Characterization of transient elasticity in neutrophils

Leukocyte deformability is important for the function of these cells as they undergo passage through the vasculature and adhere and deform on the endothelium, particularly in the microvasculature. Existing mechanical descriptions of the cell fail to account completely for cell behavior, even when cells are in the passive state. We present new evidence of a persistent elastic contribution to the stresses within the cell throughout continuous deformation of the cell into a cylindrical pipette. We have developed a model of the cell behavior based on transient elasticity, and we have performed measurements to characterize cell properties in terms of material coefficients. We used an approximate model of the cell deformation to estimate each of these coefficients from experimental measurements. The elastic portion of the cell response shows strain stiffening and is modeled as an incompressible Gent material that includes two material constants, an elastic modulus (∼28 ± 7 Pa) and a strain stiffening parameter that corresponds to a maximum elastic extension of approximately 80%. The steady-state flow of the cell into the pipette reflects a force-dependent relaxation of the elastic stress and an underlying cytoplasmic viscosity (32.1 ± 0.2 Pa s). The stress-relaxation model contains two coefficients, a network stability coefficient corresponding to the spontaneous rate of network relaxation at zero stress (0.62 ± 0.49 s^-1) and a constant characterizing the dependence of the relaxation rate on stress (0.0026 ± 0.0014 Pa^-1). This new model provides a comprehensive description of the mechanical response of passive neutrophils to applied force and establishes a new paradigm for modeling the mechanical responses of leukocytes and similar cells to applied force.