Regulation of Discrete Functional Responses by Syk and Src Family Tyrosine Kinases in Human Neutrophils

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.

Phosphoinositol 3-kinase-driven NET formation involves different isoforms and signaling partners depending on the stimulus

Neutrophil extracellular traps (NETs) serve to immobilize and kill pathogens, but also can contribute to the progression of several inflammatory and auto-immune diseases, as well as cancer. Whence the importance of elucidating the mechanisms underlying NET formation. In this regard, the PI3K signaling pathway has been shown to be crucial; yet little is known about which of its components are involved. Here, we identified the PI3K isoforms and associated signaling partners that are mobilized in response to different classes of physiological NET inducers (inflammatory cytokines, growth factors, chemoattractants). NET generation was assessed by microscopy and signalling molecule activation by immunoblot using phospho-antibodies. Across the various stimuli, PI3Kα and PI3Kγ isoforms clearly contributed to NET induction, while the participation of other isoforms was stimulus-dependent. Some PI3K isoforms were also found to signal through Akt, the canonical downstream effector of PI3K, while others did not. Downstream of PI3K, mTOR and PLCγ2 were used by all stimuli to control NET generation. Conversely, the involvement of other kinases depended on the stimulus - both TNFα and GM-CSF relied on PDK1 and Akt; and both TNFα and fMLP additionally used S6K. We further established that all PI3K isoforms and downstream effectors act belatedly in NET generation, as reported previously for PI3K. Finally, we revisited the PI3K-PDK1-Akt signaling hierarchy in human neutrophils and again found stimulus-dependent differences. Our data uncover unsuspected complexity and redundancy in the signaling machinery controlling NET formation through the all-important PI3K pathway. Conserved signaling molecules represent therapeutic targets for pathologies involving NETs and in this regard, the existence of drugs currently used in the clinic or undergoing clinical trials (which target PI3K isoforms, mTOR or Akt), underscores the translational potential of our findings.

Human Neutrophils Generate Extracellular Vesicles That Modulate Their Functional Responses

Neutrophils influence innate and adaptive immunity by releasing various cytokines and chemokines, by generating neutrophil extracellular traps (NETs), and by modulating their own survival. Neutrophils also produce extracellular vesicles (EVs) termed ectosomes, which influence the function of other immune cells. Here, we studied neutrophil-derived ectosomes (NDEs) and whether they can modulate autologous neutrophil responses. We first characterized EV production by neutrophils, following MISEV 2018 guidelines to facilitate comparisons with other studies. We found that such EVs are principally NDEs, that they are rapidly released in response to several (but not all) physiological stimuli, and that a number of signaling pathways are involved in the induction of this response. When co-incubated with autologous neutrophils, NDE constituents were rapidly incorporated into recipient cells and this triggered and/or modulated neutrophil responses. The pro-survival effect of GM-CSF, G-CSF, IFNγ, and dexamethasone was reversed; CXCL8 and NET formation were induced in otherwise unstimulated neutrophils; the induction of inflammatory chemokines by TNFα was modulated depending on the activation state of the NDEs' parent cells; and inducible NET generation was attenuated. Our data show that NDE generation modulates neutrophil responses in an autocrine and paracrine manner, and indicate that this probably represents an important aspect of how neutrophils shape their environment and cellular interactions.

Early and Late Processes Driving NET Formation, and the Autocrine/Paracrine Role of Endogenous RAGE Ligands

Neutrophil extracellular trap (NET) formation has emerged as an important response against various pathogens; it also plays a role in chronic inflammation, autoimmunity, and cancer. Despite a growing understanding of the mechanisms underlying NET formation, much remains to be elucidated. We previously showed that in human neutrophils activated with different classes of physiological stimuli, NET formation features both early and late events that are controlled by discrete signaling pathways. However, the nature of these events has remained elusive. We now report that PAD4 inhibition only affects the early phase of NET generation, as do distinct signaling intermediates (TAK1, MEK, p38 MAPK). Accordingly, the inducible citrullination of residue R2 on histone H3 is an early neutrophil response that is regulated by these kinases; other arginine residues on histones H3 and H4 do not seem to be citrullinated. Conversely, elastase blockade did not affect NET formation by several physiological stimuli, though it did so in PMA-activated cells. Among belated events in NET formation, we found that chromatin decondensation is impaired by the inhibition of signaling pathways controlling both early and late stages of the phenomenon. In addition to chromatin decondensation, other late processes were uncovered. For instance, unstimulated neutrophils can condition themselves to be poised for rapid NET induction. Similarly, activated neutrophils release endogenous proteic factors that promote and largely mediate NET generation. Several such factors are known RAGE ligands and accordingly, RAGE inbibition largely prevents both NET formation and the conditioning of neutrophils to rapidly generate NETs upon stimulation. Our data shed new light on the cellular processes underlying NET formation, and unveil unsuspected facets of the phenomenon that could serve as therapeutic targets. In view of the involvement of NETs in both homeostasis and several pathologies, our findings are of broad relevance.

Bletinib ameliorates neutrophilic inflammation and lung injury by inhibiting Src family kinase phosphorylation and activity

BACKGROUND AND PURPOSE

Neutrophil overactivation is crucial in the pathogenesis of acute lung injury (ALI). Bletinib (3,3'-dihydroxy-2',6'-bis(p-hydroxybenzyl)-5-methoxybibenzyl), a natural bibenzyl, extracted from the Bletilla plant, exhibits anti-inflammatory, antibacterial, and antimitotic effects. In this study, we evaluated the therapeutic effects of bletinib in human neutrophilic inflammation and LPS-mediated ALI in mice.

EXPERIMENTAL APPROACH

In human neutrophils activated with the formyl peptide (fMLP), we assessed integrin expression, superoxide anion production, degranulation, neutrophil extracellular trap (NET) formation, and adhesion through flow cytometry, spectrophotometry, and immunofluorescence microscopy. Immunoblotting was used to measure phosphorylation of Src family kinases (SFKs) and downstream proteins. Finally, a LPS-induced ALI model in male BALB/c mice was used to investigate the potential therapeutic effects of bletinib treatment.

KEY RESULTS

In activated human neutrophils, bletinib reduced degranulation, respiratory burst, NET formation, adhesion, migration, and integrin expression; suppressed the enzymic activity of SFKs, including Src, Lyn, Fgr, and Hck; and inhibited the phosphorylation of SFKs as well as Vav and Bruton's tyrosine kinase (Btk). In mice with ALI, the pulmonary sections demonstrated considerable amelioration of prominent inflammatory changes, such as haemorrhage, pulmonary oedema, and neutrophil infiltration, after bletinib treatment.

CONCLUSION AND IMPLICATIONS

Bletinib regulates neutrophilic inflammation by inhibiting the SFK-Btk-Vav pathway. Bletinib ameliorates LPS-induced ALI in mice. Further biochemical optimisation of bletinib may be a promising strategy for the development of novel therapeutic agents for inflammatory diseases.

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.

Pro-Inflammatory Response of Bovine Polymorphonuclear Cells Induced by Mycoplasma mycoides subsp. mycoides

Mycoplasma mycoides subsp. mycoides (Mmm) is the etiological agent of contagious bovine pleuropneumonia (CBPP), one of the major diseases affecting cattle in sub-Saharan Africa. Some evidences suggest that the immune system of the host (cattle) plays an important role in the pathogenic mechanism of CBPP, but the factors involved in the process remain largely unknown. The present study aimed to investigate the cell response of bovine polymorphonuclear neutrophils (PMNs) after Mmm in vitro exposure using one step RT-qPCR and Western blotting. Data obtained indicate that gene and protein expression levels of some pro-inflammatory factors already change upon 30 min of PMNs exposure to Mmm. Of note, mRNA expression level in Mmm exposed PMNs increased in a time-dependent manner and for all time points investigated; targets expression was also detected by Western blotting in Mmm exposed PMNs only. These data demonstrate that when bovine PMN cells are triggered by Mmm, they undergo molecular changes, upregulating mRNA and protein expression of specific pro-inflammatory factors. These results provide additional information on host-pathogen interaction during CBPP infection.

Cytokine Production and NET Formation by Monosodium Urate-Activated Human Neutrophils Involves Early and Late Events, and Requires Upstream TAK1 and Syk

Gout is a prevalent and incapacitating disease triggered by the deposition of monosodium urate (MSU) crystals in joints, which are also massively infiltrated by neutrophils. The interaction of the latter with MSU crystals triggers several responses, including the generation of inflammatory mediators and of neutrophil extracellular traps (NETs). Though some of the signaling events mobilized by MSU in neutrophils have been described (e.g., Src family kinases, Syk, PKC, PI3K), the picture remains fragmentary. Likewise, the impact of these signaling events on cellular responses is incompletely understood. In this study, we examined transcriptomic changes triggered by MSU in neutrophils and their impact on the corresponding proteins, as well as the role of various signaling pathways in prominent functional responses. We report for the first time that neutrophils can secrete the monocyte chemoattractant, CCL4, in response to MSU. Accordingly, we found that transcription factors NF-κB, CREB, and C/EBP are belatedly activated by MSU crystals, and at least the former is involved in chemokine generation. Moreover, we show that MAPKs and Akt are activated by MSU in neutrophils, that they are under the control of TAK1 and Syk, and that they participate in cytokine generation and NETosis. In the latter instance, we found the phenomenon to be independent of endogenous ROS, but under the control of PAD4. We finally provide evidence that endogenous factors contribute to the belated phosphorylation of kinases and transcription factors in response to MSU. Collectively, our findings unveil potentially important therapeutic targets for gouty arthritis.

Physiological Stimuli Induce PAD4-Dependent, ROS-Independent NETosis, With Early and Late Events Controlled by Discrete Signaling Pathways

Neutrophils are known to extrude decondensed chromatin, thus forming NETs (neutrophil extracellular traps). These structures immobilize pathogens, thereby preventing their spreading, and are also adorned with antimicrobial molecules. NETs can also influence pathogenesis in chronic inflammation, autoimmunity, and cancer. Despite the importance of NETs, the molecular mechanisms underlying their formation, as well as the upstream signaling pathways involved, are only partially understood. Likewise, current methodological approaches to quantify NETs suffer from significant drawbacks, not the least being the inclusion of a significant non-specific signal. In this study, we used novel, fluorescent polymers that only bind extruded chromatin, allowing a specific and standardized quantification of NETosis. This allowed us to reliably rank the relative potency of various physiologic NET inducers. In neutrophils activated with such stimuli, inhibition of the Syk or PI3K pathways blocked NETosis by acting upon late events in NET formation. Inhibition of the TAK1, p38 MAPK, or MEK pathways also hindered NETosis, but by acting on early events. By contrast, inhibiting PKC, Src family kinases, or JNK failed to prevent NETosis; cycloheximide or actinomycin D were also ineffective. Expectedly, NET formation was deeply compromised following inhibition of the NADPH oxidase in PMA-activated neutrophils, but was found to be ROS-independent in response to physiological agonists. Conversely, we show for the first time in human neutrophils that selective inhibition of PAD4 potently prevents NETosis by all stimuli tested. Our data substantially extends current knowledge of the signaling pathways controlling NETosis, and reveals how they affect early or late stages of the phenomenon. In view of the involvement of NETs in several pathologies, our findings also identify molecular targets that could be exploited for therapeutic intervention.