Human neutrophil surface protrusion under a point load: location independence and viscoelasticity

Mechanical properties of neutrophils have been recognized as key contributors to stabilizing neutrophil rolling on the endothelium during the inflammatory response. In particular, accumulating evidence suggests that surface protrusion and tether extraction from neutrophils facilitate stable rolling by relieving the disruptive forces on adhesive bonds. Using a customized optical trap setup, we applied piconewton-level pulling forces on targeted receptors that were located either on the microvillus tip (CD162) or intermicrovillus surface of neutrophils (CD18 and CD44). Under a constant force-loading rate, there always occurred an initial tent-like surface protrusion that was terminated either by rupture of the adhesion or by a "yield" or "crossover" to tether extraction. The corresponding protrusional stiffness of neutrophils was found to be between 0.06 and 0.11 pN/nm, depending on the force-loading rate and the cytoskeletal integrity, but not on the force location, the medium osmolality, nor the temperature increase from 22 degrees C to 37 degrees C. More importantly, we found that neutrophil surface protrusion was accompanied by force relaxation and hysteresis. In addition, the crossover force did not change much in the range of force-loading rates studied, and the protrusional stiffness of lymphocytes was similar to that of neutrophils. These results show that neutrophil surface protrusion is essentially viscoelastic, with a protrusional stiffness that stems primarily from the actin cortex, and the crossover force is independent of the receptor-cytoskeleton interaction.