Specific recognition of macroscopic objects by the cell surface: evidence for a receptor density threshold revealed by micrometric particle binding characteristics

Measuring αβ T-Cell Receptor-Mediated Mechanosensing Using Optical Tweezers Combined with Fluorescence Imaging

T-cell antigen receptors (TCRs) are mechanosensors, which initiate a signaling cascade upon ligand recognition resulting in T-cell differentiation, homeostasis, effector and regulatory functions. An optical trap combined with fluorescence permits direct monitoring of T-cell triggering in response to force application at various concentrations of peptide-bound major histocompatibility complex molecules (pMHC). The technique mimics physiological shear forces applied as cells crawl across antigen-presenting surfaces during immune surveillance. True single molecule studies performed on single cells profile force-bond lifetime, typically seen as a catch bond, and conformational change at the TCR-pMHC bond on the surface of the cell upon force loading. Together, activation and single molecule single cell studies provide chemical and physical triggering thresholds as well as insight into catch bond formation and quaternary structural changes of single TCRs. The present methods detail assay design, preparation, and execution, as well as data analysis. These methods may be applied to a wide range of pMHC-TCR interactions and have potential for adaptation to other receptor-ligand systems.

Direct Comparison of B Cell Surface Receptors as Therapeutic Targets for Nanoparticle Delivery of BTK Inhibitors

Targeting different cell surface receptors with nanoparticle (NP)-based platforms can result in differential particle binding properties that may impact their localization, bioavailability, and, ultimately, the therapeutic efficacy of an encapsulated payload. Conventional in vitro assays comparing the efficacy of targeted NPs often do not adequately control for these differences in particle-receptor binding, potentially confounding their therapeutic readouts and possibly even limiting their experimental value. In this work, we characterize the conditions under which NPs loaded with Bruton's Tyrosine Kinase (BTK) inhibitor differentially suppress primary B cell activation when targeting either CD19 (internalizing) or B220 (noninternalizing) surface receptors. Surface binding of fluorescently labeled CD19- and B220-targeted NPs was analyzed and quantitatively correlated with the number of bound particles at given treatment concentrations. Using this binding data, suppression of B cell activation was directly compared for differentially targeted (CD19 vs B220) NPs loaded with a BTK inhibitor at a range of particle drug loading concentrations. When NPs were loaded with lower amounts of drug, CD19-mediated internalization demonstrated increased inhibition of B cell proliferation compared with B220 NPs. However, these differences were mitigated when particles were loaded with higher concentrations of BTK inhibitor and B220-mediated "paracrine-like" delivery demonstrated superior suppression of cellular activation when cells were bound to lower overall numbers of NPs. Taken together, these results demonstrate that inhibition of B cell activation can be optimized for NPs targeting either internalizing or noninternalizing surface receptors and that particle internalization is likely not a requisite endpoint when designing particles for delivery of BTK inhibitor to B cells.

A new model for cell division and migration with spontaneous topology changes

Tissue topology, in particular proliferating epithelium topology, is remarkably similar between various species. Understanding the mechanisms that result in the observed topologies is needed for better insight into the processes governing tissue formation. We present a two-dimensional single-cell based model for cell divisions and tissue growth. The model accounts for cell mechanics and allows cell migration. Cells do not have pre-existing shapes or topologies. Shape changes and local rearrangements occur naturally as a response to the evolving cellular environment and cell-cell interactions. We show that the commonly observed tissue topologies arise spontaneously from this model. We consider different cellular rearrangements that accompany tissue growth and study their effects on tissue topology.

Cholesterol loading and ultrastable protein interactions determine the level of tumor marker required for optimal isolation of cancer cells

Cell isolation via antibody-targeted magnetic beads is a powerful tool for research and clinical applications, most recently for isolating circulating tumor cells (CTC). Nonetheless fundamental features of the cell-bead interface are still unknown. Here we apply a clinically relevant antibody against the cancer target HER2 (ErbB2) for magnetic cell isolation. We investigate how many target proteins per cell are sufficient for a cell to be isolated. To understand the importance of primary antibody affinity, we compared a series of point mutants with known affinities and show that even starting with subnanomolar affinity, improving antibody affinity improved cell isolation. To test the importance of the connection between the primary antibody and the magnetic bead, we compared bridging the antibody to the beads with Protein L, secondary antibody, or streptavidin: the high-stability streptavidin-biotin linkage improved sensitivity by an order of magnitude. Cytoskeletal polymerization did not have a major effect on cell isolation, but isolation was inhibited by cholesterol depletion and enhanced by cholesterol loading of cells. Analyzing a panel of human cancer cell lines spanning a wide range of expression showed that the standard approach could only isolate the highest expressing cells. However, our optimization of cholesterol level, primary antibody affinity, and antibody-bead linkage allowed efficient and specific isolation of cells expressing low levels of HER2 or epithelial cell adhesion molecule. These insights should guide future approaches to cell isolation, either magnetically or using other means, and extend the range of cellular antigens and biomarkers that can be targeted for CTC isolation in cancer research and diagnosis.

Theoretical study of the competition between cell-cell and cell-matrix adhesions

Adhesions between neighboring cells or between cells and their surrounding tissue/matrix play a crucial role in a wide range of biological processes. In order to investigate the competitive mechanisms between cell-cell and cell-matrix adhesions, we here develop a theoretical framework for multiple interacting cells lying on a planar matrix coated with distributed ligands. This model allows us to study, from the viewpoints of energy and statistics, the effects of such physical mechanisms as binding energy of bonds, nonspecific interactions, elastic deformation of cell membranes, and mixing entropy. Our calculations show that cell-matrix adhesion cannot occur when the ligand density on the matrix is lower than a threshold value, and cell-cell adhesion does not happen for a high ligand density. Glycocalyx repulsion plays a more important role in cell-matrix adhesion than in cell-cell adhesion. In addition, it is found that the cell-cell adhesion density decreases as the number of cells increases.

T-cell artificial focal triggering tools: linking surface interactions with cell response

T-cell activation is a key event in the immune system, involving the interaction of several receptor ligand pairs in a complex intercellular contact that forms between T-cell and antigen-presenting cells. Molecular components implicated in contact formation have been identified, but the mechanism of activation and the link between molecular interactions and cell response remain poorly understood due to the complexity and dynamics exhibited by whole cell-cell conjugates. Here we demonstrate that simplified model colloids grafted so as to target appropriate cell receptors can be efficiently used to explore the relationship of receptor engagement to the T-cell response. Using immortalized Jurkat T cells, we monitored both binding and activation events, as seen by changes in the intracellular calcium concentration. Our experimental strategy used flow cytometry analysis to follow the short time scale cell response in populations of thousands of cells. We targeted both T-cell receptor CD3 (TCR/CD3) and leukocyte-function-associated antigen (LFA-1) alone or in combination. We showed that specific engagement of TCR/CD3 with a single particle induced a transient calcium signal, confirming previous results and validating our approach. By decreasing anti-CD3 particle density, we showed that contact nucleation was the most crucial and determining step in the cell-particle interaction under dynamic conditions, due to shear stress produced by hydrodynamic flow. Introduction of LFA-1 adhesion molecule ligands at the surface of the particle overcame this limitation and elucidated the low TCR/CD3 ligand density regime. Despite their simplicity, model colloids induced relevant biological responses which consistently echoed whole cell behavior. We thus concluded that this biophysical approach provides useful tools for investigating initial events in T-cell activation, and should enable the design of intelligent artificial systems for adoptive immunotherapy.

Compatibility between itinerant synaptic receptors and stable postsynaptic structure

The density of synaptic receptors in front of presynaptic release sites is stabilized in the presence of scaffold proteins, but the receptors and scaffold molecules have local exchanges with characteristic times shorter than that of the receptor-scaffold assembly. We propose a mesoscopic model to account for the regulation of the local density of receptors as quasiequilibrium. It is based on two zones (synaptic and extrasynaptic) and multilayer (membrane, submembrane, and cytoplasmic) topological organization. The model includes the balance of chemical potentials associated with the receptor and scaffold protein concentrations in the various compartments. The model shows highly cooperative behavior including a "phase change" resulting in the formation of well-defined postsynaptic domains. This study provides theoretical tools to approach the complex issue of synaptic stability at the synapse, where receptors are transiently trapped yet rapidly diffuse laterally on the plasma membrane.

Manipulating microparticles with single surface-immobilized nanoparticles

This experimental study explores the capture and manipulation of micrometer-scale particles by single surface-immobilized nanoparticles. The nanoparticles, approximately 10 nm in diameter, are cationic and therefore attract the micrometer-scale silica particles in an analyte suspension. The supporting surface on which the nanoparticles reside is negative (also silica) and repulsive toward approaching microparticles. In the limit where there are as few as 9 nanoparticles per square micrometer of collector, it becomes possible to capture and hold micrometer-scale silica particles with single nanoparticles. The strong nanoparticle-microparticle attractions, their nanometer-scale protrusion forward of the supporting surface, and their controlled density on the supporting surface facilitate microparticle-surface contact occurring through a single nanoelement. This behavior differs from most particle-particle, cell-cell, or particle (or cell)-surface interactions that involve multiple ligand-receptor bonds or much larger contact areas. Despite the limited contact of microparticles with surface-immobilized nanoparticles, microparticles resist shear forces of 9 pN or more but can be released through an increase in the ionic strength. The ability of nanoparticles to reversibly trap and hold much larger targets has implications in materials self-assembly, cell capture, and sorting applications, whereas the single point of contact affords precision in particle manipulation.

Molecular bond formation between surfaces: anchoring and shearing effects

Specific molecular bonds between apposing surfaces play a central role in many biological structures and functions. They display a widely varying anchoring to the cell surface, and they are subject to forces that affect their binding characteristics due to their hydrodynamic environments. Here, we examine both anchoring and shearing aspects using simplified model systems aimed at gaining insight into the formation of a 2D bond collection under stress using two different surface anchors. The highly specific streptavidin-biotin molecular bond was chosen as the model receptor-ligand pair, and grafted colloids were used as model surfaces. To explore the role of the surface anchor, we grafted biotin onto the particle surface following two different approaches: first, the grafting was performed directly on the particle amine functions; second, a 35-nm-long PEG spacer was used. Hybrid particle classes were brought into contact in a homogeneous shear (between 200 s(-)(1) and 1200 s(-)(1)) using a cone plate geometry. The bond association and dissociation kinetics were given by the time course assemblage of hybrid particles into doublets. We observed saturating kinetics profiles that we interpreted as a linkage-breakage equilibrium, which yielded the on and off rates. We found that the biotin-PEG spacer was needed in order to observe significant binding at any shear rate. We also showed that only the number of collisions per unit time, generated by the shear, affected the on rate of the binding. Neither the exerted forces nor the collision lifetime had any effect. The off rate decreased with shear, possibly because of the shortening of the force duration, which results from the increasing shear rate.