Local force induced conical protrusions of phagocytic cells

Engineering physical microenvironments to study innate immune cell biophysics

Innate immunity forms the core of the human body's defense system against infection, injury, and foreign objects. It aims to maintain homeostasis by promoting inflammation and then initiating tissue repair, but it can also lead to disease when dysregulated. Although innate immune cells respond to their physical microenvironment and carry out intrinsically mechanical actions such as migration and phagocytosis, we still do not have a complete biophysical description of innate immunity. Here, we review how engineering tools can be used to study innate immune cell biophysics. We first provide an overview of innate immunity from a biophysical perspective, review the biophysical factors that affect the innate immune system, and then explore innate immune cell biophysics in the context of migration, phagocytosis, and phenotype polarization. Throughout the review, we highlight how physical microenvironments can be designed to probe the innate immune system, discuss how biophysical insight gained from these studies can be used to generate a more comprehensive description of innate immunity, and briefly comment on how this insight could be used to develop mechanical immune biomarkers and immunomodulatory therapies.

Size-Dependent Internalization Efficiency of Macrophages from Adsorbed Nanoparticle-Based Monolayers

Functional coatings based on the assembly of submicrometric or nanoparticles are found in many applications in the biomedical field. However, these nanoparticle-based coatings are particularly fragile since they could be exposed to cells that are able to internalize nanoparticles. Here, we studied the efficiency of RAW 264.7 murine macrophages to internalize physisorbed silica nanoparticles as a function of time and particle size. This cell internalization efficiency was evaluated from the damages induced by the cells in the nanoparticle-based monolayer on the basis of scanning electron microscopy and confocal laser scanning microscopy observations. The internalization efficiency in terms of the percentage of nanoparticles cleared from the substrate is characterized by two size-dependent regimes. Additionally, we highlighted that a delay before internalization occurs, which increases with decreasing adsorbed nanoparticle size. This internalization is characterized by a minimal threshold that corresponds to 35 nm nanoparticles that are not internalized during the 12-h incubation considered in this work.

Macrophages lift off surface-bound bacteria using a filopodium-lamellipodium hook-and-shovel mechanism

To clear pathogens from host tissues or biomaterial surfaces, phagocytes have to break the adhesive bacteria-substrate interactions. Here we analysed the mechanobiological process that enables macrophages to lift-off and phagocytose surface-bound Escherichia coli (E. coli). In this opsonin-independent process, macrophage filopodia hold on to the E. coli fimbriae long enough to induce a local protrusion of a lamellipodium. Specific contacts between the macrophage and E. coli are formed via the glycoprotein CD48 on filopodia and the adhesin FimH on type 1 fimbriae (hook). We show that bacterial detachment from surfaces occurrs after a lamellipodium has protruded underneath the bacterium (shovel), thereby breaking the multiple bacterium-surface interactions. After lift-off, the bacterium is engulfed by a phagocytic cup. Force activated catch bonds enable the long-term survival of the filopodium-fimbrium interactions while soluble mannose inhibitors and CD48 antibodies suppress the contact formation and thereby inhibit subsequent E. coli phagocytosis.

Duration of microbead seeding on endothelial cells significantly affects their response to magnetic excitation

Our investigation of endothelial cell rheology using optical magnetic twisting cytometry revealed that with time following incubation of ferromagnetic beads on the cells, beads were sinking into the cells and an increasing number of beads demonstrated apparent absurd negative rheological properties. In parallel, the beads' average rheological response changed considerably over time, both in magnitude and in distribution. It was hypothesized that the apparent negative rheological response was related to the above sinking process of seeded beads into the cells, resulting in an elevation of the beads' rotation axis, thus causing a reversal of the beads' lateral movement direction in response to twisting external magnetic excitation. The results suggest that microbead-based rheological characterization of cells should be interpreted with caution, while considering the time of data acquisition.

On the mechanical stabilization of filopodia

We studied force-induced elongation of filopodia by coupling magnetic tweezers to the tip through the bacterial coat protein invasin, which couples the force generator to the actin bundles (through myosin X), thus impeding the growth of the actin plus end. Single force pulses (15-30 s) with amplitudes between 20 and 600 pN and staircase-like force scenarios (amplitudes, ∼50 pN; step widths, 30 s) were applied. In both cases, the responses consist of a fast viscoelastic deflection followed by a linear flow regime. The deflections are reversible after switching off the forces, suggesting a mechanical memory. The elongation velocity exhibits an exponential distribution (half-width <v(1/2)>, ∼0.02 μm s(-1)) and did not increase systematically with the force amplitudes. We estimate the bending modulus (0.4 × 10(-23) J m) and the number of actin filaments (∼10) by analyzing filopodium bending fluctuations. Sequestering of intracellular Ca(2+) by BAPTA caused a strong reduction in the amplitude of elongation, whereas latrunculin A resulted in loss of the elastic response. We attribute the force-independent velocity to the elongation of actin bundles enabled by the force-induced actin membrane uncoupling and the reversibility by the treadmilling mechanism and an elastic response.

Manipulation of gold colloidal nanoparticles with atomic force microscopy in dynamic mode: influence of particle-substrate chemistry and morphology, and of operating conditions

One key component in the assembly of nanoparticles is their precise positioning to enable the creation of new complex nano-objects. Controlling the nanoscale interactions is crucial for the prediction and understanding of the behaviour of nanoparticles (NPs) during their assembly. In the present work, we have manipulated bare and functionalized gold nanoparticles on flat and patterned silicon and silicon coated substrates with dynamic atomic force microscopy (AFM). Under ambient conditions, the particles adhere to silicon until a critical drive amplitude is reached by oscillations of the probing tip. Beyond that threshold, the particles start to follow different directions, depending on their geometry, size and adhesion to the substrate. Higher and respectively, lower mobility was observed when the gold particles were coated with methyl (-CH(3)) and hydroxyl (-OH) terminated thiol groups. This major result suggests that the adhesion of the particles to the substrate is strongly reduced by the presence of hydrophobic interfaces. The influence of critical parameters on the manipulation was investigated and discussed viz. the shape, size and grafting of the NPs, as well as the surface chemistry and the patterning of the substrate, and finally the operating conditions (temperature, humidity and scan velocity). Whereas the operating conditions and substrate structure are shown to have a strong effect on the mobility of the particles, we did not find any differences when manipulating ordered vs random distributed particles.

Influence of type IV pilus retraction on the architecture of the Neisseria gonorrhoeae-infected cell cortex

Early in infection, Neisseria gonorrhoeae can be observed to attach to the epithelial cell surface as microcolonies and induce dramatic changes to the host cell cortex. We tested the hypothesis that type IV pili (Tfp) retraction plays a role in the ultrastructure of both the host cell cortex and the bacterial microcolony. Using serial ultrathin sectioning, transmission electron microscopy and 3D reconstruction of serial 2D images, we have obtained what we believe to be the first 3D reconstructions of the N. gonorrhoeae-host cell interface, and determined the architecture of infected cell microvilli as well as the attached microcolony. Tfp connect both wild-type (wt) and Tfp retraction-deficient bacteria with each other, and with the host cell membrane. Tfp fibres and microvilli form a lattice in the wt microcolony and at its periphery. Wt microcolonies induce microvilli formation and increases of surface area, leading to an approximately ninefold increase in the surface area of the host cell membrane at the site of attachment. In contrast, Tfp retraction-deficient microcolonies do not affect these parameters. Wt microcolonies had a symmetrical, dome-shaped structure with a circular 'footprint', while Tfp retraction-deficient microcolonies were notably less symmetrical. These findings support a major role for Tfp retraction in microvilli and microcolony architecture. They are consistent with the biophysical attributes of Tfp and the effects of Tfp retraction on epithelial cell signalling.

Differential force microscope for long time-scale biophysical measurements

Force microscopy techniques including optical trapping, magnetic tweezers, and atomic force microscopy (AFM) have facilitated quantification of forces and distances on the molecular scale. However, sensitivity and stability limitations have prevented the application of these techniques to biophysical systems that generate large forces over long times, such as actin filament networks. Growth of actin networks drives cellular shape change and generates nano-Newtons of force over time scales of minutes to hours, and consequently network growth properties have been difficult to study. Here, we present an AFM-based differential force microscope with integrated epifluorescence imaging in which two adjacent cantilevers on the same rigid support are used to provide increased measurement stability. We demonstrate 14 nm displacement control over measurement times of 3 hours and apply the instrument to quantify actin network growth in vitro under controlled loads. By measuring both network length and total network fluorescence simultaneously, we show that the average cross-sectional density of the growing network remains constant under static loads. The differential force microscope presented here provides a sensitive method for quantifying force and displacement with long time-scale stability that is useful for measurements of slow biophysical processes in whole cells or in reconstituted molecular systems in vitro.

Micromechanics of filopodia mediated capture of pathogens by macrophages

The biological function of filopodia has been extensively studied while only little work has been done on their mechanical properties. In the present study, we apply magnetic microbeads to explore the capturing and initial step of phagocytosis of pathogens by macrophages through filopodia. Microbeads were covered by the bacterial coat protein invasin which is known to trigger the invasion of the intestine by the bacteria Yersinia enterocolitica. These mimetics of bacteria were placed in the vicinity of J774 mouse macrophages exhibiting long filopodia. The specific adhesion of beads to the tip of a filopodium induced the retraction of the protrusion resulting in the dragging of the bead towards the cell body. The dynamics of the retraction process was analyzed by following the in-plane motion of the bead. We estimated the minimal force developed by filopodia and compared the results with previous magnetic tweezer studies of mechanical force induced growth of protrusions (Vonna et al. 2003). We show that very thin filopodia can generate astonishingly large retraction forces over large distances (>10 microm) and can act as an efficient mechanical tool to detach pathogens adhering on surfaces.

Active mechanical stabilization of the viscoplastic intracellular space of Dictyostelia cells by microtubule-actin crosstalk

The micro-viscoelasticity of the intracellular space of Dictyostelium discoideum cells is studied by evaluating the intracellular transport of magnetic force probes and their viscoelastic responses to force pulses of 20-700 pN. The role of the actin cortex, the microtubule (MT) aster and their crosstalk is explored by comparing the behaviour of wild-type cells, myosin II null mutants, latrunculin A and benomyl treated cells. The MT coupled beads perform irregular local and long range directed motions which are characterized by measuring their velocity distributions (P(v)). The correlated motion of the MT and the centrosome are evaluated by microfluorescence of GFP-labelled MTs. P(v) can be represented by log-normal distributions with long tails and it is determined by random sweeping motions (v approximately 0.5 microm/s) of the MTs (caused by tangential forces on the filament ends coupled to the actin cortex) and by intermittent bead transports parallel to the MTs (v(max) approximately 1.5 microm/s). The tails are due to spontaneous filament deflections (with speeds up to 10 microm/s) attributed to pre-stressing of the MT by local cortical tensions, generated by dynactin motors generating plus-end directed forces in the MTs. The viscoelastic responses are strongly non-linear and are mostly directed opposite or perpendicular to the force, showing that the cytoplasm behaves as an active viscoplastic body with time and force dependent drag coefficients. Nano-Newton loads exerted on the soft MT are balanced by traction forces arising at the MT ends coupled to the actin cortex and the centrosome, respectively. The mechanical coupling between the soft microtubules and the viscoelastic actin cortex provides cells with high mechanical stability despite the softness of the cytoplasm.

Mechanics of neutrophil phagocytosis: experiments and quantitative models

To quantitatively characterize the mechanical processes that drive phagocytosis, we observed the FcγR-driven engulfment of antibody-coated beads of diameters 3 μm to 11 μm by initially spherical neutrophils. In particular, the time course of cell morphology, of bead motion and of cortical tension were determined. Here, we introduce a number of mechanistic models for phagocytosis and test their validity by comparing the experimental data with finite element computations for multiple bead sizes. We find that the optimal models involve two key mechanical interactions: a repulsion or pressure between cytoskeleton and free membrane that drives protrusion, and an attraction between cytoskeleton and membrane newly adherent to the bead that flattens the cell into a thin lamella. Other models such as cytoskeletal expansion or swelling appear to be ruled out as main drivers of phagocytosis because of the characteristics of bead motion during engulfment. We finally show that the protrusive force necessary for the engulfment of large beads points towards storage of strain energy in the cytoskeleton over a large distance from the leading edge (∼0.5 μm), and that the flattening force can plausibly be generated by the known concentrations of unconventional myosins at the leading edge.

Weak force stalls protrusion at the leading edge of the lamellipodium

Protrusion, the first step of cell migration, is driven by actin polymerization coupled to adhesion at the cell's leading edge. Polymerization and adhesive forces have been estimated, but the net protrusion force has not been measured accurately. We arrest the leading edge of a moving fish keratocyte with a hydrodynamic load generated by a fluid flow from a micropipette. The flow arrests protrusion locally as the cell approaches the pipette, causing an arc-shaped indentation and upward folding of the leading edge. The effect of the flow is reversible upon pipette removal and dependent on the flow direction, suggesting that it is a direct effect of the external force rather than a regulated cellular response. Modeling of the fluid flow gives a surprisingly low value for the arresting force of just a few piconewtons per micrometer. Enhanced phase contrast, fluorescence, and interference reflection microscopy suggest that the flow does not abolish actin polymerization and does not disrupt the adhesions formed before the arrest but rather interferes with weak nascent adhesions at the very front of the cell. We conclude that a weak external force is sufficient to reorient the growing actin network at the leading edge and to stall the protrusion.

Viscoelasticity of entangled actin networks studied by long-pulse magnetic bead microrheometry

We studied the viscoelastic response of entangled actin networks using embedded microbeads driven by force pulses with amplitudes in the range from 3 to 120 pN and durations up to 60 s. We distinguished three regimes in the time dependence of the compliance J(t) of the network. These were characterized by specific power laws J(t) approximately t(alpha)(i) (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent alpha1 approximately 0.75. In the long-time regime (i=3), we find that alpha3 approximately 1. For the intermediate-time interval (i=2), we observed a novel dynamic regime: for all actin concentrations and all applied forces, it was characterized by the exponent alpha3 approximately 0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J approximately c(-gamma)(i) with gamma2 approximately 1.1 and gamma 3 approximately 1.4. Using these results, we calculated the shear modulus in the frequency domain and found that the intermediate-time regime in the t domain corresponds to its plateau behavior.

An introduction to cell motility for the physical scientist

Directed, purposeful movement is one of the qualities that we most closely associate with living organisms, and essentially all known forms of life on this planet exhibit some type of self-generated movement or motility. Even organisms that remain sessile most of the time, like flowering plants and trees, are quite busy at the cellular level, with large organelles, including chloroplasts, constantly racing around within cellular boundaries. Directed biological movement requires that the cell be able to convert its abundant stores of chemical energy into mechanical energy. Understanding how this mechanochemical energy transduction takes place and understanding how small biological forces generated at the molecular level are marshaled and organized for large-scale cellular or organismal movements are the focus of the field of cell motility. This tutorial, aimed at readers with a background in physical sciences, surveys the state of current knowledge and recent advances in modeling cell motility.

Active microrheology of networks composed of semiflexible polymers: theory and comparison with simulations

Based on the results of our computer simulation [Ter-Oganessian et al., Phys. Rev. E 72, 041510 (2005)], we have developed a theoretical description of the motion of a bead, embedded in a network of semiflexible polymers (filaments) and responding to an applied force. The theory reveals the existence of an osmotic restoring force, generated by the piling up of filaments in front of the moving bead and first deduced through computer simulations. The theory predicts that the bead displacement scales like x approximately t(alpha) with time, where alpha = (1/2) in an intermediate-time regime and alpha = 1 in a long-time regime. It also predicts that the compliance varies with a concentration like c(4/3) in agreement with experiment.

Protrusion force transmission of amoeboid cells crawling on soft biological tissue

We applied a colloidal force microscopy technique to measure the spreading and retraction forces generated by protrusions (pseudopodia) of vegetative amoeboid cells (Dictyostelium discoideum) adhering on soft tissue analogues composed of 2-mm thick hydrogels of hyaluronic acid exhibiting Young's moduli between 10 and 200 Pa. Local shear deformations of the polymer films evoked by magnetic tweezers and by cellular protrusions were determined by analyzing the deflections of colloidal beads randomly deposited on the surface of the polymer cushions, which enabled us to measure forces generated by advancing ("pushing" forces) and retracting ("pulling" forces) protrusions in a direct way. We found that the maximum amplitudes generated by the advancing protrusions (pushes) decrease with increasing stiffness of the HA substrate while the amplitudes of the retractions do not show such a dependence. The maximum forces transmitted by the advancing and retracting protrusions increase with increasing stiffness of the HA films (from 0.02 to 1 nN for the case of pushing). The protrusions spread or retract with constant velocities which are higher for retractions (100 nm s(-1)) than for spreadings (50 nm s(-1)) and are not significantly influenced by the substrate rigidity. We provide evidence that elastic equilibrium during protrusion formation and retraction is maintained by local elastic dipole fields generated at the rim of the protrusions. A model of protrusion force transmission by coupling of growing actin gel in the cytoplasm of the protrusions to cell surface receptors through talin clutches is proposed.

Microviscoelasticity of the apical cell surface of human umbilical vein endothelial cells (HUVEC) within confluent monolayers

We studied the local viscoelasticity of the apical membrane of human umbilical vein endothelial cells within confluent layers by magnetic tweezers microrheometry. Magnetic beads are coupled to various integrins by coating with fibronectin or invasin. By analyzing the deflection of beads evoked by various force scenarios we demonstrate that the cell envelope behaves as a linear viscoelastic body if forces up to 2 nN are applied for short times (<20 s) but can respond in an adaptive way if stress pulses are applied longer (>30 s). The time-dependent shear relaxation modulus G(t) exhibits three time regimes: a fast response (t < 0.05 s) where the relaxation modulus G(t) obeys a power law G(t) approximately t(-0.82+/-0.02); a plateau-like behavior (at 0.05 s < t < 0.15 s); and a slow flow-like response which is, however, partially reversible. Strain field mapping experiments with colloidal probes show that local forces induce a strain field exhibiting a range of zeta = 10 +/- 1 microm, but which could only be observed if nonmagnetic beads were coupled to the cell surface by invasin. By application of the theory of elasticity of planar bodies we estimated a surface shear modulus of 2.5 x10(-4) N/m. By assuming a thickness of the actin cortex of approximately 0.5 microm we estimate a Young modulus micro approximately 400 Pa for the apical membrane. The value agrees with a plateau modulus of an entangled or weakly cross-linked actin network of an actin concentration of 100 microM (mesh size 0.2 microm). This result together with our observation of a strong reduction of the shear modulus by the actin destabilizing agent latrunculin A suggests that the shear modulus measured by our technique is determined by the actin cortex. The effect of two ligands inducing actin stress fiber formation and centripetal contraction of cells (associated with the formation of gaps in the confluent cell monolayer) on the viscoelastic responses were studied: histamine and lysophosphatidic acid (LPA). Histamine evoked a dramatic increase of the cell stiffness by >1 order of magnitude within <30 s, which is attributed to a transient rise of the intracellular Ca(2+) level, since DMSO exerted a similar effect. The stiffening is accompanied by a concomitant rounding of the cells as observed by microinterferometry and relaxes partially in the timescale of 5 min, whereas gaps between cells close after approximately 30 min. LPA did not exert a remarkable and reproducible effect other than an occasional very weak transient increase of the shear stiffness, which shows that the gap formation activated by LPA is mediated by a different mechanism than that induced by histamine.

Crawling toward a unified model of cell mobility: spatial and temporal regulation of actin dynamics

Crawling cells of various morphologies displace themselves in their biological environments by a similar overall mechanism of protrusion through actin assembly at the front coordinated with retraction at the rear. Different cell types organize very distinct protruding structures, yet they do so through conserved biochemical mechanisms to regulate actin polymerization dynamics and vary the mechanical properties of these structures. The moving cell must spatially and temporally regulate the biochemical interactions of its protein components to exert control over higher-order dynamic structures created by these proteins and global cellular responses four or more orders of magnitude larger in scale and longer in time than the individual protein-protein interactions that comprise them. To fulfill its biological role, a cell globally responds with high sensitivity to a local perturbation or signal and coordinates its many intracellular actin-based functional structures with the physical environment it experiences to produce directed movement. This review attempts to codify some unifying principles for cell motility that span organizational scales from single protein polymer filaments to whole crawling cells.

Regulation of pseudopodia localization in lymphocytes through application of mechanical forces by optical tweezers

T-lymphocytes are responsible for cell-mediated immunity, and recognize antigens on target cells (e.g., tumor cells, virus-infected cells) and antigen presenting cells (e.g., macrophages, dendritic cells). While mechanical forces applied to a cell surface can produce alterations in the cytoskeletal structure, leading to global structural rearrangements and changes in the intracellular biochemistry and gene expression, it remains unknown if local mechanical forces acting at the lymphocyte-antigen interaction site play any role in lymphocyte activation following antigen recognition. In this study we investigate the effect of such forces induced by optical tweezers on the lymphocyte's morphological response. We brought optically trapped polystyrene beads, coated with a specific antibody against a clonotypic epitope of the T-cell receptors (TCRs), in contact with individual lymphocytes and applied mechanical forces at the TCR-antibody interaction site. Although bead size was a factor, simple bead contact tended to induce formation of pseudopodia that appeared randomly over the cell's surface, while application of tangential forces at the interaction site redirected pseudopodia formation toward that site and promoted endocytosis activity. We propose that local forces play a key role in the initial lymphocyte adhesion to antigen-bearing cells, and may be implicated in antigen-specific motility, transendothelial migration, and tissue homing to sites of inflammation.