E-selectin-mediated rapid NLRP3 inflammasome activation regulates S100A8/S100A9 release from neutrophils via transient gasdermin D pore formation.

S100A8/S100A9 is a proinflammatory mediator released by myeloid cells during many acute and chronic inflammatory disorders. However, the precise mechanism of its release from the cytosolic compartment of neutrophils is unclear. Here, we show that E-selectin-induced rapid S100A8/S100A9 release during inflammation occurs in an NLRP3 inflammasome-dependent fashion. Mechanistically, E-selectin engagement triggers Bruton's tyrosine kinase-dependent tyrosine phosphorylation of NLRP3. Concomitant potassium efflux via the voltage-gated potassium channel KV1.3 mediates ASC oligomerization. This is followed by caspase 1 cleavage and downstream activation of pore-forming gasdermin D, enabling cytosolic release of S100A8/S100A9. Strikingly, E-selectin-mediated gasdermin D pore formation does not result in cell death but is a transient process involving activation of the ESCRT III membrane repair machinery. These data clarify molecular mechanisms of controlled S100A8/S100A9 release from neutrophils and identify the NLRP3/gasdermin D axis as a rapid and reversible activation system in neutrophils during inflammation.

CCR3-dependent eosinophil recruitment is regulated by sialyltransferase ST3Gal-IV.

Eosinophil recruitment is a pathological hallmark of many allergic and helminthic diseases. Here, we investigated chemokine receptor CCR3-induced eosinophil recruitment in sialyltransferase St3gal4-/- mice. We found a marked decrease in eosinophil extravasation into CCL11-stimulated cremaster muscles and into the inflamed peritoneal cavity of St3gal4-/- mice. Ex vivo flow chamber assays uncovered reduced adhesion of St3gal4-/- compared to wild type eosinophils. Using flow cytometry, we show reduced binding of CCL11 to St3gal4-/- eosinophils. Further, we noted reduced binding of CCL11 to its chemokine receptor CCR3 isolated from St3gal4-/- eosinophils. This was accompanied by almost absent CCR3 internalization of CCL11-stimulated St3gal4-/- eosinophils. Applying an ovalbumin-induced allergic airway disease model, we found a dramatic reduction in eosinophil numbers in bronchoalveolar lavage fluid following intratracheal challenge with ovalbumin in St3gal4-deficient mice. Finally, we also investigated tissue-resident eosinophils under homeostatic conditions and found reduced resident eosinophil numbers in the thymus and adipose tissue in the absence of ST3Gal-IV. Taken together, our results demonstrate an important role of ST3Gal-IV in CCR3-induced eosinophil recruitment in vivo rendering this enzyme an attractive target in reducing unwanted eosinophil infiltration in various disorders including allergic diseases.

Direct Rap1/Talin1 interaction regulates platelet and neutrophil integrin activity in mice.

Targeting Talin1 to the plasma membrane is a crucial step in integrin activation, which in leukocytes is mediated by a Rap1/RIAM/Talin1 pathway, whereas in platelets, it is RIAM independent. Recent structural, biochemical, and cell biological studies have suggested direct Rap1/Talin1 interaction as an alternative mechanism to recruit Talin1 to the membrane and induce integrin activation. To test whether this pathway is of relevance in vivo, we generated Rap1 binding-deficient Talin1 knockin (Tln13mut) mice. Although Tln13mut mice showed no obvious abnormalities, their platelets exhibited reduced integrin activation, aggregation, adhesion, and spreading, resulting in prolonged tail-bleeding times and delayed thrombus formation and vessel occlusion in vivo. Surprisingly, neutrophil adhesion to different integrin ligands and ß2 integrin-dependent phagocytosis were also significantly impaired, which caused profound leukocyte adhesion and extravasation defects in Tln13mut mice. In contrast, macrophages exhibited no defect in adhesion or spreading despite reduced integrin activation. Taken together, our findings suggest that direct Rap1/Talin1 interaction is of particular importance in regulating the activity of different integrin classes expressed on platelets and neutrophils, which both depend on fast and dynamic integrin-mediated responses.

Neutrophil recruitment and intracellular vesicle transport: A short overview.

Recruitment of neutrophils from the intravascular compartment into injured tissue is an essential component of the inflammatory response. It involves intracellular trafficking of vesicles within neutrophils and endothelial cells, both containing numerous proteins that have to be distributed in a tightly controlled and precise spatiotemporal fashion during the recruitment process. Rab proteins, a family of small GTPases, together with their effectors, are the key players in guiding and regulating the intracellular vesicle trafficking machinery during neutrophil recruitment. This review will provide a short overview on this process and highlight new findings as well as current controversies in the field.

Src family kinase-mediated vesicle trafficking is critical for neutrophil basement membrane penetration.

Leukocyte recruitment into inflamed tissue is highly dependent on the activation and binding of integrins to their respective ligands, followed by the induction of various signaling events within the cell referred to as outside-in signaling. Src family kinases (SFK) are the central players in the outside-in signaling process, assigning them a critical role for proper immune cell function. Our study investigated the role of SFK on neutrophil recruitment in vivo using Hck-/- Fgr-/- Lyn-/- mice, which lack SFK expressed in neutrophils. We show that loss of SFK strongly reduces neutrophil adhesion and post-arrest modifications in a shear force dependent manner. Additionally, we found that in the absence of SFK, neutrophils display impaired Rab27a-dependent surface mobilization of neutrophil elastase, VLA3 and VLA6 containing vesicles. This results in a defect in neutrophil vascular basement membrane penetration and thus strongly impaired extravasation. Taken together, we demonstrate that SFK play a role in neutrophil post-arrest modifications and extravasation during acute inflammation. These findings may support the current efforts to use SFK-inhibitors in inflammatory diseases with unwanted neutrophil recruitment.

Maturation of Platelet Function During Murine Fetal Development In Vivo.

OBJECTIVE: Platelet function has been intensively studied in the adult organism. However, little is known about the function and hemostatic capacity of platelets in the developing fetus as suitable in vivo models are lacking. APPROACH AND RESULTS: To examine fetal platelet function in vivo, we generated a fetal thrombosis model and investigated light/dye-induced thrombus formation by intravital microscopy throughout gestation. We observed that significantly less and unstable thrombi were formed at embryonic day (E) 13.5 compared with E17.5. Flow cytometry revealed significantly lower platelet counts in E13.5 versus E17.5 fetuses versus adult controls. In addition, fetal platelets demonstrated changed activation responses of surface adhesion molecules and reduced P-selectin content and mobilization. Interestingly, we also measured reduced levels of the integrin-activating proteins Kindlin-3, Talin-1, and Rap1 during fetal development. Consistently, fetal platelets demonstrated diminished spreading capacity compared with adults. Transfusion of adult platelets into the fetal circulation led to rapid platelet aggregate formation even in young fetuses. Yet, retrospective data analysis of a neonatal cohort demonstrated no correlation of platelet transfusion with closure of a persistent ductus arteriosus, a process reported to be platelet dependent. CONCLUSIONS: Taken together, we demonstrate an ontogenetic regulation of platelet function in vivo with physiologically low platelet numbers and hyporeactivity early during fetal development shedding new light on hemostatic function during fetal life.

LST1 promotes the assembly of a molecular machinery responsible for tunneling nanotube formation.

Carefully orchestrated intercellular communication is an essential prerequisite for the development of multicellular organisms. In recent years, tunneling nanotubes (TNT) have emerged as a novel and widespread mechanism of cell-cell communication. However, the molecular basis of their formation is still poorly understood. In the present study we report that the transmembrane MHC class III protein leukocyte specific transcript 1 (LST1) induces the formation of functional nanotubes and is required for endogenous nanotube generation. Mechanistically, we found that LST1 induces nanotube formation by recruiting the small GTPase RalA to the plasma membrane and promoting its interaction with the exocyst complex. Furthermore, we determined that LST1 recruits the actin-crosslinking protein filamin to the plasma membrane and interacts with M-Sec, myosin and myoferlin. These results allow us to suggest a molecular model for nanotube generation. In this proposal LST1 functions as a membrane scaffold mediating the assembly of a multimolecular complex, which controls the formation of functional nanotubes.

A20 and the noncanonical NF-κB pathway are key regulators of neutrophil recruitment during fetal ontogeny.

Newborns are at high risk of developing neonatal sepsis, particularly if born prematurely. This has been linked to divergent requirements the immune system has to fulfill during intrauterine compared with extrauterine life. By transcriptomic analysis of fetal and adult neutrophils, we shed new light on the molecular mechanisms of neutrophil maturation and functional adaption during fetal ontogeny. We identified an accumulation of differentially regulated genes within the noncanonical NF-κB signaling pathway accompanied by constitutive nuclear localization of RelB and increased surface expression of TNF receptor type II in fetal neutrophils, as well as elevated levels of lymphotoxin α in fetal serum. Furthermore, we found strong upregulation of the negative inflammatory regulator A20 (Tnfaip3) in fetal neutrophils, which was accompanied by pronounced downregulation of the canonical NF-κB pathway. Functionally, overexpressing A20 in Hoxb8 cells led to reduced adhesion of these neutrophil-like cells in a flow chamber system. Conversely, mice with a neutrophil-specific A20 deletion displayed increased inflammation in vivo. Taken together, we have uncovered constitutive activation of the noncanonical NF-κB pathway with concomitant upregulation of A20 in fetal neutrophils. This offers perfect adaption of neutrophil function during intrauterine fetal life but also restricts appropriate immune responses particularly in prematurely born infants.

The voltage-gated potassium channel KV1.3 regulates neutrophil recruitment during inflammation.

AIMS: Neutrophil trafficking within the vasculature strongly relies on intracellular calcium signalling. Sustained Ca2+ influx into the cell requires a compensatory efflux of potassium to maintain membrane potential. Here, we aimed to investigate whether the voltage-gated potassium channel KV1.3 regulates neutrophil function during the acute inflammatory process by affecting sustained Ca2+ signalling. METHODS AND RESULTS: Using in vitro assays and electrophysiological techniques, we show that KV1.3 is functionally expressed in human neutrophils regulating sustained store-operated Ca2+ entry through membrane potential stabilizing K+ efflux. Inhibition of KV1.3 on neutrophils by the specific inhibitor 5-(4-Phenoxybutoxy)psoralen (PAP-1) impaired intracellular Ca2+ signalling, thereby preventing cellular spreading, adhesion strengthening, and appropriate crawling under flow conditions in vitro. Using intravital microscopy, we show that pharmacological blockade or genetic deletion of KV1.3 in mice decreased neutrophil adhesion in a blood flow dependent fashion in inflamed cremaster muscle venules. Furthermore, we identified KV1.3 as a critical component for neutrophil extravasation into the inflamed peritoneal cavity. Finally, we also revealed impaired phagocytosis of Escherichia coli particles by neutrophils in the absence of KV1.3. CONCLUSION: We show that the voltage-gated potassium channel KV1.3 is critical for Ca2+ signalling and neutrophil trafficking during acute inflammatory processes. Our findings do not only provide evidence for a role of KV1.3 for sustained calcium signalling in neutrophils affecting key functions of these cells, they also open up new therapeutic approaches to treat inflammatory disorders characterized by overwhelming neutrophil infiltration.

Binding of Rap1 and Riam to Talin1 Fine-Tune ß2 Integrin Activity During Leukocyte Trafficking.

ß2 integrins mediate key processes during leukocyte trafficking. Upon leukocyte activation, the structurally bent ß2 integrins change their conformation towards an extended, intermediate and eventually high affinity conformation, which mediate slow leukocyte rolling and firm arrest, respectively. Translocation of talin1 to integrin adhesion sites by interactions with the small GTPase Rap1 and the Rap1 effector Riam precede these processes. Using Rap1 binding mutant talin1 and Riam deficient mice we show a strong Riam-dependent T cell homing process to lymph nodes in adoptive transfer experiments and by intravital microscopy. Moreover, neutrophils from compound mutant mice exhibit strongly increased rolling velocities to inflamed cremaster muscle venules compared to single mutants. Using Hoxb8 cell derived neutrophils generated from the mutant mouse strains, we show that both pathways regulate leukocyte rolling and adhesion synergistically by inducing conformational changes of the ß2 integrin ectodomain. Importantly, a simultaneous loss of both pathways results in a rolling phenotype similar to talin1 deficient neutrophils suggesting that ß2 integrin regulation primarily occurs via these two pathways.

Extratubular Polymerized Uromodulin Induces Leukocyte Recruitment and Inflammation In Vivo.

Uromodulin (UMOD) is produced and secreted by tubular epithelial cells. Secreted UMOD polymerizes (pUMOD) in the tubular lumen, where it regulates salt transport and protects the kidney from bacteria and stone formation. Under various pathological conditions, pUMOD accumulates within the tubular lumen and reaches extratubular sites where it may interact with renal interstitial cells. Here, we investigated the potential of extratubular pUMOD to act as a damage associated molecular pattern (DAMP) molecule thereby creating local inflammation. We found that intrascrotal and intraperitoneal injection of pUMOD induced leukocyte recruitment in vivo and led to TNF-α secretion by F4/80 positive macrophages. Additionally, pUMOD directly affected vascular permeability and increased neutrophil extravasation independent of macrophage-released TNF-α. Interestingly, pUMOD displayed no chemotactic properties on neutrophils, did not directly activate ß2 integrins and did not upregulate adhesion molecules on endothelial cells. In obstructed neonatal murine kidneys, we observed extratubular UMOD accumulation in the renal interstitium with tubular atrophy and leukocyte infiltrates. Finally, we found extratubular UMOD deposits associated with peritubular leukocyte infiltration in kidneys from patients with inflammatory kidney diseases. Taken together, we identified extratubular pUMOD as a strong inducer of leukocyte recruitment, underlining its critical role in mounting an inflammatory response in various kidneys pathologies.

LST1: A multifunctional gene encoded in the MHC class III region.

The LST1 gene is located in the MHC class III cluster between the MHC class I and II regions. While most genes in this cluster have been sufficiently characterised, a definitive function and expression pattern for LST1 still remains elusive. In the present review we describe its promotor, gene organisation, splice variants and expression in human tissues, cell lines and cancer. We focus on LST1 expression in inflammation and discuss known correlations with autoimmune diseases and cancer. Current data on LST1 polymorphisms and their known associations with pathologies are also discussed in detail. We summarize the potential functions that have been described for the full-length LST1 protein including its function as a transmembrane adaptor protein with inhibitory signal transduction and its role as a membrane scaffold facilitating the formation of tunnelling nanotubes. We also discuss further potential functions by compiling all known LST1-interacting proteins. Furthermore, we address knowledge gaps and conflictive issues regarding disease association, non-hematopoietic expression and the discrepancy between RNA and protein expression data.

The Yin and Yang of Tyrosine Kinase Inhibition During Experimental Polymicrobial Sepsis.

Neutrophils are the first cells of our immune system to arrive at the site of inflammation. They release cytokines, e.g., chemokines, to attract further immune cells, but also actively start to phagocytose and kill pathogens. In the case of sepsis, this tightly regulated host defense mechanism can become uncontrolled and hyperactive resulting in severe organ damage. Currently, no effective therapy is available to fight sepsis; therefore, novel treatment targets that could prevent excessive inflammatory responses are warranted. Src Family tyrosine Kinases (SFK), a group of tyrosine kinases, have been shown to play a major role in regulating immune cell recruitment and host defense. Leukocytes with SFK depletion display severe spreading and migration defects along with reduced cytokine production. Thus, we investigated the effects of dasatinib, a tyrosine kinase inhibitor, with a strong inhibitory capacity on SFKs during sterile inflammation and polymicrobial sepsis in mice. We found that dasatinib-treated mice displayed diminished leukocyte adhesion and extravasation in tumor necrosis factor-α-stimulated cremaster muscle venules in vivo. In polymicrobial sepsis, sepsis severity, organ damage, and clinical outcome improved in a dose-dependent fashion pointing toward an optimal therapeutic window for dasatinib dosage during polymicrobial sepsis. Dasatinib treatment may, therefore, provide a balanced immune response by preventing an overshooting inflammatory reaction on the one side and bacterial overgrowth on the other side.

MST1-dependent vesicle trafficking regulates neutrophil transmigration through the vascular basement membrane.

Neutrophils need to penetrate the perivascular basement membrane for successful extravasation into inflamed tissue, but this process is incompletely understood. Recent findings have associated mammalian sterile 20-like kinase 1 (MST1) loss of function with a human primary immunodeficiency disorder, suggesting that MST1 may be involved in immune cell migration. Here, we have shown that MST1 is a critical regulator of neutrophil extravasation during inflammation. Mst1-deficient (Mst1-/-) neutrophils were unable to migrate into inflamed murine cremaster muscle venules, instead persisting between the endothelium and the basement membrane. Mst1-/- neutrophils also failed to extravasate from gastric submucosal vessels in a murine model of Helicobacter pylori infection. Mechanistically, we observed defective translocation of VLA-3, VLA-6, and neutrophil elastase from intracellular vesicles to the surface of Mst1-/- neutrophils, indicating that MST1 is required for this crucial step in neutrophil transmigration. Furthermore, we found that MST1 associates with the Rab27 effector protein synaptotagmin-like protein 1 (JFC1, encoded by Sytl1 in mice), but not Munc13-4, thereby regulating the trafficking of Rab27-positive vesicles to the cellular membrane. Together, these findings highlight a role for MST1 in vesicle trafficking and extravasation in neutrophils, providing an additional mechanistic explanation for the severe immune defect observed in patients with MST1 deficiency.

Extracellular MRP8/14 is a regulator of ß2 integrin-dependent neutrophil slow rolling and adhesion.

Myeloid-related proteins (MRPs) 8 and 14 are cytosolic proteins secreted from myeloid cells as proinflammatory mediators. Currently, the functional role of circulating extracellular MRP8/14 is unclear. Our present study identifies extracellular MRP8/14 as an autocrine player in the leukocyte adhesion cascade. We show that E-selectin-PSGL-1 interaction during neutrophil rolling triggers Mrp8/14 secretion. Released MRP8/14 in turn activates a TLR4-mediated, Rap1-GTPase-dependent pathway of rapid ß2 integrin activation in neutrophils. This extracellular activation loop reduces leukocyte rolling velocity and stimulates adhesion. Thus, we identify Mrp8/14 and TLR4 as important modulators of the leukocyte recruitment cascade during inflammation in vivo.

Sphingosine-1-phosphate receptor 3 promotes leukocyte rolling by mobilizing endothelial P-selectin.

Sphingosine-1-phosphate (S1P) participates in inflammation; however, its role in leukocyte rolling is still unclear. Here we use intravital microscopy in inflamed mouse cremaster muscle venules and human endothelial cells to show that S1P contributes to P-selectin-dependent leukocyte rolling through endothelial S1P receptor 3 (S1P3) and Gαq, PLCß and Ca(2+). Intra-arterial S1P administration increases leukocyte rolling, while S1P3 deficiency or inhibition dramatically reduces it. Mast cells involved in triggering rolling also release S1P that mobilizes P-selectin through S1P3. Histamine and epinephrine require S1P3 for full-scale effect accomplishing it by stimulating sphingosine kinase 1 (Sphk1). In a counter-regulatory manner, S1P1 inhibits cAMP-stimulated Sphk1 and blocks rolling as observed in endothelial-specific S1P1(-/-) mice. In agreement with a dominant pro-rolling effect of S1P3, FTY720 inhibits rolling in control and S1P1(-/-) but not in S1P3(-/-) mice. Our findings identify S1P as a direct and indirect contributor to leukocyte rolling and characterize the receptors mediating its action.

Angiopoietin 2 mediates microvascular and hemodynamic alterations in sepsis.

Septic shock is characterized by increased vascular permeability and hypotension despite increased cardiac output. Numerous vasoactive cytokines are upregulated during sepsis, including angiopoietin 2 (ANG2), which increases vascular permeability. Here we report that mice engineered to inducibly overexpress ANG2 in the endothelium developed sepsis-like hemodynamic alterations, including systemic hypotension, increased cardiac output, and dilatory cardiomyopathy. Conversely, mice with cardiomyocyte-restricted ANG2 overexpression failed to develop hemodynamic alterations. Interestingly, the hemodynamic alterations associated with endothelial-specific overexpression of ANG2 and the loss of capillary-associated pericytes were reversed by intravenous injections of adeno-associated viruses (AAVs) transducing cDNA for angiopoietin 1, a TIE2 ligand that antagonizes ANG2, or AAVs encoding PDGFB, a chemoattractant for pericytes. To confirm the role of ANG2 in sepsis, we i.p. injected LPS into C57BL/6J mice, which rapidly developed hypotension, acute pericyte loss, and increased vascular permeability. Importantly, ANG2 antibody treatment attenuated LPS-induced hemodynamic alterations and reduced the mortality rate at 36 hours from 95% to 61%. These data indicate that ANG2-mediated microvascular disintegration contributes to septic shock and that inhibition of the ANG2/TIE2 interaction during sepsis is a potential therapeutic target.

Plasma fibronectin deficiency impedes atherosclerosis progression and fibrous cap formation.

Atherosclerotic lesions are asymmetric focal thickenings of the intima of arteries that consist of lipids, various cell types and extracellular matrix (ECM). These lesions lead to vascular occlusion representing the most common cause of death in the Western world. The main cause of vascular occlusion is rupture of atheromatous lesions followed by thrombus formation. Fibronectin (FN) is one of the earliest ECM proteins deposited at atherosclerosis-prone sites and was suggested to promote atherosclerotic lesion formation. Here, we report that atherosclerosis-prone apolipoprotein E-null mice lacking hepatocyte-derived plasma FN (pFN) fed with a pro-atherogenic diet display dramatically reduced FN depositions at atherosclerosis-prone areas, which results in significantly smaller and fewer atherosclerotic plaques. However, the atherosclerotic lesions from pFN-deficient mice lacked vascular smooth muscle cells and failed to develop a fibrous cap. Thus, our results demonstrate that while FN worsens the course of atherosclerosis by increasing the atherogenic plaque area, it promotes the formation of the protective fibrous cap, which in humans prevents plaques rupture and vascular occlusion.