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First published online April 24, 2006
doi: 10.1242/10.1242/jcs.02876

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Mechanics of neutrophil phagocytosis: experiments and quantitative models

¹ Biomedical Engineering Department, Boston University, 44 Cummington Street, Boston, MA 02215, USA
² Department of Biomedical Engineering, University of California, 451 East Health Sciences Drive, Davis, CA 95616, USA

View larger version (13K): [in a new window]   Fig. 1. (A) The pedestal-phase, during which the cell body acquires a slight indentation. (B) The cup-phase, during which lamellae sharply emerge from the cell body to spread over the bead. Notice the distinctive contact angle between cell body and lamella (inset). (C) The rounding-phase, during which the cell rounds up, the contact angle between cell body and lamella vanishes and the bead is pulled inwards.

View larger version (37K): [in a new window]   Fig. 2. Schematic description of a mechanical model of neutrophil phagocytosis. The cell-bead interface is stabilized by adhesion sites that act as anchors to the internal cytoskeleton. Myosin motors draw down F-actin, thus pulling the bead into the cell or, equivalently, the cell onto the bead. Polymerization of new actin near the contact line (the leading edge) drives protrusion around the bead.

View larger version (58K): [in a new window]   Fig. 3. Phagocytosis of 3.2 µm (top) and 4.6 µm (bottom) beads. Experimental and simulation images shown at synchronized times indicated by orange arrows on line-plots. Color inside the simulated cells indicates the cytoskeletal volume fraction in percent (% network). Tension plots show experimental data in grey (filled squares correspond to shown micrographs, empty squares to other phagocytoses of same diameter beads purposefully selected for significant difference). For graphs on the left, the heavy black line shows total surface tension from simulation (red is the elastic contribution el and blue is the viscous contribution vis). On the right, bead center position is shown with respect to the back of the cell. A magnification of the boxed area shown in the middle image of the second half of this figure is shown in Fig. 5.

View larger version (57K): [in a new window]   Fig. 4. Same as previous figure for the phagocytosis of 6.2 µm (top) and 11.2 µm (bottom) beads.

View larger version (63K): [in a new window]   Fig. 5. Structure of the lamella for the 4.6 µm phagocytosis at 55 seconds (see boxed area in Fig. 3). (Top) Cytoskeletal volume fraction and velocity field with respect to the bead (substratum frame). (Bottom) Polymerization messenger concentration and cytoskeletal velocity field with respect to the cell (lamella frame). The white patch of membrane corresponds to the region of enhanced messenger emissivity. Notice that, the velocity of the cytoskeleton vanishes in the substratum frame at the cell-bead interface due to the no-slip boundary condition. Notice also the high density of cytoskeleton at that same interface due to the contractile force directed down toward the substratum.

View larger version (37K): [in a new window]   Fig. 6. Comparison of simulations with and without contractile flattening force; 3.2 µm bead (left), 6.2 µm bead (right); with contractility (top), without contractility (bottom).

View larger version (20K): [in a new window]   Fig. 7. Bead position vs time for simulations of the phagocytosis of a 3.2 µm bead. Solid lines correspond to cytoskeleton-membrane repulsion models, dashed lines to cytoskeletal swelling models. For each model, the flattening force was varied by ±33% around the best fit (thick line). Notice the strong dependence of the bead-trajectory in the cytoskeleton swelling model. Notice also that, for the best fit in this model, the shape of the cell at 20 seconds shows a phagocytic cup of excessive thickness compared with observations.

View larger version (18K): [in a new window]   Fig. 8. Impact of cytoplasmic viscosity and cortical tension on transitional cell shapes. (Top) Two patterns in the spreading of an initially hemispherical droplet. (Bottom) Simulations of 11-µm-bead phagocytosis with two different levels of membrane slack (26% and 52%). Tension is plotted [black, normal slack (taut), grey, increased slack] together with cell shapes at 60 seconds. Notice the effacement of the body-lamella contact angle in the taut simulation.

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