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 K_V1.3 regulates neutrophil function during the acute inflammatory process by affecting sustained Ca²⁺ signalling. Using in vitro assays and electrophysiological techniques, we show that K_V1.3 is functionally expressed in human neutrophils regulating sustained store-operated Ca²⁺ entry through membrane potential stabilizing K⁺ efflux. Inhibition of K_V1.3 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 K_V1.3 in mice decreased neutrophil adhesion in a blood flow dependent fashion in inflamed cremaster muscle venules. Furthermore, we identified K_V1.3 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 K_V1.3. We show that the voltage-gated potassium channel K_V1.3 is critical for Ca²⁺ signalling and neutrophil trafficking during acute inflammatory processes. Our findings do not only provide evidence for a role of K_V1.3 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.

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.

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.

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 recruitmentin vivoand 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 β2 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.

Newborns are at high risk to develop sepsis. This is linked to innate immune responses at birth which are not completely adapted to postnatal life. Neutrophils are key players of innate immunity and exhibit a marked ontogenetic regulation of their functionality. Here, we studied the NLRP3 inflammasome in neonatal neutrophils and found lower baseline expression of NLRP3, pro-caspase-1 and the K⁺-channel K_V1.3 compared to adult neutrophils. Following stimulation with LPS/Nigericin, ASC oligomerization, caspase-1 activation and IL-1β release were significantly reduced in neonatal compared to adult neutrophils. Similarly, stimulation of neonatal neutrophils with E-selectin led to reduced NLRP3 inflammasome activation accompanied by diminished release of the alarmin S100A8/A9. Taken together, our results strongly indicate diminished NLRP3 inflammasome activation in neonatal neutrophils leading to a significant reduction of released IL-1β and S100A8/A9. These findings identify reduced neutrophil NLRP3 inflammasome activation as critical component contributing to the inherent susceptibility to infections in neonates.

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.