Signal inhibitory receptor on leukocytes-1 (SIRL-1) negatively regulates the oxidative burst in human phagocytes

Soluble signal inhibitory receptor on leukocytes-1 reflects disease activity and assists diagnosis of patients with rheumatoid arthritis

BACKGROUND

SIRL-1, an immunosuppressive receptor encoded by the VSTM1 gene, has recently been linked to rheumatoid arthritis (RA) due to its association with activated polymorphonuclear neutrophils (PMNs). Considering that the activated PMNs play a crucial role in the pathogenesis of rheumatoid arthritis (RA), we aimed to measure the levels of soluble SIRL-1, investigating whether they add value to RA in the clinical diagnosis.

METHODS

Utilizing an enzyme-linked immunosorbent assay, the concentration of sSIRL-1 was measured in serum samples from cohort 1 diagnosed with RA (n = 96), gout (n = 54), osteoarthritis (n = 47), healthy controls (n = 86) and synovial fluid samples from OA (n = 8) and RA (n = 8) patients, respectively. Additionally, an external validation in cohort 2 (n = 156) comprising various inflammatory diseases was employed.

RESULTS

The study revealed a distinctive upregulation of sSIRL-1 in the serum of RA compared to HC and other arthralgia diseases (p < 0.0001), which also displayed a significant elevation in synovial fluid from RA compared to OA (p < 0.05). Notably, sSIRL-1 levels exhibited a significant decrease in patients who achieved disease remission (p < 0.05). Furthermore, the diagnostic accuracy of RA was enhanced when sSIRL-1 was combined with anti-CCP and RF, yielding an impressive AUC value of 0.950.

CONCLUSION

The expression pattern of sSIRL-1 in RA, coupled with its correlation with disease activity, underscores its potential clinical utility for both diagnosis and disease monitoring in RA patients. This study offers valuable insights into the evolving diagnostic landscape of RA.

Oxidized galectin-1 in SLE fails to bind the inhibitory receptor VSTM1 and increases reactive oxygen species levels in neutrophils

Inhibitory immune receptors set thresholds for immune cell activation, and their deficiency predisposes a person to autoimmune responses. However, the agonists of inhibitory immune receptors remain largely unknown, representing untapped sources of treatments for autoimmune diseases. Here, we show that V-set and transmembrane domain-containing 1 (VSTM1) is an inhibitory receptor and that its binding by the competent ligand soluble galectin-1 (Gal1) is essential for maintaining neutrophil viability mediated by downregulated reactive oxygen species production. However, in patients with systemic lupus erythematosus (SLE), circulating Gal1 is oxidized and cannot be recognized by VSTM1, leading to increased intracellular reactive oxygen species levels and reduced neutrophil viability. Dysregulated neutrophil function or death contributes significantly to the pathogenesis of SLE by providing danger molecules and autoantigens that drive the production of inflammatory cytokines and the activation of autoreactive lymphocytes. Interestingly, serum levels of glutathione, an antioxidant able to convert oxidized Gal1 to its reduced form, were negatively correlated with SLE disease activity. Taken together, our findings reveal failed inhibitory Gal1/VSTM1 pathway activation in patients with SLE and provide important insights for the development of effective targeted therapies.

Soluble Signal Inhibitory Receptor on Leukocytes-1 Is Released from Activated Neutrophils by Proteinase 3 Cleavage

Signal inhibitory receptor on leukocytes-1 (SIRL-1) is an immune inhibitory receptor expressed on human granulocytes and monocytes that dampens antimicrobial functions. We previously showed that sputum neutrophils from infants with severe respiratory syncytial virus (RSV) bronchiolitis have decreased SIRL-1 surface expression compared with blood neutrophils and that SIRL-1 surface expression is rapidly lost from in vitro activated neutrophils. This led us to hypothesize that activated neutrophils lose SIRL-1 by ectodomain shedding. Here, we developed an ELISA and measured the concentration of soluble SIRL-1 (sSIRL-1) in patients with RSV bronchiolitis and hospitalized patients with COVID-19, which are both characterized by neutrophilic inflammation. In line with our hypothesis, sSIRL-1 concentration was increased in sputum compared with plasma of patients with RSV bronchiolitis and in serum of hospitalized patients with COVID-19 compared with control serum. In addition, we show that in vitro activated neutrophils release sSIRL-1 by proteolytic cleavage and that this diminishes the ability to inhibit neutrophilic reactive oxygen species production via SIRL-1. Finally, we found that SIRL-1 shedding is prevented by proteinase 3 inhibition and by extracellular adherence protein from Staphylococcus aureus. Notably, we recently showed that SIRL-1 is activated by PSMα3 from S. aureus, suggesting that S. aureus may counteract SIRL-1 shedding to benefit from preserved inhibitory function of SIRL-1. In conclusion, we report that SIRL-1 is released from activated neutrophils by proteinase 3 cleavage and that endogenous sSIRL-1 protein is present in vivo.

Understanding inhibitory receptor function in neutrophils through the lens of CLEC12A

Neutrophils are the first leukocytes recruited from the circulation in response to invading pathogens or injured cells. To eradicate pathogens and contribute to tissue repair, recruited neutrophils generate and release a host of toxic chemicals that can also damage normal cells. To avoid collateral damage leading to tissue injury and organ dysfunction, molecular mechanisms evolved that tightly control neutrophil response threshold to activating signals, the strength and location of the response, and the timing of response termination. One mechanism of response control is interruption of activating intracellular signaling pathways by the 20 inhibitory receptors expressed by neutrophils. The two inhibitory C-type lectin receptors expressed by neutrophils, CLEC12A and DCIR, exhibit both common and distinct molecular and functional mechanisms, and they are associated with different diseases. In this review, we use studies on CLEC12A as a model of inhibitory receptor regulation of neutrophil function and participation in disease. Understanding the molecular mechanisms leading to inhibitory receptor specificity offers the possibility of using physiologic control of neutrophil functions as a pharmacologic tool to control inflammatory diseases.

A 69 kb Deletion in chr19q13.42 including PRPF31 Gene in a Chinese Family Affected with Autosomal Dominant Retinitis Pigmentosa

We aimed to identify the genetic cause of autosomal dominant retinitis pigmentosa (adRP) and characterize the underlying molecular mechanisms of incomplete penetrance in a Chinese family affected with adRP. All enrolled family members underwent ophthalmic examinations. Whole-genome sequencing (WGS), multiplex ligation-dependent probe amplification (MLPA), linkage analysis and haplotype construction were performed in all participants. RNA-seq was performed to analyze the regulating mechanism of incomplete penetrance among affected patients, mutation carriers and healthy controls. In the studied family, 14 individuals carried a novel heterozygous large deletion of 69 kilobase (kb) in 19q13.42 encompassing exon 1 of the PRPF31 gene and five upstream genes: TFPT, OSCAR, NDUFA3, TARM1, and VSTM1. Three family members were sequenced and diagnosed as non-penetrant carriers (NPCs). RNA-seq showed significant differential expression of genes in deletion between mutation carriers and healthy control. The RP11 pedigree in this study was the largest pedigree compared to other reported RP11 pedigrees with large deletions. Early onset in all affected members in this pedigree was considered to be a special phenotype and was firstly reported in a RP11 family for the first time. Differential expression of PRPF31 between affected and unaffected subjects indicates a haploinsufficiency to cause the disease in the family. The other genes with significant differential expression might play a cooperative effect on the penetrance of RP11.

Inhibitory pattern recognition receptors

Pathogen- and damage-associated molecular patterns are sensed by the immune system's pattern recognition receptors (PRRs) upon contact with a microbe or damaged tissue. In situations such as contact with commensals or during physiological cell death, the immune system should not respond to these patterns. Hence, immune responses need to be context dependent, but it is not clear how context for molecular pattern recognition is provided. We discuss inhibitory receptors as potential counterparts to activating pattern recognition receptors. We propose a group of inhibitory pattern recognition receptors (iPRRs) that recognize endogenous and microbial patterns associated with danger, homeostasis, or both. We propose that recognition of molecular patterns by iPRRs provides context, helps mediate tolerance to microbes, and helps balance responses to danger signals.

VSTM1 regulates monocyte/macrophage function via the NF-κB signaling pathway

Objective

V-set and transmembrane domain-containing protein 1 (VSTM1) is negatively correlated with inflammation. However, its effect on atherosclerosis (AS) remains largely unexplored. In this study, we aimed to assess the effect of VSTM1 on the biological function of human peripheral blood mononuclear cells /macrophages stimulated by oxidized low-density lipoprotein (ox-LDL).

Methods

U937 cells were divided into three groups as follows: control group, pLenti-VSTM1 shRNA group (VSTM1 depletion), and pLenti-VSTM1 group (VSTM1 overexpression). Cellular migration, chemotaxis, apoptosis, and secretion of inflammatory factors of monocytes/macrophages stimulated by ox-LDL were studied.

Results

Overexpression of VSTM1 decreased the proliferation of U937 cells and induced cellular apoptosis. Depletion of VSTM1 enhanced the invasiveness and chemotaxis, increased the inflammatory response, and reduced the incidence of cell necrosis and apoptosis. Nuclear factor κ of B cells (NF-κB) was activated in VSTM1-depleted U937 cells.

Conclusion

VSTM1 might play an important role in the activation of monocytes/macrophages and participate in the pathogenesis of AS via regulating NF-κB activity.

Neutrophil extracellular traps: from physiology to pathology

At the frontline of the host defense response, neutrophil antimicrobial functions have adapted to combat infections and injuries of different origins and magnitude. The release of web-like DNA structures named neutrophil extracellular traps (NETs) constitutes an important mechanism by which neutrophils prevent pathogen dissemination or deal with microorganisms of a bigger size. At the same time, nuclear and granule proteins with microbicidal activity bind to these DNA structures promoting the elimination of entrapped pathogens. However, these toxic properties may produce unwanted effects in the host, when neutrophils uncontrollably release NETs upon persistent inflammation. As a consequence, NET accumulation can produce vessel occlusion, tissue damage, and prolonged inflammation associating with the progression and exacerbation of multiple pathologic conditions. This review outlines recent advances in understanding the mechanisms of NET release and functions in sterile disease. We also discuss mechanisms of physiological regulation and the importance of neutrophil heterogeneity in NET formation and composition.

The Regulation of Neutrophil Extracellular Trap-induced Tissue Damage by Human CD177

Background

Neutrophil-induced tissue damage contributes to the rejection in xenotransplantation. Therefore, suppressing neutrophil function could be effective in suppressing xenogeneic rejection. In a previous study, we demonstrated that the ectopic expression of human cluster of differentiation 31 (CD31) on porcine endothelial cells (PEC) significantly suppressed neutrophil-mediated cytotoxicity through the homophilic binding of CD31. Cluster of differentiation 177 (CD177) was recently reported to be a high-affinity heterophilic binding partner for CD31 on endothelial cells. Thus, we hypothesized that human CD177 on PEC might induce a stronger suppression in neutrophil-mediated cytotoxicity compared with CD31. In this study, the inhibitory function of human CD177 on PEC in neutrophil-mediated cytotoxicity was investigated.

Methods

PEC were transfected with a cloning plasmid containing cDNA inserts that encoded for hCD177 and hCD31 genes. Neutrophil-induced cytotoxicity was evaluated by flow cytometry after coculturing with PEC or PEC/CD177 in the presence of phorbol 12-myristate 13-acetate. To elucidate the mechanisms responsible for hCD177-induced suppression, the phosphorylation of src homology region 2 domain containing phosphatase 1 was measured by immunoblot analysis.

Results

Human CD177 on PEC induced a significant reduction in neutrophil-induced cytotoxicity. In addition, CD177 on PEC induced a significant increase in the phosphorylation of src homology region 2 domain-containing phosphatase 1 in neutrophils and suppressed NETosis.

Conclusions

These findings suggest that human CD177 suppresses neutrophil-mediated cytotoxicity through the inhibition of NETosis.

Signal inhibitory receptor on leukocytes-1 recognizes bacterial and endogenous amphipathic α-helical peptides

Signal inhibitory receptor on leukocytes-1 (SIRL-1) is a negative regulator of myeloid cell function and dampens antimicrobial responses. We here show that different species of the genus Staphylococcus secrete SIRL-1-engaging factors. By screening a library of single-gene transposon mutants in Staphylococcus aureus, we identified these factors as phenol-soluble modulins (PSMs). PSMs are amphipathic α-helical peptides involved in multiple aspects of staphylococcal virulence and physiology. They are cytotoxic and activate the chemotactic formyl peptide receptor 2 (FPR2) on immune cells. Human cathelicidin LL-37 is also an amphipathic α-helical peptide with antimicrobial and chemotactic activities, structurally and functionally similar to α-type PSMs. We demonstrate that α-type PSMs from multiple staphylococcal species as well as human cathelicidin LL-37 activate SIRL-1, suggesting that SIRL-1 recognizes α-helical peptides with an amphipathic arrangement of hydrophobicity, although we were not able to show direct binding to SIRL-1. Upon rational peptide design, we identified artificial peptides in which the capacity to ligate SIRL-1 is segregated from cytotoxic and FPR2-activating properties, allowing specific engagement of SIRL-1. In conclusion, we propose staphylococcal PSMs and human LL-37 as a potential new class of natural ligands for SIRL-1.

Oxidative stress specifically inhibits replication of dengue virus

Reactive oxygen species (ROS) are chemically active species which are involved in maintaining cellular and signalling processes at physiological concentrations. Therefore, cellular components that regulate redox balance are likely to play a crucial role in viral life-cycle either as promoters of viral replication or with antiviral functions. Zinc is an essential micronutrient associated with anti-oxidative systems and helps in maintaining a balanced cellular redox state. Here, we show that zinc chelation leads to induction of reactive oxygen species (ROS) in epithelial cells and addition of zinc restores ROS levels to basal state. Addition of ROS (H₂O₂) inhibited dengue virus (DENV) infection in a dose-dependent manner indicating that oxidative stress has adverse effects on DENV infection. ROS affects early stages of DENV replication as observed by quantitation of positive and negative strand viral RNA. We observed that addition of ROS specifically affected viral titres of positive strand RNA viruses. We further demonstrate that ROS specifically altered SEC31A expression at the ER suggesting a role for SEC31A-mediated pathways in the life-cycle of positive strand RNA viruses and provides an opportunity to identify drug targets regulating oxidative stress responses for antiviral development.

Recognition of S100 proteins by Signal Inhibitory Receptor on Leukocytes-1 negatively regulates human neutrophils

Signal inhibitory receptor on leukocytes‐1 (SIRL‐1) is an inhibitory receptor with a hitherto unknown ligand, and is expressed on human monocytes and neutrophils. SIRL‐1 inhibits myeloid effector functions such as reactive oxygen species (ROS) production. In this study, we identify S100 proteins as SIRL‐1 ligands. S100 proteins are composed of two calcium‐binding domains. Various S100 proteins are damage‐associated molecular patterns (DAMPs) released from damaged cells, after which they initiate inflammation by ligating activating receptors on immune cells. We now show that the inhibitory SIRL‐1 recognizes individual calcium‐binding domains of all tested S100 proteins. Blocking SIRL‐1 on human neutrophils enhanced S100 protein S100A6‐induced ROS production, showing that S100A6 suppresses neutrophil ROS production via SIRL‐1. Taken together, SIRL‐1 is an inhibitory receptor recognizing the S100 protein family of DAMPs. This may help limit tissue damage induced by activated neutrophils.

TARM-1 Is Critical for Macrophage Activation and Th1 Response in Mycobacterium tuberculosis Infection

T cell-interacting activating receptor on myeloid cells 1 (TARM-1) is a novel leukocyte receptor expressed in neutrophils and macrophages. It plays an important role in proinflammatory response in acute bacterial infection, but its immunomodulatory effects on chronic Mycobacterium tuberculosis infections remain unclear. TARM-1 expression was significantly upregulated on CD14^high monocytes from patients with active pulmonary tuberculosis (TB) as compared that on cells from patients with latent TB or from healthy control subjects. Small interfering RNA knockdown of TARM-1 reduced expression levels of proinflammatory cytokines IL-12, IL-18, IL-1β, and IL-8 in M. tuberculosis-infected macrophages, as well as that of HLA-DR and costimulatory molecules CD83, CD86, and CD40. Moreover, TARM-1 enhanced phagocytosis and intracellular killing of M. tuberculosis through upregulating reactive oxygen species. In an in vitro monocyte and T cell coculture system, blockade of TARM-1 activity by TARM-1 blocking peptide suppressed CD4⁺ T cell activation and proliferation. Finally, administration of TARM-1 blocking peptide in a mouse model of M. tuberculosis infection increased bacterial load and lung pathology, which was associated with decreased macrophage activation and IFN-γ production by T cell. Taken together, these results, to our knowledge, demonstrate a novel immune protective role of TARM-1 in M. tuberculosis infection and provide a potential therapeutic target for TB disease.

Enhanced expression of immune checkpoint receptors during SARS-CoV-2 viral infection

The immune system is tightly regulated by the activity of stimulatory and inhibitory immune receptors. This immune homeostasis is usually disturbed during chronic viral infection. Using publicly available transcriptomic datasets, we conducted in silico analyses to evaluate the expression pattern of 38 selected immune inhibitory receptors (IRs) associated with different myeloid and lymphoid immune cells during coronavirus disease 2019 (COVID-19) infection. Our analyses revealed a pattern of overall upregulation of IR mRNA during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. A large number of IRs expressed on both lymphoid and myeloid cells were upregulated in nasopharyngeal swabs (NPSs), while lymphoid-associated IRs were specifically upregulated in autopsies, reflecting severe, terminal stage COVID-19 disease. Eight genes (BTLA, LAG3, FCGR2B, PDCD1, CEACAM1, CTLA4, CD72, and SIGLEC7), shared by NPSs and autopsies, were more expressed in autopsies and were directly correlated with viral levels. Single-cell data from blood and bronchoalveolar samples also reflected the observed association between IR upregulation and disease severity. Moreover, compared to SARS-CoV-1, influenza, and respiratory syncytial virus infections, the number and intensities of upregulated IRs were higher in SARS-CoV-2 infections. In conclusion, the immunopathology and severity of COVID-19 could be attributed to dysregulation of different immune inhibitors. Targeting one or more of these immune inhibitors could represent an effective therapeutic approach for the treatment of COVID-19 early and late immune dysregulations.

Signal Inhibitory Receptor on Leukocytes-1 is highly expressed on lung monocytes, but absent on mononuclear phagocytes in skin and colon

Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1) is expressed on human blood monocytes and granulocytes and inhibits myeloid effector functions. On monocytes, but not granulocytes, SIRL-1 expression is low or absent in individuals with the single nucleotide polymorphism (SNP) rs612529C. The expression of SIRL-1 in tissue and the influence of rs612529 hereon is currently unknown. Here, we used flow cytometry to determine SIRL-1 expression on immune cells in human blood and three barrier tissues; skin, colon and lung. SIRL-1 was expressed by virtually all neutrophils and eosinophils in these tissues. In contrast, SIRL-1 was not expressed by monocyte-derived cells in skin and colon, whereas it was highly expressed by lung classical monocytes. Lung monocytes from individuals with a rs612529C allele had decreased SIRL-1 expression, consistent with the genotype association in blood. Within the different monocyte subsets in blood and lung, SIRL-1 expression was highest in classical monocytes and lowest in nonclassical monocytes. SIRL-1 was not expressed by dendritic cells in blood and barrier tissues. Together, these results indicate that SIRL-1 is differentially expressed on phagocyte subsets in blood and barrier tissues, and that its expression on monocytes is genotype- and tissue-specific. Immune regulation of monocytes by SIRL-1 may be of particular importance in the lung.

Genetics and Epigenetics of Atopic Dermatitis: An Updated Systematic Review

BACKGROUND

Atopic dermatitis is a common inflammatory skin disorder that affects up to 15-20% of the population and is characterized by recurrent eczematous lesions with intense itching. As a heterogeneous disease, multiple factors have been suggested to explain the nature of atopic dermatitis (AD), and its high prevalence makes it necessary to periodically compile and update the new information available. In this systematic review, the focus is set at the genetic and epigenetic studies carried out in the last years.

METHODS

A systematic literature review was conducted in three scientific publication databases (PubMed, Cochrane Library, and Scopus). The search was restricted to publications indexed from July 2016 to December 2019, and keywords related to atopic dermatitis genetics and epigenetics were used.

RESULTS

A total of 73 original papers met the inclusion criteria established, including 9 epigenetic studies. A total of 62 genes and 5 intergenic regions were described as associated with AD.

CONCLUSION

Filaggrin (FLG) polymorphisms are confirmed as key genetic determinants for AD development, but also epigenetic regulation and other genes with functions mainly related to the immune system and extracellular matrix, reinforcing the notion of skin homeostasis breakage in AD.

Development of a Bioinformatics Framework for Identification and Validation of Genomic Biomarkers and Key Immunopathology Processes and Controllers in Infectious and Non-infectious Severe Inflammatory Response Syndrome

Sepsis is defined as dysregulated host response caused by systemic infection, leading to organ failure. It is a life-threatening condition, often requiring admission to an intensive care unit (ICU). The causative agents and processes involved are multifactorial but are characterized by an overarching inflammatory response, sharing elements in common with severe inflammatory response syndrome (SIRS) of non-infectious origin. Sepsis presents with a range of pathophysiological and genetic features which make clinical differentiation from SIRS very challenging. This may reflect a poor understanding of the key gene inter-activities and/or pathway associations underlying these disease processes. Improved understanding is critical for early differential recognition of sepsis and SIRS and to improve patient management and clinical outcomes. Judicious selection of gene biomarkers suitable for development of diagnostic tests/testing could make differentiation of sepsis and SIRS feasible. Here we describe a methodologic framework for the identification and validation of biomarkers in SIRS, sepsis and septic shock patients, using a 2-tier gene screening, artificial neural network (ANN) data mining technique, using previously published gene expression datasets. Eight key hub markers have been identified which may delineate distinct, core disease processes and which show potential for informing underlying immunological and pathological processes and thus patient stratification and treatment. These do not show sufficient fold change differences between the different disease states to be useful as primary diagnostic biomarkers, but are instrumental in identifying candidate pathways and other associated biomarkers for further exploration.

The Orphan Immune Receptor LILRB3 Modulates Fc Receptor-Mediated Functions of Neutrophils

Neutrophils are critical to the generation of effective immune responses and for killing invading microbes. Paired immune receptors provide important mechanisms to modulate neutrophil activation thresholds and effector functions. Expression of the leukocyte Ig-like receptor (LILR)A6 (ILT8/CD85b) and LILRB3 (ILT5/CD85a) paired-receptor system on human neutrophils has remained unclear because of the lack of specific molecular tools. Additionally, there is little known of their possible functions in neutrophil biology. The objective of this study was to characterize expression of LILRA6/LILRB3 receptors during human neutrophil differentiation and activation, and to assess their roles in modulating Fc receptor-mediated effector functions. LILRB3, but not LILRA6, was detected in human neutrophil lysates following immunoprecipitation by mass spectrometry. We demonstrate high LILRB3 expression on the surface of resting neutrophils and release from the surface following neutrophil activation. Surface expression was recapitulated in a human PLB-985 cell model of neutrophil-like differentiation. Continuous ligation of LILRB3 inhibited key IgA-mediated effector functions, including production of reactive oxygen species, phagocytic uptake, and microbial killing. This suggests that LILRB3 provides an important checkpoint to control human neutrophil activation and their antimicrobial effector functions during resting and early-activation stages of the neutrophil life cycle.

The Role of Inhibitory Receptors in Monosodium Urate Crystal-Induced Inflammation

Inhibitory receptors are key regulators of immune responses. Aberrant inhibitory receptor function can either lead to an exacerbated or defective immune response. Several regulatory mechanisms involved in the inflammatory reaction induced by monosodium urate crystals (MSU) during acute gout have been identified. One of these mechanisms involves inhibitory receptors. The engagement of the inhibitory receptors Clec12A and SIRL-1 has opposing effects on the responses of neutrophils to MSU. We review the general concepts of inhibitory receptor biology and apply them to understand and compare the modulation of MSU-induced inflammation by Clec12A and SIRL-1. We also discuss gaps in our knowledge of the contribution of inhibitory receptors to the pathogenesis of gout and propose future avenues of research.

Human CD31 on porcine cells suppress xenogeneic neutrophil-mediated cytotoxicity via the inhibition of NETosis

BACKGROUND

Xenotransplantation is one of the promising strategies for overcoming the shortage of organs available for transplant. However, many immunological obstructions need to be overcome for practical use. Increasing evidence suggests that neutrophils contribute to xenogeneic cellular rejection. Neutrophils are regulated by activation and inhibitory signals to induce appropriate immune reactions and to avoid unnecessary immune reactivity. Therefore, we hypothesized that the development of neutrophil-targeted therapies may have the potential for increased graft survival in xenotransplantation.

METHODS

A plasmid containing a cDNA insert encoding the human CD31 gene was transfected into swine endothelial cells (SEC). HL-60 cells were differentiated into neutrophil-like cells by culturing them in the presence of 1.3% dimethyl sulfoxide for 48 hours. The cytotoxicity of the differentiated HL-60 cells (dHL-60) and peripheral blood-derived neutrophils was evaluated by WST-8 assays. To investigate the mechanism responsible for hCD31-induced immunosuppression, citrullinated histone 3 (cit-H3) and phosphorylation of SHP-1 were detected by a cit-H3 enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively.

RESULTS

A significant decrease in dHL-60 and neutrophil-mediated cytotoxicity in SEC/hCD31 compared with SEC was seen, as evidenced by a cytotoxicity assay. Furthermore, the suppression of NETosis and the induction of SHP-1 phosphorylation in neutrophils that had been co-cultured with SEC/CD31 were confirmed by cit-H3 ELISA and Western blotting with an anti-phosphorylated SHP-1.

CONCLUSION

These data suggest that human CD31 suppresses neutrophil-mediated xenogenic cytotoxicity via the inhibition of NETosis. As CD31 is widely expressed in a variety of inflammatory cells, human CD31-induced suppression may cover the entire xenogeneic cellular rejection, thus making the generation of human CD31 transgenic pigs very attractive for use in xenografts.

Synergistic antitumor activity of triple-regulated oncolytic adenovirus with VSTM1 and daunorubicin in leukemic cells

V-set and transmembrane domain-containing 1 (VSTM1), which is downregulated in bone marrow cells from leukemia patients, may provide a diagnostic and treatment target. Here, a triple-regulated oncolytic adenovirus was constructed to carry a VSTM1 gene expression cassette, SG611-VSTM1, and contained the E1a gene with a 24-nucleotide deletion within the CR2 region under control of the human telomerase reverse transcriptase promoter, E1b gene directed by the hypoxia response element, and VSTM1 gene controlled by the cytomegalovirus promoter. Real-time quantitative PCR and Western blot analyses showed that SG611-VSTM1 expressed VSTM1 highly efficiently in the human leukemic cell line K562 compared with SG611. In Cell Counting Kit-8 and flow cytometric assays, SG611-VSTM1 exhibited more potent anti-proliferative and pro-apoptotic effects in leukemic cells compared with SG611 and exerted synergistic cytotoxicity with low-dose daunorubicin (DNR) in vitro. In xenograft models, SG611-VSTM1 intratumorally injected at a dose of 1 × 10(9) plaque forming units combined with intraperitoneally injected low-dose DNR displayed significantly stronger antitumor effects than either treatment alone. Histopathologic examination revealed that SG611-VSTM1 induced apoptosis of leukemic cells. These results implicate an important role for VSTM1 in the pathogenesis of leukemia, and SG611-VSTM1 may be a promising agent for enhancing chemosensitivity in leukemia therapy.

Neutrophil extracellular traps in immunity and disease

Neutrophils are innate immune phagocytes that have a central role in immune defence. Our understanding of the role of neutrophils in pathogen clearance, immune regulation and disease pathology has advanced dramatically in recent years. Web-like chromatin structures known as neutrophil extracellular traps (NETs) have been at the forefront of this renewed interest in neutrophil biology. The identification of molecules that modulate the release of NETs has helped to refine our view of the role of NETs in immune protection, inflammatory and autoimmune diseases and cancer. Here, I discuss the key findings and concepts that have thus far shaped the field of NET biology.

A functional SNP associated with atopic dermatitis controls cell type-specific methylation of the VSTM1 gene locus

BACKGROUND

Expression quantitative trait loci (eQTL) databases represent a valuable resource to link disease-associated SNPs to specific candidate genes whose gene expression is significantly modulated by the SNP under investigation. We previously identified signal inhibitory receptor on leukocytes-1 (SIRL-1) as a powerful regulator of human innate immune cell function. While it is constitutively high expressed on neutrophils, on monocytes the SIRL-1 surface expression varies strongly between individuals. The underlying mechanism of regulation, its genetic control as well as potential clinical implications had not been explored yet.

METHODS

Whole blood eQTL data of a Chinese cohort was used to identify SNPs regulating the expression of VSTM1, the gene encoding SIRL-1. The genotype effect was validated by flow cytometry (cell surface expression), correlated with electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation (ChIP) and bisulfite sequencing (C-methylation) and its functional impact studied the inhibition of reactive oxygen species (ROS).

RESULTS

We found a significant association of a single CpG-SNP, rs612529T/C, located in the promoter of VSTM1. Through flow cytometry analysis we confirmed that primarily in the monocytes the protein level of SIRL-1 is strongly associated with genotype of this SNP. In monocytes, the T allele of this SNP facilitates binding of the transcription factors YY1 and PU.1, of which the latter has been recently shown to act as docking site for modifiers of DNA methylation. In line with this notion rs612529T associates with a complete demethylation of the VSTM1 promoter correlating with the allele-specific upregulation of SIRL-1 expression. In monocytes, this upregulation strongly impacts the IgA-induced production of ROS by these cells. Through targeted association analysis we found a significant Meta P value of 1.14 × 10^-6 for rs612529 for association to atopic dermatitis (AD).

CONCLUSION

Low expression of SIRL-1 on monocytes is associated with an increased risk for the manifestation of an inflammatory skin disease. It thus underlines the role of both the cell subset and this inhibitory immune receptor in maintaining immune homeostasis in the skin. Notably, the genetic regulation is achieved by a single CpG-SNP, which controls the overall methylation state of the promoter gene segment.

Fine-tuning neutrophil activation: Strategies and consequences

In spite of their important role in host defense, neutrophils can also cause severe morbidity and mortality. Neutrophils have an extensive armory necessary to eradicate pathogens, but neutrophil infiltration and activation also induces major tissue injury associated with acute and chronic inflammatory disorders. Here, we review neutrophil anti-microbial functions and discuss their individual contribution to disease pathogenesis. Furthermore, we provide an overview of the anti-inflammatory drugs that can dampen neutrophil transmigration, elastase activity, and the production of reactive oxygen species which are already in clinical trials. The discovery of potential inhibitors of the release of neutrophil extracellular trap is still in its infancy. Here, we discuss small molecule inhibitors and inhibitory receptors that show promising results in reducing neutrophil extracellular trap formation in vitro and in vivo and discuss the advantages and drawbacks of inhibiting the release of neutrophil extracellular traps as a therapeutic treatment. Specific suppression of neutrophil extracellular trap formation, preferably while other antimicrobial functions are preserved, would be an ideal approach to treat neutrophilic inflammation, since it prevents acute as well as chronic neutrophil-associated pathology.

Surveillance of cell and tissue perturbation by receptors in the LRC

The human leukocyte receptor complex (LRC) encompasses several sets of genes with a common evolutionary origin and which form a branch of the immunoglobulin superfamily (IgSF). Comparisons of LRC genes both within and between species calls for a high degree of plasticity. The drive for this unprecedented level of variation is not known, but it relates in part to interaction of several LRC products with polymorphic human leukocyte antigen (HLA) class I molecules. However, the range of other proposed ligands for LRC products indicates a dynamic set of receptors that have adapted to detect target molecules relating to numerous cellular pathways. Several receptors in the complex bind a molecular signature in collagenous ligands. Others detect a variety of motifs relating to pathogens in addition to cellular stress, attesting to the opportunistic versatility of LRC receptors.

Immunoreceptors on neutrophils

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Neutrophil activities must be tightly controlled to maintain immune homeostasis.

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Activating and inhibitory receptors balance the outcome of immune cell activation.

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Immunoreceptors contain Ig-like extracellular domains and signal via ITAMs or ITIMs.

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Syk or SHP/SHIP mediate downstream signaling after immunoreceptor activation.

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Targeting immunoreceptors provides opportunities for therapeutic interventions.

Neutrophils play a critical role in the host defense against infection, and they are able to perform a variety of effector mechanisms for this purpose. However, there are also a number of pathological conditions, including autoimmunity and cancer, in which the activities of neutrophils can be harmful to the host. Thus the activities of neutrophils need to be tightly controlled. As in the case of other immune cells, many of the neutrophil effector functions are regulated by a series of immunoreceptors on the plasma membrane. Here, we review what is currently known about the functions of the various individual immunoreceptors and their signaling in neutrophils. While these immunoreceptors allow for the recognition of a diverse range of extracellular ligands, such as cell surface structures (like proteins, glycans and lipids) and extracellular matrix components, they commonly signal via conserved ITAM or ITIM motifs and their associated downstream pathways that depend on the phosphorylation of tyrosine residues in proteins and/or inositol lipids. This allows for a balanced homeostatic regulation of neutrophil effector functions. Given the number of available immunoreceptors and their fundamental importance for neutrophil behavior, it is perhaps not surprising that pathogens have evolved means to evade immune responses through some of these pathways. Inversely, some of these receptors evolved to specifically recognize these pathogens. Finally, some interactions mediated by immunoreceptors in neutrophils have been identified as promising targets for therapeutic intervention.

TARM1 Is a Novel Leukocyte Receptor Complex-Encoded ITAM Receptor That Costimulates Proinflammatory Cytokine Secretion by Macrophages and Neutrophils

We identified a novel, evolutionarily conserved receptor encoded within the human leukocyte receptor complex and syntenic region of mouse chromosome 7, named T cell-interacting, activating receptor on myeloid cells-1 (TARM1). The transmembrane region of TARM1 contained a conserved arginine residue, consistent with association with a signaling adaptor. TARM1 associated with the ITAM adaptor FcRγ but not with DAP10 or DAP12. In healthy mice, TARM1 is constitutively expressed on the cell surface of mature and immature CD11b(+)Gr-1(+) neutrophils within the bone marrow. Following i.p. LPS treatment or systemic bacterial challenge, TARM1 expression was upregulated by neutrophils and inflammatory monocytes and TARM1(+) cells were rapidly recruited to sites of inflammation. TARM1 expression was also upregulated by bone marrow-derived macrophages and dendritic cells following stimulation with TLR agonists in vitro. Ligation of TARM1 receptor in the presence of TLR ligands, such as LPS, enhanced the secretion of proinflammatory cytokines by macrophages and primary mouse neutrophils, whereas TARM1 stimulation alone had no effect. Finally, an immobilized TARM1-Fc fusion protein suppressed CD4(+) T cell activation and proliferation in vitro. These results suggest that a putative T cell ligand can interact with TARM1 receptor, resulting in bidirectional signaling and raising the T cell activation threshold while costimulating the release of proinflammatory cytokines by macrophages and neutrophils.

Neutrophils in respiratory syncytial virus infection: A target for asthma prevention

Lower respiratory tract infections by respiratory syncytial virus (RSV) are the foremost cause of infant hospitalization and are implicated in lasting pulmonary impairment and the development of asthma. Neutrophils infiltrate the airways of pediatric patients with RSV-induced bronchiolitis in vast numbers: approximately 80% of infiltrated cells are neutrophils. However, why neutrophils are recruited to the site of viral respiratory tract infection is not clear. In this review we discuss the beneficial and pathologic contributions of neutrophils to the immune response against RSV infection. Neutrophils can limit viral replication and spread, as well as stimulate an effective antiviral adaptive immune response. However, low specificity of neutrophil antimicrobial armaments allows for collateral tissue damage. Neutrophil-induced injury to the airways during the delicate period of infant lung development has lasting adverse consequences for pulmonary architecture and might promote the onset of asthma in susceptible subjects. We suggest that pharmacologic modulation of neutrophils should be explored as a viable future therapy for severe RSV-induced bronchiolitis and thereby prevent the inception of subsequent asthma. The antiviral functions of neutrophils suggest that targeting of neutrophils in patients with RSV-induced bronchiolitis is best performed under the umbrella of antiviral treatment.

VSTM-v1, a potential myeloid differentiation antigen that is downregulated in bone marrow cells from myeloid leukemia patients

Leukocyte differentiation antigens often represent important markers for the diagnosis, classification, prognosis, and therapeutic targeting of myeloid leukemia. Herein, we report a potential leukocyte differentiation antigen gene VSTM1 (V-set and transmembrane domain-containing 1) that was downregulated in bone marrow cells from leukemia patients and exhibited a higher degree of promoter methylation. The expression level of its predominant encoded product, VSTM1-v1, was positively correlated with myeloid cell maturation state. Restoration of VSTM1-v1 expression inhibited myeloid leukemia cells' growth. Therefore, VSTM1-v1 might represent an important myeloid leukocyte differentiation antigen and provide a potential target for the diagnosis and treatment of leukemia.

V‑set and transmembrane domain‑containing 1 is silenced in human hematopoietic malignancy cell lines with promoter methylation and has inhibitory effects on cell growth

Numerous leukocyte differentiation antigens act as important markers for research, diagnosis, triage and eventually treatment targets for hematopoietic malignancies. V‑set and transmembrane domain‑containing 1 (VSTM1) was identified by immunogenomic analysis as a potential leukocyte differentiation antigen gene. VSTM1 is located at 19q13.4 on human chromosomes, an important genomic region prone to genetic and epigenetic modifications in numerous hematopoietic malignancies. VSTM1‑v1, a primary splicing form encoded by VSTM1, is a type I transmembrane molecule with an extracellular immunoglobulin V‑like domain and two cytoplasmic immunoreceptor tyrosine-based inhibitory motifs. In the present study, VSTM1 expression was examined in normal human peripheral leukocytes and hematopoietic tumor cell lines; in addition, the aberrant methylation of the VSTM1 gene was evaluated using methylation‑specific polymerase chain reaction (MSP). The results of the present study demonstrated that VSTM1 was widely expressed in normal human peripheral blood leukocytes, including granulocytes and monocytes, in concurrence with previous studies, as well as lymphocytes; in addition, the molecular size and expression levels of VSTM1 varied considerably between leukocytes. However, VSTM1 was undetectable in numerous hematopoietic tumor cell lines following promoter hypermethylation. The effects of pharmacologically‑induced demethylation of the VSTM1 gene and promoter region were analyzed using MSP and biosulfite genomic sequencing, and the results revealed that VSTM1 expression was restored in methylation‑silenced Jurkat cells. In addition, CKK‑8 assays revealed that VSTM1‑v1 overexpression in Jurkat cells resulted in growth suppression. Furthermore, the inhibitory effect on cell growth was enhanced following antibody‑induced cross‑linking of VSTM1‑v1. In conclusion, the results of the present study indicated that promoter methylation silenced VSTM1 and negatively regulated cell growth in human hematopoietic malignancy cell lines.

Tissue-specific responses of oxidative stress biomarkers and antioxidant defenses in rainbow trout Oncorhynchus mykiss during a vaccination against furunculosis

The present study was conducted to evaluate the effects of vaccination against furunculosis on responses of oxidative stress and antioxidant defenses in rainbow trout Oncorhynchus mykiss muscle, gills, liver, and brain tissues. The oxidative stress markers (malondialdehyde and carbonyl derivatives of protein oxidative destruction levels), antioxidant defenses (superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase), and total antioxidant capacity in different tissues of rainbow trout were measured. Our data showed that exposure of trout to vaccine against furunculosis produced changes (either increase or decrease) in oxidative stress and antioxidant enzymes responses, and these responses showed marked organ differences, associated with tissue patterns. Our study demonstrated that vaccinated trout showed alteration in antioxidant defenses and oxidative stress responses, with higher severity in the liver, compared with other tissues. Our data also suggest that vaccination against furunculosis induced lipid peroxidation in gill and liver tissues. However, muscle and brain tissue are capable of restoring its pro- and antioxidant balance after vaccination.

Leukocyte-associated Ig-like receptor-1 is a novel inhibitory receptor for surfactant protein D

The collagenous C-type lectin, SP-D, is a multitrimeric glycoprotein present at mucosal surfaces and is involved in host defense against infections in mammals. SP-D has immunomodulatory properties, but the underlying mechanisms are incompletely understood. SP-D contains collagen domains. LAIR-1 is an inhibitory immune receptor at the cell surface of various immune-competent cells that binds collagen. We hypothesized that the immunomodulatory functions of SP-D can be mediated via interactions between its collagen domain and LAIR-1. Binding assays show that SP-D interacts via its collagenous domain with LAIR-1 and the related LAIR-2. This does not affect the mannan-binding capacities of SP-D, which induces cross-linking of LAIR-1 in a cellular reporter assay. Functional assays show that SP-D inhibits the production of FcαR-mediated reactive oxygen via LAIR-1. Our studies indicate that SP-D is a functional ligand of the immune inhibitory receptor LAIR-1. Thus, we have identified a novel pathway for the immunomodulatory functions of SP-D mediated via binding of its collagenous domains to LAIR-1. This may provide a mechanism for the unexplained immunomodulatory function of the collagenous domains of SP-D.

Ligation of signal inhibitory receptor on leukocytes-1 suppresses the release of neutrophil extracellular traps in systemic lupus erythematosus

Neutrophil extracellular traps (NETs) have been implicated in the pathogenesis of systemic Lupus erythematosus (SLE), since netting neutrophils release potentially immunogenic autoantigens including histones, LL37, human neutrophil peptide (HNP), and self-DNA. In turn, these NETs activate plasmacytoid dendritic cells resulting in aggravation of inflammation and disease. How suppression of NET formation can be targeted for treatment has not been reported yet. Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1) is a surface molecule exclusively expressed on phagocytes. We recently identified SIRL-1 as a negative regulator of human neutrophil function. Here, we determine whether ligation of SIRL-1 prevents the pathogenic release of NETs in SLE. Peripheral blood neutrophils from SLE patients with mild to moderate disease activity and healthy donors were freshly isolated. NET release was assessed spontaneously or after exposure to anti-neutrophil antibodies or plasma obtained from SLE patients. The formation of NETs was determined by microscopic evaluation using DNA dyes and immunostaining of NET components, as well as by live cell imaging. We show that SLE neutrophils spontaneously release NETs. NET formation is enhanced by stimulation with antibodies against LL37. Inhibition of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and MEK-ERK signaling prevents NET release in response to these antibodies. Signaling via the inhibitory receptor SIRL-1 was induced by ligation with anti-SIRL-1 specific antibodies. Both spontaneous and anti-neutrophil antibody-induced NET formation is suppressed by engagement of SIRL-1. Furthermore, NET release by healthy neutrophils exposed to SLE plasma is inhibited by SIRL-1 ligation. Thus, SIRL-1 engagement can dampen spontaneous and anti-neutrophil antibody-induced NET formation in SLE, likely by suppressing NAPDH oxidase and MEK-ERK activity. Together, these findings reveal a regulatory role for SIRL-1 in NET formation, potentially providing a novel therapeutic target to break the pathogenic loop in SLE.