Biomedical subjects

S Neelamegham

Publications and source records attributed to S Neelamegham.

9 recordsLinked to original sources

PubMed1998

Venous levels of shear support neutrophil-platelet adhesion and neutrophil aggregation in blood via P-selectin and beta2-integrin.

BACKGROUND: After activation, platelets adhere to neutrophils via P-selectin and beta2-integrin. The molecular mechanisms and adhesion events in whole blood exposed to venous levels of hydrodynamic shear in the absence of exogenous activation remain unknown. METHODS AND RESULTS: Whole blood was sheared at approximately 100 s(-1). The kinetics of neutrophil-platelet adhesion and neutrophil aggregation were measured in real time by flow cytometry. P-selectin was upregulated to the platelet surface in response to shear and was the primary factor mediating neutrophil-platelet adhesion. The extent of neutrophil aggregation increased linearly with platelet adhesion to neutrophils. Blocking either P-selectin, its glycoprotein ligand PSGL-1, or both simultaneously by preincubation with a monoclonal antibody resulted in equivalent inhibition of neutrophil-platelet adhesion (approximately 30%) and neutrophil aggregation (approximately 70%). The residual amount of neutrophil adhesion was blocked with anti-CD11b/CD18. Treatment of blood with prostacyclin analogue ZK36374, which raises cAMP levels in platelets, blocked P-selectin upregulation and neutrophil aggregation to baseline. Complete abrogation of platelet-neutrophil adhesion required both ZK36374 and anti-CD18. Electron microscopic observations of fixed blood specimens revealed that platelets augmented neutrophil aggregation both by forming bridges between neutrophils and through contact-mediated activation. CONCLUSIONS: The results are consistent with a model in which venous levels of shear support platelet adherence to neutrophils via P-selectin binding PSGL-1. This interaction alone is sufficient to mediate neutrophil aggregation. Abrogation of platelet adhesion and aggregation requires blocking Mac-1 in addition to PSGL-1 or P-selectin. The described mechanisms are likely of key importance in the pathogenesis and progression of thrombotic disorders that are exacerbated by leukocyte-platelet aggregation.

Abciximab↗

PubMed1998

Hydrodynamic shear shows distinct roles for LFA-1 and Mac-1 in neutrophil adhesion to intercellular adhesion molecule-1.

The binding of neutrophil beta2 integrin to intercellular adhesion molecule-1 (ICAM-1) expressed on the inflamed endothelium is critical for neutrophil arrest at sites of tissue inflammation. To quantify the strength and kinetics of this interaction, we measured the adhesion between chemotactically stimulated neutrophils and ICAM-1-transfected mouse cells (E3-ICAM) in suspension in a cone-plate viscometer at shear rates typical of venular blood flow (100 s-1 to 500 s-1). The kinetics of aggregation were fit with a mathematical model based on two-body collision theory. This enabled estimation of adhesion efficiency, defined as the probability with which collisions between cells resulted in firm adhesion. The efficiency of beta2-integrin-dependent adhesion was highest ( approximately 0.2) at 100 s-1 and it decreased to approximately zero at 400 s-1. Both LFA-1 and Mac-1 contributed equally to adhesion efficiency over the initial 30 seconds of stimulation, but adhesion was entirely Mac-1-dependent by 120 seconds. Two hydrodynamic parameters were observed to influence integrin-dependent adhesion efficiency: the level of shear stress and the intercellular contact duration. Below a critical shear stress (<2 dyn/cm2), contact duration predominantly limited adhesion efficiency. The estimated minimum contact duration for beta2-integrin binding was approximately 6.5 ms. Above the critical shear stress (>2 dyn/cm2), the efficiency of neutrophil adhesion to E3-ICAM was limited by both the contact duration and the tensile stress. We conclude that at low shear, neutrophil adhesion is modulated independently through either LFA-1 or Mac-1, which initially contribute with equal efficiency, but differ over the duration of chemotactic stimulation.

Animals↗

PubMed1997

Synergy between L-selectin signaling and chemotactic activation during neutrophil adhesion and transmigration.

L-selectin enables capture and rolling of neutrophils on inflamed endothelium. This may facilitate the binding of agonists such as IL-8 and platelet-activating factor (PAF), which signal CD18-mediated firm adhesion and transmigration. Recent studies demonstrate that L-selectin can mediate transmembrane signaling. However, the functional effects of costimulation through agonist and L-selectin require further study. Here, we quantify cell adhesion, motility, and transmigration in response to co-activation through L-selectin and agonist. The surface expression of CD11b/CD18 increased and L-selectin decreased in proportion to the extent of L-selectin cross-linking. A flow cytometric assay was used to measure CD11b/CD18-dependent adhesion to fluorescent beads adsorbed with albumin. Neutrophil adhesion was detected within seconds of adding PAF (20 pM), IL-8 (50 pM), or cross-linking L-selectin. Costimulation through agonist and L-selectin potentiated by up to threefold the rate and extent of bead capture. Stimulation through L-selectin induced membrane ruffling, whereas PAF or IL-8 induced bipolar shape change. L-selectin cross-linking sustained the transient shape change induced by low concentrations (10-50 pM) of agonist. Chemokinesis stimulated by IL-8 was inhibited in the presence of cross-linking L-selectin. This was attributed to enhanced cell spreading following costimulation. Migration across HUVEC monolayers stimulated with IL-1 was also potentiated in the presence of L-selectin cross-linking. We propose that cross-linking of L-selectin and binding of agonist receptors may act synergistically to amplify neutrophil activation and emigration in the inflamed vasculature.

Cell Adhesion↗

PubMed1997

Regulation of human T lymphocyte coactivation with an alpha4 integrin antagonist peptide.

The cyclic hexapeptide CWLDVC (TBC 772) is an antagonist of alpha4 integrins and a potent inhibitor of lymphocyte interactions with fibronectin, vascular cell adhesion molecule-1, and muscosal vascular addressin cell adhesion molecule-1 (MAdCAM-1). As such, peptide TBC 772 effectively inhibits the activation of freshly isolated human T lymphocytes stimulated with purified vascular cell adhesion molecule-1 coimmobilized with anti-CD3 mAb. The influence of peptide binding on distinct sites of the alpha4beta1 complex was determined by flow cytometry and cellular adhesion assays employing a panel of mAbs. Binding of the alpha4-specific mAb L25 and the beta1-specific mAb 33B6 was not altered by the peptide; however, binding of mAb 19H8, which is specific for a combinatorial epitope of alpha4beta1, was dramatically inhibited. Treatment of lymphocytes with the peptide caused an increase in a ligand-induced epitope on beta1 integrin defined by mAb 15/7. In T cell activation studies using coimmobilized anti-CD3 mAb and the anti-integrin mAbs, the peptide had broader inhibitory activity, suppressing costimulation induced by all the integrin mAbs. The peptide was not generally toxic and was integrin selective in its suppressive activity, as coactivation by ligation of CD3 in conjunction with CD28 or CD26 was not affected. These results suggest that the antagonist peptide CWLDVC can effectively neutralize integrin coactivation systems by a mechanism independent of competitive binding.

Antibodies, Monoclonal↗

PubMed1997

A model for the kinetics of homotypic cellular aggregation under static conditions.

We present the formulation and testing of a mathematical model for the kinetics of homotypic cellular aggregation. The model considers cellular aggregation under no-flow conditions as a two-step process. Individual cells and cell aggregates 1) move on the tissue culture surface and 2) collide with other cells (or aggregates). These collisions lead to the formation of intercellular bonds. The aggregation kinetics are described by a system of coupled, nonlinear ordinary differential equations, and the collision frequency kernel is derived by extending Smoluchowski's colloidal flocculation theory to cell migration and aggregation on a two-dimensional surface. Our results indicate that aggregation rates strongly depend upon the motility of cells and cell aggregates, the frequency of cell-cell collisions, and the strength of intercellular bonds. Model predictions agree well with data from homotypic lymphocyte aggregation experiments using Jurkat cells activated by 33B6, an antibody to the beta 1 integrin. Since cell migration speeds and all the other model parameters can be independently measured, the aggregation model provides a quantitative methodology by which we can accurately evaluate the adhesivity and aggregation behavior of cells.

Antibodies↗

PubMed1997

Modeling the reversible kinetics of neutrophil aggregation under hydrodynamic shear.

Neutrophil emigration into inflamed tissue is mediated by beta 2-integrin and L-selectin adhesion receptors. Homotypic neutrophil aggregation is also dependent on these molecules, and it provides a model system in which to study adhesion dynamics. In the current study we formulated a mathematical model for cellular aggregation in a linear shear field based on Smoluchowski's two-body collision theory. Neutrophil suspensions activated with chemotactic stimulus and sheared in a cone-plate viscometer rapidly aggregate. Over a range of shear rates (400-800 s-1), approximately 90% of the single cells were recruited into aggregates ranging from doublets to groupings larger than sextuplets. The adhesion efficiency fit to these kinetics reached maximum levels of > 70%. Formed aggregates remained intact and resistant to shear up to 120 s, at which time they spontaneously dissociated back to singlets. The rate of cell disaggregation was linearly proportional to the applied shear rate, and it was approximately 60% lower for doublets as compared to larger aggregates. By accounting for the time-dependent changes in adhesion efficiency, disaggregation rate, and the effects of aggregate geometry, we succeeded in predicting the reversible kinetics of aggregation over a wide range of shear rates and cell concentrations. The combination of viscometry with flow cytometry and mathematical analysis as presented here represents a novel approach to differentiating between the effects of hydrodynamics and the intrinsic biological processes that control cell adhesion.

Cell Adhesion↗

PubMed1996

Induction of homotypic lymphocyte aggregation: evidence for a novel activation state of the beta1 integrin.

Intercellular adhesion of Jurkat lymphocytic cells was investigated by use of monoclonal antibodies 33B6 and 18D3, which bind to the beta1 integrin receptor. 33B6 induced homotypic aggregation of Jurkat cells, whereas 18D3 inhibited this aggregation. Jurkat cells could he induced to aggregate at low 33B6 concentrations corresponding to 5% beta1 integrin site occupancy, and the rate of aggregation was maximum at 30% occupancy. Simultaneous addition of mAb 18D3 and 33B6 demonstrated that the two antibodies mediate changes in the beta1 integrin activation state that are competitive in nature. Aggregation through beta1 integrin induced by 33B6 was reversed by subsequent addition of 18D3. To further examine the mechanism by which 33B6 and 18D3 affect cell adhesion function, we explored the binding of monoclonal antibody (mAb) 15/7. This mAb recognizes an activation epitope of the beta1 integrin and has been shown to sustain cell adhesion to vascular cell adhesion molecule 1 (VCAM-1) and fibronectin. Activation of Jurkat cells with Mn2+ caused a 2.5-fold increase in 15/7 binding but did not increase binding of 33B6. 33B6 partially blocked 15/7 binding to beta1 integrin on unstimulated and Mn2+-activated Jurkat cells. 18D3 did not affect mAb 15/7 binding. These results indicate that 33B6 and 18D3 modulated homotypic aggregation by inducing a novel activation state of the very late activation integrin distinct from the state recognized by 15/7, which supports cell binding to VCAM-1 and fibronectin.

Antibodies, Monoclonal↗

PubMed1996

Molecular dynamics of the transition from L-selectin- to beta 2-integrin-dependent neutrophil adhesion under defined hydrodynamic shear.

Homotypic adhesion o2 neutrophils stimulated with chemoattractant is analogous to capture on vascular endothelium in that both processes depend on L-selectin and beta 2-integrin adhesion receptors. Under hydrodynamic shear, cell adhesion requires that receptors bind sufficient ligand over the duration of intercellular contact to withstand hydrodynamic stresses. Using cone-plate viscometry to apply a uniform linear shear field to suspensions of neutrophils, we conducted a detailed examination of the effect of shear rate and shear stress on the kinetics of cell aggregation. A collisional analysis based on Smoluchowski's flocculation theory was employed to fit the kinetics of aggregation with an adhesion efficiency. Adhesion efficiency increased with shear rate from approximately 20% at 100 s-1 to approximately 80% at 400 s-1. The increase in adhesion efficiency. Adhesion efficiency increased with shear rate from approximately 20% at 100 s-1 to approximately 80% at 400 s-1. The increase in adhesion efficiency with shear was dependent on L-selectin, and peak efficiency was maintained over a relatively narrow range of shear rates (400-800 s-1) and shear stresses (4-7 dyn/cm2). When L-selectin was blocked with antibody, beta 2-integrin (CD11a, b) supported adhesion at low shear rates (< 400 s-1). The binding kinetics of selectin and integrin appear to be optimized to function within discrete ranges of shear rate and stress, providing an intrinsic mechanism for the transition from neutrophil tethering to stable adhesion.

Antibodies, Monoclonal↗

PubMedDate not supplied

A quantitative assay for intercellular aggregation.

In an earlier communication (Munn et al., J Immunol. Methods 166: 11-25, 1993), we presented the initial development of a quantitative assay for monitoring the rates of cellular aggregation based on digital image processing and video microscopy. This study describes some important enhancements and modifications to the procedure. A new index is introduced to characterize the three-dimensional morphology of the aggregates. This index is based on temporal changes in the projected area of the cells and cell aggregates during the course of the experiment. By drawing an analogy with the kinetic theory of gases, we have also introduced a procedure to normalize for variations in cell seeding density among different experiments. In addition, the image analysis technique has been improved by introducing a background subtraction algorithm to remove illumination defects and an adaptive segmentation procedure. These improvements allowed us to completely automate the image analysis procedure, thus minimizing user intervention and improving the reproducibility of the measurements. The enhanced visual assay is evaluated using some recent results from our studies on homotypic lymphocyte aggregation.

Cell Aggregation↗