferocytosis prevents chronic inflammatory disease and autoimmunity.

Near-infrared benzodiazoles as small molecule environmentally-sensitive fluorophores

The development of fluorophores emitting in the near-infrared spectral window has gained increased attention given their suitable features for biological imaging. In this work, we have optimised a general and straightforward synthetic approach to prepare a small library of near-infrared-emitting C-bridged nitrobenzodiazoles using commercial precursors. C-bridged benzodiazoles have low molecular weight and neutral character as important features that are not common in most near-infrared dyes. We have investigated their fluorescence response in the presence of a wide array of 60 different biomolecules and identified compound 3i as a potential chemosensor to discriminate between Fe2+ and Fe3+ ions in aqueous media.

HIV-1 Vpu Promotes Phagocytosis of Infected CD4+ T Cells by Macrophages through Downregulation of CD47

Human immunodeficiency virus (HIV) remodels the cell surface of infected cells to facilitate viral dissemination and promote immune evasion. The membrane-associated viral protein U (Vpu) accessory protein encoded by HIV-1 plays a key role in this process by altering cell surface levels of multiple host proteins. Using an unbiased quantitative plasma membrane profiling approach, we previously identified CD47 as a putative host target downregulated by Vpu. CD47 is a ubiquitously expressed cell surface protein that interacts with the myeloid cell inhibitory receptor signal regulatory protein-alpha (SIRPα) to deliver a "don't-eat-me" signal, thus protecting cells from phagocytosis. In this study, we investigate whether CD47 modulation by HIV-1 Vpu might promote the susceptibility of macrophages to viral infection via phagocytosis of infected CD4⁺ T cells. Indeed, we find that Vpu downregulates CD47 expression on infected CD4⁺ T cells, leading to enhanced capture and phagocytosis by macrophages. We further provide evidence that this Vpu-dependent process allows a C-C chemokine receptor type 5 (CCR5)-tropic transmitted/founder (T/F) virus, which otherwise poorly infects macrophages in its cell-free form, to efficiently infect macrophages. Importantly, we show that HIV-1-infected cells expressing a Vpu-resistant CD47 mutant are less prone to infecting macrophages through phagocytosis. Mechanistically, Vpu forms a physical complex with CD47 through its transmembrane domain and targets the latter for lysosomal degradation. These results reveal a novel role of Vpu in modulating macrophage infection, which has important implications for HIV-1 transmission in early stages of infection and the establishment of viral reservoir. IMPORTANCE Macrophages play critical roles in human immunodeficiency virus (HIV) transmission, viral spread early in infection, and as a reservoir of virus. Selective capture and engulfment of HIV-1-infected T cells was shown to drive efficient macrophage infection, suggesting that this mechanism represents an important mode of infection notably for weakly macrophage-tropic T/F viruses. In this study, we provide insight into the signals that regulate this process. We show that the HIV-1 accessory protein viral protein U (Vpu) downregulates cell surface levels of CD47, a host protein that interacts with the inhibitory receptor signal regulatory protein-alpha (SIRPα), to deliver a "don't-eat-me" signal to macrophages. This allows for enhanced capture and phagocytosis of infected T cells by macrophages, ultimately leading to their productive infection even with transmitted/founder (T/F) virus. These findings provide new insights into the mechanisms governing the intercellular transmission of HIV-1 to macrophages with implications for the establishment of the macrophage reservoir and early HIV-1 dissemination in vivo.

Identification of the Crucial Gene in Overflow Arteriovenous Fistula by Bioinformatics Analysis

Objective: The aim was to study the preliminary screening of the crucial genes in intimal hyperplasia in the venous segment of arteriovenous (AV) fistula and the underlying potential molecular mechanisms of intimal hyperplasia with bioinformatics analysis. Methods: The gene expression profile data (GSE39488) was analyzed to identify differentially expressed genes (DEGs). We performed Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of DEGs. Gene set enrichment analysis (GSEA) was used to understand the potential activated signaling pathway. The protein-protein interaction (PPI) network was constructed with the STRING database and Cytoscape software. The Venn diagram between 10 hub genes and gene sets of 4 crucial signaling pathways was used to obtain core genes and relevant potential pathways. Furthermore, GSEAs were performed to understand their biological functions. Results: A total of 185 DEGs were screened in this study. The main biological function of the 111 upregulated genes in AV fistula primarily concentrated on cell proliferation and vascular remodeling, and the 74 downregulated genes in AV fistula were enriched in the biological function mainly relevant to inflammation. GSEA found four signaling pathways crucial for intimal hyperplasia, namely, MAPK, NOD-like, Cell Cycle, and TGF-beta signaling pathway. A total of 10 hub genes were identified, namely, EGR1, EGR2, EGR3, NR4A1, NR4A2, DUSP1, CXCR4, ATF3, CCL4, and CYR61. Particularly, DUSP1 and NR4A1 were identified as core genes that potentially participate in the MAPK signaling pathway. In AV fistula, the biological processes and pathways were primarily involved with MAPK signaling pathway and MAPK-mediated pathway with the high expression of DUSP1 and were highly relevant to cell proliferation and inflammation with the low expression of DUSP1. Besides, the biological processes and pathways in AV fistula with the high expression of NR4A1 similarly included the MAPK signaling pathway and the pathway mediated by MAPK signaling, and it was mainly involved with inflammation in AV fistula with the low expression of NR4A1. Conclusion: We screened four potential signaling pathways relevant to intimal hyperplasia and identified 10 hub genes, including two core genes (i.e., DUSP1 and NR4A1). Two core genes potentially participate in the MAPK signaling pathway and might serve as the therapeutic targets of intimal hyperplasia to prevent stenosis after AV fistula creation.

Near-Infrared Fluorescent Probes for the Detection of Cancer-Associated Proteases

Proteases are enzymes capable of catalyzing protein breakdown, which is critical across many biological processes. There are several families of proteases, each of which perform key functions through the degradation of specific proteins. As our understanding of cancer improves, it has been demonstrated that several proteases can be overactivated during the progression of cancer and contribute to malignancy. Optical imaging systems that employ near-infrared (NIR) fluorescent probes to detect protease activity offer clinical promise, both for early detection of cancer as well as for the assessment of personalized therapy. In this Review, we review the design of NIR probes and their successful application for the detection of different cancer-associated proteases.

Macrophage Regulation of Granulopoiesis and Neutrophil Functions

Significance: Neutrophils are potent effector cells of innate immunity requiring precise regulation of their numbers and functions in blood and tissues. Recent Advances: Macrophages have emerged as modulators of neutrophil properties. In inflammatory conditions, tissue macrophages modulate neutrophil trafficking and activation. Further, macrophages govern granulopoiesis in the bone marrow hematopoietic niche. Interactions of macrophages and neutrophils can be induced by cytokines and damage-associated molecular patterns, and they are also regulated by oxidative signaling. Critical Issues: We review the impact of macrophages on neutrophil development and function, and its consequences in health and disease. Future Directions: Targeting the liaison between macrophages and neutrophils might provide an interesting therapeutic strategy to reduce tissue inflammation and promote immune tolerance. Antioxid. Redox Signal. 35, 182-191.

AXL kinase-mediated astrocytic phagocytosis modulates outcomes of traumatic brain injury

Background

Complex changes in the brain microenvironment following traumatic brain injury (TBI) can cause neurological impairments for which there are few efficacious therapeutic interventions. The reactivity of astrocytes is one of the keys to microenvironmental changes, such as neuroinflammation, but its role and the molecular mechanisms that underpin it remain unclear.

Methods

Male C57BL/6J mice were subjected to the controlled cortical impact (CCI) to develop a TBI model. The specific ligand of AXL receptor tyrosine kinase (AXL), recombinant mouse growth arrest-specific 6 (rmGas6) was intracerebroventricularly administered, and selective AXL antagonist R428 was intraperitoneally applied at 30 min post-modeling separately. Post-TBI assessments included neurobehavioral assessments, transmission electron microscopy, immunohistochemistry, and western blotting. Real-time polymerase chain reaction (RT-PCR), siRNA transfection, and flow cytometry were performed for mechanism assessments in primary cultured astrocytes.

Results

AXL is upregulated mainly in astrocytes after TBI and promotes astrocytes switching to a phenotype that exhibits the capability of ingesting degenerated neurons or debris. As a result, this astrocytic transformation promotes the limitation of neuroinflammation and recovery of neurological dysfunction. Pharmacological inhibition of AXL in astrocytes significantly decreased astrocytic phagocytosis both in vivo and in primary astrocyte cultures, in contrast to the effect of treatment with the rmGas6. AXL activates the signal transducer and activator of the transcription 1 (STAT1) pathway thereby further upregulating ATP-binding cassette transporter 1 (ABCA1). Moreover, the supernatant from GAS6-depleted BV2 cells induced limited enhancement of astrocytic phagocytosis in vitro.

Conclusion

Our work establishes the role of AXL in the transformation of astrocytes to a phagocytic phenotype via the AXL/STAT1/ABCA1 pathway which contributes to the separation of healthy brain tissue from injury-induced cell debris, further ameliorating neuroinflammation and neurological impairments after TBI. Collectively, our findings provide a potential therapeutic target for TBI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02201-3.

Endothelial Senescence and Chronic Fatigue Syndrome, a COVID-19 Based Hypothesis

Myalgic encephalomyelitis/chronic fatigue syndrome is a serious illness of unknown etiology, characterized by debilitating exhaustion, memory impairment, pain and sleep abnormalities. Viral infections are believed to initiate the pathogenesis of this syndrome although the definite proof remains elusive. With the unfolding of COVID-19 pandemic, the interest in this condition has resurfaced as excessive tiredness, a major complaint of patients infected with the SARS-CoV-2 virus, often lingers for a long time, resulting in disability, and poor life quality. In a previous article, we hypothesized that COVID-19-upregulated angiotensin II triggered premature endothelial cell senescence, disrupting the intestinal and blood brain barriers. Here, we hypothesize further that post-viral sequelae, including myalgic encephalomyelitis/chronic fatigue syndrome, are promoted by the gut microbes or toxin translocation from the gastrointestinal tract into other tissues, including the brain. This model is supported by the SARS-CoV-2 interaction with host proteins and bacterial lipopolysaccharide. Conversely, targeting microbial translocation and cellular senescence may ameliorate the symptoms of this disabling illness.

Having an Old Friend for Dinner: The Interplay between Apoptotic Cells and Efferocytes

Apoptosis, the programmed and intentional death of senescent, damaged, or otherwise superfluous cells, is the natural end-point for most cells within multicellular organisms. Apoptotic cells are not inherently damaging, but if left unattended, they can lyse through secondary necrosis. The resulting release of intracellular contents drives inflammation in the surrounding tissue and can lead to autoimmunity. These negative consequences of secondary necrosis are avoided by efferocytosis—the phagocytic clearance of apoptotic cells. Efferocytosis is a product of both apoptotic cells and efferocyte mechanisms, which cooperate to ensure the rapid and complete removal of apoptotic cells. Herein, we review the processes used by apoptotic cells to ensure their timely removal, and the receptors, signaling, and cellular processes used by efferocytes for efferocytosis, with a focus on the receptors and signaling driving this process.

Platelets orchestrate the resolution of pulmonary inflammation in mice by T reg cell repositioning and macrophage education

Beyond hemostasis, platelets actively participate in immune cell recruitment and host defense, yet their potential in the resolution of inflammatory processes remains unknown. Here, we demonstrate that platelets are recruited into the lung together with neutrophils during the onset of inflammation and alongside regulatory T (T reg) cells during the resolution phase. This partnering dichotomy is regulated by differential adhesion molecule expression during resolution. Mechanistically, intravascular platelets form aggregates with T reg cells, a prerequisite for their recruitment into the lung. This interaction relies on platelet activation by sCD40L and platelet P-selectin binding to PSGL-1 on T reg cells. Physical platelet–T reg cell interactions are necessary to modulate the transcriptome and instruct T reg cells to release the anti-inflammatory mediators IL-10 and TGFβ. Notably, the presence of platelet–T reg cell aggregates in the lung was also required for macrophage transcriptional reprogramming, polarization toward an anti-inflammatory phenotype, and effective resolution of pulmonary inflammation. Thus, platelets partner with successive immune cell subsets to orchestrate both the initiation and resolution of inflammation.

Dermal fibroblast phagocytosis of apoptotic cells: A novel pathway for wound resolution

The effective clearance of apoptotic cells is an essential step in the resolution of healing wounds. In particular, blood vessel regression during wound resolution produces a significant number of apoptotic endothelial cells (ApoEC) that must be cleared. In considering the fate of ApoEC and the presence of fibroblasts during wound resolution, we hypothesized that fibroblasts might serve as phagocytes involved in endothelial cell removal. The current study investigated whether dermal fibroblasts engulf ApoEC, whether this uptake alters the phenotype of dermal fibroblasts, and the biological molecules involved. In both in vitro and in vivo studies, following ApoEC engulfment, fibroblasts acquired a pro-healing phenotype (increased cell migration, contractility, α-smooth muscle actin expression, and collagen deposition). In addition, fibroblast uptake of ApoEC was shown to be mediated in part by the milk fat globule-EGF factor 8 protein/integrin αv β5 pathway. Our study demonstrates a novel function of fibroblasts in the clearance of ApoEC and suggests that this capability has significant implications for tissue repair and fibrosis.

Beta-2-Glycoprotein-I Deficiency Could Precipitate an Antiphospholipid Syndrome-like Prothrombotic Situation in Patients With Coronavirus Disease 2019

Objective

Patients with coronavirus disease 2019 (COVID‐19) present coagulation abnormalities and thromboembolic events that resemble antiphospholipid syndrome (APS). This work has aimed to study the prevalence of APS‐related antigens, antibodies, and immune complexes in patients with COVID‐19 and their association with clinical events.

Methods

A prospective study was conducted on 474 adults with severe acute respiratory syndrome coronavirus 2 infection hospitalized in two Spanish university hospitals. Patients were evaluated for classic and extra‐criteria antiphospholipid antibodies (aPLs), immunoglobulin G (IgG)/immunoglobulin M (IgM) anticardiolipin, IgG/IgM/immunoglobulin A (IgA) anti‐β2‐glicoprotein‐I (aβ2GPI), IgG/IgM antiphosphatidylserine/prothrombin (aPS/PT), the immune complex of IgA aβ2GPI (IgA‐aβ2GPI), bounded to β2‐glicoprotein‐1 (β2GPI) and β2GPI levels soon after COVID‐19 diagnosis and were followed‐up until medical discharge or death.

Results

Prevalence of aPLs in patients with COVID‐19 was as follows: classic aPLs, 5.8%; aPS/PT, 4.6%; IgA‐aβ2GPI, 15%; and any aPL, 21%. When patients were compared with individuals of a control group of a similar age, the only significant difference found was the higher prevalence of IgA‐aβ2GPI (odds ratio: 2.31; 95% confidence interval: 1.16‐4.09). No significant differences were observed in survival, thrombosis, or ventilatory failure in aPL‐positive versus aPL‐negative patients. β2GPI median levels were much lower in patients with COVID‐19 (15.9 mg/l) than in blood donors (168.8 mg/l; P < 0.001). Only 3.5% of patients with COVID‐19 had normal levels of β2GPI (>85 mg/l). Low levels of β2GPI were significantly associated with ventilatory failure (P = 0.026).

Conclusion

β2GPI levels were much lower in patients with COVID‐19 than in healthy people. Low β2GPI‐levels were associated with ventilatory failure. No differences were observed in the COVID‐19 evolution between aPL‐positive and aPL‐negative patients. Functional β2GPI deficiency could trigger a clinical process similar to that seen in APS but in the absence of aPLs.

Phagocytosis: Sweet Repulsions via the Glycocalyx

New work shows that the glycocalyx meshwork on the surface of macrophages prevents phagocytic receptors from binding their ligands by two means - electrostatic charge and steric hindrance. Components of this barrier are present on pathogenic and malignant targets that elude phagocytosis.

The promise of macrophage directed checkpoint inhibitors in myeloid malignancies

To date, many of the most successful checkpoint inhibitor-based therapies in cancer have targeted T-cells and adaptive immunity. However, there is an ongoing search for novel checkpoints with targeting innate immunity via activation of macrophage phagocytosis representing an exciting therapeutic strategy. CD47 is the dominant negative macrophage immune checkpoint expressed on cancer cells which acts as a "don't eat me signal", preventing phagocytosis via its interaction with SIRP-α on macrophages. CD47 has been shown to be upregulated in many cancer types including myeloid malignancies with increased expression associated with inferior OS. Magrolimab, an anti-CD47 antibody, has shown proof-of-principle of efficacy in this therapeutic class with promising early results in both higher risk myelodysplastic syndromes (MDS) and TP53 mutant acute myeloid leukemia (AML). The toxicity profile to date has been shown safe and manageable with on-target anemia related to CD47 being present on aged red blood cells and without evidence of immune related toxicities. Investigation of novel agents targeting this pathway and novel combinations are ongoing. New strategies targeting macrophage checkpoints are encouraging and likely will lead a paradigm shift in the current treatment of myeloid malignancies.

Novel C1q receptor-mediated signaling controls neural stem cell behavior and neurorepair

C1q plays a key role as a recognition molecule in the immune system, driving autocatalytic complement cascade activation and acting as an opsonin. We have previously reported a non-immune role of complement C1q modulating the migration and fate of human neural stem cells (hNSC); however, the mechanism underlying these effects has not yet been identified. Here, we show for the first time that C1q acts as a functional hNSC ligand, inducing intracellular signaling to control cell behavior. Using an unbiased screening strategy, we identified five transmembrane C1q signaling/receptor candidates in hNSC (CD44, GPR62, BAI1, c-MET, and ADCY5). We further investigated the interaction between C1q and CD44 , demonstrating that CD44 mediates C1q induced hNSC signaling and chemotaxis in vitro, and hNSC migration and functional repair in vivo after spinal cord injury. These results reveal a receptor-mediated mechanism for C1q modulation of NSC behavior and show that modification of C1q receptor expression can expand the therapeutic window for hNSC transplantation.

Sensing and clearance of apoptotic cells

Macrophages specifically engulf apoptotic cells but not healthy cells. Phosphatidylserine (PtdSer) is localized at the inner leaflet of plasma membranes as a result of the action of flippases (ATP11A and 11C). When cells undergo apoptosis, caspase 3 cleaves and inactivates the flippases, while simultaneously cleaving XKR8 to activate its phospholipid scramblase activity. PtdSer is thus swiftly and irreversibly exposed to the cell surface as an 'eat me' signal. Tissue resident macrophages recognize the apoptotic cells using a PtdSer-receptor TIM4 and engulf them with TAM tyrosine-kinase receptors, and integrins. The PtdSer 'eat me' signal appears to override 'don't eat me' signals in most cases.

Airinemes: thin cellular protrusions mediate long-distance signalling guided by macrophages

Understanding the mechanisms of cell-to-cell communication is one of the fundamental questions in biology and medicine. In particular, long-range signalling where cells communicate over several cell diameters is vital during development and homeostasis. The major morphogens, their receptors and intracellular signalling cascades have largely been identified; however, there is a gap in our knowledge of how such signalling factors are propagated over a long distance. In addition to the diffusion-based propagation model, new modalities of disseminating signalling molecules have been identified. It has been shown that cells can communicate with direct contact through long, thin cellular protrusions between signal sending and receiving cells at a distance. Recent studies have revealed a type of cellular protrusion termed 'airinemes' in zebrafish pigment cell types. They share similarities with previously reported cellular protrusions; however, they also exhibit distinct morphology and features. Airinemes are indispensable for pigment pattern development by mediating long-distance Delta-Notch signalling between different pigment cell types. Notably, airineme-mediated signalling is dependent on skin-resident macrophages. Key findings of airineme-mediated intercellular signalling in pattern development, their interplay with macrophages and their implications for the understanding of cellular protrusion-mediated intercellular communication will be discussed.

A fluorogenic cyclic peptide for imaging and quantification of drug-induced apoptosis

Programmed cell death or apoptosis is a central biological process that is dysregulated in many diseases, including inflammatory conditions and cancer. The detection and quantification of apoptotic cells in vivo is hampered by the need for fixatives or washing steps for non-fluorogenic reagents, and by the low levels of free calcium in diseased tissues that restrict the use of annexins. In this manuscript, we report the rational design of a highly stable fluorogenic peptide (termed Apo-15) that selectively stains apoptotic cells in vitro and in vivo in a calcium-independent manner and under wash-free conditions. Furthermore, using a combination of chemical and biophysical methods, we identify phosphatidylserine as a molecular target of Apo-15. We demonstrate that Apo-15 can be used for the quantification and imaging of drug-induced apoptosis in preclinical mouse models, thus creating opportunities for assessing the in vivo efficacy of anti-inflammatory and anti-cancer therapeutics.

Programmed cell death or apoptosis is an essential biological process that is impaired in some diseases and can be used to assess the effectiveness of drugs. Here the authors design Apo-15 as a fluorogenic peptide for the detection and real-time imaging of apoptotic cells.

Physical Constraints and Forces Involved in Phagocytosis

Phagocytosis is a specialized process that enables cellular ingestion and clearance of microbes, dead cells and tissue debris that are too large for other endocytic routes. As such, it is an essential component of tissue homeostasis and the innate immune response, and also provides a link to the adaptive immune response. However, ingestion of large particulate materials represents a monumental task for phagocytic cells. It requires profound reorganization of the cell morphology around the target in a controlled manner, which is limited by biophysical constraints. Experimental and theoretical studies have identified critical aspects associated with the interconnected biophysical properties of the receptors, the membrane, and the actin cytoskeleton that can determine the success of large particle internalization. In this review, we will discuss the major physical constraints involved in the formation of a phagosome. Focusing on two of the most-studied types of phagocytic receptors, the Fcγ receptors and the complement receptor 3 (αMβ2 integrin), we will describe the complex molecular mechanisms employed by phagocytes to overcome these physical constraints.

Phagocytic Integrins: Activation and Signaling

Phagocytic integrins are endowed with the ability to engulf and dispose of particles of different natures. Evolutionarily conserved from worms to humans, they are involved in pathogen elimination and apoptotic and tumoral cell clearance. Research in the field of integrin-mediated phagocytosis has shed light on the molecular events controlling integrin activation and their effector functions. However, there are still some aspects of the regulation of the phagocytic process that need to be clarified. Here, we have revised the molecular events controlling phagocytic integrin activation and the downstream signaling driving particle engulfment, and we have focused particularly on α_Mβ₂/CR3, α_Xβ₂/CR4, and a brief mention of α_Vβ₅/α_Vβ₃integrins.

Phagocytosis of Apoptotic Cells in Resolution of Inflammation

Efficient inflammation resolution is important not only for the termination of the inflammatory response but also for the restoration of tissue integrity. An integral process to resolution of inflammation is the phagocytosis of dying cells by macrophages, known as efferocytosis. This function is mediated by a complex and well-orchestrated network of interactions amongst specialized phagocytic receptors, bridging molecules, as well as “find-me” and “eat-me” signals. Efferocytosis serves not only as a waste disposal mechanism (clearance of the apoptotic cells) but also promotes a pro-resolving phenotype in efferocytic macrophages and thereby termination of inflammation. Alterations in cellular metabolism are critical for shaping the phenotype and function of efferocytic macrophages, thus, representing an important determinant of macrophage plasticity. Impaired efferocytosis can result in inflammation-associated pathologies or autoimmunity. The present mini review summarizes current knowledge regarding the mechanisms regulating macrophage efferocytosis during clearance of inflammation.

Microglia in the developing retina couple phagocytosis with the progression of apoptosis via P2RY12 signaling

BACKGROUND

Microglia colonize the developing vertebrate central nervous system coincident with the detection of developmental apoptosis. Our understanding of apoptosis in intact tissue in relation to microglial clearance of dying cells is largely based on fixed samples, which is limiting given that microglia are highly motile and mobile phagocytes. Here, we used a system of microglial depletion and in vivo real-time imaging in zebrafish to directly address microglial phagocytosis of apoptotic cells during normal retinal development, the relative timing of phagocytosis in relation to apoptotic progression, and the contribution of P2RY12 signaling to this process.

RESULTS

The depletion of microglia resulted in accumulation of numerous apoptotic cells in the retina. Real-time imaging revealed precise timing of microglial engulfment with the progression of apoptosis, and dynamic movement and displacement of engulfed apoptotic cells. Inhibition of P2RY12 signaling delayed microglial clearance of apoptotic cells.

CONCLUSIONS

Microglial engulfment of dying cells is coincident with apoptotic progression and requires P2RY12 signaling, indicating that microglial P2RY12 signaling is shared between development and injury response. Our work provides important in vivo insight into the dynamics of apoptotic cell clearance in the developing vertebrate retina and provides a basis to understand microglial phagocytic behavior in health and disease.

Physical Constraints and Forces Involved in Phagocytosis

Phagocytosis is a specialized process that enables cellular ingestion and clearance of microbes, dead cells and tissue debris that are too large for other endocytic routes. As such, it is an essential component of tissue homeostasis and the innate immune response, and also provides a link to the adaptive immune response. However, ingestion of large particulate materials represents a monumental task for phagocytic cells. It requires profound reorganization of the cell morphology around the target in a controlled manner, which is limited by biophysical constraints. Experimental and theoretical studies have identified critical aspects associated with the interconnected biophysical properties of the receptors, the membrane, and the actin cytoskeleton that can determine the success of large particle internalization. In this review, we will discuss the major physical constraints involved in the formation of a phagosome. Focusing on two of the most-studied types of phagocytic receptors, the Fcγ receptors and the complement receptor 3 (αMβ2 integrin), we will describe the complex molecular mechanisms employed by phagocytes to overcome these physical constraints.