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Neutrophil Traction Stresses are Concentrated in the Uropod during Migration

Lee A. Smith ^*, Helim Aranda-Espinoza , Jered B. Haun , Micah Dembo  and Daniel A. Hammer 

^* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania; Department of Bioengineering, University of Maryland, College Park, Maryland; Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania; and Department of Biomedical Engineering, Boston University, Boston, Massachusetts

Correspondence: Address reprint requests and inquiries to Daniel A. Hammer, Tel.: 215-573-6361; E-mail: hammer{at}seas.upenn.edu.

We find that in contrast to strongly adherent, slow moving cells such as fibroblasts, neutrophils exert contractile stresses largely in the rear of the cell (uropod) relative to the direction of motion. Rather than the leading edge pulling the cell, the rear is both anchoring the cell and the area in which the contractile forces are concentrated. These tractions rapidly reorient themselves during a turn, on a timescale of seconds to minutes, and their repositioning precedes and sets the direction of motion during a turn. We find the total average root mean-squared traction force to be 28 ± 10 nN during chemokinesis, and 67 ± 10 nN during chemotaxis. We hypothesize that the contraction forces in the back of the neutrophil not only break uropodial adhesive contacts but also create a rearward squeezing contractility, as seen in amoeboid or amoeboidlike cells and the formation of blebs in cells, causing a flow of intracellular material to the fluidlike lamellipod. Our findings suggest an entirely new model of neutrophil locomotion.

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