Beta2-integrin-mediated adhesion and intracellular Ca2+ release in human eosinophils

Airways exposure of bacterial superantigen SEB enhances bone marrow eosinophil population and facilitates its egress to blood and lung tissue

BACKGROUND

Differentiation of bone marrow eosinophils (BM-EO) and its trafficking to peripheral blood and respiratory mucosa are a hallmark of inflammatory diseases. Staphylococcal enterotoxin B (SEB) has been shown to aggravate airways eosinophilic inflammation. This study aimed to investigate the effects of mouse airways SEB exposure on BM-EO population, as well as its adhesive properties and release of cytokines/chemokines that orchestrate BM-EO trafficking to lungs.

METHODS

Male BALB/c mice were intranasally exposed to SEB (1 μg), and at 4, 16, 24 and 48 h thereafter, bone marrow (BM), circulating blood and bronchoalveolar lavage (BAL) fluid were collected. Levels of cytokines/chemokines and expressions of VLA-4 and CCR3 in BM were evaluated. Adhesion of BM to ICAM-1 and VCAM-1 were also evaluated.

RESULTS

SEB exposure promoted a marked eosinophil influx to BAL at 16 and 24 h after exposure, which was accompanied by significant increases in counts of immature (16 h) and mature (4 to 48 h) forms of eosinophil in BM, along with blood eosinophilia (16 h). In BM, higher levels of eotaxin, IL-5, IL-4, IL-3 and IL-7 were detected at 16 to 48 h. SEB also significantly increased CCR3 expression and calcium levels in BM-EO, and enhanced the cell adhesion to ICAM-1 (24 h) and ICAM-1 (48 h).

CONCLUSION

Airways SEB exposure increases the number of eosinophils in BM by mechanisms involving a network of cytokine and chemokine release, facilitating the BM-EO adhesion to ICAM-1 and VCAM-1 to gain access to the peripheral blood and lung tissues.

The eosinophil actin cytoskeleton undergoes rapid rearrangement in response to fluid shear stress

The regulatory processes involved in eosinophil trafficking into tissues are poorly understood; therefore, it is crucial to elucidate these mechanisms to advance the quality of clinical care for patients with eosinophil-mediated diseases. The complex interactions between eosinophil integrin receptors and their corresponding ligands on the post-capillary venules of the bronchial endothelium result in distinct modifications to the cytoskeletal architecture that occur in coordinated, temporally regulated sequences. The current study utilizes real-time confocal microscopy and time-based immunofluorescence staining to further characterize the effects of physiologically relevant fluid shear stress on this novel phenomenon of perfusion-induced calcium response. We found that the mere perfusion of fluid over adhered human eosinophils induced a release of intracellular calcium observed in conjunction with changes in cell morphology (flattening onto the coverslip surface, an increase in surface area, and a loss of circularity), suggesting a previously unknown mechanosensing aspect of eosinophil migration out of the vasculature. Although changes in morphology and degree of calcium release remained consistent across varying perfusion rates, the latency of the response was highly dependent on the degree of shear stresses. Eosinophils were fixed post-perfusion at specific timepoints for immunofluorescence staining to track proteins of interest over time. The distribution of proteins was diffuse throughout the cell prior to perfusion; however, they quickly localized to the periphery of the cell within 5 min. The actin cytoskeleton became markedly built up at the cell edges rapidly after stimulation, forming punctate dots by 4 min, suggesting a pivotal role in directed cell motility.

Integrins are Mechanosensors That Modulate Human Eosinophil Activation

Eosinophil migration to the lung is primarily regulated by the eosinophil-selective family of eotaxin chemokines, which mobilize intracellular calcium (Ca²⁺) and orchestrate myriad changes in cell structure and function. Eosinophil function is also known to be flow-dependent, although the molecular cognate of this mechanical response has yet to be adequately characterized. Using confocal fluorescence microscopy, we determined the effects of fluid shear stress on intracellular calcium concentration ([Ca²⁺]_i) in human peripheral blood eosinophils by perfusing cells in a parallel-plate flow chamber. Our results indicate that fluid perfusion evokes a calcium response that leads to cell flattening, increase in cell area, shape change, and non-directional migration. None of these changes are seen in the absence of a flow stimulus, and all are blocked by chelation of intracellular Ca²⁺ using BAPTA. These changes are enhanced by stimulating the cells with eotaxin-1. The perfusion-induced calcium response (PICR) could be blocked by pre-treating cells with selective (CDP-323) and non-selective (RGD tripeptides) integrin receptor antagonists, suggesting that α₄β₇/α₄β₁ integrins mediate this response. Overall, our study provides the first pharmacological description of a molecular mechanosensor that may collaborate with the eotaxin-1 signaling program in order to control human eosinophil activation.

Cystic fibrosis transmembrane conductance regulator is involved in airway epithelial wound repair

The role of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) in airway epithelial wound repair was investigated using normal human bronchial epithelial (NHBE) cells and a human airway epithelial cell line (Calu-3) of serous gland origin. Measurements of wound repair were performed using continuous impedance sensing to determine the time course for wound closure. Control experiments showed that the increase in impedance corresponding to cell migration into the wound was blocked by treatment with the actin polymerization inhibitor, cytochalasin D. Time lapse imaging revealed that NHBE and Calu-3 cell wound closure was dependent on cell migration, and that movement occurred as a collective sheet of cells. Selective inhibition of CFTR activity with CFTR(inh)-172 or short hairpin RNA silencing of CFTR expression produced a significant delay in wound repair. The CF cell line UNCCF1T also exhibited significantly slower migration than comparable normal airway epithelial cells. Inhibition of CFTR-dependent anion transport by treatment with CFTR(inh)-172 slowed wound closure to the same extent as silencing CFTR protein expression, indicating that ion transport by CFTR plays a critical role in migration. Moreover, morphologic analysis of migrating cells revealed that CFTR inhibition or silencing significantly reduced lamellipodia protrusion. These findings support the conclusion that CFTR participates in airway epithelial wound repair by a mechanism involving anion transport that is coupled to the regulation of lamellipodia protrusion at the leading edge of the cell.