Syk is required for integrin signaling in neutrophils

Loss of neutrophil Shp1 produces hemorrhagic and lethal acute lung injury

The acute respiratory distress syndrome (ARDS) is associated with significant morbidity and mortality and neutrophils are critical to its pathogenesis. Neutrophil activation is closely regulated by inhibitory tyrosine phosphatases including Src homology region 2 domain containing phosphatase-1 (Shp1). Here, we report that loss of neutrophil Shp1 in mice produced hyperinflammation and lethal pulmonary hemorrhage in sterile inflammation and pathogen-induced models of acute lung injury (ALI) through a Syk kinase-dependent mechanism. We observed large intravascular neutrophil clusters, perivascular inflammation, and excessive neutrophil extracellular traps in neutrophil-specific Shp1 knockout mice suggesting an underlying mechanism for the observed pulmonary hemorrhage. Targeted immunomodulation through the administration of a Shp1 activator (SC43) reduced agonist-induced reactive oxygen species in vitro and ameliorated ALI-induced alveolar neutrophilia and NETs in vivo. We propose that the pharmacologic activation of Shp1 has the potential to fine-tune neutrophil hyperinflammation that is central to the pathogenesis of ARDS.

De novo steroidogenesis in tumor cells drives bone metastasis and osteoclastogenesis

Osteoclasts play a central role in cancer-cell-induced osteolysis, but the molecular mechanisms of osteoclast activation during bone metastasis formation are incompletely understood. By performing RNA sequencing on a mouse breast carcinoma cell line with higher bone-metastatic potential, here we identify the enzyme CYP11A1 strongly upregulated in osteotropic tumor cells. Genetic deletion of Cyp11a1 in tumor cells leads to a decreased number of bone metastases but does not alter primary tumor growth and lung metastasis formation in mice. The product of CYP11A1 activity, pregnenolone, increases the number and function of mouse and human osteoclasts in vitro but does not alter osteoclast-specific gene expression. Instead, tumor-derived pregnenolone strongly enhances the fusion of pre-osteoclasts via prolyl 4-hydroxylase subunit beta (P4HB), identified as a potential interaction partner of pregnenolone. Taken together, our results demonstrate that Cyp11a1-expressing tumor cells produce pregnenolone, which is capable of promoting bone metastasis formation and osteoclast development via P4HB.

The Syk Inhibitor Entospletinib Abolishes Dermal-Epidermal Separation in a Fully Human Ex Vivo Model of Bullous Pemphigoid

Bullous pemphigoid (BP) is an autoantibody-mediated blistering skin disease characterized by local inflammation and dermal-epidermal separation, with no approved targeted therapy. The Syk tyrosine kinase is critical for various functions of the immune response. Second-generation Syk inhibitors such as entospletinib are currently being tested for hematological malignancies. Our aim was to test the effect of entospletinib in a fully human model system of BP. Incubating BP serum-treated human frozen skin sections with normal human granulocytes and fresh plasma triggered dermal-epidermal separation that was dependent on complement, NADPH oxidase, and protease activity. Entospletinib dramatically reduced dermal-epidermal separation with a half-maximal inhibitory concentration of ≈16 nM. Entospletinib also reduced ROS production, granule release, and spreading of human granulocytes plated on immobilized immune complexes consisting either of a generic antigen-antibody pair or of recombinant collagen type XVII (BPAg2) and BP serum components (supposedly autoantibodies). However, entospletinib did not affect the chemotactic migration of human granulocytes or their responses to nonphysiological stimulation by phorbol esters. Entospletinib had no effect on the survival of granulocytes either. Taken together, entospletinib abrogates dermal-epidermal separation, likely through inhibition of granulocyte responsiveness to deposited immune complexes. Entospletinib or other Syk inhibitors may provide therapeutic benefits in BP.

Phagocytosis

One hundred years have passed since the death of Élie Metchnikoff (1845-1916). He was the first to observe the uptake of particles by cells and realized the importance of this process, named phagocytosis, for the host response to injury and infection. He also was a strong advocate of the role of phagocytosis in cellular immunity, and with this, he gave us the basis for our modern understanding of inflammation and the innate immune response. Phagocytosis is an elegant but complex process for the ingestion and elimination of pathogens, but it is also important for the elimination of apoptotic cells and hence fundamental for tissue homeostasis. Phagocytosis can be divided into four main steps: (i) recognition of the target particle, (ii) signaling to activate the internalization machinery, (iii) phagosome formation, and (iv) phagolysosome maturation. In this chapter, we present a general view of our current knowledge on phagocytosis performed mainly by professional phagocytes through antibody and complement receptors and discuss aspects that remain incompletely understood.

The selective inhibition of the Syk tyrosine kinase ameliorates experimental autoimmune arthritis

Autoimmune arthritis - such as rheumatoid arthritis - affect a significant proportion of the population, which can cause everyday joint pain, decreased mobility and reduced quality of life. Despite having more and more therapeutic options available, there are still a lot of patients who cannot reach remission or low disease activity by current therapies. This causes an urgent need for the development of new treatment options. The Syk tyrosine kinase plays an essential role in B cell receptor, Fc receptor and integrin signaling. It has been shown that the hematopoietic cell-specific deletion of Syk resulted in a complete protection against autoantibody-induced experimental arthritis. This prompted us to test the effect of entospletinib, a second generation, Syk-selective inhibitor, which has a tolerable safety profile according to hematological clinical trials, in experimental autoimmune arthritis. We found that entospletinib dose-dependently decreased the macroscopic signs of joint inflammation, while it did not affect the health status of the animals. In line with these findings, local neutrophil accumulation and cytokine levels were reduced compared to the vehicle-treated group, while macrophage accumulation and synovial fibroblast numbers were not significantly altered. Meanwhile, entospletinib dose-dependently decreased the cell responses of immune complex- or integrin ligand-activated neutrophils. Overall, we found that selective Syk inhibition by entospletinib reduced the activity of autoantibody-induced experimental arthritis, which seems to be based mainly on the effect of the inhibitor on neutrophil functions. Our data raise the possibility that entospletinib could be a good drug candidate in the treatment of human autoimmune arthritis.

Neuronal-specific methylome and hydroxymethylome analysis reveal replicated and novel loci associated with alcohol use disorder

Alcohol use disorder (AUD) is a complex condition associated with adverse health consequences that affect millions of individuals worldwide. Epigenetic modifications, including DNA methylation (5mC), have been associated with AUD and other alcohol-related traits. Epigenome-wide association studies (EWAS) have identified differentially methylated genes associated with AUD in human peripheral and brain tissue. More recently, epigenetic studies of AUD have also evaluated DNA hydroxymethylation (5hmC) in the human brain. However, most of the epigenetic work in postmortem brain tissue has examined bulk tissue. In this study, we investigated neuronal-specific 5mC and 5hmC alterations at CpG sites associated with AUD in the human orbitofrontal cortex (OFC). Neuronal nuclei from the OFC were evaluated in 34 human postmortem brain samples (10 AUD, 24 non-AUD). Reduced representation oxidative bisulfite sequencing was used to assess 5mC and 5hmC at the genome-wide level. Differential 5mC and 5hmC were evaluated using the methylKit R package and significance was set at false discovery rate <0.05 and differential methylation >2. Functional enrichment analyses were performed and replication was evaluated replication in an independent dataset that assessed 5mC and 5hmC of AUD in bulk cortical tissue. We identified 417 5mC and 363 5hmC genome-wide significant differential CpG sites associated with AUD, with 59% in gene promoters. We also identified genes previously implicated in alcohol consumption, such as SYK, CHRM2, DNMT3A, and GATA4, for 5mC and GATA4, and GAD1, GATA4, DLX1 for 5hmC. Replication was observed for 28 CpG sites from a previous AUD 5mC and 5hmC study, including FOXP1. Lastly, GWAS enrichment analysis showed an association with AUD for differential 5mC genes. This study reveals neuronal-specific methylome and hydroxymethylome dysregulation associated with AUD. We replicated previous findings and identified novel associations with AUD for both 5mC and 5hmC marks within the OFC. Our findings provide new insights into the epigenomic dysregulation of AUD in the human brain.

The biology of TREM receptors

Triggering receptors expressed on myeloid cells (TREMs) encompass a family of cell-surface receptors chiefly expressed by granulocytes, monocytes and tissue macrophages. These receptors have been implicated in inflammation, neurodegenerative diseases, bone remodelling, metabolic syndrome, atherosclerosis and cancer. Here, I review the structure, ligands, signalling modes and functions of TREMs in humans and mice and discuss the challenges that remain in understanding TREM biology.

Inflammatory ER stress responses dictate the immunopathogenic progression of systemic candidiasis

Recognition of pathogen-associated molecular patterns can trigger the inositol-requiring enzyme 1 α (IRE1α) arm of the endoplasmic reticulum (ER) stress response in innate immune cells. This process maintains ER homeostasis and also coordinates diverse immunomodulatory programs during bacterial and viral infections. However, the role of innate IRE1α signaling in response to fungal pathogens remains elusive. Here, we report that systemic infection with the human opportunistic fungal pathogen Candida albicans induced proinflammatory IRE1α hyperactivation in myeloid cells that led to fatal kidney immunopathology. Mechanistically, simultaneous activation of the TLR/IL-1R adaptor protein MyD88 and the C-type lectin receptor dectin-1 by C. albicans induced NADPH oxidase-driven generation of ROS, which caused ER stress and IRE1α-dependent overexpression of key inflammatory mediators such as IL-1β, IL-6, chemokine (C-C motif) ligand 5 (CCL5), prostaglandin E2 (PGE2), and TNF-α. Selective ablation of IRE1α in leukocytes, or treatment with an IRE1α pharmacological inhibitor, mitigated kidney inflammation and prolonged the survival of mice with systemic C. albicans infection. Therefore, controlling IRE1α hyperactivation may be useful for impeding the immunopathogenic progression of disseminated candidiasis.

GM-CSF receptor/SYK/JNK/FOXO1/CD11c signaling promotes atherosclerosis

Atherosclerosis complicates chronic inflammatory diseases, such as rheumatoid arthritis and systemic lupus erythematosus, suggesting that a shared physiological pathway regulates inflammatory responses in these diseases wherein spleen tyrosine kinase (SYK) is involved. We aimed to identify a shared therapeutic target for atherosclerosis and inflammatory diseases. We used Syk-knockout atherosclerosis-prone mice to determine whether SYK is involved in atherosclerosis via the inflammatory response and elucidate the mechanism of SYK signaling. The Syk-knockout mice showed reduced atherosclerosis in vivo, and macrophages derived from this strain showed ameliorated cell migration in vitro. CD11c expression decreased on Syk-knockout monocytes and macrophages; it was upregulated by forkhead box protein O1 (FOXO1) after stimulation with granulocyte-macrophage colony-stimulating factor (GM-CSF), and c-Jun amino-terminal kinase (JNK) mediated SYK signaling to FOXO1. Furthermore, FOXO1 inhibitor treatment mitigated atherosclerosis in mice. Thus, GM-CSF receptor/SYK/JNK/FOXO1/CD11c signaling in monocytes and macrophages and FOXO1 could be therapeutic targets for atherosclerosis and inflammatory diseases.

Siglec cis-ligands and their roles in the immune system

Sialic acid-binding immunoglobulin-like lectins are a family of membrane molecules primarily expressed in immune cells. Most of them are inhibitory receptors containing immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic tail. On the cell surface, sialic acid-binding immunoglobulin-like lectins are mostly bound by sialylated glycans on membrane molecules expressed in the same cell (cis-ligands). Although ligands of sialic acid-binding immunoglobulin-like lectins are not efficiently identified by conventional methods such as immunoprecipitation, in situ labeling including proximity labeling is useful in identifying both cis-ligands and the sialylated ligands expressed by other cells (trans-ligands) of sialic acid-binding immunoglobulin-like lectins. Interaction of the inhibitory sialic acid-binding immunoglobulin-like lectins with cis-ligands including both those with and without signaling function modulates the inhibitory activity of sialic acid-binding immunoglobulin-like lectins by multiple different ways. This interaction also modulates signaling function of the cis-ligands. So far, little is known about the role of the interaction between sialic acid-binding immunoglobulin-like lectins and the cis-ligands. Nonetheless, recent studies showed that the inhibitory activity of CD22 (also known as Siglec-2) is regulated by endogenous ligands, most likely cis-ligands, differentially in resting B cells and those in which B-cell antigen receptor is ligated. This differential regulation plays a role in quality control of signaling-competent B cells and also partial restoration of B-cell antigen receptor signaling in immunodeficient B cells.

Myeloid Src-family kinases are critical for neutrophil-mediated autoinflammation in gout and motheaten models

Autoinflammatory diseases include a number of monogenic systemic inflammatory diseases, as well as acquired autoinflammatory diseases such as gout. Here, we show that the myeloid Src-family kinases Hck, Fgr, and Lyn are critical for experimental models of gout, as well as for genetically determined systemic inflammation in the Ptpn6me-v/me-v (motheaten viable) mouse model. The Hck-/-Fgr-/-Lyn-/- mutation abrogated various monosodium urate (MSU) crystal-induced pro-inflammatory responses of neutrophils, and protected mice from the development of gouty arthritis. The Src-family inhibitor dasatinib abrogated MSU crystal-induced responses of human neutrophils and reduced experimental gouty arthritis in mice. The Hck-/-Fgr-/-Lyn-/- mutation also abrogated spontaneous inflammation and prolonged the survival of the Ptpn6me-v/me-v mice. Spontaneous adhesion and superoxide release of Ptpn6me-v/me-v neutrophils were also abolished by the Hck-/-Fgr-/-Lyn-/- mutation. Excessive activation of tyrosine phosphorylation pathways in myeloid cells may characterize a subset of autoinflammatory diseases.

Neutrophil-Specific Syk Expression Is Crucial for Skin Disease in Experimental Epidermolysis Bullosa Acquisita

Autoantibodies against the dermal-epidermal junction component type VII collagen (C7) trigger skin disease in the inflammatory form of epidermolysis bullosa acquisita. We have previously identified the Syk tyrosine kinase as a crucial participant in anti-C7 antibody-induced experimental epidermolysis bullosa acquisita. However, it is still unclear which cellular lineage needs to express Syk during the disease process. In this study, we show that the loss of Syk, specifically from neutrophils, results in complete protection from the anti-C7 antibody-initiated skin disease both macroscopically and microscopically. Mice with a neutrophil-specific Syk deletion had decreased neutrophil accumulation and abrogated CXCL2 and IL-1β levels in the skin upon anti-C7 treatment, whereas isolated Syk-deficient neutrophils had decreased superoxide release, cell spreading, and cytokine release on C7-anti-C7 immune complex surfaces. Entospletinib and lanraplenib, two second-generation Syk-specific inhibitors, effectively abrogated immune complex-induced responses of human neutrophils and decreased the anti-C7 antibody-initiated, neutrophil-mediated ex vivo dermal-epidermal separation in human skin samples. Taken together, these results point to a crucial role for Syk in neutrophils in the development and progression of epidermolysis bullosa acquisita and suggest Syk inhibition as a potential therapeutic strategy.

Targeting Neutrophil β2-Integrins: A Review of Relevant Resources, Tools, and Methods

Neutrophils are important innate immune cells that respond during inflammation and infection. These migratory cells utilize β₂-integrin cell surface receptors to move out of the vasculature into inflamed tissues and to perform various anti-inflammatory responses. Although critical for fighting off infection, neutrophil responses can also become dysregulated and contribute to disease pathophysiology. In order to limit neutrophil-mediated damage, investigators have focused on β₂-integrins as potential therapeutic targets, but so far these strategies have failed in clinical trials. As the field continues to move forward, a better understanding of β₂-integrin function and signaling will aid the design of future therapeutics. Here, we provide a detailed review of resources, tools, experimental methods, and in vivo models that have been and will continue to be utilized to investigate the vitally important cell surface receptors, neutrophil β₂-integrins.

New small molecules in dermatology: for the autoimmunity, inflammation and beyond

OBJECTIVE AND DESIGN

The discovery of new inflammatory pathways and the mechanism of action of inflammatory, autoimmune, genetic, and neoplastic diseases led to the development of immunologically driven drugs. We aimed to perform a narrative review regarding the rising of a new class of drugs capable of blocking important and specific intracellular signals in the maintenance of these pathologies: the small molecules.

MATERIALS/METHODS

A total of 114 scientific papers were enrolled in this narrative review.

RESULTS

We describe in detail the families of protein kinases-Janus Kinase (JAK), Src kinase, Syk tyrosine kinase, Mitogen-Activated Protein Kinase (MAPK), and Bruton Tyrosine Kinase (BTK)-their physiologic function and new drugs that block these pathways of intracellular signaling. We also detail the involved cytokines and the main metabolic and clinical implications of these new medications in the field of dermatology.

CONCLUSIONS

Despite having lower specificity compared to specific immunobiological therapies, these new drugs are effective in a wide variety of dermatological diseases, especially diseases that had few therapeutic options, such as psoriasis, psoriatic arthritis, atopic dermatitis, alopecia areata, and vitiligo.

Analysis of intracellular tyrosine phosphorylation in circulating neutrophils as a rapid assay for the in vivo effect of oral tyrosine kinase inhibitors

Tyrosine kinases are crucial signaling components of diverse biological processes and are major therapeutic targets in various malignancies and immune-mediated disorders. A critical step of development of novel tyrosine kinase inhibitors is the transition from the confirmation of the in vitro effects of drug candidates to the analysis of their in vivo efficacy. To facilitate this transition, we have developed a rapid in vivo assay for the analysis of the effect of oral tyrosine kinase inhibitors on basal tyrosine phosphorylation of circulating mouse neutrophils. The assay uses a single drop of peripheral blood without sacrificing the mice. Flow cytometry using intracellular staining by fluorescently labeled anti-phosphotyrosine antibodies revealed robust basal tyrosine phosphorylation in resting circulating neutrophils. This signal was abrogated by the use of isotype control antibodies or by pre-saturation of the anti-phosphotyrosine antibodies with soluble phosphotyrosine amino acids or tyrosine-phosphorylated peptides. Basal tyrosine phosphorylation was dramatically reduced in neutrophils of triple knockout mice lacking the Src-family tyrosine kinases Hck, Fgr, and Lyn. Neutrophil tyrosine phosphorylation was also abrogated by oral administration of the Abl/Src-family inhibitor dasatinib, a clinically used anti-leukemic agent. Detailed dose-response and kinetic studies revealed half-maximal reduction of neutrophil tyrosine phosphorylation by 2.9 mg/kg dasatinib, with maximal reduction observed 2 h after inhibitor administration. Taken together, our assay allows highly efficient analysis of the in vivo effect of orally administered tyrosine kinase inhibitors, and may be used as a suitable alternative to other existing approaches.

Exploring the mechanism of action of Hedyotis diffusa Willd on acne using network analysis

In this study, we used a network pharmacological method to explore the active ingredients of Hedyotis diffusa Willd (HDW) in the treatment of acne and elucidated the physiological mechanisms in the human body in which they are involved. We identified the active compounds of HDW that are expected to act effectively in the human body using the Traditional Chinese Medicine Systems Pharmacology database and analysis platform and extracted potential interacting proteins for each active compound using the Swiss Target Prediction platform. Next, we analyzed the potential mechanisms of action of the protein targets shared by HDW and each standard drug on acne and assessed the possibility of spontaneous occurrence of the binding between proteins and active compounds through the molecular docking process. Seven active compounds were selected according to the oral bioavailability and drug-likeness criteria of the Traditional Chinese Medicine Systems Pharmacology database and analysis platform. Subsequently, 300 protein targets were collected from the Swiss Target Prediction. Using the Search Tool for the Retrieval of Interacting Genes/Proteins database, a protein-protein interaction network was constructed by analyzing the relationship between HDW, acne, and each standard drug. By analyzing the gene ontology terms and Kyoto Encyclopedia of Genes and Genomes pathway, the "positive regulation of lipid metabolic process" was found to be the most involved pathway shared by HDW, acne, and isotretinoin. An analysis of the protein targets shared by the antibiotic agents with HDW and acne found that "cholesterol storage" in tetracycline, "icosacoid transport" in azithromycin, "steroid hydroxylase activity" in erythromycin, "positive regulation of leukocyte tethering or rolling" in clindamycin, "response to UV-A" in minocycline, "steroid 11-beta-monooxygenase activity" in doxycycline, and "neutrophil-mediated immunity" in trimethoprim were the most involved. Virtual molecular docking analysis showed that all proteins spontaneously bound to their corresponding active compounds. Our analysis suggests that HDW can, directly and indirectly, suppress sebum secretion and exert antiinflammatory effects on acne. Further, HDW may regulate free radicals and suppress apoptosis. Therefore, HDW can be used as an alternative or supplement to standard drugs for acne treatment in patients who cannot use standard treatments due to side effects.

Sialylation regulates neutrophil transepithelial migration, CD11b/CD18 activation, and intestinal mucosal inflammatory function

Polymorphonuclear neutrophils (PMNs) play a critical role in clearing invading microbes and promoting tissue repair following infection/injury. However, dysregulated PMN trafficking and associated tissue damage is pathognomonic of numerous inflammatory mucosal diseases. The final step in PMN influx into mucosal lined organs (including the lungs, kidneys, skin, and gut) involves transepithelial migration (TEpM). The β2-integrin CD11b/CD18 plays an important role in mediating PMN intestinal trafficking, with recent studies highlighting that terminal fucose and GlcNAc glycans on CD11b/CD18 can be targeted to reduce TEpM. However, the role of the most abundant terminal glycan, sialic acid (Sia), in regulating PMN epithelial influx and mucosal inflammatory function is not well understood. Here we demonstrate that inhibiting sialidase-mediated removal of α2-3-linked Sia from CD11b/CD18 inhibits PMN migration across intestinal epithelium in vitro and in vivo. Sialylation was also found to regulate critical PMN inflammatory effector functions, including degranulation and superoxide release. Finally, we demonstrate that sialidase inhibition reduces bacterial peptide-mediated CD11b/CD18 activation in PMN and blocks downstream intracellular signaling mediated by spleen tyrosine kinase (Syk) and p38 MAPK. These findings suggest that sialylated glycans on CD11b/CD18 represent potentially novel targets for ameliorating PMN-mediated tissue destruction in inflammatory mucosal diseases.

Asebogenin suppresses thrombus formation via inhibition of Syk phosphorylation

BACKGROUND AND PURPOSE

Thrombosis is a major cause of morbidity and mortality worldwide. Platelet activation by exposed collagen through glycoprotein VI (GPVI) and formation of neutrophil extracellular traps (NETs) are critical pathogenic factors for arterial and venous thrombosis. Both events are regulated by spleen tyrosine kinase (Syk)-mediated signalling events. Asebogenin is a dihydrochalcone whose pharmacological effects remain largely unknown. This study aims to investigate the antithrombotic effects of asebogenin and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Platelet aggregation was assessed using an aggregometer. Platelet P-selectin exposure, integrin activation and calcium mobilization were determined by flow cytometry. NETs formation was assessed by SYTOX Green staining and immunohistochemistry. Quantitative phosphoproteomics, microscale thermophoresis, in vitro kinase assay and molecular docking combined with dynamics simulation were performed to characterize the targets of asebogenin. The in vivo effects of asebogenin on arterial thrombosis were investigated using FeCl₃ -induced and laser-induced injury models, whereas those of venous thrombosis were induced by stenosis of the inferior vena cava.

KEY RESULTS

Asebogenin inhibited a series of GPVI-induced platelet responses and suppressed NETs formation induced by proinflammatory stimuli. Mechanistically, asebogenin directly interfered with the phosphorylation of Syk at Tyr525/526, which is important for its activation. Further, asebogenin suppressed arterial thrombosis demonstrated by decreased platelet accumulation and fibrin generation and attenuated venous thrombosis determined by reduced neutrophil accumulation and NETs formation, without increasing bleeding risk.

CONCLUSION AND IMPLICATIONS

Asebogenin exhibits potent antithrombotic effects by targeting Syk and is a potential lead compound for the development of efficient and safe antithrombotic agents.

Neutrophils inhibit γδ T cell functions in the imiquimod-induced mouse model of psoriasis

Background

Psoriasis is a chronic skin disease associated with deregulated interplays between immune cells and keratinocytes. Neutrophil accumulation in the skin is a histological feature that characterizes psoriasis. However, the role of neutrophils in psoriasis onset and development remains poorly understood.

Methods

In this study, we utilized the model of psoriasiform dermatitis, caused by the repeated topical application of an imiquimod containing cream, in neutrophil-depleted mice or in mice carrying impairment in neutrophil functions, including p47phox -/- mice (lacking a cytosolic subunit of the phagocyte nicotinamide adenine dinucleotide phosphate - NADPH - oxidase) and Sykfl/fl MRP8-cre+ mice (carrying the specific deletion of the Syk kinase in neutrophils only), to elucidate the specific contribution of neutrophils to psoriasis development.

Results

By analyzing disease development/progression in neutrophil-depleted mice, we now report that neutrophils act as negative modulators of disease propagation and exacerbation by inhibiting gammadelta T cell effector functions via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-mediated reactive oxygen species (ROS) production. We also report that Syk functions as a crucial molecule in determining the outcome of neutrophil and γδ T cell interactions. Accordingly, we uncover that a selective impairment of Syk-dependent signaling in neutrophils is sufficient to reproduce the enhancement of skin inflammation and γδ T cell infiltration observed in neutrophil-depleted mice.

Conclusions

Overall, our findings add new insights into the specific contribution of neutrophils to disease progression in the IMQ-induced mouse model of psoriasis, namely as negative regulatory cells.

TMT-Based Quantitative Proteomic Analysis Reveals Downregulation of ITGAL and Syk by the Effects of Cycloastragenol in OVA-Induced Asthmatic Mice

BACKGROUND

Cycloastragenol (CAG) has been reported to alleviate airway inflammation in ovalbumin- (OVA-) induced asthmatic mice. However, its specific mechanisms remain unclear.

OBJECTIVE

This study is aimed at investigating the effects of CAG on asthma, comparing its efficacy with dexamethasone (DEX), and elucidating the mechanism of CAG's regulation.

METHODS

The asthma mouse model was induced by OVA. CAG at the optimal dose of 125 mg/kg was given every day from day 0 for 20-day prevention or from day 14 for a 7-day treatment. We observed the preventive and therapeutic effects of CAG in asthmatic mice by evaluating the airway inflammation, AHR, and mucus secretion. Lung proteins were used for TMT-based quantitative proteomic analysis to enunciate its regulatory mechanisms.

RESULTS

The early administration of 125 mg/kg CAG before asthma happened prevented asthmatic mice from AHR, airway inflammation, and mucus hypersecretion, returning to nearly the original baseline. Alternatively, the administration of CAG during asthma also had the same therapeutic effects as DEX. The proteomic analysis revealed that the therapeutical effects of CAG were associated with 248 differentially expressed proteins and 3 enriched KEGG pathways. We then focused on 3 differentially expressed proteins (ITGAL, Syk, and Vav1) and demonstrated that CAG treatment downregulated ITGAL, Syk, and Vav1 by quantitative real-time PCR, western blot analysis, and immunohistochemical staining.

CONCLUSION

These findings suggest that CAG exerts preventive and protective effects on asthma by inhibiting ITGAL, Syk, and the downstream target Vav1.

The Role of Reactive Species on Innate Immunity

This review examines the role of reactive species RS (of oxygen ROS, nitrogen RNS and halogen RHS) on innate immunity. The importance of these species in innate immunity was first recognized in phagocytes that underwent a “respiratory burst” after activation. The anion superoxide ^•O₂⁻ and hydrogen peroxide H₂O₂ are detrimental to the microbial population. NADPH oxidase NOx, as an ^•O₂⁻ producer is essential for microbial destruction, and patients lacking this functional oxidase are more susceptible to microbial infections. Reactive nitrogen species RNS (the most important are nitric oxide radical -^•NO, peroxynitrite ONOO^— and its derivatives), are also harmful to microorganisms, including bacteria, viruses, and parasites. Hypochlorous acid HOCl and hypothiocyanous acid HOSCN synthesized through the enzyme myeloperoxidase MPO, which catalyzes the reaction between H₂O₂ and Cl⁻ or SCN⁻, are important inorganic bactericidal molecules, effective against a wide range of microbes. This review also discusses the role of antimicrobial peptides AMPs and their induction of ROS. In summary, reactive species RS are the heart of the innate immune system, and they are necessary for microbial lysis in infections that can affect mammals throughout their lives.

β2-Integrin Adhesive Bond Tension under Shear Stress Modulates Cytosolic Calcium Flux and Neutrophil Inflammatory Response

On arrested neutrophils a focal adhesive cluster of ~200 high affinity (HA) β₂-integrin bonds under tension is sufficient to trigger Ca²⁺ flux that signals an increase in activation in direct proportion to increments in shear stress. We reasoned that a threshold tension acting on individual β₂-integrin bonds provides a mechanical means of transducing the magnitude of fluid drag force into signals that enhance the efficiency of neutrophil recruitment and effector function. Tension gauge tethers (TGT) are a duplex of DNA nucleotides that rupture at a precise shear force, which increases with the extent of nucleotide overlap, ranging from a tolerance of 54pN to 12pN. TGT annealed to a substrate captures neutrophils via allosteric antibodies that stabilize LFA-1 in a high- or low-affinity conformation. Neutrophils sheared on TGT substrates were recorded in real time to form HA β₂-integrin bonds and flux cytosolic Ca²⁺, which elicited shape change and downstream production of reactive oxygen species. A threshold force of 33pN triggered consolidation of HA β₂-integrin bonds and triggered membrane influx of Ca²⁺, whereas an optimum tension of 54pN efficiently transduced activation at a level equivalent to chemotactic stimulation on ICAM-1. We conclude that neutrophils sense the level of fluid drag transduced through individual β₂-integrin bonds, providing an intrinsic means to modulate inflammatory response in the microcirculation.

Soluble uric acid inhibits β2 integrin-mediated neutrophil recruitment in innate immunity

Neutrophils are key players during host defense and sterile inflammation. Neutrophil dysfunction is a characteristic feature of the acquired immunodeficiency during kidney disease. We speculated that the impaired renal clearance of the intrinsic purine metabolite soluble uric acid (sUA) may account for neutrophil dysfunction. Indeed, hyperuricemia (HU, serum UA of 9-12 mg/dL) related or unrelated to kidney dysfunction significantly diminished neutrophil adhesion and extravasation in mice with crystal- and coronavirus-related sterile inflammation using intravital microscopy and an air pouch model. This impaired neutrophil recruitment was partially reversible by depleting UA with rasburicase. We validated these findings in vitro using either neutrophils or serum from patients with kidney dysfunction-related HU with or without UA depletion, which partially normalized the defective migration of neutrophils. Mechanistically, sUA impaired β2 integrin activity and internalization/recycling by regulating intracellular pH and cytoskeletal dynamics, physiological processes that are known to alter the migratory and phagocytic capability of neutrophils. This effect was fully reversible by blocking intracellular uptake of sUA via urate transporters. In contrast, sUA had no effect on neutrophil extracellular trap formation in neutrophils from healthy subjects or patients with kidney dysfunction. Our results identify an unexpected immunoregulatory role of the intrinsic purine metabolite sUA, which contrasts the well-known immunostimulatory effects of crystalline UA. Specifically targeting UA may help to overcome certain forms of immunodeficiency, for example in kidney dysfunction, but may enhance sterile forms of inflammation.

Neutrophil Protein Kinase R Mediates Endothelial Adhesion and Migration by the Promotion of Neutrophil Actin Polymerization

Neutrophils protect against bacterial and fungal infections, but tight regulation of cell activation is essential for avoiding tissue damage in autoimmune disorders. Protein kinase R (PKR) is a serine/threonine kinase originally characterized by its role in the defense mechanisms against viral infection. Although PKR is involved in the signaling pathways of neurodegenerative diseases and metabolic disorders, its function in neutrophils is not well delineated. In this study, we demonstrate that human neutrophil PKR mediates adhesion to endothelial cells under physiological flow conditions but does not mediate rolling on those cells. Also, neutrophil PKR activation contributes to migration toward chemoattractants. Mechanistically, neutrophil PKR mediates the cell spreading and binding to ICAM-1 in static condition. Moreover, Ab microarray reveals that calcium/calmodulin-dependent protein kinase II is phosphorylated downstream of PKR and affects actin polymerization that is a cytoskeleton rearrangement indispensable for neutrophil migration induced by fMLF. In vivo, neutrophil recruitment into the dorsal air pouch of mice is reduced by PKR inhibitor treatment. Also, in mice with nephrotoxic serum nephritis, the compound treatment suppresses neutrophil accumulation in kidney glomerulus and subsequent development of albuminuria. Thus, in vascular inflammation, neutrophil PKR plays a critical role in the recruitment process, including endothelial adhesion and migration via leukocyte actin polymerization.

TLR Signaling Rescues Fungicidal Activity in Syk-Deficient Neutrophils

An impaired neutrophil response to pathogenic fungi puts patients at risk for fungal infections with a high risk of morbidity and mortality. Acquired neutrophil dysfunction in the setting of iatrogenic immune modulators can include the inhibition of critical kinases such as spleen tyrosine kinase (Syk). In this study, we used an established system of conditionally immortalized mouse neutrophil progenitors to investigate the ability to augment Syk-deficient neutrophil function against Candida albicans with TLR agonist signaling. LPS, a known immunomodulatory molecule derived from Gram-negative bacteria, was capable of rescuing effector functions of Syk-deficient neutrophils, which are known to have poor fungicidal activity against Candida species. LPS priming of Syk-deficient mouse neutrophils demonstrates partial rescue of fungicidal activity, including phagocytosis, degranulation, and neutrophil swarming, but not reactive oxygen species production against C. albicans, in part due to c-Fos activation. Similarly, LPS priming of human neutrophils rescues fungicidal activity in the presence of pharmacologic inhibition of Syk and Bruton's tyrosine kinase (Btk), both critical kinases in the innate immune response to fungi. In vivo, neutropenic mice were reconstituted with wild-type or Syk-deficient neutrophils and challenged i.p. with C. albicans. In this model, LPS improved wild-type neutrophil homing to the fungal challenge, although Syk-deficient neutrophils did not persist in vivo, speaking to its crucial role on in vivo persistence. Taken together, we identify TLR signaling as an alternate activation pathway capable of partially restoring neutrophil effector function against Candida in a Syk-independent manner.

Albumin Nanoparticle Endocytosing Subset of Neutrophils for Precision Therapeutic Targeting of Inflammatory Tissue Injury

The complex involvement of neutrophils in inflammatory diseases makes them intriguing but challenging targets for therapeutic intervention. Here, we tested the hypothesis that varying endocytosis capacities would delineate functionally distinct neutrophil subpopulations that could be specifically targeted for therapeutic purposes. By using uniformly sized (∼120 nm in diameter) albumin nanoparticles (ANP) to characterize mouse neutrophils in vivo, we found two subsets of neutrophils, one that readily endocytosed ANP (ANPhigh neutrophils) and another that failed to endocytose ANP (ANPlow population). These ANPhigh and ANPlow subsets existed side by side simultaneously in bone marrow, peripheral blood, spleen, and lungs, both under basal conditions and after inflammatory challenge. Human peripheral blood neutrophils showed a similar duality. ANPhigh and ANPlow neutrophils had distinct cell surface marker expression and transcriptomic profiles, both in naive mice and in mice after endotoxemic challenge. ANPhigh and ANPlow neutrophils were functionally distinct in their capacities to kill bacteria and to produce inflammatory mediators. ANPhigh neutrophils produced inordinate amounts of reactive oxygen species and inflammatory chemokines and cytokines. Targeting this subset with ANP loaded with the drug piceatannol, a spleen tyrosine kinase (Syk) inhibitor, mitigated the effects of polymicrobial sepsis by reducing tissue inflammation while fully preserving neutrophilic host-defense function.

Decoding the signaling profile of hematopoietic progenitor kinase 1 (HPK1) in innate immunity: a proteomic approach

Signaling via β₂ integrins (CD11/CD18) as well as T and B cell receptors involves similar pathways. However, the activation of the same signaling molecule can result in opposing effects. One such example is the hematopoietic progenitor kinase 1 (HPK1), which negatively regulates T and B cell activation but enforces neutrophil adhesion via β₂ integrins. This difference may be defined by specific HPK1 interacting networks in different leukocyte subsets which have already been described in the adaptive immune system. Here, we set out to identify interacting proteins of HPK1 in neutrophil-like differentiated HL-60 cells exposed to immobilized fibrinogen and left non-activated or Mn²⁺ -activated to allow β₂ integrin-dependent adhesion. Co-immunoprecipitation experiments followed by mass spectrometry led to the identification of 115 HPK1-interacting proteins. 58 proteins were found only in non-activated cells and 39 proteins only in Mn²⁺ -activated adherent cells. From these results we decoded a pre-existing signaling cluster of HPK1 in non-activated cells encompassing proteins essential for β₂ integrin-mediated signaling during neutrophil trafficking, namely DNAX-activation protein 12 (DAP12), spleen tyrosine kinase (Syk) and Rac1. Thus, our study provides novel insights into the complex architecture of the signaling processes during neutrophil activation and the complex signaling profile of HPK1 in leukocytes. This article is protected by copyright. All rights reserved.

Spleen tyrosine kinase facilitates neutrophil activation and worsens long-term neurologic deficits after spinal cord injury

Background

Spinal cord injury elicits widespread inflammation that can exacerbate long-term neurologic deficits. Neutrophils are the most abundant immune cell type to invade the spinal cord in the early acute phase after injury, however, their role in secondary pathogenesis and functional recovery remains unclear. We have previously shown that neutrophil functional responses during inflammation are augmented by spleen tyrosine kinase, Syk, a prominent intracellular signaling enzyme. In this study, we evaluated the contribution of Syk towards neutrophil function and long-term neurologic deficits after spinal cord injury.

Methods

Contusive spinal cord injury was performed at thoracic vertebra level 9 in mice with conditional deletion of Syk in neutrophils (Syk^f/fMRP8-Cre). Hindlimb locomotor recovery was evaluated using an open-field test for 35 days following spinal cord injury. Long-term white matter sparing was assessed using eriochrome cyanide staining. Blood-spinal cord barrier disruption was evaluated by immunoblotting. Neutrophil infiltration, activation, effector functions, and cell death were determined by flow cytometry. Cytokine and chemokine expression in neutrophils was assessed using a gene array.

Results

Syk deficiency in neutrophils improved long-term functional recovery after spinal cord injury, but did not promote long-term white matter sparing. Neutrophil activation, cytokine expression, and cell death in the acutely injured spinal cord were attenuated by the genetic loss of Syk while neutrophil infiltration and effector functions were not affected. Acute blood-spinal cord barrier disruption was also unaffected by Syk deficiency in neutrophils.

Conclusions

Syk facilitates specific neutrophil functional responses to spinal cord injury including activation, cytokine expression, and cell death. Long-term neurologic deficits are exacerbated by Syk signaling in neutrophils independent of acute blood-spinal cord barrier disruption and long-term white matter sparing. These findings implicate Syk in pathogenic neutrophil activities that worsen long-term functional recovery after spinal cord injury.

The Roles of Neutrophils in Cytokine Storms

A cytokine storm is an abnormal discharge of soluble mediators following an inappropriate inflammatory response that leads to immunopathological events. Cytokine storms can occur after severe infections as well as in non-infectious situations where inflammatory cytokine responses are initiated, then exaggerated, but fail to return to homeostasis. Neutrophils, macrophages, mast cells, and natural killer cells are among the innate leukocytes that contribute to the pathogenesis of cytokine storms. Neutrophils participate as mediators of inflammation and have roles in promoting homeostatic conditions following pathological inflammation. This review highlights the advances in understanding the mechanisms governing neutrophilic inflammation against viral and bacterial pathogens, in cancers, and in autoimmune diseases, and how neutrophils could influence the development of cytokine storm syndromes. Evidence for the destructive potential of neutrophils in their capacity to contribute to the onset of cytokine storm syndromes is presented across a multitude of clinical scenarios. Further, a variety of potential therapeutic strategies that target neutrophils are discussed in the context of suppressing multiple inflammatory conditions.

Phospholipase Cγ2 is Essential for Experimental Models of Epidermolysis Bullosa Acquisita

Phospholipase Cγ2 (PLCγ2) mediates tyrosine kinase‒coupled receptor signaling in various hematopoietic lineages. Although PLCγ2 has been implicated in certain human and mouse inflammatory disorders, its contribution to autoimmune and inflammatory skin diseases is poorly understood. In this study, we tested the role of PLCγ2 in a mouse model of epidermolysis bullosa acquisita triggered by antibodies against type VII collagen (C7), a component of the dermo-epidermal junction. PLCγ2-deficient (Plcg2^-/-) mice and bone marrow chimeras with a Plcg2^-/- hematopoietic system were completely protected from signs of anti-C7-induced skin disease, including skin erosions, dermal‒epidermal separation, and inflammation, despite normal circulating levels and skin deposition of anti-C7 antibodies. PLCγ2 was required for the tissue infiltration of neutrophils, eosinophils, and monocytes/macrophages as well as for the accumulation of proinflammatory mediators (including IL-1β, MIP-2, and LTB₄) and reactive oxygen species. Mechanistic experiments revealed a role for PLCγ2 in the release of proinflammatory mediators and reactive oxygen species but not in the intrinsic migratory capacity of leukocytes. The phospholipase C inhibitor U73122 inhibited dermal-epidermal separation of human skin sections incubated with human neutrophils in the presence of anti-C7 antibodies. Taken together, our results suggest a critical role for PLCγ2 in the pathogenesis of the inflammatory form of epidermolysis bullosa acquisita.

Siglec-8 Signals Through a Non-Canonical Pathway to Cause Human Eosinophil Death In Vitro

Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is a glycan-binding receptor bearing immunoreceptor tyrosine-based inhibitory and switch motifs (ITIM and ITSM, respectively) that is selectively expressed on eosinophils, mast cells, and, to a lesser extent, basophils. Previous work has shown that engagement of Siglec-8 on IL-5-primed eosinophils causes cell death via CD11b/CD18 integrin-mediated adhesion and NADPH oxidase activity and identified signaling molecules linking adhesion, reactive oxygen species (ROS) production, and cell death. However, the proximal signaling cascade activated directly by Siglec-8 engagement has remained elusive. Most members of the Siglec family possess similar cytoplasmic signaling motifs and recruit the protein tyrosine phosphatases SHP-1/2, consistent with ITIM-mediated signaling, to dampen cellular activation. However, the dependence of Siglec-8 function in eosinophils on these phosphatases has not been studied. Using Siglec-8 antibody engagement and pharmacological inhibition in conjunction with assays to measure cell-surface upregulation and conformational activation of CD11b integrin, ROS production, and cell death, we sought to identify molecules involved in Siglec-8 signaling and determine the stage of the process in which each molecule plays a role. We demonstrate here that the enzymatic activities of Src family kinases (SFKs), Syk, SHIP1, PAK1, MEK1, ERK1/2, PLC, PKC, acid sphingomyelinase/ceramidase, and Btk are all necessary for Siglec-8-induced eosinophil cell death, with no apparent role for SHP-1/2, SHIP2, or c-Raf. While most of these signaling molecules are necessary for Siglec-8-induced upregulation of CD11b integrin at the eosinophil cell surface, Btk is phosphorylated and activated later in the signaling cascade and is instead necessary for CD11b activation. In contrast, SFKs and ERK1/2 are phosphorylated far earlier in the process, consistent with their role in augmenting cell-surface levels of CD11b. In addition, pretreatment of eosinophils with latrunculin B or jasplakinolide revealed that actin filament disassembly is necessary and sufficient for surface CD11b integrin upregulation and that actin polymerization is necessary for downstream ROS production. These results show that Siglec-8 signals through an unanticipated set of signaling molecules in IL-5-primed eosinophils to induce cell death and challenges the expectation that ITIM-bearing Siglecs signal through inhibitory pathways involving protein tyrosine phosphatases to achieve their downstream functions.

Signaling through Syk or CARD9 Mediates Species-Specific Anti-Candida Protection in Bone Marrow Chimeric Mice

The spleen tyrosine kinase (Syk) and the downstream adaptor protein CARD9 are crucial signaling molecules in antimicrobial immunity. Candida parapsilosis is an emerging fungal pathogen with a high incidence in neonates, while Candida albicans is the most common agent of candidiasis. While signaling through Syk/CARD9 promotes protective host mechanisms in response to C. albicans, its function in immunity against C. parapsilosis remains unclear. Here, we generated Syk-/- and CARD9-/- bone marrow chimeric mice to study the role of Syk/CARD9 signaling in immune responses to C. parapsilosis compared to C. albicans. We demonstrate various functions of this pathway (e.g., phagocytosis, phagosome acidification, and killing) in Candida-challenged, bone marrow-derived macrophages with differential involvement of Syk and CARD9 along with species-specific differences in cytokine production. We report that Syk-/- or CARD9-/- chimeras rapidly display high susceptibility to C. albicans, while C. parapsilosis infection exacerbates over a prolonged period in these animals. Thus, our results establish that Syk and CARD9 contribute to systemic resistance to C. parapsilosis and C. albicans differently. Additionally, we confirm prior studies but also detail new insights into the fundamental roles of both proteins in immunity against C. albicans. Our data further suggest that Syk has a more prominent influence on anti-Candida immunity than CARD9. Therefore, this study reinforces the Syk/CARD9 pathway as a potential target for anti-Candida immune therapy. IMPORTANCE While C. albicans remains the most clinically significant Candida species, C. parapsilosis is an emerging pathogen with increased affinity to neonates. Syk/CARD9 signaling is crucial in immunity to C. albicans, but its role in in vivo responses to other pathogenic Candida species is largely unexplored. We used mice with hematopoietic systems deficient in Syk or CARD9 to comparatively study the function of these proteins in anti-Candida immunity. We demonstrate that Syk/CARD9 signaling has a protective role against C. parapsilosis differently than against C. albicans. Thus, this study is the first to reveal that Syk can exert immune responses during systemic Candida infections species specifically. Additionally, Syk-dependent immunity to a nonalbicans Candida species in an in vivo murine model has not been reported previously. We highlight that the contribution of Syk and CARD9 to fungal infections are not identical and underline this pathway as a promising immune-therapeutic target to fight Candida infections.

Neutrophils require SKAP2 for reactive oxygen species production following C-type lectin and Candida stimulation

Signaling cascades converting the recognition of pathogens to efficient inflammatory responses by neutrophils are critical for host survival. SKAP2, an adaptor protein, is required for reactive oxygen species (ROS) generation following neutrophil stimulation by integrins, formyl peptide receptors, and for host defense against the Gram-negative bacterial pathogens, Klebsiella pneumoniae and Yersinia pseudotuberculosis. Using neutrophils from murine HoxB8-immortalized progenitors, we show that SKAP2 in neutrophils is crucial for maximal ROS response to purified C-type lectin receptor agonists and to the fungal pathogens, Candida glabrata and Candida albicans, and for robust killing of C. glabrata. Inside-out signaling to integrin and Syk phosphorylation occurred independently of SKAP2 after Candida infection. However, Pyk2, ERK1/2, and p38 phosphorylation were significantly reduced after infection with C. glabrata and K. pneumoniae in Skap2-/- neutrophils. These data demonstrate the importance of SKAP2 in ROS generation and host defense beyond antibacterial immunity to include CLRs and Candida species.

Neutrophil-specific deletion of Syk results in recruitment-independent stabilization of the barrier and a long-term improvement in cognitive function after traumatic injury to the developing brain

While traumatic brain injury (TBI) is the leading cause of death and disability in children, we have yet to identify those pathogenic events that determine the extent of recovery. Neutrophils are best known as "first responders" to sites of infection and trauma where they become fully activated, killing pathogens via proteases that are released during degranulation. However, this activational state may generate substantial toxicity in the young brain after TBI that is partially due to developmentally regulated inadequate antioxidant reserves. Neutrophil degranulation is triggered via a downstream signaling pathway that is dependent on spleen tyrosine kinase (Syk). To test the hypothesis that the activational state of neutrophils is a determinant of early pathogenesis and long-term recovery, we compared young, brain-injured conditional knockouts of Syk (syk^f/fMRP8-cre⁺) to congenic littermates (syk^f/f). Based upon flow cytometry, there was an extended recruitment of distinct leukocyte subsets, including Ly6G⁺/Ly6C^- and Ly6G⁺/Ly6C^int, over the first several weeks post-injury which was similar between genotypes. Subsequent assessment of the acutely injured brain revealed a reduction in blood-brain barrier disruption to both high and low molecular weight dextrans and reactive oxygen species in syk^f/fMRP8-cre⁺ mice compared to congenic littermates, and this was associated with greater preservation of claudin 5 and neuronal integrity, as determined by Western blot analyses. At adulthood, motor learning was less affected in brain-injured syk^f/fMRP8-cre⁺ mice as compared to syk^f/f mice. Performance in the Morris Water Maze revealed a robust improvement in hippocampal-dependent acquisition and short and long-term spatial memory retention in syk^f/fMRP8-cre⁺ mice. Subsequent analyses of swim path lengths during hidden platform training and probe trials showed greater thigmotaxis in brain-injured syk^f/f mice than sham syk^f/f mice and injured syk^f/fMRP8-cre⁺ mice. Our results establish the first mechanistic link between the activation state of neutrophils and long-term functional recovery after traumatic injury to the developing brain. These results also highlight Syk kinase as a novel therapeutic target that could be further developed for the brain-injured child.

The influence of extracellular tissue on neutrophil function and its possible linkage to inflammatory diseases

BACKGROUND

Migration, production of reactive oxygen species (ROS), release of myeloperoxidase (MPO), and NETosis are functional immunological reactions of elementary importance for polymorphonuclear neutrophils (PMN). Unregulated inflammatory response of PMN within tissues plays a key role in the pathophysiology of several diseases. However, little is known about the behavior of PMN after migration through blood vessel walls. Therefore, we investigated the influence of the extracellular matrix (ECM) on PMN function.

MATERIALS AND METHODS

We established an in vitro chemotaxis model of type I and III collagen, fibrin, and herbal agarose tissues using µ-slide chemotaxis devices and N-formylmethionine-leucyl-phenylalanine (fMLP). PMN within the matrices were assessed with a fluorescent time-lapse microscope for live-cell imaging.

RESULTS

PMN function was obviously influenced by the ECM. Type III collagen had an inhibitory effect on PMN migration regarding track length, direction, and targeting. Type III collagen also had an accelerating effect on neutrophil ROS production. Agarose had an inhibitory effect on MPO release and fibrin a retarding effect on NETosis.

CONCLUSION

Because of the high abundance of type III collagen in lung and skin matrices, the interaction of PMN with the respective matrix could be an important mechanism in the pathophysiology of acute respiratory distress syndrome and pyoderma gangrenosum.

The 5-Phosphatase SHIP2 Promotes Neutrophil Chemotaxis and Recruitment

Neutrophils, the most abundant circulating leukocytes in humans have key roles in host defense and in the inflammatory response. Agonist-activated phosphoinositide 3-kinases (PI3Ks) are important regulators of many facets of neutrophil biology. PIP3 is subject to dephosphorylation by several 5’ phosphatases, including SHIP family phosphatases, which convert the PI3K product and lipid second messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3) into PI(3,4)P2, a lipid second messenger in its own right. In addition to the leukocyte restricted SHIP1, neutrophils express the ubiquitous SHIP2. This study analyzed mice and isolated neutrophils carrying a catalytically inactive SHIP2, identifying an important regulatory function in neutrophil chemotaxis and directionality in vitro and in neutrophil recruitment to sites of sterile inflammation in vivo, in the absence of major defects of any other neutrophil functions analyzed, including, phagocytosis and the formation of reactive oxygen species. Mechanistically, this is explained by a subtle effect on global 3-phosphorylated phosphoinositide species. This work identifies a non-redundant role for the hitherto overlooked SHIP2 in the regulation of neutrophils, and specifically, neutrophil chemotaxis/trafficking. It completes an emerging wider understanding of the complexity of PI3K signaling in the neutrophil, and the roles played by individual kinases and phosphatases within.

Neutrophil and Eosinophil DNA Extracellular Trap Formation: Lessons From Pathogenic Fungi

Fungal infections represent a worldwide health problem. Fungal pathogens are responsible for a variety of conditions, including superficial diseases, allergic pathologies and potentially lethal invasive infections. Neutrophils and eosinophils have been implicated as effector cells in several pathologies. Neutrophils are major effector cells involved in the control of fungal infections and exhibit a plethora of antifungal mechanisms, such as phagocytosis, reactive oxygen species production, degranulation, extracellular vesicle formation, and DNA extracellular trap (ET) release. Eosinophils are polymorphonuclear cells classically implicated as effector cells in the pathogenesis of allergic diseases and helminthic infections, although their roles as immunomodulatory players in both innate and adaptive immunity are currently recognized. Eosinophils are also endowed with antifungal activities and are abundantly found in allergic conditions associated with fungal colonization and sensitization. Neutrophils and eosinophils have been demonstrated to release their nuclear and mitochondrial DNA in response to many pathogens and pro-inflammatory stimuli. ETs have been implicated in the killing and control of many pathogens, as well as in promoting inflammation and tissue damage. The formation of ETs by neutrophils and eosinophils has been described in response to pathogenic fungi. Here, we provide an overview of the mechanisms involved in the release of neutrophil and eosinophil ETs in response to fungal pathogens. General implications for understanding the formation of ETs and the roles of ETs in fungal infections are discussed.

Neutrophil Phospholipase Cγ2 Drives Autoantibody-Induced Arthritis Through the Generation of the Inflammatory Microenvironment

OBJECTIVE

Gain-of-function mutations and genome-wide association studies have linked phospholipase Cγ2 (PLCγ2) to various inflammatory diseases, including arthritis in humans and mice. PLCγ2-deficient (Plcg2^-/- ) mice are also protected against experimental arthritis. This study was undertaken to test how PLCγ2 triggers autoantibody-induced arthritis in mice.

METHODS

PLCγ2 was deleted from various mouse cellular lineages. Deletion efficacy and specificity were tested by immunoblotting and intracellular flow cytometry. Autoantibody-induced arthritis was triggered by K/BxN serum transfer. The role of neutrophil PLCγ2 was further investigated by analysis of the inflammatory exudate, competitive in vivo migration assays, and in vitro functional studies.

RESULTS

PLCγ2 deficiency in the entire hematopoietic compartment completely blocked autoantibody-induced arthritis. Arthritis development was abrogated by deletion of PLCγ2 from myeloid cells or neutrophils but not from mast cells or platelets. Neutrophil infiltration was reduced in neutrophil-specific PLCγ2-deficient (Plcg2^{Δ PMN} ) mice. However, this was not due to an intrinsic migration defect since Plcg2^{Δ PMN} neutrophils accumulated normally when wild-type cells were also present in mixed bone marrow chimeras. Instead, the Plcg2^{Δ PMN} mutation blocked the accumulation of interleukin-1β, macrophage inflammatory protein 2 (MIP-2), and leukotriene B₄ (LTB₄ ) in synovial tissues and reduced the secondary infiltration of macrophages. These findings were supported by in vitro studies showing normal chemotactic migration but defective immune complex-induced respiratory burst and MIP-2 or LTB₄ release in PLCγ2-deficient neutrophils.

CONCLUSION

Neutrophil PLCγ2 is critical for arthritis development, supposedly through the generation of the inflammatory microenvironment. PLCγ2-expressing neutrophils exert complex indirect effects on other inflammatory cells. PLCγ2-targeted therapies may provide particular benefit in inflammatory diseases with a major neutrophil component.

β2 Integrin Signaling Cascade in Neutrophils: More Than a Single Function

Neutrophils are the most prevalent leukocytes in the human body. They have a pivotal role in the innate immune response against invading bacterial and fungal pathogens, while recent emerging evidence also demonstrates their role in cancer progression and anti-tumor responses. The efficient execution of many neutrophil effector responses requires the presence of β2 integrins, in particular CD11a/CD18 or CD11b/CD18 heterodimers. Although extensively studied at the molecular level, the exact signaling cascades downstream of β2 integrins still remain to be fully elucidated. In this review, we focus mainly on inside-out and outside-in signaling of these two β2 integrin members expressed on neutrophils and describe differences between various neutrophil stimuli with respect to integrin activation, integrin ligand binding, and the pertinent differences between mouse and human studies. Last, we discuss how integrin signaling studies could be used to explore the therapeutic potential of targeting β2 integrins and the intracellular signaling cascade in neutrophils in several, among other, inflammatory conditions in which neutrophil activity should be dampened to mitigate disease.

Structural and Signaling Events Driving Aspergillus fumigatus-Induced Human Eosinophil Extracellular Trap Release

Eosinophils are granulocytes classically involved in allergic diseases and in the host immune responses to helminths, fungi, bacteria and viruses. The release of extracellular DNA traps by leukocytes is an important mechanism of the innate immune response to pathogens in various infectious conditions, including fungal infections. Aspergillus fumigatus is an opportunistic fungus responsible for allergic bronchopulmonary aspergillosis (ABPA), a pulmonary disease marked by prominent eosinophilic inflammation. Previously, we demonstrated that isolated human eosinophils release extracellular DNA traps (eosinophil extracellular traps; EETs) when stimulated by A. fumigatus in vitro. This release occurs through a lytic non-oxidative mechanism that involves CD11b and Syk tyrosine kinase. In this work, we unraveled different intracellular mechanisms that drive the release of extracellular DNA traps by A. fumigatus-stimulated eosinophils. Ultrastructurally, we originally observed that A. fumigatus-stimulated eosinophils present typical signs of extracellular DNA trap cell death (ETosis) with the nuclei losing both their shape (delobulation) and the euchromatin/heterochromatin distinction, followed by rupture of the nuclear envelope and EETs release. We also found that by targeting class I PI3K, and more specifically PI3Kδ, the release of extracellular DNA traps induced by A. fumigatus is inhibited. We also demonstrated that A. fumigatus-induced EETs release depends on the Src family, Akt, calcium and p38 MAPK signaling pathways in a process in which fungal viability is dispensable. Interestingly, we showed that A. fumigatus-induced EETs release occurs in a mechanism independent of PAD4 histone citrullination. These findings may contribute to a better understanding of the mechanisms that underlie EETs release in response to A. fumigatus, which may lead to better knowledge of ABPA pathophysiology and treatment.

Pseudomonas aeruginosa induces p38MAP kinase-dependent IL-6 and CXCL8 release from bronchial epithelial cells via a Syk kinase pathway

Pseudomonas aeruginosa (Pa) infection is a major cause of airway inflammation in immunocompromised and cystic fibrosis (CF) patients. Mitogen-activated protein (MAP) and tyrosine kinases are integral to inflammatory responses and are therefore potential targets for novel anti-inflammatory therapies. We have determined the involvement of specific kinases in Pa-induced inflammation. The effects of kinase inhibitors against p38MAPK, MEK 1/2, JNK 1/2, Syk or c-Src, a combination of a p38MAPK with Syk inhibitor, or a novel narrow spectrum kinase inhibitor (NSKI), were evaluated against the release of the proinflammatory cytokine/chemokine, IL-6 and CXCL8 from BEAS-2B and CFBE41o- epithelial cells by Pa. Effects of a Syk inhibitor against phosphorylation of the MAPKs were also evaluated. IL-6 and CXCL8 release by Pa were significantly inhibited by p38MAPK and Syk inhibitors (p<0.05). Phosphorylation of HSP27, but not ERK or JNK, was significantly inhibited by Syk kinase inhibition. A combination of p38MAPK and Syk inhibitors showed synergy against IL-6 and CXCL8 induction and an NSKI completely inhibited IL-6 and CXCL8 at low concentrations. Pa-induced inflammation is dependent on p38MAPK primarily, and Syk partially, which is upstream of p38MAPK. The NSKI suggests that inhibiting specific combinations of kinases is a potent potential therapy for Pa-induced inflammation.

Identification of Inhibitors of Integrin Cytoplasmic Domain Interactions With Syk

Leukocyte inflammatory responses require integrin cell-adhesion molecule signaling through spleen tyrosine kinase (Syk), a non-receptor kinase that binds directly to integrin β-chain cytoplasmic domains. Here, we developed a high-throughput screen to identify small molecule inhibitors of the Syk-integrin cytoplasmic domain interactions. Screening small molecule compound libraries identified the β-lactam antibiotics cefsulodin and ceftazidime, which inhibited integrin β-subunit cytoplasmic domain binding to the tandem SH2 domains of Syk (IC₅₀ range, 1.02–4.9 µM). Modeling suggested antagonist binding to Syk outside the pITAM binding site. Ceftazidime inhibited integrin signaling via Syk, including inhibition of adhesion-dependent upregulation of interleukin-1β and monocyte chemoattractant protein-1, but did not inhibit ITAM-dependent phosphorylation of Syk mediated by FcγRI signaling. Our results demonstrate a novel means to target Syk independent of its kinase and pITAM binding sites such that integrin signaling via this kinase is abrogated but ITAM-dependent signaling remains intact. As integrin signaling through Syk is essential for leukocyte activation, this may represent a novel approach to target inflammation.

In Vivo Functions of Mouse Neutrophils Derived from HoxB8-Transduced Conditionally Immortalized Myeloid Progenitors

Although neutrophils play important roles in immunity and inflammation, their analysis is strongly hindered by their short-lived and terminally differentiated nature. Prior studies reported conditional immortalization of myeloid progenitors using retroviral expression of an estrogen-dependent fusion protein of the HoxB8 transcription factor. This approach allowed the long-term culture of mouse myeloid progenitors (HoxB8 progenitors) in estrogen-containing media, followed by differentiation toward neutrophils upon estrogen withdrawal. Although several reports confirmed the in vitro functional responsiveness of the resulting differentiated cells (HoxB8 neutrophils), little is known about their capacity to perform in vivo neutrophil functions. We have addressed this issue by an in vivo transplantation approach. In vitro-generated HoxB8 neutrophils showed a neutrophil-like phenotype and were able to perform conventional neutrophil functions, like respiratory burst, chemotaxis, and phagocytosis. The i.v. injection of HoxB8 progenitors into lethally irradiated recipients resulted in the appearance of circulating donor-derived HoxB8 neutrophils. In vivo-differentiated HoxB8 neutrophils were able to migrate to the inflamed peritoneum and to phagocytose heat-killed Candida particles. The reverse passive Arthus reaction could be induced in HoxB8 chimeras but not in irradiated, nontransplanted control animals. Repeated injection of HoxB8 progenitors also allowed us to maintain stable circulating HoxB8 neutrophil counts for several days. Injection of arthritogenic K/B×N serum triggered robust arthritis in HoxB8 chimeras, but not in irradiated, nontransplanted control mice. Taken together, our results indicate that HoxB8 progenitor-derived neutrophils are capable of performing various in vivo neutrophil functions, providing a framework for using the HoxB8 system for the in vivo analysis of neutrophil function.

ORAI1 and ORAI2 modulate murine neutrophil calcium signaling, cellular activation, and host defense

Calcium signals are initiated in immune cells by the process of store-operated calcium entry (SOCE), where receptor activation triggers transient calcium release from the endoplasmic reticulum, followed by opening of plasma-membrane calcium-release activated calcium (CRAC) channels. ORAI1, ORAI2, and ORAI3 are known to comprise the CRAC channel; however, the contributions of individual isoforms to neutrophil function are not well understood. Here, we show that loss of ORAI1 partially decreases calcium influx, while loss of both ORAI1 and ORAI2 completely abolishes SOCE. In other immune-cell types, loss of ORAI2 enhances SOCE. In contrast, we find that ORAI2-deficient neutrophils display decreased calcium influx, which is correlated with measurable differences in the regulation of neutrophil membrane potential via KCa3.1. Decreased SOCE in ORAI1-, ORAI2-, and ORAI1/2-deficient neutrophils impairs multiple neutrophil functions, including phagocytosis, degranulation, leukotriene, and reactive oxygen species (ROS) production, rendering ORAI1/2-deficient mice highly susceptible to staphylococcal infection. This study demonstrates that ORAI1 and ORAI2 are the primary components of the neutrophil CRAC channel and identifies subpopulations of neutrophils where cell-membrane potential functions as a rheostat to modulate the SOCE response. These findings have implications for mechanisms that modulate neutrophil function during infection, acute and chronic inflammatory conditions, and cancer.

Spleen Tyrosine Kinase Is a Critical Regulator of Neutrophil Responses to Candida Species

Invasive fungal infections constitute a lethal threat, with patient mortality as high as 90%. The incidence of invasive fungal infections is increasing, especially in the setting of patients receiving immunomodulatory agents, chemotherapy, or immunosuppressive medications following solid-organ or bone marrow transplantation. In addition, inhibitors of spleen tyrosine kinase (Syk) have been recently developed for the treatment of patients with refractory autoimmune and hematologic indications. Neutrophils are the initial innate cellular responders to many types of pathogens, including invasive fungi. A central process governing neutrophil recognition of fungi is through lectin binding receptors, many of which rely on Syk for cellular activation. We previously demonstrated that Syk activation is essential for cellular activation, phagosomal maturation, and elimination of phagocytosed fungal pathogens in macrophages. Here, we used combined genetic and chemical inhibitor approaches to evaluate the importance of Syk in the response of neutrophils to Candida species. We took advantage of a Cas9-expressing neutrophil progenitor cell line to generate isogenic wild-type and Syk-deficient neutrophils. Syk-deficient neutrophils are unable to control the human pathogens Candida albicans, Candida glabrata, and Candida auris Neutrophil responses to Candida species, including the production of reactive oxygen species and of cytokines such as tumor necrosis factor alpha (TNF-α), the formation of neutrophil extracellular traps (NETs), phagocytosis, and neutrophil swarming, appear to be critically dependent on Syk. These results demonstrate an essential role for Syk in neutrophil responses to Candida species and raise concern for increased fungal infections with the development of Syk-modulating therapeutics.IMPORTANCE Neutrophils are recognized to represent significant immune cell mediators for the clearance and elimination of the human-pathogenic fungal pathogen Candida The sensing of fungi by innate cells is performed, in part, through lectin receptor recognition of cell wall components and downstream cellular activation by signaling components, including spleen tyrosine kinase (Syk). While the essential role of Syk in macrophages and dendritic cells is clear, there remains uncertainty with respect to its contribution in neutrophils. In this study, we demonstrated that Syk is critical for multiple cellular functions in neutrophils responding to major human-pathogenic Candida species. These data not only demonstrate the vital nature of Syk with respect to the control of fungi by neutrophils but also warn of the potential infectious complications arising from the recent clinical development of novel Syk inhibitors for hematologic and autoimmune disorders.

Yersinia pseudotuberculosis YopH targets SKAP2-dependent and independent signaling pathways to block neutrophil antimicrobial mechanisms during infection

Yersinia suppress neutrophil responses by using a type 3 secretion system (T3SS) to inject 6–7 Yersinia effector proteins (Yops) effectors into their cytoplasm. YopH is a tyrosine phosphatase that causes dephosphorylation of the adaptor protein SKAP2, among other targets in neutrophils. SKAP2 functions in reactive oxygen species (ROS) production, phagocytosis, and integrin-mediated migration by neutrophils. Here we identify essential neutrophil functions targeted by YopH, and investigate how the interaction between YopH and SKAP2 influence Yersinia pseudotuberculosis (Yptb) survival in tissues. The growth defect of a ΔyopH mutant was restored in mice defective in the NADPH oxidase complex, demonstrating that YopH is critical for protecting Yptb from ROS during infection. The growth of a ΔyopH mutant was partially restored in Skap2-deficient (Skap2KO) mice compared to wild-type (WT) mice, while induction of neutropenia further enhanced the growth of the ΔyopH mutant in both WT and Skap2KO mice. YopH inhibited both ROS production and degranulation triggered via integrin receptor, G-protein coupled receptor (GPCR), and Fcγ receptor (FcγR) stimulation. SKAP2 was required for integrin receptor and GPCR-mediated ROS production, but dispensable for degranulation under all conditions tested. YopH blocked SKAP2-independent FcγR-stimulated phosphorylation of the proximal signaling proteins Syk, SLP-76, and PLCγ2, and the more distal signaling protein ERK1/2, while only ERK1/2 phosphorylation was dependent on SKAP2 following integrin receptor activation. These findings reveal that YopH prevents activation of both SKAP2-dependent and -independent neutrophilic defenses, uncouple integrin- and GPCR-dependent ROS production from FcγR responses based on their SKAP2 dependency, and show that SKAP2 is not required for degranulation.

Pathogenic Yersinia species carry a virulence plasmid encoding a type 3 secretion system that translocates 6–7 effector Yops into host cells. We demonstrate that YopH protects Yersinia pseudotuberculosis from neutrophil-produced reactive oxygen species (ROS) and degranulation by interfering with signaling pathways downstream of three major receptor classes in neutrophils. We show that a previously identified target of YopH, SKAP2, controls some of the pathways essential for YopH to inactivate during infection. SKAP2 is essential in mediating ROS production downstream of two major receptors; however, it is dispensable for degranulation from the three major receptors tested. Our study illustrates that YopH protects Y. pseudotuberculosis by blocking both SKAP2-dependent and independent signaling pathways that regulate several neutrophil functions.

SKAP2 is required for defense against K. pneumoniae infection and neutrophil respiratory burst

Klebsiella pneumoniae is a respiratory, blood, liver, and bladder pathogen of significant clinical concern. We show that the adaptor protein, SKAP2, is required for protection against K. pneumoniae (ATCC 43816) pulmonary infections. Skap2-/- mice had 100-fold higher bacterial burden when compared to wild-type and burden was controlled by SKAP2 expression in innate immune cells. Skap2-/- neutrophils and monocytes were present in infected lungs, and the neutrophils degranulated normally in response to K. pneumoniae infection in mice; however, K. pneumoniae-stimulated reactive oxygen species (ROS) production in vitro was abolished. K. pneumoniae-induced neutrophil ROS response required the activity of SFKs, Syk, Btk, PLCγ2, and PKC. The loss of SKAP2 significantly hindered the K. pneumoniae-induced phosphorylation of SFKs, Syk, and Pyk2 implicating SKAP2 as proximal to their activation in pathogen-signaling pathways. In conclusion, SKAP2-dependent signaling in neutrophils is essential for K. pneumoniae-activated ROS production and for promoting bacterial clearance during infection.

Klebsiella pneumoniae is a type of bacteria that can cause life-threatening infections – including pneumonia, blood stream infections, and urinary tract infections – in hospitalized patients. These infections can be difficult to treat because some K. pneumoniae are resistant to antibiotics. The bacteria are normally found in the human intestine, and they do not usually cause infections in healthy people. This implies that healthy people’s immune systems are better able to fend off K. pneumoniae infections; learning how could help scientists develop new ways to treat or prevent infections in hospitalized patients.

In healthy people, a type of immune cell called neutrophils are the first line of defense against bacterial infections. Several different proteins are needed to activate neutrophils, including a protein called SKAP2. But the role of this protein in fighting K. pneumoniae infections is not clear.

To find out what role SKAP2 plays in the defense against pneumonia caused by K. pneumoniae, Nguyen et al. compared infections in mice with and without the protein. Mice lacking SKAP2 in their white blood cells had more bacteria in their lungs than normal mice. The experiments showed that neutrophils from mice with SKAP2 produce a burst of chemicals called “reactive oxygen species”, which can kill bacteria. But neutrophils without the protein do not. Without SKAP2, several proteins that help produce reactive oxygen species do not work.

Understanding the role of SKAP2 in fighting infections may help scientists better understand the immune system. This could help clinicians to treat conditions that cause it to be hyperactive or ineffective. More studies are needed to determine if SKAP2 works the same way in human neutrophils and if it works against all types of K. pneumoniae. If it does, then scientists might be able use this information to develop therapies that help the immune system fight infections.

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.

Ets-2 deletion in myeloid cells attenuates IL-1α-mediated inflammatory disease caused by a Ptpn6 point mutation

The SHP-1 protein encoded by the Ptpn6 gene has been extensively studied in hematopoietic cells in the context of inflammation. A point mutation in this gene (Ptpn6^spin) causes spontaneous inflammation in mice, which has a striking similarity to neutrophilic dermatoses in humans. Recent findings highlighted the role of signaling adapters and kinases in promoting inflammation in Ptpn6^spin mice; however, the underlying transcriptional regulation is poorly understood. Here, we report that SYK is important for driving neutrophil infiltration and initiating wound healing responses in Ptpn6^spin mice. Moreover, we found that deletion of the transcription factor Ets2 in myeloid cells ameliorates cutaneous inflammatory disease in Ptpn6^spin mice through transcriptional regulation of its target inflammatory genes. Furthermore, Ets-2 drives IL-1α-mediated inflammatory signaling in neutrophils of Ptpn6^spin mice. Overall, in addition to its well-known role in driving inflammation in cancer, Ets-2 plays a major role in regulating IL-1α-driven Ptpn6^spin-mediated neutrophilic dermatoses. Model for the role of ETS-2 in neutrophilic inflammation in Ptpn6^spin mice. Mutation of the Ptpn6 gene results in SYK phosphorylation which then sequentially activates MAPK signaling pathways and activation of ETS-2. This leads to activation of ETS-2 target genes that contribute to neutrophil migration and inflammation. When Ets2 is deleted in Ptpn6^spin mice, the expression of these target genes is reduced, leading to the reduced pathology in neutrophilic dermatoses.