Mechanical shedding of L-selectin from the neutrophil surface during rolling on sialyl Lewis x under flow

Role of c-Abl in L-selectin shedding from the neutrophil surface

L-selectin is a key molecule that participates in neutrophil tethering and subsequent rolling. It is cleaved from the surface of neutrophils activated in the presence of lipopolysaccharides, N-formyl-methionine-leucine-phenylalanine (fMLP), or Interleukin-8 (IL-8). We previously showed that L-selectin is also shed from the neutrophil surface during rolling on sialyl Lewis-x coated surfaces in a force-, ADAM-17 sheddase-, and p38 MAP kinase-dependent manner under flow. c-Abl tyrosine kinase is phosphorylated when L-selectin on the surface of neutrophils is cross-linked with anti-L-selectin antibodies. Here, we study the effect of c-Abl inhibition on L-selectin shedding from primary human neutrophils in static conditions following exposure to fMLP, IL-8, and hypotonic buffer and under flow through sialyl Lewis-x coated microtubes. Results indicate that c-Abl inhibition by STI571 significantly affects neutrophil adhesion via L-selectin, by decreasing the average rolling velocity and increasing the flux of rolling cells. The change in surface receptor expression was verified by flow cytometry. Interestingly, other forms of L-selectin shedding induced by fMLP, IL-8 or osmotic swelling were unaffected by STI571 treatment. These findings implicate the c-Abl signaling molecule in regulating L-selectin mechanical shedding in response to shear stress, setting this type of signaling apart from those triggered by the presence of a hypotonic environment, fMLP, or IL-8. This study sheds light on the role of c-Abl in neutrophil adhesion not previously reported in the literature.

Leukocytes of pregnant women with small-for-gestational age neonates have a different phenotypic and metabolic activity from those of women with preeclampsia

OBJECTIVE

Preeclampsia and pregnancies complicated by small-for-gestational age (SGA) neonates share several underlying mechanisms of disease. However, while an exaggerated systemic maternal inflammatory response is regarded as one of the hallmarks of the pathogenesis of preeclampsia, the presence of a similar systemic intra-vascular inflammation in mothers of SGA neonates without hypertension is controversial. The aim of this study was to determine phenotypic and metabolic changes in granulocytes and monocytes of women who develop preeclampsia and those who deliver an SGA neonate, compared to normal pregnant women.

METHODS

This cross-sectional study included patients with a normal pregnancy (n = 33), preeclampsia (n = 33), and an SGA without preeclampsia (n = 33), matched for gestational age at blood sample collection. Granulocyte and monocyte phenotypes were determined by flow cytometry, using monoclonal antibodies against selective cluster of differentiation (CD) antigens. The panel of antibodies included the following: CD11b, CD14, CD16, CD18, CD49d, CD62L, CD64, CD66b, and HLA-DR. Intracellular reactive oxygen species (iROS) were assessed at the basal state and after stimulation (oxidative burst). Results were reported as mean channel brightness (MCB) or intensity of detected fluorescence. Analysis was conducted with non-parametric statistics. A p-value < 0.01 was considered statistically significant.

RESULTS

(1) Women who delivered an SGA neonate had a higher MCB of CD11b in granulocytes and monocytes than those with a normal pregnancy (p < 0.001 for both); (2) patients with preeclampsia had a lower median MCB of CD62L in granulocytes (p = 0.006) and a higher median basal iROS and oxidative burst in monocytes than women with an SGA neonate (p = 0.003 and p = 0.002, respectively).

CONCLUSION

Pregnancies complicated by the delivery of an SGA neonate are characterized by a higher activation of maternal peripheral leukocytes than in normal pregnancies, but lower than in pregnancies complicated by preeclampsia.

The cytoplasmic domains of TNFalpha-converting enzyme (TACE/ADAM17) and L-selectin are regulated differently by p38 MAPK and PKC to promote ectodomain shedding

L-selectin mediates the initial tethering and subsequent rolling of leucocytes along luminal walls of inflamed venules. TACE [TNFalpha (tumour necrosis factor alpha)-converting enzyme] is responsible for cleaving the membrane-proximal extracellular domain of L-selectin (also known as shedding), which reduces the efficiency of leucocyte recruitment to sites of inflammation. Many reports have highlighted roles for PKC (protein kinase C) and p38 MAPK (mitogen-activated protein kinase) in promoting L-selectin shedding with little insight into the mechanism involved. By using PMA and the phosphatase inhibitors cantharidin and calyculin A, we could selectively activate PKC or p38 MAPK respectively to promote TACE-dependent shedding of L-selectin. Interestingly, the intracellular mechanisms leading to the shedding event differed dramatically. For example, regulatory elements within the L-selectin cytoplasmic tail, such as ERM (ezrin/radixin/moesin)-binding and serine residues, were important for PKC- but not p38 MAPK-dependent shedding. Also, increased and sustained cell surface levels of TACE, and phosphorylation of its cytoplasmic tail (a hallmark of TACE activation), occurred in lymphocytes and monocytes following p38 MAPK activation. Finally, we showed that TNFalpha-induced shedding of L-selectin in monocytes was strikingly similar to cantharidin-induced shedding and suggest that this newly characterized mechanism could be physiologically relevant in inflammatory cells.

A congenital activating mutant of WASp causes altered plasma membrane topography and adhesion under flow in lymphocytes

Leukocytes rely on dynamic actin-dependent changes in cell shape to pass through blood vessels, which is fundamental to immune surveillance. Wiskott-Aldrich Syndrome protein (WASp) is a hematopoietic cell-restricted cytoskeletal regulator important for modulating cell shape through Arp2/3-mediated actin polymerization. A recently identified WASp(I294T) mutation was shown to render WASp constitutively active in vivo, causing increased filamentous (F)-actin polymerization, high podosome turnover in macrophages, and myelodysplasia. The aim of this study was to determine the effect of WASp(I294T) expression in lymphocytes. Here, we report that lymphocytes isolated from a patient with WASp(I294T), and in a cellular model of WASp(I294T), displayed abnormal microvillar architecture, associated with an increase in total cellular F-actin. Microvillus function was additionally altered as lymphocytes bearing the WASp(I294T) mutation failed to roll normally on L-selectin ligand under flow. This was not because of defects in L-selectin expression, shedding, cytoskeletal anchorage, or membranal positioning; however, under static conditions of adhesion, WASp(I294T)-expressing lymphocytes exhibited altered dynamic interaction with L-selectin ligand, with a significantly reduced rate of adhesion turnover. Together, our results demonstrate that WASp(I294T) significantly affects lymphocyte membrane topography and L-selectin-dependent adhesion, which may be linked to defective hematopoiesis and leukocyte function in affected patients.

Nanoparticle coatings for enhanced capture of flowing cells in microtubes

Recently, a flow-based selectin-dependent method for the capture and enrichment of specific types of cells (CD34+ hematopoetic stem and progenitor cells and human leukemia HL60) from peripheral blood was demonstrated. However, these devices depend on a monolayer of selectin protein, which has been shown to have a maximum binding efficiency as a function of surface area. A novel surface coating of colloidal silica nanoparticles was designed that alters the surface roughness resulting in increased surface area. The nanoparticles were adhered using either an inorganic titanate resinous coating or an organic polymer of poly-L-lysine. Using Alexa Fluor 647 conjugated P-selectin, an increase in protein adsorption of up to 35% when compared to control was observed. During perfusion experiments using P-selectin-coated microtubes, we observed increased cell capture and greatly decreased rolling velocity at equivalent protein concentration compared to nonparticle control. Atomic force microscopy showed increased surface roughness consistent with the nanoparticle mean diameter, suggesting a monolayer of particles. These results support the coating's potential to improve existing cell capture implantable devices for a variety of therapeutic and scientific uses.

Myeloid-derived suppressor cells down-regulate L-selectin expression on CD4+ and CD8+ T cells

Effective cell-mediated antitumor immunity requires the activation of tumor-reactive T cells and the trafficking of activated T cells to tumor sites. These processes involve the extravasation of lymphocytes from the blood and lymphatics, and their homing to lymph nodes and tumors. L-selectin (CD62L) is an important molecule in these processes. It directs naive lymphocytes to peripheral lymph nodes where they become activated and it traffics naive lymphocytes to inflammatory environments, such as tumors. Individuals with advanced cancer are immune suppressed due to myeloid-derived suppressor cells (MDSC), a population of immature myeloid cells that accumulate to high levels in response to tumor-secreted and proinflammatory factors. We now demonstrate that the reduction in T cell levels of L-selectin that is commonly seen in individuals with cancer inversely correlates with MDSC levels. Three lines of evidence demonstrate that MDSC directly down-regulate L-selectin on naive T cells: 1) naive T cells cocultured with tumor-induced MDSC have reduced L-selectin; 2) T cells in tumor-free aged mice with elevated levels of MDSC have reduced L-selectin, and 3) peritoneal exudate T cells of tumor-free mice treated with plasminogen activator urokinase to elevate MDSC have reduced levels of L-selectin. MDSC are likely to down-regulate L-selectin through their plasma membrane expression of ADAM17 (a disintegrin and metalloproteinase domain 17), an enzyme that cleaves the ectodomain of L-selectin. Therefore, MDSC down-regulate L-selectin levels on naive T cells, decreasing their ability to home to sites where they would be activated. This is another mechanism by which MDSC inhibit antitumor immunity.

Adhesive dynamics simulations of the mechanical shedding of L-selectin from the neutrophil surface

Here we accurately recreate the mechanical shedding of L-selectin and its effect on the rolling behavior of neutrophils in vitro using the adhesive dynamics simulation by incorporating the shear-dependent shedding of L-selectin. We have previously shown that constitutively expressed L-selectin is cleaved from the neutrophil surface during rolling on a sialyl Lewis x-coated planar surface at physiological shear rates without the addition of exogenous stimuli. Utilizing a Bell-like model to describe a shedding rate which presumably increases exponentially with force, we were able to reconstruct the characteristics of L-selectin-mediated neutrophil rolling observed in the experiments. First, the rolling velocity was found to increase during rolling due to the mechanical shedding of L-selectin. When most of the L-selectin concentrated on the tips of deformable microvilli was cleaved by force exerted on the L-selectin bonds, the cell detached from the reactive plane to join the free stream as observed in the experiments. In summary, we show through detailed computational modeling that the force-dependent shedding of L-selectin can explain the rolling behavior of neutrophils mediated by L-selectin in vitro.

Microcontact printing of P-selectin increases the rate of neutrophil recruitment under shear flow

The local variation of P-selectin expression on inflamed endothelial layers affects leukocyte recruitment in vivo. As an initial study of the spatially heterogeneous presentation of P-selectin in vitro, the influence of microcontact printing (microCP) of P-selectin on a planar surface in neutrophil recruitment was investigated using a parallel-plate flow chamber. Microline patterned and nonpatterned P-selectin were prepared using PDMS, Poly(dimethylsiloxane) (PMDS) stamps and isolated neutrophils perfused over the surface to quantify the level of neutrophil recruitment. We first found a significant increase in cell rolling flux and a decrease in cell rolling velocity on the microcontact printed P-selectin-surfaces compared with a randomly adsorbed P-selectin-surface. However, the increase in rolling adhesion under shear on the surfaces prepared by microCP was not proportional to the number of functional sites of P-selectin transferred using immunofluorescent labeling. Interestingly, the relative immunofluorescent intensities of the nonfunctional regions of microcontact printed P-selectin-surfaces were substantially lower than that that of randomly adsorbed P-selectin. Taken together, these data indicate that the microCP of selectin increases the transfer rate of the adhesion molecule on a surface in the functionally correct orientation and, consequently, improves the recruitment of leukocytes to the selectin surface under flow. It is concluded that microCP may be a more general technique to control protein orientation on a substrate.

Inhibition of Na+/H+ exchanger enhances low pH-induced L-selectin shedding and beta2-integrin surface expression in human neutrophils

Ischemia-reperfusion injury is a common pathological occurrence causing tissue damage in heart attack and stroke. Entrapment of neutrophils in the vasculature during ischemic events has been implicated in this process. In this study, we examine the effects that lactacidosis and consequent reductions in intracellular pH (pH(i)) have on surface expression of adhesion molecules on neutrophils. When human neutrophils were exposed to pH 6 lactate, there was a marked decrease in surface L-selectin (CD62L) levels, and the decrease was significantly enhanced by inclusion of Na(+)/H(+) exchanger (NHE) inhibitor 5-(N,N-hexamethylene)amiloride (HMA). Similar effects were observed when pH(i) was reduced while maintaining normal extracellular pH, by using an NH(4)Cl prepulse followed by washes and incubation in pH 7.4 buffer containing NHE inhibitors [HMA, cariporide, or 5-(N,N-dimethyl)amiloride (DMA)]. The amount of L-selectin shedding induced by different concentrations of NH(4)Cl in the prepulse correlated with the level of intracellular acidification with an apparent pK of 6.3. In contrast, beta(2)-integrin (CD11b and CD18) was only slightly upregulated in the low-pH(i) condition and was enhanced by NHE inhibition to a much lesser extent. L-selectin shedding was prevented by treating human neutrophils with inhibitors of extracellular metalloproteases (RO-31-9790 and KD-IX-73-4) or with inhibitors of intracellular signaling via p38 MAP kinase (SB-203580 and SB-239063), implying a transmembrane effect of pH(i). Taken together, these data suggest that the ability of NHE inhibitors such as HMA to reduce ischemia-reperfusion injury may be related to the nearly complete removal of L-selectin from the neutrophil surface.

Shear-induced capping of L-selectin on the neutrophil surface during centrifugation

L-selectin on leukocytes is critical in leukocyte tethering and adhesion to inflamed endothelium and lymphocyte homing to lymphoid organs. The spatial distribution of L-selectin on leukocytes controls cellular adhesive function in hydrodynamic shear. How L-selectin changes its position on the cell membrane remains an open question, but a possible candidate is shear stress encountered on the cell surface. Here we demonstrate shear-induced L-selectin polarization on the membrane during the process of centrifugation of resting neutrophils via immunofluorescent microscopy. It was found that randomly distributed L-selectin on neutrophils moves to a polar cap at one end of the cell after centrifugation (300 x g for 2 min) without inflammatory stimuli. This L-selectin redistribution under shear was predicted by Monte Carlo simulations that show how convection dominates over diffusion, leading to L-selectin cap formation during centrifugation at 280 x g or during leukocyte adhesion to the endothelial wall at 1 dyn/cm(2). Those results point to a role for shear stress in the modulation of L-selectin distribution, and suggest a possible alternate mechanism and reinterpretation of previous in vitro studies of L-selectin mediated adhesion of neutrophils isolated via centrifugation.

Phosphoinositide 3-Kinase γ Inhibition Plays a Crucial Role in Early Steps of Inflammation by Blocking Neutrophil Recruitment

Leukocyte trafficking to inflammatory sites is a gradual process, which is dominated in its early phases by chemokine- and cytokine-mediated neutrophil recruitment. The chemokine regulated on activation normal T cell expressed and secreted (RANTES) has been shown to be highly expressed in the joints of patient with rheumatoid arthritis and to promote leukocyte trafficking into the synovial tissue. In this study, we investigated the effect of RANTES in a murine model of peritoneal chemotaxis, and we found that RANTES dose-dependently induces neutrophil recruitment. Then, through morphological and histological analyses, we observed that activated neutrophils represent the major infiltrating population in response to RANTES chemotactic stimulus. Furthermore, we demonstrated that oral administration of either nonisoform-specific phosphoinositide 3-kinase (PI3K) inhibitor LY294002 (morpholin-4-yl-8-phenylchromen-4-one) or selective PI3Kgamma inhibitor AS041164 (5-benzo[1,3]dioxol-5-ylmethylene-thiazolidine-2,4-dione) blocks RANTES-induced chemotaxis and reduces the level of AKT phosphorylation. Because the two compounds showed a similar pharmacokinetic profile in terms of bioavailability and half-life after oral route administration, the selective inhibition of the PI3Kgamma-isoform pathway through AS041164 was three times more potent in reducing neutrophil recruitment. Finally, to confirm the blockade of neutrophil infiltration that occurs in the early phase of the inflammatory response, AS041164 was also tested in a model of carrageenan-induced paw edema in rats. Therefore, the PI3Kgamma pathway plays an important role in controlling neutrophil chemotaxis during early steps of inflammation.