Micromanipulation of adhesion of phorbol 12-myristate-13-acetate-stimulated T lymphocytes to planar membranes containing intercellular adhesion molecule-1

DNA Aptamers in the Detection of Leukemia Cells by the Thickness Shear Mode Acoustics Method

By using the thickness shear mode acoustics method (TSM) and single-molecule force spectroscopy (SMFS) we studied the interactions between DNA aptamers (sgc8c) specific to the protein tyrosine kinase 7 (PTK7), which is localized in the membranes of leukemia lymphoblastics (MOLT-4), and lymphocyte (Jurkat) cell lines, as well with PTK7-negative U266 myeloid leukemia cells. The TSM method allowed the development of a highly sensitive, label-free biosensor for the detection leukemia cells with a limit of detection of (195±20) cells/mL. SMFS approved the high selectivity of the sgc8c aptamers to the PTK7 receptors at the cell surface and allowed determining the binding probability of the aptamers to the PTK7 receptors at different cell lines.

Assay of Adhesion Under Shear Stress for the Study of T Lymphocyte-Adhesion Molecule Interactions

Overall, T cell adhesion is a critical component of function, contributing to the distinct processes of cellular recruitment to sites of inflammation and interaction with antigen presenting cells (APC) in the formation of immunological synapses. These two contexts of T cell adhesion differ in that T cell-APC interactions can be considered static, while T cell-blood vessel interactions are challenged by the shear stress generated by circulation itself. T cell-APC interactions are classified as static in that the two cellular partners are static relative to each other. Usually, this interaction occurs within the lymph nodes. As a T cell interacts with the blood vessel wall, the cells arrest and must resist the generated shear stress.(1,2) These differences highlight the need to better understand static adhesion and adhesion under flow conditions as two distinct regulatory processes. The regulation of T cell adhesion can be most succinctly described as controlling the affinity state of integrin molecules expressed on the cell surface, and thereby regulating the interaction of integrins with the adhesion molecule ligands expressed on the surface of the interacting cell. Our current understanding of the regulation of integrin affinity states comes from often simplistic in vitro model systems. The assay of adhesion using flow conditions described here allows for the visualization and accurate quantification of T cell-epithelial cell interactions in real time following a stimulus. An adhesion under flow assay can be applied to studies of adhesion signaling within T cells following treatment with inhibitory or stimulatory substances. Additionally, this assay can be expanded beyond T cell signaling to any adhesive leukocyte population and any integrin-adhesion molecule pair.

Micro-PTV measurement of the fluid shear stress acting on adherent leukocytes in vivo

Leukocyte adhesion is determined by the balance between molecular adhesive forces and convective dispersive forces. A key parameter influencing leukocyte adhesion is the shear stress acting on the leukocyte. This measure is indispensable for determining the molecular bond forces and estimating cell deformation. To experimentally determine this shear stress, we used microparticle tracking velocimetry analyzing more than 24,000 images of 0.5 microm fluorescent microbeads flowing within mildly inflamed postcapillary venules of the cremaster muscle in vivo. Green fluorescent protein, expressed under the lysozyme-M promoter, made leukocytes visible. After applying stringent quality criteria, 3 of 69 recordings were fully analyzed. We show that endothelial cells, but not leukocytes, are covered by a significant surface layer. The wall shear rate is nearly zero near the adherent arc of each leukocyte and reaches a maximum at the apex. This peak shear rate is 2-6-fold higher than the wall shear rate in the absence of a leukocyte. Microbead trajectories show a systematic deviation toward and away from the microvessel axis upstream and downstream from the leukocyte, respectively. The flow field around adherent leukocytes in vivo allows more accurate estimates of bond forces in rolling and adherent leukocytes and improved modeling studies.

Cooperative adhesion of ligand-receptor bonds

Cooperative (simultaneous) breakage of multiple adhesive bonds has been proposed as a mechanism for enhanced binding strength between adhesion molecules on apposing cell surfaces. In this report, we used the atomic force microscopy (AFM) to study how changes in binding affinity and separation rate of force-induced ligand-receptor dissociation affect binding cooperativity. The AFM force measurements were carried out using (strept)avidin-functionalized cantilever tips and biotinylated agarose beads under conditions where multiple (strept)avidin-biotin linkages were formed following surface contact. At slow surface separation of the AFM cantilever from the bead's surface, the (strept)avidin-biotin linkages appeared to rupture sequentially. Increasing the separation rate from 210 to 1950 nm/s led to a linear increase in the average rupture force. Moreover, force histograms revealed a quantized force distribution that shifted toward higher values with increasing separation rate. In measurements of streptavidin-iminobiotin adhesion, the force distribution also shifted toward higher values when the buffer was adjusted to a higher pH to raise the binding affinity. Together, these results demonstrate that the cooperativity of ligand-receptor bonds is significantly enhanced by increases in surface separation rate and/or binding affinity.

Reaction of the skin fibroblast cytoskeleton to micromanipulation interventions

Micromanipulation is a strong mechanical intervention into cellular integrity and induces large changes in the fine structure of the treated cells. Human diploid skin fibroblasts (KF1 and KF2 cell lines) were chosen as an experimental model. Special hatching needles were used for defined micromanipulation interventions (deformation of plasma membrane). Changes in cytoskeletal structures were visualized by using fluorescent and confocal microscopy. The actin cytoskeleton showed a more sensitive response to micromanipulation than microtubules. Characteristic changes in microfilaments, i.e., thickenings and knot formation, were visible in treated cells fixed immediately after micromanipulation and were the result of hatching-needle pressure on the plasma membrane as well as a reaction of actin filaments localized near the plasma membrane deformation. These direct changes and also other specific alterations in the actin filament network were detectable 14 to 16 h after treatment, but they were not observed when longer reparation intervals were used.

Intrinsic activating properties of GP IIb/IIIa blockers

Potential intrinsic activating properties are probably the most controversially discussed issues with respect to GP IIb/IIIa blockers, especially since clinical trials with oral GP IIb/IIIa blockers revealed disappointing results. Based on the finding that currently clinically used GP IIb/IIIa blockers are ligand mimetics, experimental data are discussed, demonstrating an intrinsic activating effect of ligand mimetic GP IIb/IIIa blockers that potentially results in fibrinogen binding to alpha(IIb)beta(3) and in platelet aggregation. Furthermore, the inhibitory effect of aspirin on GP IIb/IIIa blocker-induced platelet aggregation is discussed as a clinically relevant finding. Finally, the potential association of GP IIb/IIIa blocker-induced thrombocytopenia with platelet activation is described.

Adhesion energy of receptor-mediated interaction measured by elastic deformation

We investigated the role of receptor binding affinity in surface adhesion. A sensitive technique was developed to measure the surface energy of receptor-mediated adhesion. The experimental system involved a functionalized elastic agarose bead resting on a functionalized glass coverslip. Attractive intersurface forces pulled the two surfaces together, deforming the bead to produce an enlarged contact area. The Johnson-Kendall-Roberts (JKR) model was used to relate the surface energy of the interaction to the elasticity of the bead and the area of contact. The surface energies for different combinations of modified surfaces in solution were obtained from reflection interference contrast microscopy (RICM) measurements of the contact area formed by the bead and the coverslip. Studies with surfaces functionalized with ligand-receptor pairs showed that the relationship between surface energy and the association constant of the ligand binding has two regimes. At low binding affinity, surface energy increased linearly with the association constant, while surface energy increased logarithmically with the association constant in the high affinity regime.

Development of a side-view chamber for studying cell-surface adhesion under flow conditions

Observing microscopic specimens is often useful in studies of cellular interaction with a vascular wall. We have developed an in vitro side-view flow chamber that permits observations from the side of the cell's contact with various adhesive surfaces under dynamic flow conditions. This side-view flow chamber consists of two precision rectangular glass tubes called microslides. A smaller microslide is inserted into a larger one to create a flow channel with a flat surface on which either cultured vascular endothelium can be grown or purified adhesion molecules can be coated. Two optical prisms with a 45 degrees chromium-coated surface are used along the flow channel to generate light illumination and observation pathways. The side-view images of cell-substrate contact can be obtained using a light microscope. This design allows us not only to measure the effects of flow on cell-surface adhesion strength, but also to have close observation of cell deformation and adhesive contact to various surfaces in shear flow. In addition, this chamber can readily serve for a conventional top-view flow channel, similar to the parallel-plate flow chambers used in many areas. The development of such a side-view flow chamber can be beneficial to various in vitro applications in cellular studies that require an edge view, especially for various cell interactions with cultured vascular endothelium or surfaces containing single-type adhesive molecules under flow conditions.

Variation in the velocity, deformation, and adhesion energy density of leukocytes rolling within venules

Leukocyte rolling along the endothelium in inflammation is caused by continuous formation and breakage of bonds between selectin adhesion molecules and their ligands. We investigated trauma-induced leukocyte rolling in venules (diameter, 23 to 58 microns; wall shear stress, 1.2 to 35 dyne/cm2) of the exteriorized rat mesentery using high-resolution intravital microscopy. While rolling, the leukocytes deformed into a tear-droplike shape. Deformation continued to increase with shear stress up to the highest values observed (35 dyne/cm2). Successive leukocytes had similar rolling velocities at the same axial positions along each vessel, suggesting that heterogeneity of endothelial adhesiveness is responsible for velocity variation. Adhesion energy density varied inversely with instantaneous rolling velocity and directly with instantaneous deformation. Adhesion energy density reached a maximum of 0.36 dyne/cm, similar to values found for lymphocyte function-associated antigen-1-dependent adhesion of stimulated T cells to isolated intercellular adhesion molecule-1. We conclude that selectin-mediated adhesion during rolling produces adhesion energy densities comparable to those observed for integrin-mediated adhesion events in other experimental systems.

Physical response of collagen gels to tensile strain

Extra cellular matrix, which provides physical support to epithelial and endothelial cells and to fibroblasts, also affects a number of important cell biological phenomena, such as cell motility and angiogenesis. Although type I collagen has long been recognized as the primary structural component of the extra cellular matrix, little is known about the physical properties of collagen gels. In this study, we used a servo-controlled linear actuator to impose quick stretches on dilute collagen gels. An axial strain imposed on the gel within few milliseconds resulted in a rapid development of gel tension in the direction of the strain. The gel tension then decayed toward a steady-state value within several seconds. The instantaneous gel stiffness increased and the relaxed gel stiffness decreased with the extent of gel stretching. These rheological parameters were also dependent on the density of the collagen network. Taken together the results indicated that collagen gels possess nonlinear viscoelastic properties.

Role of E-cadherin in the response of tumor cell aggregates to lymphatic, venous and arterial flow: measurement of cell-cell adhesion strength

Defects in the expression or function of the calcium dependent cell-cell adhesion molecule E-cadherin are common in invasive, metastatic carcinomas. In the present study the response of aggregates of breast epithelial cells and breast and colon carcinoma cells to forces imposed by laminar flow in a parallel plate flow channel was examined. Although E-cadherin negative tumor cells formed cell aggregates in the presence of calcium, these were significantly more likely than E-cadherin positive cell aggregates to disaggregate in response to low shear forces, such as those found in a lymphatic vessel or venule (< 3.5 dyn/cm2). E-cadherin positive normal breast epithelial cells and E-cadherin positive breast tumor cell aggregates could not be disaggregated when exposed to shear forces in excess of those found in arteries (> 100 dyn/cm2). E-cadherin negative cancer cells which had been transfected with E-cadherin exhibited large increases in adhesion strength only if the expressed protein was appropriately linked to the cytoskeleton. These results show that E-cadherin negative tumor cells, or cells in which the adhesion molecule is present but is inefficiently linked to the cytoskeleton, are far more likely than E-cadherin positive cells to detach from a tumor mass in response to low shear forces, such as those found in a lymphatic vessel or venule. Since a primary route of dissemination of many carcinoma cells is to the local lymph nodes these results point to a novel mechanism whereby defects in cell-cell adhesion could lead to carcinoma cell dissemination.

Modulation of cell adhesion by changes in alpha L beta 2 (LFA-1, CD11a/CD18) cytoplasmic domain/cytoskeleton interaction

The integrin alpha L beta 2 (leukocyte function-associated molecule 1, CD11a/CD18) mediates activation-dependent adhesion of leukocytes. The cytoplasmic domains of alpha L beta 2 have been demonstrated to modulate adhesiveness of alpha L beta 2. Affinity changes of alpha L beta 2 for its ligand or postreceptor events can be responsible for this modulation of adhesiveness. To investigate the possible role of the alpha L beta 2 cytoplasmic domains in postreceptor events we constructed cDNA encoding chimeric proteins with intracellular alpha L beta 2 domains, which are responsible for alpha L beta 2 specific intracellular interactions, and extracellular alpha IIIb beta 3 (GP IIb/IIIa) domains, which allow the assessment of the receptor affinity state. The cDNA was stably transfected in Chinese hamster ovary cells and chimeric heterodimer formation proven by immunoprecipitations and flow cytometry. The chimeric receptors mediate adhesion to immobilized fibrinogen, and this adhesion is increased by phorbol myristate acetate and abolished by cytochalasin D. However, neither treatment affects the affinity state of the chimeric receptor, suggesting involvement of the cytoskeleton in the regulation of alpha L beta 2 mediated cell adhesion. To exclude the possibility of postoccupancy affinity changes of the chimeric receptors, we locked the receptors into a high affinity state by creating a deletion variant. The region deleted (VGFFK) is highly conserved in integrin alpha subunit cytoplasmic domains. Cotransfection of this deletion variant with a beta subunit truncation (beta 3 delta 724) and a triple mutation at 758-760 (TTT to AAA) of beta 2 abolishes adhesion without changing the affinity state. A single mutation (TTT to TAT) reduces adhesion by half without affinity change. Scanning electron microscopy reveals impaired spreading of these truncated/mutated chimeras. Immunofluorescence microscopy demonstrates a correlation between impaired adhesion and a decrease in the ability to form focal adhesions and to organize the cytoskeleton into stress fibers. These results describe the integrin/cytoskeleton interaction, the organization of the cytoskeleton, and cell spreading as postreceptor events modulating alpha L beta 2 cytoplasmic domain mediated cell adhesion. Furthermore, we demonstrate that the cytoplasmic domain of the beta 2 subunit, and within it the TTT region, are required for these postreceptor events. Additionally, we present a new approach, using deletion variants to lock integrins in a high affinity state without interfering with the investigated integrin/cytoskeleton interaction. This approach may be generally useful to investigate the role of postreceptor events in integrin-mediated cell adhesion and migration.

Stimulation of integrin-mediated adhesion of T lymphocytes and monocytes: two mechanisms with divergent biological consequences

We show that the adhesion of T lymphoid cells to immobilized fibronectin can be increased by two distinct mechanisms. The first is by increasing the affinity of the fibronectin receptor/ligand interaction using the anti-beta 1 integrin monoclonal antibody 8A2. The second is by treating the cells with phorbol 12-myristate 13-acetate (PMA), which alters events that occur after receptor occupancy (e.g., cell spreading) without affecting receptor affinity. The effects of these two mechanisms on adhesion in the presence of physiological concentrations of soluble fibronectin suggest that they have different biological consequences. Under these conditions, the net effect of increasing the affinity of the fibronectin receptors is to decrease cell adhesion, whereas the increase in adhesion induced by PMA is unaffected. This suggests that the high affinity receptors are not primarily available for cell adhesion under these circumstances, and that they have an alternative function. We further show that high affinity binding of soluble fibronectin can be induced by either differentiation of the monocytic cell line THP-1 or by cross-linking the T cell receptor complexes on the T lymphoid cell line HUT-78. The differentiated monocytic cells express two populations of fibronectin receptors: a minority in a high affinity state, and the majority in a low affinity state. Thus they will both continue to adhere in the presence of physiological concentrations of soluble fibronectin and bind significant amounts of soluble fibronectin at the cell surface.

Interaction forces between red cells agglutinated by antibody. IV. Time and force dependence of break-up

We report on an extension of a previously described method to measure the hydrodynamic force to separate doublets of fixed, sphered and swollen red cells cross-linked by antibody (S. P. Tha, J. Shuster, and H. L. Goldsmith. 1986. Biophys. J. 50:1117-1126). With a traveling microtube apparatus, doublets are tracked and videotaped in a slowly accelerating Poiseuille flow in 150-microns-diameter tubes, and the hydrodynamic normal force at break-up, Fn, is computed from the measured doublet velocity and radial position. Previous results showed a large range of Fn, the mean of which increased with [antiserum], and an absence of clustering at discrete values of Fn. Since it was assumed that the cells separate the instant a critical force to break all crossbridges was reached, lack of clustering could have been due to the use of a polyclonal antiserum. We therefore studied the effect of monoclonal IgM or IgA antibody on the distribution of Fn. The results showed that the data are as scattered as ever, with Fn varying from 2 to 200 pN, and exhibit no evidence of clustering. However, the scatter in Fn could be due to the stochastic nature of intercellular bonds (E. Evans, D. Berk, and A. Leung. 1991a. Biophys. J. 59:838-848). We therefore studied the force dependence of the time to break-up under constant shear stress (Fn from 30 to 200 pN), both in Poiseuille and Couette flow, the latter by using a counter-rotating cone and plate rheoscope. When 280 doublets were rapidly accelerated in the traveling microtube and then allowed to coast in steady flow for up to 180 s, 91% survived into the constant force region; 16% of these broke up after time intervals, tP, of 2-30s. Of 340 doublets immediately exposed to constant shear in the rheoscope, 37% broke after time intervals, tc, from < 1 to 10 s. Thus, doublets do indeed break up under a constant shear stress, if given time. The average time to break-up decreased significantly with increasing force, while the fraction of doublets broken up increased. At a given Fn, the fraction of break-ups decreased with increasing [IgM], suggesting that the average number of bonds had also increased. Using a stochastic model of break-up (G. I. Bell. 1978. Science (Washington DC). 200:618-627; E. Evans, D. Berk,and A. Leung. 1991. Biophys. J. 59:838-848) and a Poisson distribution for the number of bonds, Nb, break-up in slowly accelerating Poiseuille flow and in immediate shear application in Couette flow was simulated. In Poiseuille flow, the observed range and scatter in Fn could be reproduced assuming (Nb) > 5. In the rheoscope, the time intervals and number of rotations to break-up, tc, were quite well reproduced assuming (Nb) = 4. The similarity of (Fn) for monoclonal IgM and IgA for doublet break-up under constant slow acceleration is compatible with the conclusion of Evans et al. (1991 a) for normal red cells and Xia et al. (manuscript submitted for publication) for sphered and swollen red cells, that the applied force extracts the antigen from the cell membrane.

Viscosity of passive human neutrophils undergoing small deformations

At issue is the type of constitutive equation that can be used to describe all possible types of deformation of the neutrophil. Here a neutrophil undergoing small deformations is studied by aspirating it into a glass pipet with a diameter that is only slightly smaller than the diameter of the spherically shaped cell. After being held in the pipet for at least seven seconds, the cell is rapidly expelled and allowed to recover its undeformed, spherical shape. The recovery takes approximately 15 s. An analysis of the recovery process that treats the cell as a simple Newtonian liquid drop with a constant cortical (surface) tension gives a value of 3.3 x 10(-5) cm/s for the ratio of the cortical tension to cytoplasmic viscosity. This value is about twice as large as a previously published value obtained with the same model from studies of large deformations of neutrophils. This discrepancy indicates that the cytoplasmic viscosity decreases as the amount of deformation decreases. An extrapolated value for the cytoplasmic viscosity at zero deformation is approximately 600 poise when a value for the cortical tension of 0.024 dyn/cm is assumed. Clearly the neutrophil does not behave like a simple Newtonian liquid drop in that small deformations are inherently different from large deformations. More complex models consisting either of two or more fluids or multiple shells must be developed. The complex structure inside the neutrophil is shown in scanning electron micrographs of osmotically burst cells and cells whose membrane has been dissolved away.