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

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

Halloysite Nanotube Coatings Suppress Leukocyte Spreading

The nanoscale topography of adhesive surfaces is known to be an important factor governing cellular behavior. Previous work has shown that surface coatings composed of halloysite nanotubes enhance the adhesion, and therefore capture of, rare target cells such as circulating tumor cells. Here we demonstrate a unique feature of these coatings in their ability to reduce the adhesion of leukocytes and prevent leukocyte spreading. Surfaces were prepared with coatings of halloysite nanotubes and functionalized for leukocyte adhesion with E-selectin, and the dilution of nanotube concentration revealed a threshold concentration below which cell spreading became comparable to smooth surfaces. Evaluation of surface roughness characteristics determined that the average distance between discrete surface features correlated with adhesion metrics, with a separation distance of ∼2 μm identified as the critical threshold. Computational modeling of the interaction of leukocytes with halloysite nanotube-coated surfaces of varying concentrations demonstrates that the geometry of the cell surface and adhesive counter-surface produces a significantly diminished effective contact area compared to a leukocyte interacting with a smooth surface.

Five days of head-down-tilt bed rest induces noninflammatory shedding of L-selectin

Head-down-tilt bed rest (HDTBR) is a popular model, simulating alterations of gravitation during space missions. The aim of this study was to obtain a better insight into the complexly orchestrated regulations of HDTBR-induced immunological responses, hypothesizing that artificial gravity can mitigate these HDTBR-related physiological effects. This crossover-designed 5 days of HDTBR study included three protocols with no, or daily 30 min of centrifugation or 6 × 5 min of centrifugation. Twelve healthy, male participants donated blood pre-HDTBR, post-HDTBR, and twice during HDTBR. Cellular immune changes were assessed either by enumerative and immune cell phenotyping assays or by functional testing of responses to either recall antigens or receptor-dependent activation by chemotactic agents N-formyl-methionyl-leucyl-phenylalanine (fMLP) and with TNF-α. The expression of the adhesion molecule L-selectin (CD62L) on the surface of granulocytes and its shedding into plasma samples were measured. In parallel, other humoral factor, such as interleukin-6 and interleukin-8, parameters of endothelial damage (glycocalyx) were determined. Hematocrit and hemoglobin were significantly increased during HDTBR. Although immune functional tests did not indicate a change in the immune performance, the expression of CD62L on resting granulocytes was significantly shed by 50% during HDTBR. Although the latter is normally associated to an activation of inflammatory innate immune responses and during interaction of granulocytes with the endothelium, CD62L shedding was, however, not related either to a systemic inflammatory alteration or to shedding of the endothelial glycocalyx during bed rest. This suggests a noninflammatory or “mechanical” shedding related to fluid shifts during head-down intervention and not to an acute inflammatory process.

Glycomechanics of the metastatic cascade: tumor cell-endothelial cell interactions in the circulation

Hydrodynamic shear force plays an important role in the leukocyte adhesion cascade that involves the tethering and rolling of cells along the endothelial layer, their firm adhesion or arrest, and their extravasation or escape from the circulatory system by inducing passive deformation, or cell flattening, and microvilli stretching, as well as regulating the expression, distribution, and conformation of adhesion molecules on leukocytes and the endothelial layer. Similarly, the dissemination of circulating tumor cells (CTCs) from the primary tumor sites is believed to involve tethering, rolling, and firm adhesion steps before their eventual extravasation which leads to secondary tumor sites (metastasis). Of particular importance to both the leukocyte adhesion cascade and the extravasation of CTCs, glycoproteins are involved in all three steps (capture, rolling, and firm adhesion) and consist of a variety of important selectin ligands. This review article provides an overview of glycoprotein glycosylation associated with the abnormal glycan expression on cancer cell surfaces, where well-established and novel selectin ligands that are cancer related are discussed. An overview of computational approaches on the effects of fluid mechanical force on glycoprotein mediated cancer cell rolling and adhesion is presented with a highlight of recent flow-based and selectin-mediated cell capturing/enriching devices. Finally, as an important branch of the glycoprotein family, mucins, specifically MUC1, are discussed in the context of their aberrant expression on cancer cells and their role as cancer cell adhesion molecules. Since metastasis relies heavily on glycoprotein interactions in the bloodstream where the fluid shear stress highly regulates cell adhesion forces, it is important to study and understand the glycomechanics of all relevant glycoproteins (well-established and novel) as they relate to the metastatic cascade.

Vascular Recruitment of Human Retinoblastoma Cells by Multi-Cellular Adhesive Interactions with Circulating Leukocytes

Retinoblastoma (RB) is a retinal cancer of childhood. RB survivors tend to develop additional tumors later in life, although the physical mechanisms of RB metastatic spread are largely unknown. One step in metastasis through the blood stream is tumor cell adherence to the blood vessel wall through specific receptor:ligand interactions. Yet, human RB cell lines RB143 and WERI-Rb27 do not express selectin ligands or beta-2 integrins and cannot directly interact with inflamed endothelium. In this study, we show that RB cells express ICAM-1, a beta-2 integrin ligand that correlates with metastasis and is preferentially co-expressed on RB cells that also express ABCG2, a stem cell marker associated with chemoresistance and metastasis. Based on the presence of ICAM-1+ RB cells, we tested the hypothesis that RB cells could be recruited to an E-selectin surface via attachment to activated polymorphonuclear cells (PMNs). We characterized the dynamic adhesion between RB cells and PMNs within E-selectin coated microtubes under a physiological range of wall shear stress values (0.2-5 dyn/cm²). We show that activated PMNs are necessary for the recruitment of RB cells through ICAM-1:LFA-1 binding. Results from this work may lead to new strategies that target the metastatic spread of tumor cells.

Mechanical stress induces tumor necrosis factor-{alpha} production through Ca2+ release-dependent TLR2 signaling

We studied centrifugation-mediated mechanical stress-induced tumor necrosis factor-alpha (TNF-alpha) production in the monocyte-like cell line THP-1. The induction of TNF-alpha by mechanical stress was dependent on the centrifugation speed and produced the highest level of TNF-alpha after 1 h of stimulation. TNF-alpha production returned to normal levels after 24 h of stimulation. Mechanical stress also induced Toll-like receptor-2 (TLR2) mRNA in proportion to the expression of TNF-alpha. The inhibition of TLR2 signaling by dominant negative myeloid differentiation factor 88 (MyD88) blocked TNF-alpha expression response to mechanical stress. After transient overexpression of TLR2 in HEK-293 cells, mechanical stress induced TNF-alpha mRNA production. Interestingly, mechanical stress activated the c-Src-dependent TLR2 phosphorylation, which is necessary to induce Ca(2+) fluxes. When THP-1 cells were pretreated with BAPTA-AM, thapsigargin, and NiCl(2).6H(2)O, followed by mechanical stimulation, both TLR2 and TNF-alpha production were inhibited, indicating that centrifugation-mediated mechanical stress induces both TLR2 and TNF-alpha production through Ca(2+) releases from intracellular Ca(2+) stores following TLR2 phosphorylation. In addition, TNF-alpha treatment in THP-1 cells induced TLR2 production in response to mechanical stress, whereas the preincubation of anti-TNF-alpha antibody scarcely induced the mechanical stress-mediated production of TLR2, indicating that TNF-alpha produced by mechanically stimulated THP-1 cells affected TLR2 production. We concluded that TNF-alpha production induced by centrifugation-mediated mechanical stress is dependent on MyD88-dependent TLR2 signaling that is associated with Ca(2+) release and that TNF-alpha production induced by mechanical stress affects TLR2 production.